Table of Content

28 August 2021, Volume 42 Issue 4

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Petroleum Geology

Progress and key research directions of tight gas exploration and development inXujiahe Formation, Sinopec exploration areas, Sichuan Basin

Herong Zheng, Zhongqun Liu, Shilin Xu, Zhenfeng Liu, Junlong Liu, Zhiwen Huang, Yanqing Huang, Zhiliang Shi, Qingzhao Wu, Lingxiao Fan, Jinhui Gao

2021, 42(4):  765-783.  doi:10.11743/ogg20210401

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Gas exploration in tight sandstones of the Xujiahe Formation in Sinopec's exploration areas, Sichuan Basin, has been fruitful.However, gas in the formation is pervasive but only locally enriched in rather tight layers, which results in low reserve abundance, water production being common and gas yields being controlled by fractures.Exploitation of the gas reservoirs in the formation has therefor been a challenge.With exploration and productive capacity building being pursued strategically in an efficient way by the group in the areas in recent years, Sinopec has followed a roadmap leading to an integrated geology and engineering during a new round of exploration and development activities in the areas of the basin.Significant progresses have been made in the basic geological theories and key technologies of exploration and development.The gas accumulation pattern, summarized as "charging prior to tightening and overpressure-driven with late adjustment", and the enrichment law, briefly described as "early structure controlling enrichment zones, differential tightening process controlling reservoir physical properties and late structural activities determining well performance", are combined to establish a trap dynamic evaluation method based on paleo and modern structures and deformation history.Sweet spot classification criteria have been established with 2 categories and 4 types of sweet-spot modes being built.For tight sandstone fracture prediction, an integrated technology of multi-attributes, including maximum likelihood, structural entropy, shape index and curvature, was used to describe fault-fold fractured reservoirs.To better identify the components of tight sandstone and fluids, we performed continuous logging evaluation of grain-size parameter and neutron acoustic envelope logging evaluation of gas-bearing properties.A nitrogen gas drilling technology has been proposed to enhance well performance and stimulation procedures have been tailored for different types of reservoirs.At present, the key exploration and development issue of the Xujiahe Formation is how to realize a large-scale and effective development.Based on the research in recent years, we suggest the following domains that require a continuous attention: a basin-wide study of differential enrichment of gas in the Xujiahe Formation; a fine geological evaluation of different types of sweet spots; fine description of the interior of fracture bodies, accurate prediction of high-quality reservoirs and development of fluid and engineering technologies for production enhancement and optimization; as well as an integration of geology and engineering combined with differential exploitation strategy according to different types of reserves.Only with development priority being given to sweet spots with well-developed fractures and study being focused on the improvement of production from sweet spots in porous matrix, will an effective exploration and production of gas from these hard-to-get reserves in the Xujiahe Formation be possible.

Advances and trending topics in sedimentary reservoir research ofthe Upper Triassic Xujiahe Formation, Sichuan Basin

Zhensheng Shi, Xiaomin Zhu, Yaxiong Zhang, Hui Jin

2021, 42(4):  784-800.  doi:10.11743/ogg20210402

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The Upper Triassic in the Sichuan Basin is rich in both conventional and unconventional gases, the temporal and spatial distribution and enrichment of which are controlled by sedimentary reservoirs under an isochronous stratigraphic framework.Exploration activities in the basin have drawn greatly from sedimentary reservoir research and highlighted its scientific and strategic significance.Progresses made in the domain can be summarized as establishment of symmetrical and less symmetrical sequence stratigraphic models and their evolutionary patterns of the foreland basin based on previous Upper Triassic sequence stratigraphic frameworks.The Ma'antang Formation to the third member of Xujiahe Formation are marine deposits, and the fourth to the sixth member of Xujiahe Formation are continental deposits, though still under the effect of sea transgression.The Ma'antang Formation mainly consists of reef-shoal facies, lagoon facies, tidal flat lake and delta facies; the Xiaotangzi Formation mainly consists of barrier sand bar-lagoon facies with small deltas developed around basin.The second, fourth and sixth members of Xujiahe Formation are dominated by alluvial fans, fan deltas and braided river delta facies, the third member consists of costal-delta facies, and the fifth member is mainly composed of lacustrine-delta facies.Five major provenances existed during the Late Triassic in Sichuan Basin, including Qinling ancient land, Dabashan ancient land, Songpan-Ganzi fold belt, Kangdian ancient land, and Jianglan ancient land, with each providing sediment inputs of temporal and spatial differences in terms of nature, scope and extent.Sediments were largely sourced from the north part of the basin during the deposition of the second and third members of Xujiahe Formation.Tight sandstone reservoirs in the Xujiahe Formation are closely linked to compaction which caused great primary porosity loss and the quartz overgrowth in Ⅱ and Ⅲ phases that further reduced the primary porosity.The closed underground fluid environment also made its contribution to the formation of tight reservoirs.Sweet spots in the formation were controlled by microfacies, dissolution, chlorite lining, and fractures, and thus making the underwater branch channels at delta fronts the most promising targets for tight gas discovery.With major progresses in shallow water delta research and shale gas exploration in recent years, shallow water deltas, fine-grain sedimentology and deposition mechanisms, sedimentary process simulation and fine-grained reservoir characterization of the Upper Triassic in Sichuan Basin, have become the trending topics and future research directions.

