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Article

Development Characteristics of Natural Fractures in Metamorphic Basement Reservoirs and Their Impacts on Reservoir Performance: A Case Study from the Bozhong Depression, Bohai Sea Area, Eastern China

by
Guanjie Zhang
1,2,
Jingshou Liu
1,2,*,
Lei Zhang
3,
Elsheikh Ahmed
1,2,
Qi Cheng
3,
Ning Shi
1,2 and
Yang Luo
1,2
1
School of Earth Resources, China University of Geosciences, Wuhan 430074, China
2
Hubei Key Laboratory of Oil and Gas Exploration and Development Theory and Technology (China University of Geosciences), Wuhan 430074, China
3
Tianjin Branch of China National Offshore Oil Corporation, Tianjin 841000, China
*
Author to whom correspondence should be addressed.
J. Mar. Sci. Eng. 2025, 13(4), 816; https://doi.org/10.3390/jmse13040816
Submission received: 9 March 2025 / Revised: 10 April 2025 / Accepted: 16 April 2025 / Published: 19 April 2025
(This article belongs to the Special Issue Advances in Offshore Oil and Gas Exploration and Development)
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Abstract

:
Archaean metamorphic basement reservoirs, characterized by the development of natural fractures, constitute the primary target for oil and gas exploration in the Bozhong Depression, Bohai Bay Basin, Eastern China. Based on analyses of geophysical image logs, cores, scanning electron microscopy (SEM), and laboratory measurements, tectonic fractures are identified as the dominant type of natural fracture. Their development is primarily controlled by lithology, weathering intensity, and faulting. Fractures preferentially develop in metamorphic rocks with low plastic mineral content and are positively correlated with weathering intensity. Fracture orientations are predominantly parallel or subparallel to fault strikes, while localized stress perturbations induced by faulting significantly increase fracture density. Open fractures, constituting more than 60% of the total reservoir porosity, serve as both primary storage spaces and dominant fluid flow conduits, fundamentally governing reservoir quality. Consequently, spatial heterogeneity in fracture distribution drives distinct vertical zonation within the reservoir. The lithological units are ranked by fracture development potential (in descending order): leptynite, migmatitic granite, gneiss, cataclasite, diorite-porphyrite, and diabase. Diabase represents the lower threshold for effective reservoir formation, whereas overlying lithologies may function as reservoirs under favorable conditions. The large-scale compressional orogeny during the Indosinian period marked the primary phase of tectonic fracture formation. Subsequent uplift and inversion during the Yanshanian period further modified and overlaid the Indosinian structures. These structures are characterized by strong strike-slip strain, resulting in a series of conjugate shear fractures. During the Himalayan period, preexisting fractures were primarily reactivated, significantly influencing fracture effectiveness. The development model of the fracture network system in the metamorphic basement reservoirs of the study area is determined by a coupling mechanism of dominant lithology and multiphase fracturing. The spatial network reservoir system, under the control of multistage structure and weathering, is key to the formation of large-scale effective reservoirs in the metamorphic basement.

