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Article

Distribution Characteristics and Hydrocarbon Significance of Deep-Water Fine-Grained Sedimentary Rocks in the Steep-Slope Zone of a Graben Lake Basin: A Case Study of Es3l sub-Member in the Jiyang Depression, Bohai Bay Basin, China

by
Qi Zhong
1,2,3,
Wangpeng Li
1,2,4,
Hui Huang
5,
Jianhui Jiang
3,
Jianguo Zhang
1,2,3,*,
Pinxie Li
3,
Yali Liu
1,2,4,
Jiabin Wu
3,
Fenghua Wang
6,
Bintian Tan
6 and
Ruo Jia
6
1
State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Efficient Development, Beijing 102206, China
2
Sinopec Key Laboratory of Shale Oil/Gas Exploration and Production Technology, Beijing 102206, China
3
School of Energy Resources, China University of Geosciences, Beijing 100083, China
4
Petroleum Exploration and Production Research Institute, SINOPEC, Beijing 102206, China
5
Research Institute of Petroleum Exploration and Development, Shengli Oilfield Company, SINOPEC, Dongying 257015, China
6
Shengli Oil Production Plant, Shengli Oilfield Company, SINOPEC, Dongying 257000, China
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(9), 882; https://doi.org/10.3390/min14090882
Submission received: 3 July 2024 / Revised: 13 August 2024 / Accepted: 23 August 2024 / Published: 29 August 2024
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Abstract

:
The high exploration and development production capacity of the Jiyang Depression, Bohai Bay Basin, China in the early stage confirms the huge exploration and development potential of shale oil in the study area. Due to the complexity of the depositional mechanism in the study area, the distribution law of fine-grained sedimentary rocks is not well understood, which restricts further exploration breakthroughs. This paper comprehensively observes rock cores and thin sections, combines mineral components, Rock-Eval pyrolysis, rock-cutting logging and logging data to classify lithofacies, and clarifies the distribution law of various lithofacies. The research results show that, according to lithological characteristics, various lithofacies origins are classified into three categories: terrigenous, mixed, and endogenous sources, and six lithofacies types are distinguished: terrigenous low-organic-matter massive siltstone (LF1), terrigenous low-organic-matter massive mudstone (LF2), mixed-source medium-organic-matter massive mudstone (LF3), mixed-source medium-to-high-organic matter laminated-massive mudstone (LF4), mixed-source medium-to-high-organic-matter laminated mudstone (LF5), and endogenous-sourced medium-to-high-organic matter laminated limestone (LF6). The distribution of lithofacies in plane is symmetrical in the east–west direction and is characterized by a banded distribution; the distribution in profile shows a stable depositional process and a continuous depositional sequence. The various lithofacies depositional models have been summarized; the terrigenous input from the northern steep-slope zone has influenced the hydrodynamic conditions of the lake basin, significantly affecting the lithofacies depositional variations from the steep-slope zone to the deep-sag area. The geological evaluation of each lithofacies has been conducted; LF1 + LF4 + LF5 are classified as Class I—target reservoirs for shale oil development, while LF3 + LF6 are considered Class II—favorable reservoirs. The result of the study provide a reference for the classification of fine-grained sedimentary-rock facies and distribution characteristics, and the evaluation of shale-oil-reservoir sweet spots in graben lake basins.

1. Introduction

Shale oil has become a hot spot for the exploration and development of unconventional oil and gas resources worldwide [1,2], and the oil and gas exploration field is gradually focusing on the exploration of unconventional fine-grained sedimentary-rock oil and gas reservoirs [3,4,5]. Recently, multiple exploration wells in the Bonan Sag of Ji’yang Depression in eastern China have tested and obtained industrial oil flows. The peak oil and gas equivalent of five horizontal wells exceeded 100 m3/d [6,7], confirming the enormous exploration and development potential of highly evolved shale oil in the Bonan Sag of the Paleogene. Bonan Sag is a faulted depression basin [8,9], with complex depositional mechanisms, and unclear understanding of lithofacies deposition processes and distribution models [10], limiting further exploration breakthroughs in the area.
As exploration deepens, the rapid changes in lithofacies in the study area, along with the increasingly prominent issues of vertical superposition and planar crossover, are becoming more pronounced [11]. Additionally, in the process of oil and gas exploitation, the entire rock mass is fractured, with vertical fracturing thicknesses reaching tens of meters, and horizontal well-drilling distances of several kilometers [12], highlighting the urgent need to resolve lithofacies distribution issues. Previous studies have addressed the above issues in the study area. They have conducted research on aspects such as lithofacies division of fine-grained sedimentary rocks [13], sedimentary environments [14,15], etc. Liu et al. (2022) further established a zonal distribution model of mudrock in the graben basin [6]. Cao et al. (2021) studied the characteristics of gravity flow deposition in the faulted basin of the area [11]. Zhang et al. (2021) summarized the lithofacies characteristics and depositional evolution of the Es3l Sub-Member in the study area [16]. Wang et al. (2019) studied the fine-grained sedimentary system of mud shale in the area [17]. Some scholars have also conducted research on various lithofacies combinations [18,19,20], oil-bearing properties, and mobility, among others. Previous studies have provided initial insights into the lithofacies characteristics of the study area. However, the understanding of the distribution patterns, superposition relationships, and depositional model of fine-grained sedimentary-rock lithofacies in the study area remains unclear, which hinders further evaluation of reservoir quality and prediction of favorable areas.
Therefore, based on core and thin-section observations, combined with material composition, Rock-Eval pyrolysis, cutting logging and logging data, this paper classifies fine-grained sedimentary-rock lithofacies, analyzes the causal mechanisms, depositional processes, and models of each lithofacies, clarifies the lithofacies distribution in profiles and plans, conducts geological evaluations of each lithofacies, identifies lithofacies types which are favorable for shale oil development, and provides references for further exploration breakthroughs in the Bonan Sag.

