Unconventional Petroleum Sedimentology: A Key to Understanding Unconventional Hydrocarbon Accumulation

Caineng Zou , Zhen Qiu , Jiaqiang Zhang , Zhiyang Li , Hengye Wei , Bei Liu , Jianhua Zhao , Tian Yang , Shifa Zhu , Huifei Tao , Fengyuan Zhang , Yuman Wang , Qin Zhang , Wen Liu , Hanlin Liu , Ziqing Feng , Dan Liu , Jinliang Gao , Rong Liu , Yifan Li

Engineering ›› 2022, Vol. 18 ›› Issue (11) : 62 -78.

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Engineering ›› 2022, Vol. 18 ›› Issue (11) :62 -78. DOI: 10.1016/j.eng.2022.06.016
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Unconventional Petroleum Sedimentology: A Key to Understanding Unconventional Hydrocarbon Accumulation
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Abstract

The commercial exploitation of unconventional petroleum resources (e.g., shale oil/gas and tight oil/gas) has drastically changed the global energy structure within the past two decades. Sweet-spot intervals (areas), the most prolific unconventional hydrocarbon resources, generally consist of extraordinarily high organic matter (EHOM) deposits or closely associated sandstones/carbonate rocks. The formation of sweet-spot intervals (areas) is fundamentally controlled by their depositional and subsequent diagenetic settings, which result from the coupled sedimentation of global or regional geological events, such as tectonic activity, sea level (lake level) fluctuations, climate change, bottom water anoxia, volcanic activity, biotic mass extinction or radiation, and gravity flows during a certain geological period. Black shales with EHOM content and their associated high-quality reservoir rocks deposited by the coupling of major geological events provide not only a prerequisite for massive hydrocarbon generation but also abundant hydrocarbon storage space. The Ordovician–Silurian Wufeng–Longmaxi shale of the Sichuan Basin, Devonian Marcellus shale of the Appalachian Basin, Devonian–Carboniferous Bakken Formation of the Williston Basin, and Triassic Yanchang Formation of the Ordos Basin are four typical unconventional hydrocarbon systems selected as case studies herein. In each case, the formation of sweet-spot intervals for unconventional hydrocarbon resources was controlled by the coupled sedimentation of different global or regional geological events, collectively resulting in a favorable environment for the production, preservation, and accumulation of organic matter, as well as for the generation, migration, accumulation, and exploitation of hydrocarbons. Unconventional petroleum sedimentology, which focuses on coupled sedimentation during dramatic environmental changes driven by major geological events, is key to improve the understanding of the formation and distribution of sweet-spot intervals (areas) in unconventional petroleum systems.

Keywords

Sedimentology / Black shales / Fine-grained sediments / Organic matter accumulation / Extraordinarily high organic matter / Unconventional petroleum

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Caineng Zou, Zhen Qiu, Jiaqiang Zhang, Zhiyang Li, Hengye Wei, Bei Liu, Jianhua Zhao, Tian Yang, Shifa Zhu, Huifei Tao, Fengyuan Zhang, Yuman Wang, Qin Zhang, Wen Liu, Hanlin Liu, Ziqing Feng, Dan Liu, Jinliang Gao, Rong Liu, Yifan Li. Unconventional Petroleum Sedimentology: A Key to Understanding Unconventional Hydrocarbon Accumulation. Engineering, 2022, 18(11): 62-78 DOI:10.1016/j.eng.2022.06.016

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1. Introduction

Unconventional petroleum resources are abundant and widely distributed (Fig. S1(a) in Appendix A), represented by shale oil/gas, tight oil/gas, coalbed methane/oil, heavy oil, and oil sand [1]. The emergence of new production technologies (e.g., horizontal drilling and multi-stage hydraulic fracturing) has allowed the commercial exploitation and production of oil and gas from tight (lowpermeability) unconventional reservoirs that were unprofitable decades ago. In the United States, between 2009 and 2019, the annual production of shale gas and tight oil (including shale oil) increased from 1.4 × 1011 to 7.2 × 1011 m3 and from 3.2 × 107 to 3.9 × 108 t, respectively [2]. Outside North America, the exploitation of unconventional petroleum resources is the greatest in China. In 2020, China’s annual shale gas production exceeded 2.0 × 1010 m3 , and tight gas and tight oil production exceeded 4.5 × 1010 m3 and 3.0 × 106 t, respectively [3]. Although several technical challenges remain, the discovery of unconventional petroleum resources has fundamentally altered the global energy structure.

According to the US Energy Information Administration, the technically recoverable resources of shale gas and shale oil discovered in the continental United States are 2.1 × 1013 m3 and 3.3 × 109 t, respectively, which are mainly distributed over 22 shale plays [4] (Fig. S1(b) in Appendix A). Regarding geological conditions, these shale plays are distributed over the Canadian Shield and its periphery, and accumulated in foreland or cratonic basins (Table S1 in Appendix A). China has 35 organic-rich shale strata (intervals) spanning from the Mesoproterozoic to the Cenozoic [5]. These shale strata were deposited under drastically different environmental settings (marine, marine–continental, and lacustrine) in different types of sedimentary basins (Fig. S1(c) and Table S1 in Appendix A). Shale gas is one of the first unconventional resources exploited industrially in China, with an estimated technically recoverable resource volume of 2.2 × 1013 m3 , whereof the largest proportion (1.3 × 1013 m3 ) originates from marine shales [6]. Conversely, shale oil resources in China mainly comprise lacustrine organic-rich shale and mudstone. The estimated total mass of the technically recoverable resources of shale oil in China is between 7.4 × 109 and 3.7 × 1010 t.

Unconventional petroleum exploration and development mainly target certain sweet-spot intervals and areas[1–6]. The term ‘‘sweet-spot interval” refers to a stratigraphic interval enriched in unconventional oil and gas that can be economically exploited under current technical conditions. The sweet-spot interval typically accounts for a small portion of the total thickness of the shale strata[1–6], and its accumulation area in geography is called the ‘‘sweet-spot area” [6]. Accurately determining sweet-spot intervals (areas) for shale oil and gas has been historically difficult because their formation requires a suite of geological factors, including a high total organic carbon (TOC) content, porosity, gas or oil content, brittle mineral content, high strata pressure coefficient, and abundant lamination fissures and microfractures[6–7]. Therefore, improving the understanding of the accumulation mechanisms of unconventional oil and gas is critical to improve the accuracy of the prediction of the spatial distribution of sweet-spot intervals (areas) in any unconventional reservoir.

A prerequisite for the accumulation of unconventional petroleum in sweet-spot intervals (areas) is a source rock with substantial amounts of retained hydrocarbons[6–8]. The TOC content is the primary parameter when evaluating source rocks, and a threshold concentration of 0.5 wt% TOC characterizes mudstone as a hydrocarbon source rock in conventional petroleum geology [9]. However, shale oil sweet-spot intervals in immature to low-mature stages of organic matter (OM) are usually characterized by TOC ≥ 6.0 wt%, and shale gas sweet-spot intervals in high-mature to over-mature stages of OM are characterized by TOC ≥ 3.0 wt%[6,10]. OM-rich intervals with a TOC content ≥ 3.0 wt% were first proposed as extraordinarily high OM (EHOM) deposits [11], wherein OM enrichment is a fundamental factor controlling the formation and distribution of sweet-spot intervals (areas) of shale oil/gas. Similar to tight oil, oil-producing or sweet-spot intervals correspond to sandstones or carbonates with better reservoir properties than those of shales, and are within or immediately adjacent to organic-rich shales. The key to understanding unconventional hydrocarbon accumulation is resolving the mechanism of enrichment of the EHOM and the formation of coexisting high-quality reservoir rocks.

