Preamble

Discussion on the Existence of Bedding Fractures Under Formation Conditions Based on the Coupling Relationship Between Multi-Stage Stress Fields and Clastic Particle Contact

National Engineering Laboratory for Exploration and Development of Low-Permeability Oil and Gas Fields
Exploration and Development Research Institute of PetroChina Changqing Oilfield Company

Abstract

The existence of bedding fractures under deep formation conditions remains a critical subject of debate in the exploration and development of unconventional reservoirs. This study investigates the mechanical mechanisms governing bedding fracture behavior by analyzing the coupling relationship between multi-stage tectonic stress fields and the microscopic contact mechanics of clastic particles. By integrating geological observations with theoretical geomechanical models, we explore the conditions under which bedding fractures can remain open or be generated under high effective overburden pressure. Our findings suggest that the preservation of these fractures is not merely a function of depth but is significantly influenced by the heterogeneity of particle-scale stress distribution and the evolutionary history of the regional stress field.

1. Introduction

In low-permeability and unconventional oil and gas reservoirs, bedding fractures (also referred to as foliation fractures or laminations) serve as vital pathways for fluid migration and contribute significantly to the overall permeability of the formation. However, the traditional view in geomechanics often assumes that such horizontal or sub-horizontal features should be tightly closed due to the immense vertical lithostatic pressure at depth. Despite this, core samples and production data frequently indicate the presence of open or partially open bedding planes that contribute to reservoir productivity.

This paper aims to resolve this discrepancy by examining the multi-stage stress history of the formation and the specific contact mechanics between clastic particles. We propose that the interplay between macroscopic tectonic forces and microscopic grain-to-grain interactions determines the existence and conductivity of bedding fractures under true formation conditions.

2. Multi-Stage Stress Field Evolution

The stress state of a reservoir is rarely the result of a single geological event. Instead, it is the cumulative product of multiple tectonic stages, including burial, uplift, and lateral compression.

2.1 Tectonic Loading and Unloading

During the burial process, the vertical stress $\sigma_v$ increases linearly with depth. However, subsequent tectonic uplift can lead to a reduction in $\sigma_v$ while the horizontal stresses $\sigma_h$ and $\sigma_H$ may relax at different rates due to the elastic-plastic properties of the rock matrix. This differential relaxation often creates a "stress memory" effect within the bedding planes.

摘要

To clarify the existence of bedding fractures (including bedding planes) under formation conditions in tectonically stable areas of sedimentary basins, and to identify the discrepancies between fractures observed in cores and the actual subsurface fracture systems, this study aims to provide guidance for the exploration of shale-hosted shale oil. Applying principles of sedimentology and rock mechanics, we employ a research methodology that integrates macro- and micro-scale observations. We analyze the coupling relationship between clastic grain contacts and multi-stage stress fields, combined with core fracture observations from hydraulic fracturing test sites, to systematically investigate the response mechanisms of fractures within multi-stage stress fields.

The research results indicate that high-angle tensile-shear fractures can form under high confining pressure during the burial stage. During the uplift stage, differential strain generated by the unloading of partial effective stress can form two types of secondary shear fractures. In the subsurface stage (prior to surface exposure), differential strain is primarily concentrated at lithological interfaces, resulting in high-angle shear fractures. Upon exposure at the surface, low-angle shear fractures (i.e., pseudo-bedding fractures) are formed. Under the formation conditions of tectonically stable areas in sedimentary basins, low-angle bedding fractures (including bedding planes) do not exist in an open state. The bedding fractures observed in surface cores are the result of unloading, which causes irreversible damage to the internal structure of the core. This leads to a weakening of mechanical parameters such as the elastic modulus and tensile strength (with the degree of weakening perpendicular to the bedding plane being significantly greater than that parallel to it), ultimately resulting in "distorted" mechanical properties.

There is a fundamental difference between the bedding fractures observed in surface cores and the true subsurface fracture system; the former cannot directly represent the original underground state. Currently, there is a disciplinary divide regarding the understanding of bedding fractures:

Sedimentary geology focuses on the primary static characteristics of clastic grain contact relationships, often ignoring their dynamic response within multi-stage stress fields. Conversely, rock mechanics emphasizes the regulatory mechanisms of the current stress field on grain contacts, while neglecting the constraints imposed by the multi-stage stress history during the sedimentary and diagenetic processes. For the exploration of shale-hosted shale oil, it is necessary to break through the traditional understanding of matrix pore systems and shift exploration targets toward tectonically-derived high-angle fracture systems. Developing prediction technologies for high-angle fractures is of significant guiding importance for the exploration of shale oil in areas lacking large-scale sand bodies.

关键词

Exploring the Existence of Parting Fractures under Stratigraphic Conditions Based on Multi-Stage Stress Field Clastic Particle Contact Coupling Relationships

QI Yalin
National Engineering Laboratory for Exploration and Development of Low Permeability Oil Fields, Xi'an, Shaanxi 710018, China
Exploration & Development Research Institute of PetroChina Changqing Oilfield Company, Xi'an, Shaanxi 710018, China

Abstract

Understanding the genetic mechanisms of fractures is critical for shale oil exploration. This study investigates the existence of parting fractures under stratigraphic conditions by analyzing the coupling relationship between multi-stage stress fields and clastic particle contacts. Through mechanical modeling and analysis of stress unloading processes, we examine the transition between tensile and shear fracture modes. The research highlights the significance of the dual-porosity system in shale reservoirs and provides a theoretical basis for identifying parting fractures, which are essential for optimizing shale oil recovery.

Keywords: Genetic mechanism; Parting fractures; Stress unloading; Tensile-shear fractures; Dual-porosity system; Shale oil exploration

1. Introduction

In the context of unconventional hydrocarbon exploration, shale reservoirs have become a primary focus globally. Unlike conventional reservoirs, shale systems are characterized by extreme heterogeneity and low permeability, making natural fracture networks the primary conduits for fluid flow. Among these, parting fractures—fractures that develop along bedding planes or laminations—play a decisive role in enhancing the storage capacity and permeability of the reservoir.

However, the existence and preservation of these parting fractures under high-pressure stratigraphic conditions remain subjects of intense debate. Traditional views often suggest that vertical lithostatic pressure would keep such horizontal features closed. This paper explores the mechanical feasibility of these fractures by examining the coupling between clastic particle contacts and the evolution of multi-stage stress fields, particularly during periods of tectonic uplift and stress unloading.