2. Geological Settings

Bohnan Sag is located in the west-central part of the Zhanhua Depression of the Jiyang Depression in the Bohai Bay Basin, covering an area of about 600 km2 [21], starting from the Chengdong Bulge in the north, neighboring the Guxi Depression in the east, the Sanhecun Sag in the south, and the Yihezhuang Bulge in the west, and transitioning in the form of gentle slopes to the Chenjiazhuang Bulge in the southwest [22] (Figure 1a), and it is a typical representative of the terrestrial graben-shaped lake basins of eastern China [16,23] (Figure 1b). From bottom to top, the sag successively developed the Paleogene Shahejie Formation, Dongying Formation, Neogene Guantao Formation, Minghuazhen Formation, and Quaternary Pingyuan Formation. Among them, the third member of the Paleogene Shahejie Formation is divided into lower, middle, and upper sub-sections [24,25]. The lower-third sub-member of Shahejie Formation (abbreviated as Es3l) is the target strata for the study area (Figure 1c). The depositional period of the Es3l sub-member was characterized by a humid climate, situated in a stable and enduring deep-to-semi-deep lacustrine depositional environment [26,27], and developed mudrock with thicknesses ranging from 200 to 1300 m (Figure 1c) [16].

3. Materials and Methods

This paper focuses on the most promising shale oil exploration in the Es3l sub-member of the Bonan Sag. A total of 96 wells were analyzed, with core and logging data collected. Key observations and analyses were conducted on the cores from the Es3l sub-member in 5 key mudrock-system core wells—BY 6, BYP 5, Y 1-1, Y 1-2, and YX 291. The continuous core lengths for these wells are 89 m, 71 m, 320 m, 300 m, and 359 m, respectively. Based on high-precision vertical sampling of the core, the analyzed data include observations of 695 cast thin sections, approximately 345 X-ray diffraction analyses, 180 total organic carbon (TOC) content analyses, and Rock-Eval pyrolysis analysis, among other related observations and analyses.
For thin-section preparation, firstly, samples of distinctly characterized rocks were collected, cut and roughly ground, thin-section adhesive was applied to the roughly ground rock samples, which were then pasted on a glass sheet, and the prepared thin sections were further ground and polished to obtain smooth surfaces for observation and analysis under a microscope.
X-ray diffraction analysis was performed using a D/max-2500 TTR system at Shengli Oilfield, SINOPEC (Dongying, China). The X-ray diffraction data were analyzed in subsequent diffractograms to identify and semi-quantitatively study the relative abundance (weight percent) of various mineral components, which is important in lithofacies delineation, determining the source of the material, and evaluating the brittleness of shale oil reservoirs. The TOC data were collected at the Research Institute of Exploration and Development of Shengli Oilfield Company, Sinopec Group (Dongying, China), and were statistically calculated from bulk rock samples, which were crushed and milled to less than 100-mesh powder. The TOC data mainly respond to the abundance of organic matter in the rocks.

4. Petrological Characteristics and Analysis of Lithofacies Genesis

4.1. Petrological Characteristics

4.1.1. Mineralogical Characteristics

Through statistical analysis of 637 X-ray diffraction data from multiple wells such as L 69, JYC 1, BYP 5, Y 1-1, and Y 1-2 in the Bonan Sag, it was found that the mineral composition of the fine-grained sedimentary rocks in the Es3l sub-member is relatively rich, mainly consisting of carbonate minerals (calcite + dolomite), felsic minerals (quartz + feldspar), and clay minerals, with an average total content exceeding 95%. The variation range of felsic minerals is 7%–61%, averaging 23.59%; the clay mineral content ranges from 1.9% to 46.6%, averaging 22.31%; the carbonate mineral content ranges from 5% to 79%, averaging 48.12% (Figure 2); detailed mineral percentage data are shown in Table 1. The content of pyrite ranges from 0% to 14%, averaging 2.06%, making it a secondary mineral; TOC content ranges from 0.46% to 7.45%, averaging 2.14% (Table 1).
Statistical analysis of Rock-Eval pyrolysis data indicates that the organic matter content in the Es3l sub-member in the Bonan Sag ranges from 0.46% to 7.45%, with an average of 2.14%, mainly distributed within the range of 2.12%–3.24% (Table 1).

4.1.2. Sedimentary Structural Characteristics

Depositional structures can reflect the genesis and depositional environment of fine-grained sedimentary rocks [29,30], and are one of the important characteristics for distinguishing lithofacies [31]. The cores and thin sections of fine-grained sedimentary rocks from the Es3l sub-member of the Bonan Sag indicate that the rocks are dense and structurally characterized mainly by layer-massive formations, with a minor portion of laminated shale (Figure 3). Therefore, the depositional structures of fine-grained sedimentary rocks in the study area are mainly divided into laminated, layered, and massive fabric types.

4.2. Lithofacies Classification and Genesis Analysis

4.2.1. Classification of Lithofacies Types

Lithofacies classification is mainly based on the specific geologic features of the study area, such as the graben lake basin, multi-source background, salinization features, and other factors. The source of materials, TOC and depositional structure were considered. Based on the differences in mineral composition, organic matter, and structure, the fine-grained sedimentary rocks of the Es3l sub-member of the Bonan Sag can be classified into six lithofacies. The criteria for lithofacies classification include three aspects: first, when classifying the lithofacies of the fine-grained sedimentary rocks in the Es3l sub-member of the Bonan Sag, the source of materials was considered. Based on the characteristics of sources of different minerals, with carbonate mineral content thresholds of 25% and 75%, the fine-grained sedimentary rocks are divided into three main categories: terrigenous, mixed-source, and endogenic fine-grained sedimentary rocks. Among them, the content of carbonate minerals is 0%–25% from terrestrial sources, 25%–75% from mixed sources, and 75%–100% from endogenous sources (Figure 2) [27]. Secondly, the classification of lithofacies considers the depositional structural features of the study area, mainly including massive, layer-massive, and laminated types. Finally, based on the characteristics of organic matter content in the study area and the needs of source-rock evaluation, with organic matter content thresholds of 2% and 4%, they are classified into low, medium, and high organic-matter types (Figure 4).
In summary, the mudrock of the Es3l sub-member of the Bonan Sag can be classified into six lithofacies, namely terrigenous low-organic-matter massive siltstone (LF1), terrigenous low-organic-matter massive mudstone (LF2), mixed-source medium-organic-matter massive mudstone (LF3), mixed-source medium-to-high-organic-matter layer-massive mudstone (LF4), mixed-source medium-to-high-organic-matter laminated mudstone (LF5), and endogenic medium-to-high-organic-matter laminated marl (LF6) (Table 2).

