Lithologic Reservoirs

Based on the theory of continuous accumulation and the data of seismic，drilling ，master logging， well logging and core testing，the hydrocarbon accumulation conditions of Jiufotang Formation in Ludong Sag of Kailu Basin were studied from multiple aspects，including the tectonic sedimentary evolution background and its controlled source rocks，reservoirs，and reservoir characteristics of the Jiufotang Formation. The continuous accumulation patterns and favorable exploration targets in this area were clarified. The results show that：（1）Ludong Sag presents a single-fault half-graben configuration，where the steep slope zone serves as the main source supply area of the original sedimentary source，while the gentle slope zone is the main uplift and denudation area in the late stage，the main part of the original sedimentary subsidence center exists in current structure. The sedimentation of Jiufotang Formation is controlled by the staged activities of the sag-controlling fault. In the sedimentary stage with low sedimentation rate and relatively shallow water conditions，large fan deltas are developed in the steep slope zone，and the sand bodies are in the state of“pervasive sand distribution”.（2）Two source rock intervals are identified in Jiufotang Formation in the research area：tuffaceous mudstones in lower member of Jiufotang Formation and oil shales（Oil Groups Ⅰ-Ⅲ）in upper member of Jiufotang Formation，both of them are good source rocks with low maturity to maturity. The principal source rocks are the oil shales（Oil Groups Ⅰ- Ⅲ），with the chloroform bitumen‘A’of 0.48%，TOC of 4.47%，total hydrocarbon of 2 377 mg/L，organic matter type mainly consisting of type Ⅰ and Ⅱ1，and vitrinite reflectance of 0.64%. A small amount of conventional sandstone reservoirs are developed in the underwater distributary channel of the fan delta front. The tight sandstone reservoirs are widely and continuously distributed，and the lacustrine carbonate reservoirs are developed in the center of the subsag. Influenced by the effects of provenance supply and water environment，the storage space of different types of reservoirs develop sequentially. Intergranular（dissolved）pores in coarse sandstones， intragranular dissolved pores in fine sandstones，matrix dissolution pores in dolomitic mudstones，and carbonate dissolution pores are widely developed. Thick dark mudstone developed in Shahai Formation can be used as regional cap rock（. 3）The source-reservoir in the subsag area is integrated，and with superior preservation conditions. Different types of reservoirs are continuously developed. Conventional oil-tight oil-shale reservoirs are successively developed on the plane，and multiple types are superimposed vertically. Houhe and Jiaolige fan bodies have shown promising exploration results，while Kuluntala and Qinghe fan bodies are favorable areas for exploration.

Characteristics of Triassic paleostructure and their control on sedimentation in Kuqa Depression，Tarim Basin

XIE Huiwen, ZHANG Liang, WANG Bin, LUO Haoyu, ZHANG Ke, ZHANG Guowei, LI Ling, SHEN Lin

2025, Vol.37(3): 13–22 Abstract ( 6 ) PDF (20858 KB) ( 8 )

doi: https://doi.org/10.12108/yxyqc.20250302

Based on the fine structural interpretation of seismic data，Triassic synsedimentary faults in Kuqa Depression of Tarim Basin were studied by using the impression method，and the pre-sedimentary paleomorphology of Triassic was restored，and their controlling effects on sedimentation were clarified. The results show that：（1）Triassic of Kuqa Depression develops four rows of synsedimentary reverse faults，including Kubei fault， Bashi-Yiqicreek fault，Kela-Yangbei fault and Keshen fault，which are structurally stacked，and the front zone reaches the south of Kelasu tectonic belt. （2） Triassic paleogeomorphology is distributed in an east-west direction，with the South Tianshan orogenic belt in the north and the foreland basin uplift in the south. The central part is the structural low part of Kuqa Depression，which is constrained by the local low bulge，and develops Wushi Sag，Baicheng Sag and Yangxia Sag. The northern boundary of the prototype basin is located 24-51 km north of the current basin boundary，and the area of the prototype basin is 11 913 km2 larger than that of the present basin.（3）The sedimentation and evolution of Triassic in the study area are controlled by the paleotectonic pattern，which is generally manifested as a wedge-shape that gradually thickening from south to north，with a tendency of thickening from west to east，and the distribution of strata in the north-south direction is asymmetrical. Due to the weakening orogeny of South Tianshan，Triassic has a complete foreland basin extrusion-relaxation pattern.Okhobrak Formation develops fan delta，the lower part of Karamay Formation and Huangshanjie Formation are semi-deep lake and deep lake sediments.The upper part of Huangshanjie Formation and Tarichik Formation are the transformation of the flood plain from the semi-deep lake and deep lake to the meandering river，which vertically constitutes a complete sedimentary cycle of continental lake basin evolution. The ascending walls of the syngenetic reverse faults all have the function of secondary provenance，and form a good spatial combination of reservoirs and caps in the lower wall，which is a favorable potential exploration field.

