石油实验地质

2021, 43(1): .

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Discovery and significance of Cretaceous-Paleogene freshwater foraminifera in Subei-South Yellow Sea Basin

LIU Yurui

2021, 43(1): 1-13. doi: 10.11781/sysydz202101001

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In previous studies, the "marine facies" fossils in the second member of Taizhou Formation and the second and fourth members of Funing Formation of the Subei-South Yellow Sea Basin were generally considered to be organisms associated with transgression and organisms that lived in saline lakes. However, after reviewing the data, this theory proved to be wrong: most of the foraminifera do not exist in the above mentioned members. The distributions of "marine facies" fossils are not concentrated, and the surrounding rocks do not contain gypsum-salt. The evidence of paleoclimate and paleohydrology is also contrary to lake salinization theory. Formation comparison results show that 97 foraminifera are from 4 new strata, and the remaining 3 are in the second member of Funing Formation. The source of transgression, the ancient East China Sea, does not exist, and there are no marine fossils here. The "marine facies" fossils found here are all euryhaline organisms that can also live in fresh water. During the Late Cretaceous-Paleocene lake invasion period, the ancient climate was humid and the ancient lake was deep, and the water could not be salinized. Glauconite, apatite and trace elements cannot be used as a criterion for distinguishing between marine and terrestrial facies. Different types of euryhaline organisms are usually flourishing separately, and co-exist with a large number of freshwater organisms; meanwhile, the surrounding rocks do not contain gypsum-salt. According to the evidence above, the euryhaline organisms in this area are interpreted to live in freshwater, since foraminifera can live in a freshwater environment, and there was no water salinization in the three stages, of regional lake invasion. The large scale, high quality source rocks in Mesozoic-Cenozoic terrestrial basins in China are mainly freshwater lacustrine sediments, which can provide rich oil and gas resources.

Development characteristics and formation mechanism of ultra-deep carbonate fault-dissolution body in Shunbei area, Tarim Basin

LÜ Haitao, HAN Jun, ZHANG Jibiao, LIU Yongli, LI Yingtao

2021, 43(1): 14-22. doi: 10.11781/sysydz202101014

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As a spatial geological body, a strike-slip fault zone has a physically and chemically controlled compart- mentalization effect, which can form a unique fracture-cavity system with associated pore spaces inside the fault zone. A fracture-cavity system related to ultra-deep carbonate "fault-dissolution body" developed in Shunbei area of the Tarim Basin, and its "fault-dissolution body" reservoir is closely related to fault activities. The Shunbei strike-slip fault zone was studied with previous analyses and practices. The inner structure of the strike -slip fault zone was discussed, and the origin and characteristics of "fault-dissolution body" were analyzed. The spatial distribution of the "fault-dissolution body" reservoir is constrained by the boundary of the strike-slip fault. The external contour of the reservoir is mainly controlled by the fault zone, and the width is narrow while the depth is large. The reservoir space is mainly composed of "cavity" type caverns and a seam network formed by multi-stage structural rupture, and has undergone erosion and cementation via multiple types of fluids, forming dissolution holes and various pore spaces along the fracture system, and it leads to the complexity of the internal reservoir structure of fault- dissolution body. According to the dynamic changes of fluid participation and fluid properties in the fault zone, three genetic mechanisms can be identified: fault-capacity, karst and hydrothermal fluid erosion. The formation mechanism of the "fault-dissolution body" may be one of them, or a combination of two or three types. The most essential difference between "fault-dissolution body" and karst cave reservoir is that the composition of the former fracture-cavity system and the formation of reservoir scale are closely related to the fault activity, while the latter depends mainly on the water-rock interaction and the total matter volume removed.

