In Daniudi gasfield, Ordos Basin, production differs greatly in porous carbonate reservoirs even after hydraulic sand fracturing due to strong reservoir heterogeneity. Thus, a model was established to compute the production contribution of each layer, and a heat map matrix chart was created on both geology-engineering parameters and production data. Moreover, the major geology-engineering factors influencing on the production were analyzed to evolve into a formula for evaluating double sweetspots and calculating productivity. Results show that (i) the production contribution in fifth submember of Majiagou 5 Member (Ma 5⁵, hereinafter same) negatively correlates with gamma ray and positively with interval transit time, bulk density and gas saturation. While that of Ma 5⁶⁺⁷ is positively correlated with reservoir thickness, bulk density and neutron porosity; (ii) the production in Ma 5^5-7 is in high correlation with reservoir thickness, maximum total hydrocarbon, porosity, gas saturation, and injected fracturing-liquid, and acid proppant volume; (iii) the established double-sweetspot index calculation model for Ma 5^5-7 is dependent on multiple factors, and the calculated index has the correlation of 0.937 5 with the production; and (iv) the created production prediction model is on the basis of the double-sweetspot index, and the error between the predicted and actual production is less than 2%. In conclusion, with high coincidence with reality, both geology-engineering sweetspot and productivity prediction models which provide active guidance for selecting staged fracturing and designing optimal fracturing are worth popularizing.

There are two problems appeared in four gas logging charts (triangle, 3H, specific value, and logarithmic charts) which are currently used in Shunbei field, for example inconsistent conclusions interpreted on oil and gas show, and great difference among completing test results. Thus, taken 157 wells in this field as examples, some data on mud-logging display and completing test information in some of the wells were gathered together and then selected to analyze the diversity in the occurrence of natural gas in reservoirs, the difference in the dissolution and precipitation of various component in water and crude oil, and the gas chromatography principle of composite logging instrument. In addition, two identifying parameters of humidity and oil-water separation coefficient were determined, and a crossplot chart was also developed. Better endings have been achieved in the trial of 28 wells newly drilled in this field. Results show that (i) the proportion of each component in total hydrocarbon can be calculated according to its molar mass even though total hydrocarbon and component are attached to different analysis systems; and (ii) the humidity can effectively separate a gas layer from oil or water layers in the chart, while the separation coefficient can obviously separate a water layer from an oil layer in the chart. In conclusion, the difference between total hydrocarbon and the sum of each component can qualitatively reflect the content of heavy component in the fluid. What's more, the chart with the humidity as the vertical axis and the separation coefficient as the horizontal axis can better discriminate fluid properties in Shunbei field.

At present, the Lower Paleozoic carbonate gas reservoirs are regarded as the main productivity substitute in Daniudi gasfield. But how each interval dedicates to the production in carbonate horizontal wells is unclear, and the main controls on this dedication are ambiguous as well, all conditioning the production increase, cost reduction and efficiency improvement. To this end, some difference among these intervals was found out through liquid-producing profile tested by coiled tubing. In addition, the main controls was made clear. Results show that (i) the stimulation size exerts little impact on each interval production when one horizontal interval is located in certain reservoirs with better physical properties, and these properties and gas content as fundamental factors can affect the productivity in each interval; and (ii) when one interval is located in other reservoirs whose physical properties are poor with small difference, the physical properties, gas content and fracturing scale still have little impact on each interval production. Nevertheless, stimulation may be gained in such poor reservoirs through special fracturing. In conclusion, it is recommended for carbonate gas reservoirs in horizontal wells to prioritize the deployment in favorable blocks developed with fractured-porous reservoirs, followed by appropriately increasing the stimulation scale for efficiency improvement in tight carbonate gas reservoirs. What's more, for a certain block developed with porous reservoirs, it is supposed to make differentiated stimulation measures for various reservoir types to achieve effective production in each interval.

A lack of data on initial pressure in sour gas reservoirs, T block, northeastern Sichuan Basin, may bring about much hardship in sorting out change laws of formation pressure during reservoir development. Thus, a model was built to predict the sulfur solubility in such reservoirs according to dynamical data. Then, gas composition and some critical parameters were computed by means of gas chromatography experiments, and one approach to correct a deviation factor was selected to analyze the material-balance equation with consideration of sulfur deposition in these reservoirs. Finally, a formation-pressure calculation method which combines the equation with the correction was proposed. Results show that (i) the minimum relative error from DPR and Gopal approaches to measure the deviation factor is respectively 0.39% and 2.85% on average, indicating that these two approaches are more suitable for sour gas reservoirs in northeastern basin; (ii) the average error between calculations from the proposed method and the measured formation pressure ranges from 1.93% to 3.15%, standing for this newly method with smaller error than that from traditional ones, is more accuracy; and (iii) since there is no need for this method to hold that much data, it can reduce loss caused by shut-in pressure measurement in sour gas reservoirs with high yield.

The fifth member of the Ordovician Majiagou Formation (Ma 5 Member) in Daniudi gasfield is standing in regression. Vertically, it is divided into 10 submembers (Ma 5¹ to Ma 5¹⁰), among which Ma 5¹⁺² is still in relative regression. Numerous dissolved pores, vugs and fractures induced from fresh water leaching and karstification are extended into Ma 5¹⁺² where gray dolomite is dominantly developed with a little limestone to accumulate hydrocarbon. Therefore, cores, well- and mud-logging data from over 580 wells drilling through the target were analyzed to predict geological sweetspots of Ma 5¹⁺² in this field. And that there developed 4 types of sedimentary microfacies, including calcareous dolomite flat, dolomite flat, dolomitic limestone flat, and argillaceous dolomite flat, was pointed out. Furthermore, combined with paleogeomorphological natures, the thickness distribution of Ma 5¹⁺² reservoirs on the plane was characterized. By referring to production data of gas wells, the extension of quality reservoirs was predicted, and the sweetspots were delineated. Results show that (i) 7 episodes of sedimentary cycle are developed vertically in Ma 5¹⁺² where shale content increases from below, meaning the decreasing trend of waterbody depth and salinity while this content decreases from north to south on profile, implying the increasing trend of waterbody depth and salinity; and (ii) Ma 5¹⁺² reservoirs overall get thicker to the west, and their thickness varies greatly along north to south, with the maximum thickness difference of 24 m, but gently along east to west, with the maximum difference of 8 m. Six geomorphological units are identified, i.e., karst platform mound, monadnock, slope, highland, low-lying land and trough, and the reservoirs are mainly located in the first two units. In conclusion, the quality reservoirs are mainly deposited in both calcareous dolomite flat and dolomite flat. Rich in dolomite extended from northeast to southwest, the residual reservoirs are thick. This thickness often exceeds 4 m in both platform mound and monadnock, and that from 2 to 6 m in slope. Additionally, three sweetspot zones are selected on the basis of predominant microfacies, lithofacies and paleogeomorphological units. With the porosity exceeding 2.5%, the quality reservoirs are over 4 m in thickness. Imaging logging presents that fractures, dissolved pores and vugs are well developed, indicating better physical properties. Both gas saturation and total hydrocarbon content are over 15% and 20%, respectively. The testified cumulative production is more than 2000×10⁴ m³.

