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01 October 2024, Volume 45 Issue 5 Previous Issue    Next Issue

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OIL AND GAS EXPLORATION

Layered Structural Deformation Characteristics of Kuqa Foreland Thrust Belt XU Zhenping, YANG Xianzhang, NENG Yuan, DUAN Yunjiang, ZHANG Wen, HU Jianning, ZHANG Mengyang 2024, 45 (5):  505-515.  doi: 10.7657/XJPG20240501 Abstract ( 255 )   HTML ( 14 )   PDF (5903KB) ( 337 )   Save The seismic data acquired from Kuqa foreland thrust belt is characterized by low signal-to-noise ratio and high interpretive ambiguity. By using high-resolution 3D seismic data, drilling and lab hydrocarbon analysis data, the stratigraphic assemblages of Kuqa foreland thrust belt were systematically described, the structural model was detailedly interpreted, and the hydrocarbon accumulation system was deeply analyzed. It is found that the Kuqa foreland thrust belt develops two sets of detachment layers: Paleogene and Neogene gypsum-salt rocks, and Triassic and Jurassic coal measures, all of which feature stratified detachment, vertical stacking, and multiphase deformation. Detachment folds in caprocks are found in the shallow structures, while basement-involved imbricate thrust structures are developed in deep strata. Detachment plastic deformation occured in the gypsum-salt and coal layers. Faulting occured in three phases including Caledonian, late Hercynian-Indosinian, and Yanshanian-Himalayan. The late Hercynian-Indosinian tectonics controlled the Mesozoic sedimentation, showing a north-to-south onlap thinning feature. Layered structural deformation in the Kuqa foreland thrust belt governs the stratified accumulation and migration of hydrocarbons. Hydrocarbons in the strata above the coal seam predominantly originated from the Jurassic source rocks, whereas oil and gas in the strata below the coal seam mainly came from the Triassic source rocks which contributs 60% of the hydrocarbons. A substantial quantity of hydrocarbon remains trapped in the formation below the coal layer. Figures and Tables | References | Related Articles | Metrics

Fracture Characteristics and Seismic Prediction of Z4 Metamorphic Buried-Hill Reservoir DING Sheng, LIU Jinhua, SHANG Yamin, PENG Pai, FU Jinxiang 2024, 45 (5):  516-521.  doi: 10.7657/XJPG20240502 Abstract ( 243 )   HTML ( 7 )   PDF (3488KB) ( 182 )   Save Seismic prediction of fractures is the foundation of fractured reservoir evaluation. Metamorphic buried-hill reservoirs exhibit diverse fracture types, significant variations in fracture development at different reservoir parts, and difficulties in describing fracture heterogeneity. The Z4 metamorphic buried-hill reservoir was investigated for its fracture characteristics and seismic prediction. The development of fractures in the Z4 reservoir has layering characteristics and can be divided into four sections such as weathered-semi-filled fractures at the top, highly developed net-like fractures in the upper part, moderately developed low-angle fractures in the middle part, and poorly developed high-angle fractures at the bottom. A comprehensive fracture prediction technique was proposed, which integrates multi-scale general spectral decomposition, dip-oriented eigenvalue coherent processing, and iterative ant analysis. The fracture orientations and development revealed by cores were compared with the results of seismic prediction, suggesting a high consistency. It is believed that the multi-approach comprehensive fracture seismic prediction technology proposed in this study has high accuracy. Figures and Tables | References | Related Articles | Metrics