Characteristics and main controlling factors of carbonate reservoirs of Middle-Lower Ordovician, Shunnan area, Tarim Basin

LIU Jun, CHEN Qianglu, WANG Peng, YOU Donghua, XI Binbin, GONG Hanning

2021, 43(1): 23-33. doi: 10.11781/sysydz202101023

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Natural gas breakthroughs have been made in many wells recently in the Shunnan in the southern part of the Shuntoguole low uplift in the Tarim Basin. With the advancement of exploration, the understanding of reservoir development is one of the key issues affecting exploration evaluation. Based on the research of sedimentary facies and tectonic evolution, core description, thin section analysis, physical property analysis, comprehensive logging and reservoir prediction were used to study the carbonate reservoirs in the Middle-Lower Ordovician in the study area. The reservoir spaces include pores and fractures controlled by fault and hydrothermal fluid activities, and also intercrystalline pores and intercrystalline solution pores in dolomite reservoirs, micropores in limestone reservoirs. There are three types of reservoirs, including fracture-cavity type controlled by faults, fracture-cavity type controlled by hydrothermal fluid activities, and fractures-pores. The fractures-pores mainly distribute in limestones in the Yijianfang Formation and the upper Yingshan Formation and in dolomites in the Lower Yingshan Formation. They have poor physical properties, and are featured by extra-low porosity and permeability. The fracture-cavity types controlled by faults and hydrothermal fluid activities mainly distribute along the NE strike-slip fault zone. Strike-slip fractures and associated fractures developed in multiple stages are the main controlling factors for large-scale effective reservoirs. They formed fractured-vuggy reservoirs. Meanwhile, they connected pores and micropores, intercrystalline pores, and intercrystalline dissolution pores, forming connected effective reservoirs, which was also a prerequisite for hydrothermal fluid activities.

Geological characteristics and optimization of favorable areas in the upper gas reservoir of Jiaoshiba block in the Fuling Shale Gas Field, Sichuan Basin

SHU Zhiguo, WANG Jin

2021, 43(1): 34-44. doi: 10.11781/sysydz202101034

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The shale quality characteristics such as lithofacies, geochemistry, porosity and gas bearing properties, some evaluation standards of gas bearing properties and compressibility of the upper gas layer were studied for the upper gas reservoir of Jiaoshiba block in the Fuling Shale Gas Field. The upper gas layer in Jiaoshiba block is mainly of type Ⅱ, and the layer No. 8 has the best gas bearing properties. of five catogaries including shale hydrocarbon source quality, gas bearing capacity, structural deformation strength, fracture and buried depth were optimized, and an evaluation system for development selection of the upper gas reservoir in Jiaoshiba block was established. The upper gas reservoir in Jiaoshiba block was divided into four areas, among which the well block JY-A is the most favorable area, which can be considered to be a standard for gas reservoir development in the upper part of Jiaoshiba block. In addition, the study revealed that organic matter abundance, porosity and preservation conditions are the main factors affecting shale gas enrichment in the upper gas layer of Jiaoshiba block, while burial depth, crustal stress and the extent of natural fracture development are the main factors controlling gas well production in the upper gas reservoir.

New insights into structural characteristics in central-northern Longmen Mountains: implications for multiple-decollement deformation

DENG Peng, FANG Chengming, DENG Mingzhe, ZHAO Li

2021, 43(1): 45-55. doi: 10.11781/sysydz202101045

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The central-northern segment of the Longmen Mountains is located in the transfer zone, for which the structural characteristics are little studied. Based on the reprocessed two-dimensional seismic data, this work used methods such as field geological investigation, drilling analysis and structural interpretation to comprehensively study the structural characteristics in the central-northern segment of the Longmen Mountains. The thick belt is characterized by deep and ductile deformation and shows the combination styles of a series of tight anticlines and synclines. The transition belt I developed many imbricate faults which cut to the ground surface. The main faults in the transition belt I derived many secondary gently dipping faults in the footwall, which controlled the development of the relative fault-propagation folds. The transition belt Ⅱ is a buried structure that includes the No.1 fault. The intense imbricate thrust structures occurred in its hanging wall and the autochthonous structure with weak fold deformation developed in its footwall. This study provides new insight that the high-angle thrust faults commonly derive secondary gently dipping faults in the footwall, and both of them formed the falling-γ type of the structure style. According to the analysis of the structure characteristics and mechanism origin, it is suggested that the composite structure styles combining the imbricate and falling-γ type are the basic structure styles in the study area, which is controlled by the multiple-decollement deformation from the deep layer to the shallow layer as well as from the trailing edge to the leading edge.