Characteristics and main controlling factors of Triassic Chang 8 tight sandstone reservoir in Huanqing area，Ordos Basin

XIAO Wenhua, YANG Jun, YAN Baonian, WANG Jianguo, LI Shaoyong, MA Qilin, LI Zonglin, XUE Huanzhao

2025, Vol.37(3): 23–32 Abstract ( 4 ) PDF (20023 KB) ( 6 )

doi: https://doi.org/10.12108/yxyqc.20250303

Based on the data of drilling，logging and core，the characteristics and formation mechanism of Triassic Chang 8 tight sandstone reservoir in Huanqing area of Ordos Basin were analyzed by thin section observation，mercury intrusion experiment，nuclear magnetic resonance，then the main controlling factors for enrichment of tight oil were clarified. The results show that：（1）Chang 8 tight sandstone reservoir in Huanqing area is mainly composed of lithic feldspar sandstone and feldspar lithic sandstone，with low compositional maturity （average 0.97）. The main reservoir space types are secondary pores，and intragranular pores and kaolinite intergranular pores can be seen . The porosity ranges from 5.50% to 12.60%，with an average value of 9.50%. The permeability ranges from 0.09 mD to 3.43 mD，with an average value of 0.79 mD. The reservoir is commonly of small pore throat radius，with an average pore throat radius of 0.12-0.16 um and the maximum pore throat radius of 0.49-0.62 μm. The high permeability is mainly contributed by the large pore throat.（2）The evolution about diagenetic and reservoir of Chang 8 reservoir in the study area are as follow. In the early diagenetic stage A，calcareous cemented tight sandstone was formed by injection of calcareous carbonate supersaturated pressure release water into sandy sediments，and the cementation was mainly calcite. In the middle diagenetic stage A，the organic acid fluid was injected into the tight sandstone along the hydrocarbon generation pressurized fracture to dissolve calcite cementation and feldspar detrital particles，and some of the reservoir space were formed.（3）The formation of Chang 8 tight oil reservoirs are controlled by slope background，high-quality source rocks，largescale distribution of river sand bodies and good source-reservoir configuration.The oil and gas are concentrated in the tight sandstone reservoirs close to the source rocks，and the sandstone with large thickness has a better oily content.

Main controlling factors of hydrocarbon accumulation and enrichment of Jurissic Xishanyao Formation in Monan Slope，Junggar Basin

YIN Zhaopu, ZHU Feng, ZHOU Zhiyao, WANG Lili, LIU Xiaoye, Nazyman, WANG Yuting, HUANG Darui

2025, Vol.37(3): 33–46 Abstract ( 3 ) PDF (13772 KB) ( 7 )

doi: https://doi.org/10.12108/yxyqc.20250304

Based on drilling, logging，2D and 3D seismic，core analysis，well tie comparison and seismic inversion，the research of hydrocarbon reservoirs in Yongjin Oilfield of Jurissic Xishanyao Formation in Monan Slope of Junggar Basin was carried out. The main controlling factors of hydrocarbon accumulation and enrichment of Monan Slope were analyzed from perspectives of sequence stratigraphy，sedimentary system，trap conditions and transportation system，and favorable zones were predicted. The results show that：（1）Jurassic Xishanyao Formation in Monan Slope of Junggar Basin is a third-order sequence，which has undergone a process of progradationretrogradation.The red mudstone developed across time at the top boundary of Jurassic in Monan Slope is widely distributed throughout the area，and can be used as a regional cap rock.（2）The high-quality sand bodies are developed on large scale in the third member of Jurassic Xishanyao Formation.The beach sand in the inner leading edge and the shore-face sand bar in the outer leading edge of the wave-dominated delta are effective reserviors. The internal mudstone serves as the top and bottom plates，lake-bay mudstone laterally seals，providing a superior conditions for the development of lithologic traps.（3）The Indosinian-Yanshanian strike-slip faults connect deep oil sources and can serve as oil source faults. During the Himalayan，low-angle large-scale thrusting nappe faults horizontally developed and transported oil and gas upward，serving as favorable channels for hydrocarbon migration.（4）The intersection of the shore-face sand dam trap zones in the outer leading edge of the wave-dominated delta in the third member of Xishanyao Formation in the north of Monan Slope and the Dadonggou fault zones of Himalayan is favorable zone for further exploration.

Accumulation condition of “below the source rock” tight oil in the 4th member of Cretaceous Quantou Formation in Putaohua Oilfield， northern Songliao Basin