Depositional responses of Xujiahe Formation to the uplifting ofLongmenshan during the Late Triassic, Western Sichuan Depression

Hongde Chen, Lei Liu, Liangbiao Lin, Xinglong Wang, Zhiwei Wang, Yu Yu, Jian Zeng, Pengwei Li

2021, 42(4):  801-815.  doi:10.11743/ogg20210403

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The Upper Triassic Xujiahe Formation in Western Sichuan Depression, Sichuan Basin, was deposited against a marine to terrestrial backdrop.With nearby Longmenshan uplifting during the period, the formation received various provenance inputs that led to a complex sandstone distribution and high reservoir heterogeneity.To reveal the distribution of sand bodies as well as the depositional history and response of the formation to the uplifting, we took the second to fourth members as study objects and analyzed their drilling, logging and seismic data.The results suggest that the uplifting of northern Longmenshan with weak extrusion and intense sediment flux led to a major supply of sands from Micangshan-Dabashan during the deposition of the second member.The stacked channels at the transitional delta front show a relative consistency and inheritance, with the main drainage line migrating eastward.The underwater uplifting of northern and middle parts of the mountain with intensive extrusion and weak sediment flux resulted in a supply of sediments from ancient Kangdian land and Micangshan-Dabashan during the deposition of the third member, with deltaic sand bodies mainly distributing in the north and south of the basin and sand bars in the center of the basin.During the deposition of the fourth member, the northern, central and southern parts of the mountain had all been uplifted.And with a weak extrusion and intensive sediment flux, the uplifting introduced a supply of sediments from both Micangshan-Dabashan and Longmenshan, resulting in a wide distribution of lacustrine delta sandstones of NW-SE and NE-SW orientations.It concludes that the uplifting intensity disparity and rising sequence of the different parts of Longmenshan during the episodic extrusion process are probably the joint controlling factors on the depositional processes and accumulation of sediments in the Western Sichuan Depression.

Reservoir tightening process and its coupling relationship with hydrocarbon accumulation in the fourth member of Upper Triassic Xujiahe Formation in theWestern Sichuan Depression, Sichuan Basin

Liangbiao Lin, Yu Yu, Hongli Nan, Hongde Chen, Lei Liu, Dong Wu, Zhikang Wang

2021, 42(4):  816-828.  doi:10.11743/ogg20210404

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The Upper Triassic Xujiahe Formation in Western Sichuan Depression is rich in hydrocarbon resources and has been one of the major tight gas pay zones in the area. By focusing on the tight sandstone of the fourth member of the formation, this study aims to answer why the stone is so tight and what factors had affected its tightening-up process based on petrological characteristics, reservoir physical property and diagenesis analyses, which are also combined with hydrocarbon accumulation history to confirm a possible coupling relationship between the two processes. It reveals that compaction caused great porosity loss in the stone. However, it was the late-stage precipitation of carbonate cements in a semi-closed to a closed setting that played a key role in the process by reducing the porosity of high-quality reservoirs in the formation to less than 10 % during the middle and late stages of mesodiagenesis (corresponding to the Early Cretaceous). Fluid inclusions and laser Raman compositional analyses suggest two hydrocarbon accumulation periods, of which the second period (the Late Jurassic-Early Cretaceous) contributed more. It also suggests that when other types of reservoirs in the basin were entering the tightening-up stage, i.e. when their porosity was reduced to less than 10 %, hydrocarbon accumulation in the fourth member of Xujiahe Formation was still ongoing, indicating a precedence of reservoir tightening to hydrocarbon accumulation.