2. Genetic Mechanisms of Parting Fractures

2.1 Multi-Stage Stress Field Evolution

The formation of fractures in shale is rarely the result of a single tectonic event. Instead, it is the product of a complex history of burial, lithification, and subsequent tectonic deformation. During the burial phase, increasing vertical stress $\sigma_v$ leads to the compaction of clastic particles. As the burial depth increases, the contact relationships between particles transition from point contacts to line and even convex-concave contacts.

[FIGURE:1]

When tectonic

Abstract

To clarify the existence of bedding fractures (including stratification fractures) under the s tructural stability conditions of sedimentary basins, Revealing the differences between surface core observed fractures and the underground real fracture system, and to guide the exploration of shale type shale oil. Based on sedimentology and rock mechanic s principles, a macro microscopic combined research approach was adopted to conduct a multi stage stress field clastic particle contact coupling relationship analysis, combined with hydraulic fracturing test field core fracture observations, systematically studying the response mechanism of fractures in multi stage stress fields. The research results indicate that:

During the burial stage, high angle tensile shear fractures can form under high confining pressure. During the uplift stage, differential strain resulting from partial effective stress unloading can generate two types of secondary shear fractures: in the s tage before exposure to the surface, differential strain mainly concentrates at lithological interfaces, forming high angle shear fractures; while in the surface exposure stage, low angle shear fractures (i.e., pseudo bedding parallel fractures) can form. angle bedding parallel fractures (including bedding fractures) cannot form under formation conditions of stable tectonic regions in sedimentary basins bedding parallel fractures observed in surface cores are products of unloading (stress unloadin ). Stress unloading causes irreversible damage to the internal structure of the core, leading to the weakening of mechanical parameters such as elastic modulus and tensile strength (the degree of weakening is significan greater in the direction perpend icular to the bedding planes than parallel to them). Consequently, the mechanical properties become

"distorted" and cannot accurately represent the in situ conditions.

There is an essential difference between the bedding parallel fractures observed in surf ace cores and the real fracture system, and they cannot directly represent the original underground state.

Current understanding of bedding parallel fractures suffers from compartmentalization between disciplines: sedimentary geology focuses on the primar y static characteristics of clastic particle contacts, neglecting their dynamic response in multi stage stress fields; while rock mechanics emphasizes the regulatory mechanism of the current stress field on clastic particle contacts, overlookin g the constr aints imposed by the multi stage stress fields throughout the sedimentary diagenetic history.

For shale type shale oil exploration, it is necessary to break through the traditional understanding of matrix pore systems and shift exploration targ ets to cturally formed high angle fracture systems, developing high angle fracture prediction technologies. This understanding has guiding significance for the exploration of shale type shale oil in areas without large scale sand bodies.

Keywords

fracture formation mechanism; parting fracture; stress unloading; tensile shear fracture; dual porosity system; shale oil exploration; multi stage stress field

0 引言

Shale is a typical fine-grained sedimentary rock characterized by the layered enrichment of components, forming what are known as laminae—features that differ significantly in color and composition from the adjacent matrix. Shale is primarily composed of a matrix and millimeter-to-centimeter scale laminae. The matrix consists of a disordered mixture of detrital grains (such as quartz and feldspar), clay minerals (illite/smectite mixed-layers), and organic matter. In contrast, laminae consist of ordered, layered accumulations of single components such as felsic materials, tuffaceous materials (predominantly felsic minerals, illite, and pyrite), or organic matter. These compositional differences between the matrix and laminae lead to markedly different geomechanical properties and weathering resistance. When such lithological combinations are exposed at the surface, differential weathering produces a "book-page" structure. Laminae are primary sedimentary structures that can exist under both deep subsurface and surface conditions; however, foliation (shaly cleavage) is a secondary structure that develops only under surface conditions. Foliation fractures, as a natural fracture system developing along shale bedding planes, are generally considered the manifestation of bedding fractures in shale. The conceptual evolution of these fractures has progressed through several developmental stages. The concept of bedding fractures originated in the study of dolomite and sandstone reservoirs, where they were regarded as important storage spaces, though a complete conceptual framework had not yet been established at that time.

During this period, foliation fractures were explicitly identified as products of tectonic stress or dissolution processes that modify sedimentary bedding. The phenomenon of widespread bedding fracture development in rocks was recorded, and these features became a primary object of study in shale reservoir research.

In recent years, core observations and simulation tests focusing on foliation fractures have revealed that they serve as potential mechanical planes of weakness that can significantly enhance the storage and seepage performance of shale. Furthermore, foliation fractures are easily activated, which promotes the expansion of hydraulic fractures and the formation of complex fracture networks, thereby improving the effectiveness of hydraulic fracturing treatments. Consequently, these fractures have become a focal point of exploration and development. Ongoing debates regarding the existence of foliation fractures (including bedding fractures) under the formation conditions of tectonically stable regions in sedimentary basins continue to drive deeper research in this field.

Departing from traditional empirical analogies based on core appearances, this study strictly adheres to first principles. By systematically analyzing the evolution of stress fields and the contact mechanics of detrital grains, we step-by-step derive and demonstrate the existence of foliation fractures (including bedding fractures) under the formation conditions of tectonically stable regions in sedimentary basins. This work provides a solid theoretical foundation and scientific basis for shale reservoir evaluation and hydraulic fracturing design.

1 页理缝

Engineering Identification Criteria

Discontinuous interfaces serve as preferential flow paths for fluids. Under formation conditions, the developmental state of fractures is essentially a reflection of the contact relationships between clastic grains. The presence of bedding fractures under formation conditions indicates that vertically adjacent clastic grains have lost contact, and that this phenomenon extends laterally to a certain degree.

The contact relationships between clastic grains are controlled by the stress interactions between them. When core samples are brought to the surface, their constituent clastic grains have undergone stages of sedimentation and hydrocarbon evolution, as well as a physical transition from the surface to the subsurface and back to the surface. Correspondingly, the stress experienced by these grains undergoes a dynamic process of gradual increase followed by decrease. Therefore, analyzing the existence of bedding fractures requires tracing the evolution of the contact states between clastic grains and making a comprehensive judgment based on the characteristics of the multi-stage stress field evolution.

2.1 应力加载

The burial process stage can be categorized into four distinct sub-stages based on the coupling relationship between the stress field and the contact mechanics of the clastic particles:

[FIGURE:1] Schematic diagram of the coupling relationship between the stress field and clastic particle contacts.

