Multi-source genesis of continental carbonate-rich fine-grained sedimentary rocks and hydrocarbon sweet spots
School of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, China
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Received: 2020-08-14 Online: 2021-01-15
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This paper systematically discusses the multiple source characteristics and formation mechanisms of carbonate-rich fine-grained sedimentary rocks through the analysis of material source and rock formation. The hydrocarbon accumulation characteristics of carbonate-rich fine-grained sedimentary rocks are also summarized. The results show that the main reason for the enrichment of fine-grained carbonate materials in rift lake basins was the supply of multiple material sources, including terrestrial material input, formation of intrabasinal authigenic carbonate, volcanic-hydrothermal material feeding and mixed source. The development of carbonate bedrock in the provenance area controlled the filling scale of carbonate materials in rift lake basins. The volcanic-hydrothermal activity might provide an alkaline fluid to the lake basins to strengthen the material supply for the formation of carbonate crystals. Authigenic carbonate crystals induced by biological processes were the main source of long-term accumulation of fine-grained carbonate materials in the lake basins. Carbonate-rich fine-grained sedimentary rocks with multiple features were formed through the interaction of physical, biochemical and chemical processes during the deposition and post-deposition stages. The source and sedimentary origin of the fine-grained carbonate rock controlled the hydrocarbon accumulation in it. In the multi-source system, the types of “sweet spots” of continental shale oil and gas include endogenous type, terrigenous type, volcanic-hydrothermal type and mixed source type.
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JIANG Zaixing, KONG Xiangxin, YANG Yepeng, ZHANG Jianguo, ZHANG Yuanfu, WANG Li, YUAN Xiaodong.
Introduction
Shale oil and gas are important replacement hydrocarbon resources, which commonly occur in fine-grained sedimentary rocks with particle size smaller than 62.5 μm[1,2]. Following the successful commercial development of marine shale gas in China and abroad, China has started theoretical research and tentative exploration of continental shale oil and gas since 2010, and has found abundant hydrocarbon resources in continental shale strata to date[3].
Different from marine shale, continental shale has unique characteristics such as unstable depositional environments, complex source materials, and diverse components[4,5]. Based on the source type, the fine particles in shale can be divided into terrigenous, endogenous, vol-canic, and ‘deep’ sources. The terrigenous-source material includes quartz, feldspar, and clay minerals formed by physical weathering of the parent rocks in the provenance region[1]. The endogenous-source material refers to authigenic minerals that precipitate directly from the intrabasinal lake water. The formation of such materials is largely related to the metabolism of biological organisms[6]. Volcanic-source material refers to the volcanic ash transported into the lake basins by wind. The deep- source material refers to the crystalline deposits from hydrothermal fluids upwelling along faults and/or blowing as springs. Volcanic and deep source materials commonly occur together in lake basins as a result of volcanic eruptions and are collectively known as volcanic-hydrothermal materials[7,8]. These fine-grained materials have undergone physical, chemical, and biological processes during depositional and post-depositional stages[9], leading to differentiation in their particle structure, accumulation patterns, coexistence with organic matter, and diagenetic pathways. Thus fine-grained sedimentary rocks formed by them have widely different physical properties and hydrocarbon accumulation capacities[1]. Understanding lithofacies types of fine-grained sedimentary rocks is the base and core of evaluating integrated source-reservoir[10,11]. Based on composition, the fine-grained sedimentary rock can be classified into carbonate-rich, silty, and clayey and mixed-source fine-grained sedimentary rocks[2].
Carbonate-rich fine-grained sedimentary rock, widely distributed in rift lake basins of China, is an important carrier of shale oil and gas[4,8,12]. The reasons of the deposition of fine-grained carbonate materials and the origins of fine-grained carbonate sedimentary rocks in continental lake basins haven’t been studied systematically so far, limiting further understanding on their lithofacies types and characteristics, and hindering the study on the mechanisms of hydrocarbon accumulation in carbonate-rich fine-grained sedimentary rocks. Based on previous understandings of our study, we examined systematically the forming mechanisms of multi-source fine- grained carbonate materials in continental rift lake basins and sorted the hydrocarbon accumulation types in carbonate-rich fine-grained sedimentary rocks in this work, in the hope to propel the discovery of more hydrocarbon resources in the carbonate-rich fine-grained sedimentary rocks in lacustrine rift basins of China.