Palaeokarst characteristics and effects on reservoirs in the fourth member of Leikoupo Formation, Longmen Mountain front, western Sichuan Basin

LI Yong, DENG Meizhou, LI Guorong, GAO Hengyi, WANG Qiongxian, HE Sai

2021, 43(1): 56-63. doi: 10.11781/sysydz202101056

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Dolostones of tidal flat facies are the major reservoir rock type of the fourth member of Leikoupo Formation, Longmen Mountain front, western Sichuan Basin. The reservoir controls are dominated by phase belt, dolomitization and denudation. Core observation, cast thin section identification, laser carbon and oxygen isotopes, cathode luminescence, electron probe and inclusion thermometry were used to study the denudation effect. This reservoir went through three periods of denudation, including penecontemporaneous period, epidiagenetic period and buried period, which separately formed intragranular and moldic pores, gypsum caves, and fault-line cavities. The studies of karstification and gypsum dissolution in the paleo-epigenetic period showed that, the distinguishing signs of meteoric water impacts in episode I of the Indosinian Movement include the irregular network of weathering cracks, karst and gypsum breccia, de-anointed calcite maintaining gypsum board shape or right-angle edge shape, widely developed paste mold holes and paste-dissolved holes. These products are featured by low carbon and oxygen stable isotope values, a wide range of strontium isotope values, low content of Sr-Mn-Fe-K-Na trace elements, low-temperature single-phase water-soluble inclusions. The dissolution of gypsum in episode I of the Indosinian Movement is the key for reservoir generation.

Characteristics and main controlling factors of tight oil reservoirs in Cretaceous Tengger Formation, A'nan Sag, Erlian Basin

FU Xiaodong, ZHANG Tianfu, WU Jianping, WANG Xiaofang, ZHOU Jingao, JIANG Tao, LI Bing, LI Changxin

2021, 43(1): 64-76. doi: 10.11781/sysydz202101064

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A set of "special lithology" tight reservoirs related to volcanic activity developed widely in the lower submember of the first member of Cretaceous Tengger Formation (K₁bt^1x) in the A'nan Sag, Erlian Basin, which is the main target for tight oil exploration. Based on section observation, physical property analysis, scanning electron microscopy (SEM), mercury intrusion porosimetry, CT scanning and nuclear magnetic resonance (NMR), the characteristics and main controlling factors of "sweet spots" of tight oil reservoirs with different lithologies in the K₁bt^1x were studied. Tuffaceous rocks, sandstones, dolomites and mudstones developed in the K₁bt^1x, and tuffaceous rocks and sandstones are the favorable lithologies for the development of tight oil reservoirs. The reservoir space consists of dissolved pores, intercrystalline micropores, residual intergranular pores, drainage pores and microfractures. The main reservoir space of tuff includes devitrified intercrystalline pores and dissolution pores, whereas that of sandstone includes residual intergranular pores and dissolution pores. The reservoir pore structures can be divided into 3 types and 4 subtypes. The thin-layered tuff and fine sandstone with solution pores have better pore structures, but the sedimentary tuff and dolomite have poor pore structures. The reservoir has a strong oil content heterogeneity controlled by reservoir lithology, porosity and permeability. The development of "sweet spots" in tuffaceous tight reservoirs is mainly controlled by the volcaniclastic deposition, devitrification and dissolution, while the development of "sweet spots" in sandstone reservoirs is mainly controlled by sedimentary microfacies, rock structure and dissolution.