RAN Yixuan, DU Changpeng, ZHANG Jingjing

2025, Vol.37(3): 47–58 Abstract ( 3 ) PDF (9498 KB) ( 7 )

doi: https://doi.org/10.12108/yxyqc.20250305

In order to clarify the genesis and favorable exploration area of“below the source rock”tight oil of the 4th member of Cretaceous Quantou Formation in Putaohua Oilfield of northern Songliao Basin，tight oil types and distribution characteristics of different structural units were identified through fine characterization for source rock characteristics，migration channels，hydrocarbon-generating pressurization，high-quality reservoir characteristics. Hydrocarbon accumulation models were established for each structural unit. The results show that：（1）High-mature source rock in the 1st member of Qingshankou Formation of Gulong Sag is the material basis for the tight oil reservoir accumulation of the 4th member of Cretaceous Quantou Formation in Putaohua Oilfield. High-quality reservoirs with porosity greater than 10% are good for the enrichment of tight oil. The “L-type” migration channels composed of faults and river sandbodies are the necessary guarantees for tight oil lateral migration. （2）The 4th member of Cretaceous Quantou Formation underwent 2 main accumulation period. The overpressure of 19.6-23.5 MPa formed by hydrocarbon-generating pressurization in late Mingshui Formation，is the main power for oil and gas to flow backward and form “below the source rock” tight oil （. 3）Four kinds of tight oil reservoirs，such as fault-lithologic oil reservoirs，fault reservoirs，sandstone lens oil reservoirs，updip pinch-out lithologic oil reservoirs，are developed in the 4th member of Quantou Formation. Maoxing syncline mainly develops small-scale sandstone lens and fault reservoirs，with scattered oil bearing areas. While Toutao nose-bulge develops updip pinchout lithologic oil and fault reservoirs，with good oil bearing. And Punan nose-bulge mainly develops fault and fault-lithologic oil reservoirs，with oil and gas continuously distributed in large expanse.（4）The accumulation of tight oil reservoir is controlled by hydrocarbon generation centers，“L-type” migration channels，overpressure and reservoir quality. The accumulation models of Maoxing syncline，Punan nose-bulge and Toutai nose-bulge are “upper-generation and lower-storage, overpressure drive within source rock，oil and gas backflow，and part ‘sweet spot’enrichment”， “source-reservoir adjoining，fault-sandstone relay L-type migration，upper aggregation”，and “source-reservoir separation，fault-river lateral migration and accumulation，and high-quality reservoir enrichment”，respectively. Punan nose-bulge is the favorable exploration area for tight oil.

Hydrocarbon accumulation conditions and exploration potential of Triassic Baikouquan Formation in Shawan Sag，Junggar Basin

DENG Gaoshan, DONG Xuemei, YU Haitao, ZHANG Jie, YUE Xiwei, REN Junmin, JIANG Tao

2025, Vol.37(3): 59–72 Abstract ( 2 ) PDF (46228 KB) ( 6 )

doi: https://doi.org/10.12108/yxyqc.20250306

Based on TOC testing，rock pyrolysis，casting thin section，scanning electron microscope analysis， the source rocks，reservoirs and hydrocarbon accumulation conditions of Shawan Sag in Junggar Basin were comprehensively studied. The results show that：（1）Triassic Baikouquan Formation in Shawan Sag of Junggar Basin is a gentle slope shallow water retrograding fan delta deposits，and develops two types of favorable sand bodies：fan delta front sand body and shore shallow lake facies coastal sand dam sand body，with the distribution characteristics of horizontal connection and vertical superposition. The lithology is a positive cycle of “coarse at the bottom and fine at the top”. The overall physical properties of the reservoir are ultra-low to low porosity permeability，with an average porosity of 7.8% and an average permeability of 2.38 mD. The physical properties gradually improve from bottom to top，the third member of Baikouquan Formation are with the best physical properties，obviously controlled by sedimentary facies. Moreover，the inhibition of cementation by chlorite cladding，the porosity enlargement by albite dissolution，and the overpressure formed by continuous hydrocarbon injection all have important effects on the development of high-quality reservoirs.（2）Baikouquan Formation reservior has continuous hydrocarbon injection from multiple sets of source rocks（mainly of high-quality source rocks in salt lakes of Fengcheng Formation）. The source-reservoir configuration type can be clasified as vertical configuration and lateral configuration，and their tightness controls the degree of oil and gas enrichment. The subfacies reserviors of fan delta front are widely distributed，with a large area and porosity mostly greater than 15%. It also develops efficient transport system such as high-angle deep faults，unconformity surface，sand body. The overlying regional cap rocks provide superior reservoir conditions.（3）A series of nose-bulges developed along NW direction，close to the source，are favorable structure exploration areas，and the slope break zones are favorable exploration area for lithology.

Characteristics and quantitative prediction of structural fractures in the second member of Triassic Xujiahe Formation in Xinchang area， western Sichuan Basin

