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28 October 2021, Volume 42 Issue 5
Petroleum Geology
Genesis of rich hydrocarbon resources and their occurrence and accumulation characteristics in the Ordos Basin
Chiyang Liu, Jianqiang Wang, Dongdong Zhang, Hongge Zhao, Junfeng Zhao, Lei Huang, Wenqing Wang, Yang Qin
2021, 42(5):  1011-1029.  doi:10.11743/ogg20210501
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Despite a long history of oil and gas exploration and production, the Ordos Basin is still a place full of surprises that attract numerous geologists and petroleum engineers.By the year of 2020, the basin has accumulatively produced more than 79 million tons of oil equivalent, ranking the first among all the petroliferous basins in China.This study relates the genesis, occurrence and accumulation characteristics of abundant hydrocarbons in the basin to four features that define it.①The hydrocarbon-rich sag in the southern basin during the deposition of the Yanchang Formation in Mesozoic is formed in a dynamic environment with obvious tectonic activities and active deep actions.The basin in that period belongs to a type of large-scale, extensional backland prototype basin against a favorable geothermal setting.②The basin at present is a multiple superimposed basin as a result of composite superimposition of three successive large cratonic basins during the Mesozoic, early-Paleozoic and late-Paleozoic, respectively.③High-quality Triassic Yanchang Formation source rocks and Carboniferous-Permian coal-measure source rocks are both rich in uranium and multiple interlayered tuffs.④During a late evolution, the basin, reshaped by various reformation featuring intense on borders and weak in interior, was uplifted into a plateau and eventually evolved into a residual basin.The above four distinctive attributes and characteristics work in unison in shaping the basin into a treasure house of hydrocarbons.The occurrence and accumulation of oil and gas in the basin of different periods have their own characteristics and spatial division types.Conventional and unconventional hydrocarbon resources also show obvious spatial-temporal transition and coexistence compatibility in terms of accumulation and distribution, which are characterized by vertical migration over short distances and near-source accumulation.Low-and ultra-low-permeability tight reservoirs are widespread and tight stratigraphic and lithological reservoirs with low permeability, low pressure, and low yield dominate.Based on the above analysis, it can be foreseen that the exploration potential of the basin is still huge and worthy of broader investigation.

Thermodynamic evolution and hydrocarbon accumulation in the Ordos Basin
Zhanli Ren, Kai Qi, Jinbu Li, Xiaoju Huo, Junping Cui, Peng Yang, Kun Wang, Zhanjun Chen, Guilin Yang
2021, 42(5):  1030-1042.  doi:10.11743/ogg20210502
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The thermal history and thermal lithospheric thickness restoration for the Ordos Basin indicate that tectonic thermal events and dynamic processes of deep thermal lithosphere thinning in the Early Cretaceous result in thin lithosphere, high heat flow values and geothermal gradient at the southern part of the basin.Source rocks at different horizons there also show high maturity and obvious thermal anomalies.Thermal simulation for the source rocks and thermodynamic history of the basin indicate strong influence of the thermal events and thermal anomalies in the Early Cretaceous on the significantly increasing amount of hydrocarbon generation, which is also interpreted to be one of the important reasons behind the high oil and gas enrichment in the basin.The large-scale hydrocarbon generation and accumulation from these source rocks in the Early Cretaceous are mainly controlled by the deep thermodynamic process of lithosphere thinning and tectonic thermal events during the period.The distribution of oil and gas fields in different Paleozoic and Mesozoic layers is mainly controlled by the distribution of source rocks and their thermal maturity as well as reservoirs.From the perspective of source rock development and thermal maturity, tight oil and shale oil in the Chang 7 member (Yanchang Formation) of southern basin have high thermal maturity, and are widely distributed, making the member a quite promising exploration target.The Carboniferous-Permian with its basin-wide occurring coal-measure source rocks has high exploration potential for natural gas.The deep Lower Paleozoic Ordovician carbonate sequences have the conditions favorable for large gas fields to form but further study is needed to clarify the hydrocarbon generation potential of source rocks.The deep Proterozoic is also worthy of an exploratory gaze with the distribution of fault depressions and the hydrocarbon generation capacity of source rocks being recommended as the focuses.