Micro-scale pore-throat distributions in tight sandstone reservoirs and its constrain to movable fluid

ZHONG Hongli, ZHANG Fengqi, ZHAO Zhenyu, WEI Chi, LIU Yang

2021, 43(1): 77-85. doi: 10.11781/sysydz202101077

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To understand the constrains of multi-scale microscopic pore-throat distributions to the movable fluid in tight sandstone reservoirs, high-pressure mercury injection and nuclear magnetic resonance (NMR) were employed to obtain the distribution characteristics of micro-scale pore-throat distributions of Chang 6, Chang 7 and Chang 8 oil-bearing sections of the Yanchang Formation in the southeastern part of the Yishan slope, Ordos Basin. The occurrences feature of movable fluid in tight sandstone reservoirs were also investigated by centrifugal experiment and the T2 spectrum analysis of NMR. Results show that the microscopic pore-throat radius distribution in the tight sandstone reservoirs of Yanchang Formation has a wide distribution (ranging from 1. 13 to 3 050. 80 nm), mainly distributed from 10 to 500 nm, referring to micro- and nano-scale pore-throats and mainly were nano-scale. The movable fluid saturation in tight sandstone reservoirs ranged from 9. 83% to 25. 64% with an average value of 17.53%, indicating a low content in tight sandstone reservoirs. The porosity and permeability of studied reservoirs were positively correlated to the pore-throats having movable fluid, indicating the physical properties of reservoirs playing a role in controlling the distribution of movable fluid in tight sandstone reservoirs.The ratio of pore-throat which greater than 50 nm, and ratio of pore-throat greater than 100 nm, the maximum pore-throat radius, peak pore throat radius etc., showed a positive correlation with the porosity of movable fluid in tight sandstone reservoirs. It can be indicated that the distribution of relative larger pore-throasts, especially those larger than 100 nm, have strong controlling affection to the relative content of movable fluid in tight sand- stone reservoirs. The sorting coefficient of pore-throat is positively correlated with the content of flexible fluid due to the well-sorted tight sandstones with a wide pore-throat radius distribution in tight sandstone reservoirs are dominated by fine pore-throats.

Densification process of Huagang Formation in northern and central Xihu Sag of East China Sea Shelf Basin

ZHANG Wu, JIANG Yiming, XIAO Xiaoguang, CHEN Hao, MIAO Qing, XU Zhixing

2021, 43(1): 86-95. doi: 10.11781/sysydz202101086

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Technical methods such as cast thin section identification, scanning electron microscopy, constant rate mercury intrusion, X-ray diffraction, fluid inclusion homogenization temperature measurement, laser Raman component analysis and illite isotope dating were used to analyze the densification process of Huagang Formation in the northern and central Xihu Sag of the East China Sea Shelf Basin. The controlling factors of reservoir densification were discussed. The reservoir heterogeneity of the Huagang Formation is controlled by the pore throat structure, and the reservoir with a throat radius less than 1 μm is regarded as a tight reservoir. Burial compaction is the main reason for the generally low permeability and tightness of reservoirs, and differential diagenesis aggravates the differential evolution of reservoirs. When the formation temperature exceeds 160℃, the inhibition of cementation by the ring chlorite is significantly weakened. The alkaline closed environment of the middle diagenesis B stage makes a large amount of iron-containing carbonate, pom-shaped chlorite and bridging illite enriched and directly blocks the throats, resulting in large-scale tight reservoirs in the Huagang Formation. Recovering the key diagenetic evolution process of the Huagang Formation reservoirs, it is speculated that the H4 and H5 sand formations of the Huagang Formation entered large-scale compaction during 9-7 Ma, and the strong lateral compression of the Longjing Movement accelerated the densification process.

Oil source correlation and its relationship with deeply buried hydrocarbon accumulations in Tianjingshan Paleo-uplift area, northern segment of Western Sichuan Depression

LIANG Xiao, WU Liangliang, LI Yading, LI Jing, FANG Xinyan, PU Boyu, WANG Hua, LIU Shugen