YANG Xu, BAI Mingsheng, GONG Hanbo, LI Gao, TAO Zuwen

2025, Vol.37(3): 73–83 Abstract ( 4 ) PDF (21092 KB) ( 5 )

doi: https://doi.org/10.12108/yxyqc.20250307

The second member of Triassic Xujiahe Formation in Xinchang area of western Sichuan Basin has dense reservoirs，the development and distribution of structural fractures are crucial for the efficient development of gas reservoirs. Based on imaging logging data，the development characteristics of reservoir structural fractures in the study area were analyzed. Based on the heterogeneous rock mechanics model，the numerical simulation and quantitative prediction of the tectonic stress field and structural fractures in the target layer during Himalayan period were carried out by using the finite element simulation method，rock fracture criterion and elastic strain energy. The results show that：（1）The fractures in the second member of Triassic Xujiahe Formation in Xinchang area of western Sichuan are mainly structural shear fractures，with fractures trending in NWW direction，NEE direction，near EW direction，and SN direction according to their development degree. The formation stages of the fractures mainly include the late Indosinian，Yanshanian，early Himalayan，and late Himalayan. The fractures formed in the late Indosinian and Yanshanian filled with calcite or quartz，and only the fractures formed in Himalayan were effective.（2）During the Himalayan movement，the maximum horizontal principal stress of the second member of Xujiahe Formation was mainly 65-100MPa，and the minimum horizontal principal stress was mainly 50-85MPa. The tectonic position and lithofacies of the strata controlled the stress distribution.（3）The average density of structural fractures in the research area is relatively small，at 0.28 fractures per meter，and the density of fractures near the faults is greater than 1.20 fractures per meter. The density of fractures in different lithofacies varies significantly. The distribution of fractures is jointly controlled by factors such as faults，tectonic position，and lithofacies.（4）The average absolute percentage error between the fracture prediction results and the measured results is 11.40%，and the prediction results are reliable.

Characteristics and main controlling factors of Archean buried hill reservoir in Bohai Sea

WANG Lijuan, HAN Denglin, MA Liangshuai, HU Rongrong, WANG Chenchen, MA Binyu, YAN Kang, ZHU Chaobin

2025, Vol.37(3): 84–94 Abstract ( 5 ) PDF (20165 KB) ( 5 )

doi: https://doi.org/10.12108/yxyqc.20250308

Based on core thin section analysis，X-ray diffraction analysis and logging data，the reservoir structure characteristics and main control factors of the M oilfield median buried hill and the BZ19-6 low buried hill in Bohai Sea area were studied. The results show that：（1）Longitudinally，the weathering zone reservoir in M oilfield median buried hill reservoir is well developed，and the pore-fracture type is dominant. The fractures in BZ19-6 low buried hill reservoir are particularly developed，and the weathering zone and the inner zone are dominated by fractured reservoirs. There are obvious differences in burial depth，source-reservoir contact and vertical zonation among different types of buried hill，lead to the differences in the microstructure of reservoirs in terms of pore throat space.（2）Multi-stage tectonic movement causes the differences in the zonation of different types buried hill reservoirs，paleogeomorphological structure restricts the plane distribution of buried hill reservoirs and the migration direction of organic acids，and weathering leaching indirectly improves the reservoir structure of different types of buried hills，and its improvement degree is controlled by buried hill fractures and weathering environment.（3）Different types of buried hill high quality reservoir development zone，have different characteristics. The BZ19-6 low buried hills high quality reservoirs are mainly concentrated in the deep inner fracture zone，and the M oilfield median buried hills are mainly distributed in the weathering zone at the top of the buried hills.

Lithofacies identification using conventional logging curves and its exploration significance，Triassic Chang 8₁ sub-member，Longdong area，Ordos Basin

ZHANG Zhaohui, ZHANG Jiaosheng, LIU Jungang, ZOU Jiandong, ZHANG Jianwu, LIAO Jianbo, LI Zhiyong, ZHAO Wenwen

2025, Vol.37(3): 95–107 Abstract ( 4 ) PDF (6644 KB) ( 5 )

doi: https://doi.org/10.12108/yxyqc.20250309

Using core description，outcrop analysis，combined with logging interpretation，the types and logging response characteristic of lithofacies in tight sandstone of Chang 81 sub-member of the Yanchang Formation in the Longdong area of Ordos Basin have been classified and analyzed. Based on principal component analysis， sensitive logging parameters related to lithofacies were optimized. By leveraging the small variance characteristics of the random forest classifier and the minimal deviation of the XGBoost algorithm，a Stacking ensemble learning model was constructed to intelligently identify lithofacies. The results show that：（1）The Chang 81 submember in the Longdong area comprises six types of lithofacies：homogeneous fluvial channel lithofacies，heterogeneous fluvial channel lithofacies，homogeneous distributary mouth bar lithofacies, heterogeneous distributary mouth bar lithofacies，heterogeneous overtopping lithofacies，and mudstone lithofacies. The homogeneous fluvial channel and homogeneous distributary mouth bar facies exhibit high quartz and feldspar content，with relatively well-developed intergranular and intragranular solution pores. The average porosity is 8.32%，and the average permeability is 1.81 mD. These lithofacies represent a favorable lithofacies type，characterized by homogeneous blocky bedding formed in high-energy sedimentary environments. In contrast，the heterogeneous distributary channel lithofacies and heterogeneous distributary mouth bar lithofacies are distinguished by heterogeneous bedding formed under medium-low energy conditions.（2）In comparison to the Random Forest and XGBoost algorithms，the accuracy of the Stacking learning model in identifying lithofacies reached 94.2%. This result provides reliable methodological and technical support for characterizing lithofacies distribution.（3）The sedimentary process governs the spatial distribution of lithofacies，which serve as the material basis for sedimentary beddings. Homogeneous fluvial channel facies are predominantly developed in the central part of the distributary fluvial channel， exhibiting a banded structure that extends to the average low water level. In contrast，heterogeneous fluvial channel lithofacies are primarily located at the edges of the fluvial channel，surrounding the homogeneous fluvial channel lithofacies. The most favorable reservoir petrophysical properties，characterized by good physical attributes，are found in the overlapping areas of multiple periods of homogeneous fluvial channel lithofacies，marking them as potential regions for exploration and development.

