Tectonic Controls on Late Paleozoic Shale Gas Preservation in Western Shandong, China

Abstract

Based on structural geology, petrology, shale gas geology, and basin modeling, this study investigates the characteristics of tectonic evolution and sedimentary responses in Western Shandong since the Late Paleozoic. Additionally, this study systematically investigates the hydrocarbon generation and reservoir formation conditions of Late Paleozoic shale gas and analyzes the controlling effects of tectonic activities on shale gas preservation. It is concluded that the TOC of the Late Paleozoic source rocks in this area ranges from 1.0% to 4.5%, considered as moderate to good source rocks. The Ro ranges from 0.7% to 3.5%, indicating a low to overmature stage. Deep concave source rocks have strong hydrocarbon generation potential. The Late Paleozoic shale gas reservoirs in Western Shandong are affected by superposed multiphase tectonic movements, where the activity of fault systems exerts dual controls on shale gas preservation.

1. Introduction

Shale gas is an unconventional natural gas that is “self-generated and self-stored” within mudshales. According to expert estimates, China’s technically recoverable shale gas resources are approximately 25 × 10¹² m³ (these data are from the Report on the Evaluation Results of National Oil and Gas Resources during the 13th Five Year Plan Period), roughly equivalent to those of the United States. However, the United States has been a global leader in understanding shale gas distribution patterns, evaluation systems, development technologies, and reservoir formation mechanisms [1,2,3,4,5]. A report published by the U.S. Energy Information Administration (EIA) in October 2024 predicts that U.S. shale gas production in 2024 will exceed 1 trillion cubic meters, far surpassing China’s total natural gas production of 246.4 billion cubic meters in the same year.

Relatively speaking, China’s shale gas exploration and research started later, with limited understanding of shale gas reservoir formation conditions and controlling factors—especially, the impact of tectonic activities on shale gas preservation—which is still being developed. The accumulation and reservoir formation of shale gas primarily depend on the presence, quality, and mutual matching of fundamental geological conditions such as “hydrocarbon generation, reservoir storage, and preservation” [6]. Historically, more attention has been paid to the “generation and storage” conditions for shale gas enrichment, while preservation conditions remain underexplored in most studies [7,8,9]. Compared with shale gas reservoir formation conditions in the United States [10], those in China have experienced multiple phases of tectonic movements with high intensity, leading to poorer “preservation” conditions. Therefore, China should prioritize research on shale gas preservation conditions—a fundamental geological issue in shale gas exploration and development—especially regarding the critical controlling role of tectonic movements around the peak hydrocarbon generation period of mudshales on reservoir formation and preservation.

2. Regional Geological Setting

Western Shandong refers to the extensive area bounded by the TanLu Fault to the east, the Lanliao Fault to the west, the Bohai Bay Basin to the north, and the Huangkou Sag of the South North China Basin to the south. The Upper Paleozoic coal-measure source rocks in this region are well developed, which contains multiple sags (Figure 1). Larger sags include the Wenshang, Jining, Juye, Chengwu, and Yutai Sags overlying the Western Shandong uplift [11,12,13]. The periphery of the sags is enclosed by the Heze uplift, Longwangmiao uplift, Yanzhou uplift, and Dongping uplift. The central region is the Jiaxiang uplift. Late Paleozoic shale is mainly distributed in these sags. Three shale gas exploration wells have been drilled here: CYC-1 Well, LYC-1 Well, and YYC-1 Well. Geologically, Western Shandong differs from typical shale gas regions like the Sichuan Basin in that it lacks extensive, thick, and continuously distributed marine shale formations with high organic carbon content and optimal thermal maturity; instead, its strata are dominated by continental clastics and carbonate rocks with less favorable conditions for large-scale shale gas accumulation.

In Western Shandong, structural ore-controlling and ore-forming processes are highly significant. Deep-source hydrocarbon-generating organic matter or ore-bearing fluids accumulate and form reservoirs through brittle–ductile to ductile–brittle detachment zones and structural fracture zones within the tectonic framework [11,12,13]. The geological structures in Western Shandong are dominated by faulted blocks, with rare fold structures. In Linzi District, younger sedimentary rocks overlie the ancient crystalline basement, and magmatic rocks, including volcanic rocks, are widely distributed, characterized by multiple phases of magmatic activity. The strata in Western Shandong are well developed, encompassing metamorphic rocks, limestone, shale, coal seams, and volcanic extrusive rocks, which reflect the sedimentary environments of different geological periods.

3. Materials and Methods

The data and samples used in this study were all derived from the Western Shandong, primarily involving the Upper Carboniferous Taiyuan Formation and the Lower Permian Shanxi Formation. They included cores and thin sections from three shale gas parameter wells (the CYC-1 Well, LYC-1 Well, and YYC-1 Well), as well as some outcrop rock samples. In addition, this study refers to some previous research data and achievements and collected information such as source rock and reservoir analysis results and testing data required for the research.

3.1. Research Methods for Geological Structure Evolution

This study employed the balanced cross-section method to reconstruct the evolutionary history of geological structures. Research steps typically include data collection, geological cross-sections construction, decompaction correction, fault restoration, and balance verification. Among these steps, structural modeling software is commonly utilized to perform the forward modeling of deformation processes, verifying whether the restored cross-sections conform to physical mechanisms. The basic geological data required for creating a balanced profile include geological profile maps, seismic interpretation profiles, drilling data, etc. Among these, seismic profiles provide the most fundamental and important information.