Characteristics of unconformity resulted from Huaiyuan Movement in Ordos Basin and its significance for oil and gas exploration
Xiangbo Li, Hongbo Wang, Junping Huang, Caili Zhang, Yan Zhang, Yating Wang, Lei Zhang, Jing Wang, Huaqing Liu
2021, 42(5):  1043-1055.  doi:10.11743/ogg20210503
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The unconformity formed out of Huaiyuan Movement is one of the important regional unconformities in the North China Platform.Following the investigation on and analysis of the occurrence and geological characteristics of the movement, we make an in-depth analysis of the evidences and characteristics manifesting the movement-caused unconformity and its correlation with reservoirs and oil and gas accumulation in the Ordos Basin, with new understandings obtained.The results show that: ①There is a regional unconformity formed by the Huaiyuan Movement in the deep Ordovician of Ordos Basin, and it perhaps associates with a large-scale geomorphology system of karst ravines; ②The regional unconformity and ravine system serve as the paleogeomorphologic setting for the deposition of the overlying strata, to be specific, the middle-to-bottom parts of Majiagou Formation (namely, from Ma 4 to Ma 1 Members), and control the depositional pattern and favorable reservoir facies distribution, as well as the development of dolomitized and weathered crust reservoirs in the underlying Lower Ordovician to Cambrian and to Changcheng Systems; ③The regional unconformity acts as a favorable channel for the lateral migration of natural gas from different sources in certain regions, known as "highway of hydrocarbon migration"; meanwhile, it is also a favorable place for hydrocarbon accumulation, the upper and lower layers near which are conducive to natural gas accumulation and enrichment in large scale.It is possibly a new exploration area worthy of attention in replacement of weathering crust on top of the Ordovician.However, the current understandings have to be deepened on the unconformity in the study area, for which a 3D seismic acquisition is recommended to promote creative geological understandings and new exploration discoveries in the Ordos Basin.

Tectonic genesis of Triassic Yanchang Formation valley systems, southern Ordos Basin
Faqi He, Rong Qi, Fubin Wang, Jie Deng, Li Cheng, Tianle Hu
2021, 42(5):  1056-1062.  doi:10.11743/ogg20210504
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The valley systems widely developed at the top of Yanchang Formation in the Zhenjing area, southern Ordos Basin, is previously considered to be a product of long denudation and river erosion.However, deeper investigation indicates otherwise.In order to gain clearer insights into the connection between the development characteristics and occurrence of the systems with regional tectonics, this study characterizes the systems and discusses their tectonic genesis through analyses of fault interpretation and pre-Jurassic paleogeomorphy restoration based on 3D seismic data.It shows that the main fault (Yudu) experiences a sinistral strike-slip due to a NE-SW oblique compression during the late deposition stage of the Yanchang Formation, causing some layers of the formation that are still in the process of diagnesis to be plastically deformed to form a series of NW-SW trending compressional uplifts and valleys.An almost synchronously occurring NW-SE extension eventually results in the formation of this broadly distributed valley systems.The understanding can be used to guide the exploration of reservoirs in both the Yanchang Formation and overlying Yan'an Formation in the area.

Features and origin of deep-water debris flow deposits in the Triassic Chang 7 Member, Ordos Basin
Xinping Zhou, Qing He, Jiangyan Liu, Shixiang Li, Tian Yang
2021, 42(5):  1063-1077.  doi:10.11743/ogg20210505
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The deep-water gravity flow deposits in the 7th member of the Triassic Yanchang Formation (Chang 7 Member), Ordos Basin, are taken as the main research object to investigate the sedimentary characteristics and genesis of different types of subaqueous debris flow deposits.An integration of log data, core observation and analysis, thin section analysis and quantitative statistics, is applied to discuss the features, sedimentary sequences and genetic mechanisms of debris flow deposits.The results show that the deep-water debris flows in the study area mainly consists of 3 types, that is sandy debris flow, muddy debris flow and muddy flow.Deposits of the first type feature massive sandstone with floating argillaceous debris and a single layer thickness of 0.24 m to 1.10 m, averaging 0.55 m.Those of the second type occur either in isolated massive deposition or in massive argillaceous deposition paired with the underlying massive sandstone: the former is rich in floating mudstone tearing debris and deformation structures of soft sediments, while the latter is rich in floating muddy debris and sandy agglomerates with a single layer thickness ranging from 0.21 m to 1.29 m, averaging 0.60 m.The occurrence of the third type resembles the first type, while its former is composed of sandy mudstone or argillaceous sandstone, and its latter has floating argillaceous chips of millimeter scale in laminar, with a single layer thickness ranging from 0.20 m to 0.60 m, averaging 0.30 m.The flow transformation caused by the involvement of environmental water body in the process of high-concentration sandy or muddy sediment transport, is the main reason for the formation of sandy debris flow deposits, isolated massive muddy debris flow deposits, and mud flow deposits.The muddy debris flow deposits paired with the underlying massive sandstone is mainly caused by flow erosion or sand body liquefaction; and the mud flow deposits paired with the underlying massive sandstone may be caused by flow transformation resulted from deceleration and expansion, and differential settlement of clastic particles.