Characteristics and formation mechanism of Permian marine shale of Kaijiang-Liangping trough in northern Sichuan Basin

YANG Xue, YANG Yuran, ZHANG Jingyuan, TIAN He, WANG Qing, SONG Fang, ZHANG Saike, CHEN Yao

2025, Vol.37(3): 108–119 Abstract ( 2 ) PDF (9608 KB) ( 7 )

doi: https://doi.org/10.12108/yxyqc.20250310

In order to clarify the formation mechanism of Permian Wujiaping formation organic rich shale of KaijiangLiangping trough in northern Sichuan, based on the drilling core data. The characteristics of geochemical elements，macro and micro evidences of rocks are analyzed，and the research is carried out from the aspects of tectonic evolution，sedimentary environment and the impact of events. The results show that：（1）The tectonic evolution of the Kaijiang-Liangping trough in northern Sichuan controls the sedimentary environment of shale，and the formation of the Kaijiang-Liangping trough provides basic geological conditions for shale deposition. Many sets of shale deposited in the trough are thicker than those outside the trough. The vertical lithologic combination is complex，and the plane heterogeneity is very strong. Relatively stable organic rich shale was formed in the deep-water shelf facies.（2）The lithology of the Permian Wusan member shale in the study area is mainly siliceous shale，with the characteristics of“one thin and four high”，that is，the continuous shale is thin，high TOC，high porosity，high gas content and high brittleness. According to the comprehensive evaluation，it is a Permian high-quality shale reservoir and has realized the commercial development of natural gas.（3）Volcanic activity in event deposits is one of the important factors affecting the formation of high-quality reservoirs. The multi-stage tuff belt developed in the shale layer of the third member of Wu corresponds to the peak period of biological development，and primary productivity controls the formation of high organic carbon.（4）Affected by tectonic activities and sea area closure，the original sedimentary layers in the Kaijiang-Liangping trough are clearly visible. The original sedimentary framework formed the pore framework. The upper Wusan member of the Permian system is the best quality reservoir. The organic rich shale was formed in a deep-water reducing environment，and the formation and enrichment of high organic matter in the shale were promoted by remote volcanic activity. The original mineral composition and its accumulation mode control the distribution of high porosity and high gas content areas，thus creating natural gas wells with high and stable production.

Characteristics and reservoir control of the strike-slip salt arch belt of Shahejie fourth Member Formation of Paleogene system in Laizhou Bay Depression，Bohai Bay Basin

CHEN Huaiyi, LI Long, BAI Bing, YUE Junpei, KANG Rong, ZHANG Xingqiang

2025, Vol.37(3): 120–128 Abstract ( 4 ) PDF (15655 KB) ( 4 )

doi: https://doi.org/10.12108/yxyqc.20250311

Based on the interpretation results of new 3D seismic data in Kenli 17-1 tectonic area of Laizhou Bay Depression，the distribution，formation mechanism，evolution model and reservoir formation significance of strike-slip salt arch belt are studied. The results show that：（1）The salt sources of the fourth member of Shahejie Formation of Paleogene in Laizhou Bay Depression are mainly divided into two types：deep hydrothermal and terrigenous sources. The supervised KNN model multi-attribute recognition method is used to describe the shape boundary of the salt arch，and the salt structure is identified to be almost north-south distribution. By intercepting the seismic sections in the south，middle and north，the salt structure can be divided into three types：integrated contact non-puncturing type，hidden puncturing type and non-integrated contact puncturing type.（2）The strikeslip salt arch belt in the study area has a three-part genetic pattern of“strike-slip - inversion-salt arch”. Under the coupling and superposition of gravity sedimentary differential load and strike-slip extrusion，the salt arch structure forms a north-south shape difference，and the salt structure mainly goes through three main evolution stages：development stage，maturity stage and re-active stage（. 3）The strike-slip salt arch belt can accelerate the maturation of the overlying source rocks，reduce the hydrocarbon expulsion threshold，and have a good capping effect，and control the migration and accumulation of oil and gas. The formation of the third member and below of the Paleogene Shahejie formation in the south of the study area，Keli 17-1，mostly formed salt wall side sealing reservoirs. Lithologic updip pinch-out or tectono-lithologic reservoirs are mostly formed in the deep Dongying Formation，the first Member and the second Member of the Paleogene Shahejie formation in Kenli 11-2 area in the north of central China，and punctured graben-fault-block reservoirs are mostly formed in Guantao Formation and Minghuazhen Formation.