3.2. Testing and Analysis Methods for Hydrocarbon Source Rocks

The hydrocarbon source rock testing and analysis included total organic carbon content (TOC), kerogen type, thermal evolution degree (Ro), and hydrocarbon generation potential.

The TOC of the source rocks was tested using a Rock-Eval pyrolyzer with the rock pyrolysis method. Samples were heated to 550 °C to release free hydrocarbons (S1) and pyrolyzed hydrocarbons (S2), and results were calculated using specific formulas.

Maceral groups (e.g., sapropelinite, exinite, vitrinite, inertinite) were identified, and their proportions were statistically analyzed under transmitted/reflected light microscopy to classify the kerogen type.

The degree of organic matter thermal evolution (vitrinite reflectance) was measured using a vitrinite reflectance meter. Since vitrinite reflectance increases with maturity during thermal evolution, maturity stages were determined by measuring reflectance values (Ro%) of vitrinite under oil-immersed reflected light.

Hydrocarbon generation potential was primarily evaluated via rock pyrolysis combined with TOC measurement for rapid assessment, with thermal simulation experiments and gas chromatography–mass spectrometry (GC-MS) for a more in-depth analysis of the hydrocarbon generation mechanisms. These methods are widely applied for evaluating unconventional oil and gas resources such as shale gas and tight oil, providing critical insights for exploration and development.

3.3. Reservoir Testing and Analysis Methods

Reservoir testing analysis included determining the mineral composition and content, natural fracture characteristics in rock cores, and thin-section fractures and pores.

X-ray diffraction (XRD) analysis was conducted on 89 rock samples to determine their mineral composition and content.

Characteristics of natural fractures in cores—including orientation, dip angle, density, aperture, and filling conditions—were analyzed on site during coring. Notably, this fracture study method has limitations; for example, secondary fractures may be induced during coring. Natural fractures and pore characteristics in thin sections were identified and quantitatively analyzed using an OLYMPUS BH-2 polarizing microscope.

3.4. Basin Simulation Methods

Basin simulation software (e.g., PetroMod) was employed to assess stratigraphic thickness, denudation thickness, lithological characteristics, compaction degree, and rock physical properties derived from outcrop observations and seismic interpretation. Integrating one-dimensional well data, two-dimensional seismic line interpretations, and geochemical research results on Carboniferous–Permian shale gas generation in the study area, burial history and thermal evolution history were modeled in PetroMod to enhance the simulation’s reliability. This approach determined the burial process and peak gas generation period of the Carboniferous–Permian mudshales.

This PetroMod modeling involved the following key assumptions: stratigraphic layers are laterally continuous and can be correlated across the modeled basin, thermal properties are uniform within the same set of rock layers, source rock maturation is primarily temperature-driven, and the pressure effects on activation energy can be ignored.

The key modeling parameters used in this basin simulation can be divided into stratigraphic, thermal, and fluid migration parameters, among others. Among them, geological parameters included layer thickness, lithology, porosity–permeability relationship, and other parameters. The layer thickness parameter was taken from seismic data interpretation results. The lithological parameters were mainly acquired from the core logging of parameter wells. The relationship between porosity and permeability was derived from the laboratory test results. Thermal parameters included present-day geothermal gradient and thermal conductivity. The present-day geothermal gradient was measured based on the bottomhole temperature (BHT). The thermal conductivity comes from empirical values in the laboratory. Finally, the Darcy method was used to assess fluid transport, considering buoyancy and capillary forces as the primary driving mechanisms for fluid transport.

4. Results

4.1. Late Paleozoic Shale Gas Source and Reservoir Formation Conditions in Western Shandong

This study used field geological surveys, the collation of regional shale gas-related geological data and basic geological exploration results, testing and analysis, and basin modeling to systematically analyze the development characteristics of Late Paleozoic source rocks and reservoir properties in Western Shandong to elucidate the conditions for shale gas generation and accumulation.

4.1.1. Source Rock Development Characteristics

The Early Paleozoic in Western Shandong was affected by regional subsidence, resulting in slow subsidence throughout the area. Marine sedimentation occurred, with the development of tidal flats, lagoons, and platform carbonate rocks. Starting in the Middle-Late Ordovician, plate convergence and subduction caused regional uplift and erosion, resulting in the complete absence of Upper Ordovician to Devonian strata. A large-scale marine transgression occurred in the Early Permian, followed by a widespread marine regression by the end of the Taiyuan Formation (Early Permian). By the Shanxi Formation (Early Permian), the sea had retreated to the southern margin of the North China Basin, and the influence of sea-level changes was significantly reduced. Driven by sea-level fluctuations, multiple marine transgressions occurred from the Late Carboniferous to Early Permian, depositing a suite of paralic transitional facies dark sandstones, mudstones, limestones, and coal seams. Continental fluvial-lacustrine sedimentary formations emerged in the Middle Permian.

Carboniferous–Permian strata in Western Shandong, from bottom to top, include the Benxi Formation, Taiyuan Formation, Shanxi Formation, and Shihezi Group (Figure 2). Field outcrop and drill core observations indicate that several layers of dark mudshales in the Taiyuan and Shanxi Formations are favorable for shale gas formation. Core analysis results from Well CYC-1 shows that the Taiyuan Formation is dominated by dark mudshales, limestones, and coal seams, representing the most organic-rich mudstone interval. The Lower Shanxi Formation is primarily composed of dark mudstones (Figure 3 right), while the upper part consists of sandstones and argillaceous sandstones (Figure 3 left), making it the second-most significant interval for dark mudshale accumulation after the Taiyuan Formation.