Shale oil exploration potential in central Ordos Basin: A case study of Chang 7 lacustrine shale
Bojiang Fan, Yue Jin, Liang Shi, Yating Li, Weichang Chen
2021, 42(5):  1078-1088.  doi:10.11743/ogg20210506
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The Chang 7 member (the seventh member of Yanchang Formation) lacustrine shale in the central Ordos Basin is largely thin and made up of either pure shale or shale interbedded with sandy laminas or thin sand layers of shallow to semi-deep lake facies.Its high content of sand seems quite ideal for achieving positive stimulation responses through fracturing.A discussion and comparison of shale oil exploration targets in the area is recommended for gaining such response.However, no such efforts have been made so far.To judge the oil potential of the shale, samples are collected from both pure shale and shale interbeded with sandy lamina to compare hydrocarbon generation and hydrocarbon expulsion capacities based on geochemical tests and analyses.The results show that shale mainly composed of sapropelic organic matter has higher organic matter abundance than shale interbeded with sandy lamina composed of mixed organic matters.Both are at a low to moderate maturity stage but shale interbeded with sandy lamina is inferior to pure shale in terms of hydrocarbon generation capacity.Both can effectively expel hydrocarbons.Shale interbeded with sandy lamina contains oil at 7.22 mg/g and pure shale 9.42 mg/g.Both can be economically developed and share similar oil saturation index, indicating great oil potential.

Source rock characterization: The dark mudstone in Chang 7 Member of Triassic, central Ordos Basin
Yaxiong Zhang
2021, 42(5):  1089-1097.  doi:10.11743/ogg20210507
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The seventh member of Yanchang Formation (Chang 7 Member) in Ordos Basin is typical lacustrine dark mudstone, which slims down while approaching lake rim at the central part of the basin.The study of source rocks, especially the dark mudstone as source rock in that part, has been very limited due to its thinness.Dark mudstone samples from 15 exploration wells in the area are collected and analyzed through core observation, TOC determination, rock pyrolysis, GC-MS and organic carbon isotope analysis.The results indicate a sedimentary environment of weak reduction and oxidation with mixed inputs of plankton and terrestrial plants for the dark mudstone and provide parameters that define the dark mudstone as a good source rock: high TOC (averaged at 2.39%), type Ⅱ2 and type Ⅲ dominated organic matter with an average S1+S2 at 7.65 mg/g, and moderate thermal evolution with Ro averaged at 0.98%.The total hydrocarbon generation amount, hydrocarbon expulsion amount and residual hydrocarbon amount of the dark mudstone are respectively measured to be 793, 337 and 456 million tons.And the comprehensive hydrocarbon expulsion efficiency is 42.50%.The contribution ratio of dark mudstone and shale to the oil and gas resources in the area is 1:1.8.All indicate great source rock potential of the stone.