Taken fourteen geophysical prospecting projects in ST block as examples, three appraisal indexes including technical, application and economic effect were discussed for these projects during their full lifecycle in an effort to create the post evaluation system of prospecting effect, optimize exploration and development strategies, select the project access to investment, increase technical process and expand application profits. In addition, both route plan and computing workflow were proposed for the appraisal index system with five levels in the transverse direction. Results show that (i) among these fourteen projects, three ones like SN41N are regarded as the type I project due to excellent effect, nine ones such as AD with moderate effect as the type II, and the rest two projects with poor effect as the type III; and (ii) the re-inspection also demonstrates the appraisal results in line with practice in the study area. In conclusion, it is essential to continuously upgrade technological level in acquisition and interpretation of seismic data, to deepen the research on geological conditions to form reservoirs, to improve reservoir-protection skills, and to speed up the popularization of prospecting achievements. Figuring out the similarity and difference and their origin, our methodology may support technical advancement and is conducive to enhancing both management efficiency and exploration benefits for the projects. With quantifiable and attainable natures and strong practicality, the scientific and rational methodology gives expression to output efficiency and contributory value in projects' total process.

In an effort to speed up the productivity construction of tight gas in Sichuan Basin, gas reservoirs of the first member of the Middle Jurassic Shaximiao Formation (Shaximiao 1 Member) in Zitong block of northwestern basin were taken as examples to analyze geological and development characteristics. With complex geological conditions, the Shaximiao 1 reservoirs are quite different from the same reservoirs and Shaximiao 2 reservoirs individually in adjacent Jianyang and Jinqiu blocks. So, it's necessary to make available measures on development technologies through pilot tests. Results show that (i) developed with vertically superimposed multiple-episode channel sandstone, the Shaximiao 1 reservoirs in Zitong block are characterized by poor physical properties, small thickness, and strong heterogeneity; and (ii) there exist numerous pressure systems in these reservoirs with high water saturation, leading to big incidence of water production. In conclusion, Zitong block has a great prospect in natural-gas exploration and development, and will be a remarkably new domain for enlarging reserves and production in the future. Nevertheless, a shortage of static and dynamic data and inadequate understanding on reservoir geology make it essential to conduct pilot tests. What's more, there massively extended two suits of reservoirs in Zitong block, such as No. 1 and No. 4 sandbodies superimposed in Shaximiao 1 Member. And industrial gas flow has been received. Both sandbodies are selected as the target in pilot demonstration areas to make clear gas content in them. On top of that, learned from development experience in adjacent blocks, 8 horizontal wells with big included angle have been deployed and some efficient stimulation technologies have been optimized to establish a geology-engineering integration model, among which the designed output in single well is (6-9)×10⁴ m³/d, and the predicted cumulative production also in single well of (1.06-1.30)×10⁸ m³, respectively.

During west testing, there exists large difference in single-well productivity and poor effect in stable production for gas reservoirs of the fifth member of Xujiahe Formation (Xujiahe 5 Member) in Dongfengchang area. So, the main controls on the productivity and reasons on production decline were discussed in the light of geological, dynamic monitoring and experimental data. And some countermeasures were made for stabilizing production. Results show that (i) the productivity is principally affected by both types of gas wells and extensional degree of quality reservoirs. The productivity in horizontal wells is significantly higher than that in vertical ones. The more developed the quality reservoirs, the higher the productivity; (ii) for these wells, the stabilizing effect is poor with the average stable period of 10 months, and the production decline index of 0.0017 per day on average, creating low effective permeability and finite gas supply; (iii) induced from big production pressure difference caused by high proration in the early days of development, the reservoirs feature strong stress sensitivity. Moreover, small dew-point pressure difference results that condensate oil is easy to be precipitated to cause anti condensation damage, to bringing about permeability loss, and finally the rapid productivity decline; (iv) fracturing sand’s backflow results in the decrease of the conductivity in induced fractures and these fractures accumulated near wellbore area. And it’s hard for fracturing liquid to flow back, resulting in liquid accumulation or even water flooding and well shutdown; and (v) the managed-pressure production can effectively reduce the permeability loss. And liquid nitrogen cleanup and production-string optimization are conducive to liquid drainage. As a result, the productivity decline may be put off. All results may provide important reference on technical measures made for the development of tight sandstone gas reservoirs in similar areas.

Taken Dugui gas province as an example, the production traits were discussed for tight sandstone gas wells rich in water at various water/gas ratio to hold up the efficient development in such gas reservoirs. On this basis, the effect of stress sensitivity and pressure difference on the production was also analyzed. Moreover, production outcome was accessed for gas wells with different proration working systems, and differentiated proration plans and standards were proposed to condition the pressure-drop rate. Results show that (i) with the unique pore-throat assemblage dominated by micropore and fine throat, the reservoirs in the study area feature low to ultra-low porosity, low permeability and strong heterogeneity; (ii) the development in these wells is seriously affected by water production. The higher the water/gas ratio, the larger the production decline and pressure-drop rate, and the lower the elastic yield; and (iii) with the increasing division in production pressure, part of capillary water and bound-water membrane may turn into movable water. As a result, water production increases, both elastic yield and cumulative production decrease, creating further complication in stabilizing production to deteriorate the development. In conclusion, for this kind of tight sandstone reservoirs rich in water in the mode of depletion development, controlling the proration and pressure-drop rate is an important means for gas wells to prolong their stable period and improve recovery. What's more, after the proposed differentiated proration plan and pressure-drop rate control chart have been in practice, the coincidence rate achieves up to 87.5%. So, these plan and chart may be popularized to the development in similar reservoirs and provide better guidance for the efficient development.

In this study, two suites of gas reservoirs in Sichuan Basin were taken as objectives to identify controls on the productivity in tight sandstone reservoirs with underdeveloped natural fractures, including the Upper Xujiahe 2 Member in Yuanba area of northern basin and Xujiahe 5 Member in Dongfengchang area in central basin. Their geological characteristics, drilling techniques and fracturing stimulation were discussed to ascertain the productivity controls. Results show that (i) vastly enriched in natural gas, both ancient and current structural highs with a great possibility in big output are available for hydrocarbon migration and accumulation. While there exists a poor effect in structural lows; (ii) the better the reservoirs' physical properties, the higher the enrichment in the reservoir-forming process, and the stronger the flowing rate in the production process, also implying the thicker the developed reservoirs with high quality, the higher the output; (iii) it is essential to perform the stimulation in such reservoirs with underdeveloped fractures. Especially, to expand fracturing scale may create an obvious stimulation effect in the developed quality reservoirs; (iv) not all of tight sandstone reservoirs are developed everywhere, creating more quality ones encountered in horizontal wells. So, an evidently incremental production can be attained in vertical wells; and (v) reservoir protection shall be stressed because some reservoirs with poor to moderate physical properties are contaminated by barite in drilling fluid to further condition the output. In conclusion, for the porous tight sandstone gas reservoirs, the productivity is mainly influenced by ancient and current structural highs, growth of quality reservoirs, stimulation scale and well types. What's more, the priority for well deployment shall be set in some superimposed zones developed with quality source rocks, positive structures and comparatively quality reservoirs.

Porosity is a decisive parameter in the evaluation on reservoirs' physical properties. Only a little data has been acquired from drilling and coring in NC area, central Sichuan Basin. In particular, it was a direct challenge to receive this parameter. And traditional methods for porosity prediction based on conventional logging data give rise to a large error or poor accuracy. So, taken the reservoirs of Xujiahe 4 Member as examples, an improved machine-learning algorithm, namely SAO-LightGBM, was put forward to clarify the physical properties of tight sandstone reservoirs in this area. Then, the algorithm was utilized to analyze the latent relationship of porosity to logging parameters, indicating its strong correlation with acoustic time difference, density, neutron porosity, resistivity and natural gamma. Finally, a prediction model was built on account of these parameters. Results show that (i) owing to the exclusive dual swarm mechanism together with both efficient enquiry and utilization strategy, SAO algorithm can quickly search an optimal hyper-parameter combination of LightGBM, so as to scale up this model's prediction ability; and (ii) both mean absolute error and determination coefficient of SAO-LightGBM are 3.37% and 0.92, respectively. In conclusion, boasting more reliable ability in comparison with other regular models, SAO-LightGBM can predict the porosity very well, which plays a guiding role in reservoir research and later exploitation in NC area.