In terms of thickness distribution, the cumulative thickness of dark shale in the Taiyuan and Shanxi Formations within the study area can reach 40–200 m: the Taiyuan Formation has a slightly thicker dark shale thickness, averaging around 20–220 m; and the average thickness of a single layer of dark shale in the Shanxi and Taiyuan Formations is about 20–80 m. From a planar distribution perspective, the areas with thick dark mudstone in the Taiyuan and Shanxi Formation are mainly located in the western part of the study area, west of Cao County in the Wenshang-Ningyang area in the northern part of the study area, respectively.

4.1.2. Organic Geochemical Characteristics of Source Rocks

The analysis of organic geochemical characteristics of source rocks is based on field geological surveys, data collection from Wells CYC-1, LYC-1, and YYC-1, and laboratory testing. Evaluation focused on organic matter abundance, kerogen type, thermal maturity of organic matter, and hydrocarbon generation potential [14,15,16].

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Organic Matter Abundance

Late Paleozoic shales in Western Shandong generally exhibit high total organic carbon (TOC) content. Data from Wells CYC-1, LYC-1, and YYC-1 show that TOC in dark mudshales of the Taiyuan and Shanxi Formations ranges from 0.7% to 3.5%, with an average of ~2.8%, classifying them as moderate to good source rocks [17,18,19,20,21,22,23,24,25]. Laterally, TOC decreases gradually from the Shanxian-Yutai line toward both sides.

In the Taiyuan Formation, dark mudshales have TOC values of 1.5–4.5%, with averages of 2.5–3.5%. In the center of sedimentation, the top black shales of the Taiyuan Formation reach TOC values of 4.0–6.0% and locally exceed 8.0% in intervals enriched with plankton during marine transgressions. The Shanxi Formation dark mudshales have lower TOCs (1.0–2.5%, averaging 1.5–2.0%), significantly lower than those in the Taiyuan Formation. In the center of sedimentation, delta-front mudshales in the middle Shanxi Formation have TOC of 2.0–3.0%, with local interbeds (e.g., carbonaceous mudstone) reaching 4.0%. Collectively, these results indicate that the organic carbon content (TOC) of high-quality shale from Taiyuan Formation to Shanxi Formation in the study area is relatively high, reflecting a high degree of organic matter enrichment (Figure 4, yellow lines).

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Kerogen Types

Kerogen sample analyses indicate that the organic matter in dark mudshales of the Taiyuan and Shanxi Formations is primarily Type II₂ and Type III, with Type III dominating, reflecting a gas-prone hydrocarbon generation potential [26,27,28,29,30].

In the Taiyuan Formation of Western Shandong, Type II kerogen accounts for ~30–50%. Microscopic components of mudshales in the Yanzhou Coalfield show exinite contents of 25–40%, with H/C atomic ratios of 0.9–1.3, characteristic of typical Type II₂ kerogen. This kerogen type originates from the mixed deposition of planktonic and terrigenous organic matter in tidal flat-lagoon environments, exhibiting high hydrocarbon generation potential with hydrocarbon generation yields (S1 + S2) of ~2.5–5.0 mg/g [16]. Type III kerogen constitutes 50–70%, dominated by vitrinite (>50% content) and with H/C atomic ratios <0.8. In western Heze, Taiyuan Formation shales have vitrinite contents exceeding 60% in kerogen microcomponents, with O/C atomic ratios of 0.15–0.25, indicating the dominant input of terrigenous higher plants [17].

Kerogen in the Shanxi Formation is overwhelmingly humic (Type III), with vitrinite contents generally >70%, inertinite 10–20%, and exinite <5%. Kerogen in Shanxi Formation shales from the Jining Coalfield is dominated by structured and homogeneous vitrinite, with H/C atomic ratios of 0.6–0.8 and O/C atomic ratios of 0.2–0.3 [31].

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Thermal Maturity of Organic Matter

The geothermal gradient across the study area is generally uniform. Dark mudshales in the Taiyuan–Shanxi Formations enter the maturity stage when vitrinite reflectance (Ro) reaches 0.5%, generating minor hydrocarbons. The Indosinian orogeny at the end of the Triassic caused strata regional uplift and erosion; due to varying degrees of erosion, subsequent sedimentary basins entered stages of differential subsidence. Presently, Ro values in the Taiyuan–Shanxi dark mudshales range from 0.7% to 3.5% (average 1.53%), classifying them as the dry gas evolution stage and spanning low-maturity to overmaturity stages of organic matter maturation. Laterally, in the sedimentary centers of the Jining and Yutai Sag areas, Ro exceeds 2.0% (Figure 4, purple lines), indicating entry into the major hydrocarbon generation stage (high-maturity stage, corresponding to the gas generation window). These values reflect the high thermal maturity of the shales and strong hydrocarbon generation potential [26,27,28,29,30].

