Advances in Geo-Energy Research Vol. 5, No. 2, p. 121-126, 2021 Perspective Key issues and development direction of petroleum geology research on source rock strata in China
Jiarui Li, Zhi Yang *, Songtao Wu, Songqi Pan
Research Institute of Petroleum Exploration & Development, PetroChina, Beijing 100083, P. R. China
Keywords: Abstract: Tight oil and gas After more than 20 years of technological advancements, the novel field of oil and gas shale oil and gas production from source rock strata, which comprise tight and shale oil and gas reservoirs, coal-bed methane has become the major contributor to the increase in unconventional oil and gas reserves source rock strata in China. Accordingly, this field has gradually entered a new stage of revolutionary development. The oil and gas production in China from source rock strata will achieve unconventional oil and gas sustainable development in the future. Different types of source rock strata present distinct sweet area challenges and require diverse development paths. Based on the geological conditions of Cited as: source rock strata in China, this study focuses on identifying the “sweet areas” among Li, J., Yang, Z., Wu, S., Pan, S. Key hydrocarbon accumulations. It specifically analyzes the key development issues of tight oil, tight gas, shale oil, shale gas, and coal-bed methane, while proposing potential solutions issues and development direction of and identifying the possible directions for future development. This study aims to provide petroleum geology research on source a reference for scientists concerned with the development of unconventional oil and gas rock strata in China. Advances in reserves in China. Geo-Energy Research, 2021, 5(2): 121-126, doi: 10.46690/ager.2021.02.02
1. Introduction al., 2021; Zou et al., 2021). Meanwhile, shale oil production has shown potential to allow further exploration (Du et al., Source rock strata can be used to obtain a continuous 2019; Yang et al., 2019c; Hu et al., 2020; Zhao et al., 2020; distribution of oil and gas resources that can be exploited Zou et al., 2020), and manufacturing of coal-bed methane has industrially by using existing technologies, in which hydrocar- demonstrated stable development (Sun et al., 2021). Overall, bons are generated, retained, or gathered in the source rocks or oil and gas derived from source rock strata have become major in the tight reservoirs adjacent to the source rocks (Yang et al., contributors for the increase in unconventional oil and gas 2019b, 2021). Oil and gas are derived from source rock strata reserves in China (Yang et al., 2021). using two types of resources: source rock reservoirs including shale oil, shale gas, and coal-bed methane (Yang et al., 2019a; 2. Petroleum geology characteristics of source Zou et al., 2019b), as well as tight oil and gas reservoirs rock strata in China adjacent to the source rocks (Yang et al., 2015; Sun et al., 2019). The overall strategic development of unconventional The core of hydrocarbon accumulation in source rock strata production of oil and gas in China is now being realized, is the source rock. Within the strata, a large amount of oil and with the utilization of source rock strata. Furthermore, the gas resources are gathered in the source rocks and in the tight field of tight sandstone gas production has seen large-scale reservoirs adjacent to the source rocks. The onshore source development (Dai et al., 2012; Sun et al., 2019). The need for rock strata in China are characterized by varied sedimentary an accelerated development of shale gas and tight oil reservoirs environments and complex lithofacies, ranging from the Meso- has garnered significant attention (Hao et al., 2013; Chen et al., Neoproterozoic to the Quaternary. The hydrocarbon accumu- 2015; Yang et al., 2015; Jin et al., 2016; Ma et al., 2018; Jiao lation distribution in the source rock strata can be roughly et al., 2020; Xu et al., 2020; Horsfield et al., 2021; Venieri et divided into Cenozoic, Cretaceous, Jurassic-Triassic, Upper
∗Corresponding author. E-mail address: [email protected] (J. Li); [email protected] (Z. Yang); [email protected] (S. Wu); [email protected] (S. Pan). 2207-9963 © The Author(s) 2021. Received March 5, 2021; revised March 13, 2021; accepted March 14, 2021; available online March 16, 2021. 122 Li, J., et al. Advances in Geo-Energy Research, 2021, 5(2): 121-126
Age Accumulation Stratum Lithology Source rock Reservoir series Main hydrocarbon accumulation types and geological ages /Ma domain
Quaternary Q 2.5 Gas hydrate in northern slope of South China Sea(N-Q)
Pliocene N2 Neogene
Cenozoic Miocene N1 2 Cenozoic Tight oil and shale oil in depression-slope area of western Qaidam Basin(E3 /N1) 23.0
Oligocene E3 Tight oil shale oil in depression-slope area of Bohai Bay Basin(E) Paleogene Eocene E2 Heavy oil asphalt in gentle slope area of Bohai Bay Basin(E)
Paleocene E1 65.5 Oil shale in Songliao Basin(K) Upper K2 Cretaceous Cretaceous Tight oil and shale oil in depression area of Songliao Basin(K1) Lower K 1 Heavy oil sand in northwestern margin of Junggar Basin(K) 145.5
Upper J3
Tight sandstone gas in Kuqa and Northern Xinjiang slope areas of Tarim Basin(J) Mesozoic Jurassic Middle J2