OM in sediments is mainly derived from primary producers, such as algae, in overlying surface waters. Under normal marine conditions, approximately 10% of the photosynthesized OM migrates from the photic zone and settles in the underlying sediment [12]. Upon the settling of OM at the sediment–water interface, active degradation by aerobic and anaerobic remineralization processes causes less than 1% of the photosynthesized OM to be retained and buried in sediments [13]. The enrichment of OM in sediments and the formation of EHOM deposits are thus likely controlled collectively by high primary productivity, anoxic bottom water conditions, and appropriate sedimentation rates [14], signifying that EHOM deposits only occur in specific depositional environments. For instance, large-scale EHOM deposits are usually closely related to several global or regional geological events, such as sea (or lake) level fluctuations (e.g., Ref. [15]), climate change (e.g., Ref. [16]), oceanic anoxic events (e.g., Ref. [17]), volcanic activity (e.g., Ref. [18]), and biotic events (e.g., Ref. [19]). Similarly, the formation of tight oil sweet-spot intervals (areas) is usually caused by a dramatic change in the sedimentary system associated with tectonic events (e.g., Ref. [20]), climate, sea level changes (e.g., Ref. [21]), gravity flow events (e.g., Ref. [22]), and other geological processes. Therefore, the formation of unconventional hydrocarbon sweet-spot intervals (areas) may occur from the coupling of sedimentation controlled by major geological events[6,23].

The quality of petroleum reservoir rocks is controlled by the original sediment composition and diagenetic overprinting. Diagenetic processes in sandstones and carbonates, including compaction, cementation, and recrystallization, also occur in fine-grained mudstones/shales. However, the diagenesis of organic-rich black shales involves a complex mass transfer, such as OM thermal maturation (e.g., Ref. [24]) and clay mineral transformation (e.g., Ref. [25]). The diagenetic evolutionary pathway of shales is controlled by shale lithofacies, which are fundamentally governed by depositional processes, and the environment because sedimentation provides the materials that diagenesis acts upon during burial. Integrating the deposition and diagenesis of unconventional reservoir rocks can help determine potential sweet-spot intervals (areas) in unconventional petroleum systems.

In this study, first, we review recent progresses in the understanding of depositional and diagenetic processes in the fine-grained sediments of unconventional petroleum systems. Subsequently, we discuss the role of geological events in controlling the formation of unconventional petroleum sweet-spot intervals (areas) by presenting four case studies: the Late Ordovician–early Silurian Wufeng–Longmaxi shale of the Sichuan Basin, the Middle Devonian Marcellus shale of the Appalachian Basin, the Late Devonian–Early Carboniferous Bakken Formation of the Williston Basin, and the Middle–Late Triassic Yanchang Formation of the Ordos Basin. This study aims to review relevant sedimentological data obtained from these shale strata to improve the understanding of unconventional hydrocarbon accumulations from a new perspective—unconventional petroleum sedimentology—and hopefully define new perspectives for the future exploration and development of unconventional petroleum resources.

2. Depositional processes in unconventional petroleum systems

2.1. OM-rich shales or mudstones

2.1.1. Deposition of shales or mudstones

Shales and mudstones are fine-grained sedimentary rocks composed of ≥ 50% particles with a grain size < 62.5 μm. Although they are both widely used terms for fine-grained terrigenous clastic rocks (Fig. 1), the word ‘‘shale” is commonly used in the petroleum industry. Nevertheless, the term ‘‘shale” should be used with caution because it implies fissility, usually a byproduct of weathering, which is not always observed in freshly exposed rocks [26]. Typically, mudstones contain components of varying grain sizes and compositions derived from weathering, primary production, and diagenesis [27]. Because of the large cryptic variation in grain size and sedimentary features, mudstones have historically been considered to be mostly deposited through suspension fallout under low-energy hydrodynamic conditions. The well-developed fine laminations (undisrupted by macrobenthos)[28,29] (Fig. 1), which are commonly present in many organic-matter-rich mudstones, have been used as key evidence to suggest that these sediments were deposited under persistently stagnant (and anoxic) bottom water conditions through the continuous suspension settling of mineral grains and OM (e.g., Ref. [30]). However, this simple model of mud deposition is being increasingly challenged.

Fig. 1. Some typical sedimentary structures in shale/mudstone. (a) Parallel lamination, small ripple lamination, Wufeng Formation, Ordovician, China; (b) parallel lamination, Marcellus shale, Devonian, southwestern Pennsylvania, USA [28]; (c) parallel lamination and ripple lamination, Bakken Formation, Mississippian, Montana, USA; (d) current ripple lamination, Qiongzhusi Formation, Cambrian, Sichuan Basin, China; (e) current ripple lamination, Yanchang Formation, Triassic, Ordos Basin, China; (f) current ripple cross lamination, Lucaogou Formation, Permian, Junggar Basin, China; (g) wave ripple lamination, Eagle Ford shale, Cretaceous, Texas, USA [29]; (h) lag layer consisting of fossil fragments and reworked pyrite (black-colored particles), Wufeng Formation, Ordovician, Sichuan Basin, China; and (i) lag layers of fossil fragments, Bakken Formation, Mississippian, Montana, USA.

Over the past few decades, a paradigm shift has occurred for clarifying the processes that govern the transport, deposition, and erosion of fine-grained sediments, owing to studies on mud transport and deposition from multiple aspects. The dispersal and accumulation of mud by several hydrodynamic processes (e.g., river floods and oceanic currents) in relatively energetic environments have been directly documented in many modern muddy shelves [31]. Mud in rivers and seas—regardless of salinity—is likely to form flocculated aggregates, termed ‘‘flocs” [32]. The transport and deposition of mud as flocculated aggregates have also been commonly observed in many modern mud-dominated depositional systems [33].

Advances in the understanding of the depositional processes of fine-grained sediments have also been complemented by findings from flume experiments. These findings indicate that flocculated muds can be transported in bedload and form floccule ripples at sufficiently strong flow velocities (~25 cm·s 1 at 5 cm flow depth), thereby facilitating the bedload transport of sand[32,34]. The increasing number of careful facies analyses of ancient mudstone successions has also revealed many sedimentary and biogenic features in mudstones. The common presence of low-angle ripplescale cross lamination and localized erosional features in mudstones indicates the influence of flows that carry sediment across the seafloor or lake bottom [35]. The wide range of sedimentary structures (e.g., current and wave ripple cross lamination, parallel lamination, and grading and loading structures) identified in mudstones indicate that the deposition of fine-grained sediments is subject to various types of gravity flows, such as turbidity currents, muddy hyperpycnal flows, storm-induced currents, tide-related currents, or a combination of two or more flow types [28,29,36– 39] (Fig. 1). Features such as erosional surfaces, lag layers (of detrital, biogenic, or diagenetic origin), and enriched early diagenetic products (e.g., carbonate concretions, pyrite nodules, and phosphatic particles) indicate erosion, hiatuses, or condensation within fine-grained sedimentary successions [22].