2021, 43(1): 96-111. doi: 10.11781/sysydz202101096

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The Tianjingshan Paleo-uplift (TJS PU) area in the northern segment of Western Sichuan Depression (NSWSCD) has abundant oil and gas shows. The traceability of paleo-oil reservoirs and their destruction are of great significance to the study of the characteristics and mechanism of deep hydrocarbon accumulations. This study focuses on the tectonic evolution of Mianyang-Changning Intracratonic Sag (MY-CN IS) and TJS PU. The Cambrian-Ordovician tectonic movement was significant in the NSWSCD, as indicated by the formation of the TJS PU. From tension to compression tectonic-sedimentary evolution, the TJS PU is an important node in the Early Paleozoic. The analysis of deep marine hydrocarbon geological characteristics in the NSWSCD shows that the Lower Cambrian Maidiping-Qiongzhusi formations are the best source rock. Reservoir bitumen in the Dengying Fm is used as a template to identify organic geochemical parameters of paleo-oil reservoir systems with different layers and different phases in the TJS PU area. Organic carbon isotope and biomarker compounds indicate that the main source of paleo oil reservoirs is the Lower Cambrian organic-rich black shale, and the concept of "Tianjingshan Paleo-uplift paleo-reservoir system" is finally established. According to the corresponding classification of source-reservoir-caprock combination, oil source identification and tectonic evolution period study, it is once again confirmed that the deep marine oil and gas multi-phase and diverse accumulation process under the complex structural background of the northern segment of the Western Sichuan Depression can be divided into "primary oil reservoir-primary gas reservoir" and "secondary oil reservoir-primary gas reservoir" types.

Hydrocarbon charging events and timing in Paleogene Hetaoyuan Formation, Biyang Sag, Nanxiang Basin

ZHU Yan, LI Fengxun, CHEN Honghan, ZHAO Yingbin, LI Lijuan, ZHENG Rong, BU Xuqiang

2021, 43(1): 112-120. doi: 10.11781/sysydz202101112

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Previous studies showed that the structure of Nanxiang Basin controlled the formation of oil and gas reservoirs, but the exact formation time of the reservoir and its coupling relationship with structural movement are not clear yet. Therefore, by analyzing the fluid inclusion system and combining with the projection method of burial history, this paper analyzed the times of oil and gas charging and accumulation in the Paleogene Hetaoyuan Formation of the Biyang Sag, and determined the age of oil and gas accumulation. The results showed that there were four phases of oil charging in two stages and one phase of natural gas charging in the Biyang Sag. The first stage mainly occurred during fault depression and uplift, including the first phase (36.1-23.5 Ma), the second phase (34.1-21.2 Ma) and the third phase (30.9-16.2 Ma) of oil charging, with the characteristics of multi-phase continuous charging. The second stage occurred during depression subsidence, including the fourth phase (7.9-0.2 Ma) of oil charging and the first phase (3.0-0.8 Ma) of natural gas charging. The early stage (the first phase) was the main formation time of oil and gas reservoirs in the Biyang Sag. The charging time of oil and gas was different in different structural zones in the Biyang Sag. The central deep subsided area and the southern slope were closer to the hydrocarbon-generating center, and were charged earlier, while the northern slope was charged relatively late. Therefore, it is recommended to focus on strengthening the vertical multi-layer and multi-stage three-dimensional exploration in the southern part of the Biyang Sag, strengthening the exploration of subtle lithology in the center, and focusing on the exploration of secondary or adjusted structural reservoirs in the north.

Effective techniques for seismic description of reservoirs in Jiyang Depression, Bohai Bay Basin

GUO Yuanling, LIU Cuirong, LI Hongmei, JIANG Jie

2021, 43(1): 121-127. doi: 10.11781/sysydz202101121

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A series of 3D seismic descriptions for five reservoir types in the Jiyang Depression of Bohai Bay Basin was proposed based on the comparison and analysis of a large amount of drilling and seismic data. The effective techniques for seismic description of channel sandstone reservoirs include amplitude preserving and frequency extension processing based on high-precision isochronous stratigraphic framework, fusion of pre-stack and post-stack sensitive attributes, and comprehensive identification of oil and gas potential. Logging constrained sequence subdivision, sedimentary microfacies and lithofacies identifications by stochastic simulation and probability body subdivision of impedance lithofacies, and phase constrained multi-parameter prediction of effective reservoir can be effectively applied to seismic description of a conglomeritic fan reservoir. On the basis of improving processing resolution, multi-attribute fusion technology can effectively predict the effective beach bar sandstone reservoirs. The techniques such as "sweet spot" attribute analysis, seismic waveform indication inversion, pre-stack elastic parameter inversion, lithology probability inversion can effectively describe turbidite sandstone reservoirs. Fault enhancement processing, multi-attribute body joint identification of faults, optimization of fault plane combination and spatial three-dimensional interpretation can precisely describe complex fault block reservoirs. The technical effectiveness varies with the accuracy and quality of seismic data and the changes of geological conditions.