Tight and shale oil and gas in depression-slope area of Sichuan Basin(J1) Lower J1 199.6 Jurassic-Triassic Tight sandstone gas in depression slope area of Sichuan Basin(T3) Upper T3 Tight oil and shale oil in depression slope area of Ordos Basin(T3)
Triassic Middle T2
Lower T1 252.2 Upper P3
Tight oil and shale oil in depressions of Northern Xinjiang(P1-2) Permian Middle P2 Lower P 1 299.0 Tight sandstone gas in depression-slope area of Ordos Basin(P1)
Upper C2 Coal-bed methane in Syncline area of Qinshui Basin(C -P ) Carboniferous 1 1 Upper Paleozoic
Lower C1 Coal-bed methane and shale gas in Ordos Basin(C1-P1) 359.6 Upper D3 Devonian Middle D2
Lower D1 Paleozoic 416.0 Upper S3
Silurian Middle S2 S Lower 1 443.8 Shale gas in shelf area of Sichuan Basin(O3-S1) Upper O3
Ordovician Middle O2 Lower Paleozoic
Lower O1 485.4 Upper Є3
Cambrian Middle Є2 Shale gas in shelf area of Sichuan Basin(Є) Lower Є1 541.0 Proterozoic Proterozoic shale gas(Pt?)
Sandstone-mudstone Mudstone Sandstone Dolomite Limestone interbed
Coal bed Marl Source rock Gas-bearing strata Oil-bearing strata
Fig. 1. Comprehensive histogram of unconventional oil and gas resources in China (Yang et al., 2021).
Paleozoic, and Lower Paleozoic accumulation domains (Fig. 1) source rock strata are distributed continuously in the center and (Yang et al., 2021). Among these, the Cenozoic is represented the slope area of the basin over a large range. Furthermore, by tight oil and shale oil in the Paleogene strata of the Bohai the source rocks and reservoirs coexist without obvious trap Bay and Qaidam basins. The Cretaceous is represented by boundaries, which breaks through the concept of conventional tight oil and shale oil in the Songliao Basin. The Triassic- oil and gas reservoirs; consequently, the objective of the Jurassic is represented by tight oil and shale oil in the Ordos research shifts from “trap and reservoir” to “reservoir sweet Basin, tight oil and shale oil in the Sichuan Basin, and tight area” (Zou et al., 2019b; Yang et al., 2021). The core focus in sandstone gas in the Tarim Basin. The Upper Paleozoic is the research on the oil and gas geology of source rock strata is represented by Carboniferous-Permian tight sandstone gas and to study and evaluate the sweet area (section) of hydrocarbon coal-bed methane in the Ordos Basin and tight oil and shale oil accumulations in the source rock strata; that is, on the plane in the Junggar Basin. The Lower Paleozoic is represented by (section) of the source rock strata, artificially stimulations are Wufeng-Longmaxi Formation shale gas in the Sichuan Basin. induced throughout by using existing technologies to produce The study of oil and gas deposits in source rock strata a favorable zone (section) that comprises industrial oil and in China is a new area of research in petroleum geology, gas deposits (Yang et al., 2021). The identification of the which studies the types, occurrence mechanisms, enrichment “sweet area” (section) is carried out using the product of laws, distribution characteristics, production mechanisms, as- the six-property relationship, which includes the source rock, sessment methodologies, key technologies, and development reservoir, oil-bearing, liquidity, fracture, and economy. The strategies of oil and gas resources in China’s onshore source source rock property represents the hydrocarbon generation rock strata (Yang et al., 2021). The oil and gas derived from potential and spatial distribution of high-quality source rocks; Li, J., et al. Advances in Geo-Energy Research, 2021, 5(2): 121-126 123 it is used to find organic-rich source rocks. The reservoir prop- low, and the content of plastic clay minerals is high; this erty represents the formation and distribution of lithology and renders the creation of artificial fractures difficult. lithofacies, reservoir capacity, and connectivity of pore throat- • The underground processing of shale oil is a physical and fracture network systems in tight reservoirs, with the aim of chemical process, in which the “horizontal well electric finding effective reservoir development areas and screening heating upgrading” technology is used to constantly heat sweet reservoir are as that exhibit relatively good porosity the organic-rich shale intervals at buried depths ranging and permeability (Ijeje et al., 2019; Cai et al., 2020; Liang et from 300 m to 3000 m, thereby converting various types al., 2021). The oil-bearing property represents the occurrence of organic matter into light fractions (Zhao et al., 2020). state and filling degree of oil and gas in tight reservoirs, At present, there are still some uncertainties in the pro- with the aim of optimizing reservoirs with good oiliness. motion of an underground in situ conversion processing The fluidity property represents the formation energy, fluid technology that uses heating. composition, oil and gas quality, and the seepage-diffusion- Tight sandstone gas adsorption characteristics of tight reservoirs, with the aim of • The exploration targets of continental and transitional determining oil and gas reservoirs that exhibit strong mobility facies are deeper and more complex, and the natural and can be easily exploited. The fracture property represents gas accumulation patterns are changeable; therefore, it