Characteristics and distribution of "sweet spot" reservoirs in the third and fifth members of Upper Triassic Xujiahe Formation, Western Sichuan Depression, Sichuan Basin

Sujuan Ye, Yingtao Yang, Ling Zhang

2021, 42(4):  829-840, 862.  doi:10.11743/ogg20210405

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The third and fifth members of Xujiahe Formation in Western Sichuan Depression are characterized by thick source rocks with high TOC and medium- to large-scale delta distributary channel and sheet sandstones of huge in-source accumulation potential.A systematic study was carried out on the characteristics, formation mechanism and distribution of "sweet spot" reservoirs in the two members based on an integration of petrography, pore structure, and petrophysical property analyses with well logging interpretations.The results show that fine-grained thin sandstone interbeds against a delta front to shore-shallow lacustrine environment dominate the two members, with the third member in southern Chengdu Sag containing some single- or multiple-layered thick sandstone against delta distributary channels.The thin sandstone reservoirs are characterized by diverse rock types, fine grain size, and high contents of clay matrix and carbonate minerals, and rather tight due to intensive compaction and carbonate cementation."Sweet spot" reservoirs in the thin sandstones are mainly the results of hydrocarbon generation, dissolution and fracturing processes.Assemblages of organic-rich shale and fine- to medium-grained sandstone with high content of quartz (feldspar) and small amount of carbonate debris (cement) are considered potential sweet spots.The thick sandstone reservoirs are dominated by secondary pores and fractures.Sweet spots are mostly controlled by granularity, the content of feldspar (quartz) and other soluble (brittle) minerals as well as dissolution intensity, of which the latter is the key factor.The fifth member in the south of Longmenshan front structural belt and Xinchang structural belt is most likely to contain thin sandstone sweet spots because of its high-quality source rocks, high content of quartz (feldspar and clay), and low content of carbonate minerals.While the third member in southern Chengdu Sag is ideal for the thick sandstone sweet spots to develop in quartz (feldspar)-rich medium- to coarse-grained sandstones adjacent to shales vertically and laterally.

Dynamic chemical characteristics and origin of formation water in the second member of Xujiahe Formation, Xinchang structural belt, Sichuan Basin

Zhanghua Lou, Yizhe Su, Rong Zhu, Yifeng Liu, Shilin Xu, Wangpeng Li

2021, 42(4):  841-851.  doi:10.11743/ogg20210406

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Formation water from the second member of Xujiahe Formation, Xinchang structural belt, Sichuan Basin, is chemically complicated and highly variable in salinity, with some wells recording dynamic changes of chemical characteristics of produced waters.This paper discusses the migration of formation water based on a thorough analysis of the chemical characteristics of formation water from each interval of the Xujiahe Formation by using salinity data, chlorine/magnesiumionic ratios, metamorphism coefficients, Novák's phase diagrams and etc. The results show that the formation water of the second member has salinity values ranging between 70 and 110 g/L, chlorine/magnesium ionic ratios greater than 50 and metamorphic coefficients greater than 10; and that of the fourth member has salinity values ranging between 50 and 90 g/L, chlorine/magnesium ionic ratios less than 50 and metamorphism coefficients less than 10.Their Novak's phase diagrams also have obvious differences.These characteristics can be used to determine the stratigraphic origins of formation waters.The produced water from most wells in the second member show relatively stable chemical characteristics, indicating that the waters are sourced from the same stratigraphic interval. However, the produced water from some wells, such as Xin 856, Xin 2, Xin 10-1H and Xin 5, show significant variation of chemical characteristics. Judging by the graphs, the waters are multi-sourced. Alien waters of high salinity and high bromide ion concentration from marine Leikoupo Formation had been detected in wells Xin 2 and Xin 856 after Wenchuan Earthquake. Waters from the fourth member were also observed in wells Xin 10-1H and Xin 5.It is suggested that faults are the most likely pathways for this cross-layer migration of formation water.

Gas pool sweet spot models and their forming mechanism in the Xu 2 Member in Xinchang area, Western Sichuan Depression, Sichuan Basin

Junlong Liu, Zongquan Hu, Zhongqun Liu, Wujun Jin, Kaihua Xiao, Youyi Bi, Jitong Li