Interstitial matter and its impact on reservoir development in Chang 6 deepwater tight sandstone in Huaqing area, Ordos Basin
Zhaobing Chen, Zhenyu Zhao, Ling Fu, Jianrong Gao, Wei Song, Xinjing Chen
2021, 42(5):  1098-1111.  doi:10.11743/ogg20210508
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Unconventional tight sandstone reservoir resulting from deepwater gravity flow, as a new hotspot for hydrocarbon exploration and development, has attracted much attention.The deepwater depositional environment is characterized by complex hydrodynamics, resulting in large variation in type and content of interstitials, and strong heterogeneity of reservoirs under microscopic observation, thus challenging petroleum E & P.In this study, an integration of casting thin section and SEM observation, Quantitative Evaluation of Materials by Scanning Electron Microscopy (QEMSCAN), electron microprobe and nano CT analysis, is applied to finely characterize the microscopic features of various interstitial matter in the deepwater tight sandstone of the 6th member of Yanchang Formation (Chang 6 Member) in Huaqing area, Ordos Basin, and discuss the effects of interstitials on reservoir pore structure and physical properties.The results show that the matrix is not completely tight, which could, under diagenetic modification, generate a complex secondary pore network consisting of intercrystalline and dissolved pores with a pore size ranging between 20 nm and 1 000 nm.With a matrix content of less than or equal to 7%, the matrix contains well-developed secondary pores, which has a positive impact on reservoir quality; with a matrix content of greater than 7%, the secondary pore development is undermined, exacerbating reservoir compaction.The intercrystalline pores in clay mineral cements, however, could alleviate the reduction of porosity to some extent; on the other hand, the morphology, occurrence of different clay minerals, and their effect on reservoir sensitivity, function to increase the complexity of clay mineral-permeability relationship.In areas with high content of illite, I/S mixed layer and thick chlorite, the reservoirs are commonly poor in physical properties; while high-quality reservoirs are most seen in areas of well-developed kaolinite.Carbonate and siliceous cements are key in undermining the physical properties of deepwater tight sandstone reservoirs.In all, the area with a matrix content of 7% or less and relatively well-developed kaolinite cement is a focus for the exploration and development of tight sandstone reservoirs of deepwater facies.

Insights on factors causing differential enrichment of Chang 6 Member in Jiyuan area, Ordos Basin
Baohong Shi, Xinyu Qin, Caili Zhang, Wen Liu, Gang Liu, Chanyuan Shi, Lei Zhang, Zishu Yong
2021, 42(5):  1112-1123.  doi:10.11743/ogg20210509
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The 6th Member of Yanchang Formation (Chang 6 Member) is a key target layer for petroleum exploration in Jiyuan area, Ordos Basin.The reservoirs of Chang 6 Member are characterized by large burial depth and complex oil-water contact, resulting in markedly differential distribution both vertically and laterally.In order to find out the factors causing the differential enrichment of reservoirs in the study area, we make a comparative analysis of the geological conditions based on core and casting thin section observation, formation testing, and geochemical analysis of inclusions and crude oil biomarker compounds.The results show that the key factors of differential enrichment are the source rock quality, type of sand bodies and physical properties of reservoirs.As a result, high-quality source rocks, favorable sand bodies and relatively high permeability, work in unison to generate the enriched zone of Chang 6 reservoirs.In the particular sedimentary environment, vertically speaking, the Chang 61 submember with the thickest sand bodies dominated by Type Ⅰ and in steady lateral distribution, features larger radius of throats, better physical properties, high hydrocarbon abundance in the reservoirs; in planar view, sand bodies developed in the east and west of the study area, feature large thickness, favorable type, less argillaceous interlayers, and better physical properties.In addition, source rocks have large thickness and strong hydrocarbon generation capacity, so they are favorable play fairways with favorable source-reservoir configuration.While the sand bodies of large thickness in central Jiyuan area are characterized by multiple beds thin for each single, multiple argillaceous interlayers, strong reservoir heterogeneity, and the source rocks are thin, so although the reservoirs therein are of better physical properties, their oil-bearing properties are poor, with most wells producing water.