2.1.1 沉积阶段

Under surface conditions, the gravitational load of the overburden must be transmitted downward through the contact points between particles, forming what are known as force chains. Any complete "loss of contact" between vertically adjacent clastic particles would lead to an interruption of these force chains, resulting in a localized stress collapse. Such a state is mechanically unstable and impossible to maintain. To preserve mechanical equilibrium, clastic particles must rearrange themselves to reconstruct the force chain network. During this stage, vertically adjacent clastic particles maintain point contact; because loss of contact is physically untenable \cite{2,12}, low-angle bedding fractures are unlikely to form.

2.1.2 埋藏阶段

As burial depth increases, the weight of the overlying sediment rises, and the spaces between clastic particles become fully saturated with water. According to Terzaghi's effective stress principle, the total weight of the rock strata is supported by two components: the effective stress transmitted through the contact points of the particles, and the pore pressure borne by the pore water. The effective stress directly controls particle compression, particle sliding, and the expulsion of pore water, whereas the pore water pressure does not directly participate in the forces between particles. Together, the effective stress sustained by the particles and the pore pressure sustained by the fluid balance the overburden load. Compressive deformation is primarily driven by the effective stress; as effective stress increases, the contact between particles becomes tighter and the pore volume decreases. This principle links macroscopic loads with microscopic particle contacts, establishing a macro-mechanical framework for the study of particle contact relationships.

As the burial depth of clastic particles increases, the vertical effective stress gradually grows, eventually approaching and exceeding the horizontal effective stress. During this stage, the clastic particles have not yet consolidated into rock. Under the action of vertical effective stress, the complete separation of vertically adjacent particles would violate the principles of force chain transmission; therefore, absolute separation can only exist transiently. Subsequently, the clastic particles undergo rotational adjustment to re-establish contact and restore the force chains. Consequently, the contact mode gradually transforms from initial loose point contacts to tighter line contacts, making it impossible to form low-angle bedding fractures with rhythmic characteristics. Shale is characterized by a low elastic modulus and strong stress sensitivity. With the increase of the overlying vertical effective stress, its overburden porosity ($\phi$) and permeability ($k$) decrease exponentially. This reflects the further reduction of intergranular pores and the continued compaction of clastic particles as burial depth and effective stress increase. During this stage, it remains difficult for low-angle bedding fractures to form.

2.1.3 成岩阶段

Diagenesis primarily involves cementation and dissolution. Cementation consolidates clastic grains through mineral precipitation, inheriting and reinforcing the point or point-line contacts established during the burial stage. Conversely, dissolution adjusts the contact relationships between clastic grains through varying degrees of etching of the grains themselves or the surrounding cement. The morphological characteristics of clastic grains serve as microscopic evidence for assessing dissolution intensity and fluid activity history. Under formation conditions, if clastic rocks undergo only weak dissolution, the grains typically retain angular outlines. However, when dissolution is intense, grain edges tend to become rounded due to selective dissolution, resulting in a larger external morphology. The re-contact process of clastic grains is jointly controlled by chemical dissolution and elasto-plastic mechanical feedback.

When the volume of dissolution is less than the original elastic closure between grains (defined as the micrometer-scale Hertzian elastic deformation produced by external effective stress between two spherical grains in the absence of cement), dissolution occurs primarily at stress concentration sites near the contact zone. As material is removed, the elastic strain energy previously stored at the grain contacts is released, leading to elastic rebound. During this process, the grains restore their point or point-line contact configurations, though the contact area decreases. Since the cement is not completely destroyed, the rock mass maintains its original overall strength. This stage can be summarized as "slight dissolution with contact restoration."

As the volume of dissolution continues to increase beyond the limit compensable by elastic rebound, several phenomena occur: (1) original quartz or carbonate cements are completely dissolved, causing grains to lose their rigid "welding"; (2) local suspended grain arches (temporary self-stabilizing structures formed by several grains after cement dissolution) are created, leading to a redistribution of stress paths; and (3) under the influence of differential stress, these arch structures become unstable, causing grains to undergo minute displacements and rotations as the system adjusts toward a new mechanical equilibrium configuration. Although the adjusted grains still tend toward point-line contacts, the contact directions, coordination numbers, and force chain networks are redistributed. This stage can be summarized as "intense dissolution, arch instability, and new point-line contact formation." Dissolution alone cannot cause vertically adjacent clastic grains to completely lose contact; therefore, the mechanical conditions necessary for the formation and preservation of low-angle bedding fractures are lacking.

2.1.4 生烃阶段

As burial depth increases, source rocks enter the hydrocarbon generation stage. If the pore pressure exceeds the formation fracture pressure ($\sigma_f$), tensile fractures are generated within the formation rock. The propagation direction of these natural fractures follows the Hubbert-Willis principle, which dictates that fractures always propagate along the path of least resistance and minimum energy consumption. This direction is determined by the minimum sum of the principal stresses and the rock's tensile strength. Consequently, the macroscopic propagation direction of a fracture is the result of the combined effects of the in-situ stress field and the rock's inherent tensile strength.

$$P_f > \sigma' + T \tag{3}$$

The tensile strength of rock is primarily controlled by the type of cement and the rhythmic structure determined by clastic particles [FIGURE:1]. Among these factors, the cement type and bonding mode are the dominant determinants of rock tensile strength. At the microscopic scale, these factors do not exhibit strong directionality. Although the distribution of clastic particles shows strong microscopic directionality [FIGURE:2], its influence on the tensile strength of the rock is relatively minor. Even if directional differences exist, they are weakened macroscopically by the homogeneity of the cement, resulting in a lack of significant directionality in the overall tensile strength of the rock.

Under in situ formation conditions, the difference in tensile strength between two specific directions is far smaller than the difference measured under surface conditions (where stress unloading causes the weakening of vertical tensile strength to be much greater than that of horizontal tensile strength). The vertical effective principal stress ($\sigma_v'$) originates primarily from the self-weight of the overlying rock mass and pore pressure. The horizontal effective principal stress ($\sigma_h'$), specifically the minimum effective principal stress ($\sigma_{hmin}'$), is contributed by the component generated from the vertical stress via the Poisson effect combined with tectonic stress. Consequently, the minimum effective principal stress depends on burial depth and the intensity of tectonic activity. Samples from the Mesozoic strata of the Ordos Basin exhibit various characteristics: rhythmic laminations at $1991.15\text{ m}$; calcite filling intergranular pores at $1993.28\text{ m}$; ankerite filling intergranular pores at $2008.78\text{ m}$; quartz filling intergranular pores; carbonate filling intergranular pores; and filamentous illite filling intergranular pores at $1973.27\text{ m}$. In regions with intense tectonic activity, the horizontal effective principal stress is dominated by tectonic stress and its value is typically greater than the vertical effective principal stress, making the vertical stress the minimum principal stress.