4.2.2. Main Lithofacies Characteristics and Genesis Mechanism

The lithology of wells BY 6, BYP 5, Y 1-1, Y 1-2, and YX 291 mainly consists of terrigenous, mixed-source, endogenous mudstone, and a small amount of siltstone. Mixed-source mudrock is widely distributed, indicating deposition environments in semi-deep lakes to deep lakes [18]. Marl is generally closely related to shallow water and is not conducive to organic-matter enrichment [32]; due to the weaker hydrodynamics, combined with higher productivity on the gently sloping area, there is a certain level of organic-matter enrichment. Siltstone is generally found in shallow-water environments with a certain source of material supply, and its organic-matter migration and enrichment differ greatly from mudrock [33,34]. BYP 5, Y 1-1, Y 1-2, and the YX 291 well are dominated by LF4 as the predominant lithofacies, followed by LF6, and it is occasionally developed in LF5 and LF1. Small faults, fractures filled with calcite, muddy gravel, and sandy streaks are visible in the core, with layered or massive structures dominating the core sections, accounting for about 70%, while laminated structures account for about 30%.
  • Terrigenous low-organic rich massive siltstone (LF1)
LF1 in the core appears grayish-white, with massive structures. The single-period massive structure varies in thickness from a few centimeters to several tens of centimeters, showing obvious torn clasts, deformation structures, and bioturbation phenomena (Figure 3a). Microscopically, poorly rounded and poorly sorted quartz grains are observed, exhibiting angular to sub-angular shapes, with grain sizes ranging from 20 to 250 μm, displaying characteristics of scour surfaces (Figure 5a,b). A small number of foraminiferal shells can also be observed (Figure 5c).
The structures of scour surfaces and tearing debris in the core indicates that the deposition of this lithofacies was influenced by event deposition such as landslides, turbidity currents, and seasonal flooding (Figure 3a). Upon entering the basin, terrigenous minerals increased hydraulic conditions, causing sediment to settle down without timely differentiation, forming massive structures. Hydrodynamic conditions refer to the loaded transport of terrestrial-sourced debris as it is transported by rivers into the lake basin by a combination of gravity, buoyancy, drag forces due to bottom-bed shear, and uplift forces. Turbulence caused by terrigenous input disrupted foraminifera, which then settled, together with terrigenous clasts [35]. Terrigenous input into the basin also introduces oxygen from the atmosphere into the water. While increased oxygen levels in the water benefit basin organism growth, biological activities alter hydraulic conditions, disrupting original sedimentary structures [36]. Moreover, enhanced oxidation and hydrodynamic forces in the water column are not conducive to organic-matter enrichment and conservation [37,38,39].
2.
Terrigenous low-organic-matter massive mudstone (LF2)
LF2 appears dark gray with massive structures on the core, with relatively uniform material distribution and no significant differences (Figure 3b), showing no effervescence upon dripping hydrochloric acid. Under the microscope, quartz, feldspar, and clay minerals are intermixed deposits, with angular to subangular quartz grains, ranging in size from 10 to 50 μm, exhibiting poor roundness and sorting (Figure 5d).
During the event deposition process, larger mineral particles from terrestrial-source clastic materials are deposited first, but, as the transport distance increases, smaller mineral particles gradually deposit, as well [40,41]. At this stage, hydrodynamic conditions of the water body remain strong, leading to the formation of massive structures during deposition. Although the oxygen content in the water decreases at this point, the inhibitory effect on organic-matter enrichment remains strong [42], resulting in relatively low organic-matter content in the rock.
3.
Mixed-source medium-organic-matter massive mudstone (LF3)
LF3 exhibits a massive structure in the core, with a relatively uniform material distribution without significant differences (Figure 3c). Under the microscope, one can see a mixed accumulation of terrigenous materials (quartz, feldspar, and clay minerals) and endogenous carbonate minerals (calcite, dolomite), with detrital quartz chaotically distributed in sub-angular to angular forms, having a larger grain size, ranging from 20–80 μm (Figure 5e,f). Smaller particles of micritic calcite partially gather to form micro-lens bodies (Figure 5f).
LF3 is formed by mixed-source, event deposition, which occurs due to changes in hydrodynamic conditions caused by the input of terrigenous material into the lake basin. This results in the disturbance of endogenous deposits (containing large amounts of calcite), which are then remixed with terrigenous material and redeposited. As the terrigenous material advances towards the center of the lake basin, the size of the mineral particles decreases. Although the hydrodynamics weaken, they remain strong enough to stir the bottom sediments, causing the already deposited endogenous minerals to rapidly deposit with the terrigenous detritus. The reduction of terrigenous material, coupled with a decrease in oxygen input, reduces the oxidation of the lake water [43], improving the conditions for the enrichment and conservation of organic matter.
4.
Mixed-source medium-to-high-organic-matter layer-massive mudstone (LF4)
LF4, a layer-massive structure of mixed-source, high-organic-matter-content mudstone, was observed on the core, with vigorous effervescence upon dripping dilute hydrochloric acid. Various minerals are evenly distributed under the microscope, with quartz mostly sub-rounded-to-rounded, with particle sizes ranging from a few micrometers to over ten micrometers, mostly surrounded by clay minerals.
During the late stage of event deposition, substances such as clay minerals with smaller densities and volumes were abundantly deposited [44,45]. At this time, although relatively far away from the source area, the hydrodynamics caused by the input of terrestrial sources is weakened, but it still affects the deposition process of sediment, and the fine-grained long quartzite minerals are subject to storm currents, bottom currents, and other roles of stripping and re-suspension; long-distance transportation occurs at the bottom of the lake [46,47,48] and the minerals are fully mixed with endogenous carbonate minerals and then settle to form massive, layer-massive mixed-source mudrock, so the main development is of a layer-massive structure [49]. At this point, the minimal amount of oxygen brought by terrestrial inputs struggles to alter the redox environment of the water body significantly. Moreover, with greater water depth, the overall environment remains reducing, facilitating the enrichment and conservation of organic matter [50], with organic-matter content ranging from 2% to 6% via Rock-Eval pyrolysis analysis (Table 1).
5.
Mixed-source medium-to-high-organic-matter laminated mudstone (LF5)
LF5 exhibits a laminated structure on the core (Figure 3e). Under the microscope, quartz appears sub-rounded-to-rounded, surrounded by clay minerals, with well-developed micaceous textures. Dark stripes formed by the mixture of clay minerals and organic matter alternate with brighter mud microcrystalline calcite laminae, forming a distinct varve, with a single-layer thickness ranging from 2 to 40 μm (Figure 5i,j). Continuous arrangements of calcite lenses and sub-rounded-to-rounded quartz grains can also be observed (Figure 5i).
The well-developed laminated structure indicates weak hydrodynamic conditions, suggesting an overall quiet reducing environment in the basin. The sub-rounded-to-rounded quartz grains observed in this lithofacies are formed during the process of clay mineral alteration or biogenic activities. The distinct varves observed under the microscope are due to sedimentary differences caused by seasonal changes: mud microcrystalline calcite laminae form more favorably in summer, while other seasons favor the formation of relatively darker clay mineral and organic-matter-mix laminae [51]. Additionally, seasonal variations cause differences in basin water temperature, resulting in water pressure during the process of temperature homogenization in the basin [52], which affects the original sedimentary structures. Therefore, well-developed micaceous textures are visible under the microscope.
6.
Endogenic medium-to-high-organic-matter laminated marl (LF6)
LF6 exhibits a laminated structure on the core, with a relatively uniform distribution of laminae (Figure 3f). It vigorously effervesces with dilute hydrochloric acid. Under the microscope, horizontal or wavy laminae are well developed, with a single-layer thickness ranging from 40 to 100 μm, and a high content of calcite minerals, mainly mud microcrystalline calcite (Figure 5k,l).
Abundant calcite laminae observed under the microscope (Figure 5k,l) are direct evidence of biogeochemical deposition. The formation of LF6 is closely related to the life activities of algae. Within the basin, autochthonous organisms, during their life activities, influence the aquatic environment, initiating a series of biogeochemical reactions that subsequently affect mineral deposition [53,54]. The biogeochemical deposition processes affecting LF6 in the study area are complex. On the one hand, the metabolism of planktonic algae and bacteria increases the pH of the surrounding water, leading to the conversion of HCO3− to CO32−, and the higher concentration of CO32− favors the saturation deposition of carbonate minerals such as calcite [55,56]. On the other hand, the mucilage secreted during the life activities of algae promotes the aggregation and bonding of calcite particles, facilitating the deposition of carbonate minerals [57]. The enrichment of organic matter in LF6 is mainly provided by algae, and the relatively quiet and reducing aquatic environment favors the conservation of organic matter [39].