In recent years, high yield has been acquired from the Upper Ordovician Wufeng Formation-the first submember of first member of the Lower Silurian Longmaxi Formation (Long 1₁ submember) in shale gas wells, Luzhou area, revealing promising prospects in shale gas exploration and development. So, taking the organic-rich shales of Wufeng Formation-Long 1₁ submember in this area as examples, we discussed the extension of minor layers and microfacies together with the quality difference of shale reservoirs with different microfacies in an effort to better promote both efficient exploration and fine development in Luzhou area. And then, the predominant microfacies and the main controls on shale gas enrichment and high yield were made clear. Results show that, (i) Wufeng Formation-Long 1₁ submember can be subdivided into Wufeng Formation, Long 1₁¹, Long 1₁², Long 1₁³ and Long 1₁⁴, which individually represent their own characteristics in lithology, electric property and fossil; (ii) totally, there developed 9 microfacies from Wufeng Formation to Long 1₁ submember, among which, strongly reduced argillaceous siliceous shelf, strongly reduced silty siliceous shelf, and strongly reduced silty calcareous-siliceous shelf function as the most dominant microfacies, where shale features four highs in TOC, porosity, gas content and brittleness. And it is mainly developed in Long 1₁¹, Long 1₁² and Long 1₁³; and (iii) the strongly restricted setting of deepwaters is the foundation for the extension of quality shale with high yield, abnormal pressure and rich gas content as the guarantee, and effective fracturing as the key, respectively.

To figure out the lithofacies distribution, main controls on reservoir extension, and reservoir-forming modes of volcanic rocks in eastern Sichuan Basin, both forming modes and hydrocarbon accumulation were analyzed for the Permian volcanic rocks in this area according to outcrop, drilling, logging and experimental data. Moreover, favorable exploration belts were pointed out. Results show that, affected by the Emei taphrogenesis, there developed two sets of favorable volcanic reservoirs in the Middle Permian Maokou Formation and the Upper Permian Wujiaping Formation of eastern basin. Mainly located in Dazhu-Liangping-Xinglongchang effusive-facies block of northern Fuling area, the volcaniclastic lava reservoirs around volcano seat are developed at the middle and upper parts of the third member of Maokou Formation (Maokou 3 Member). And they are 10 m to 35 m in thickness and their logging-interpreted porosity between 3.4% and 8.0%, respectively. Situated in Puguang-Fuling North-Jiannan area, the volcanic-sedimentary clastic reservoirs, also as the compound deposition of volcanic ash and regular sediments, far away from the seat, are developed at Maokou 3 Member bottom and in Wujiaping Formation. And their thickness varies from 16 m to 35 m. These volcanic-sedimentary clastic rocks are regarded as not only reservoirs but also quality source rocks. For sake of their own natures in high porosity and TOC, self-generation and self-storage, and massive and successive extension, it is possible to form reservoirs inside source. The volcaniclastic lava reservoirs of Maokou 3 Member are trapped by the source rocks of both underlying Maokou 3 Member bottom and overlying Wujiaping Formation, forming one source-reservoir assembly like a sandwich available for reservoirs near source. It is concluded that (i) in northern Fuling area, as the favorable exploration belt for the volcaniclastic lava reservoirs, old wells' resurvey and retesting or deploying exploration wells at advantageous structural points can be carried out; and (ii) surrounding Kaijiang-Liangping continental shelf, Puguang-Fuling North-Jiannan area is believed as the favorable belt for the volcanic-sedimentary clastic reservoirs. Unconventional thinks may be adopted, for example bypassing high-steep structural belts or selecting sweet spots with better preservation and buried depth less than 6,000 m for in-depth exploitation.

There exist many problems in drawing sedimentary-facies plans, including confused facies or facies deficiency, too simple lithological icon style, facies division in chaos, and informal color filling. So, taking drawing specifications as objectives, we analyzed the main issues in SY/T5615-2004 Drawing Specification and Pattern for Oil and Gas Geology Exploration (hereinafter referred to as Drawing Specification and Pattern), and pointed out how to make use of the relatively complex marine carbonate facies model, establish lithology pattern library through systematic rock classification, and form coloring principles by gathering colors filling in different facies zones from deep-sea basins to ancient continents. Results show that (i) three conceptual facies patterns, including the newly revised Wilson facies pattern, the carbonate platform facies pattern and the carbonate gentle-slope facies pattern, are regarded as the most important reference basis to divide relatively complex marine sedimentary facies; (ii) referring to the classification scheme of both rock icon style and rock formation in Drawing Specification and Pattern, we have established over 2,200 lithology patterns through the five-step way, which may roughly meet the current demand for drawing sedimentary-facies plans with high precision and achieve the aim of "lithology namely lithofacies"; (iii) characterization on thin-section microfacies is conducted after accurate lithologic filling, which considers not only microfacies classification but micro scale in this thin section; and (iv) coloring principles are as follows: blue towards the deep-sea, brown towards the continent, and the marine-terrigenous facies in orange, respectively. In conclusion, to properly take advantage of facies patterns may improve scientific natures in drawing sedimentary-facies plans. Setting up the lithology pattern library which meets the needs of production and scientific research is conducive to this drawing timely. Accurate lithologic filling is helpful to perform facies classification, especially for microfacies with inconsistent definition. And these principles make the plans in better versatility and pleasing to the eye.

For reasons of fine characterization on favorable belts available for adjustment of tight gas reservoirs rich in water and making clear reservoirs' spatial distribution, the gas reservoirs of the Permian Lower Shihezi Formation, A well area, Dongsheng gasfield, Ordos Basin, were taken as objectives to analyze their modeling tools. In addition, a 3D geological model was established for the Lower Shihezi reservoirs by virtue of Petrel software based on core, well-logging and geological data. Results show that (i) the overall study area is just a wide and gentle monocline with northeast high and southwest low. There developed two normal faults, small nose-like uplift and anticlinal structure; (ii) massive deposits of both channel and composite channel bar in the first member of Shihezi Formation (Shihezi 1 Member) represent the pattern of "mudstone-wrapped sandstone", while those in the Shihezi 2 and Shihezi 3 Members are smaller in size with the "sandstone-wrapped mudstone" pattern. In this study area, mudstone interbed accounts for 50%, and both siltstone and tight interbed of 30%, respectively; and (iii) data from the established model is relatively accordant with drilling practice, so the natural gas in place here can be calculated from the model rapidly and efficiently. The calculated ending in the Lower Shihezi reservoirs is 341.16×108 m3. Three conclusions are made. First, subdividing stratigraphic microstructure based on fine single-layer division of horizontal wells makes it possible to build a fine structure model for the whole area. Second, creating a sedimentary microfacies model and a barrier-interbed model under the control of horizontal well configuration can effectively improve the accuracy and effectiveness of physical parameter models. Third, the parameter modeling under the constraint of influential factors (such as sedimentary microfacies, lithofacies and faults) can also enhance the model's accuracy and effectiveness, so as to make it well applicable to the tight gas reservoirs rich in water with "multiple, thin, narrow, tight, strong and complex" reservoir space.