Reservoir identification and prediction of He 7 Member in Binchang block, southern Ordos Basin
Ling Liu, Linlin Wang, Jiang Wu, Xia Chen, Jiaqi Yang
2021, 42(5):  1124-1135.  doi:10.11743/ogg20210510
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The 7th member of Upper Palaeozoic Shihezi Formation (He 7 Member), is a new favorable play for gas exploration in Binchang block, southern Ordos Basin.However, the reservoir prediction and selection of exploration targets therein have become quite hard given multiple factors including thin reservoir, rapid change of lithology, strong heterogeneity, and low signal-to-noise ratio and resolution.The exploration progress, thus, has been severely restricted.The study aims to improve the accuracy of sweet spot prediction and the success rate of natural gas exploration in the study area.A progressive prediction technique from "configuration, attribute to inversion" is proposed regarding reservoir characteristics therein.The results indicate that the internal reflection configuration of intervals with well-developed sand bodies is defined through seismic response characteristic analysis together with seismic forward modeling, the sensitive elastic parameters of sand bodies are clarified by petrophysical analysis; the channel shapes are characterized effectively by neural network clustering of reflection configuration in frequency domain; and the sand body thickness is predicted accurately with Bayesian stochastic inversion.Moreover, the sweet spots in the reservoir are identified by gas-bearing potential prediction.Under the influence of paleogeomorphology, there are a total of 4 SW-NE-striking channel sand bodies of strip-shaped distribution identified in the Upper Palaeozoic He 7 Member in the south of Binchang block, which are the favorable play fairways in exploration.While the channel sand bodies to the east are in isolated and scattered distribution.In practice, the targets selected under the guidance of the technique are proved to be high in quality by drilling.The lateral resolution of seismic prediction is high, serving for reflecting the change features of the channels and channel sand bodies truthfully.The sweet spot prediction in tight and thin sandstone reservoirs of strong heterogeneity can, thereby, be solved effectively, which is of guiding significance to future exploration and development.

Paleogeomorphologic restoration and its controlling effect on deposition of delta-front sand bodies: A case study of Shan 13 sub-member of the Permian Shanxi Formation, Qingyang gas field, Ordos Basin
Jinbu Li, Jiping Wang, Long Wang, Bin Fu, Hui Xia, Zhixiao Li
2021, 42(5):  1136-1145, 1158.  doi:10.11743/ogg20210511
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Micro-paleogeomorphology within basin, a key factor controlling the deposition of sand bodies, determines the development scale and position of reservoir sand bodies.In the study, an integration of 3D seismic and log data is applied to the restoration of micro-paleogeomorphology during the Shan 13 sub-member deposition in the Permian Shanxi Formation in Qingyang gas field, Ordos Basin, by considering the residual stratum thickness, decompression correction and paleo-current direction.Furthermore, the study analyzes the effect of micro-paleogeomorphology on delta-front sand body deposition, which in turn serves to predict the distribution pattern of favorable sand bodies and guide well emplacement therein.The results show that during the Shan 13 depositional period, the western and southern parts of the study area are topographical highs, the northeast is the bottom land, and the central area is a transitional slope zone; the paleo-current during the depositional period trends mainly SW-NE, and the No.3 to 4 distributary channels sourced from the southern topographical highs converge in the central and northern parts, forming a zone of superimposed sand bodies; with the emplacement of wells in line with the predicted distribution pattern of Shan 13 sand bodies, the average sandstone thickness penetrated by vertical wells increases by 0.6 m, and the rate of sandstone encountering in drilling of horizontal wells increases by 20%, further suggesting the reliability of paleogeomorphology restoration-based sand body prediction.The method proposed in the study can be of referential and guiding significance to the emplacement of wells in other blocks of Qingyang gas field and the development of tight gas under similar geological conditions.

Upper Paleozoic tight gas sandstone reservoirs and main controls, Linxing block, Ordos Basin
Chang Liu, Daomin Zhang, Chao Li, Yuanyuan Lu, Shanshan Yu, Mingqiang Guo
2021, 42(5):  1146-1158.  doi:10.11743/ogg20210512
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The Upper Palaeozoic tight sandstone in the Linxing block at the east margin of Ordos Basin is estimated to have a great gas potential of over 100 BCM.However, complex gas-water distribution by highly heterogeneous reservoirs against a special structural background and changing reservoir-forming settings with controls hinder the gas exploration and development in the block.From a perspective of tight sandstone gas reservoiring conditions, this study starts from establishing lateral sedimentary models, analyzing the vertical coupling relationship between source rocks, reservoirs and cap rocks, to summarizing the gas-water spatial-temporal distribution pattern and differential characteristics of gas accumulation process.The differential enrichment law in the block revealed throught the study can be used to guide an effective release of the huge gas potential from the east margin of Ordos Basin.The configuration of source rocks, reservoirs and carrier beds is defined as that the coal-measure source rocks characterized by high organic matter abundance and hydrocarbon generation potential is connected vertically with the sandstone reservoirs of marine barrier-coast tidal flat facies and continental rivers-shallow delta facies via the moderately developed faults, thus vertically constituting three plays that can be described as lower indigenous play, middle near-source rock play and upper far-source rock play.The study proposes that tight sandstone gas reservoirs in the block are formed after tightening-up process based on a deeper understanding of tightening-up mechanism of sandstone reservoirs and the coupling relationship between diagenesis and reservoir-forming.It suggests that it is overpressure that drives the gas to migrate after analyzing the migration and accumulation mechanism.It establishes a quasi-continuous selective accumulation model of multi-layer systems in different structural zones with sufficient gas source.It also summarizes the main controlling factors of multi-layer system and boils down the characteristics of the reservoirs as "source rock controlling gas potential, structure controlling play, micro-phase controlling reservoir, and physical property controlling accumulation".It finally concludes that high volume of gas continuously gushed from source rocks would preferentially migrate to and accumulate in areas with the right faults and high permeability sand bodies as well as moderate tectonic activities.