1. Carbonate-rich fine-grained sedimentary rocks and continental shale oil and gas
Carbonate-rich fine-grained sedimentary rocks are defined as the fine-grained sedimentary rocks dominated by carbonate minerals[4], which are widely distributed in continental rift basins in eastern and western China (Fig. 1). Based on the authors’ database[13], the calcite-rich fine-grained sedimentary rocks are mainly distributed in the Eocene lacustrine strata of the Bohai Bay Basin, and the Oligocene lacustrine strata of the Nanxiang Basin. The dolomitic fine-grained sedimentary rocks are mainly distributed in the Eocene lacustrine strata of the Jianghan Basin and the Cangdong Sag of the Bohai Bay Basin as well as in the Middle Permian lacustrine strata of the Junggar and Santanghu Basin (Table 1). Oil and gas exploration in shale formations dominated by carbonate-rich fine-grained sedimentary rock was first started in the Henan Oilfield and has revealed good prospects. For example, the carbonate fine-grained sedimentary rock in third member of the Oligocene Hetaoyuan Formation in the Biyang Sag yielded high oil and gas production after fracturing[3]. Subsequently, shale oil and gas exploration in the carbonate-rich fine-grained sedimentary rocks was conducted in the Jiyang Depression[11] and in the Shulu Sag by the Huabei Oilfield Company[14]. In recent years, breakthroughs in shale oil and gas exploration in carbonate-rich fine-grained sedimentary rocks have been made successively in the Jimsar Sag [15], the Cangdong Sag[16], and the Qianjiang Sag[12], rendering the carbonate-rich fine-grained sedimentary rocks the major source of shale oil and gas in the continental rift basins of eastern and western China.
Fig. 1.
Fig. 1.
Carbonate content of petroliferous fine-grained sedimentary rocks in continental basins of China[13].
Table 1 Mineral compositions of shales in continental basins in China.
Basin | Sag | Formation | Mineral composition/% | |||||||
---|---|---|---|---|---|---|---|---|---|---|
Calcite | Dolomite | Clay | Quartz | Feldspar | Pyrite | Analcite | Evaporitic mineral | |||
Bohai Bay Basin | Dongying Sag[4, 13] | Upper part of 4th member and lower part of 3rd member of Eocene Shahejie Formation (Es3L & Es4U) | 37.5 | 10.3 | 20.3 | 23.1 | 6.3 | 2.5 | ||
Zhanhua Sag[4, 13] | Lower part of 3rd member of Eocene Shahejie Formation (Es3L) | 30.8 | 8.3 | 25.4 | 24.0 | 6.0 | 4.1 | 1.3 | ||
Shulu Sag[4, 13] | Lower part of 3rd member of Eocene Shahejie Formation (Es3L) | 54.3 | 18.3 | 14.3 | 10.3 | 1.5 | 1.3 | |||
Cangdong Sag[13] | 2nd member of Eocene Kongdian Formation (Ek2) | 8.0 | 25.9 | 14.2 | 20.5 | 23.7 | 0.6 | 6.8 | ||
Nanxiang Basin | Biyang Sag[4, 13] | 3rd member of Oligocene Hetaoyuan Formation | 17.4 | 10.4 | 28.9 | 19.5 | 18.8 | 2.8 | ||
Jianghan Basin | Qianjiang Sag[4] | 3rd member of Eocene Qianjiang Formation (Eq3) | 9.8 | 30.9 | 12.4 | 6.7 | 7.6 | 5.3 | 27.2 | |
Junggar Basin | Jimusaer Sag[13] | Middle Permian Lucaogou Formation (P2l) | 10.6 | 26.7 | 10.4 | 20.2 | 30.9 | 0.9 | 0.6 | |
Luanping Basin[13] | Lower Cretaceous Xiguayuan Formation (C1x) | 10.0 | 18.3 | 21.7 | 19.2 | 28.9 | 1.2 | 0.4 | 0.1 | |
Songliao Basin | Gulong Sag[13] | Upper Cretaceous Qingshankou Formation | 2.4 | 9.1 | 34.8 | 36.2 | 16.7 | |||
Ordos Basin[13] | 7th member of Upper Triassic Yanchang Formation (T3y7) | 0.7 | 57.0 | 22.5 | 15.0 |
2. Multi-sourced geneses of carbonate-rich fine-grained sedimentary rocks
2.1. Sources of fine-grained carbonates
The common occurrence of fine-grained carbonate sediments in continental rift basins is related to multiple sources of Ca2+, Mg2+, and CO32- [7,17]. Through this study, we find that the major factors for the large-scale development of fine-grained carbonate sediments in the continental rift basins of China include rock composition of the provenances, biochemical conditions of the water body in the basin, and volcanic-hydrothermal activity during the depositional period.