Under this stress state, fractures propagate perpendicular to the direction of the minimum principal stress, thereby forming low-angle fractures. In tectonically stable areas, the stress state is simultaneously influenced by burial depth and tectonic stress. Statistical results based on borehole data from mainland China indicate that at burial depths of less than $500\text{ m}$, the ratio of the minimum horizontal principal stress to the vertical principal stress ($k$) is greater than $1.0$, indicating that the vertical principal stress is the minimum principal stress. When the depth is between $500\text{ m}$ and $3000\text{ m}$, this ratio is primarily distributed between $0.5$ and $1.0$; after the depth exceeds $3000\text{ m}$, the ratio further converges toward $0.5$ to $0.7$. This suggests that as burial depth increases, the horizontal effective principal stress becomes the minimum principal stress. The critical depth range for the transition of fracture morphology from low-angle to high-angle is approximately $0.5$ to $1.0\text{ km}$. (1) When the burial depth is less than the critical depth, the horizontal effective principal stress is mainly controlled by tectonic stress, and the vertical principal stress is the minimum effective principal stress, which theoretically favors the development of low-angle fractures. However, the original formation pore pressure in this depth interval is usually lower than the rock fracture pressure, making it difficult for fractures to form spontaneously. Even in cases of local overpressure, the rock will preferentially fracture along the parts with the weakest tensile strength. Limited by pressure conditions, these tend to form isolated single fractures rather than developing into a low-angle fracture system with a rhythmic structure. (2) When the burial depth is greater than the critical depth, the influence of tectonic stress weakens significantly, the horizontal effective principal stress becomes less than the vertical effective principal stress, and the minimum effective principal stress shifts to the horizontal direction. If fracturing occurs at this stage, fractures will propagate perpendicular to the direction of the minimum horizontal principal stress, forming high-angle fractures rather than low-angle ones. It should be noted that although differences may exist between the paleostress field during hydrocarbon generation and the present-day stress field, the laws governing stress changes with burial depth remain consistent, and the opening and extension of fractures under present-day conditions are still controlled by the current stress field.

The fabric of minerals determines properties such as hardness, density, and crystalline structure, which in turn dictate mechanical properties including the elastic modulus, shear modulus, and Poisson's ratio. Clastic rocks are composed of various minerals, and their rock mechanical parameters depend on mineral composition, diagenetic intensity, and fabric. Under in situ formation stress fields, clastic rocks are in a state of compression, similar to a compressed spring ($F=k \cdot \Delta x$, where $k$ is the stiffness coefficient and $\Delta x$ is the deformation). The stress-strain response (unloading effect) during the process of formation uplift is controlled by the degree of confining pressure release and lithological differences. Based on the coupling relationship of clastic grain contacts, this process can be divided into two stages: sub-surface (not exposed) and surface exposure [FIGURE:3].

2.2.1 未出露地表阶段（围压部分解除）

The spatial variation of lithology in actual strata can be summarized by two patterns: vertical mutation and lateral mutation. Different lithological variation patterns result in distinct mechanical properties, contact relationships between clastic grains, and rock deformation characteristics.

Vertical Lithological Mutation

A typical example of vertical lithological mutation is the interbedding of sandstone and mudstone. For this lithological combination, the strata remain constrained by horizontal effective stress during uplift and erosion, while the vertical effective stress decreases. Due to the difference in elastic moduli between sandstone and mudstone, the vertical rebound strain of low-modulus mudstone is significantly greater than that of high-modulus sandstone under conditions of partial vertical effective stress unloading. This difference in rebound magnitude between the two lithologies generates "interlayer incompatible strain." Because the strata are laterally constrained, this incompatible strain differential is converted into interlayer shear stress at the sandstone-mudstone interface, triggering interfacial instability and micro-shear slippage.

According to the Coulomb shear failure criterion, when the accumulated interlayer shear stress exceeds the Coulomb shear strength of the rock interface ($\tau = \sigma_n \tan \phi + c$),...

$$\tau = G\gamma \tag{4}$$

$$\tau = \sigma_n \tan \phi + c \tag{5}$$

Coulomb shear strength is primarily governed by the cohesion at the interface (determined by the intensity of diagenesis), the normal stress acting on the slip surface (determined by effective stress), and the internal friction angle of the bedding plane (determined by factors such as rigid grain content, diagenetic intensity, and pressure).

When the interface between sandstone and mudstone is unbonded or weakly cemented, the interfacial cohesion is minimal. Furthermore, if the interface is rich in clay minerals, smooth, and possesses high water content, the internal friction angle remains low. Significant tectonic uplift can lead to a substantial reduction in normal stress, thereby markedly decreasing shear strength and facilitating the occurrence of bedding-parallel shear slippage.

Conversely, when the sandstone-mudstone interface is strongly cemented, the interfacial cohesion is relatively high. A high content of rigid grains (such as quartz) and strong cementation at the interface result in a larger internal friction angle. In cases where the magnitude of strata uplift is small, the reduction in normal stress is limited, and the shear strength remains significant, making bedding-parallel shear slippage less likely to occur.

When interfacial instability triggers micro-shear slippage, particles within the micro-shear slip zone rotate and reorganize to form force-chain arch structures. These arches can only form within regions where particle rotation and reorganization occur, with their thickness corresponding to the thickness of the micro-shear slip zone. The length, or span, of a force-chain arch is self-organized by the mechanical system it supports to achieve effective stress transmission and load-bearing. Through the arch footings, vertical effective stress is converted into lateral thrust and transmitted to the undisturbed domains, facilitating stress redistribution and load continuity. The force-chain arch is not synonymous with the micro-shear slip zone itself; rather, it represents the internal load-transfer mechanism. The micro-shear zone is composed of several force-chain arch structures connected end-to-end and nested within one another. Thus, the micro-shear slip zone serves as the carrier for the formation and existence of force-chain arches, while the arch structures constitute the core mechanism for stress transfer and load-bearing within the zone.