To comprehend the major interesting reservoirs for both exploration and development of unconventional oil and gas in Sichuan Basin, natural fractures in the Longmaxi Formation, Weiyuan block, southern Sichuan Basin, were taken as objectives. Their development characteristics and forming timing were analyzed by means of core observation, thin-section identification and geochemical experiments. Additionally, both development types and forming episodes were defined. Results show that (i) these fractures can be mainly classified into three types of structural fracture, hydrocarbon-generating overpressured fracture and bedding fracture; and (ii) there exist multi-episode fractures in this formation. Three conclusions have been made. First, Weiyuan area often witnesses natural fractures with smaller width and high proportion of horizontal-bedding fractures while occasionally high-angle structural ones with low effectiveness. Moreover, the density of fracture extension increases gradually with the increasing burial depth of the Longmaxi Formation. Second, based on temperature calculation and structural depth evolution, it is demonstrated that these fractures were dominantly formed in the Indosinian, Yanshanian and late Himalayanian epochs. And carbon/oxygen isotope and inclusion experiments all confirm their existence in these three epochs. Third, in the study area, it can be found a small number of NW-strike shear fractures and bedding fractures of sandstone interbedded with mudstone developed in the Indosinian epoch, NW-trending shear and hydrocarbon-generating overpressured fractures near faults or at anticline high in the Yanshanian epoch, nearly NW- and NWW-strike shear fractures also close to faults or at anticline high in the late Himalayanian epoch, and nearly S-N-strike tensile fractures developed at anticlinal core in the late Himalayanian epoch, respectively. Finally, based on the geomechanical setting of Weiyuan block, the characteristics, forming episodes and distribution patterns have been figured out for these natural fractures, which may provide technical support for the reservoir evaluation, favorable selection, sweetspot assessment together with exploitation of the Longmaxi Formation in this block.

It is rather unusual and complex for underground gas storages (UGSs) rebuilt from low-permeability gas reservoirs with water. To provide guidance for feasibility evaluation on the rebuilding from similar reservoirs, the second member of Xujiahe Formation (Xujiahe 2 Member), Zhongba gasfield, Sichuan Basin, was taken as an objective to clarify its geological feasibility in UGS construction. And then, some dynamic characteristics were summed up, such as water invasion and waterbody scale. Thirdly, by means of relative-permeability experiments on cores and numerical simulation, both rebuilding feasibility and effective storage capacity were discussed. Finally, one method was created for this feasibility evaluation. Results show that (i) regional faults and roof or floor enjoy better sealing capacity; (ii) the mobilization ratio of reservoir space is still able to maintain at a higher level; (iii) all the gas reservoirs represent weak to medium water driving with edge and bottom water in subactive to inactive state, which exerts less influence on the main UGS site; and (iv) static and dynamic characteristics suggest that both pure gas area and gas-water transitional area in the main structure are available for UGS construction. In conclusion, rebuilding one UGS in Zhongba gasfield from Xujiahe 2 gas reservoirs is feasible. What's more, the effective storage capacity is 106.80×108 m3 under the original formation pressure of 27.25 MPa.

The top Ordovician, Daniudi gasfield, northeastern Ordos Basin, possesses poor seismic resolution which is influenced by coal seams of the Taiyuan Formation, resulting that paleogeomorphic restoration is not accurate only dependent on its seismic interpretation. So, in the light of drilling, well logging and seismic data, integrated with regional, depositional and structural setting, the key geological interface was explored precisely for this field from two technologies, such as post-stack interpretation and multi-parameter inversion. Several karst predominance areas were pointed out through karst restoration. Finally, the effect on the extension of karst reservoirs was analyzed. Results show that (i) for this sort of reservoirs, the paleogeomorphic restoration based on multi-parameter inversion should be kept the predicted error of residual thickness no higher than 4 m; and (ii) based on percolation of paleogeomorphic units, combined one relation of gas layer thickness to residual weathering-crust thickness with productivity-well distribution, the evaluated zonal paleogeomorphology suggests one area with the residual thickness less than 58 m comparatively favorable for karst extension. In conclusion, the paleogeomorphic restoring technology based on multi-parameter inversion may effectively improve the interpretation for the interface to further raise the restoring precision, which is conducive to clarifying the distribution of karst reservoirs and offering assistance for the next exploitation.

Pores in shale are complex and ever-changing due to mixed sedimentation and diagenesis, which makes their thorough characterization quite difficult via just one means. So, taken the shale in the Lower Silurian Longmaxi Formation of Weiyuan area with more exploration and development practices as an example, micropores among the shale were characterized through various tools, such as argon ion polishing scanning electron microscope, quantitative image mosaic, connection survey of both gas adsorption and high-pressure mercury injection, and fluid tracer. In addition, the main controls on their development were analyzed. Results show that (i) in the Longmaxi Formation, Weiyuan area, the pores boast various types with organic pore as the major one, which achieves 92.89%; (ii) pore diameter is dominated by micropore and mesopore, contributing a lot to specific surface area and pore volume; (iii) tracer is always enriched in somewhere to develop with bedding, indicating greater porosity and permeability between bedding, which may bring about better migration pathway for shale gas; and (iv) also a greater contribution to micropore volume and specific surface area, TOC content together with quartz content exert a joint impact on pore development. In conclusion, to characterize micropore types via multiple tools as well as to sum up structural and percolation laws are conducive to figuring out micro characteristics of shale gas reservoirs. Meanwhile, analyzing influential factors on pores may provide important support for the later reservoir evaluation and development deployment of shale gas .

In April 2022, FB1 well in Qijiang area of southeastern Sichuan Basin achieved the gas production of 6.42×104 m3 /d from the second member of Qixia Formation (Qixia 2 Member) after acid-fracturing test. It was the first considerable exploration breakthrough for the Qixia Formation in this basin made in intra-platform and platform-depression margin shoals followed by major commercial top in both marginal platform shoal and intra-platform shoal around paleouplift. So, in order to comprehend sedimentary and reservoir characteristics of the Qixia Formation, Qijiang area, ascertain favorable exploration zones and guide the next exploration deployment, both sedimentary-facies distribution and reservoir extension were explored for this formation by making full use of field survey, core observation, thin-section identification, assay analysis and well-logging data. Results show that (i) during the deposition of the Qixia Formation, a sort of uplift-depression pattern was inherited in this area before the Permian deposition to further grow up the stable deposition of carbonate platform where platform-depression margin shoal, inter-shoal and intra-platform depression were developed. Qixia 1 Member belongs to the inter-shoal deposition, and high-energy shoal deposits were extended at the top of the upper submember of Qixia 2 Member and the top of the lower submember; (ii) reservoir rocks are mostly composed of sparry grainstone, aplite dolomite and limy dolomite, and reservoir space can be classified into two types, i.e., pore (vug) and fracture. Imaging well-logging and physical property data indicates the overall reservoir characteristics of low porosity and extra low permeability; and (iii) with an obvious facies control, Qixia reservoirs get better under karstification. The transitional belt of platform-depression margin during the deposition of Qixia Formation is regarded as a favorable exploration area. It is deemed that these results offer a scientific decision-making basis for the following deployment in the Qixia Formation of Qijiang area.