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Summary

A comprehensive evaluation of the organic geochemical characteristics of shale gas source rocks in Western Shandong region reveals the following: The Taiyuan Formation of the Upper Carboniferous is dominated by medium to high TOC (1.5~4.5%), and the Type II kerogen enrichment zone has potential for shale oil development. The Shanxi Formation is dominated by low to medium TOC (1.0~2.5%), and the III type kerogen dominated area is the focus of the shale gas exploration (

Table 1. Organic geochemical characteristics evaluation of shale gas source rocks in Western Shandong.

Parameter	Upper Carboniferous Taiyuan Formation	Lower Permian Shanxi Formation
TOC Range	1.5–4.5% (Average: 2.5–3.5%)	1.0–2.5% (Average: 1.5–2.0%)
Kerogen Type	Type II (30–50%) + Type III (50–70%)	Type III (>90%)
H/C Atomic Ratio	Type II: 0.9–1.3;Type III: <0.8	0.6–0.8
O/C Atomic Ratio	0.15–0.25	0.2–0.3
Hydrocarbon Generation Potential	Moderate (Yield: 2.0–4.0 mg/g)	Low (Yield: 0.5–1.5 mg/g)
Thermal Evolution Degree	High-maturity stage (Ro > 1.3%)	Maturity-high maturity stage (Ro > 0.8%)

).

4.1.3. Hydrocarbon Generation Evolution of Source Rocks

The Taiyuan Shanxi Formation dark shale reached the mature hydrocarbon generation stage in the early Indosinian period in the deeply buried Chengwu and Yutai Sags. Subsequently, due to the influence of regional compression and uplift, the hydrocarbon generation process of the source rock was interrupted. The relative subsidence movement caused by regional subsidence during the Yanshanian period led to the Taiyuan Shanxi Formation dark shale entering the deep burial stage again, resulting in a secondary hydrocarbon generation process [18] (Figure 5).

4.1.4. Shale Gas Reservoir Characteristics

Shale gas reservoirs in Western Shandong are primarily concentrated in the Taiyuan–Shanxi Formation shales, with a thickness of ~80–120 m. Reservoir properties are controlled by pore types, fracture development characteristics, and mineral composition, among others.

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Mineralogical Composition

The X-ray diffraction (XRD) analysis of mudshales from the Shanxi and Taiyuan Formations in the study area shows that they are dominated by clay-rich fractions, with brittle minerals including quartz, feldspar, calcite, dolomite, and pyrite—of which quartz has the highest content. The clay minerals are mainly kaolinite, illite, and illite-smectite mixed layers.

Microscopic observations reveal that both the Taiyuan and Shanxi Formations have high average contents of brittle minerals and low average contents of clay minerals (predominantly illite and montmorillonite), with minor pyrite in localized areas. High brittle mineral content is conducive to forming complex fracture networks during hydraulic fracturing, though high clay mineral content may induce reservoir sensitivity (water sensitivity, velocity sensitivity). Calculations of brittleness index II show values of 13.97–80.73% (average 52.85%) and 48.97–89.15% (average 70.11%) for the Taiyuan and Shanxi Formation (

Table 2. Statistical table of mineral contents in Carboniferous–Permian mudshales of Wester Shandong (part of the data from reference [15], with permission from CNKI, 2018).

Stratum	Number of Samples	Quartz (%)	Feldspar (%)	Calcite (%)	Dolomite (%)	Pyrite (%)	Clay Minerals (%)	Brittleness Index II (%)
Taiyuan Formation	31	12.75–73.51 Average: 42.09	0–25.93 Average: 8.69	0–4.85 Average: 0.16	0–9.71 Average: 0.76	0–13.15 Average: 2.15	12.79–83.15 Average: 43.21	13.97–80.73 Average: 52.85
Shanxi Formation	6	45.95–66.87 Average: 52.37	1.99–28.76 Average: 16.95	0–0.53 Average: 0.09	0–1.73 Average: 0.59	0–1.99 Average: 0.97	8.15–47.29 Average: 28.13	48.97–89.15 Average: 70.11

Note: Brittleness Index II = (Quartz + Feldspar + Calcite + Dolomite)/(Quartz + Feldspar + Calcite + Dolomite + Clay Minerals) × 100%.

). Since there is less clay mineral content in both formations than brittle mineral content and the brittleness index II is high, these conditions are favorable for shale gas storage and subsequent fracturing-based development.

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Fracture and Pore Development Characteristics

Since the Indosinian period, faults in Western Shandong have been predominantly NW-trending. Following the Indosinian orogeny, deep crustal detachment faults controlled the sedimentation of the Western Shandong Block since the Late Mesozoic, forming a series of NW-trending half-grabens. Influenced by the regional stress field and faults, the dominant fracture orientations in the study area are NNW-, SEE-, and NEE-trending, primarily shear fractures that commonly exhibit conjugate X-shaped patterns in plain view. From the on-site sampling photos of the rock core in Well CYC-1 (Figure 6), it can be seen that most fractures remain unfilled, enhancing pore connectivity, while the rest are almost entirely filled with calcite (Figure 6).

Shaped by regional tectonic movements (e.g., Indosinian and Yanshanian compression), natural fractures are predominantly high-angle structural fractures, with localized interlayer fractures. Fracture density ranges from 0.5 to 2 fractures per meter, and aperture measures 10–50 μm. Some fractures are filled with calcite or clay, with effective fractures accounting for ~30–50% of the total.