1. Introduction

Basement reservoirs exhibit pronounced heterogeneity in their properties, with varied storage spaces and significant thicknesses of reservoir rock [1,2,3]. These reservoirs are widespread globally and are recognized as important sources of oil and gas [4,5]. The lithology of basement reservoirs is notably diverse, predominantly comprising metamorphic rocks, igneous rocks, sandstones, and carbonates [6,7]. Typically, basement reservoirs are classified into weathered crust reservoirs, located at structural elevations, and interior basement reservoirs. Basement reservoirs typically have undergone multiple phases of tectonic activity and diagenesis, leading to the development of natural fractures and secondary pores, making them highly heterogeneous [8,9,10].
The Bozhong Depression in the Bohai Bay Basin ranks among the most productive oil and gas regions in China. The extensive distribution of Archaean metamorphic basement reservoirs in deeper sections of the Bozhong Depression has greatly facilitated progress in oil and gas exploration [11,12]. Because primary pores are poorly developed in metamorphic rocks, natural fractures and associated secondary porosity are prevalent in these reservoirs. These natural fractures serve as primary storage spaces, playing a critical role in enhancing fluid migration and flow capacity within reservoirs [4,9]. Furthermore, fractures exert significant influence on reservoir fracturing reconstruction and the optimized design of petroleum engineering schemes [13,14,15,16,17]. Therefore, accurately characterizing the fracture system within these reservoirs is essential for the execution of fracturing and horizontal drilling operations, as well as for the strategic deployment of well patterns [18,19].
With advancements in oil and gas exploration and the development of metamorphic basement reservoirs, numerous studies have been conducted from both geological and engineering perspectives. Scholars have primarily investigated the formation mechanisms of these reservoirs and the key factors influencing their effectiveness. Furthermore, the studies that have been conducted on metamorphic basement reservoirs have explained mainly the strong heterogeneity and prominent vertical stratification of reservoirs [20,21,22,23,24,25,26]. Research has shown that metamorphic basement reservoirs are controlled to varying degrees by fracture distribution, and that their oil and gas bearing capacity and productivity strongly depend on natural fractures and later development plans [27,28,29]. However, while current research focuses on the use of geophysical and geological data to predict and characterize fractures in metamorphic basement reservoirs [30,31], the development characteristics of natural fractures, the controlling factors governing their formation, and their impact on reservoir quality remain incompletely understood. The fundamental causes of the distinct vertical zonation observed in metamorphic basement reservoirs also require further investigation [3]. Furthermore, the mechanisms underlying the genesis and development patterns of natural fractures in deep metamorphic basement reservoirs have not been thoroughly investigated [9].
This study investigates the metamorphic basement reservoirs in the Bozhong Depression by utilizing geophysical log data, cores, fluorescence scanning microscopy, SEM, and experimental data from 18 wells. It aims to elucidate the development characteristics of natural fractures in metamorphic basement reservoirs in the Bozhong Depression, and the main factors controlling the formation and development of fractures are discussed on the basis of geophysical log data. On the basis of the experimental data, the influence of open fractures on the storage space and fluid flow of metamorphic basement reservoirs is discussed, and the root cause of obvious vertical zonation of metamorphic basement reservoirs is discussed. Drawing on these results, spatial fracture network phylogenetic models are established for metamorphic basement reservoirs at different structural positions. These results can provide insights for the future exploration and exploitation of other metamorphic basement reservoirs.

2. Geological Setting

The Bohai Bay Basin, located in eastern China, has a rhombic shape and covers an area of approximately 20 × 104 km2, trending in a NW–SE direction (Figure 1A). It is a Mesozoic–Cenozoic superimposed basin that developed on the Paleozoic cratonic basement of North China through strike-slip and pull-apart tectonics [32,33,34,35]. The Bozhong Depression, the largest hydrocarbon-generating depression within the Bohai Bay Basin (Figure 1B), has developed a complex fault system since the Mesozoic. This system is characterized by early extensional faults, mid-phase strike-slip extensional faults, and late-stage strike-slip faults in localized areas. Multiple phases of fault activity control the distributions of various source rock sequences, influencing their thermal evolution. Additionally, the diverse trap types formed by faulting have provided favorable conditions for hydrocarbon accumulation, with faults acting as effective conduits for the vertical migration of hydrocarbons [36,37].
The Bozhong Depression is underlain by basement rocks, with Paleozoic and Proterozoic strata entirely absent, and parts of the Mesozoic strata are also missing (Figure 2). The stratigraphic sequence encountered in boreholes, from top to bottom, includes the Quaternary Pingyuan Formation; the Neogene Minghuazhen and Guantao Formations; the Paleogene Dongying, Shahejie, and Kongdian Formations; Mesozoic strata; and the Archean metamorphic basement [38,39]. Owing to multiple phases of intense tectonic activity, the structural characteristics of the basement strata in the Bozhong Depression are highly complex, with different stratigraphic sequences in direct contact with each other (Figure 1C). The Archean metamorphic rocks, which constitute the most extensive and oldest crystalline basement in the Bozhong Depression, constitute the primary focus of this study. These rocks are typically buried at depths greater than 4000 m, with exploratory wells revealing a maximum thickness exceeding 1300 m [40,41]. Hydrocarbons in the metamorphic basement reservoirs originate from three major source rocks: the third member of the Paleogene Shahejie Formation, the first member of the Shahejie Formation, and the third member of the Dongying Formation. The primary cap rocks for these reservoirs are the thick lacustrine mudstones of the Shahejie and Dongying Formations. This combination of source rocks, cap rocks, and reservoirs has ensured the preservation of oil and gas within metamorphic basement reservoirs over long geological periods, providing favorable conditions for hydrocarbon accumulation [42,43].
Owing to multiple phases of intense compressional uplift, metamorphic basement reservoirs have experienced complex weathering processes, leading to the formation of weathered crust reservoirs at structural highs and interior basement reservoirs [44]. The storage spaces in these metamorphic basement reservoirs are diverse and primarily consist of natural fractures and secondary porosity. The physical properties of these reservoirs exhibit significant heterogeneity, with porosities ranging from 1% to 10% and an average porosity of 5.22%, while permeabilities range from 0.003 mD to 20.33 mD, with an average permeability of 0.37 mD [26].