2.1.1. Terrigenous source
The carbonate accumulation in lakes depends on the amount of carbonate or calcium-rich rocks in the provenances or underground[17]. Limestone, dolomite, marble, basalt, and carbonatite all can release calcium ions through weathering and leaching, which can provide material base for crystallization of carbonate minerals[18]. If carbonate rocks prevail in the bedrock of provenance region, the basin will mainly be filled by carbonate-rich sediments. If carbonate and siliceous rocks jointly exist in the bedrock, the distribution of carbonate sediments in the basin will be affected by the type of terrigenous sources. If there is only a small amount of carbonate rocks in the provenance, sediments in the basin would only contain a small amount of carbonate minerals[17]. Most of the Eocene lacustrine strata in the Bohai Bay Basin overlie the basement of Cambrian-Ordovician marine carbonate rocks. The marine carbonate rocks exposed on the surface can supply carbonate debris and dissolved ions to the lake basin. For example, the Shulu Sag is a typical rift basin dominated by carbonate rock provenances-the Ningjin Uplift in the west and the Xinhe Uplift in the east[19]. Input of carbonate debris is an important source of carbonate sediments in the basin; as a result, a large amount of carbonate breccia and carbonate-rich fine-grained sedimentary rocks developed in the Es3L[20]. A large amount of terrigenous carbonate debris decompose into carbonate silt and mud during transportation and form carbonate-rich fine-grained sedimentary rocks in the deepwater (Fig. 2a, b). In addition, under warm and humid climates, the carbonate is supplied in the form of ions or colloids through chemical weathering, which is an another efficient mechanism for carbonate deposition in the lake[20]. As a result, in the Es3L of the Shulu Sag, carbonate materials are the dominant components of the fine-grained sedimentary rocks (Table 1). It should be noted that availability of carbonate bedrock in a provenance is not mandatory for development of carbonate-rich fine-grained sedimentary rocks in the basin; rather it mainly controls the development scale of the carbonate-rich fine-grained sedimentary rock.
2.1.2. Intrabasinal biogenic source
Intrabasinal biogenic source is an important source of authigenic carbonate in lacustrine basins. Carbonate minerals can be formed by evaporation, the chemomotive force and solution concentration required for calcite crystallisation do not control the calcite precipitation. Without the participation of organisms such as algae, it is difficult to form large-scale chalk deposits in lakes[21]. The planktonic algae and bacteria in the lake can reduce the partial pressure of CO2 in the water body through their metabolism, which increases the pH of the water body and makes the chemical equilibrium shift from HCO3- toward CO32-. They create a favourable microenvironment for calcite crystallisation[6,20]. Since the cell surface of bacteria is negatively charged, positively charged metal ions such as Ca2+ would be adsorbed on their surface[22], making the cytoderm the base for calcite nucleation[23]. Calcite formed in this way is called biologically-induced calcite. The polysaccharides in the extracellular polymer secreted by organisms, as catalysts, can weaken the bond between Mg2+ and H2O, reducing the energy required for dolomite crystallisation[24]. The microorganisms and the organic matter they made of are essential for dolomite crystallisation in lacustrine basins. Dolomite crystallisation resulting from an organic environment is also considered to be a form of biologically influenced precipitation[25]. During the carbonate rock genesis, authigenic carbonate minerals are closely related to the organisms and the organic matter they are made of. Such coexistence of inorganic and organic matter is also present in resulting sediments. For example, intracrystalline pores are formed in the interior of fine-grained carbonate crystals due to the degradation of residual organic matter[26]. In addition, organic films and organic filaments from the residual organisms surround the carbonate crystals (Fig. 2c). Organisms can also control biomineralisation through an organic matrix composed of organic macromolecules to form calcareous organisms, which would evolve into ultramicrofossils in special forms (Fig. 2d). Changes in the chemistry conditions of the water caused by intrabasinal biological activities are the main mechanism that causes the pre-existing Ca2+ and Mg2+ to form carbonate crystals.
2.1.3. Volcanic-hydrothermal source
Hydrothermal fluid accompanied by volcanic eruption could increase the CO2 content and alkalinity of the water body by introducing Ca2+ and Mg2+, creating a suitable environment for carbonate mineral crystallization[7]. Volcanic activity is common in continental rift basins due to their active tectonic activity. For example, a large amount of tuffaceous fine-grained clastic material has been found in the Jimsar Sag of Junggar Basin and Luanping Basin[27,28]. Volcanic-hydrothermal materials also provide material sources for the development of fine-grained carbonate sediments in continental rift basins. On one hand, the influx of hydrothermal fluids rich in Ca2+, Mg2+, Fe2+, and CO32- increases the ion concentration and temperature required for mineral crystallisation of the water near the spring or fault in a short time, thus directly leading to precipitation of carbonate minerals rich in Fe and Mg[29]. On the other hand, hydrothermal fluids diffuse around and increase the temperature of the surrounding water, promoting the growth of microorganisms, and providing substances for biochemical reactions related to biological activities in the water[30]. Influenced by volcanic activity, fine-grained sedimentary rocks with microcrystalline dolomite laminae developed in the Lower Cretaceous Xiguayuan Formation of the Luanping Basin (Fig. 2e).
Fig. 2.
Fig. 2.
The fine-grained carbonates formed from different sources.