Based on some investigations on geological setting of Huolinhe sag, Erlian Basin, influential factors on pore and fracture structure, permeability, as well as porosity and permeability were analyzed for the low-rank coal of the Low Cretaceous Damoguaihe Formation in this sag through low-temperature nitrogen adsorption experiments, optical microscopic method, and porosity and permeability test in order to reveal characteristics of coalbed methane (CBM) reservoirs with low rank in this area. In addition, both reservoir properties and influential factors were pointed out. Results show that (i) there developed micropores (pore diameter<100 nm) predominantly in these CBM reservoirs, accounting for over 90%. Most of them are gas impermeable with one end closed. And their porosity is between 11.33% and 26.32%, 18.62% on average, belonging to medium porosity; (ii) there largely extended II-type fractures with the width more than 5 μm and the length less than 10 mm; (iii) the permeability varies from 0.067 mD to 38.550 mD, 5.440 mD on average, often in the range of 0.067-1.830 mD, staying at low level; and (iv) not only porosity but permeability are principally affected by maceral, ash content and fracture development. In conclusion, these findings may provide reference for the exploration and development of the low-rank CBM in Huolinhe sag and other areas of Erlian Basin.

Taken western Sichuan gasfield as an example, such critical geological factors as reservoir characteristics and gas-water distribution affecting development benefits were analyzed on the basis of thin-section, mercury-injection and test data to make development technological strategies in an effort to assure gas reservoirs of Leikoupo Formation in western Sichuan Basin with benefits. Results show that (i) this field is located in the thrust nappe structure belt of Longmenshan piedmont, which is structurally a NE-SW trending long-axis faulted anticline, emerging as a framework of north high and south low. There developed interlayer minor faults and other minor faults accompanying with Pengxian fault respectively at flanks of Jinma and Yazihe structures; (ii) there are tidal-flat sedimentary facies whereas mainly intertidal-subtidal subfacies deposited in this area with dolomitic flat and algae dolomitic flat as the favorable microfacies featured by a stable lateral extension; (iii) as a sort of thin dolomite interlayer with the characteristics of overlap-maps, these reservoirs are vertically able to be divided into two members, namely upper thin and the lower thick. They character low porosity and permeability. And their space can be classified into three types of dissolved pore and vug as well as microfracture; and (iv) the gas-water contact within the reservoirs descends gradually in different structures southwards, being distributed like the pattern of north high and south low. Four conclusions are made, the first is to take structural highs as the priority target zone of productivity construction, to adopt one suite of development series with the lower reservoir as the principal while taking of the upper one into consideration, and to subdivide the series locally in line with the principles of "benefits first, fewer wells with high production, effective replacement, and continuous stable production"; the second to adopt cluster wells and deploy intensively reach-extended wells, and long target spacing and highly deviated wells in this gasfield even at a densely populated area in order to raise single-well productivity and reserve-producing degree; the third to optimize the reasonable production rate in different types of wells so as to ensure an overall uniform advancement of edge water; and the last to provide ordered replacement in the periphery when the main area stepping into the final stable-production phase in an effort to realize these gas reservoirs in the long-term stable production and further enhance their recovery factor.

Impacted by high stress and drilling-fluid hydration, some horizontal wells in southern Sichuan Basin suffer wellbore instability in deep shale of Longmaxi Formation. So, the deep shale of Longmaxi Formation in Yongchuan southern area was taken as an example to probe the feasibility of slimhole shaft building with 6.5-in in these deep wells. And XRD and SEM, along with basic physical-chemical tests of expansion ratio and rock mechanical parameters were conducted under different working-fluid conditions. Additionally, change laws of wellbore stability under different sizes of hole and rock before and after being soaked by drilling fluid were pointed out through combining with instability models under the action of mechanical-chemical coupling and fluid-solid coupling. Results show that (i) the shale in this formation is overall tight whose minerals emerge as a kind of oriented extension with rich quartz and calcite, and it is hard and brittle; (ii) there exists a little clay in the shale, being dominated by illite and chlorite with a little illite and smectite interlayer. The swelling strain is generally lower than 5‰ with the poor water-swelling capacity under conditions of water- and oil-based drilling fluid; and (iii) in the environment of downhole pressure, the mechanical strength is stronger, and the influential change of oil-based drilling fluid is within 10% globally. In conclusion, various hole sizes before and after drilling-fluid soaking exert less influence on the wellbore stability. What's more, superior to 8.5-in one, 6.5-in slimhole can meet requirements for the wellbore stability of slimhole shaft building.

Western Guizhou Province is a key area for early exploration of high-rank coalbed methane (CBM) in China. Hydrogeological units with differential distribution may seriously condition CBM exploration and development in this area. Thus, Longtan Formation in Bide-Santang Basin was taken as an objective to analyze large amount of geological data on coalfields in an effort to figure out hydrogeological effects on CBM exploitation. Furthermore, according to exploitation practices of CBM wells, the units were reasonably divided, and the influence of different units on CBM enrichment and production was clarified. Results show that (i) formation water in Longtan Formation of Bide-Santang Basin is initially stemmed from atmospheric precipitation after inferring from hydrogen and oxygen isotopes as identification indexes; and (ii) according to five parameters of permeability coefficient, unit water inflow, fault extension, water head elevation and salinity, there divided four hydrogeological units, including recharge-strong runoff zone, strong runoff zone induced from fracture disturbance, weak runoff zone, and weak runoff-detention zone. In conclusion, the combination of hydrogeological units and producers suggests that the CBM production rate in the weak runoff-detention zone is high with single peak resulting in the greatest development potential with Zhuzang syncline as the soul. This rate in the weak runoff zone is medium and stable, holding in the second place of development potential. As for the other two units of both retention and runoff zones, it is advised to improve their own CBM exploitation effects from some aspects of fracturing scale, production system and well pattern & well type.

Intra-platform mound and shoal emerges as a newly important domain for natural-gas exploration in the Upper Sinian Dengying Formation of Sichuan Basin. In order to determine the main influential factors and extensional characteristics of quality Dengying 4 Member, both intra-platform drilling and field outcrop on the member in northern basin were taken as objectives to comprehensively analyze basic characteristics and main control factors of Dengying 4 reservoirs through core observation, thin-section identification and reservoirs' physical property. Results show that (i) lithologically, intra-platform mound and shoal reservoirs of Dengying 4 Member in northern Sichuan Basin are dominated by algae bound dolomite, algal dolarenite, algae stromatolite dolomite and algae laminated dolomite, and their space is elementally composed of intercrystalline (dissolution) pore, algae frame pore, intergranular dissolution pore and fracture; (ii) for those intra-platform mound and shoal reservoirs with high quality, the distribution is primarily affected by sedimentary facies belts, like algae mound and shallow shoal microfacies conducive to reservoir development; and (iii) multi-episode dissolution can improve reservoir performance with effect, among which karst in the early diagenesis period is the foundation to form quality reservoirs, and dissolution in the burial period can actively expand reservoir space. The intensity in the first buried dissolution period fells into a decline downward from the top Dengying 4 Member, and the second period causes early porous-vuggy reservoirs further dissolved and expanded, forming today's reservoir space. In conclusion, controlled by Hannan ancient land and Xuanhan-Kaijiang paleo-uplift, the near NS-strike intra-platform mound and shoal belt developed in slope break zones, grows into a favorable area for oil and gas exploration in Dengying 4 Member.