Characteristics of Maze Karst Cave System in Lianglitage Formation of Tahe Oilfield, Tarim Basin ZHANG Changjian, JIANG Lin, WANG Yan, ZENG Qingyong, MA Xuejian 2024, 45 (5):  522-532.  doi: 10.7657/XJPG20240503 Abstract ( 203 )   HTML ( 7 )   PDF (7293KB) ( 89 )   Save To understand the styles and structures of the maze karst cave system in the Upper Ordovician Lianglitage formation in the Tahe oilfield, Tarim basin, the palaeohydrological and geomorphological restoration, karst framework construction, karst cave identification, and genetic model analysis were performed for Block 11 of the oilfield by using the methods such as paleolandform restoration, paleo-water system characterization, log-based stratigraphic correlation, structural fracture analysis, and seismic attribute characterization. The results show that during the Episode Ⅱ of the Middle Caledonian, the southern Tahe oilfield was higher in the northwest than in the southeast geomorphically, with developed NNW-SSE dendritic incised valleys. A subhorizontal maze karst cave system with closed conduit structures and high intensity of erosion are found in the Lianglitage formation, which is a typical maze karst cave system formed by epigenetic karst diffusion and infiltration and shares similarities in genesis with the Bullita karst cave system in the Judbarra region of Australia. The mudstone interval in the Upper Ordovician Qiaerbake formation serves as an aquiclude, which controls the lateral erosion of the karst cave system in the Lianglitage formation. The faults connecting surface water systems provide primary channels for karst water infiltration and erosion. As the regional base level drops, karst water infiltrates downwards along the fractures into the Middle Ordovician strata, forming fault-karst reservoirs in the Yijianfang formation. Understanding the “double-layer” maze epigenetic karst cave system of the Episode Ⅱof the Middle Caledonian in the Tahe oilfield is crucial for the development of Upper Ordovician reservoirs. Figures and Tables | References | Related Articles | Metrics

New Insights into Eruption Model of Huoshiling Formation Volcanic Rocks in Chaganhua Subsag of Changling Fault Depression XU Fangzhe, ZHU Jianfeng, LIU Yuhu, LENG Qinglei 2024, 45 (5):  533-540.  doi: 10.7657/XJPG20240504 Abstract ( 160 )   HTML ( 6 )   PDF (7496KB) ( 59 )   Save The exploration and development practice has suggested that the volcanic rocks of the Huoshiling formation in the Chaganhua subsag of the Changling fault depression are distinct in rock types and lithologic assemblages from typical volcanoclastic rocks. Core samples and thin sections from the Huoshiling formation volcanic rocks reveal unique structures such as volcanic ash balls and quench-fractured breccia, with generally poor reservoir physical properties. Typical hummocky envelopes of volcanic edifice cannot be identified. All these phenomena indicate a potential unique eruption environment in this area. By analyzing seismic reflection characteristics, core descriptions, thin section features, and laboratory test results, typical indicators of the eruption environments were identified. Combining lithologic assemblages from existing wells and the regional tectonic setting, a volcanic eruption and deposition model for this area was established. The study reveals that the volcanic rocks in this area experienced three periods of eruption in onshore, land-water and underwater environments, respectively. Underwater volcanic rocks exhibit low aspect ratio, and is often in wide and gentle shield shape, with poorly developed primary pores, but developed dissolution and devitrification pores. The overall reservoir space is dominated by medium to small pores. The onshore volcanic rock strata are selected as exploration targets due to developed pores and good rock physical properties. Figures and Tables | References | Related Articles | Metrics

Genesis and Identification of Low Resistivity of Oil Layers in Badaowan Formation on Southern Slope of Zhongguai Bulge, Junggar Basin LI Fengling, FANG Xinxin, ZHANG Zhen, MA Sijie, LIU Rongjun 2024, 45 (5):  541-551.  doi: 10.7657/XJPG20240505 Abstract ( 238 )   HTML ( 15 )   PDF (4996KB) ( 213 )   Save Compared to other low-resistivity oil layers, the low-resistivity oil layers in the Lower Jurassic Badaowan formation on the southern slope of the Zhongguai bulge in the Junggar basin are characterized by early hydrocarbon accumulation, deep burial, large grain size, and low mud content, showing a unique low-resistivity genesis. Based on a comprehensive analysis on the genetic mechanisms of typical low-resistivity oil layers globally, together with the data of drilling, logging, well testing, and core analysis in the study area, the main controlling factors of the low-resistivity oil layers in the Badaowan formation were investigated from various perspectives including tectonics, sedimentation, diagenesis, reservoir characteristics, and hydrocarbon accumulation conditions. It is found that low resistivity of the oil layers in the study area is jointly controlled by macroscopic and microscopic factors. In a macroscopic setting with low tectonic amplitude and weak hydrodynamic sedimentation, low oil-water differentiation degree, high formation water salinity, and low tuff debris content are the main controlling factors for low resistivity, while low saturation of bound water is a secondary controlling factor. Accordingly, a chart illustrating the relationship between formation resistivity and oil/gas indicator coefficient was established, which matches the formation/production testing data in the study area by 92.9%. The study results provide a basis for identifying low-resistivity oil layers in the Badaowan formation on the southern slope of the Zhongguai bulge. Figures and Tables | References | Related Articles | Metrics