(a) Well ST3, 3 816.86 m, the Es3L of Shulu Sag, terrigenous detrital carbonate particles; (b) Well ST3, 3 817.01 m, the Es3L of Shulu Sag, terrigenous dolomitic debris[20]; (c) Well ST1H, 4206.60 m, the Es3L of Shulu Sag, organic ribbon and biologically induced calcite[20]; (d) Well NY1, 3 333.71 m, the Es4U of Dongying Sag, calcareous nannofossils[4]; (e) Well LT1, 1 136.73 m, the C1x of Luanping Basin, microcrystalline dolomite; (f) Well LT1, 958.06 m, the C1x of Luanping Basin, tuffaceous microcrystalline dolomite.
2.1.4. Mixed source
Studying sources of fine-grained sediments in continental basins is a challenging task because different sources can exist in the same area at the same time, leading to the formation of sediments with mixed sources[31]. For example, sediments that comprise a mixture of terrigenous clastic and carbonate sediments, tuffs, hydrothermal minerals have been found. Such sediment combinations are characterised by multi-source sediments mixed within one layer, which is different from sediments formed by a single component from a single source. Taking the Luanping Basin as an example, authigenic microcrystalline dolomite and volcanic ash coexist in some of the laminae (Fig. 2f). Multi-source substances not only form a sediment combination but also may be mutually supplemental in genesis. For example, tuff can release Mg2+ through hydrolysis, which contributes to dolomite precipitation[27]. Mixed-source sediment combinations have been found in many continental basins, such as the combination of authigenic dolomite and authigenic albite in the Jianghan Basin[4] and the combination of terrigenous carbonate debris and authigenic calcite in the Bohai Bay Basin[20].
2.1.5. Other sources
Sea water transgressing into lacustrine rift basins is another source of fine-grained carbonate sediments[32]. It affects the salinity of water in the lake and brings about a large amount of ions (SO42-, Mg2+, etc.), nutrients, and marine organisms. In addition, during burial diagenesis, organic matter evolution may give rise to fluids rich in HCO3—, which can transform or have carbonate precipitated[2].
2.2. Formation of carbonate-rich sedimentary rocks
After entering the lake basin, multi-source fine-grained carbonate materials can form carbonate-rich fine-grained sedimentary rocks with different characteristics through interactions of physical, chemical and biological processes in the depositional and diagenetic stages[4].
2.2.1. Physical process
Physical processes refer to mechanical transportation in the depositional stage, including suspended deposition, bottom current, and density current deposition induced by waves, storms, and earthquakes[33,34,35]. For example, in the Shulu Sag, a large amount of terrigenous carbonate clastic material entered the lake basin in form of clastic flows[19] and then evolved into turbidite flows. The turbidites transported fine clastic materials into the deep area of the lake basin, which were deposited into silty lamina with graded beddings (Fig. 3a, b). Clay-sized carbonate mud was suspended in the water layer and precipitated to form laminated carbonate-rich fine- grained sedimentary rocks (Fig. 3c). The content and grain size of terrigenous fine-grained carbonate clasts in different laminae varied (Fig. 3d), reflecting the periodic change of terrigenous material supply. The original carbonate-rich fine-grained sedimentary rocks could be eroded, transported, and redeposited by underflow to form laminae with lenticular structures[36] (Fig. 3b). In addition, it has been found that the tectonic activity of the rift lake basin can cause earthquakes and lake seiche, resulting in the formation of thick turbidite layers due to liquefaction and resuspension, leading to rapid settlement in form of massive fine-grained sedimentary rocks[33] (Fig. 3e, f).
Fig. 3.
Fig. 3.
Carbonate-rich fine-grained rocks formed by physical processes.
(a) Well ST3, 3 817.90 m, the Es3L of Shulu Sag, laminated carbonate siltstone; (b) Well ST3, 3 797.77 m, the Es3L of Shulu Sag, laminated carbonate siltstone with graded bedding and lenticular structure; (c) Well ST3, 3 809.50-3 809.80 m, the Es3L of Shulu Sag, laminated carbonate mudstone; (d) Well ST3, 3676.60 m, the Es3L of Shulu Sag, laminated carbonate mudstone; (e) Well ST3, 3 897.30-3 897.60 m, the Es3L of Shulu Sag, massive laminated carbonate mudstone; (f) Well ST3, 3 997.31 m, the Es3L of Shulu Sag, massive laminated carbonate mudstone.
According to the genesis and sedimentary characteristics, the carbonate-rich fine-grained sedimentary rocks formed by physical processes can be divided into laminated carbonate siltstones, laminated marlstones, and massive marlstones (Fig. 3).
2.2.2. Biochemical process
The carbonate-rich fine-grained sedimentary rocks formed that way are characterised by the development of authigenic carbonate laminae. For example, in the Bohai Bay Basin, carbonate-rich fine-grained sedimentary rocks with regular laminae are characterised by a combination of microbial induced calcite laminae and clay laminae containing organic matter[38] (Fig. 4a, b). Formation of this rock is related to periodic change of material supply and sedimentation in the lake basin caused by climate and hydrological environment influences[20]. In the Eocene lacustrine strata of the Jianghan Basin, carbonate-rich fine-grained sedimentary rocks with high total organic carbon (TOC) values are presented by the com-bination of yellow dolomitic laminae and grey marl laminae (Fig. 4c). The genesis of the dolomitic laminae is considered to be related to microorganisms[12]. The supply of volcanic-hydrothermal materials can also lead to the enrichment of nutrients in the basin[7], resulting in algal blooms, biochemical reactions and the formation of authigenic carbonate lamina (Fig. 4d).