2021, 42(4):  852-862.  doi:10.11743/ogg20210407

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Sweet spot evaluation of the tight sandstone reservoirs is of great importance to effectively developing reserves.The elaborate description of the gas pools in the 2^nd member of Xujiahe Formation (Xu 2 Member/T₃x²), serves to establish the sweet spot model and formulate the criteria for identification, and clarify the geological distribution of the sweet spots.Furthermore, the forming mechanism of the sweet spots in tight sandstone gas pools is discussed.The results show that: (1) the effective reservoirs can be grouped into two categories, namely the medium-to-coarse-grained sandstone with flat fractures (with a dip angle less than or equal to 10°), and the massive medium-to-coarse-grained sandstone; the effective fractures are structural fractures with a dip angle greater than 30°.(2) The sweet spots can be classified into three categories, namely the fault-, bedding fissure-, and pore-dominated categories.The first one is characterized by high dip angle fractures, mainly developed in the fault-fracture body dominated by SN-oriented faults; the last two are characterized by less developed high dip angle fractures, mainly distributed in high-quality formations outside of the fault-fracture body.(3) Gas-bearing zones of relatively high tectonic position serve as the basis factor for the sweet spot development in T₃x², the fault-fracture body affected by later faulting is a major factor controlling sweet spot growth, and the quality of favorable formations controls the formation of the other two categories.In all, it is of great significance to building the sweet spot models and identifying its forming mechanism for the gas pools, which could deepen our understanding on the sweet spot-related theories of tight sandstone gas pools, and is of strong support to the sweet spot distribution prediction and movable reserves evaluation.

Lithofacies-based prediction of relatively high-quality reservoirs of the Xu 3 Member in Yuanba area, northeastern Sichuan Basin

Yanqing Huang, Zhongqun Liu, Tian Lin, Kaihua Xiao, Ai Wang, Mingxi Xue, Zhizhou Huo

2021, 42(4):  863-872.  doi:10.11743/ogg20210408

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The 3P reserves of the Xujiahe Formation are large in scale in Yuanba area.However, due to the strong heterogeneity of tight sandstones, and less understanding on the distribution of relatively high-quality reservoirs, the proportion of proven reserves is low in the 3P reserves and the production tests are poor in effect.It is necessary to accurately predict the distribution of coarse siliciclastic rocks, from which relatively high-quality reservoirs are to be discovered.On the basis of sedimentary microfacies division by using well data, precise lithologic interpretation is achieved by gradual lithologic identification.The geological significance of peaks and troughs on seismic sections is clarified by analyzing the lithological associations on the top and bottom surfaces for various sand groups in the 3rd member of Xujiahe Formation (hereinafter referred to as Xu 3 Member); meanwhile, multiple methods are applied to select proper time window for extracting seismic attributes given the gap in seismic-well layering, and several seismic attributes that are not influenced by the overlying and underlying formations are extracted within the time window.In regard of the drilling data, we select the seismic attributes that are the most sensitive to coarse siliciclastic rocks and use them to map the distribution of the coarse siliciclastic rocks via seismic-well tie.On the other hand, to identify the relatively high-quality reservoirs within coarse siliciclastic rocks, we sub-divide the coarse siliciclastic rocks in lithology with core data, confirming that there are two categories of high-quality reservoirs well developed in the Xu 3 Member, that is, the medium-to-coarse-grained calcarenaceous sandstone reservoir and fine-grained arenaceous conglomerate reservoir.Furthermore, the distribution of the two categories of high-quality reservoirs is finely depicted under the constraint of the coarse siliciclastic rock distribution while establishing well logging interpretation criteria.The results serve to lay a sound basis for the large-scale development of the Xu 3 Member gas pools in high efficiency.

Characteristics and genesis of fault-fracture reservoirs in the Xujiahe Formation, Tongjiang-Malubei area, northeastern Sichuan Basin

Renchun Huang, Ruobing Liu, Ming Liu, Huanyu Cao, Sai Su, Hongquan Du

2021, 42(4):  873-883.  doi:10.11743/ogg20210409

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Affected by the superposition of multi-phase tectonic movements, the Xujiahe Formation in Tongjiang-Malubei area of the northeastern Sichuan Basin contains many sets of large-scale sand layers and special fault-fracture reservoirs in highly deformed structures.A study of reservoir characteristics, formation mechanisms and development control factors, in combination with drilling data and regional tectonic evolution for the formation, shows that the reservoirs are highly porous with well-developed fractures and feature in close relation between matrix pores and pore-fracture networks.The formation of fault-fracture reservoirs is closely linked to the triple-layer tectonic pattern and multi-phase tectonic movements in the area.The regional tectonic stress field and deep fluid activities serve respectively as the external mechanical background and internal genetic mechanism for the development of reservoirs.With source rock-rooted faults having dual functions of connecting source rock with reservoir and controlling the development of reservoirs, large-scale sand layers providing the basis for fracturing and dissolution, and local structures controlling the formation of complex fracture networks, the fault-fracture reservoirs with hydrocarbon came into being in the formation.