Pore types primarily include intergranular pores in minerals, microfractures (Figure 7a,b), and dissolution pores (Figure 7c,d). Among these, organic matter pores characterized by nanoscale honeycomb and strip-shaped structures, and others, develop with increasing maturity and serve as the main storage space for shale gas. Scanning electron microscopy (SEM) analysis of dark mudshales from the Taiyuan–Shanxi Formations in Wells CYC-1 and LYC-1 reveals well-developed microfractures, along with secondary pores such as intergranular and dissolution pores, which facilitate the storage and migration of free gas [22,32].

Porosity is generally low, dominated by matrix pores. The porosity of the Taiyuan Formation is mostly between 3% and 8%, with an average porosity of 6.6% and a standard deviation of about 1.8%. That of the Shanxi Formation shale is mostly between 4% and 9%, with an average porosity of 6.8% and a standard deviation of about 1.6%. Micropores (<2 nm), mesopores (2–50 nm), and macropores (>50 nm) all develop, with micropores and mesopores accounting for relatively higher proportions. Macropores enhance pore connectivity.

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Permeability

Matrix permeability is extremely low, relying on natural fractures and fracturing stimulation to enhance fluid conductivity. In the Taiyuan Formation, the higher content of brittle minerals in marine mudshales may result in better fracture development compared to the continental mudshales of the Shanxi Formation. Marine depositional environments typically enrich brittle minerals like quartz and feldspar, which are more prone to fracturing under stress to form natural or induced fractures. Thus, both the Taiyuan and Shanxi Formations depend on natural fractures and hydraulic fracturing to improve conductivity and reservoir permeability.

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Summary

The physical properties of shale gas reservoirs in the Taiyuan–Shanxi Formations of Western Shandong can be summarized as follows:

• Marine mudshales in the Taiyuan Formation exhibit good lateral continuity and stable physical properties, while Shanxi Formation mudshales, affected by fluvial erosion, contain interbedded sandstones and coal seams, leading to strong vertical heterogeneity in reservoir properties;
• Target intervals are mostly buried at 1500–3000 m, with stress regimes dominated by compressional-shear stress. The horizontal principal stress difference ranges from 5 to 15 MPa, influencing the propagation direction of hydraulic fractures;
• Reservoir compaction and cementation (carbonate cementation) reduce porosity, while late-stage dissolution (organic acid alteration) locally improves storage space;
• The Taiyuan–Shanxi Formations feature moderate organic matter maturity, high brittle mineral content, and a foundation for fracture stimulation, endowing this shale interval with exploration potential. However, challenges such as low porosity/permeability and strong heterogeneity remain.

4.2. Structural Evolution and Sedimentary Response Characteristics

The structural evolution characteristics of Western Shandong embody a series of complex geological processes spanning from the Neoarchean to the Precambrian and even the Mesozoic, including early Neoarchean komatiite activity, the emergence of island arcs and subduction mechanisms in the late Neoarchean, large-scale ductile shear zones and terrane collage events at the end of the Neoarchean, Paleoproterozoic extension events and cratonization processes, the formation of early Precambrian intrusive rocks, extensional phases of supercontinent breakup during the mid-late Precambrian, the tectonic evolution stage of the Tan-Lu Fault Zone in the Jurassic, and their linkages to global tectonic events. These structural evolution features are crucial for understanding the early geological evolution of the North China Craton and even the global geosphere, while also providing insights into how tectonic movements controlled the preservation of Late Paleozoic shale gas [11,12,13,26,27,28,29].

4.2.1. Structural Evolution Processes

Western Shandong is situated on the North China Plate. Since the Archean, it has undergone the Caledonian, Hercynian, Indosinian, and Yanshanian orogenies, which can be divided into five structural stages [30,31] as follows:

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Crustal Block Formation Stage (Pre-Caledonian Period)

From the Archean to the Early Paleozoic, the primitive crust was influenced by mantle convection, inducing extensional tectonism that formed a series of tensional fractures. Mantle magmas intruded along these fractures, leading to the collision, assembly, and cementation of paleocontinental nuclei into new ones, eventually forming the paleocraton;

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Crustal Block Development Stage (Caledonian to Hercynian Periods)

Following the Early Paleozoic, the North China Platform began to subside. During the Caledonian orogeny, regional uplift occurred, subjecting the study area to prolonged weathering, erosion, and sedimentary hiatus. After the Middle Carboniferous, the crust transitioned from uplift to subsidence: seawater transgressed from the southern and western margins, accompanied by frequent sea-level fluctuations that fostered paralic (coastal-marine) deposits. In later stages, as the crustal block rose again, continental sedimentation became dominant;

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Compressional Uplift Stage (Indosinian to Yanshanian Periods)

During the Indosinian period, the North China Plate to the north collided compressively with the Yangtze Plate to the south, causing folding and uplift in the study area and forming a series of nearly N-S trending faults. Take the Central Uplift as an example: in the Chiping Uplift Area—an upthrown block of the Lanliao Fault (main fault)—the Middle-Lower Triassic strata were denuded, whereas in the Shenxian Sag Area—a downthrown block—these strata were partially preserved (Figure 8b).