3. Dataset and Methodology

The data and samples used in this study were collected from metamorphic basement reservoirs in the Bozhong Depression. These include cores, thin sections, SEM images, and geophysical log data from 18 vertical wells. A total of 109 thin sections (30 μm thick) were stained with blue or red resin dyes to enhance the visibility of fractures and pores [45]. Geophysical log data were employed to identify strong weathering fracture zones (SWFZs), weak weathering fracture zones (WWFZs), and basement zones (BZs). Additionally, this study utilized data from the Tianjin Branch of the China National Offshore Oil Corporation, along with relevant literature, to gather information on faults, stratigraphy, paleogeomorphology, and production data necessary for the research.
The characterization of natural fractures in metamorphic basement reservoirs is based on their orientation, dip angle, density, aperture, and filling conditions. Core observations and analyses were conducted at the core repository of the Tianjin Branch of the China National Offshore Oil Corporation. Notably, this method of studying fractures has certain limitations; for example, fracture information is only within the core scale because of the influence of core size, and some secondary fractures may be induced during the coring procedure. To distinguish between natural and induced fractures in the cores, the criteria established by Sangree (1969) [46] and Nelson (1985) [47] were applied.
Natural fractures identified in the image logs were processed and interpreted following established methods [48,49,50]. The results indicate that conductive fractures are typically filled with conductive mud or low-resistivity minerals, appearing as low-resistivity black sinusoidal curves in the image logs. In contrast, resistive fractures are generally filled with high-resistivity minerals, appearing as light-colored or white sinusoidal curves [45]. Fracture linear density was defined as the number of fractures observed per unit length in a specific direction, as observed in both cores and image logs. The aperture of fractures identified in the image logs was calculated via the formula proposed by Luthi and Souhaite (1990) [51]. Additionally, the porosities of the fractures in the image logs were calculated via the method proposed by Wang et al. (2022) [52].
At the Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences (Wuhan), natural fractures and pore characteristics in thin sections were identified and quantitatively measured using an OLYMPUS BH-2 polarizing microscope (Leica Company, Wetzlar, Germany) and a Philips XL30 SEM (Royal Philips, Eindhoven, The Netherlands) [53]. Additionally, fluorescence scanning of cores and thin sections was performed to analyze the hydrocarbon compounds present in natural fractures (Liu et al., 2020a) [25].
X-ray diffraction (XRD) analysis was conducted to determine the weight percentages of 43 metamorphic basement reservoir rock samples from 18 wells. The tests were performed using a D/max-2500 X-ray diffractometer (Rigaku Corporation, Tokyo, Japan). The test conditions included a tube voltage of 40 kV, a tube current of 200 mA, a temperature of 25 °C, and a humidity of 40%. The distribution of samples by lithology was as follows: leptynite (5), migmatitic granite (14), gneiss (9), cataclasite (6), diorite-porphyrite (5), and diabase (4). The permeability and porosity of 51 metamorphic basement reservoir rock samples from 18 wells were measured using a helium porosimeter [54,55,56,57]. These standard core samples, each with a length of 5 cm (2 in) and a radius of 2.5 cm (1 in), were saturated with oil and dried prior to testing. The number of samples per lithology was as follows: leptynite (4), migmatitic granite (14), gneiss (18), cataclasite (6), diorite-porphyrite (5), and diabase (4).

4. Results

Since the beginning of the Cenozoic era, the metamorphic basement reservoirs in the Bozhong Depression have undergone prolonged and complex tectonic movements. Intense compression during the Indosinian period, in particular, provided the external mechanical conditions necessary for fracture formation. A significant number of fractures developed during the folding and uplift of these reservoirs. These fractures, which are closely associated with tectonic activity, are referred to as tectonic fractures [58] and constitute the primary type of natural fracture in the metamorphic basement reservoirs of the study area. Additionally, during the extensive burial and diagenetic evolution of the reservoirs, dissolution fractures formed due to the action of subsurface fluids associated with mineral dissolution. A systematic characterization of natural fractures in the metamorphic basement reservoirs of the Bozhong Depression was conducted through detailed observations and analyses of image logs, cores, thin sections, and scanning electron microscopy (SEM) data, in combination with the regional geological context.