Gas-bearing property evaluation by petrophysical logging in shale gas reservoirs: a case study in J area of Fuling shale gas field, Sichuan Basin

LIU Yun, ZHANG Mengyin

2021, 43(1): 128-135. doi: 10.11781/sysydz202101128

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The lithology of shale reservoirs is complex and the types of storage porosity are diverse. The existing gas-bearing property evaluation system by petrophysical logging is not perfect. In order to deepen the understanding of the gas-bearing capacity evaluation of shale reservoirs in J area of the Fuling shale gas field in the Sichuan Basin, the density-neutron curve superposition method, natural gamma-de-uranium gamma curve superposition method, and uranium-thorium ratio method are applied in qualitative recognition. Quantitative logging research was carried out on gas-bearing parameters such as adsorbed gas, free gas, porosity, and gas saturation. Based on the multi-parameter comparative study, organic carbon content, porosity, gas saturation, siliceous content, and burial depth are selected as the logging evaluation parameters that affect gas-bearing properties. A logging evaluation index (GQ) for gas-bearing capacity is constructed using fuzzy correlation analysis. The gas-bearing evaluation criterion is established, that is, GQ ≥ 0.57 for type Ⅰ gas-bearing reservoirs, 0.37 < GQ < 0.57 for type Ⅱ gas-bearing reservoirs, and GQ ≤ 0.37 for type Ⅲ gas-bearing reservoirs. The application of this method in J area shows that the accuracy of the quantitative prediction model for gas-bearing parameters is high, and the gas-bearing evaluation criteria can effectively evaluate and classify the gas-bearing properties of the reservoir.

Representation of gas hydrate fracture migration system by seismic

SONG Ruiyou, CHEN Kui, LI Anqi, MAO Huan, LIU Yunzhi

2021, 43(1): 136-143. doi: 10.11781/sysydz202101136

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The South China Sea is one of the four major offshore oil and gas accumulation centers in the world. Many oil and gas fields and hydrate deposits have been found on the northern slope of the South China Sea. The amount of hydrate resources is huge. Relying on high-precision seismic exploration technology, the exploration of hydrate deposits in the South China Sea has progressed greatly in recent years, but the success rate of seismic detection technology needs to be improved. Scientific drillings have proved that there are multiple solutions to find hydrate only by identifying BSR in the stable zone, while the research on hydrate accumulation system is not deep enough, especially the migration, accumulation and dispersion system. Fractures are the main systems of hydrate accumulation and mineralization in the north of the South China Sea. In order to effectively characterize the fracture system of hydrate, this paper, combined with the successful experience of oil and gas reservoir migration and accumulation system research, applied the fracture identification technology used to study fracture channels to discover the diapir gas field to the BSR area of Qiongdongnan Basin, and successfully identified the hydrate fracture transport system. Based on the results, the migration efficiency was discussed. In the study of gas hydrate accumulation systems, the seismic characterization of fractures can be used as a means to study the transport system.

Forced imbibition in tight sandstone cores

JIANG Yun, XU Guoqing, SHI Yang, YU Yue, WANG Tianyi, ZENG Xinghang, ZHENG Wei

2021, 43(1): 144-153. doi: 10.11781/sysydz202101144

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Spontaneous imbibition (SI) generally occurs under forced pressure (pressure difference between hydraulic fluid pressure and original pore pressure) during a shut-in period. However, the experimental study of SI is commonly performed at atmospheric pressure and the effect of forced pressure is often neglected. In this study, the mechanism of SI in tight sandstone samples under forced pressure (forced imbibition, FI) was studied. A new experimental method for forced imbibition was firstly constructed based on low-field nuclear magnetic resonance(LF-NMR) measurements. After that, a correlation between SI and FI was discussed. Finally, a new dimensionless time model considering the effect of forced pressure for FI was constructed. The results showed that 96.76%-97.25% wt% of the oil was distributed in nano-pores (0.1 ms ≤ T₂ ≤ 100 ms) of core samples, occupying the major pore space. The ultimate oil recovery for FI was significantly improved relative to that of SI, which was associated with the synergetic effect of enhanced SI and compaction. The new dimensionless time model for FI was proved to be effective and it provides a new method to calculate shut-in time at field scale.