RESERVOIR ENGINEERING

Development Parameters of Chang 6 Reservoir in Shuanghexi Block of Yanchang Oilfield, Ordos Basin CHEN Junjun, YANG Xingli, XIN Yichao, LIU Zhaoyang, TONG Bowen 2024, 45 (5):  552-559.  doi: 10.7657/XJPG20240506 Abstract ( 242 )   HTML ( 9 )   PDF (794KB) ( 140 )   Save The Chang 6 reservoir in the Shuanghexi block of Yanchang oilfield in the Ordos basin is characterized by low permeability. Conventional calculation methods for development indices are not conducive to geological research, policy formulation and cost control for oilfield development. The production decline patterns, producing degree of reserves by water flooding, injection-production ratio, water cut, injected water utilization, and recovery of the Chang 6 reservoir were analyzed. The results show that the production of the Chang 6 reservoir follows a hyperbolic decline pattern. The block has significant potential for water injection development, with the current control degree and producing degree of reserves by water flooding at 74.54% and 36.94%, respectively, and an injection-production connection rate of 27.27%. The optimal injection-production ratio is approximately 2.5. As the recovery efficiency increases, the water cut rises rapidly at the first and then slows down. Based on the water retention rate, water consumption index, and water flooding index, it is evident that in the late stage of development, the water injection effectiveness improves, leading to an increase in ultimate recovery. During the development process, the water cut rise rate should ideally be kept below 6.1%, and the reasonable formation pressure should be maintained above 9.1 MPa. Under these conditions, the final recovery in the study area is approximately 23%. Figures and Tables | References | Related Articles | Metrics

Factors Influencing Productivity of Edge Waterflood in Elongated Anticlinal Reservoirs XIE Qichao, TIAN Yafei, YUE Ping, SONG Peng, LIU Xinju, LIU Jian, LIU Wantao 2024, 45 (5):  560-566.  doi: 10.7657/XJPG20240507 Abstract ( 220 )   HTML ( 3 )   PDF (2616KB) ( 108 )   Save The Y reservoir in the JY oilfield is a typical elongated anticlinal structure, where the injected water readily advances along channel centerline, resulting in rapid water-flooding and rapid production decline in producers. Development of such reservoirs is challenging due to unclear factors influencing productivity, such as water body size, structural amplitude, and reservoir physical property. To address these issues, a fine numerical model was established for the elongated anticlinal reservoir, and an “edge waterflood + progressive producer-injector conversion” process was proposed. On this basis, the influences of water body size, structural amplitude, and reservoir physical property on productivity were analyzed. The results indicate that the “edge waterflood + progressive producer-injector conversion” process enhances the edge water energy to allow for bidirectional responses of well patterns, and also delays water breakthrough in producers at the structural high to significantly reduce the water cut of oil well. Furthermore, considering the structural characteristics of the reservoir, the production performance under different factors were quantified, reasonable limits for the parameters such as water body size, structural amplitude, and the ratio of vertical permeability to horizontal permeability were defined, and the adaptability of reservoir area under different reservoir physical properties was demonstrated. The study results provide valuable insights for improving waterflood effects in similar reservoirs. Figures and Tables | References | Related Articles | Metrics