Origin mechanisms on abnormal high pressure are fundamental contents in petroleum geology. Whether hydrothermal pressurization is one of effective mechanisms is still an unsolved academic problem. So, preconditions to cause the pressurization were analyzed according to thermal physical principles of rock and fluid in order to quantitatively evaluate pressurization effects on formation pressure and pressure coefficient. Then a model to quantitatively assess both formation pressure and pressure coefficient was established in the context of hydrothermal pressurization. Additionally, one relation of hydrothermal pressurization to abnormal high pressure was discussed. Results show that (i) there emerge two conditions in the pressurization, such as strata in enclosure and their increased burial depth; (ii) it is essential to exclude an undercompaction effect when evaluating the pressurization impact on formation pressure; (iii) at a fixed increase of burial depth, the greater the geothermal gradient and the thermal expansion coefficient of formation fluid, the lower the thermal expansion coefficient of rock and the compression coefficient of formation fluid, and the greater the formation pressure increase and the pressure coefficient; (iv) in the range of reach-extended depth, the strain of fluid volume induced by rising temperature is generally less than 2.0%; and (v) on the one hand, the hydrothermal pressurization may lead to rapid increase in formation pressure. On the other hand, given formation depth adds correspondingly, the formation pressure coefficient changes a little with the coefficient increment generally no more than 0.1. To sum up, only hydrothermal pressurization cannot grow up abnormal high pressure.

In order to accelerate the exploration and development process of shale gas in Qiongzhusi Formation, Sichuan Basin, with JS103 well recently made a significant breakthrough as an example, the mineralogical, organic geochemical and reservoir characteristics were analyzed for silty shale in this formation in Jingyan-Qianwei area. Influential factors on reservoir development were explored, and some differences between the silty shale in Qiongzhusi Formation and the high-quality shale in Longmaxi Formation were determined. Results show that (i) the silty shale in the study area is characterized by low TOC and gas content, moderate porosity and high content of siliceous minerals, apparently different from the high-quality shale in Longmaxi Formation with high TOC and gas content, high porosity and high content of brittle minerals; (ii) with developed inorganic pores as the soul, the silty shale reservoirs character mesopores and macropores as the main reservoir space along with the pores primarily in inkpot and slot shapes; and (iii) the anti-compaction and dissolution of siliceous minerals and the presence of moderate and stable clay minerals are conducive to pore extension, and the cementation of carbonate minerals is an important factor affecting mineral deterioration. In conclusion, adjacent to quality source rocks, the silty shale enjoys a certain hydrocarbon-generating capacity and relatively good physical properties, creating favorable conditions for shale-gas formation and enrichment. The gas content in field, as an important parameter for evaluating marine shale reservoirs, is not at all applicable to the silty shale. The coupling between active gas-testing shows while drilling and better reservoir conditions can be taken as a critical indicator to discriminate the gas content in the silty shale now.

In Daniudi gasfield, northeastern Ordos Basin, the Upper Paleozoic faults are almost underdeveloped. In DK13 well area just at the southwest of this field and accounting to one-sixth of Daniudi field, Shanxi 2 Member is one of the major payzones and has obvious features with the deposition of braided-river delta plain. In order to figure out its sedimentary characteristics in this well area for optimally selecting principal layers, sedimentary microfacies in Shanxi 2 reservoirs were identified by using core and well-logging data. There are five types of microfacies, including mid-channel bar, distributary channel, natural levee, abandoned channel, and interdistributary bay. Results show that (i) Shanxi 2 Member can be divided into Shanxi 2-1 and Shanxi 2-2 submembers, which are subdivided into four layers from Shanxi 2-1-1 to Shanxi 2-2-2, and vertically developed with 7-epsode channels; (ii) in N-E direction, there exist sufficient water energy and dense mid-channel bar with long longitudinal and horizontal axes around 2.5-4.0 km and 1.5-2.0 km as well as the thickness of 10-14 m, respectively. While in S-W direction, concentric shoal represents a few scattered extension with a small scale and an increasing shale content; and (iii) with often more than 4 m in thickness, both Shanxi 2-1-1 and Shanxi 2-2-1 layers are characterized by GR value ranging from 60 API to 75 API, shale content less than 13%, good results in gas testing, and a stable yield more than 0.9×10⁴ m³/d. It is concluded that various stages of sandbodies in Shan-2 Member exhibit great differences in their thickness, migrate frequently in horizontal direction, and are mostly separated by thick muddy deposits. Moreover, from the northeast to the southwest, the water energy weakens gradually, regression occurs initially, and channel scope reduces gradually in the horizontal. On top of that, both Shanxi 2-1-1 and Shanxi 2-2-1 layers are developed and reveal a higher production in gas testing, so they can be selected as the principal layers for subsequent exploitation.

Sichuan Basin is the main battlefield to increase shale gas reserves and production in China. As the development and productivity construction are advanced in its deep shale gas, the shale gas exploration in Wufeng Formation-Longmaxi Formation has shifted to complex structures along basin margin, whose representative is Lintanchang structure of Chishui exploration area in southeastern basin margin. Furthermore, successfully drilling in LY1HF and LY3HF wells proves formation conditions of shale gas in Lintanchang structure and also reveals complex structural characteristics and preservation conditions in Chishui exploration area. To ascertain favorable belts for shale gas, a few wells were evaluated and analyzed. Then, both structural characteristics and preservation conditions were summed up on the basis of formation conditions of shale gas. Results show that (1) in the study area, quality shale of deepwater continental shelf facies in Longmaxi Formation is thicker with high organic carbon abundance, porosity and gas content, providing right conditions for shale gas enrichment; (2) in Lintanchang structure as a nappe fault anticline controlled by nappe faults, preservation conditions are different in different zones. In the complex positive structure belt along basin margin, thrust faults serve as barriers in the updip direction through strata to offer favorable preservation; and (3) taking into consideration the development degree, burial depth, preservation and development conditions of shale in Longmaxi Formation, Nan 1 and Bei 1 belts can be selected as I-type favorable exploration belts of shale gas in Longmaxi Formation, Lintanchang structure of Chishui area, owing to their huge exploration and development potential.

There often exist well-developed fractures in Lower Paleozoic carbonate rocks, Ordos Basin, which function as one of the key factors affecting high yield and enrichment of natural gas. In order to precisely predict plane distribution laws of fractures with different sizes as well as provide reference for selecting exploration areas and deploying development wells, the Ordovician in Daniudi gasfield of Ordos Basin was taken as an objective to analyze development characteristics of the fractures in the Ordovician Majiagou Formation by using core and imaging logging data. Then, seismic response characteristics were analyzed for fracture zones and seismic attributes sensitive to fracture prediction were chosen dependant on quality analysis on seismic data and forward modeling. In addition, one method was established to predict multi-scale fractures based on seismic poststack attributes. Results show that (1) for developed fractures in the study area, their logging characteristics are featured by high interval transit time, low resistivity, compensation density and wave impedance, while implicit in seismic response characteristics only emerging as worsening continuity and differential torsion of waves; (2) the boundary of great fractures can be depicted by using dip angle and coherence, and the distribution of small fractures and the intensity in a certain zone with these small ones can be predicted by using high-accuracy Likelihood fusion attributes. Results predicted from those three sorts of fractures can prove each other, and the depicted accuracy is improved a lot; and (3) these results are in line with actual geological situations and better accordant with lost-circulation points. They are conducive to selecting areas and deploying development wells. In summary, the comprehensive fracture prediction method used in Daniudi gasfield is of guiding significance to other blocks with similar geological situations and seismic data.