Fig. 4.
Fig. 4.
Carbonate-rich fine-grained rocks formed by biochemical and chemical processes.
(a) Well ST1H, 4 206.37 m, the Es3L of Shulu Sag, rhythmic laminated marlstone; (b) fluorescence photo of (a), organic lamina showing bright yellow light, calcite lamina showing yellow green light; (c) Well BY2, 2 816.49-2 816.59 m, lower part of Eq3 of Qianjiang Sag, rhythmic laminated dolomitic mudstone; (d) Well LT1, 1 241.40 m, the Xiguayuan Formation of Luanping Basin, rhythmic laminated dolomitic mudstone with dolomicritic laminae and tuffaceous laminae; (e) Well FY1, 3 332.64 m, the Es4U of Dongying Sag, rhythmic laminated marlstone with sparry calcite laminae; (f) Well FY1, 3 397.56 m, the Es4U of Dongying Sag, sparry calcite laminae and organic-rich clay laminae.
2.2.3. Chemical process
The chemical process of carbonate-rich fine-grained sedimentary rocks can occur in a special environment, such as strong evaporation and hydrothermal influx, resulting in supersaturation of the local ion concentration. Carbonate materials can be directly precipitated through chemical reactions, forming carbonate-rich fine-grained sedimentary rocks mainly composed of inorganic crystalline materials. For example, in the Eocene lacustrine salt strata of the Jianghan Basin, the dolomitic fine-grained sedimentary rock near evaporite has low organic matter content and evaporite minerals, such as glauberite and anhydrite. The chemical process caused by evaporation and concentration changes are possible factors for their formation[12].
Chemical processes are also the main mechanism of material transformation in the diagenetic stage. The HCO3- rich fluids can cause a change in the original carbonate materials[37]. During the thermal maturation of organic matter, the discharged organic acid can dissolve the original calcite laminae, resulting in recrystallisation of sparry calcite[10]. Carbonate-rich fine-grained rocks with sparry calcite laminae are formed through chemical processing (Fig. 4e, f).
2.3. Multi-source genetic model of carbonate-rich fine-grained sedimentary rocks
The multi-source material is the main supply for deposition of fine-grained carbonate materials in continental rift basins. Main continental rift basins in China often have provenance regions mainly composed of carbonate rocks, basalts, and other calcium-bearing bedrock in surrounding areas[19,39]. Physical and chemical weathering products of these rocks can provide basic materials for carbonate filling in the basins. Volcanic activities caused by tectonic evolution can further enhance or compensate the material supply of the provenance regions by providing alkaline fluids to the basin, resulting in the enrichment of Ca2+, Mg2+, and CO32- in the basins. In the Bohai Bay Basin, Jianghan Basin, Junggar Basin, and Luanping Basin, sediments related to volcanic activities are found in the formations of carbonate-rich fine-grained sedimentary rock[4,28-29,39-40]. Aided by microbial activities in the basin, endogenous authigenic carbonate materials can crystallise from the ionic materials introduced into the basin from land and volcanic activities. The formation of endogenous materials is a stable mechanism of carbonate material filling in rift lake basins, while terrigenous and volcanic-hy-drothermal sources are the basic guarantee for generation of this kind of material. These material sources are closely related with each other and form mixed material sources in the basin. The multi-source material supply also led to multiple depositional processes in the basins. Under the combined influence of physical, chemical, and biochemical processes, a variety of different types of carbonate-rich fine-grained sedimentary rocks were formed (Fig. 5). Just because of the multiple sources, multiple depositional processes, and their mutual influences, the carbonate-rich fine-grained sedimentary rocks formed in the rift lake basins have quick changes in material composition and sedimentary characteristics in space and multiple lithofacies types in coexistence, and thus strong heterogeneity, which further affect the generation and enrichment of shale oil and gas in the fine-grained carbonate rocks.
Fig. 5.
Fig. 5.
Multi-source genetic model of carbonate-rich fine- grained sedimentary rocks[4].