Differences and development patterns of karst reservoirs in Majiagou Formation, northern Ordos Basin
Juntao Zhang, Xiaohui Jin, Ning Gu, Changrong Bian, Jiaqi Yang, Zhiliang He
2021, 42(5):  1159-1168, 1242.  doi:10.11743/ogg20210513
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Karst reservoirs are widely developed in the Majiagou Formation, northern Ordos Basin, though they are markedly different from the classic karst model.An integrated analysis of petrology and geochemistry functions to identify 3 types of karst reservoirs related in genesis and affected by gypsum-bearing strata, and a karst model of strata bearing evaporite minerals in the Majiagou Formation is established.The results show that the gypsum-bearing dolomite reservoirs with gypsum mold pores are formed under sabkha-associated dolomitization, featuring richness in evaporate minerals, sedimentary setting of gypsum-bearing dolomite flat, and reservoir space of dissolved gypsum mold pores.Dolomite reservoirs with dissolved-intercrystalline (dissolved) pores are generated under seepage-reflux dolomitization, with the fluid coming from overlying strata bearing gypsum, and mainly occur in dolomite of shallow shoal facies with reservoir space dominated by dissolved pores and intercrystalline (dissolved) pores.Fractured-vuggy limestone reservoirs are mainly developed in (dolomitic) limestone formations, which originally are deposited in limestone flat, with cracks and dissolved-intercrystalline (dissolved) pores as dominant reservoir space, and the development of primary cracks within is associated with the dissolution and deformation of underlying gypsum-bearing rocks.In terms of spatial distribution in the study area, reservoirs of the 3 types are all discovered in the central and eastern parts of northern Ordos Basin; while dolomite reservoirs with dissolved-intercrystalline (dissolved) pores are popular in the western part, and the fractured-vuggy limestone reservoirs dominate the northern part.The reservoirs of the same type also exhibit different petrological and geochemical characteristics in different paleo-geomorphic units, which could be in turn subdivided into seepage zones of the high and catchment zones of the low.The former rarely impacted by atmospheric precipitation, is characterized by well-preserved reservoirs filled with a small amount of dolomite and of low impurity level (Al, K, and Sr, etc.).The latter under the significant impact of atmospheric precipitation, features seriously-damaged reservoirs filled with a large amount of calcite, and of high impurity level and generally negative δ18O.

Genetic types and characteristics of deep oil and gas plays
Dongdong Zhang, Wenhui Liu, Xiaofeng Wang, Houyong Luo, Qingtao Wang, Yining Li, Fengjiao Li
2021, 42(5):  1169-1180.  doi:10.11743/ogg20210514
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Significant oil and gas discoveries have been obtained in deep plays in succession, making the deep plays become an important strategic domain for oil and gas exploration both in China and around the world.However, we are not quite clear about its accumulation mechanism under the high-temperature and high-pressure environment, following long evolutionary history and complex organic-inorganic interactions.The deep plays could be divided into 3 types considering the major genesis, evolutionary history and accumulation characteristics, namely, plays of pre-deep-burial enrichment, plays of syn-deep-burial enrichment, and plays of post-deep-burial enrichment.Their genetic mechanism and main accumulation characteristics are sorted out and characterized.Meanwhile, the study focuses on the mysteries concerning source rocks and hydrocarbon generation of the deep plays; among others, the involvement of deep-sourced exogenous hydrogen, calcium-rich source materials and inorganic gas, is of great significance to hydrocarbon generation from high-to-over-mature organic matter.In all, discussion on the mechanism of deep hydrocarbon accumulation is sure to help lay a solid theoretical foundation to promote deep petroleum exploration and industrial development in China.