Rapid evaluation of probable recoverable reserves in tight reservoirs: a case study of Chang 8 reservoir (eighth member of Yanchang Formation) in Honghe oil field

GUO Mingli, CHEN Yan, ZHENG Zhenheng, LI Jun, YU Lian, LIU Liqong

2021, 43(1): 154-160. doi: 10.11781/sysydz202101154

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Tight oil reservoirs are featured by tight formations, large oil area, locally enriched "sweet spots", complex oil-water distribution, and complicated reservoir conditions brought by later engineering transformation, which determines the development performance of single well. The distribution of recoverable reserves has great variability and uncertainty, so it is difficult to obtain a reasonable and definite understanding using conventional reserve evaluation methods. By introducing the evaluation methods of unconventional shale oil and gas reserves in North America, the developed wells were used to determine the reserves of single wells, and the probability distribution characteristics of reserves of developed wells were used to characterize the reserves of blocks or reservoirs. The Chang 8 (the eight member of Yanchang Formation) tight and ultra-low permeability reservoir in the Honghe oil field was studied. The initial production performance and production decline characteristics of a single well were tracked, and the correlations between recoverable reserves and geological characteristics, oil-bearing properties and initial production performance were analyzed to determine the key factors affecting the estimation of recoverable reserves. The P₁₀/P₉₀ values were used to judge the uncertainty of the estimates. The minimum sample size required for 90% confidence can be obtained when the value is 3-8; if the value is greater than 10, it indicates that the reservoir has the complexity of geological and engineering transformation and the difference of different blocks. The key influencing parameters and the probability distribution of recoverable reserves of single well were established by using the uncertainty method, and the rapid evaluation of recoverable reserves of a single well were realized.

Shale gas EUR estimation based on a probability method: a case study of infill wells in Jiaoshiba shale gas field

WEI Shaolei, HUANG Xuebin, LI Jun, SU Yinghong, PAN Lisheng

2021, 43(1): 161-168. doi: 10.11781/sysydz202101161

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The characteristics of shale gas reservoirs and the engineering characteristics of "factory" operation lead to repeated production performance among wells, which is the theoretical basis of a probability method to calculate estimated ultimate recovery (EUR). Compared to the conventional deterministic method, the probability method can calculate a reserve probability distribution. The well spacing of the Jiaoshiba shale gas field in the Sichuan Basin is 600 m, leading to low production extent. Infill wells have been introduced in order to improve gas recovery. In this paper, a probability method was adopted to calculate the EUR for the infill adjustment. Suitable analogy wells were selected by taking geological characteristics, engineering conditions, testing data and well locations into consideration. Based on the EUR probability distribution of analogy wells, the EUR probability curves of infill wells were calculated with the Monte-Carlo method. Typical well curves were established by analyzing the decline parameters of analogy wells. Future production performance and EUR of new wells or undrilled wells can be estimated with this probability method, which is the basis for future strategic decisions.

A method for evaluating the potential of reserve upgrading based on quantitative uncertainty