5. Spatiotemporal Distribution Patterns and Depositional Models

5.1. Distribution Characteristics of Profile Lithofacies

The development of fine-grained sedimentary rock deposition in the study area is minimally influenced by tectonics, with stable depositional processes and continuous sedimentary sequences (Figure 6). Near the northern source area, the steep-slope zone develops terrigenous low-organic-matter massive siltstone (LF1) at the base of the Es3l sub-member, and terrigenous low-organic-matter massive mudstone (LF2) in the middle-to-upper part of the Es3l sub-member, which are vertically overlain by terrestrial-source siltstones (Figure 6, near well Y 109). At the bottom of the slope to the deep-sag zone, the entire Es3l sub-member mainly develops mixed-source medium-organic-matter massive mudstone (LF3), as seen near wells Y 289 and YX 291 (Figure 6). In the deep-sag zone, the bottom of the Es3l sub-member mainly develops mixed-source medium-to-high-organic-matter layer-massive mudstone (LF4), and the middle-to-upper part mainly develops mixed-source medium-to-high-organic-matter laminated mudstone (LF5), as observed near well Y 1-1. Near the central basin, adjacent to the gentle-slope zone, the area mainly develops endogenic medium-to-high-organic-matter laminated marl (LF6). There is no significant lithofacies change in the vertical direction in the deep-sag zone of the basin, mainly developing LF3 + LF4 + LF5 (Figure 6).
The general law is that from the steep-slope zone in the north to the gentle-slope zone in the south, there is a succession of mechanical sedimentary rocks formed, including terrestrial fine-grained sedimentary rocks (LF1 + LF2), mudrocks deposited by mechanical and biogeochemical processes (LF3 + LF4 + LF5), and endogenous-source laminated marl deposited by biological and biogeochemical sedimentation (LF6).

5.2. Distribution Characteristics of Planar Lithofacies

Based on the analysis of the vertical variation of fine-grained sedimentary-rock lithofacies in five key wells such as Y1-1, and based on the dominant lithofacies identified by logging and core data, the plan distribution map of high-quality reservoirs in the Es3l sub-member of the Bonan Sag has been delineated (Figure 7).
The lithofacies exhibit symmetry in the east–west direction and show a belt-like distribution pattern (Figure 7). Developed on the forefront and flanks of coarse clastic deposition are the smaller-scale LF1 and larger-scale LF2. LF3 develops at the bottom of the slope zone in the northern steep-slope zone, while LF4 develops in the deep-sag zone. Transitional zones from deep-sag to gentle-slope zones develop LF5. Extensive LF6 develops in the southern gentle-slope zone. In the deep-sag zone of the depression, influenced by gravity flow from event deposition, LF3 + LF4 are mainly developed. In the deep-sag zone near the gentle-slope zone, unaffected by event deposition, terrestrial fine-grained minerals are still transported to the central basin, mainly forming LF5. Transitioning to the gentle-slope zone, almost unaffected by terrestrial input from steep-slope zones, the mineral composition is primarily endogenous carbonate minerals, ultimately depositing as LF6.