Characteristics of calcareous interlayer in narrow channel sand body and their influence on reservoir quality：A case study of the first member of Jurassic Shaximiao Formation in Zitong area，northwestern Sichuan Basin

HE Xiaolong, ZHANG Bing, XU Chuan, XIAO Bin, TIAN Yunying, LI Zhuo, HE Yifan

2025, Vol.37(3): 129–139 Abstract ( 2 ) PDF (27343 KB) ( 4 )

doi: https://doi.org/10.12108/yxyqc.20250312

The calcareous interlayers in the first member of Jurassic Shaximiao Formation in Zitong area have significant implications for favorable reservoir prediction. By combining core observation，well logging data analysis，X-ray diffraction（XRD），and mercury intrusion porosimetry，a systematic study was conducted on the characteristics of calcareous interlayer in narrow channel sand body and their influence on reservoir quality. The results show that：（1）The reservoirs in the first member of Jurassic Shaximiao Formation are primarily com‐posed of lithic feldspar sandstone and feldspar-lithic sandstone，with primary porosity as the main pore space. The petrographic and physical properties exhibit strong heterogeneity，with an average porosity of 7.21% and an average permeability of 0.380 mD.（2）Based on the core observation，the calcareous interlayers in study area can be directly identified through their light gray-white color，compactness，high hardness，and effervescence in acid. Well logging data show low natural gamma，high resistivity，and low acoustic time difference，with these interlayers mainly found at the top and bottom of the sand bodies，particularly in mouth bar sedimentary microfacies.（3）Two stages of calcareous cementation are identified in the sand bodies：Stage I calcite cementation is closely related to the depositional environment，while Stage Ⅱ calcite cementation is a product of later diagenetic evolution.（4）Based on the development of calcareous interlayers and the stacking relationships of sand bodies，three types of reservoirs are classified. Type Ⅰ，consisting of vertically stacked sand bodies in a straight river channel，has the best reservoir quality. Type Ⅱ，consisting of laterally cut sand bodies in meandering river channels，has moderate reservoir quality. Type Ⅲ，consisting of compound sand bodies in barrier-bar river systems，has the worst reservoir quality.

Types and genetic models of sand bodies in shallow water delta front of extremely gentle slope lake basin：A case study of Chang 8₁ submember of Triassic in Haotan area，Ordos Basin

LIU Yingbao, LI Yuanhao, ZHAI Wenbin, ZHAO Wenxuan, LI Zhe, YANG Longhua, GUO Yaqian

2025, Vol.37(3): 140–152 Abstract ( 4 ) PDF (25134 KB) ( 5 )

doi: https://doi.org/10.12108/yxyqc.20250313

Based on core，well logging and seismic data，the sand body types and genetic models of extremely gentle slope shallow water delta front of Chang 81 submember of Triassic in Ordos Basin were studied. The results show that：（1）Sedimentary period of Chang 8 in Haotan area is characterized by stable structure，very slow slope（only 0.03°），shallow water and large shoreline migration and swing. Chang 81 submember mainly develops three types of sand bodies：underwater distributary channel，mouth bar，sheet sand，and composite superimposed sand bodies types such as overlapped underwater distributary channel，overlapped underwater distributary channel-mouth bar，and overlapped mouth bar.（2）The proximal delta inner front mainly develops underwater distributary channel sand bodies，and the type of superposition of sand bodies is dominated by multistage superposition of underwater distributary channels，dominated by fluvial action. The logging curves are mainly box or bell shapes，and the thickness of single stage sand bodies（sand thickness > 10 m）is large，with plane distribution in strip. The distal delta inner front mainly develops underwater distributary channels and mouth bar sand bodies，forming three main types of composite superimposed sand bodies types such as overlapped underwater distributary channels，overlapped underwater distributary channel-mouth bars and overlapped mouth bars. The river and lake waves act together. The logging curve shapes are mainly box，bell and funnel， and the single-stage sand body（with a thickness of 5-10 m）is relatively thick，with plane distribution of lobate delta.The outer front of the delta dominated by the action of lake waves mainly develops sheet sand bodies. Vertically，they exhibit multi-stage isolated finger shaped sand bodies，the single-stage sand bodies（thickness < 2 m） are relatively thin，and plane distribution appear as well connected sheet sand bodies.