The accumulation of mud, as well as the facies variation in mudstones, is governed by the rate and characteristics of the mud supply and energy levels in the depositional environment. Recent multifaceted studies have proven that the model of suspension settling of mud under completely stable and anoxic conditions is an exception rather than the norm. In general, mud accumulation appears to be much more episodic and instantaneous than conventional considerations. Many mudstone successions, even those that are OM-rich, have been re-interpreted to have been deposited under the influence of bottom currents in oxygen-rich conditions after sedimentary facies analyses and petrographic studies (e.g., Ref. [40]).

2.1.2. OM accumulation

OM accumulation in sediments is mainly controlled by primary productivity, bottom water redox conditions, and sedimentation rate [14]. Primary productivity represents the rate of the photosynthetic conversion of solar energy to chemical energy stored in the plant biomass [41]. Bottom water redox conditions are typically determined between 0.5 and 1.0 m above the sediment–water interface [42]. Redox conditions can be categorized into oxic (> 2.0 mL·L 1 O2), dysoxic (2.0–0.2 mL·L 1 O2), suboxic (0.2– 0 mL·L 1 O2), anoxic (0 mL·L 1 O2), and euxinic (anoxic and free H2S) conditions[12,43]. However, suboxic conditions are generally considered to be anoxic. Thus, we subdivided the redox conditions into oxic, dysoxic, anoxic, and euxinic conditions. In principle, redox conditions in a water column are determined by physical and biogeochemical processes. Physical processes involve oxygen solubility and oxygen resupply from vertical mixing and horizontal ocean circulation (including bottom currents), and biogeochemical processes involve oxygen demand during OM decomposition. Although more than 90% of the OM derived from primary productivity can be decomposed under both oxic and anoxic conditions, anoxic environments preserve more residual OM than oxic environments because anoxic conditions can decrease the degradation rate of some metabolizable materials [42]. The gradual buildup of anoxic conditions can lead to a TOC content of 1–3 wt%, and a TOC content higher than 3–6 wt% mostly occurs because of primary productivity[11,42]. As elevated levels of primary productivity lead to a high in situ oxygen demand, marine anoxia is often accompanied by high primary productivity.

The sedimentation rates often control the dilution effect and decomposition time of OM in different bacterial zones. High sedimentation rates (e.g., > 100 cm per thousand years (kyr)) in carbonate rocks or sandstones dilute OM concentrations in sediments. In modern sediments, a strong correlation is often observed between high productivity and high sediment accumulation rate [42]. OM associated with high sedimentation rates is generally reactive. The decomposition of OM in sediments can rapidly consume oxygen in the porewater. Thus, rapidly accumulating sediments tend to have anoxic pore waters regardless of the redox conditions in the overlying bottom water [42]. Water column redox conditions play an important role in OM accumulation when sedimentation rates are < 100 cm·kyr 1 , especially at < 10 cm·kyr 1 . Comparing the effects of primary productivity and redox conditions on OM enrichment under low sedimentation rates is thus necessary. The most important factors for OM enrichment are enhanced primary productivity and suppressed OM decomposition.

2.2. Deposition of siltstones and fine sandstones

Another important type of unconventional hydrocarbon resource comprises siltstone and fine sandstone reservoirs; generally, tight sandstone reservoirs are defined as low-permeability sandstone reservoirs with an in situ matrix permeability to gas of less than 0.1 millidarcys (MD, 1 mD = 1 × 10 3 μm2 ), exclusive of the natural fracture permeability [44]. From a depositional perspective, tight sandstones can be formed by the co-deposition of sand and mud: The excessive muddy matrix fills the pore spaces, significantly decreasing the permeability of sandstones. Thus, tight sandstones may occur as immature sandstones deposited close to the source area or as muddy debris flows, hybrid flows, or hyperpycnal flows [45]. However, tight sandstone reservoirs have been found in a wide range of depositional environments (e.g., fluvial, shallow marine, and deep-water environments) [46]. This indicates that rather than specific depositional processes, sediment composition and depositional conditions exert a first-order control on subsequent diagenetic alterations, which further alters porosity and permeability characteristics and leads to the formation of tight sandstones. For example, tight sandstone reservoirs in the United States are generally clean sandstones deposited in high-energy depositional environments, wherein intergranular pores are occluded by authigenic cement rather than immature muddy sandstones [47]. The formation of tight sandstone reservoirs also requires relatively quiescent tectonics and the deposition of regionally continuous sandstones with low permeability. Later tectonic alterations, such as faulting and fracturing, as well as overpressure caused by hydrocarbon generation, can then enhance the quality of tight sandstone reservoirs [48].

Deep-water gravity flows represent one of the most important sediment transport mechanisms on a global scale [49] and can yield large, thick sand bodies of coarse-grained and associated fine-grained clastic sediments on submarine slopes, abyssal plains, and deep lakes [50]. Thick-bedded coarse-grained clastic rocks, such as tight sandstone reservoirs, are usually close to highquality source rocks. The hydrocarbons generated from the source rocks readily migrate to these sandstones and form high-yield lithologic reservoirs [50]. Fine-grained deposits produced by deep-water gravity flows are usually rich in organic material and are potential sweet-spot intervals (areas) for unconventional petroleum systems [50].

3. Diagenetic processes in unconventional petroleum systems

3.1. Evolution of OM and organic pores

OM in black shales is a source of oil and gas in conventional and unconventional petroleum systems[24,51]. Dispersed OM in black shales is composed of macerals of different origins[14,24,52]. During thermal maturation, oil-prone macerals, such as amorphous OM and alginite, first transform to pre-oil bitumen through a bituminization process with little or no migration involved and subsequently transform into oil and post-oil bitumen in the oil window and condensate-wet gas window[24,51,53]. The post-oil bitumen continues to transform into gas and pyrobitumen in the dry gas window. The generated oil can also secondarily crack into gas and pyrobitumen in the dry gas window[51,53]. However, gasprone macerals (e.g., vitrinite), inert macerals (e.g., inertinite), and zooclasts (e.g., graptolites) do not exhibit significant changes in morphology and occurrence during thermal maturation because of their low hydrocarbon generation potential[24,53]. These macerals can still be observed in high-maturity shales.

Secondary maceral solid bitumen becomes the dominant OM in black shales when the thermal maturity reaches the peak oil window maturity (vitrinite reflectance (Ro) 0.8%–1.0%)[24,51,53] (Fig. S2 in Appendix A). Solid bitumen occurs as a speckled, wispy, and interconnected OM. It occupies the interparticle spaces between mineral grains (Fig. 2) and intraparticle spaces within mineral grains and fossil shells, such as in foraminifera tests[54,55]. As solid bitumen was once liquid, it may form a threedimensional interconnected OM pore network with organic pores at high maturity, preserving methane and improving porosity[24,51].

Fig. 2. Photomicrographs (reflected white light and oil immersion) of pyrobitumen (red arrows) in black shales. (a) Wufeng–Longmaxi shale (Ro 3.07%). Sample from Changning County, Sichuan, China. (b) Marcellus shale (Ro 2.41%). Sample from Canastota, New York, USA.

In unconventional petroleum systems, OM-hosted pores are an important constituent of the pore network (Fig. 3) and can be the dominant pore type in some gas shales [56]. The gas content and methane adsorption capacity of gas shales are positively correlated with the TOC content, indicating that organic pores play an important role in shale gas preservation[6,57]. The development of organic pores is controlled by both thermal maturity and OM type, and their preservation is controlled by thermal maturity, OM content, shale composition, and pore pressure[53,56,58].