5.3. Depositional Model

The Bonan Sag is a typical graben basin [58,59], with an exceptionally complex depositional mechanism, leading to the subversion of the conventional understanding of the distribution law of fine-grained sedimentary-rock lithofacies and mineral compositions. For example, the mudstones in the deep-sag zone exhibit characteristics such as massive structure, event deposition, high organic matter, and high-carbonate and high-felsic minerals. Combining the results of previous studies, the depositional models are summarized by analyzing the sedimentary dynamics and distribution law of lithofacies in the steep-slope zone, deep-sag zone, and gentle-slope zone.
During the deposition period of the Es3l sub-member in the Bonan Sag, while terrestrial minerals from the northern steep-slope zone were input into the sag, they also affected the hydrodynamic conditions of the basin, playing a significant role in the lithofacies changes in different areas of the Bonan Sag [59]. Influenced by intense terrestrial input, the hydrodynamic conditions near the northern steep-slope zone were strong [7], which was unfavorable for the enrichment and conservation of organic matter [42]. When felsic coarse-clastic particles were transported into the basin by terrestrial rivers, they were subjected to the combined action of gravity, buoyancy, drag force caused by bed shear, and uplift force. As the hydrodynamic load on them weakened and their velocity decreased, gravity gradually became dominant. Felsic coarse-clastic sedimentary particles underwent mechanical differentiation near the shore and deposited to form massive siltstone and massive mudstone with wave-like cross-bedding layers [48]. The main lithofacies developed were LF1 and LF2 (Figure 8a). With increasing transport distance, the hydrodynamic conditions gradually weakened towards the center of the basin, and most of the coarse-clastic minerals, primarily composed of quartz minerals, had been deposited, resulting in a decrease in grain size until transitioning to mudstone deposits.
However, in the actual distribution of fine-grained sedimentary rocks in the lake basin, it was found that mixed mudrocks containing felsic and carbonate minerals with massive, laminated-massive, or even striated laminations representing rapid deposition and higher-energy currents are also present at the bottom of the lake-basin steep slopes, and even in the deep-sag zones. It is demonstrated that, in addition to the above mechanisms, other dynamical mechanisms still exist to transport long-distance felsic minerals to the depths of the lake basin and for deposition to occur [40]. The main reason is that the fine-grained felsic minerals can be stripped and re-suspended by subsequent storm currents and bottom currents, and transported over long distances at the bottom of the lake [46,47,48], mixed with endogenous carbonate minerals, and then deposited to form massive, layer-massive mixed-source mudrocks (LF3 + LF4) (Figure 8a). Evidence from cores, thin-section observations, and previous research results demonstrated that the fluids capable of long-distance transport of sediments in the graben lake basin include flood-induced density current, hyperpycnal current [60], turbidity currents [61,62,63], wind-driven circulation of the flocculating plumes [64,65], and other long-distance transport. Meanwhile, in the region from the bottom of the steep-slope zone of the Bonan Sag to the deep-sag zone, the deeper water environment provides a favorable reducing environment for organic matter enrichment and conservation, but the hydrodynamic conditions are still strong, and thus the deep-sag zone mainly develops mixed-source massive and laminated-massive mudrocks with medium-high organic matter (LF3 + LF4) (Figure 8a).
In the deep-sag zone, with increasing distance from the source area, the hydrodynamics becomes weaker, and the lithofacies type is LF5. This results in the development of laminated structures, with the lithofacies types being LF5. In the transitional zone from the deep-sag to the near gentle-slope zone, the area is almost unaffected by terrestrial input. The relatively clear, low-energy shallow-lake sedimentary environment is highly favorable for the deposition of carbonate minerals [66,67] and the conservation of organic matter [42]. Therefore, the facies zone mainly develops endogenous medium-to-high-organic laminated marlstone (LF6). When land-source debris is transported into the lake basin by rivers, it is subjected to the joint action of gravity, buoyancy, drag force caused by bottom-bed shear, and uplift force. When the hydrodynamic force loading it is weakened, the particle movement velocity is reduced, gravity gradually dominates, and the land-source sediment undergoes mechanical differentiation and settles to form a long quartzite mudstone, thus increasing the fraction of organic matter.
In terms of the change in mineral components, it shows a different law of change due to the influence of terrigenous input and water environment. Quartz from a terrigenous source is mainly affected by hydrodynamics, and its content gradually decreases due to the increase in the transport distance during the process of transporting to the center of the lake basin (Figure 8b). In the process of terrigenous input, clay minerals, due to their lower density, are in suspension under hydrodynamic action; they float and are transported toward the deep-sag zone of the lake basin [68,69]. In the deep-sag zone, the hydrodynamic force is weak and not enough for the clay minerals to continue to be transported forward, at which time the clay minerals and the weathered carbonate minerals from the host rock are mainly deposited by suspension deposition [70] (Figure 8b). Some carbonate minerals are also authigenic from the lake basin, and in the steep-slope zone the stronger hydrodynamic forces mainly deposited terrestrial-source detrital materials, so the carbonate mineral content is lower (Figure 8b). Along with the input from terrestrial sources, the strong hydrodynamics will cause the already deposited carbonate minerals such as calcite to be stirred up [67] and continue to be redeposited as the clay minerals are transported forward, which is an important reason for the higher content of carbonate minerals in the deep-sag zone.
The variation in organic-matter content depends greatly on the water body environment [71]; the water body in the region of strong input from land sources is turbulent and oxidizing, which is unfavorable to the enrichment and conservation of organic matter [38], while the strong reducing environment in the deep-sag zone enables the organic matter to be better conserved, and thus is a relatively high-value area of the organic matter content (Figure 8b).