Characteristics of and main factors controlling the tight sandstone reservoir fractures in the 2^nd member of Xujiahe Formation in Xinchang area, Western Sichuan Depression, Sichuan Basin

Wangpeng Li, Zhongqun Liu, Zongquan Hu, Wujun Jin, Pengwei Li, Junlong Liu, Shilin Xu, Anlai Ma

2021, 42(4):  884-897, 1010.  doi:10.11743/ogg20210410

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The sandstone reservoirs in the 2^nd member of Xujiahe Formation (hereafter referred to as Xu 2 Member) in Xinchang area, Western Sichuan Depression, are characterized by deep burial depth and tightness, to which the natural fractures are critical. An integration of outcrop survey, core description, conventional and imaging logging interpretation is applied to analyze the development characteristics of natural fractures in the gas reservoirs therein. It is believed that these fractures can be categorized as network fractures, vertical fractures, high dip angle fractures, horizontal fractures, oblique fractures, and low dip angle fractures, with the last two in the dominant position. Mainly developed in the middle and upper submembers of Xu 2 Member in central and eastern Xinchang tectonic belt, these fractures generally trend NE-SW, NW-SE and EW, basically consistent with that of the present maximum principal stress within, with the EW-striking structural fractures in the late period the most developed. On the other hand, the cross-plotting analysis of fracture and productivity parameters, serves to clarify the contribution of different types of fractures to productivity, and the results show that the effective fractures in the Xu 2 Member gas pool are structural ones with a dip angle of over 30° (namely, vertical, high dip angle and oblique fractures). The development of effective fractures in the study area is controlled by many factors, including faulting, tectonic deformation, grain size, rock composition, and stratum thickness, etc., and the distance from the late SN-striking reverse fault is the most critical factor controlling the development of natural structural fractures, followed by structural deformation and structural pattern in combination. In conclusion, the area with well-developed folds and within 200 m away from the hanging wall of the late SN-striking reverse fault can be regarded as a key zone for future fracture prediction, evaluation and natural gas development in Xinchang area.

Characteristics and favorable plays for high-quality reservoirs in the Middle Triassic Lei 4 Member, Yuanba area, northeastern Sichuan Basin

Jinbao Duan, Mindong Jin, Zhiwei Fan, Xiang Zhu, Yanting Liu

2021, 42(4):  898-908.  doi:10.11743/ogg20210411

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For the purpose of further enhancing the success rate of exploratory wells emplaced in the superposed shallow-karst reservoirs in the 4th member of Leikoupo Formation (hereinafter referred to as Lei 4 Member) to achieve efficient exploration and development in northeastern Sichuan, we comprehensively analyze the reservoir characteristics and major control factors based on rich data from borehole observation, coring and lab analysis.The results show that (1) the Lei 4 Member in the study area is of intraclastic grainstone shoal facies with reservoirs mainly developed in the doloarenite and karst breccia; and the dominant reservoir space is karst caverns and fractures, featuring relatively strong heterogeneity.(2) The analysis of the reservoir genetic mechanism shows that the original pore system of the shoals of tidal flat facies serves as the material basis for high-quality reservoir development.A mass of reservoir space formed by the transformation of karstification is a factor critical for the reservoir development.Thermal dissolution in the burial stage could further improve the reservoir quality.(3) The karstification shows identification markers special to mechanical filling and chemical reaction, and the property of karst fluid and its activity routines in the different karst geomorphic units serve to determine the reservoir quality.The karst mound-platform, which is near source rock laterally and/or has well-developed fracture system, is a desirable zone for future exploration in the northeastern Sichuan Basin.

Pore structure characteristics of shale reservoirs in the Ziliujing Formation in Yuanba area, Sichuan Basin

Tao Jiang, Zhijun Jin, Guangxiang Liu, Zongquan Hu, Quanyou Liu, Zhongbao Liu, Pengwei Wang, Ruyue Wang, Tao Yang, Guanping Wang

2021, 42(4):  909-918.  doi:10.11743/ogg20210412

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Shale of the Ziliujing Formation in Yuanba area of Sichuan Basin, currently at a critical stage of exploration, is one of the main research targets of terrestrial shale gas.As a significant index to evaluate the storage capacity of shale reservoirs, shale pore structure is the key to clarify the enrichment mechanism of shale gas.In this study, the pore structure is quantitatively characterized through an integration of total organic carbon (TOC) content tests, whole rock X-ray diffraction, combined experiment of N₂ adsorption and high-pressure mercury intrusion for pore size and scanning electron microscopy (SEM) after argon ion milling.The results show that the Ziliujing Formation shale is dominated by clay minerals, accounting for 38.8% to 67.3% with an average of 52.8%;the pores are mainly matrix pores, followed by organic pores, and with micro-fractures developed locally.The hysteresis loops of N₂ adsorption reflect the well development of parallel plate-like slit pores together with a small amount of ink bottle-shaped pores.The main interval of pore size distribution falls in mesopores.Compared with Dongyuemiao Member, Da'anzhai Member contains fewer micropores and more macropores, leading to a higher median value of pore size and pore volume, while a relatively smaller specific surface area (SSA).The pore volume and SSA of shale reservoirs in the Ziliujing Formation depend on clay minerals.The pore volume and SSA are in a negative correlation or irrelevant with the TOC content, mainly due to the infertility of organic pores in vitrinite and filament and the occupation of pore volume and adsorption sites by soluble organic matters.These will be of referential significance to the exploration of terrestrial shale gas in the study area.