3. Types and characteristics of sweet spots in carbonate-rich fine-grained sedimentary rocks
Fine-grained sediments in rift lake basins are from multiple sources and have experienced complex depositional and diagenetic processes. Hence, although generally rich in carbonate minerals, carbonate-rich fine- grained sedimentary rocks in rift lake basins of different sources and origins differ significantly in material composition, sedimentary structure, and organic matter enrichment degree[4], which would further result in differences in reservoir quality, oil-bearing property, and hydrocarbon accumulation mechanisms[41]. Influenced by the supply of materials from multiple sources, the strata of carbonate-rich fine-grained sedimentary rock may have various other types of sedimentary rocks, such as siltstones, tuff, and other coarse clastic sedimentary rocks[20,27]. These sedimentary units, together with limy mudstone rich in fine-grained carbonate content, dolomitic mudstone, and carbonate siltstone, constitute main reservoirs of continental shale oil and gas. The sweet spots of continental shale oil and gas are characterised by diverse lithologies. According to the material sources, the sweet spots of continental shale oil and gas can be divided into four types: endogenous, terrigenous, volcanic-hydrothermal, and mixed source ones.
3.1. Endogenous type
The “sweet spot” of endogenous type refers to hydrocarbon accumulation in fine-grained sedimentary rock composed mainly of authigenic minerals. The petrographic carrier of this “sweet spot” is laminated carbonate-rich mudstone containing authigenic carbonate laminae. They mainly occur in the Bohai Bay Basin, Nanxiang Basin, and Jianghan Basin in eastern China.
Due to the low maturity of the continental shale reservoirs, the development of organic matter pores is limited[42]. Hydrocarbons are mostly accumulated in the intercrystalline and intracrystalline pores of inorganic minerals[43,44]. Moreover, as carbonate minerals have weak adsorption to hydrocarbons, the oil and gas in these reservoirs are more likely in free state[43,44]. In the Eocene lacustrine strata of the Bohai Bay Basin, authigenic calcite laminae have intercrystalline and intracrystalline pores, of which intercrystalline pores are filled with hydrocarbons (Fig. 6a). In addition, the sparry calcite laminae formed in the diagenesis stage can develop more interlaminar fractures, which together with intercrystalline pores and fractures make the migration and accumulation of hydrocarbons easier (Fig. 6b).
Fig. 6.
Fig. 6.
Reservoir spaces and hydrocarbon accumulation in the “sweet spot” of endogenous type.
(a) Well ST1H, 4 209.30 m, the Es3L of Shulu Sag, intercrystalline and intracrystalline pores in calcite[42]; (b) Well NY1, 3 464.89 m, the Es4U of Shulu Sag, interlaminar fractures between sparry calcite laminae; (c) Well ST1H, 4 207.89 m, the Es3L of Shulu Sag, organic matter-rich laminae[4]; (d) Well BY1, 3 126.32 m, lower part of the Eq3 of Qianjiang Sag, with scattered organic matter[4]; (e) Well FY1, 3 182.69-3 183.69 m, the Es3L of Dongying Sag, rhythmic laminated marlstone and rhythmic laminated marlstone with sparry calcite laminae, red arrows indicating the potential direction of hydrocarbon migration.
Controlled by the origins of authigenic carbonate laminae, organic matter is mostly enriched near or within the laminae (Fig. 6c, d)[20, 41]. The hydrocarbons discharged from their parent materials can directly enrich in the nearby authigenic carbonate laminae to form in situ self-sourced reservoirs. Affected by the HCO3--rich fluids during the diagenetic stage, original carbonate materials in local parts can be modified to form sparry calcite (Fig. 6c). Compared with the original carbonate laminae, sparry calcite laminae have better reservoir properties and can be favourable locations for shale oil and gas accumulation[45]. Hence, hydrocarbon migration and accumulation can also occur in the self-source and self-reservoir “sweet spot” due to changes in the material structure.
Based on the hydrocarbon accumulation model of the “sweet spot” of endogenous type, it can be concluded that the generation and migration of hydrocarbons are restricted to the interior of the lithofacies. The formation of the shale oil and gas “sweet spot” depends on the hydrocarbon generation potential and reservoir property of the carrier lithofacies. Taking the Es4U in the Well NY1 of the Dongying Sag as an example, this formation is mainly composed of carbonate-rich mudstone and clay-rich mudstone with abundant organic matter (Fig. 7). According to the free hydrocarbon content (S1) and oil saturation index (OSI), it was found that hydrocarbons were mainly concentrated in the carbonate-rich fine- grained sedimentary rock with obvious sparry calcite and dolomitization, this is because they have more matrix pores developed. In addition, due to the limited migration of hydrocarbons, the maturity of organic matter is also an important factor affecting the formation of this kind of “sweet spot”. With the increase of maturity indicated by Ro value, the hydrocarbon conversion rate (S1/(S1+S2)) increases significantly, this is why the lower section has higher oil content than the upper section. The identification and evaluation of endogenous “sweet spots” should not only take into consideration the potential of hydrocarbon generation and accumulation of lithofacies but also the thermal evolution degree of the organic matter. Furthermore, it is necessary to study in detail the evolution of fine-grained carbonates in depositional and diagenetic processes, because diagenesis plays an important role in the transformation of materials and improvement of reservoir properties in lithofacies.
Fig. 7.
Fig. 7.