natural fractures, artificial fractures (stimulated throughout the is difficult to find new sources of tight sandstone gas. network), and the stress characteristics of tight reservoirs, • It is difficult to optimize well patterns and design horizon- with the aim of optimizing high-brittleness reservoirs that tal well trajectories because of the strong heterogeneity facilitate large-scale fracturing. Finally, the economy property of gas-bearing sand body distribution and the significant represents the real potential of oil and gas resources, low- number of uncertainties during reservoir prediction be- cost development paths, and economic and social benefits; it tween wells. aims to improve the efficiency of exploration and development • The percolation law of tight sandstone gas areas with dif- under the premise of ensuring sustained production. ferent water cuts is complex, and the reserve production 3. Key issues in petroleum geology of source capacity is very low. rock strata in China • Key technologies, such as reservoir reconstruction, need to be strengthened for adapting to the characteristics of Currently, the development of oil and gas production from low-grade tight sandstone gas areas and improving the onshore source rock strata in China faces many issues and single-well production. challenges, such as lack of theoretical understanding, tech- Shale gas nological research and development, and cost control; these • There are still many unknown aspects with regard to the factors seriously restrict the large-scale utilization and effective basic geological understanding of organic-rich shale dis- exploitation of source rock strata as oil and gas resources. tribution, reservoir mechanisms, and shale gas enrichment Tight oil laws of onshore and marine-onshore transitional facies. • The lithology of an onshore tight reservoir is complex • Except for the shale gas of the Wufeng-Longmaxi For- and difficult to predict, while the stability and continuity mation in the Sichuan Basin and its surrounding areas, of the reservoir distribution are poor. the multi-layer marine shale of the Cambrian and Car- • The degree of thermal evolution of onshore source rocks boniferous has not been fully discovered. is low, oil viscosity is generally large, and oil fluidity in • Research on deep marine shale fracturing, atmospheric the reservoir is poor. shale low-cost fracturing, fault and fracture prediction, • The target layer is mainly under medium or low pressure, big data, artificial intelligence, and other technologies and generally requires energy storage and pressurization; still need to be strengthened, and anew three-dimensional therefore, it is difficult to develop this layer effectively. development model of an artificially stimulated fracture • The degree of geology-engineering integration is low, present throughout the gas reservoir still needs further while the applicability of key technologies still needs to exploration and innovation. be improved and the development cost is still high. Coal-bed methane Shale oil • Insufficient preparations for medium and low rank coal- • The onshore shale strata are mainly in the medium and bed methane resource reserve areas. low maturity windows because of the low thermal evo- • The production of single wells is low, while their ineffi- lution degree, low hydrocarbon generation conversion of cient development is a significant issue. organic matter under natural conditions, high oil viscosity, • The geological conditions are complex, while coal-bed and low movable oil proportion. methane exploration and development technologies are • The porosity and permeability of shale reservoirs are difficult to replicate. extremely low. Inorganic pores in clay minerals function • Underground coal gasification refers to the process of as the main reservoir storage; however, organic pores producing CH4,H2, and other combustible gases through are generally underdeveloped, with poor storage and in situ controlled combustion of coal in the strata with an connectivity. appropriate engineering technology (Zou et al., 2019a). • The natural fractures in shale reservoirs are relatively At present, there are still some uncertainties with regard 124 Li, J., et al. Advances in Geo-Energy Research, 2021, 5(2): 121-126
oil and gas theoretical strategic representative reservoir type key technology accumulation profile understanding positioning example The whole contains organic matter. The organic-rich lithofacies of deep lake and Surface distillation and heating in- Important resources shallow lake in depression control the situ conversion. Huadian, Dunmi Basin oil shale distribution of oil shale with high oil of oil shale content. The thick and large area organic-rich shale contains organic matter and hydrocarbons Horizontal well electric heating and Strategic replacement Chang 7 Member of Ordos Basin as a whole. other underground in-situ conversion medium and low maturity resources of shale oil Nenjiang Formation of Songliao Organic-rich sections are oil-bearing and technology. Basin distributed continuously. Rich and high shale oil yield in fracture development section of lamina. Horizontal well cutting volume fract- Important resources medium and high maturity uring and large