4.1. Petrography

Analysis of core and image log data indicates that the lithologies of the metamorphic basement reservoirs in the Bohai Bay Basin primarily include gneiss, migmatitic granite, cataclasite, leptynite, and later intrusive dikes such as diorite-porphyrite and diabase. The X-ray diffraction (XRD) results (Table 1) indicate that the brittle mineral content varies among the different metamorphic rocks. In these rocks, brittle minerals are primarily composed of plagioclase, alkali feldspar, and quartz, whereas plastic minerals primarily consist of amphibole, biotite, and pyroxene. In the metamorphic basement of the Bozhong Depression, leptynite contains more than 90% brittle minerals and only a very small proportion of plastic minerals. Migmatitic granite and gneiss also exhibit high brittle mineral contents, exceeding 80%, with the remainder composed of plastic minerals. In contrast, later intrusive dikes, such as diorite-porphyrite and diabase, contain lower brittle mineral contents (less than 75%) and relatively higher proportions of plastic minerals.
The permeability and porosity test results of rock samples from metamorphic basement reservoirs show that rocks rich in brittle minerals possess better reservoir properties than those rich in plastic minerals. Leptynite exhibits the most favorable physical properties, with an average porosity of 4.6% and an average permeability of 0.578 × 10−3 μm2, followed by migmatitic granite, which has an average porosity of 3.2% and an average permeability of 0.142 × 10−3 μm2. In contrast, late intrusive diorite-porphyrite and diabase exhibit the poorest physical properties, with average porosities of 2.1% and 1.7%, respectively, and average permeabilities of 0.041 × 10−3 μm2 and 0.039 × 10−3 μm2, respectively (Table 2).

4.2. Characteristics of Natural Fractures by Image Logs

Image logs reveal that tectonic fractures in the metamorphic basement reservoirs of the study area are predominantly medium-angle fractures, with low-angle and high-angle fractures comprising a relatively small proportion. High-angle fractures (dips > 75°) account for 5.6% of all fractures, while low-angle fractures (dips < 25°) represent 12.5%. Medium-angle fractures (dipping between 25° and 75°) constitute 81.9% of all the fractures. The aperture of tectonic fractures varies significantly with dip angle: low-angle fractures typically have apertures of less than 50 µm, medium-angle fractures generally range from 50–100 µm, and high-angle fractures often exceed 100 µm. The fractures are primarily oriented in nearly E–W, NE–SW, NW–SE, and ENE–WSW directions. Among these fractures, those oriented ENE–SW and NW–SE are the most prevalent, while those oriented NE–SW and nearly E–W are less common (Figure 3). In the image logs, most tectonic fractures are unfilled, with filled fractures comprising only 38% of the total fractures (Figure 4). Dissolution fractures, formed by the dissolution of early tectonic fractures, are also observed in the image logs (Figure 5).
The image logs reveal significant variations in the degree of tectonic fracture development among the various rock types within the metamorphic basement reservoirs of the Bozhong Depression. Leptynite exhibits the highest average linear density of tectonic fractures (1.52 m−1), followed by migmatitic granite (1.12 m−1), granitic gneiss (1.10 m−1), and cataclasite (1.06 m−1). In contrast, the diorite-porphyrite and diabase samples exhibit lower average linear densities of tectonic fractures, at 0.65 m−1 and 0.58 m−1, respectively.

4.3. Characteristics of Natural Fractures by Cores

Core inspections reveal that the surfaces of tectonic fractures are typically uneven (Figure 6A,C,D) and exhibit irregular development (Figure 6B). Fractures with different orientations intersect or terminate against one another (Figure 6D), forming a network fracture system through mutual crosscutting (Figure 6B). The lengths of tectonic fractures are generally less than 30 cm, with most fractures ranging between 5 and 20 cm. Statistical analysis indicates that the distribution of tectonic fractures is highly heterogeneous, with average fracture linear densities ranging from 1–6 m−1 and maximum values reaching up to 12 m−1.