Lithofacies, organic geochemistry and mineral composition of Es4U in Well NY1 of the Dongying Sag. S1—free hydrocarbon content, mg/g; S2—cracked hydrocarbon content, mg/g; OSI—oil saturation index.
3.2. Terrigenous type
The “sweet spot” of terrigenous type refers to hydrocarbon accumulation in fine-grained sedimentary rocks mainly composed of terrigenous clastic materials. Typical example is shale oil and gas accumulated in interbedded mudstones and siltstones in the Yanchang Formation of the Ordos Basin[46]. They are also found in formations where carbonate-rich fine-grained sedimentary rocks have been developed, such as the Zhanhua Sag and Shulu Sag[47].
A rift lake basin can develop abundant clastic sedimentary units, which are controlled by the input of terrigenous materials and mechanical transport. Compared to clay-rich fine-grained sedimentary rocks, these silty grains are more resistant to compaction, which results in preserving more intergranular pore space. The overall porosity and permeability of the lower part of Shahejie 3 Formation in the Shulu Sag is low owing to the influence of compaction[14]. Rhythmic laminated marlstones formed by biochemical process cannot accumulate hydrocarbons due to their poor reservoir properties. The development of “sweet spot” of endogenous type in the Shulu Sag is limited. However, abundant carbonate siltstones and breccias are deposited in the formation of Shulu Sag due to the abundant terrigenous material input[19]. These carbonate-rich clastic sedimentary units developed abundant intergranular pores and other forms of storage space between the clastic carbonate particles (Fig. 8), which greatly improve the reservoir quality of the compact reservoir.
Fig. 8.
Fig. 8.
Reservoir spaces in the “sweet spot” of terrigenous type[42].
(a) Well ST3, 3803.80 m, the Es3L of Shulu Sag, intergranular pores between clastic particles; (b) Well ST3, 3801.57 m, the Es3L of Shulu Sag, intergranular pores and fractures.
As the input of terrigenous materials made the preservation conditions of organic matter poorer, the carbonate-rich fine-grained sedimentary rock layers adjacent to the terrigenous clastic sedimentary units have lower organic matter abundances[47]. These fine-grained sedimentary rock layers are not effective source rocks. For example, the laminated carbonate-rich siltstone and limy mudstone formed by geologic event in the lower part of Sha 3 M in the Shulu Sag have lower TOC values (Fig. 9). One of the conditions for the formation of terrigenous type “sweet spot” is the development of migration channels connecting effective reservoir rocks and source rocks. A previous study showed that the lower part of the Eq3L in the Shulu Sag had high fractures in high density and mostly high angle. The fractures can connect the effective source rocks with reservoir rocks composed of clastic materials, making it possible for hydrocarbons to accumulate in the carbonate-rich siltstone and conglomerate to form “sweet spots” of terrigenous type (Fig. 9).
Fig. 9.
Fig. 9.
Lithofacies, fracture density and organic geochemistry of Es3L in Well ST1H of Shulu Sag[42].
Rlld—deep lateral resistivity; Rlls—shallow lateral resistivity.
3.3. Volcanic-hydrothermal type
The “sweet spot” of volcanic-hydrothermal type refers to hydrocarbon accumulation in the fine-grained sedimentary rocks mainly composed of materials derived from volcanic sources or hydrothermally derived crystalline materials. Typical examples of this type sweet spot are the Lucaogou Formation in the Jimusar Sag of Junggar Basin, NW China[15] and the Xiguayuan Formation in the Luanping Basin[28]. Volcanic eruption and associated hydrothermal activities can give rise to tuff and dolomitic mudstone in rift lake basins[30]. Tuff is a direct product of volcanic activity. Dolomite can be either a direct hydro-thermal precipitation product or an endogenous mineral precipitated in the biochemical process induced by hydrothermal fluid. The main reservoir space types of sedimentary units formed by volcanic-hydrothermal events are intergranular pores in tuff and intercrystalline pores between dolomite crystals (Fig. 10). In the Jimusar Sag, tuff and tuff-dolomite in the Middle Permian Lucaogou Formation have the highest oil content[27], and the sweet spots appear at the locations with a high content of tuff (Fig. 11). This is because volcanic-hydrothermal fluids are usually rich in nutrient elements, promoting biological growth and enrichment of organic matter after the geological event. As a result, the dolomitic fine-grained sedimentary rock formed in this background has better hydrocarbon generation potential. Compared with the terrigenous type, the “sweet spot” of the volcanic-hydrothermal type has reservoir and source rocks more closer in space or lithology, and thus featuring integrated source and reservoir (Fig. 11). Moreover, the transition metal elements in volcanic eruption materials have catalytic effect on hydrocarbon generation of the source rock[48], which can offset the low maturity of continental source rock, facilitating hydrocarbon expulsion of the source rock earlier.
Fig. 10.
Fig. 10.
Reservoir spaces in the “sweet spot” of volcanic-hydrothermal type.