In order to determine the main factors influencing on productivity in gas reservoirs of Taiyuan 1 Member, Da 17 well block, Daniudi gasfield, 11 horizontal wells were taken as examples to analyze dynamic production characteristics in these reservoirs, clarify effects of both sedimentation and structure on their production and make countermeasures against development adjustment. Results show that, (1) according to dynamic characteristics, these 11 wells can be classified into type I and type II. 8 wells with type I feature high initial productivity, long stable production period, low liquid/gas ratio and better production effect while 3 wells with type II are characterized by low initial productivity, short stable production period, high liquid/gas ratio and worse production effect; (2) reservoirs in type I and type II wells are different in physical properties, but subtle disparity in their thickness. GR curves in Type I wells majorly represent low-amplitude smooth box in shape, and the reservoirs are provided with higher resistivity, porosity and permeability and better physical properties, while type II wells emerge tooth-shaped GR curves, standing for corresponding reservoirs with low resistivity, poor physical properties and high water cut; and (3) sedimentation modes are diverse, which may cause obvious difference in physical properties between two types of wells. Located at the upstream face, the type I wells boast good physical properties, high production rate, low liquid/gas ratio and high cumulative production. The type II wells are situated at the downstream face and featured by poor physical properties, interbedded mudstones and low resisitivity, as well as rich connate water bringing about high liquid/gas ratio further to affect productivity release. In conclusion, for Taiyuan 1 gas reservoirs, their productivity is mainly affected by sedimentation, and the upstream face of barrier bar serves as a profitable adjustment zone of mature area due to its good physical properties and high productivity. Additional, the recovery can be increased effectively by deploying infill wells at the upstream face of sand bar and optimizing a working system for old wells.

To accurately calculate water saturation in reservoirs and discriminate water-bearing characteristics, the gas reservoirs of both Upper and Lower Shaximiao Formation, east slope of western Sichuan Depression, Sichuan Basin, were taken as objectives to analyze the reason why both high-resistivity water layer and low-resistivity gas layer are presented in these reservoirs. And a certain deviation of water saturation was pointed out, which was individually obtained from unified formation-water resistivity and actual test result. So, it is hard to precisely discriminated the fluid properties in the reservoirs. Results show that (1) in the Upper and Lower Shaximiao Formation in this area, the formation-water characteristics vary greatly, with CaCl₂ type as the main water type, and both Na₂SO₄ and NaHCO₃ types as the minor ones. The formation-water salinity is influenced by preservation conditions and faults, representing a certain difference vertically and laterally; and (2) the formation water in the Upper and Lower Shaximiao Formation has three types of salinity (high, medium, and low), and various salinity types may substantially affect the formation-water resistivity. Therefore, when the unified formation-water resistivity is used to calculate the water saturation, an error between the calculated saturation and the real one is larger, which may affect an accurate judgement on the gas-bearing property in the reservoirs. In conclusion, a water-saturation calculation model shall be established for each flow unit based on typing of formation-water salinity, and the water saturation obtained from the formation-water resistivity, which is induced from formation-water salinity, is more accordant with the actual test result.

In Sichuan Basin, there is enormous resource potential of tight sandstone gas, also bringing about promising development prospect. So far, however, both development and production laws of tight sandstone gas reservoirs in southwestern basin have not been made clear due to only a little exploration and development practice. To further clarify the productivity characteristics and rational proration of tight sandstone gas reservoirs in this area, GQ1 well in Guanyinsi structure of southwestern Sichuan Basin, one test well targeting on the Shaximiao Formation, was taken as an objective to analyze the effect of both geological and engineering factors on well productivity according to production performance. And then, the characteristics and control factors of tight-gas productivity were pointed out. Finally, the rational proration was set for this well by using an analogy method and referring to some proration principles on JQ gasfield with similar drilling technology and reservoir and depositional characteristics. Results show that (1) the production performance of Shaximiao gas reservoirs is characterized by fast decline in the early stage, long-term stable production with low pressure and yield in the middle stage, and intermittent production in the late stage due to insufficient productivity; (2) the productivity is controlled by the development degree of quality reservoirs, and No.8 sandbody in Shaximiao 2 Member is an favorable sweetspot in Guanyinsi block; (3) the effect of both fracturing stimulation and horizontal-well stimulation is remarkable; and (4) the production test scale of GQ1 well is determined on the basis of 1/5 -1/3 of minimum absolute open flow, and the proration ranging from 3×10⁴ m³/d to 5×10⁴ m³/d is more rational. In conclusion, the tight sandstone gas reservoirs in southwestern basin are of strong heterogeneity and lower productivity. What's more, horizontal-well and fracturing stimulation can effectively improve the productivity in these tight sandstone gas reservoirs.

For most of exploration and evaluation wells in deepwater gasfields of the South China Sea, high cost in productivity test, few test wells, and shortage of productivity data bring about some challenges in uncertain productivity test. So, the main factors controlling the productivity of deepwater wells were analyzed by taking LS gasfield in deepwaters of the South China Sea as an objective. And then, two parameters on fluid-property characterization, including fluid factor and pressure factor, were proposed. In addition, based on one empirical productivity formula, another productivity evaluation formula was built up to define the productivity factor, and a new productivity evaluation chart was also developed according to the relationship of productivity factor to effective permeability. Finally, a new method to predict the productivity of untested deepwater gas wells was put forward. Results show that in LS gasfield, (1) both effective permeability and heterogeneity in poorly heterogeneous reservoirs present better regularity; (2) there is a better linear relation between fluid factor and pressure factor; and (3) with the productivity factor and effective permeability as soul, the new productivity evaluation chart has good correlation. In conclusion, after this new method is used to predict the productivity of LSX-W1 well, the error between predicted open flow and another open flow from DST is only 10%, showing a higher accuracy in this prediction. Furthermore, with its help, the productivity of untested wells in deepwater gasfields can be evaluated by using conventional well logging and pressure test even without DST. As a result, the exploration and test cost in deepwater wells may be reduced greatly, so as to provide effective support for their extensive exploration.

Daniudi gasfield is located at Yishan slope in northeastern Ordos Basin, where the Upper Paleozoic structure is relatively gentle and no faults are developed. It is a tight sandstone gas reservoir featured by strong heterogeneity and proximal oil and gas accumulation. Da 28 well area just lies in northern gasfield, in which He 1-1 submember of the Permian Lower Shihezi 1 Member is one of the principal development reservoirs and presents obvious sedimentary characteristics of braided river. In order to clarify the sedimentary characteristics of the principal development reservoirs in this well area, the sedimentary microfacies of He 1-1 submember was taken as objectives to analyze outcrop, core, and logging data. And three principal microfacies were identified, including braided channel, point bar, and flood plain. Results show that (1) this submember can be subdivided into He 1-1-1 and He 1-1-2. There is four-phase channel developed, and vertical and lateral superposition represents the nature of positive rhythm. Single-phase sandbody is preserved completely without obvious denudation. Frequent channel scouring leads to a few thick argillaceous sediments with some thin fine-grained interbed, which belong to high-energy braided channel deposits; and (2) the sandbody advances continuously southward along provenance, and the main channel stays in the same place, presenting the characteristics of thick in the middle and thin on both sides. The sandbody in the north is more developed and dominated by coarse sediments. In conclusion, the sandbody of He 1-1 submember is characterized by complex superposition and clear difference between thickness and gas content. The effective sandbody thicknesses of point bar and braided channel are 6.5 m and less than 4.0 m, respectively. what's more, gas layers are developed in the microfacies of point bar and braided channel, especially in their evolution zones, where sand is purer due to the effect of current scouring. Shale content is less than 15%, Gamma-ray value ranges from 45 API to 70 API, the sandbody's physical property is better and gas content is higher, which can be used as reference and basis for the late development and measured well deployment.