XIAO Yuru, HUANG Xuebin, LI Shu, LIU Liqiong, LI Jun

2021, 43(1): 169-175. doi: 10.11781/sysydz202101169

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Most of the initial in-place and undeveloped OOIP are low-grade reserves, and how to evaluate their potential is an urgent problem. There are many uncertainties in low-grade reserves, such as complex geological conditions, strong reservoir heterogeneity, limited data and limited geological knowledge. The reliability of the conventional evaluation method is poor. Therefore, a series of systematic evaluation methods for initial in-place and undeveloped OOIP on the basis of quantitative uncertainty was proposed, which mainly focused on three improvements. Firstly, the range value was used to describe the uncertainty of OOIP, which changed the unique value estimated by the certainty method in the past. The degree of certainty was proposed to quantitatively characterize the risk of reserves. Secondly, the evaluation index system, the evaluation method and the evaluation process of uncertainty of the key parameters were established. Thirdly, the new method was changed from the previous emphasis on reserve scale to reserve value, established a three-factor model of "the degree of certainty-unit reserves/resources value and the scale of reserves/resources" by applying risk-value assessment technology. Potential blocks were divided into four categories, namely type Ⅰ with low risk and high value, type Ⅱ with low risk and low value, type Ⅲ with high risk and high value, and type Ⅳ with high risk and low value. In this way, a potential evaluation platform combining reserves, production and value evaluation was proposed. The application of the method showed that it can effectively evaluate the reserve upgrading and development potential and optimize the blocks, and the selected blocks are basically consistent with the subsequent upgrading and development of potential blocks, which verified the reliability and rationality of the method.

Evaluation of upgrading potential and strategy for inferred initially in-place petroleum

SHI Lei, HUANG Xuebin, LIU Jingliang, ZHANG Lian, ZHUANG Li, ZHANG Fengdong

2021, 43(1): 176-183. doi: 10.11781/sysydz202101176

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Under the background of low oil price, the effective upgrading and utilization of the inferred initially in-place petroleum reserves are essential. At present, there is a large quantity of existing inferred initially in-place petroleum, but there is no systematic study on the evaluation of the upgrading potential. In this paper, a funnel screening method is used to establish a clear and effective evaluation procedure and to focus on the evaluation and control of risks in each stage of the process. In order to minimize the influence of human factors in geological risk assessment, the technique for order preference by similarity to an ideal solution (TOPSIS) is used to evaluate the geological factors that affect the reserves upgrading for the first time. Eleven potential upgrading blocks have been selected and achieved good results in clastic rock reservoirs in the Bohai Bay Basin. The research results show that the uncertainty of the predicted reserves is large. In the upgrading process, the risks and difficulties should be identified in time. The corresponding technical measures are taken and the strategy are proposed, which provides guarantee for the further efficient and economic upgrading of reserves.

Controlled reserve upgrade standard for middle-deep low permeability glutenite reservoirs in Jiyang Depression, Bohai Bay Basin

LI Shu, HUANG Xuebin, XIAO Yuru, ZHENG Zhenheng, LIU Liqiong

2021, 43(1): 184-192. doi: 10.11781/sysydz202101184

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Abstract:
Glutenite reservoirs are one of the major targets for petroleum exploration, and some important breakthroughs have been made in many oil-bearing basins in recent years. Through 2019 glutenite reservoirs in the Jiyang Depression of Bohai Bay Basin accounted for 15% of the total low permeability controlled reserves of SINOPEC. However, due to the lack of upgrade standards and the unclear main control factors, which of these controlled reserves can be upgraded is also unclear. Based on the status analysis of upgraded reserves and the characteristics of proved reserves in low permeability glutenite reservoirs, it is suggested that reservoir effectiveness, productivity and economy are the main factors that influence the controlled reserve upgrade. The key parameters for characterizing and controlling reserve upgrading were determined, and the methods for determining rational production and decline rate were studied in detail. A two-level standard for the upgrading of controlled reserves in low permeabilityglutenite reservoirs was established under different geological conditions and oil prices. First, the lowest limit for reservoir effectiveness was assessed under different permeability and burial depth, such as in situ crude oil viscosity, effective thickness, effective porosity and oil saturation. Second, the lowest limit for single well productivity and minimum economic recoverable reserves of individual wells under different burial depth and oil price was considered. The results provide a basis to evaluate the upgrading potential of controlled reserves of low permeability glutenites in the middle and deep layers of Jiyang Depression. The selected upgrade potential of a block is basically consistent with the subsequent upgrade effect of that block, which verifies the reliability and rationality of the above upgrade standard.

2021, 43(1): 193-193.

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Abstract:

2021 Vol. 43, No. 1

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