Study on NMR response mechanism and pore structure evaluation method of basic volcanic rock：A case study of Permian Emeishan Basalt Formation in southwestern Sichuan Basin

ZHAO Ailin, LAI Qiang, FAN Ruiqi, WU Yuyu, CHEN Jie, YAN Shuanglan, ZHANG Jiawei, LIAO Guangzhi

2025, Vol.37(3): 153–164 Abstract ( 3 ) PDF (14274 KB) ( 7 )

doi: https://doi.org/10.12108/yxyqc.20250314

Taking Permian Emeishan Basalt Formation in southwestern Sichuan Basin as a case study，based on petrological characteristics and nuclear magnetic resonance relaxation mechanism of basic volcanic rocks，the effects of internal magnetic field gradients and paramagnetic mineral content on T2 spectra were analyzed by using a combination of numerical simulations of internal magnetic field gradients and variable echo time NMR experiments. A new method for characterizing the pore structure of basic volcanic rocks was proposed. The results show that：（1）The volcanic reservoirs in the study area are mainly composed of basalt volcanic clastic lava，calcareous breccia lava，and basaltic rock. The mineral composition of both calcareous breccia lava and basaltic volcanic debris lava is mainly composed of calcite，quartz，plagioclase，and clay minerals. The average mass fractions of clay minerals are 27% and 32%，respectively，with chlorite accounting for 84% and 33% of clay minerals，respectively.（2）The content of paramagnetic minerals（chlorite+iron containing minerals）in basic volcanic rocks is relatively high，which can generate strong internal magnetic field gradients during NMR measurements. Under high internal magnetic field gradient，as echo time increases，the main peak of the T2 spectrum shifts toward shorter relaxation times，the overall spectrum area gradually decreases，and lead to a smaller nuclear magnetic porosity. The larger the internal magnetic field gradient value，the more distorted the geometric shape of the pores and the smaller the pore size. The internal magnetic field gradient has the greatest impact on calcareous breccia lava，followed by basaltic volcanic debris lava，and has the least impact on diabase porphyry.（3）Establishing a porosity correction formula based on relative error of nuclear magnetic porosity and paramagnetic mineral content through data fitting，converting T1 spectrum into pore size distribution based on the feature that T1 measurement is almost not affected by internal gradient magnetic field，establishing the relationship between the geometric mean value of T1 and T2 spectrum，correcting the peak shift of the T2 spectrum，converting T2 spectrum into pore size distribution，and then T2 NMR pore structure evaluation can be achieved（. 4）The relative error between nuclear magnetic porosity calculated by this method and the logging porosity is 15%，the relative error of the average pore throat radius between calculated by the method described and from CT digital core experiment is 6%，the distribution of the basic volcanic rocks in the study area are highly heterogeneous，mainly consisting of small and medium-sized pores throats.

Quantitative selection and application of well locations for encrypted wells in fractured-porosity carbonate rocks：A case study of Cambrian oil reservoirs in Tanghai Oilfield，Huanghua Depression，Bohai Bay Basin

ZHANG Qinglong, MAO Yuanyuan, FENG Jiansong, YUAN Xuansheng, ZHOU Wei, ZHU Fujin, XUAN Lingling

2025, Vol.37(3): 165–175 Abstract ( 1 ) PDF (11334 KB) ( 5 )

doi: https://doi.org/10.12108/yxyqc.20250315

The formation energy of Cambrian fractured porous carbonate reservoirs in Tanghai Oilfield，Bohai Bay Basin decreases rapidly，and the overall recovery rate is low. In order to solve the problem of deploying encrypted well locations reasonably，a new method for quantitative optimization of well locations was proposed. The results show that：（1）The cumulative oil production of a single well in a fractured porous carbonate reservoir throughout its entire life cycle is mainly influenced by the combined effects of single well controlled movable remaining oil，average permeability of reservoir fractures，average thickness of single reservoir，average porosity of reservoir fractures，and average permeability of the matrix，with single well controlled movable remaining oil having the greatest impact.（2）The quantitative optimization method for well location is to comprehensively utilize logging，seismic，core thin section and other data to establish a three-dimensional geological model and a single well controlled movable residual oil model；Sort the relative importance of each influencing factor through correlation analysis of models，reservoir numerical simulation results and single well production performance data；Utilize the improved Analytic Hierarchy Process to calculate the weights of various influencing factors and the comprehensive“sweet spot”coefficient，and select the well locations with high comprehensive“sweet spot” coefficients for deployment.（3）This method has achieved good results in the optimization of well location targets in carbonate reservoirs of Cambrian Xuzhuang Formation and Maozhuang Formation in T180X2 Block of Tanghai Oilfield. All three exploration wells have achieved high single well production，with a cumulative oil production of 20 800 tons throughout the entire life cycle. The comprehensive“sweet spot”coefficient shows a good positive correlation with the cumulative oil production of a single well throughout its life cycle.

Influence of inclined micro-fractures on shearwave attenuation in limestone

ZHOU Qingwen, WU Dong, CAI Ming, ZHANG Chengguang, CHEN Yuanbo, LIN Wang, ZHANG Yuanjun, LI Zhi