Progress of molecular simulation application research in petroleum geochemistry

Meijun Li, Xiaoqiang Liu, Qiuya Han, Hong Xiao, Ronghui Fang, Daxiang He, Zhiwei Gao

2021, 42(4):  919-930.  doi:10.11743/ogg20210413

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Molecular simulation measures molecular microstructures and calculate macroscopic properties of a given system by using computer modelling technology.It can directly reproduce intermolecular interaction and chemical reaction processes on molecular or even atomic level and is characterized by variable controllability, high operation efficiency, short research cycle, high safety, and so on.The technique has been applied in petroleum geochemistry.This study briefly summarizes the application and progress of molecular simulation in the following aspects: (1) the adsorption behavior of coalbed methane, (2) the adsorption of shale gas and shale oil, (3) the construction of molecular indicators assessing maturity of organic matters, (4) the establishment of molecular markers tracing subsurface petroleum migration orientations; and (5) the mechanisms for hydrocarbon generation and oil cracking in reservoirs.The molecular simulation technique has profound application prospects, and is playing a role of ever-growing significance, particularly in unconventional petroleum exploration and research of hydrocarbon generation mechanism and petroleum migration and accumulation in deep and ultra-deep reservoirs.

Accumulation of sediments with extraordinary high organic matter content: Insight gained through geochemical characterization of indicative elements

Zhen Qiu, Hengye Wei, Hanlin Liu, Nan Shao, Yuman Wang, Leifu Zhang, Qin Zhang

2021, 42(4):  931-948.  doi:10.11743/ogg20210414

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Organic matter (OM) accumulation plays a key role in the development of sweet spots in unconventional oil and gas plays, especially in shale. Fine-grained sedimentary rocks containing or associated with sweet spots generally have TOC content of higher than 3.0%. We define these sediments or sedimentary rocks with TOC ≥ 3.0% as the extraordinary high organic matter (EHOM) deposits, and propose conceptual hypotheses for the accumulation process of EHOM. We suggest that high primary productivity provides prerequisites for OM accumulation and that the anoxia is prone to enhance large scale accumulation of EHOM. We use the Wufeng-Longmaxi formations in the Sichuan Basin to carry out a case study for the discussion on the elemental bio-geochemical cycles during the EHOM accumulation processes. The conceptual hypothesis for EHOM enrichments constrained by elementary geochemistry is verified. Our result shows that the EHOM enrichment in the Wufeng-Longmaxi formations was the results of dynamic evolutions of the high primary productivity and the redox conditions jointly controlled by high primary productivity and water circulation. As one of the research targets of unconventional petroleum sedimentology, this study could help better understanding the forming mechanism of the "sweet-spots" and further support unconventional petroleum exploration and development.

Occurrence, composition and origin of analcime in terrestrial lacustrine sedimentary rocks in China

Ye Jia, Shifa Zhu, Yi Yang, Huan Tong, Xiaomin Zhu

2021, 42(4):  949-962.  doi:10.11743/ogg20210415

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Natural analcime closely related to oil and gas reservoirs, is quite common in sedimentary rocks.We summarize the occurrence, composition, genesis of natural analcime and discuss its impact on reservoirs based on previous researches.The results show that analcime, a kind of silicate mineral containing crystalline water and of framework structure, is characterized by a silica-oxygen tetrahedron on map view to form a closed ring, and a cage structure in 3-D visualization space, the pore pathways in which are conducive to dissolution.There are five typical types of natural analcime in terrestrial lacustrine basins, among which the analcime with intergranular cements and fracture fillings is euhedral, and the exhalative analcime comes in two kinds of occurrence, namely, porhyritic texture and discontinuous coarse-grained laminae.The compositional differences of analcime concentrate on the content of Na and K ions: the K ion content tends to be higher, and the Si/Al ratio in analcime mainly ranges from 2.0 to 2.69, when it comes to alkaline volcanic rock as the host-rock.Among the four categories of natural analcime in terms of genesis, that in terrestrial lacustrine basins is mainly derived from diagenesis and hydrothermal exhalation.In all, sedimentary facies is key in the formation and dissolution of analcime; the analcime with intergranular cements is widely spread in the fan delta front developed from volcanic materials, while the analcime out of hydrothermal exhalation is common in the deep-semi-deep lacustrine subfacies; and dissolution of analcime by acidic fluid in the later stage is different in different sedimentary facies.