platform development. Qingyi Member of Songliao Basin Organic-rich shale layers are oil-bearing of shale oil now and distributed continuously. Horizontal well complex fracture net- Important substitute medium and high pressure The formation is oil-bearing, continuously work fracturing platform development. Permian in Junggar Basin distributed and fractured. The reservoir resources for tight oil tight oil 1 2 sweet area is enriched and highly productive. Horizontal well volume fracturing Important resources Chang7 Chang7 oil layer in medium and low pressure The stratum is gas-bearing and contin- to increase production. of tight oil now Ordos basin uously distributed, and the structural Important substitute water-bearing gas layer peak is enriched and high-yielding. Water control and drainage to enha- Xujiahe Formation in central The coal measures are gas-bearing and nce recovery. resources for tight gas Sichuan Basin tight gas continuously distributed. Favorable Sand body characterization and ho- Important resources water-free gas layer facies zone is enrichment and high yield. Middle East Sulige, Ordos Basin The formation contains gas and is continuo- rizontal well volume modification. of tight gas now usly distributed. The filling area with suffici- Vertical well and other multi-well Current replacement reso- ent gas source is enriched and high-yielding. Cretaceous of Erlian Basin middle and low coal rank fracturing production. coal-bed The formation contains gas and is contin- urces of coal-bed methane uously distributed. Gas-filled and hyperto- Vertical or cluster wells. Moderate Important resources of Carboniferous - Permian in methane middle to high coal rank nic zones are enriched and high-yielding. transformation of horizontal well The rich area of organic-rich laminated drainage gas development. coal-bed methane now Qinshui Basin shale in deep water lacustrine basin is Possible alternative Da 'anzhai Segment, North- high-yielding. onshore resources of shale gas central Sichuan Basin The organic-rich laminated shale in the Horizontal well hydraulic volume Important replacement transitional deep water area is gas- Shanxi Formation, Ordos Basin shale gas transitional facies bearing and continuously distributed. fracturing and platform “artificial resources for shale gas gas reservoir” volume development. The organic-rich shale in the deep water Main resources for Wufeng - Longmaxi Formation marine shelf area is gas-bearing and continuously distributed. shale gas production in Sichuan Basin
Fig. 2. Theoretical understanding, key technologies and strategic positioning of unconventional shale strata oil and gas in China (Yang et al., 2021).
to the understanding of the mechanism for this process integration of different types of tight oils is necessitated and the developments needed in the technical equipment. to promote the exploration and development of tight oil across the country. In summary, the development of oil and gas reservoirs in different types of source rock strata faces different difficulties. Shale oil For tight oil, the main challenge is the effective development • The construction of a national-level demonstration area is of medium-and low-pressure parts; for tight sandstone gas, necessitated for continental shale oil in the Qingshankou it is the effective utilization of water-bearing parts; for shale Formation in the Songliao Basin. oil, it is the scaled development of medium and low maturity • To focus on the construction of the national demonstration parts; for shale gas, it is the development of continental zone of the Permian onshore shale oil in the Junggar and marine-continental transitional parts; and for coal-bed Basin for medium-and low-maturity shale oil, there is methane, it is the supplementary development of low-rank a need to effectively cope with the characteristics of coal. The next scientific and technological research phases will medium and low pressures, difficult seepage flow, and need to be further targeted for effectively solving these issues difficult reformation of the onshore tight oil layers, while and maximizing the potential of these difficult-to-use resources exploring effective development paths for low-grade and (Fig. 2). difficult-to-use reserves. 4. Development direction of source rock strata • For organic-rich shale exhibit medium and low maturi- ties, there is a need to focus on the construction of a oil and gas in China national continental shale oil demonstration area in the The source rock strata in China exhibit a rich accumulation seventh member of the Triassic Yanchang Formation in of hydrocarbons. Natural gas in source rock strata will become the Ordos Basin and the Cretaceous Nenjiang Formation the main force of natural gas production. Meanwhile, source in the Songliao Basin, while exploring the formation of rock strata will become the foundation of stable oil production, core technologies and equipment for underground in situ which have great development potential in the future. conversion. Tight oil Tight sandstone gas • To carry out a new round of national classification evalu- • To deepen the understanding of tight sandstone gas en- ation of tight oil resources, the main enrichment zone for richment rules, optimization