The micro-shear zone consistently remains within the regime of "frictional arch transfer." The Coulomb criterion provides the theoretical framework and primary controlling factors for the inclination angle of these micro-shear zones, preventing them from evolving into low-angle foliation.

$$\theta = \pm (45^\circ - \phi/2)$$

Lateral lithological mutations: A typical example of a lateral lithological mutation is the abrupt contact between sandstone and mudstone. For this specific lithological combination, phenomena such as partial vertical effective stress unloading and horizontal effective stress redistribution will occur.

The denudation of overlying rock leads to the unloading of vertical effective stress. Under confined conditions, vertical rebound strain is generated, which is controlled by the elastic modulus. Rocks with a high elastic modulus exhibit weak rebound strain, whereas rocks with a low elastic modulus show significant rebound strain. This lateral variation in lithology results in a vertical strain differential between layers, generating shear stress at the lithological interface (Eq. (4)). When the Coulomb failure criterion is satisfied, high-angle fractures are formed due to vertical effective stress unloading and differential rebound.

While vertical differential rebound leads to the accumulation of shear stress at the lithological interface, the horizontal effective stress is jointly controlled by the relaxation of the tectonic stress field and the Poisson effect (Eq. (5)). The redistribution of the horizontal effective stress difference further influences the fracture mode. When the horizontal effective stress difference $\Delta \sigma_h$ is large, conjugate shear fractures develop (Eq. (6)). This process characterizes the redistribution of horizontal effective stress and the resulting differentiation of fracture patterns.

$$\epsilon_v = \frac{1}{E} [\sigma_v - \nu(\sigma_H + \sigma_h)]$$

Uplift and denudation reduce the effective vertical stress, which, under the lateral constraints of the surrounding rock, induces two distinct types of localized deformation. First, non-uniform lateral rebound forms microscopic shear bands, manifested as force-chain arches and micro-slips. Although minute slips and apertures exist within these force-chain arches—indicating microscopic damage—this damage remains distributed, limited, and stable; it does not localize or coalesce into a dominant macroscopic fracture. Second, differences in vertical rebound induce high-angle fractures. Because the lithological interfaces of the interbedded layers cannot enter a tensile cracking mode, low-angle fractures are not generated, resulting in a natural absence of low-angle bedding fractures.

2.2.2 出露地表阶段（围压完全解除）

Griffith proposed that the failure of brittle materials originates from the expansion of internal micro-cracks under tensile stress. Controlled by the Poisson effect and Griffith's criterion, the synergistic unloading of vertical and horizontal effective stresses leads to tensile or shear fracturing along bedding planes.

Strain differentiation is dominated by vertical rebound. As the vertical effective stress is unloaded due to the denudation of overlying rock, vertical rebound is triggered. The rebound strain in rocks with a low elastic modulus is significantly higher than that in rocks with a high elastic modulus.

Regarding bedding plane fracturing, as confining pressure is released, vertical strain is transformed through the Poisson effect, resulting in distinct strain differentiation: rocks with a high elastic modulus exhibit small rebound strain, while those with a low elastic modulus exhibit large rebound strain. This leads to the accumulation of tensile or shear strain energy at lithological interfaces. When the tensile strain energy exceeds the tensile strength of the rock, or when the shear strain energy exceeds the rock's cohesion, "pseudo-fissures" form along the bedding planes. Once a core is brought to the surface, it loses the constraint of the surrounding rock, and the effective stress is unloaded. Similar to a spring losing its tension, the core deforms in the direction of the effective stress unloading. Because the effective stresses differ across various directions of the in-situ stress field, the core undergoes differential deformation, which generates micro-cracks. This causes irreversible damage to the internal structure and a "distortion" of mechanical properties, making it impossible to reflect the true mechanical behavior of the rock under formation conditions. During the burial stage (from surface to formation), since the clastic grains and the resulting rocks are within the in-situ stress field, the conditions for vertical separation of adjacent grains do not exist; thus, low-angle bedding fissures with rhythmic characteristics cannot form. Taking shale as an example, it is microscopically composed of a matrix and millimeter-to-centimeter-scale laminae. The matrix is a disordered mixture of clastic grains such as quartz and feldspar, clay minerals like illite and illite-smectite mixed layers, and organic matter. The laminae consist of ordered, layered accumulations of specific components such as quartz-feldspar, tuffaceous material (primarily quartz, feldspar, illite, and pyrite), or organic matter. Tensile strength exhibits weak directionality, whereas rock mechanical parameters such as Poisson's ratio are highly sensitive to structural features like grain arrangement and bedding, showing significant directionality under the assumption of an in-situ stress field at depth.

Shale is composed of organic-rich laminae (with elastic modulus $E_1$ and Poisson's ratio $\nu_1$) and felsic laminae (with elastic modulus $E_2$ and Poisson's ratio $\nu_2$). When the shale is brought to the surface, the stress field and rock mechanical properties change. First, the constraint of the surrounding rock is lost. Subsequently, due to differences in composition and structure, the elastic modulus of the laminae decays during weathering and dehydration, with the organic-rich laminae experiencing a greater reduction than the felsic laminae. This leads to vertical and lateral deformation of the laminae. Vertically, because the organic-rich laminae have a lower elastic modulus, their vertical strain is greater, resulting in a relative elongation that exceeds that of the felsic laminae. Laterally, the difference in rebound causes the accumulation of tensile or shear strain energy at the lithological interface.

If the interfacial shear strain energy exceeds the cohesion, shear sliding occurs along the laminae, forming micro-cracks (shear-dominated). If the interfacial tensile strain energy exceeds the tensile strength of the felsic material, tensile fractures form directly (tension-dominated). This process constitutes the core mechanism for the formation of "pseudo-fissures": the disparity in mechanical parameters between layers leads to vertical and lateral strain differentiation, resulting in the accumulation of shear and tensile strain energy at the interface.

The dilemma in understanding bedding fissures stems from disciplinary fragmentation. Sedimentary geology focuses on the primary, static characteristics of clastic grain contacts but ignores their dynamic response within multi-stage stress fields. Conversely, rock mechanics emphasizes the regulatory mechanisms of the present-day stress field on grain contacts but neglects the constraints imposed by the multi-stage stress fields throughout the sedimentary and diagenetic history.

This disciplinary fragmentation has led to an imbalance in the correlation between static and dynamic factors, as well as between historical and contemporary stress-fabric relationships.

3 工程验证

To reveal the spatial distribution of natural and hydraulic fractures under reservoir conditions, the first hydraulic fracturing test site in China was deployed in the Ordos Basin, drawing on the field practices of hydraulic fracturing test sites in North America.