Methods and Technologies

Assessment of anisotropic in-situ stressses in the Upper Triassic Xujiahe Formation reservoirs in Western Sichuan Depression of the Sichuan Basin

Jincai Zhang, Xin Fan, Zhiwen Huang, Zhongqun Liu, Yuanchang Qi

2021, 42(4):  963-972.  doi:10.11743/ogg20210416

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The Triassic Xujiahe Formation sandstones in the western Sichuan Depression are tight gas reservoirs, some of which contain natural fractures, particularly for the gas-rich reservoir in the 2nd member of the Xujiahe Formation (hereafter referred to as Xu 2 Member) with well-developed vertical or high dip angle fractures. Anisotropies of both rocks and horizontal principal stresses are thereby resulted. To calculate in-situ stresses in the naturally fractured formations using conventional methods obtained in the isotropic formation may overestimate the horizontal principal stress. New assessment methods of in-situ stresses in the anisotropic formations are studied in this paper. The in-situ stressess and formation pressures in the Xinchang and Hexinchang gas fields are obtained based on measured data from wellbore influxes, kicks, mini-frac tests and wellbore breakouts.The measured results indicate that natural fractures have a great impact on the in-situ stresses. The theory of anisotropy is applied to establish the in-situ stress models of VTI and HTI for the Xujiahe Formation reservoirs. The measured data in the formations with natural fractures reveal that the horizontal principal stress and formation breakdown pressure decrease as the natural fracture density, dip angle and intensity index increase. This is consistent to the derived HTI model. The assessment methods for in-situ stresses and their impact on hydraulic fracturing are proposed for the Xujiahe Formation reservoirs based on the anisotropic model and the measured data of natural fractures, horizontal stresses and breakdown pressures.

Concept and geological model of fault-fracture reservoir and their application in seismic fracture prediction: A case study on the Xu 2 Member tight sandstone gas pool in Xinchang area, Western Sichuan Depression in Sichuan Basin

Zhenfeng Liu, Zhongquan Liu, Yuanling Guo, Yuxin Ji, Wangpeng Li, Tian Lin, Tiansheng Chen, Wujun Jin

2021, 42(4):  973-980.  doi:10.11743/ogg20210417

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Fractures are one of the factors crucial for the formation of sweet spots in tight oil & gas reservoirs.Due to the great difference between the geological scale of fractures and the resolution of seismic data, and the vague understanding on development patterns of effective fractures of large scale, the effectiveness of seismic prediction methods of fractures is very limited.Based on the concept of fault-fracture reservoir put forward by some geological researchers in recent years, we summarize the connotation of fault-fracture reservoir, and establish a geological model showing their development in the tight sandstone gas pool of the second member of Xujiahe Formation (hereinafter referred to as Xu 2 Member) in Xinchang area.Furthermore, under the guide of the newly established geological model, we discuss the compositional units and geophysical response features of enclosing rocks, calculate and screen the gradient structure tensor (GST)-based chaotic attributes (chaos) and the maximum likelihood attributes for fault-fracture reservoir prediction, and put forward the technical scheme in predicting fault-fracture reservoirs in the Xu 2 Member tight sandstone pool in Xinchang area.In application of the scheme, artificial fault interpretation and electrical imaging logging are used to determine the threshold values of the GST-based chaotic attributes (chaos) and the maximum likelihood attributes.Besides, the threshold values of seismic attributes are calibrated according to the interpretation results of electrical imaging logging and artificial faults, the maximum likelihood attributes (greater than 0.15) are applied to describe the sliding surface of faults, various ranges of threshold values of chaotic attributes are applied to depict the sliding breaking zone (chaos attribute, 0.18-1.0) and induced fracture zone (chaos attribute, 0.05-0.18), and corresponding prediction results are obtained for major target sequences in the study area as guided by the geological model.The comparison of prediction results and electrical imaging log data shows the prediction is in high consistency with the actual situation, and its geological significance is obvious, indicating the concept of fault-fracture reservoir and its geological model are of application value in the practice of seismic fracture prediction.