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Faulting and Fragmentation Stage (Yanshanian Period to First Phase of the Himalayan Orogeny)

During the Yanshanian Period, intense crustal activity occurred due to the subduction of the Pacific Plate beneath the Eurasian Plate. Under the influence of SE-directed principal compressive stress, relative subsidence movements dominated Western Shandong, giving rise to NW-trending faults and a series of fluvial–lacustrine sedimentary basins aligned with these faults (Figure 8c). In the Late Cretaceous, sinistral shear along the Tan-Lu Fault Zone caused crustal uplift and subsequent erosion (Figure 8d);

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Re-subsidence Stage (Since the Second Phase of the Himalayan Orogeny)

Since the Neogene, lithospheric isostatic adjustment has induced large-scale regional subsidence, shaping the present-day structural framework of the area (Figure 8f).

4.2.2. Sedimentary Response Characteristics of the Late Paleozoic

During the formation of depressions or basins, stable subsidence environments facilitate the enrichment and preservation of organic matter, which serves as the material basis for shale gas accumulation. The sedimentary evolution of Western Shandong typifies the tectonic–sedimentary coupling in the eastern North China Craton, undergoing four stages: basement formation, epicontinental sea deposition, volcanic-sedimentary transition, and Cenozoic rifting. Its complete sedimentary sequence records multiple phases of tectonic movements, sea-level fluctuations, and paleoclimate changes, providing critical insights into the destruction mechanisms of the North China Craton, hydrocarbon exploration, and regional geological hazard prevention.

Western Shandong was among the earliest areas in North China to accumulate Late Paleozoic strata, which are well-developed and diverse. Sedimentary environments evolved from marine to paralic (coastal-marine) and finally to continental, categorized primarily into marine, paralic transitional, lacustrine, swamp, and fluvial environments [14,33].

The Late Carboniferous Taiyuan Formation and Early Permian Shanxi Formation in Western Shandong belong to the paralic sedimentary system of the southeastern North China Platform. The Taiyuan Formation is dominated by marine-to-transitional facies limestone, sandstone, mudstone, and coal seams. Mudshales formed predominantly in low-energy environments such as tidal flats and lagoons, with organic matter derived mainly from aquatic organisms and localized terrigenous input. The Shanxi Formation features continental-to-transitional fluvial and deltaic deposits, where mudshales developed in floodplains and overbank settings, with organic matter sourced primarily from higher plants and often interbedded with coal seams. Both formations provide a sedimentary foundation for shale gas formation, with cumulative thicknesses of dark mudshales reaching tens of meters and forming favorable source–reservoir–seal assemblages.

4.3. Shale Gas Preservation Conditions in the Western Shandong

4.3.1. Controlling Effects of Tectonic Activity on Shale Gas Preservation in Western Shandong

The extensional fault system in Western Shandong consists of steeply dipping normal faults (near-NW- and near-EW-trending) and shallow detachment faults. The former has large cutting depths and long activity durations, controlling the deposition of half-grabens; the latter develop along Paleozoic unconformities, accompanied by dynamic metamorphism such as silicification and marbleization. Fault breccias and slickensides in the fault zones indicate multi-phase activity [29].

Late Paleozoic shale gas reservoirs in Western Shandong are influenced by multiple phases of structural superposition, with preservation conditions controlled by both the fault system and tectonic activity. The fault system exerts dual positive–negative controls on shale gas preservation.

• Destructive effects: Active fault zones may form escape pathways for shale gas, particularly in areas with intense tectonic activity, such as around steep normal faults with high vertical movement rates.
• Constructive effects: Faults in stable periods can act as trap boundaries, and fault-block structures formed by detachment combinations, and steep faults can locally seal gas accumulations.

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Negative Impacts of Fault Systems and Tectonic Activity on Shale Gas Preservation

The NW-trending steeply dipping ductile shear zones cut through deep mantle rocks, triggering the upwelling of anatectic magmas and driving the uplift of country rocks. Detachment-tilted fault blocks at the top cause partial strata to rise to shallow depths, potentially compromising the integrity of cap rocks. Additionally, brittle–ductile shear zones and deep faults provide migration pathways for deep-source ore-bearing fluids. Hydrothermal fluids from magmatic activity may modify shale reservoir properties (e.g., micropore development), but excessive uplift can exacerbate cap rock erosion. In structural axes and fault-concentrated areas, uplift erosion and active fluid flow degrade shale gas preservation conditions.

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Positive Impacts of Multi-Phase Structural Superposition

Since the Late Paleozoic, Western Shandong has experienced the multi-phase superposition of Indosinian EW-trending folding–thrusting and Yanshanian extension–detachment, forming complex fault networks. However, the central parts of sag basins (e.g., the Upper Paleozoic Shanxi and Taiyuan Formations), characterized by weak fault activity and intact cap rocks, emerge as favorable zones for shale gas accumulation.

4.3.2. Matching Relationship Between Tectonic Movements and Gas Generation Peak of Target Intervals

Based on thermal history research combined with tectonic evolution analysis, this study examines the matching relationship between tectonic movements and the gas generation peak of dark mudshales in the Taiyuan–Shanxi Formations (Figure 9). The results show that Late Paleozoic dark mudshales in the Taiyuan–Shanxi Formations of the Western Shandong entered the maturity stage in the Early Triassic, beginning to generate hydrocarbons. Concurrently, compressional uplift induced by the Indosinian orogeny altered the regional structural framework. During the Yanshanian period, relative subsidence in the Western Shandong re-buried the Taiyuan–Shanxi dark mudshales, triggering secondary hydrocarbon generation dominated by gas formation.