Fig. 3. Nano-scale organic pores in the Wufeng–Longmaxi shale, Sichuan Basin. Sample from Changning County, Sichuan, China. Organic pores in panel (a) are protected by a rigid framework composed of diagenetic quartz (Qtz). Organic matter in panel (b) has a spongy texture.

Organic pores in black shales may have primary or secondary origins. Primary organic pores occur within structured OM, such as cellular pores derived from the cell lumen of higher terrestrial plants[53,58]. Secondary organic pores are generally thought to form when gaseous hydrocarbons are generated and expelled from residual OM (solid bitumen or pyrobitumen) [51,53,55,56,58,59]. Secondary organic pores are more abundant in the gas window than in the oil window because ① gas generation and expulsion mainly occur in the gas window, and ② the migration of bitumen and oil in the oil window can infill newly formed organic pores and mask their presence[24,53].

3.2. Diagenesis of minerals and mineral-associated pores in shales

The brittle mineral content controls the fracturing effectiveness of shale and is essential for the commercial exploitation of shale oil and gas. As the main brittle component in organic-matter-rich shales, quartz not only has a detrital origin but also an authigenic origin[54,60] (Fig. 4). The dissolution and reprecipitation of siliceous biological skeletons have been proposed to explain the formation of microcrystalline authigenic quartz infill found in intergranular pores/biocavities in shales, such as the Chattanooga, Barnett, and Mowry shales in the United States [61]. The pressure dissolution of clastic quartz particles and the transformation of clay minerals can also lead to the extensive development of quartz overgrowth, as documented for Haynesville shale in the United States [62]. Therefore, authigenic quartz from the dissolution and reprecipitation of siliceous biological skeletons and transformation of clay minerals are the main mechanisms for the formation of authigenic quartz in shale oil and gas reservoirs.

Fig. 4. Typical authigenic quartz in shales/mudstones. (a) Radiolarian fossils in Longmaxi shale, Weiyuan, Sichuan Basin. (b) Sponge spicules (s) were replaced by quartz in Barnett shale, Texas, Fort Worth Basin. For classification purposes, dolomite replacement (d) is added to the bio-siliceous grain volume [54]. (c) Silica nanospheres (Si) and plate-like authigenic quartz in Woodford shale, Texas, Permian Basin [60]. (d) Micron-scale authigenic quartz (small red triangles) aggregates in Longmaxi shale, Weiyuan, Sichuan Basin.

Calcite cementation, replacement, and recrystallization are common in shale and mudstone [63] (Fig. S3 in Appendix A). Calcareous biological components (e.g., foraminifera); seasonal carbonate deposition formed by periodic algal blooms; and biological, chemical, and mechanical compaction typically provide the materials for calcite recrystallization during diagenesis (Fig. S3) [64]. Sparry calcite usually occurs in organic-matter-rich mudstones. It is mainly distributed at the edge of the OM laminae, indicating a close relationship between sparry calcite and hydrocarbon generation. The release of CO2 and organic acids during hydrocarbon generation causes the dissolution of carbonate minerals. The resulting pressure or concentration gradient can then induce transportation in the pores and fractures over a short distance via infiltration or expansion, thus precipitating granular carbonate minerals when the binding Mg2+ is removed from fluids [65].

Clay minerals are closely related to oil and gas generation and exploration. Acidic clay minerals have been demonstrated to have catalytic effects on hydrocarbon generation reactions of OM. Therefore, the diagenetic evolution of clay minerals and their interactions with OM have attracted extensive attention [66]. The transformation of smectite into illite through the illite/smectite (I/S) mixed layer is the most common type of clay mineral diagenesis; however, the underlying mechanism remains controversial [67]. Nevertheless, the temperature of each stage of smectite transformation to illite is generally consistent among studies. The transformation of smectite to the I/S mixed layer occurs at 70–95 °C [68], and approximately 95% of smectite can be converted to illite before the temperature reaches 200 °C [69]; this process is also controlled by the chemical composition of the interlayer solution, formation pressure, and time.

The primary sedimentary components control the diagenetic pathway of shale/mudstone and may affect the pore evolution. Although authigenic minerals formed in the early diagenetic stage can reduce porosity, they effectively inhibit mechanical compaction in later stages[68,70,71] (Fig. 5). Shales rich in quartz, particularly diagenetic silica, contain more organic pores than clayrich shales because the former can form a rigid silica framework that protects organic pores from mechanical compaction [72]. Dissolution pores and OM-hosted pores typically develop in the middle and late diagenetic stages. Additionally, under the influence of pressure and tectonic stress, fractures of different scales formed during diagenesis can store oil and gas and act as the main conduits for hydrocarbon migration, greatly improving the permeability of shale and mudstone reservoirs.

Fig. 5. Mineral, OM, and pore evolution during diagenesis[68,70,71]. Vol.: volatile; bit.: bituminous; anthrac.: anthracite; Opal-A: amorphous opal; Opal-CT: crystalline opal.

4. Sedimentological implications for unconventional hydrocarbon accumulation

The formation of unconventional hydrocarbon systems is the result of the depositional and diagenetic processes of finegrained sediments coupled with hydrocarbon accumulation. Depositional environments control OM enrichment (forming source rocks and shale oil/gas reservoirs) or the concurrent deposition of coarse-grained clastics and carbonates (forming tight oil/gas reservoirs). Diagenesis involves mineral alteration, reservoir pore– throat evolution, and the thermal maturation of OM. Hydrocarbon accumulation mainly refers to the formation and distribution of sweet-spot intervals (areas) in unconventional petroleum systems, and its prerequisites include large-scale hydrocarbon expulsion and migration, effective trapping of hydrocarbons, and sealing by roof and floor strata. Fundamentally, the sedimentological history controls the development of source rocks, reservoirs, sealed roof and floor strata, diagenetic evolution, and the hydrocarbon accumulation process, which collectively control the formation of unconventional petroleum systems[6,23].

Unconventional oil and gas exploration and development target sweet-spot intervals (areas) within shale strata, that is, the EHOM intervals and coexisting tight sandstone or carbonate intervals[6,11]. The formation of the EHOM and sealed strata is closely related to their depositional settings and is the result of sedimentation coupled with several global or regional geological events, such as tectonic activity, volcanic activity, sea level (lake level) fluctuations, climate change, water column anoxia, biotic mass extinction, and gravity flow events. The coupled spatiotemporal deposition of these geological events was recorded in detail during several major transition intervals in the Phanerozoic (Fig. S4 in Appendix A). Four typical examples of unconventional hydrocarbon reservoirs are discussed in the context of how coupled sedimentation and geological histories influence the formation of their sweet-spot intervals, as well as hydrocarbon accumulation.