Characteristics and application effect of logging-based fracture identification in tight sandstones: A case study of the Upper Triassic Xu 2 Member in Western Sichuan Depression, Sichuan Basin

Zhiyuan Liu, Hao Li, Qingzhao Wu, Zeyu Nan, Junlei Su, Wujun Jin

2021, 42(4):  981-991.  doi:10.11743/ogg20210418

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Fractures are a key factor for high and stable production of tight sandstone reservoirs.The coincidence rate of fracture identification based on conventional logging is, however, low, due to the low resolution of conventional logging, rapid change of lithology and physical properties of tight sandstone, small-scale fractures with variable occurrences.Based on the concluded regional fracture types and fracture logging response characteristics, we innovatively extract multiple factors sensitive to fracture such as resistivity reduction and moveout augmentation in the tight sandstone of the second member of Xujiahe Formation (hereinafter referred to as Xu 2 Member) in Xinchang gas field, Western Sichuan Depression.Furthermore, resistivity dimension and acoustic dimension constraint index are selected to suppress the interference effect with the amplification of fracture signals; and the gray correlation is applied for the first time to determine the weight coefficient of sensitive factors, the fracture indication curve is calculated by weighting, and the threshold constraint is used to identify fractures.In comparison with the fracture response pattern library, the fracture type is identified by clustering.In comparing with electric imaging data of the 5 wells including Wells X5 and X101, the coincidence rate of fracture identification reaches up to more than 90%, providing a favorable basis for the study of fracture distribution pattern, reservoir type evaluation and productivity prediction of tight sandstones.

Exploration and prediction of promising fault-fracture reservoirs in the Xujiahe Formation, northeastern Sichuan Basin

Wei Wang, Rui Fan, Chengyin Li, Dapeng Qu, Lamei Zhang, Kelu Su

2021, 42(4):  992-1001.  doi:10.11743/ogg20210419

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Tight sandstone in the Upper Triassic Xujiahe Formation, northeastern Sichuan Basin, has been one of the exploration targets for natural gas in the area. However, the stone, as a whole, is rather tight, deeply buried and highly heterogeneous, making commercial development of gas from it quite a challenge. The formation contains many fault-fracture gas reservoirs in the Tongjiang-Malubei area of the Tongnanba structural belt, where certain lithological facies in well-fractured zones may be used as clues to the whereabouts of the reservoirs. By recognition of source rock-rooted faults based on dynamic structure restoration, nonlinear inversion and prediction of lithology and physical properties, and multi-scale fusion as well as five-dimensional pre-stack fracture prediction in OVT domain, we effectively addressed the recognition of source rock-rooted faults, the prediction of relatively high-quality reservoirs and fractures as well as "sweet spots"-several issues easily encountered in dealing with this type of reservoir. These techniques were applied well to the predication of fault-fracture reservoirs in the northeast Sichuan Basin with prediction error of reservoir thickness being reduced from 16% to 8% and predication accuracy of fractures being increased from 60% to 85%. Four wells were so successfully drilled according to the predication results that controlled reserves of 367.91×10⁸ m³ were booked with a possible gas resource base of 100 billion cubic meters readily on the way.

Reservoir engineering geological characteristics and stimulation in Xujiahe Formation, Xinchang gas field, Western Sichuan Depression, Sichuan Basin

Huashu Zhu, Lin Liu, Zhiwen Huang, Xin Fan, Jincai Zhang

2021, 42(4):  1002-1010.  doi:10.11743/ogg20210420

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Reservoir stimulation is a key technology to develop the tight gas reservoir in the Western Sichuan Basin.The second member of the Xujiahe Formation (hereinafter referred to as Tx²), a typical deep tight formation, is characterized by tightness, strike-slip fault stress state with minor principal stress close to vertical stress, and well-developed fractures within, and the effect of reservoir simulation is quite limited at the early stages.The engineering geological evaluation and analysis of the previous hydraulic fracturing operation in the Xujiahe Formation are carried out, and three types of "double sweet spots" (namely, the sweet spot both geologically and geomechanically) are determined based on lithofacies, petrology, physical properties and natural fractures.Among others, the fault-fracture type could be developed by nitrogen drilling, small-scale fracturing and acidizing; the bedding-parallel fracture type could be developed by the near-wellbore deep plugging removing and integrated stimulation technique; and the pore type needs to be fractured by volume fracturing under ultra-high pressure.In practice, Well X8-2 encountering fault-fracture type sweet spot achieves a steady open flow rate of 53×10⁴ m³/d through nitrogen drilling, and Well XS101 encountering pore type sweet spots achieves a steady production rate of 15×10⁴ m³/d through large-scale volume fracturing.The success of Wells X8-2 and SX101 shows that these technologies are effective in stimulation of the tight gas reservoirs, thus being applicable for the effective development of natural gas resources in the Xujiahe Formation, Western Sichuan Depression.