Methods and Technologies
FSV estimation and its application to development of shale oil via volume fracturing in the Ordos Basin
Fangzheng Jiao
2021, 42(5):  1181-1188.  doi:10.11743/ogg20210515
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Qingcheng oilfield in the Ordos Basin is the first shale oilfield with over one-billion tons of reserves discovered in China.Volume fracturing of horizontal well is a key technology for effective development of shale oil in the oilfield.In this study, a fracture network swept volume (FSV) model for volume fracturing is built, the relation between key geologic and engineering parameters and microseismic coverage volume is established by multivariate linear regression, actual production data are applied to correct the relation to get an empirical formula for quantitative calculation of FSV, and then the correlation template between FSV and productivity is plotted to guide the optimization of engineering parameters of volume fracturing.The results show that fractures created during the volume fracturing of shale oil reservoirs appear as a stripped fracture network dominated by main fractures together with branch fractures in the shape of cactus; and the main factors affecting FSV include fracturing fluid volume, fracture density, pumping rate, oil layer thickness, and proppant load.Field practices prove that the empirical formula and correlation template proposed in the study can predict FSV with accuracy and provide scientific basis for design optimization and effect evaluation of volume fracturing of horizontal shale oil well in Qingcheng oilfield.

Tectonic evolution analysis constrained jointly by in-situ calcite U-Pb dating and apatite fission track for southwestern Ordos Basin
Pang Yang, Zhanli Ren, Zhao Jianxin, Duc Nguyen Ai, Feng Yuexing, Kai Qi, Kun Wang
2021, 42(5):  1189-1201.  doi:10.11743/ogg20210516
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The lower Paleozoic strata of Ordos Basin is rich in oil and gas resources.The Lower Paleozoic marine carbonate in the southwestern Ordos Basin is characterized by large thickness and high abundance of organic matter.The source rocks are in dry gas window.However, the tectonic evolution has not been well evaluated.The timing of the earlier tectonic events is a key scientific problem and needs to be addressed timely.In this study, under the constraints of detailed field investigations, microstructure characteristics, in-situ trace element analysis and regional deposition-burial evolution history, the secondary calcite in-situ U-Pb dating is combined with apatite fission-track time-temperature simulation curves of the Lower Paleozoic carbonate strata and its fault facture zone to reveal four tectonic events including the Late Ordovician, Late Triassic, Late Jurassic and end of the Early Cretaceous-Cenozoic.A hydrothermal event occurred at the Late Ordovician 452±13 Ma, which might be linked to the subduction and closure of the Proto-Tethys Ocean during the Caledonian period.A limited Indosinian tectono-thermal event occurred at 214±22 Ma, which might be caused by the regional fault movement.The intense tectonic activities during the Yanshanian and Himalayan periods induced a strong uplift and denudation.The southwestern Ordos Basin experienced multi-stage complex tectonic evolution and the caprock was seriously damaged, leaving rather unfavorable preservation conditions for hydrocarbon.This study conducts a novel tectonic evolution analysis by combining low-temperature thermochronology with in-situ U-Pb dating of fault-related calcite, providing a new way to analyze tectonic evolution and later transformation process at the margin of complex superimposed basins.

Super-resolution imaging of thin sections for lacustrine shale reservoirs
Chao Guo, Qianping Zhao, Gang Liu, Shiyan Hao, Chao Gao, Jianbo Sun, Chao Liu, Yiyi Chen
2021, 42(5):  1202-1209.  doi:10.11743/ogg20210517
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Despite a great progress made in the exploration and development of shale gas and oil in recent years, the study on the pores, organic matter and mineral composition of shale reservoirs at a microscopic scale is still a challenge with thin section analysis for conventional reservoirs. In order to solve this problem, this study introduces a super-resolution technology to improve thin section image quality for revealing micro-characteristics of shale reservoirs. A set of super-resolution models are established based on generative adversarial networks and corresponding content loss functions are also set up for thin-section images. Application of the technology to the processing of actual data from lacustrine shale gas reservoirs in the Yanchang Formation in Ordos Basin has yielded positive results, demonstrating quantitatively and qualitatively its applicability, accuracy and reliability for unconventional reservoir assessment.