The hydraulic fracturing test platform is located in the core area of the Qingcheng Oilfield. The target layer, the Chang 7 member, consists of semi-deep lacustrine gravity flow deposits with an oil reservoir depth of 1800 m. Construction of the hydraulic fracturing test platform began in [Month], involving [Number] wells and the comprehensive application of fiber optics, dual-well microseismic monitoring, downhole television, tracers, and coring across [Number] well-tests. Among these, the horizontal well [Well Name] has a depth of $m$ and a horizontal section length of $m$, with fracturing conducted in [Number] clusters. Monitoring included [Number] vertical coring wells and a high-angle directional coring well of $m$, covering the dominant sweet spot area; the horizontal coring well recovered $m$ of core from the middle of the oil reservoir. Field statistics from the hydraulic fracturing test site revealed that natural fractures in the study area are predominantly high-angle shear fractures. The morphology of hydraulic fractures is controlled by the contemporary stress field, cutting through bedding interfaces to form high-angle fractures parallel to the maximum horizontal effective principal stress, rather than low-angle fractures. Furthermore, the so-called "foliation fractures" (including bedding fractures) observed in surface cores were not contaminated by drilling mud, indicating they are mostly closed under formation conditions; these fractures only open after the core is retrieved and the stress is unloaded. The dip angles are primarily distributed [Range].

The foliation fractures observed in surface cores result from unloading, which generates micro-fractures of varying scales. This causes irreversible damage to the core structure and weakens mechanical parameters such as the elastic modulus and tensile strength. Notably, the degree of weakening in mechanical parameters perpendicular to the bedding plane is significantly greater than that parallel to the bedding plane, leading to a "distortion" of mechanical properties.

There is a fundamental difference between the foliation fractures observed in surface cores and the fracture systems existing under reservoir conditions. Consequently, surface observations cannot directly represent the original state of the formation.

4 页理缝对油藏类型的控制

The existence of low-angle bedding fractures under formation conditions in tectonically stable zones of sedimentary basins directly influences the classification of reservoir types. This uncertainty presents a dual challenge for the exploration and development of shale-hosted shale oil, necessitating both theoretical re-evaluation and technical restructuring.

If low-angle bedding fractures exist under formation conditions, they combine with nano-to-microscale matrix pores to constitute a "dual-porosity system." By connecting isolated matrix pores, these fractures significantly enhance effective storage capacity and seepage capability, thereby forming matrix-type reservoirs. This scenario leads to higher estimates of technically recoverable reserves and resource abundance. From a technical perspective, the focus shifts toward the quantitative evaluation of bedding fracture development and the optimization of volume fracturing parameters, which significantly reduces technical complexity and development costs.

Conversely, if low-angle bedding fractures are absent under formation conditions, the system must rely solely on a single matrix porosity and sparse, tectonically-derived high-angle fractures. In this case, storage capacity is limited, and fluid seepage becomes highly dependent on the density and connectivity of high-angle fractures, resulting in highly heterogeneous fracture-type reservoirs. Consequently, technically recoverable reserves are significantly reduced, resource abundance estimates become more conservative, and the productivity of existing pilot wells requires re-evaluation. Under these circumstances, the technical focus shifts to the prediction of tectonically-induced high-angle fractures, leading to a marked increase in both technical complexity and development costs.

5 结论

During the burial stage of strata (stress loading), rocks can develop high-angle tensile-shear fractures under the dominance of high confining pressure and vertical effective stress. During the strata uplift stage (stress unloading), rocks that have not yet outcropped may undergo vertical differential strain due to lateral lithological variations, leading to the formation of high-angle tensile-shear fractures along lithological interfaces.

For rocks outcropping at the surface, the release of vertical effective stress and changes in horizontal effective stress can lead to fracturing along bedding planes, forming low-angle "pseudo-foliation" fractures. The orientation of fractures is controlled by the dynamic evolution of the stress state during the burial-uplift process. Under the formation conditions of tectonically stable zones in sedimentary basins, low-angle foliation fractures (including bedding fractures) cannot form or exist.

The foliation fractures observed in surface cores are the result of unloading, which causes irreversible damage to the internal structure of the core. This leads to overestimated laboratory measurements of porosity and permeability, as well as the weakening of mechanical parameters such as elastic modulus and tensile strength (the degree of weakening in vertical mechanical parameters is far greater than that in horizontal ones). Consequently, the physical and mechanical properties become "distorted." Under the formation conditions of tectonically stable zones in sedimentary basins, fracture orientation is jointly controlled by the tensile strength of the rock and the in-situ stress field, resulting primarily in high-angle tensile-shear fractures.

There is a fundamental difference between the foliation fractures observed in surface cores and the actual subsurface fracture system; the former cannot directly represent the original underground state. The cognitive dilemma regarding foliation fractures stems from disciplinary fragmentation: sedimentary geology focuses on the static lithological description of clastic grain contact relationships while ignoring their dynamic response within multi-stage stress fields; conversely, rock mechanics emphasizes the regulatory mechanism of the current stress field on grain contacts while neglecting the constraints imposed by the multi-stage stress history during burial and uplift. Therefore, it is necessary to establish a multidisciplinary cross-validation framework integrating sedimentology and mechanics. Under the formation conditions of stable zones in sedimentary basins, low-angle foliation fractures cannot form or exist, and thus cannot combine with nano-to-microscale matrix pores to constitute a "dual-porosity system" with effective storage and high seepage capacity. For shale-type shale oil lacking large-scale sand bodies, the exploration focus must shift from "matrix-type reservoirs" that rely on matrix pore systems to "fracture-type reservoirs" that rely on high-angle tensile-shear fractures. Technically, the priority should shift toward the prediction of tectonically-derived high-angle fractures.

Symbol Annotations: $\sigma'{H}$ maximum horizontal effective principal stress; $\sigma$ maximum horizontal principal stress; $\sigma'{h}$ minimum horizontal effective principal stress; $\sigma$ normal overlap; $R$ equivalent radius of curvature; $\phi$ internal friction angle, related to particle roughness and mineral composition; $\theta$ angle between the shear failure plane and the direction of maximum principal stress; $C$ cohesion, describing the chemical bonding strength between particles.}$ normal stress acting on the shear plane; $\tau$ shear stress; $\sigma_{h}$ minimum horizontal principal stress; $P_{p}$ pore pressure, describing the support effect of fluid on particles; $T_{0}$ rock tensile strength; $P_{b}$ formation breakdown pressure; $F$ elastic or contact force; $E$ Young's modulus, describing the material's ability to resist elastic deformation; $G$ shear modulus, describing the material's ability to resist shear deformation; $\nu$ Poisson's ratio, describing the correlation between lateral and longitudinal deformation; $\epsilon_{x}$ lateral strain, dimensionless; $\epsilon_{y}$ longitudinal strain, dimensionless; $\epsilon_{z}$ strain in the z-axis direction, dimensionless; $\gamma$ shear strain, describing the degree of shape change under shear force; $\delta_{n

参考文献

Geological and Engineering Integrated Evaluation of Laminated Shale Oil in the Chang 7 Sub-member of the Ordos Basin. Advances in Earth Sciences (Hans), 2024, 14(6): 851. Qi Yalin.