5.4. “Sweet Spot” Lithofacies Quality Evaluation and Prediction

5.4.1. “Sweet Spot” Lithofacies Quality Evaluation

The previous work utilized the “mineral source–organic matter content–sedimentary structure” approach to classify the lithofacies of deep-water fine-grained sedimentary rocks in the Bonan Sag. The results indicate that different lithofacies exhibit significant differences in mineral composition, sedimentary structure, and organic-matter abundance. By analyzing the existing data, it was found that different lithofacies also show considerable differences in reservoir quality (Table 3). Based on the analysis of the sedimentary model of graben lacustrine basins in the previous text, the differences in “sweet spot” quality of lithofacies with different origins are discussed. “Sweet spot” lithofacies refer to lithofacies that are favorable for shale oil storage and good shale oil reservoirs.
LF1′s deposition process involves the inflow of rivers carrying a significant amount of terrigenous minerals into the sag, creating a turbulent aquatic environment that is not conducive to the accumulation of organic matter. The input of terrigenous material also introduces a certain amount of oxygen into the water, disrupting its reducing conditions [72,73], which is unfavorable for the conservation of organic matter, resulting in a low TOC content in the sediments. The detrital quartz content is relatively high, ranging from 65% to 75%. The sediments formed in this turbulent water environment mainly exhibit massive structures with high porosity. Comprehensive analysis suggests that LF1 has good reservoir quality.
LF2 and LF1 are both formed by terrigenous-event deposition. Although the abundance of organic matter in LF2 has increased due to the longer transportation distance, its absolute content remains low. Additionally, the porosity and brittle mineral content of LF2 are lower, resulting in generally poorer reservoir quality.
LF3 is formed during event deposition processes, with a higher brittle mineral content. In the mixed-source depositional area at the base of the steep-slope, hydrodynamic and oxidative properties of the water decrease, with an average organic-matter content of 2.3% (Table 3). Compared to the lithofacies formed by terrigenous-event deposition, the connectivity of pores improves during fracturing, but its S1 (free hydrocarbon) content remains relatively low (1.8 mg/g). Therefore, the reservoir quality of LF3 is considered average.
LF4 is distributed in the deep-sag zone of the Bonan Sag. Nutrients brought in during terrigenous input processes promote biological blooms and organic-matter generation, with the deep-lake reducing environment providing favorable conditions for organic matter accumulation. The average organic matter content is 3.8%. The brittle mineral content ranges from 60% to 75% (Table 3). The relatively high porosity and S1 content of LF4 result in better reservoir quality, indicating significant exploration potential.
LF5 has a higher content of calcite and feldspathic minerals, with a combined total reaching 72%. The organic-matter content ranges from 2.1% to 4.2%, with an average content of 3.1%. The average S1 content is 2 mg/g (Table 3). Therefore, LF5 has good reservoir performance and significant potential for shale oil exploration.
LF6 is formed under biochemical deposition, with CaCO3 being the primary component provided by biological shells. Therefore, LF6 has a high content of brittle minerals, averaging 77% (Table 3). Although algae in the lake basin can generate a large amount of organic matter, some of it is oxidized and degraded, due to the influence of ancient geomorphology in the transition area from deep sag to gentle slope. The average organic matter content is 3.0%. Considering the high brittleness, high porosity, moderate organic matter abundance, and S1 content of LF6, it is believed that LF6 has good reservoir performance and a certain capacity for shale oil storage (Table 3).

5.4.2. Prediction of Favorable Lithofacies

Favorable lithofacies refer to lithofacies that are favorable for shale oil storage and good shale oil reservoirs. Based on the analysis of fine-grained sedimentary-rock lithofacies types, organic-matter abundance, and oil-bearing properties, it is believed that Class I—target reservoirs are terrigenous low-organic-matter massive siltstone (LF1), mixed-source medium-to-high-organic-matter layer-massive mudstone (LF4), and mixed-source medium-to-high-organic-matter laminated mudstone (LF5), with TOC averages greater than 3%, and with other indicators such as S1, porosity, brittle mineral content, and reservoir space type all having good display. Class II—favorable reservoirs include mixed-source medium-organic-matter massive mudstone (LF3) and endogenous medium-to-high-organic-matter laminated marl (LF6), with TOC averages between 1.5% and 2.5%. TOC is one of the most important controlling factors for shale oil in the study area.

6. Conclusions

(1) The lithological characteristics of the fine-grained sedimentary rocks in the Es3l sub-member of the Bonan Sag have been identified, and a classification scheme for fine-grained sedimentary-rock lithofacies has been established. The fine-grained sedimentary rocks in the study area are divided into six lithofacies types: terrigenous low-organic-matter massive siltstone (LF1), terrigenous low-organic-matter massive mudstone (LF2), mixed-source medium-organic-matter massive mudstone (LF3), mixed-source medium-to-high-organic-matter layer-massive mudstone (LF4), mixed-source medium-to-high-organic-matter laminated mudstone (LF5), and endogenous medium-to-high-organic-matter laminated marl (LF6). The formation of LF1 and LF2 was influenced by terrigenous-event deposition such as landslides, turbidity currents, and seasonal flooding effects; LF3 was formed by mixed-source, terrigenous-event deposition; LF4 was still influenced by terrigenous inputs; LF5 was formed under weaker hydrodynamic conditions; and the enrichment of organic matter in LF6 was mainly due to the provision of material sources by algae and a favorable reducing environment.
(2) The spatiotemporal distribution characteristics of fine-grained sedimentary rocks in the Bonan Sag have been clarified, and a depositional model has been established. In the northern steep-slope zone, terrigenous input is strong, mainly developing LF1 and LF2. In the bottom area of the steep-slope zone, with the increase in transport distance, mixed deposition of terrigenous and endogenous materials is predominant, developing LF3. In the deep-depression zone, LF4 is mainly developed. With further distance from the source, and the weakening of hydrodynamics, LF5 is primarily developed. In the transition area from the deep-depression zone to the southern gentle-slope zone, the water environment is relatively clear and calm, mainly developing LF6.
(3) The quality differences of “sweet spots” in different lithofacies types were analyzed, and favorable lithofacies for shale oil were predicted. The porosity of various lithofacies types is relatively high, with the brittle mineral content of LF1, LF4, and LF5 reaching up to 69%, organic-matter content ranging from 2.1% to 4.4%, averaging 3.8%, and an average S1 value of 3.2 mg/g, indicating good reservoir properties. LF1, LF4, and LF5 are classified as Class I—target reservoirs, while LF3 and LF6 are classified as Class II—favorable reservoirs. The result of the study provide a reference for the classification of fine-grained sedimentary rock facies and distribution characteristics and the evaluation of shale-oil-reservoir sweet spots in graben lake basins.

Author Contributions

Conceptualization, Q.Z. and J.Z.; methodology, Q.Z. and J.Z.; software, Q.Z.; validation, Q.Z., W.L., H.H., J.J., J.Z., P.L., Y.L., J.W., B.T., F.W. and R.J.; formal analysis, Q.Z. and J.Z.; investigation, Q.Z.; resources, W.L. and Y.L.; data curation, W.L. and Y.L.; writing—original draft preparation, Q.Z.; writing—review and editing, Q.Z. and J.Z.; visualization, Q.Z.; supervision, Q.Z., W.L., H.H., J.J., J.Z., P.L., Y.L., J.W., B.T., F.W. and R.J.; project administration, W.L.; funding acquisition, W.L., and Y.L. All authors have read and agreed to the published version of the manuscript.