4.1. Typical unconventional petroleum reservoirs

4.1.1. Late Ordovician–Early Silurian Wufeng–Longmaxi shale gas

From the Late Ordovician to the early Silurian, Wufeng– Longmaxi shale was widely deposited throughout the Yangtze Shelf Sea, spanning across 13 graptolite zones, during five stages [23] (Fig. 6). Although its total thickness generally exceeds 300 m, only the graptolitic black shales are excellent source rocks and the major target for shale gas exploration and development, with a sweet-spot area of approximately 2 × 104 km2 in south China [6] (Table S1). Stratigraphically, these graptolite shales (i.e., sweet-spot intervals) are mainly located at the bottom of the shale strata and comprise the most organic-rich intervals (TOC > 3.0 wt%) in the Wufeng and Long-1 member shales [23] (Fig. 6). These sweet-spot intervals are generally 10–40 m thick and have a high porosity (> 4.0%), gas content (> 3.0 m3 ·t 1 ), brittle mineral content (> 70%), and strata pressure coefficient (> 1.2), with relatively abundant lamination fissures and microfractures [6]. In 2021, the annual production of Wufeng–Longmaxi shale gas was 2.3 × 1010 m3 , and the cumulative proven reserves of shale gas were over 2.7 × 1012 m3 , making China the largest shale gas producer outside North America.

Fig. 6. Major geological events during the Ordovician and Silurian transition and characteristics of shale gas sweet-spot intervals for Wufeng–Longmaxi shale in the Sichuan Basin, south China (modified from Ref. [23]). Ma: million year; KB: Kuanyinchiao bed; LOMEI: Late Ordovician mass extinction interval.

4.1.2. Middle Devonian Marcellus shale hale gas

The largest unconventional gas-producing formation worldwide, the Marcellus shale, was deposited during the Middle Devonian in an asymmetrical northeast-trending Appalachian foreland basin in southeastern Laurentia (Fig. 7[73–81]). It contains three members: Union Springs shale, Cherry Valley limestone, and Oatka Creek shale [74]. According to the TOC content, the Marcellus shale can be divided into two intervals: an organic-rich interval and an organic-poor interval. The organic-rich interval comprises the entire Union Springs shale and the lower part of the Oatka Creek shale, characterized by high TOC contents (ranging from 5 to 13 wt%, with an average of 7 wt%) [74,78–80]. The organic-poor interval comprises the upper part of the Oatka Creek shale, with a comparatively lower TOC content (ranging from 2 to 4 wt%, with an average of 3 wt%)[74,78,80]. The organic-rich interval, which primarily consists of black shales with a low clay mineral content, is the most important gas-producing part and a sweet-spot interval in the Marcellus shale. The reservoir space in these shales consists of interparticle pores and open fractures, with a porosity of 3%–10% [82], which is significantly affected by the OM content and thermal maturity [83]. In shale gas industrial production areas, the thickness of the sweet-spot intervals generally exceeds 15 m [84]. The Marcellus shale is the largest single natural gas-producing formation in the United States, with a daily production of over 6.7 × 108 m3 in 2020, accounting for more than onethird of total daily US production [2]. 

Fig. 7. Middle Devonian geological records and characteristics of shale gas sweet-spot intervals of Marcellus shale in Appalachian Basin, North America. Generalized stratigraphic column of the Marcellus Formation is from Ref. [74], with the star indicating the age of the volcanic ash [73]. Tectonic activity intensity is modified from Ref. [74]. Bottom water redox conditions are based on Ref. [75]. The global sea level change curve is from Ref. [76]. The relative sea level change curve is from Ref. [77]. The clastic input curve is from Ref. [74]. The data on TOC contents are collected from Refs. [74,78–81].

4.1.3. Late Devonian–Early Carboniferous Bakken tight oil/shale oil

The Bakken Formation was deposited in the Williston Basin, an elliptical intracratonic basin in western Laurentia, during the Devonian–Carboniferous transitional interval (Fig. 8 [19,21,48,85–96]. Stratigraphically, the Bakken Formation can be subdivided into four stratigraphic members in ascending order: the Pronghorn, Lower Bakken, Middle Bakken, and Upper Bakken members [86] (Fig. 8). Of these four members, the Lower and Upper Bakken members have average thicknesses of 3 and 2 m, respectively [97]. However, the Lower and Upper Bakken members are world-class petroleum source rocks. They are characterized by organic-rich, massive to parallel-laminated black shales, with average TOC contents between 8 and 10 wt% [21,92,93,97].

Fig. 8. Late Devonian–Early Carboniferous geological records and characteristics of the tight oil/shale oil sweet-spot intervals in the Bakken Formation in Williston Basin, North America. Generalized stratigraphic column is modified from Ref. [86]. Tectonic event columns are based on Refs.[19,88,89]. Global sea level change curve is from Ref. [76]. Climate change information is from Refs.[19,93]. Bioturbation is redrawn from Ref. [91]; the width of bars represents the relative strength of bioturbation. Bottom water redox conditions are based on Ref. [89]. Data on TOC contents are collected from Refs.[21,92,93]. Data on porosity is collected from Refs. [48,94–96]. Distribution of sweetspot intervals is from Ref. [87]. Biodiversity affected by the Hangenberg Crisis is from Ref. [19]; crosses represent extinction, and the width of the bars represents the relative abundance of taxa. PD: present day; Iceh.: icehouse; DCB: Devonian–Carboniferous boundary.

The Bakken Formation is a representative unconventional hydrocarbon play in the United States that produces more than 1.6 × 105 tonnes of tight oil and 5.5 × 107 m3 of shale gas per day in 2020 [2]. Most of the hydrocarbon resources in the Bakken Formation were extracted from the Middle Bakken and Pronghorn members. The Middle Bakken member has an average thickness of 13 m [98]. Tight oil sweet-spot intervals within the Middle Bakken comprise clean fine-grained sandstones, sandy skeletal-lime grainstones, and microcrystalline dolomites, with an average matrix porosity of approximately 9%, and maximum up to 14% [48,87,94–96]. The estimated prospective area of the Bakken tight oil/shale oil sweet-spot area is over 3.4 × 104 km2 in the United States and more than 2.3 × 104 km2 in Canada [99] (Table S1).

4.1.4. Middle–Late Triassic Yanchang tight oil/shale oil

During the Triassic period, the Yanchang Formation was deposited in the Ordos Basin, a rectangular basin on the southwestern margin of the north China Block. The Yanchang Formation can be subdivided into ten members based on lithofacies associations, logging characteristics, and tuff marker layers: Chang 1–Chang 10 members with increasing stratigraphic depths. The Chang 7 member mainly comprises black shales and mudstones intercalated with siltstones and sandstones [23] (Fig. 9). Black shale intervals at the bottom of the Chang 7 member (Chang 73) are the sweet-spot intervals for shale oil and have high TOC contents (up to 30 wt%), with a thickness of approximately 30–60 m. Sandstones in the middle and upper parts of the Chang 7 member (Chang 72 and Chang 71), which were deposited by gravity flows, are sweet-spot intervals for tight oil (Fig. 9). Sweet-spot areas for tight oil and shale oil over approximately 1.06 × 104 and 2.60 × 104 km2 , respectively [100] (Table S1). In 2020, 9.31 × 105 t of tight oil was produced from the Chang 7 member [101]. In 2021, the cumulative proven reserves of tight oil in the Chang 7 member were over 1 × 109 t, and prospective shale oil resources were approximately 6 × 109 t [100].

Fig. 9. Major geological events during the Middle–Late Triassic and characteristics of tight oil/shale oil sweet-spot intervals of Yanchang Formation in Ordos Basin, north China (modified from Ref. [23]).