Producing Patterns and Improvement of Polymer Flooding Profile in Class ⅡB Reservoir, Daqing Oilfield ZHOU Congcong, CAO Ruibo, SUN Hongguo, FAN Yu, GUO Songlin, LIANG Guoliang 2024, 45 (5):  567-573.  doi: 10.7657/XJPG20240508 Abstract ( 197 )   HTML ( 1 )   PDF (611KB) ( 96 )   Save The application of polymer flooding is expanding in Daqing oilfield, with the target transferring to poor-quality Class ⅡB reservoir. The existing polymers exhibit poor compatibility with the reservoir, the producing patterns of the profile remain unclear, and the effects of polymer flooding development vary greatly in different areas. To address these issues, through the analysis of field profile data statistics and laboratory experiments, the producing patterns and improvement methods for the profiles in Class ⅡB reservoir were investigated. In view of the producing status from water-injected layers, Lamadian area which is characterized by thick channel sandbodies and good reservoir properties exhibits the highest proportion of net pay producing, with the reservoir exploited frequently, multiple polymer breakthrough layers, and relatively high liquid absorption. The Sazhong and Sanan areas, with limited channel sandbodies and multiple thin sand layers with poor properties, show a low net pay producing proportion in the water flooding stage, after polymer flooding which is 12.5% and 15.4% higher than those in the water flooding stage, respectively. Additionally, the reservoirs with permeability ranging from 100 to 300 mD show a significant profile improvement. In view of the producing status of water-unswept layers, the strong vertical heterogeneity of Class ⅡB reservoir results in a high net pay unproducing proportion between layers. In this case, the profile should be improved by achieving balanced interlayer production. The alternating injection of salt-resistant polymer of high and low concentrations can delay the rise of water cut, enhance the liquid absorption in low-permeability layers, and significantly improve the recovery efficiency of polymer flooding. A pilot test on alternating injection of DS1200 salt-resistant polymer of high and low concentrations was conducted in A area of Beibei block in Lamadian, achieving satisfactory water-cut reduction and oil increment. This technology provides a guidance for improving the polymer flooding profiles of Class ⅡB reservoir in Daqing oilfield. Figures and Tables | References | Related Articles | Metrics

Research and Application of a New Method for Dynamic Diagnosis of Gas-Cap Reservoirs WU Ruidong, WANG Rui, MA Lian, ZHANG Chunguang, SONG Gangxiang, SHI Meixue, LU Ying 2024, 45 (5):  574-580.  doi: 10.7657/XJPG20240509 Abstract ( 209 )   HTML ( 2 )   PDF (1046KB) ( 162 )   Save During the development of gas-cap reservoirs, crude oil, dissolved gas, gas-cap gas, condensate oil, and formation water may be produced simultaneously. Accurately calculating formation pressure and recovery percent of each phase is crucial for dynamic diagnosis and potential tapping of remaining oil and gas in such reservoirs. Current methods for calculating formation pressure fail to take water invasion into consideration, leading to uncertainty in production splitting, which increases the risks in subsequent adjustment and potential tapping. Through water influx fitting and Newton iteration methods, a new method for dynamic diagnosis of gas-cap reservoirs based on water invasion characteristic analysis and average formation pressure prediction was established. The application of this method in the Y3 gas-cap reservoir in the M oilfield indicates that crude oil and condensate oil account for 89.7% and 10.3% in the produced oil, respectively, and the produced gas contains 97.9% gas-cap gas and only 2.1% dissolved gas. The recovery efficiency of gas-cap gas and condensate oil is as high as 46.6% and 31.2%, respectively, while the recovery efficiency of crude oil and dissolved gas is merely 12.1% and 1.7%, respectively. These results are consistent with production test results. Figures and Tables | References | Related Articles | Metrics

Optimization of Perforation in CBM Horizontal Wells in Southern Qinshui Basin LI Kexin, ZHANG Cong, LI Jun, LIU Chunchun, YANG Ruiqiang, ZHANG Wuchang, LI Shaonan, REN Zhijian 2024, 45 (5):  581-589.  doi: 10.7657/XJPG20240510 Abstract ( 220 )   HTML ( 5 )   PDF (766KB) ( 258 )   Save To enhance the fracturing performance of coalbed methane (CBM) horizontal wells in the Qinshui basin, by analyzing the data of distributed optical fiber monitoring of water and gas production profiles, mud log, and well logging, the key factors influencing the fracturing performance were identified. These factors include coal quality, coal structure, drilling position, and perforation method. The middle to upper part of coal seam No. 3 in the Qinshui basin, characterized by low GR values, high coal quality, and intact coal structure, is identified as the optimal interval for fracturing stimulation. Based on the double GR curves, the drilling position of horizontal wellbore trajectory in the coal seam can be accurately determined, aiding in the selection of optimal fracturing interval and perforation method. When the drilling position is located in the middle part of the coal seam, conventional perforation method can be efficient. When the drilling position approaches the roof or is beyond the seam, downward directional perforation is preferred to effectively stimulate the high-quality upper part of the coal seam. When the drilling position is near the lower dirt band, upward directional perforation is advisable to target the high-quality middle part of the coal seam. Field application to 46 horizontal wells demonstrated that the single well production exceeded 2.5×104 m3/d and was stabilized at 2×104 m3/d, and the reservoir fracturing efficiency increased by 10% to 50%, recording a satisfactory development effect of the horizontal wells. Figures and Tables | References | Related Articles | Metrics