Funding

This article was supported by the Open Fund Project Fine characterization and reservoir formation mechanism of thick and high-quality massive shale oil reservoirs in the Bonan Sag [33550000-24-ZC0613-0022] of [Sinopec Key Laboratory of Shale Oil/Gas Exploration and Production Technology]. Funder: Sinopec Key Laboratory of Shale Oil/Gas Exploration and Production Technology. Funder number: 33550000-24-ZC0613-0022.

Data Availability Statement

Restrictions apply to the availability of these data. Data was obtained from [Research Institute of Petroleum Exploration and Development, Shengli Oilfield Company, SINOPEC] and are available [from the corresponding authors] with the permission of [Research Institute of Petroleum Exploration and Development, Shengli Oilfield Company, SINOPEC].

Conflicts of Interest

Author Qi Zhong, Wangpeng Li, Jianguo Zhang and Yali Liu were employed by the company Petroleum Exploration and Production Research Institute, SINOPEC, China. Hui Huang was employed by the company Research Institute of Petroleum Exploration and Development, Shengli Oilfield Company, SINOPEC, China. Fenghua Wang, Bintian Tan, and Ruo Jia were employed by the company Shengli Oil Production Plant, Shengli Oilfield Company, SINOPEC, China. The paper reflects the views of the scientists and not the companies.

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Figure 1. Regional geological setting map. (a) Structural setting of the Jiyang Depression, Bohai Bay Basin, China, where the Bonan Sag is located in its eastern parts. See “(b)” for detailed cross section of A-A′. (b) Cross section A-A′ in “(a)” in the Zhanhua Depression [modified from Ref. [16]]. (c) Comprehensive bar chart of stratigraphic development in the Bonan Sag [modified from Ref. [28]].
Figure 1. Regional geological setting map. (a) Structural setting of the Jiyang Depression, Bohai Bay Basin, China, where the Bonan Sag is located in its eastern parts. See “(b)” for detailed cross section of A-A′. (b) Cross section A-A′ in “(a)” in the Zhanhua Depression [modified from Ref. [16]]. (c) Comprehensive bar chart of stratigraphic development in the Bonan Sag [modified from Ref. [28]].
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Figure 2. Key-well mineral-composition ternary diagram of the Es3l sub-member in the Bonan Sag.
Figure 2. Key-well mineral-composition ternary diagram of the Es3l sub-member in the Bonan Sag.
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Figure 3. Core samples from key wells in the Bonan Sag deep-sag zone. (a) Core sample from well YX 291, characterized by LF1 of terrestrial source, featuring tearing debris (red arrows), bioturbation (yellow circles), and deformational structures. (b) Core sample from well Y 1-2, characterized by LF2 of terrestrial source, with sandstone bands and uniform material distribution. (c) Core sample from well Y 1-2, characterized by LF3, exhibiting massive structures and uniform material distribution. (d) Core sample from well Y 1-2, characterized by LF4, featuring layered and blocky structures. (e) Core sample from well L69, characterized by LF6, exhibiting laminated structures with distributed laminae. (f) Core sample from well L67, characterized by LF6, exhibiting laminated structures with distributed laminae.
Figure 3. Core samples from key wells in the Bonan Sag deep-sag zone. (a) Core sample from well YX 291, characterized by LF1 of terrestrial source, featuring tearing debris (red arrows), bioturbation (yellow circles), and deformational structures. (b) Core sample from well Y 1-2, characterized by LF2 of terrestrial source, with sandstone bands and uniform material distribution. (c) Core sample from well Y 1-2, characterized by LF3, exhibiting massive structures and uniform material distribution. (d) Core sample from well Y 1-2, characterized by LF4, featuring layered and blocky structures. (e) Core sample from well L69, characterized by LF6, exhibiting laminated structures with distributed laminae. (f) Core sample from well L67, characterized by LF6, exhibiting laminated structures with distributed laminae.
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Figure 4. Classification scheme of fine-grained sedimentary rock lithofacies in Es3l sub-member of Bonan Sag.
Figure 4. Classification scheme of fine-grained sedimentary rock lithofacies in Es3l sub-member of Bonan Sag.
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Figure 5. Thin-section characteristics of fine-grained sedimentary rocks from key wells. (a,b) Angular-to-subangular quartz grains with scour marks. (c) Foraminifera shells (red arrows). (d) Angular-to-subangular quartz grains, with particle sizes between 10and 50 μm. (e) Intermingled deposits of quartz and calcite, with angular-to-subangular quartz grains randomly distributed, with larger grain sizes between 20 and 80 μm (red arrows indicate quartz, green arrows indicate calcite). (f) Aggregation of small-grained mud microcrystalline calcite forming micro lenses (red arrows). (g,h) Uniform distribution of various minerals, with quartz mostly sub-rounded-to-rounded, smaller grains ranging from a few micrometers to over ten micrometers, mostly surrounded by clay minerals. (i) Lenticular textures of calcite. (j) Alternating layers of organic matter + clay and mud microcrystalline calcite forming distinct light and dark layers. (k) Micaceous layers, mud microcrystalline calcite. (l) Horizontal layers, mud microcrystalline calcite.
Figure 5. Thin-section characteristics of fine-grained sedimentary rocks from key wells. (a,b) Angular-to-subangular quartz grains with scour marks. (c) Foraminifera shells (red arrows). (d) Angular-to-subangular quartz grains, with particle sizes between 10and 50 μm. (e) Intermingled deposits of quartz and calcite, with angular-to-subangular quartz grains randomly distributed, with larger grain sizes between 20 and 80 μm (red arrows indicate quartz, green arrows indicate calcite). (f) Aggregation of small-grained mud microcrystalline calcite forming micro lenses (red arrows). (g,h) Uniform distribution of various minerals, with quartz mostly sub-rounded-to-rounded, smaller grains ranging from a few micrometers to over ten micrometers, mostly surrounded by clay minerals. (i) Lenticular textures of calcite. (j) Alternating layers of organic matter + clay and mud microcrystalline calcite forming distinct light and dark layers. (k) Micaceous layers, mud microcrystalline calcite. (l) Horizontal layers, mud microcrystalline calcite.
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Figure 6. Lithofacies distribution characteristics of the study area.
Figure 6. Lithofacies distribution characteristics of the study area.
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Figure 7. Distribution characteristics of planar lithofacies.
Figure 7. Distribution characteristics of planar lithofacies.
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Figure 8. Depositional models and distribution content of mineral composition. (a) Depositional models of fine-grained sedimentary rocks from the steep-slope zone and deep-sag zone, to the gentle-slope zone. (b) Changes in mineral composition from the near source area of the steep-slope zone to the far source area of the gentle-slope zone. The LF1-LF6 represent six lithofacies, and the names of each lithofacies are detailed in Table 1.
Figure 8. Depositional models and distribution content of mineral composition. (a) Depositional models of fine-grained sedimentary rocks from the steep-slope zone and deep-sag zone, to the gentle-slope zone. (b) Changes in mineral composition from the near source area of the steep-slope zone to the far source area of the gentle-slope zone. The LF1-LF6 represent six lithofacies, and the names of each lithofacies are detailed in Table 1.
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Table 1. Percentage content of mineral composition in the Es3l sub-member of the Bonan Sag. The numerator in the table represents the range of variation, and the denominator represents the average value; the statistical data come from 227 samples from well JYC 1, 49 samples from well BYP 5, 30 samples from well Y 1-1, and 331 samples from well Y 1-2.
Table 1. Percentage content of mineral composition in the Es3l sub-member of the Bonan Sag. The numerator in the table represents the range of variation, and the denominator represents the average value; the statistical data come from 227 samples from well JYC 1, 49 samples from well BYP 5, 30 samples from well Y 1-1, and 331 samples from well Y 1-2.
Clay Mineral
(%)		Quartz
(%)		Feldspar
(%)		Calcite
(%)		Dolomite (%)Pyrite
(%)		TOC
(%)
1.9 – 46.6 22.31 7 – 61 23.59 1 – 28 5.33 5 – 79 41.83 0 – 52 6.29 0 – 14 2.06 0.46 – 7.45 2.14
Table 2. Classification result of fine-grained sedimentary rock lithofacies in Es3l sub-member of Bonan Sag.
Table 2. Classification result of fine-grained sedimentary rock lithofacies in Es3l sub-member of Bonan Sag.
LithofaciesMineral SourcesTOC
(%)		Depositional StructureLithofacies Types
LF1terrigenous0–2massiveterrigenous low-organic-matter massive siltstone
LF2terrigenous0–2massiveterrigenous low-organic-matter massive mudstone
LF3mixed-source2–4massivemixed-source medium-organic-matter massive mudstone
LF4mixed-source2–6layer-massivemixed-source medium-to-high-organic-matter layer-massive mudstone
LF5mixed-source2–6laminatedmixed-source medium-to-high-organic-matter laminated mudstone
LF6endogenous2–6laminatedendogenic medium-to-high-organic-matter laminated marl
Table 3. Evaluation of reservoir space and oil-bearing properties of various lithofacies. Note: The percentage content of porosity, TOC, S1, and brittle minerals in the table is the mean value.
Table 3. Evaluation of reservoir space and oil-bearing properties of various lithofacies. Note: The percentage content of porosity, TOC, S1, and brittle minerals in the table is the mean value.
LithofaciesGenesis MechanismsType of Reservoir SpacePorosity (%)TOC (%)S1
(mg/g)		Brittle Minerals (%)
LF1Terrestrial-event depositionIntergranular pore8.80.51.273
LF2Terrestrial-event depositionIntergranular pore5.80.90.971
LF3Mixed-source-event depositionIntergranular pore6.22.31.868
LF4Mixed-source-event depositionIntergranular pore6.53.83.269
LF5Mixed-source static waterIntergranular pore, intercrystalline pore5.33.12.072
LF6Endogenous biochemicalintercrystalline pore7.33.02.177
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Zhong, Q.; Li, W.; Huang, H.; Jiang, J.; Zhang, J.; Li, P.; Liu, Y.; Wu, J.; Wang, F.; Tan, B.; et al. Distribution Characteristics and Hydrocarbon Significance of Deep-Water Fine-Grained Sedimentary Rocks in the Steep-Slope Zone of a Graben Lake Basin: A Case Study of Es3l sub-Member in the Jiyang Depression, Bohai Bay Basin, China. Minerals 2024, 14, 882. https://doi.org/10.3390/min14090882