Geological and Engineering Integrated Evaluation of Laminated Shale Oil in the Chang 7 Sub-member of the Ordos Basin. Frontiers in Earth Science (Hans), 2024, 14(6): 851. Geological and Engineering Integrated Evaluation of Laminated Shale Oil in the Chang 7 Sub-member of the Ordos Basin. Abstracts of the 17th National Conference on Palaeogeography and Sedimentology, Qi Yalin.

Geologi Engineering Integrated Evaluation of Laminated Shale Oil in the Chang 7 Formation, Ordos Basin[C].

Abstract

s of the 17th National Conference on Paleogeography and Sedimentology, 2023.

Yu Jiaren, Fan Zheren. Study on Carbonate Reservoirs of the Renqiu Buried Hill Oilfield. Acta Petrolei Sinica, 1981, 2(1).

Study on Carbonate Reservoirs of the Renqiu Ancient Uplift Oilfield. Acta Petrolei Sinica, 1981, 2(1): 57. Chen Liping, Zou Shaochun, Zhang Heqing, et al. Preliminary Study on the $T_{3}x^{2}$ and $T_{3}x^{4}$ Reservoirs of the Upper Triassic in the Suinan Gas Field, Central Sichuan. Natural Gas Industry, 1981(04): 9. Shi Baode.

Discussion on the Relationship between Seal Rocks and Hydrocarbon Accumulation. Journal of Southwest Petroleum University (Science & Technology Edition), 1989, 1(1): 28. Sun Longde, Liu He, He Wenyuan, et al. An analysis of major scientific problems and research paths of Gulong shale oil in Daqing Oilfield, NE China.

Petroleum Exploration and Development, 2021, 48(3): 453. Liu He, Meng Siwei, Wang Suling, et al. Mechanical characteristics and fracture propagation mechanisms of the Gulong shale. Oil & Gas Geology.

Breakthrough Progress in China's Continental Shale Oil Revolution. China Petrochemical News, 2024, 22(005). Cheng Qiang, Qin Zihan.

Breakthrough Progress in China's Continental Shale Oil Revo lution Sinopec News, 2024 22 (005). [9] Liu, C.

References and Bibliography

H. M. Jaeger, S. R. Nagel, D. A. Schecter, et al. Force Fluctuations in Bead Packs[J]. Science, 1995, 269(5223): 513-515.

Sun Qicheng, Wang Guangqian. The multiscale structure of granular matter and its research framework[J]. PHYSICS, 2009, 38(04): 225-233.

Sun Qicheng, Xin Haili, Liu Jianguo, et al. Skeleton and force chain network in static granular material[J]. Rock and Soil Mechanics, 2009, 30(S1): 83-88.

Wang Xiao, Chen Fanxiu, Wang Yuan, et al. Investigation of inter-particle mechanical behavior in three-dimensional granular systems via micro-CT[J]. Acta Mechanica Sinica, 2023, 55(08): 1732-1745.

Mechanica Sinica2023, 55(08): 1732 - 1741.

Wang Hongxun. Principles of Hydraulic Fracturing [M]. Beijing: Petroleum Industry Press, 1987.

References

Petroleum Industry Press. 1987.

Sun Qicheng, Wang Guangqian. Review of granular flow dynamics and its discrete models. Advances in Mechanics, 2008, 38(1): 87-100.

Chen Fanxiu, Zhuang Qi, Wang Rilong. A preliminary study of dynamic failure mechanism of force chain in a particle system based on digital image correlation. Journal of Vibration and Shock, 2016, 37(2): 563-568.

correlation[J]. Rock and Soil Mechanics, 2016, 37(2):563 - 573.

Scientific Reports ,2022 Jiang Xiaoyu, Takashi Matsushima,Raphael Blumenfeld. Coordinated stress structure self rganization in granular packing[J].Phys Rev E 111(4 2):045406.

Distribution Law of Shallow Crustal Measured Stress in Mainland China

Abstract: Based on the collection and systematic organization of measured in-situ stress data from mainland China, this paper analyzes the distribution characteristics of shallow crustal stress. By examining the relationship between stress magnitude, direction, and depth, the study reveals the fundamental patterns of the stress field across different tectonic regions. The results provide critical scientific references for crustal stability assessment, seismic hazard analysis, and underground engineering design.

1 Introduction

In-situ stress is the fundamental force driving crustal deformation, earthquake occurrence, and various geological processes. Understanding the distribution of the stress field in the shallow crust is of great significance for both fundamental geoscience research and practical engineering applications. With the rapid development of infrastructure and resource extraction in China, the demand for accurate in-situ stress data has become increasingly urgent.

This study compiles a comprehensive database of measured in-situ stress values across mainland China, utilizing methods such as hydraulic fracturing and overcoring. By synthesizing these data points, we aim to characterize the state of stress in the upper crust and discuss its relationship with regional tectonics.

2 Data Sources and Processing

The data used in this study are derived from decades of in-situ stress measurements conducted for hydropower projects, mining operations, and scientific research. To ensure data quality, a rigorous screening process was applied to the raw measurements.

[TABLE:1]

The database includes the three principal stresses: the maximum horizontal principal stress ($\sigma_H$), the minimum horizontal principal stress ($\sigma_h$), and the vertical stress ($\sigma_v$). The vertical stress is generally assumed to be equivalent to the overburden pressure, calculated as $\sigma_v = \gamma z$, where $\gamma$ is the average unit weight of the rock and $z$ is the depth.

3 Distribution Characteristics of Stress Magnitude

The analysis of the measured data indicates that stress magnitudes generally increase linearly with depth. However, significant scatter exists due to local geological structures, topography, and lithological variations.