Lithology identification in tight sandstone reservoirs using CRBM-PSO-XGBoost
Yufeng Gu, Daoyong Zhang, Zhidong Bao
2021, 42(5):  1210-1222.  doi:10.11743/ogg20210518
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Lithology identification is universally regarded as a prerequisite for some fundamental geological work such as stratigraphic correlation and analysis of sedimentary system distribution. The tight sandstone reservoirs are generally composed of multiple thin sandstone-mud alternating beds, featuring a variety of lithological types, most of which are similar in log responses, posing great difficulty for lithology identification. Given the powerful analytical performance of machine learning techniques shown on pattern recognition, XGBoost, a stable and highly efficient model is adopted to predict lithology of the tight sandstone reservoirs. However, this model needs many empirical parameters to complete its prediction, and its computing efficiency will be reduced when it deals with more independent variables. The CRBM and PSO models are, therefore, introduced to improve the prediction performance. CRBM is functional to extract features from original data so that fewer but more significant independent variables can be created by the CRBM in modeling. While PSO is effective to determine the optimal values via iterative computation for those empirical parameters used by XGBoost. Based on the data derived from some cored wells of Chang 4+5 Members in western Jiyuan oilfield, two experiments are designed to validate the prediction performance of the hybrid model proposed, CRBM-PSO-XGBoost. Furthermore, two optimized models formed by PNN and SVM are selected to make a comparison, as a way to consolidate the validation effect. The experiment results manifest that the prediction accuracy comes to peak by the proposed model, up to over 90%. In all, the hybrid model proposed in the study, capable of identifying lithology of tight sandstone reservoirs, is of wider application to lithology prediction compared with previous models.

Producing reserve estimation and design optimization of rhomboid well pattern in anisotropic reservoirs of low permeability
Minfeng Chen, Ziyou Yang, Lifeng Qin, Shixiong Fu, Jinxi Rong
2021, 42(5):  1223-1233.  doi:10.11743/ogg20210519
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Determining a well array with reasonable spacing for flooding in rhomboid well pattern, is the key to achieving effective development of reserves and well emplacement for equilibrium displacement development in regular low-permeability anisotropic reservoirs.Based on the characteristics of low-permeability reservoirs, we establish a seepage equation considering the influence of threshold pressure gradient and permeability anisotropy.Classical seepage mechanics is applied to obtain an analytical solution for the seepage field distribution in flooding unit of the rhomboid inverted nine-point well pattern.In turn, a method for evaluating the producing reserve is proposed in line with the analysis of the lateral seepage velocity variation in the flooding unit, and the main factors influencing and rules of producing reserve variation in low-permeability anisotropic reservoirs at different conditions are obtained.At last, a calculation model for optimizing the rhomboid inverted nine-point well pattern is built with the ultimate optimization purpose of meeting "requirement on effective producing reserve range and maximum equilibrium displacement coefficient", and a simultaneous optimization for the best well array with reasonable spacing in this pattern is achieved.Case analysis shows that the rhomboid well pattern optimization design method for low-permeability anisotropic reservoirs can be applied to obtain the best well array with reasonable spacing in line with the actual conditions of the reservoir and the development requirements.The design in all, is of scientific basis to the effective development of reservoirs and optimal emplacement of well patterns.

Time-varying permeability of sandstone reservoirs under waterflooding of different water quality
Jianzhong Wang, Xiankun Song, Yingchao Fu, Lianting Sun, Jinjie Xu
2021, 42(5):  1234-1242.  doi:10.11743/ogg20210520
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Water injection may lead to a chronical reduction of reservoir permeability due to the blockage of formation pores by suspended solid particles, oil, bacteria, and other impurities in the injected water.For fields under water flooding for quite a long time, it is not wise to continue sticking to the water quality determined on originally measured formation permeability.This study investigates possible formation damage caused by water injection through laboratory experiments on sandstone cores of different permeability from wells with long water injection history according to 5 different standards of water quality(SY/T 5329-2012), and put forward a concept of water quality-sensitive permeability.And through theoretical derivation, a mathematical expression describing the attenuation law of permeability is established to describe changing sandstone reservoir permeability with time during injection of water of different qualities.The criteria for water and reservoir selection are determined for better decision making on injection water quality for high water-cut wells.It provides a theoretical basis for achieving better performance of wells with long-time water flooding through water quality adjustment.

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