2025, Vol.37(3): 176–184 Abstract ( 2 ) PDF (5770 KB) ( 5 )

doi: https://doi.org/10.12108/yxyqc.20250316

To solve the problem of fine evaluation of microfractures in limestone reservoirs，based on the prin‐ciple of acoustic propagation，acoustic physical experiments were carried out under different fracture conditions （fracture dip angle and fracture aperture），and a characterization model of fracture parameters was established to quantitatively analyze the effects of fracture inclination and fracture opening on shear wave attenuation. The model has been applied to the first member of the Paleogene Shahejie Formation in Caofeidian area，Bohai Bay Basin. The experimental results show that：（1）Five groups of artificial fracture cores with different inclinations， including 0，25°，40°，55° and 70°，are designed in the acoustic physics experiment of microfracture cores in limestone reservoirs. 9 PET films of 25 μm，50 μm，100 μm，150 μm，200 μm，300 μm，400 μm，500 μm and 1 000 μm were used as 9 different fracture aperture degrees. The acoustic wave frequency was maintained at 250 kHz，and the transverse wave waveform under 9 fracture aperture conditions was measured for 5 groups of cores respectively. 32 channels of waveform data were continuously collected for each group of experiments，and the optimal waveform signal was obtained by filtering 32 channels of waveform through noise filtering program and superposition.（2）The acoustic physical test results show that the shear wave attenuation is greatly affected by fractures，and the attenuation trend gradually slows down with the increase of fracture aperture. The attenuation coefficient increases logarithmically with the increase of fracture aperture. When the fracture aperture is less than or equal to 300μm，the attenuation coefficient is more sensitive. Under the condition of the same fracture aperture degree，the shear wave attenuation coefficient decreases with the increase of fracture dip angle when the fracture dip angle is 0-40°，and increases with the increase of fracture dip angle when the fracture dip angle is greater than 40°.（3）The fracture parameter characterization model（Formula 3）is obtained based on the core physical experiment.（4）The relative error of fracture opening degree obtained by the fracture parameter characterization model in the first member of Paleogene Shahejie Formation in Caofeidian area of Bohai Bay Basin is 4.57% and the mean absolute deviation is 18.9 μm，indicating a high agreement.

PETROLEUM ENGINEERING AND OIL & GAS FIELD DEVELOPMENT

The application of random forest algorithm in predicting the casing deformation of hydraulic fracturing

LIN He, DU Jinling, XU Gang, RONG Jiaojun, LIANG Xueli, HENG Feng, GUO Junning, MA Mengxi

2025, Vol.37(3): 185–193 Abstract ( 2 ) PDF (9918 KB) ( 5 )

doi: https://doi.org/10.12108/yxyqc.20250317

Taking 12 casing-deformed horizontal wells in the Permian Lucaogou Formation of the Jimsar Sag as the research object，based on three-dimension seismic attribute data，a comparative analysis is conducted to examine the correspondence between casing deformation locations induced by geological factors and the development of natural fractures. A risk prediction model for casing deformation locations is established using the Random Forest algorithm from machine learning. The result shows that：（1）Casing deformation occurs primarily at locations where natural fracture zones develop and at boundaries of reservoir heterogeneity，with a nonlinear correlation between the two.（2）The number of decision trees and the number of features for node splitting in the Random Forest casing deformation risk prediction model are optimized using grid search and 5-fold crossvalidation. Considering both computational accuracy and efficiency，the optimal number of features for node splitting is set to 2，and the number of decision trees is set to 100.（3）The application of actual data shows that the Random Forest algorithm achieves a prediction accuracy of 87.85% for casing deformation locations in the tight oil reservoirs of the Permian Lucaogou Formation in the Jimsar Sag. The risk prediction results output by the model can provide guidance for optimizing fracturing designs and adjusting construction parameters.

Numerical well testing model of fractured well with complex fractures multi-well interference in heterogeneous tight gas reservoirs

XU Youjie, REN Zongxiao, XIANG Zuping, FAN Xiaohui, YU Mengnan

2025, Vol.37(3): 194–200 Abstract ( 3 ) PDF (16605 KB) ( 6 )

doi: https://doi.org/10.12108/yxyqc.20250318

Based on Embedded discrete crack，a numerical well testing mathematical model of complex fractures and multi well interference in heterogeneous tight gas reservoirs is established，which considers the influence of the threshold pressure gradient to the permeability calculation formula. It improved the algorithm for numerical simulation based on the Multiphase Reservoir Simulator（MRST）. The results show that：（1）Based on the MRST simulator for model calculation，the accuracy of the simplified model was verified by comparing the result with commercial software. The larger the threshold pressure gradient coefficient，the greater the upward curve of the pressure drop and pressure drop derivative curve.（2）The production type of adjacent wells（constant rate or constant pressure）mainly affects the mid to late stage characteristics of the wellbore pressure drop log-log curve. The constant rate production of adjacent wells leads the boundary reflection characteristics appears in advance， while the constant pressure production of adjacent wells causes the late-stage log-log curve of the test well to show an upward trend followed by a downward trend.（3）The influence of adjacent well fracture parameters and discrete fractures on the wellbore pressure drop curve of the test well is relatively small. The induced fracture conductivity of the test well shows a significant depression of bilinear flow stage wellbore pressure drop derivative curves.（4）When the region permeability of the test well keep constant，the higher the permeability of the adjacent well is，the earlier the interference appears during late stage. The research results give a theoretical foundation for the analysis of multiple well interference well test of fractured wells in tight gas reservoirs.