3.1 Relationship between Principal Stresses and Depth

Regression analysis was performed on the collected data to establish the relationship between the principal stresses and depth ($z$). The statistical results suggest that in the shallow crust (typically $0$ to $3000$ m), the horizontal stresses are often greater than the vertical stress.

$$\begin{aligned} \sigma_H &= A + B z \ \sigma_h &= C + D z \end{aligned}$$

Mechanics and Engineering, 2007, 26(10): 1975 - 1981.

Mechanical Mechanism of the Critical Transition Between Solid and Fluid States in Geotechnical Granular Media

Abstract

The mechanical behavior of geotechnical granular media at the transition between solid and fluid states is a critical area of study in geomechanics. This research investigates the underlying mechanisms governing the transformation from a stable solid-like state to a flowing fluid-like state. By analyzing the internal stress distribution and particle interaction dynamics, we establish a theoretical framework to describe this critical transition. The findings provide insights into the stability of granular structures and the initiation of flow-like failures in geotechnical engineering.

1. Introduction

Geotechnical granular media, such as soil, rock fragments, and sand, exhibit complex mechanical properties that vary significantly depending on their state of stress and packing density. Under certain conditions, these materials can transition from a solid state, characterized by the ability to support shear stress without continuous deformation, to a fluid state, where they flow under gravity or external loads. Understanding the critical conditions for this transition is essential for predicting landslides, debris flows, and the stability of earthworks.

2. Theoretical Framework

The transition between solid and fluid states in granular media is governed by the competition between frictional resistance and driving forces. We consider the granular assembly as a discrete system where the macroscopic behavior emerges from microscopic interactions.

2.1 Stress Distribution and Force Chains

In the solid state, granular media support loads through a network of force chains. The stability of these chains is maintained by inter-particle friction and geometric interlocking. The stress tensor $\sigma_{ij}$ within the medium can be expressed as:

$$\sigma_{ij} = \frac{1}{V} \sum_{c \in V} f_i^c l_j^c$$

where $f_i^c$ represents the contact force at contact $c$, and $l_j^c$ is the branch vector connecting the centers of the contacting particles. As the shear stress increases, these force chains undergo reconfiguration.

2.2 The Critical State Concept

The critical state is defined as the condition under which the granular medium continues to deform at constant stress and constant volume. At this point, the material loses its solid-like memory of its initial fabric. The transition to a fluid state occurs when the rate of energy dissipation through friction is exceeded by the work done by external forces, leading to a rapid increase in kinetic energy.

3. Mechanical Mechanism of State Transition

The transition from solid to

Wang Meimei. Study on the Mechanical Mechanism of Solid liquid Critical Transformation in Granular Geo materials[D]. University of Science and Technology Beijing, 2022.

Exploring Mesoscopic Deformation Mechanism of Sand in Direct Shear Test by Numerical Simulation Using Discrete Element Method

1 Introduction

The direct shear test is one of the most fundamental methods for determining the shear strength of soil in geotechnical engineering. While macroscopic experimental results provide essential design parameters, they often fail to capture the complex internal evolution of the soil fabric during shearing. With the advancement of computational mechanics, the Discrete Element Method (DEM) has emerged as a powerful tool for investigating the mechanical behavior of granular materials from a mesoscopic perspective. By simulating the interactions between individual particles, researchers can gain deeper insights into the deformation mechanisms, stress distribution, and the evolution of the contact network within the soil specimen.

2 Numerical Simulation of Direct Shear Test

In this study, a numerical model of the direct shear test was established using the discrete element framework to simulate the behavior of sandy soil. The simulation environment replicates the boundary conditions of a standard laboratory shear box, consisting of an upper and lower frame.

2.1 Contact Constitutive Model

The interaction between sand particles is governed by a linear elastic-plastic contact model. The normal force $\mathbf{F}n$ and shear force $\mathbf{F}_s$ at the contact point are calculated based on the overlap and relative displacement between particles. The normal force is defined as:
$$\mathbf{F}_n = K_n U_n \mathbf{n}$$
where $K_n$ represents the normal stiffness, $U_n$ is the normal penetration, and $\mathbf{n}$ is the unit normal vector at the contact point. The shear force is limited by the Coulomb friction law:
$$\mathbf{F}_n| + C$$} = \mu |\mathbf{F
where $\mu$ is the friction coefficient and $C$ is the cohesion (which is typically zero for clean sand).

[FIGURE:1]

2.2 Model Parameters and Boundary Conditions

The numerical specimen was generated using a radius expansion method to achieve the target void ratio. The assembly consists of spherical particles with a grain size distribution consistent with experimental sand samples. During the consolidation phase, a constant vertical pressure is applied to the top boundary. Once equilibrium is reached, the lower shear box is moved at a constant velocity while the upper box remains stationary, simulating the shearing process.

3 Mesoscopic Deformation Mechanism

The discrete element simulation allows for the visualization of internal variables that are difficult to measure in physical

Hou Mingxun .The Test on the Stress Grid of Granular Material and Characteristic Research on Arching Effect[D]. SouthChinaUni versity of Technology,2016. ] Wang Meimei. Zheng Jianwei, Xue 2024,14(6), 2485.

Pan Yuanyang, Wei Yufeng, Li Yuanzheng, et al. Study on the microscopic deformation mechanism of shear zone formation in sandy soil under direct shear tests [J]. Hydro-Science and Engineering, 2020.

Hydro-Science and Engineering, 2020; Philosophical Transactions of the Royal Society of London (Series A) [J], 1921, 221(A): 163. Mu Lijun, Qi Yin, Chen Wenbin, et al. Distribution characteristics of core fractures and proppants at the Qingcheng shale oil hydraulic fracturing test site [J], 2025, 46(9): 1764.

site[J]. Acta Petrolei Sinica, 2025, 46(9): 1764 - 1775.

Fracture Description and Insights from a High-Inclination Coring Well at the Qingcheng Shale Oil Hydraulic Fracturing Test Site [J]. China Petroleum Exploration, 2025, 30(2): 133.

Zhang Kuangsheng, Mu Lijun, Lu Hongjun, et al. Overview and Practical Understanding of the Construction of Shale Oil Hydraulic Fracturing Test Sites in the Ordos Basin [J]. Drilling & Production Technology. Research on Accumulation Conditions and Geophysical Characteristics of Mudstone Fractured Reservoirs in the Gulong Area, 2012.

Huo Fenglong. Research on reservoir forming conditions and geophysical characteristics of mudstone fractured reservoirs in Gulong area [D]. Zhejiang

University ， 2012.