AMA Style

Zhong Q, Li W, Huang H, Jiang J, Zhang J, Li P, Liu Y, Wu J, Wang F, Tan B, et al. Distribution Characteristics and Hydrocarbon Significance of Deep-Water Fine-Grained Sedimentary Rocks in the Steep-Slope Zone of a Graben Lake Basin: A Case Study of Es3l sub-Member in the Jiyang Depression, Bohai Bay Basin, China. Minerals. 2024; 14(9):882. https://doi.org/10.3390/min14090882

Chicago/Turabian Style

Zhong, Qi, Wangpeng Li, Hui Huang, Jianhui Jiang, Jianguo Zhang, Pinxie Li, Yali Liu, Jiabin Wu, Fenghua Wang, Bintian Tan, and et al. 2024. "Distribution Characteristics and Hydrocarbon Significance of Deep-Water Fine-Grained Sedimentary Rocks in the Steep-Slope Zone of a Graben Lake Basin: A Case Study of Es3l sub-Member in the Jiyang Depression, Bohai Bay Basin, China" Minerals 14, no. 9: 882. https://doi.org/10.3390/min14090882

APA Style

Zhong, Q., Li, W., Huang, H., Jiang, J., Zhang, J., Li, P., Liu, Y., Wu, J., Wang, F., Tan, B., & Jia, R. (2024). Distribution Characteristics and Hydrocarbon Significance of Deep-Water Fine-Grained Sedimentary Rocks in the Steep-Slope Zone of a Graben Lake Basin: A Case Study of Es3l sub-Member in the Jiyang Depression, Bohai Bay Basin, China. Minerals, 14(9), 882. https://doi.org/10.3390/min14090882

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