In Jian'ge block, northwestern Sichuan Basin, the bioreef of Upper Permian Changxing Formation is extensively developed at marginal platform along the western Kaijiang-Liangping continental shelf. For several large-scale bioreef zones, the internal has not been described finely due to poor seismic-data quality and shortage of research method. So, in order to make clear not only the development type of bioreef in the study area but also the areal and spatial distribution of single bioreef body inside these bioreef zones, and to further develop bioreef gas reservoirs, the single-well lithological and electrical characteristics of bioreef-beach facies and the seismic reflection and geological characteristics of various bioreef bodies were analyzed by using both drilling data and new 3D seismic data with high resolution and signal/noise ratio. In addition, to carry out the fine description of bioreef based on drilling-seismic integration was proposed. Results show that (1) in Jian’ge block, the bioreef of Changxing Formation may be influenced by palaeogeomorphology and eustasy, and there are five types of bioreef developed, presenting the characteristics of multiple phases and rows; (2) the fine interpretation on the top Changxing Formation is based on the cognition of single well-sectional bioreef characteristics, and the characterization on 3D spatial distribution of steep-slope type bioreef can be well done by means of both sedimentary paleogeomorphology restoration technology based on residual-thickness method and 3D visualization technology; (3) the seismic geometry of "uplift bulge" in the bioreef body and "lower convex lens" in the inter-bioreef current is creatively used for the bioreef description of volumetric curvature attribute based on multi-window dip scanning and structure-oriented filter, which can well describe the development boundary of single and small bioreef body inside the large-scale bioreef zones; and (4) it is expected more potential development zones in this block, which can provide support for the next development. In conclusion, the attribute description result of bioreef sensitivity, which is obtained from drilling-seismic integration and by making full use of seismic reflection dynamic and kinematic characteristics, is highly accordant with actual drilling. It is beneficial to predicting the areal distribution of single bioreef body and implementing the efficient development of bioreef gas reservoirs.

In eastern Sichuan Basin, also a prolific hydrocarbon zone in Sichuan Basin, the lower assemblage as the key exploration domain has not been made strategic breakthrough after many years of exploration practice. However, some industrial gas flow recently from the Cambrian postsalt, Pingqiao structure in eastern basin, may prove the postsalt potential. In order to figure out its reservoir-forming type, the Upper Cambrian Xixiangchi Group was taken as an example to analyze the reservoir characteristics, source-reservoir association, and dynamic reservoir-forming process by using seismic, drilling, and outcrop data. In addition, the reservoir-forming pattern of the source-reservoir connection in the study area and the main factors affecting this pattern were pointed out. Results show that (1) in the high-quality shales from Upper Ordovician Wufeng Formation to Lower Ordovician Longmaxi Formation, eastern Sichuan Basin, the total organic carbon (TOC) is generally higher than 3%, indicating an excellent gas source condition; (2) certain intraplatform shallow shoal reservoirs of Xixiangchi Group, which are developed in the periphery of micropalaeogeomorphic high of eastern basin, may be reworked by fractures and dissolution; and (3) a thrust fault developed in the middle assemblage in eastern basin may lead to a direct lateral connection between the source of Wufeng to Longmaxi formations at the fault's footwall and the reservoir of Xixiangchi Group at the hanging wall. And the Xixiangchi gas reservoir and the shale gas in Wufeng to Longmaxi formations are of the same source, which confirms the effectiveness of lateral hydrocarbon supply. In conclusion, a kind of late gas reservoir-forming pattern of "old gas source and new trap, source-reservoir connection, and lateral hydrocarbon supply" is represented, which takes the Wufeng to Longmaxi formations as effective source rocks, the dolomites of Xixiangchi Group as reservoirs, and the fault-related anticlinal trap in the high and steep structural belt of eastern Sichuan Basin as an exploration target. What's more, the better source-reservoir condition is the base to form gas reservoirs, the useful source-reservoir connection increases the reservoir-forming efficiency, and the late uplift and pressure relief of shale gas open gas charging.

In order for enhanced oil recovery (EOR) in tight sandstone gas reservoirs, some barrier sand-bar gas reservoirs of Taiyuan 2 Member, Daniudi gasfield, Ordos Basin, were taken as objectives to analyze the reservoir characteristics and clarify the reservoir heterogeneity and permeability, and gas-water two-phase seepage as the main factor affecting the recovery. Then, both remaining-gas distribution and its main types were determined by means of two methods including gas drainage area evaluation and numerical simulation. Based on different distribution types of remaining gas, the well pattern infilling and adjustment for EOR were figured out and the potential tapping countermeasures on different remaining-gas types were put forward. Finally, the EOR effects of different well pattern infilling and adjustment modes were evaluated by analyzing the production effects of infilling wells and performing the economic evaluation. Results show that (1) the remaining gas in the barrier sand bar of Taiyuan 2 Member mainly belongs to an uncontrolled type between targets or between wells with wide spacing, so its potential tapping is mainly by deployed by infilling wells; (2) when the oblique mode between targets is adopted to tap the potential of remaining gas between wells, the infilling wells are not interfered, but the included angle between horizontal section and main stress is small, resulting in a poor fracturing stimulation effect; and (3) when interline parallel infilling and interline dislocated infilling are used for potential tapping, interference will happen at the well spacing between interline parallel infilling and interline dislocated infilling less than 600 m or in the range of 400 m to 800 m, respectively. After infilling, the well spacing is 500 m, the interference is serious, and the recoverable reserves in infilling wells are decreased by 50%, but the economic requirement is still satisfied and the recovery in these wells can be improved by 5%.

In order to further clarify the development characteristics of the Ordovician karst reservoirs, northern Ordos Basin, the stratigraphic distribution, palaeogeomorphologic characteristics, and reservoir types and distribution in Shilijiahan play, Hangjinqi area, were analyzed. And both control factors and development models of these karst reservoirs were summarized. Results show that (1) bioclastic limestone widely developed in Shilijiahan play during the deposition of Majiagou 4 Member provides good material base for karst reservoirs; (2) the top Ordovician can be classified into two kinds of karst palaeogeomorphologic units, namely karst highland and karst slope. And two subunits developed in the karst slope contain karst depression and karst monadnock; and (3) two types of reservoirs are developed in the study area, i.e., porous reservoir and fractured-vuggy reservoir, among which, the latter is dominant. In conclusion, the karst geomorphology controls the preservation degree of Ordovician residual strata and the dissolution intensity. What's more, the distribution of fractured-vuggy Majiagou 4 reservoirs is controlled by palaeogeomorphology and fractures. In addition, in the transitional belt between karst monadnock and depression, the groundwater erodes vertically along faults and fractures, resulting in limestone leaching and dissolving to form dissolution vugs most favorable for reservoir development.

Most carbonate reservoirs of Lower Paleozoic Ordovician Majiagou Formation, Ordos Basin, can be hardly identified due to complex lithology and fast lithologic change. Furthermore, owing to some old wells without logging interpretation, geologists cannot conduct rapid and effective lithologic identification. So, Majiagou 5 Member in Fuxian area was taken as an objective to analyze the commonly-used identification methods. Then, the main rock types were determined based on coring data. And combined with the logging response values of the main minerals in carbonate rocks, the lithology was interpreted by means of the envelope method of photoelectric absorption cross section index curve-compensated density curve and compensated density curve-compensated neutron curve, and the clustering analysis - K-Nearest Neighbor. Results show that (1) the envelope method can easily determine dolomite, limestone, and gypsum, and especially does well in certain strata with gypsum. It is applicable to the lithological pre-judgment during production. Its key point lies in adjusting left and right scale values in the curves and its discrimination accuracy is dependent on the reliability of curve quality; (2) the clustering analysis - K-Nearest Neighbor provides more accurate results, and its prediction coincidence rate in this study is up to 92.31%. This method is more suitable for the later research, but it also needs more coring data; and (3) at present, both methods have limitations in the identification of collapsed breccia. Therefore, this identification cannot be realized without imaging logging and geological cognition.