Studies in the South Junggar Basin [34] and Sichuan Basin [35] in China highlight that trap formation must coincide with peak hydrocarbon generation to maximize accumulation efficiency. Examples of successful exploration in the Tarim Basin [36] and Ordos Basin [37] in China exemplify how secondary cracking and in situ transformation in overmature source rocks can sustain hydrocarbon supply, particularly in deep and ultradeep reservoirs. In referring to the above research results and in combination with the accumulation events diagram for dark mudshales in the Taiyuan–Shanxi Formations of Western Shandong deep sag area, the following conclusions can be drawn. A favorable match is observed between the hydrocarbon generation peak of Upper Paleozoic dark mudshales and the trap formation time during the Indosinian period. Additionally, the reburial of source rocks during the Yanshanian period facilitated secondary hydrocarbon generation. Thus, the Late Paleozoic dark mudshales in Western Shandong possess the material basis for shale gas formation. Considering their thermal maturity and matching relationship with tectonic movements around the gas generation peak, these Late Paleo-zoic coastal–marine transitional facies dark mudshales represent a favorable interval for shale gas exploration.

4.3.3. Comprehensive Evaluation of Shale Gas Preservation Conditions in Western Shandong

In integrating regional geological characteristics, tectonic geological processes, and tectonic evolution history, a comprehensive evaluation was conducted on the shale gas preservation conditions. Specific evaluation parameters include shale distribution area and burial depth, the development of large-scale fractures, erosion thickness, burial history type, and gas generation peak period. A three in one model of “structure–stratigraphy–thermal evolution” was established for shale gas preservation conditions in the Upper Paleozoic of Western Shandong (

Table 3. Comprehensive evaluation of shale gas preservation conditions in Western Shandong.

Factors	Evaluation Parameters	Evaluation Levels
		Good	Moderately Good	Average	Poor
Regional Geological Characteristics	Burial Depth (m)	2000–4000	1500–2000	1000–1500	<1000
	Areal Extent (km2)	>300	100–300	50–100	<50
Tectonic Geological Processes and Tectonic Evolution History	Distance from Throughgoing Faults (km)	>10	5–10	2–5	<2
	Development of Giant/Large Fractures	None	Mostly None	Rare	Abundant
	Erosion Thickness Since Indosinian Orogeny (m)	Thin	Thick	Relatively Thick	Thickest
	Distance from Outcrop to Target Interval (km)	>15	10–15	5–10	<5
	Burial History Type	Early long-term shallow burial—Early-middle long-term uplift—Middle-stage secondary deep burial—Late-stage rapid uplift	Early long-term shallow burial—Early-middle long-term uplift—Middle-stage secondary deep burial—Late-stage rapid uplift	Early hydrocarbon generation—Middle-stage multiple hydrocarbon generation/expulsion—Late-stage rapid uplift	Long-term continuous burial—Rapid uplift
	Gas Generation Peak (Permian shales)	Latest	Later	Moderate	Early
	Gas Generation Peak (Carboniferous shales)	Latest	Later	Moderate	Early

Notes: Giant fractures: width > 1 mm, length > 10 m; Large fractures: millimeter-scale width, length 1–10 m. (Modified from references [8,38]).

).

As shown in Table 3, the greater the burial depth of the source rock, the better the maturity of the source rock and preservation of shale gas. In terms of tectonic geological processes and tectonic evolution history, shale gas reservoirs that are more distant from large faults and less developed large fractures are, the more favorable it is for the preservation of shale gas preservation. At the same time, secondary hydrocarbon generation is more conducive to shale gas accumulation and preservation than multiple hydrocarbon generation.

Based on the comprehensive consideration of the above evaluation criteria, the Jining Sag and Yutai Sag are identified as the most favorable areas for Late Paleozoic shale gas development in Western Shandong. These areas feature organic-rich mudshales with a thickness of approximately 105–110 m and an effective burial depth of 300–3500 m. At present, they are in the major hydrocarbon generation stage, located far from major faults, and exhibit a favorable matching relationship between hydrocarbon generation and tectonic evolution.

The Chengwu Sag is classified as a sub-favorable area for Late Paleozoic shale gas development in the region. Here, organic-rich mudshales have a thickness of 60–115 m and a burial depth of 300–2500 m. Although the thermal maturity is slightly lower, the hydrocarbon generation matching relationship remains relatively good.

5. Discussion

5.1. Comprehensive Analysis of Key Findings

The study on the formation and preservation conditions of Late Paleozoic shale gas reservoirs in the western Shandong region reveals the dynamic coupling relationship between tectonic thermal evolution and source rock maturity. Research has shown that multi-stage tectonic movements (Hercynian, Indosinian, Yanshanian, and Himalayan periods) directly affect the generation, migration, and preservation of shale gas by controlling the cycles of stratigraphic uplift and subsidence. For example, the Indosinian compression and uplift interrupted primary hydrocarbon generation in source rocks, while the Yanshanian subsidence triggered secondary hydrocarbon generation, which will effectively enhance the resource potential of shale gas. This discovery is consistent with research results regarding the Sichuan Basin [35,39] and the Ordos Basin [37,40], which indicate that the contribution of tectonic thermal events to secondary hydrocarbon generation is universal.