4.2. Geological events for EHOM accumulation

4.2.1. High nutrient influx promotes primary productivity

OM in sediments is derived from primary producers, either in the overlying surface waters or on land. In open marine environments, primary producers in the ocean surface are dominated by various types of phytoplankton, and the total biomass determines the OM input. A high nutrient supply fuels primary productivity and ultimately determines the degree of OM production in the water column [102]. The production of EHOM generally requires additional nutrient sources derived from enhanced weathering on land, upwelling of nutrient-replete deep-water masses, or hydrothermal fluids. These additional nutrient sources are closely related to important geological events, including tectonic activity, climate change, sea level (lake level) fluctuations, and volcanic activity[15,18]. The production of EHOM sediments associated with climate change controls the flux and pathways of terrestrial nutrients delivered to sedimentary basins. Regional climate change from arid to more humid conditions typically enhances continental weathering and runoff, resulting in an increased input of terrestrial nutrients that induce suitable conditions for phytoplankton blooms in surface waters. For example, EHOM deposits (TOC content up to 35 wt%) of the Lower and Upper Bakken were deposited during humid climate conditions, and those in the Middle Bakken with poor OM corresponded to semiarid climatic conditions with little rainfall[19,90] (Fig. 8).

Ancient OM-rich deposits associated with upwelling are widely distributed globally [21,103]. Passive upwelling driven by sea level rise supplied large amounts of nutrients and greatly increased primary productivity, leading to EHOM enrichment in the Lower and Upper Bakken shales. Indeed, large-scale sea level changes have been invoked as a driving mechanism for enhanced nutrient input via upwelling during the Devonian–Carboniferous transitional interval (Fig. 8). A study [21] demonstrated that a relatively rapid increase in sea level may have caused an estuary-like pattern of marine circulation between the intracratonic Williston Basin and open-ocean conditions, wherein nutrient-rich upwelling seawater was injected into the semi-enclosed basin at intermediate depths (100–200 m), leading to the proliferation of phytoplankton and high primary productivity.

Volcanic and hydrothermal activities, which are closely associated with tectonic events, are also important for EHOM accumulation (Figs. 6, 7, 9, and 10). Volcanic and hydrothermal activities can enhance primary productivity via two pathways: Volcanic ash deposition and hydrothermal fluids release large amounts of nutrients directly into seawater, and volcanic eruptions and hydrothermal activities release large amounts of CO2, resulting in high partial pressure of CO2 (pCO2), eventually inducing climate warming and intense terrestrial chemical weathering, accelerating the influx of nutrients into oceans and lakes [104–108]. The marine deposition in the EHOM interval in the Wufeng Formation on the Yangtze Shelf Sea is related to volcanic eruptions (Fig. 6). Volcanic ash was extensively deposited in the Yangtze Shelf Sea of south China during the Ordovician–Silurian transition [18,23,104,109,110]. Fertilization associated with volcanism during this period of active tectonics resulted in a significant increase in the content of bioavailable nitrogen and elemental nutrients (P, K, Si, Fe, and Zn) [104] in the Yangtze Shelf Sea. These processes enhanced the primary productivity and eventually formed EHOM deposits in the upper part of the Wufeng Formation (Fig. 6). The deposition of the EHOM interval at the bottom of the Marcellus shale in the Appalachian Basin further strengthened this relationship (Fig. 7).

Fig. 10. Schematic of a typical sedimentary route for unconventional petroleum systems. Nutrients supplied by three sources—rivers, wind, and upwelling—are the key determinants of primary productivity in surface waters. Volcanism and hydrothermal activities can enhance the influx of nutrients into oceans and lakes. Anoxic or even euxinic bottom waters are favorable for the preservation of EHOM. Rapid or sudden changes in the sedimentary system associated with tectonic events, climate and sea level changes, and subaqueous gravity flow activity yield juxtaposed reservoirs. Many organic pores generated during the thermal maturation of the EHOM deposit provide abundant reservoir space for hydrocarbons. Abundant intergranular pores developed in the silty laminae associated with short-term geological events significantly improve the reservoir quality of mudrocks. Scanning electron microscopy images are from Yanchang shale (left) and Wufeng–Longmaxi shale (right) in China.

The EHOM interval (TOC up to 30 wt%) of the Yanchang Formation in the Ordos Basin was deposited in an ancient lacustrine environment (Fig. 9 [23]), closely related to both volcanic eruptions and hydrothermal activity, and probably driven by the Qinling orogenic event [111]. The EHOM deposits in the Chang 7 member contain 150 ash layers, with a single layer being up to 1 m thick [112]. Hydrothermal activity during the Chang 7 depositional period was established by several petrological and geochemical characteristics, such as deposits of bedded chert and laminar ankerite, abnormally high abundances of Fe and Mn, and the heavy sulfur isotope composition of framboidal pyrite [111]. Hydrothermal input contributes to primary productivity via significant nutrient input (N, P, Fe, Si, and Zn). This can lead to the enhanced preservation of OM through the degassing of H2S and SO2, resulting in water column stratification with anoxia at depth [113]. Moreover, the peak volcanic and hydrothermal activity occurred during the Chang 73 sub-member depositional period, which corresponds well to the deposition of the EHOM [105] (Fig. 9).

Additionally, biotic extinction/radiation events may also be linked to increased primary productivity via the significant restructuring of marine/lacustrine ecosystems and their associated food webs [19]. Such an extinction event, the ‘‘Hangenberg Crisis,” was recorded in the Devonian–Carboniferous Bakken Formation. This extinction event affected both marine and terrestrial ecosystems throughout 100–300 kyr, resulting in the extinction of about 16% of all marine families and about 21% of all marine genera, as well as the global disappearance of Archaeopteris forests on land [19] (Fig. 8). Two significant second/third-order extinctions (Annulata Event and Dasberg Event) occurred in the lower and middle parts of Lower Bakken [114]. These extinction events were associated with surface seawater warming and increased nutrient supply caused by sea level rise, leading to the proliferation of primary producers and thus EHOM accumulation in the Lower and Upper Bakken formations in the Williston Basin [83,114]. The Late Ordovician mass extinction (ca. 445 Ma), the first of the ‘‘Big Five” Phanerozoic extinction events [115], resulted in the extinction of marine benthic animals and nekton species, and survivors, including some graptolites, flourished because of reduced competition and increased niche space [116]. Gradual global warming and the removal of higher predators during the extinction event were conducive to the widespread proliferation of plankton and algae, probably resulting in the widespread deposition of EHOM and graptolite-rich intervals in the lower part of the Longmaxi Formation [23] (Fig. 6).

4.2.2. Extensive anoxia facilitates OM preservation

OM enrichment is related to the high primary productivity of OM and the effective preservation of OM due to anoxic or euxinic bottom water conditions [117]. The Late Ordovician to Early Silurian interval witnessed two global anoxic/euxinic events (Fig. 6) that contributed to the global deposition of EHOM (North America, Europe, and south China). The first of these events occurred from the Late Katian to Early Hirnantian, mainly influenced by deepwater shelf settings, and the second event was related to a rapid sea level rise and the upwelling of deep euxinic waters from the mid-late Hirnantian to the Early Rhudanian, causing distribution throughout the shelf (including the shallow-water inner shelf) [17]. The deposition of the EHOM intervals of Wufeng–Longmaxi shale in the Yangtze Shelf Sea corresponded well to these two global euxinic events (Fig. 6). The EHOM interval in the Wufeng Formation during the first euxinic event has a relatively limited spatial distribution, whereas the EHOM interval related to the second euxinic event is spatially more extensive[17,23]. These two euxinic events also promoted nutrient element recycling (e.g., phosphorus) [118], thereby further enhancing primary productivity.