of resource evaluation meth- development needs to be selected and the “sweet area” ods, implementation of national selection evaluation, and that can be drilled must be identified. large-scale application of new resources is necessitated. • Research and development of applicable drilling and • For the areas with water-bearing tight sandstone gas in completion technologies are required to effectively cope Ordos, Sichuan, and other basins where reserves are with the low-and high-viscosity characteristics of onshore difficult to use, the bottleneck theory and technologi- tight oil and to improve the production and recovery cal research must be strengthened, the organization and efficiency of single wells and blocks. management models must be optimized, tight gas fiscal • To focus on the construction of tight oil national-level policies and tax subsidy reductions must be established, development demonstration zones in the Ordos, Songliao, and the applicability of low-grade reserves must be max- and Junggar basins, a model for geology-engineering imized. Li, J., et al. Advances in Geo-Energy Research, 2021, 5(2): 121-126 125
• To develop key technologies such as high-precision, and gas reservoirs of source rock strata in China, namely: three-dimensional seismic, and large cluster horizontal volume fracturing of oil and gas underground horizontal wells wells, and to artificially stimulated fractures throughout in source rock strata, in situ heating conversion processing the gas reservoir, intelligent engineering, improvement in of continental organic-rich underground shale, and in situ the prediction accuracy of gas-bearing sand bodies, and combustion gasification of the artificially stimulated fractures. increase in the production and recovery of single wells and blocks are necessitated. 5. Conclusions Shale gas The focus of the source rock analysis is elucidating the • To strengthen the theoretical and technical research on sweet area selection and six-property relationship. These prop- the occurrence mechanism of non-marine shale gas, the erties include the source rock, reservoir, oil-bearing, liquidity, Qiongzhusi Formation in the Sichuan Basin and other fracture stimulation, and economy. unbroken marine shale gas strata must be explored along Targeted solutions to facilitate large-scale production and with thin-bed precision drilling and effective fracturing. development of medium-and low-pressure tight oil, gas from Furthermore, methods to achieve breakthroughs in new water-bearing tight sandstones, low-rank coal-bed methane, areas of shale gas extraction are required as soon as continental and transitional shale gas, and medium-low mature possible. shale oil are expected to be the key research directions for • The research and development of key technologies and petroleum geology in China’s onshore source rock strata over equipment, such as the Wufeng-Longmaxi Formation the next 15 years. If the sub-surface projects associated with marine deep shale gas volume fracturing technology and two major ongoing underground projects based on artificially rotary steering tools, must be accelerated to support stimulated fractures in the oil and gas reservoirs, which involve increase in production capacity. in situ conversion processing of organic-rich shale into oil • To establish a three-dimensional well pattern, a reason- and gas and underground coal gasification, are successful, the able amount of well spacing and a production system strategic position of oil and gas source rocks will become even template for marine shale gas blocks is required along more important. with improved reserve production and block recovery for Acknowledgement overall scale-benefit development. Coal-bed methane This work was supported by the National High Level Special Talent Support Plan (Fourth Batch), and the • The production and transformation tests must be con- China National Science and Technology Major Project (No. sistent, as they promote stable production and increase 2016ZX05046). the production in the old blocks of low-yield and low- efficiency coal-bed methane. Conflict of interest • The development of new production areas in the coal- bed methane development demonstration test will be The authors declare no competing interest. beneficial in low coal rank and complex structure areas. Open Access This article is distributed under the terms and conditions of • The production area must be expanded for growth in the Creative Commons Attribution (CC BY-NC-ND) license, which permits coal-bed methane production due to the comprehensive unrestricted use, distribution, and reproduction in any medium, provided the development of deep and coal-derived natural gas. original work is properly cited. • The construction of the underground coal gasification test References areas in the Ordos, Junggar, and Erlian basins is necessi- Cai, J., Hajibeygi, H., Yao, J., et al. Advances in porous media tated, along with the exploration of the formation theory science and engineering from InterPore2020 perspective. and core technical equipment required for underground Advances in Geo-Energy Research, 2020, 4(4): 352-355. coal gasification. Chen, X., Fan, J., Zhang, Y., et al. 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