Interphase Mass Transfer in the Petroleum System During CCUS-EOR Process SU Jinchang, LIU Bin, LI Ruguang 2024, 45 (5):  590-594.  doi: 10.7657/XJPG20240511 Abstract ( 200 )   HTML ( 1 )   PDF (524KB) ( 158 )   Save CO2 flooding is a technique that utilizes CO2 to enhance oil recovery (CCUS-EOR), and an effective means to reduce carbon emissions. To understand the interphase mass transfer in the petroleum system during CCUS-EOR, CO2/dry gas contact experiments were conducted to elucidate the changes in components of the petroleum system during the initial contact conditions at different pressures. The results indicate that during the initial contact between CO2 and oil, the interphase mass transfer for volatile and non-volatile components occurs through evaporative extraction, while the interphase mass transfer for intermediate components occurs through dissolution diffusion, which is stronger than evaporative extraction. As pressure increases, the volatile components show enhanced evaporative extraction, the non-volatile components reflect diminished evaporative extraction, and the intermediate components exhibit augmented dissolution diffusion. At relatively low pressure in the initial stage of gas injection, the interphase mass transfer of the petroleum system is dominated by evaporative extraction. As pressure increases, the mechanism of interphase mass transfer for volatile components shifts to dissolution diffusion. During the initial contact process between dry gas and oil, the interphase mass transfer for intermediate and non-volatile components is dominantly evaporative extraction. CO2 is more capable of evaporative extraction than dry gas. Figures and Tables | References | Related Articles | Metrics

APPLICATION OF TECHNOLOGY

Shale Lithology Identification Based on Improved Random Forest Algorithm：A Case of Lucaogou Formation in Junggar Basin QIN Zhijun, CAO Yingchang, FENG Cheng 2024, 45 (5):  595-603.  doi: 10.7657/XJPG20240512 Abstract ( 212 )   HTML ( 5 )   PDF (6023KB) ( 178 )   Save In the application of reservoir lithology identification, the efficiency, accuracy and effective information integration ability of machine learning algorithm have been fully verified, especially in unconventional reservoirs with strong heterogeneity such as shale. Based on the optimal selection of parameters such as natural gamma, T2 geometric mean, structural index, skeleton density index, density, and deep lateral resistivity, and using a random forest algorithm combined with recursive feature elimination (RF-RFE), major lithologies of the shale reservoirs in the Middle Permian Lucaogou formation in the Junggar basin were identified. Lithology prediction was conducted on the same dataset using conventional RF and support vector machine (SVM) algorithms, and the results were compared with those obtained from thin-section identifications. It is found that RF-RFE yields better results with only half of the logging parameters, and the parameters defined by optimal selection help reduce the algorithm’s running time. Thus, the use of RF-RFE algorithm can realize optimal selection of characteristic logging parameters, more accurate identification of shale lithology, and reduction of running time. The algorithm provides a new approach for complex lithology identification and multi-parameter selection. Figures and Tables | References | Related Articles | Metrics