In terms of preservation conditions, the throughgoing faults have significant destructive effects on shale gas reservoirs [41]. Shale reservoirs in structurally stable zones exhibit more favorable gas bearing characteristics than those in strongly deformed zones [42]. The three in one model of “structure–stratigraphy–thermal evolution “in Western Shandong shows that the preservation potential is optimal in areas with stable structures (more than 10 km away from the throughgoing fault, without large cracks), burial depths of 2000–4000 m, TOC > 2.5%, and shale thickness > 20 m. This is similar to the exploration experience of the Bakken Shale in North America [43], but the unique combination of bottom limestone and top mudstone in Western Shandong region forms a more efficient reservoir cap system, which is unique in the global land surface marine sedimentary background.

5.2. Uncertainty Analysis and Future Research Directions

We analyzed the uncertainty of this study and pointed out the direction for future research. Please refer to

Table 4. Uncertainty analysis and future research directions.

Uncertainty	Future Research Direction
The comprehensive evaluation system for shale gas preservation conditions in Western Shandong emphasizes geological factors in parameter selection but lacks integration of multi-source data (e.g., logging, seismic attributes), resulting in suboptimal prediction accuracy (compared to machine learning applications in North America [44]).	Integrate multi-source data (e.g., logging, seismic attributes) and optimize machine learning models to enhance the accuracy of preservation condition evaluations.
While the study reveals the spatiotemporal relationship between tectonic movements and secondary hydrocarbon generation, the dynamic impact of thermal fluid activity on kerogen cracking during the Yanshanian rapid subsidence stage remains unclear.	Combine basin simulation and thermal simulation experiments to develop a coupled model of “tectonic stress, organic matter evolution, and fluid migration” to quantify hydrocarbon generation contributions across different tectonic periods.
The interwoven distribution of NW-trending faults and NE-trending cracks in Western Shandong introduces significant uncertainty in preservation condition evaluations, particularly regarding prediction accuracy in “sweet-spot areas”.	Use 3D seismic attribute analysis (e.g., curvature volume, coherence volume) and machine learning algorithms (e.g., convolutional neural networks) to construct a preservation unit recognition system spanning millimeter (bedding) to kilometer (sag) scales, contingent on improved regional investigation efforts.

.

5.3. Implications for Future Exploration

Based on the three in one model of “structure–stratigraphy–thermal evolution”, the target areas are prioritized. The Jining Sag is identified as the most favorable areas for Late Paleozoic shale gas development in Western Shandong, while the Yutai Sag are is sub-favorable. However, both feature good hydrocarbon generation matching relationships. The Chengwu Sag is the third level target area. Exploration priorities should focus on regions with structural stability, moderate burial depth, and high TOC content/mudshale thickness. For the central region of the Jining Sag, it is recommended to prioritize the deployment of horizontal well segmented fracturing tests. For the central region of the Yutai Sag, it is necessary to focus on evaluating the destructive effect of deep faults on preservation conditions. Finally, for the eastern region of the Chengwu Sag, it is recommended to increase the level of investigation and strengthen research on the allocation relationship between shale gas source rock and reservoir.

6. Conclusions

This study aimed to verify the core hypothesis that the coupling relationship between the intensity of tectonic activity and burial history before and after the peak of gas generation determines the potential for shale gas preservation, with the core objective of “matching the conditions of tectonic movement and shale gas preservation”. The following innovative insights were obtained:

• Coupling analysis of burial history, thermal history, and structural history, revealed that the Carboniferous Permian shale gas reservoirs in Western Shandong region have undergone a complex evolutionary sequence of “tension–subsidence–compression erosion–resubsidence”, including basement uplift during the Hercynian period, compression folding during the Indosinian period, fault block activity during the Yanshanian period, and differential subsidence during the Himalayan period. Influenced by this tectonic evolution process, the Late Carboniferous Taiyuan Formation and Early Permian Shanxi Formation in the study area belong to the marine continental sedimentary system in the southeastern part of the North China Platform.
• Geochemical analysis showed that the organic carbon content (TOC) of shale in Taiyuan Formation and Shanxi Formation ranges from 1.0% to 4.5%, and the kerogen is mainly of type II₂–III. The Ro values show a “north–south zoning” characteristic: 0.7–1.3% in Jining Sag (mature stage), 2.0% in Yutai Sag (over mature stage), and 1.0–1.3% in Chengwu Sag (mature stage). The central region is currently in the stage of abundant shale gas generation.
• The key control factors for preservation conditions include establishing a three in one preservation evaluation model of “structure–stratigraphy–thermal evolution”: a structurally stable zone (located at a distance of more than 10 km from the throughgoing fault, without large cracks), a burial depth range of 2000–4000 m, TOC > 2.5%, and an area with a single layer thickness of shale > 20 m have the best preservation potential. Several layers of limestone have developed in the lower part of the Taiyuan Formation of the Lower Permian, while multiple sets of mudstone and argillaceous sandstone have developed in the Shihezi Formation of the Upper Permian, providing good bottom and top plates for shale gas reservoirs, which can effectively seal and protect shale gas shale reservoirs.

The results of this study provide a new paradigm for shale gas exploration in structurally complex areas of superimposed basins, including “tectonic stage matching, determining key element control reservoirs, and qualitative evaluations”. In the future, it is necessary to strengthen the micro mechanism research of fluid rock interaction in the process of structural evolution, laying the foundation for establishing a shale gas reservoir theory which is applicable to the sedimentary background of northern China’s land surface and sea.