(a) Well LT1, 992.20 m, the C1x of Luanping Basin, hydrocarbons filling intergranular pores between tuff particles; (b) Well LT1, 958.06 m, the C1x of Luanping Basin, hydrocarbons filling the dolomite intercrystalline pores.
Fig. 11.
Fig. 11.
Comprehensive geological histogram of the Lucaogou Formation in the Jimsar Sag[27].
3.4. Mixed source type
The “sweet spot” of mixed source type refers to hydrocarbon accumulation in fine-grained sedimentary rocks composed of material from multiple sources. Compared with the “sweet spots” formed under the control of a single source, the “sweet spot” of the mixed source type lacks a dominant material source during the depositional stage, and fine-grained materials of different sources generally mix together. The “sweet spot” of volcanic-hydrothermal type may contain endogenous materials formed by biochemical process besides tuff formed by volcanic eruption, hydrothermal precipitation formed by chemical process. It can be regarded as the “sweet spot” of mixed source type. In addition, the terrigenous detrital and endogenous carbonates can mix and deposit into another subtype of the “sweet spot” of mixed source type. In the Es3L of the Shulu Sag, the original rhythmic laminated marlstone was eroded and transported by debris flow, and the intrabasinal clasts deposited together with terrigenous detrital carbonates to form mixed-source sediments (Fig. 12a). The hydrocarbon released by the parent material in the intrabasinal sedimentary rock could directly transport to the terrigenous dolomitic particles and fill in the intercrystalline pores to form the “sweet spot” of mix-source type (Fig. 12b, c). Compared with the “sweet spot” of terrigenous type, the mixed source type has lower requirement on the development of structural fractures.
Fig. 12.
Fig. 12.
Reservoir lithofacies of the “sweet spot” of mixed source type[42].
(a) Well ST2X, 3 724.55-33724.94 m, the Es3L of Shulu Sag, mixed-source carbonate-rich sedimentary rocks; (b) Well ST2X, 3 722.71 m, the Es3L of Shulu Sag, detrital carbonates and abnormal pressure fractures; (c) fluorescence photo of (b).
4. Key issues and thoughts of the hydrocarbon accumulation in carbonate-rich fine-grained sedimentary rocks
Carbonate-rich fine-grained sedimentary rocks are important hydrocarbon reservoirs of continental shale oil and gas in China. With growing exploration and development, new perspectives of exploring shale oil and gas have been discovered, and some key problems have been come up. Based on the multi-source genesis of carbonate-rich fine-grained sedimentary rocks and the characteristics of hydrocarbon accumulation, the relevant problems are summarised as follows.
(1) Multi-source supply and multi-depositional dynamics lead to formation of various types of carbonate-rich fine-grained sedimentary rocks. These rocks are different from claystones in terms of their material composition and are also different from carbonate rocks in terms of their depositional models. At present, the concept and types of carbonate-rich fine-grained sedimentary rocks are still vague; thus, the definition, classification and sedimentary environment of carbonate-rich fine-grained sedimentary rocks is a fundamental problems that needs to be addressed.
(2) The wide distribution of carbonate-rich fine-grained sedimentary rocks in continental basins indicates that the formation of these rocks is controlled by many factors. To date, there have been no systematic studies on the differences in sedimentary structure, material composition and diagenetic pathways of carbonate-rich fine-grained sedimentary rocks controlled by various material sources, lake water chemistries, basin tectonics, and climate conditions.
(3) Carbonate-rich fine-grained sedimentary rocks and other genesis-related fine-grained facies have multi-hydrocarbon enrichment models. Different types of “sweet spots” have different requirements for the accumulation conditions of oil and gas. During the evaluation of “sweet spot” of shale reservoirs, we should carefully consider different types of shale reservoirs, perform targeted research correctly identifying different types of “sweet spot” of continental shale oil and gas, and finally tailor a suitable development plan for the specific accumulation.
5. Conclusions
Carbonate-rich fine-grained sedimentary rocks are important components of fine-grained sedimentary rocks in continental rift basins of China, and are important hydrocarbon reservoirs for continental shale oil and gas as well.
The source of fine-grained carbonate materials in carbonate-rich fine-grained sedimentary rocks mainly includes terrigenous input, intrabasinal biochemical crystallization, volcanic-hydrothermal material input and mixture of them. Supply of multi-source materials is the main reason for the enrichment of fine-grained carbonate in continental rift basins. The interactions of physical, chemical and biological processes lead to the multi- source fine-grained carbonate sediments and form different types of lithofacies.
In the multi-source system of continental basins, the hydrocarbon accumulation of carbonate-rich fine-grained sedimentary rocks and their genetically related fine- grained facies are largely controlled by the material source and forming mechanism. Such kind of fine- grained sedimentary rocks in continental rift basins are typified by many types, which includes the endogenous, terrigenous, volcanic-hydrothermal, mixed-source types.
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