A globally widespread oceanic anoxic event also occurred during the Devonian–Carboniferous transition [19] and was recorded in the Lower Bakken black shales of the Williston Basin[89,99]. These shales are characterized by high OM content, enrichment of pyrite, and an absence of benthic fauna or related ichnofossils (Fig. 6). Similar black shales were also widely deposited in other contemporaneous basins in Europe, North America, southeastern Asia, North Africa, Russia, Thailand, and south China (e.g., Ref. [119]). Black shales covered about 21% of the global depositional area during the Late Famennian [120]. Although the cause of this oceanic anoxic event remains controversial, the widespread deposition of black shale corresponds well to a high sea level and may be associated with an expansion of anoxic water onto shelves owing to sea level rise under redox-stratified global ocean conditions [117].

Bottom water anoxia during the deposition of Marcellus EHOM was likely caused by multiple factors (Fig. 7). In the early stage of the deposition of the Marcellus Formation, bottom water anoxia was caused by the weakening or stagnation of vertical ocean circulation, which may be related to factors such as the semi-closed nature of the Appalachian epicontinental sea, rapid tectonic subsidence, sea level rise, seasonal temperature stratification of the water column, and the rain shadow effect [74,77,121]. Subsequently, primary productivity increased, owing to a massive input of terrestrial nutrients, increasing the sinking flux of OM. OM decomposition resulted in water column oxygen depletion, leading to reducing conditions in the water column and underlying sediment pore waters[52,77]. This phenomenon is also known as the ‘‘productivity–anoxia feedback” mechanism [15,122]. 

4.2.3. Low sedimentation rate reduces dilution of sediments

Enhanced clastic inputs and associated high sedimentation rates (> 100 cm·kyr 1 ) generally dilute the OM content in sediments. In the Marcellus shale, the terrigenous input gradually increased (based on increased Al and Ti contents), and the OM content decreased from the Union Springs member to the Oatka Greek member (Fig. 7). Most ancient OM enrichment occurs in environments with low sedimentation rates. The average sedimentation rates for marine EHOM sediments are much lower than 100 cm·kyr 1 : ranging from 0.2 to 0.4 cm·kyr 1 (average TOC = 2–8 wt%) in the Wufeng–Longmaxi Formation [5], 0.1 to 0.3 cm·kyr 1 (average TOC = 8–11 wt%) in the Bakken Formation [21], and 0.17 to 1.3 cm·kyr 1 (average TOC = 8–10 w%) in the basal Marcellus Formation [73,123]. The sedimentation rate of lacustrine EHOM sediments was close to 5 cm·kyr 1 based on the sedimentation rate of the Chang 73 black shale, yielding an average TOC concentration of 15 wt% [100,112]. Changes in sedimentation rates resulting from terrigenous clastic sediment inputs have a major impact on the dilution or condensation of OM, and their processes are mainly controlled by the coupling of major geological events, including tectonic activity, climate change, and sea level fluctuation.

4.3. Geological events that induced the formation of high-quality unconventional reservoirs

4.3.1. Dramatic changes in depositional environments promote the formation of tight reservoirs

The Bakken Formation and the Chang 7 member of the Yanchang Formation are representative strata of marine and lacustrine tight oil/shale oil, respectively. The features they have in common include closely coexisting source and reservoir rocks and oil accumulation without long-distance migration. Nearby sandstone and carbonate reservoirs have significantly higher matrix porosities than shale reservoirs and are the main oil-producing intervals [20,124] (Figs. 8 and 9). These reservoir rocks were deposited during dramatic changes in the depositional environment because of the coupling of sedimentology and geological events, such as climate cooling, rapid sea level fall, and subaqueous sediment gravity flow [97,125], providing abundant reservoir space for hydrocarbon accumulation [126].

The Bakken Formation comprises two world-class petroleum source rock members (i.e., the Lower and Upper Bakken) and two tight reservoir members (the Middle Bakken and Pronghorn members). Hydrocarbons generated from the Upper and Lower Bakken organic-rich shales primarily charged the Middle Bakken and— locally—the Pronghorn member [93] (Fig. 8). Fine sandstone, bioclastic grainstone, and microcrystalline dolostone intervals within the Middle Bakken member have high matrix porosity (mean value of about 9%, maximum of 14%) and are promising targets for tight oil exploration and development in the Bakken play [127]. Sedimentological studies indicate that the Upper and Lower Bakken were deposited under relatively deep marine anoxic conditions (> 200 m depth), whereas the Middle Bakken was deposited under shallow-water high-energy conditions (< 10 m depth) [124]. Dramatic changes in the depositional environment between adjacent members in the Bakken Formation are closely related to the climate-driven sea level fluctuation, reflected by lithofacies variations from deep-water fine-grained sediments (Lower Bakken member) with a low sedimentation rate (0.1 to 0.3 cm·kyr 1 ) to shallow-water coarse clastic deposits (Middle Bakken member) with a high sedimentation rate [97,124]. Subsequently, sea level rise resulted in the slow deposition of deep-water fine-grained lithofacies (Upper Bakken member) (Fig. 8). During the Devonian–Carboniferous transition, abrupt climate cooling (and sea level fall) may be linked to high organic carbon burial in the ocean (represented by widespread black shale deposition) and the proliferation of land plants, which enhanced chemical weathering and accelerated CO2 drawdown from the atmosphere [19]. Subsequent global warming (and attendant sea level rise) may be related to an increase in atmospheric CO2, linked to volcanic activity, increased carbonate weathering, and OM oxidation during sea level fall [128].

The formation of lacustrine tight reservoirs of the Chang 7 member in the Ordos Basin is related to gravity flow activities (Fig. 9). Some geological events, such as volcanic eruptions, earthquakes, and storm waves, can trigger gravitational destabilization and the sliding of earlier sediments to form subaqueous sediment gravity flows, which can transport large amounts of sediment into semi-deep and deep lakes to form sandstone reservoirs [6,125]. These deep-water sand bodies are laterally continuous and in close vertical proximity to EHOM shales. This pattern of source–reservoir contact can shorten the migration distance of oil and improve the efficiency of oil accumulation [6,129]. Although these gravity flow sandstone reservoirs are subject to strong diagenetic alteration (e.g., compaction and cementation) and represent typical tight reservoirs, they are relatively high-quality reservoirs with an average porosity of 4% 8% and permeability of 0.02–0.10 mD, which is significantly better than those of mudstones/shales with an average porosity less than 2.5% and permeability less than 0.01 mD [130]. Therefore, tight oil sweet-spot intervals within the Chang 7 member are dominated by these sandstones (Figs. 9 and 10), the formation and distribution of which are controlled by river-fed hyperpycnal flows and intrabasinal sediment gravity flows.