A Method of Anisotropic Velocity Modeling for HTI Medium in the Surface of Piedmont Gravel Zone KONG Fanyong, XIE An, XIA Jianjun, ZHANG Lulu, WANG Liye, WANG Wei, YU Jinlin, WEI Jianbo 2024, 45 (5):  604-610.  doi: 10.7657/XJPG20240513 Abstract ( 156 )   HTML ( 3 )   PDF (17953KB) ( 110 )   Save There are widespread alluvial fans with extremely thick gravel deposits in the surface of the piedmont zones in western China. These fans exhibit strong azimuth anisotropy. The tomographic inversion velocity differs greatly from the vertical velocity, severely impacting the accuracy of static correction value of 3D seismic data and the imaging of shallow-to-medium layer in pre-stack depth migration. Based on the HTI medium theory, a method for determining the symmetry axis and constructing an anisotropic model through azimuthal tomography fitting using first-arrival forward modeling was proposed. First, based on anisotropy, the first-arrival time from both micro-logging forward modeling and actual data is examined and compared to identify the symmetry axis characteristics. Then, elliptical fitting is performed on the azimuthal first-arrival tomography inversion model to derive the initial fast and slow velocities and the symmetry axis orientation. Next, by using 2D VTI anisotropic forward modeling and tomography inversion, the correction coefficients of the velocity in the direction of the symmetry axis are obtained. Finally, an anisotropic velocity model is established for the piedmont gravel zone. Figures and Tables | References | Related Articles | Metrics

Application of Logging Data Wavelet Transform and Pseudo-Imaging to Fine Division of Deep Barrier/Interlayer SHAO Cairui, WANG Meng, CHANG Lunjie, WANG Kaiyu, ZHANG Fuming, WANG Chao 2024, 45 (5):  611-621.  doi: 10.7657/XJPG20240514 Abstract ( 182 )   HTML ( 6 )   PDF (1474KB) ( 180 )   Save Barrier/interlayer is a key factor that significantly affects fluid flow and controls the distribution of oil and water, and it serves as a crucial evidence for understanding the distribution of remaining oil. Barrier/interlayer in deep strata is difficult to identify due to the high coring cost, large depth error in logging data, low resolution of conventional logging curves, and ambiguous signals from thin interbeds. Through core analysis of key wells, the logging curves sensitive to barrier/interlayer and their response characteristics were identified. By employing wavelet decomposition and reconstruction, the conventional sensitive logging curves were processed with high resolution, which reduced the smoothing effect of adjacent layers and highlighted the logging response characteristics of thin layer interfaces, making thin layer identification resolution enhanced by nearly 100%. By integrating the vector pattern of formation dip and pseudo-imaging characteristics of barrier/interlayer, a method for identifying and dividing deep barrier/interlayer was established. Actual applications demonstrate that this method allows for precise identification of barrier/interlayer, with a much higher capability than conventional methods. This method yields an accuracy of layer correlation between wells increased by 38%, elucidating the issue of inclined oil-water contact and providing remaining oil distribution. Figures and Tables | References | Related Articles | Metrics

OVT-Domain Wide-Azimuth Seismic Forward Modeling of Glutenites in Dongying Sag LOU Fengqin, YU Jingqiang, ZHANG Yunyin, LIU Haining, WU Mingrong, GUO Zhiyang 2024, 45 (5):  622-628.  doi: 10.7657/XJPG20240515 Abstract ( 253 )   HTML ( 3 )   PDF (7909KB) ( 123 )   Save Considering the varying lithofacies and lithology of the proximal glutenites in the Dongying sag，a three-dimensional geological model of the glutenites was established for wide-azimuth seismic forward modeling. Using the simulated data cube，and through azimuthal stacking of gathers in OVT-domain，the effects of azimuth variation on parameters such as seismic travel time and amplitude were analyzed，and the relationships between azimuth/amplitude and favorable reservoirs were established. The results show that the variation in the sedimentary direction of the glutenites causes azimuth differences in seismic wave propagation，leading to azimuthal anisotropy in seismic reflections. The data cube obtained from azimuthal stacking at the azimuth perpendicular to the sedimentary boundaries is more sensitive to the responses of the top and internal boundaries of the glutenite，with stronger amplitudes. It more effectively reveals the contacts between glutenites of different periods，thereby facilitating the accurate identification of glutenite and fine prediction of favorable reservoir distribution. Wide-azimuth OVT-domain seismic data are proved effective in glutenite prediction，and have been successfully applied in predicting glutenite reservoirs in the steep slope zone of the northern Dongying sag，with the prediction results in good agreement with actual drilling results. Figures and Tables | References | Related Articles | Metrics