4.3.2. Evolution of EHOM improves shale reservoir quality

Shales, long regarded as hydrocarbon source rocks and seals, have become economic hydrocarbon reservoirs largely because of the abundance of pores in the OM. OM-hosted pores have irregular, bubble-like, elliptical shapes, and generally have sizes ranging from a few to several hundred nanometers (Fig. 3)[6,56]. These pores are widely developed in types II and I kerogen when the thermal maturity reaches an Ro of about 0.6% or higher, accounting for up to 40%–50% of the total porosity and significantly improving shale reservoir quality [56]. The porosity of the sweet-spot intervals with EHOM in shales is comparatively high because the volume of OM pores is negligible in other intervals with TOC contents less than 3.0 wt%. For example, the porosity (≥ 4.0%) of the sweet-spot intervals in Wufeng–Longmaxi shale is generally higher than that of other intervals[6,56]. The enrichment of OM is conducive to increasing OM pores when the TOC content is less than 6 wt% [72,131]. However, when the TOC content is too high, the development of OM pores is inhibited because the lower structural rigidity of sediments with high TOC contents leads to the collapse and closure of OM pores during compaction [72,131].

The OM in black shales plays an important role in controlling methane adsorption capacity and shale gas storage. Although the relationship between the total amount of methane adsorbed onto shale and its thermal maturity remains controversial, the adsorbed gas content in shales worldwide is directly proportional to their TOC content [132]. A stronger gas adsorption capacity of shale reservoirs implies a higher gas content, for example, the total gas content of the Wufeng–Longmaxi shale is positively correlated with its TOC content. Therefore, the evolution of the thermal maturity of the EHOM during diagenesis could improve the quality of shale reservoirs via two pathways: increasing total porosity and enhancing methane adsorption capacity.

4.3.3. Well-developed silica-rich laminae allow efficient development of shale reservoir

Shales usually exhibit heterogeneity and anisotropy, owing to their complex mineral composition and abundant lamina structure (Fig. 1[32,33]). According to their mineral composition, laminae in shales can be classified into four types: organic-rich, clay-rich, carbonate-rich, and silica-rich [133]. Silica-rich laminae are mainly composed of silt quartz and feldspar grains, also known as silty laminae (Fig. 10) [134]. The development of silty laminae improves the porosity and permeability of shale reservoirs and thus has a positive impact on the transport and exploitation of hydrocarbons [135]. Moreover, the high silica content makes the shale reservoir more brittle, which is conducive to the formation of complex fracture networks during reservoir fracturing reconstruction and improves the recovery efficiency of shale oil and gas [136,137].

Silty laminae in shales originate from biogenic and abiogenic sources. Biogenic silty laminae are closely associated with the proliferation of siliceous organisms, such as radiolarians and sponge spicules in surface waters, which is caused by an increase in nutrient input as a likely consequence of an increase in sea level or volcanic and hydrothermal activities [14]. A high abundance of siliceous organisms has led to the deposition of numerous organic-rich black shales worldwide (e.g. Refs. [24,136]). Conversely, abiogenic silty laminae represent short-term and relatively high siliciclastic mineral deposition rates associated with turbidity currents, hyperpycnal flows, or episodic inputs of volcanic ash [138]. Changes in the depositional setting and sedimentation rates accompanying these short-term geological events are conducive to the development of laminae in shales/mudstones. Thus, the response of geological events in sediments may improve the shale reservoir quality through various mechanisms, including increased organic pore development, reservoir adsorption capacity, reservoir percolation capacity, and brittleness. All these processes have important implications for the accumulation and industrial recovery of unconventional oil and gas.

5. Conclusions

This study provides a holistic review of the deposition and diagenesis of fine-grained sediments in unconventional petroleum systems and discusses the role of coupled sedimentation of major geological events in controlling the formation of unconventional petroleum sweet-spot intervals (areas) by considering four unconventional plays (with Ordovician to Triassic ages) from China and North America.

The following conclusions were drawn:

(1) The sweet-spot intervals (areas) of unconventional petroleum exploration and development are generally composed of EHOM deposits (TOC ≥ 3 wt%) or adjacent tight sandstone and carbonate rocks. EHOM deposits in unconventional petroleum systems provide not only a prerequisite for the massive generation and accumulation of hydrocarbons by OM enrichment during deposition but also abundant storage space for hydrocarbons by the evolution of OM-hosted pores and dissolution pores of minerals during diagenesis. Although tight sandstone reservoirs, including siltstones and fine sandstones, can form in a wide range of depositional environments, they are closely associated with EHOM deposits in shale strata because of dramatic changes in the depositional environment, such as deep-water gravity flows, providing high-quality reservoirs for unconventional petroleum resources.

(2) The sweet-spot intervals of four typical examples of unconventional hydrocarbon reservoirs, namely the Wufeng–Longmaxi shale of the Sichuan Basin, Marcellus shale of the Appalachian Basin, Bakken Formation of the Williston Basin, and Yanchang Formation of the Ordos Basin, were identified and discussed to examine the roles of major geological events in their formation. The results established that their formation is the result of the spatial and temporal coupled sedimentation of global or regional geological events, such as tectonic activity, sea level (lake level) fluctuation, climate change, bottom water anoxia, volcanic activity, biotic mass extinction or radiation, and gravity flows during key geological transitional periods.

(3) The coupled geological events collectively provide the fundamental geological settings for EHOM enrichment, including a high nutrient influx, enhanced bottom water anoxia/euxinia, and suitable sedimentation rate. Increased nutrient influx is mainly derived from enhanced weathering on land, the upwelling of nutrient-replete deep-water masses and hydrothermal fluids closely related to tectonic activity, climate change, sea level (lake level) fluctuations, or volcanic activity, which promote the proliferation of primary producers and further increase primary productivity. Extensive bottom water anoxia/euxinia facilitates OM preservation by decreasing OM decomposition. Suitable sedimentation rates controlled by the coupling of major geological events, including tectonic activity, climate change, and sea level fluctuations, are favorable for the enrichment of OM as they reduce the dilution of sediments.

(4) Coupled geological events provide favorable depositional environments for the formation of high-quality unconventional reservoirs. Dramatic environmental changes driven by major geological events, such as deep-water gravity flows, are conducive to the deposition of high-quality tight oil/gas reservoirs closely associated with EHOM deposits in shale strata. The deposition of EHOM intervals related to geological events also promotes shale oil/gas reservoir quality through the evolution of OM-hosted pores and dissolution pores of minerals during diagenesis. In addition, welldeveloped silica-rich laminae in shales related to the proliferation of siliceous organisms or bottom currents can cause shale reservoirs to become brittle, allowing the efficient development of shale oil/gas.

(5) Unconventional petroleum sedimentology, which focuses on coupled sedimentation during dramatic environmental changes driven by major geological events, is key to improve the understanding of the formation and distribution of unconventional hydrocarbon sweet-spot intervals (areas). Further research should aim to define new perspectives for the future exploration and development of unconventional petroleum resources.

Acknowledgments

This work was jointly supported by the Scientific Research and Technological Development Programs of CNPC (2021yjcq02 and 2021DJ2001). We thank Germán Otharán and anonymous reviewer(s) for helpful reviews and Editors Shasha Zhao and Lin Zhu for constructive comments.

Compliance with ethical guidelines

Caineng Zou, Zhen Qiu, Jiaqiang Zhang, Zhiyang Li, Hengye Wei, Bei Liu, Jianhua Zhao, Tian Yang, Shifa Zhu, Huifei Tao, Fengyuan Zhang, Yuman Wang, Qin Zhang, Wen Liu, Hanlin Liu, Ziqing Feng, Dan Liu, Jinliang Gao, Rong Liu, and Yifan Li declare that they have no conflicts of interest or financial conflicts to disclose.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.eng.2022.06.016.

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