4.4. Characteristics of Natural Fractures by Thin Sections and SEM Images

The development of different types of microfractures in metamorphic basement reservoirs varies significantly. Statistical analysis indicates that the average surface density of intragranular fractures is 3.7 mm mm−2, whereas the average surface density of intergranular fractures is lower, at only 0.61 mm mm−2. The aperture of intragranular fractures is generally small, typically less than 10 μm, whereas the aperture of intergranular fractures is larger, ranging between 15 and 30 μm. In terms of fracture filling, 43% of the fractures are filled primarily with clay minerals, followed by carbonate fillings (Figure 7A,B). Fractures filled with quartz and bitumen are less common (Figure 7C). Additionally, some early-filled fractures were reopened under the influence of subsequent tectonic movements, with carbonate cement precipitating to form bright calcite filling layers (Figure 7D).
Owing to prolonged weathering and leaching, metamorphic basement reservoirs commonly exhibit dissolution fractures formed by the modification of early fractures through dissolution processes [59]. Dissolution fractures are typically curved, branched, or network-like in distribution, and their edges are serrated, with dissolution traces, and the tectonic fracture surface is relatively flat. Dissolution holes often develop around dissolution fractures, such as “beaded” pores distributed along fractures. In addition, local dissolution fractures often widen, or dissolution holes can form. When early fractures are altered by subsequent fluid activity, their apertures increase, fracture surfaces become rougher, and their shapes become more irregular (Figure 8A). Additionally, as atmospheric freshwater percolates downward along tectonic fractures, the unstable minerals in the fracture fillings are either partially or completely dissolved, rendering previously filled fractures effective once again [60,61] (Figure 8B). However, owing to the lithological and structural characteristics of metamorphic basement reservoirs, dissolution fractures are less common than tectonic fractures and have relatively less significance in the processes of hydrocarbon migration and accumulation.
SEM images reveal the development of microfractures in metamorphic basement reservoirs. On the basis of the relationship between fractures and mineral grains, microfractures can be categorized into two types: (1) intergranular fractures and (2) intragranular fractures. Intergranular fractures occur along the edges of grains or cut directly through larger mineral crystals [62] (Figure 9A). These fractures can be further divided into two types on the basis of their development characteristics: the first type develops along the edges of mineral crystals, with varying lengths and curved, irregular fracture surfaces; the second type cuts directly through larger mineral crystals such as quartz and feldspar, sometimes appearing in clusters and extending over longer distances. Intragranular fractures typically occur within mineral crystals such as quartz, mica, and feldspar, and their size is constrained by the grain size. Cleavage fractures, which are found in feldspar and mica, and crack fractures, which are found in quartz, represent the primary types of intragranular fractures (Figure 9B). Generally, one set of cleavage fractures forms in mica (Figure 9C), whereas two sets of parallel cleavage fractures develop in feldspar (Figure 9D). Cleavage fractures are relatively uniformly distributed and straight, whereas crack fractures are irregularly distributed, with curved fracture surfaces.

5. Discussion

5.1. Factors Controlling Fracture Development

(1)
Lithotypes
Lithology is a fundamental factor influencing fracture development. The average linear density of fractures in different rock types is positively correlated with brittle mineral content and negatively correlated with plastic mineral content (Table 1), whereas the plastic mineral content is relatively high. Rocks with lower plastic mineral content exhibit a lower ratio of cohesion modulus to elastic modulus, resulting in greater brittleness [63,64]. Under tectonic stress, these rocks are more prone to fracturing, leading to increased fracture development. Therefore, variations in mineral composition significantly impact tectonic fracture development in metamorphic basement reservoirs.
Core and thin section observations reveal that as the content of plastic minerals in rock increases, fractures are more likely to be filled with minerals, thereby reducing fracture effectiveness. In diorite-porphyrite and diabase, more than 83% of the fractures are filled. In contrast, the proportion of filled fractures is only 26% in migmatitic granite and 35% in gneiss, with almost no filled fractures in leptynite, resulting in greater fracture effectiveness. Additionally, rocks with a high content of brittle minerals exhibit a high degree of compositional and structural heterogeneity and contain minerals of various sizes, compositions, and shapes. The differential deformation rates of these grains under stress lead to force transmission, and deformation leads to an uneven stress field distribution within the rock, promoting fracture development [65,66].
(2)
Faulting
Faults influence the local stress distribution and thereby control fracture development. They are critical factors in controlling the heterogeneity of fracture development [67]. Rose diagrams of fracture orientations from 12 wells in the study area (Figure 10) reveal that fracture orientations are parallel or nearly parallel to the adjacent faults, indicating their significant influence on fracture development. During the early Indosinian period, nearly N–S compressive stress caused fractures and faults to align parallelly. The predominant NE–E orientation of faults and fractures in the study area suggests that intense compressive stress during this period was instrumental in fracture development [9]. Notably, the rose diagrams of fracture orientations in wells B13-2, B13-6, B19-8, and B19-14 reveal a significant number of N–E-oriented fractures, which align with the adjacent N–E-oriented faults, indicating that local stresses from these faults during the Yanshanian period also played a significant role. Additionally, fracture development is greater at fault intersections. For example, Well B19-8, located at a fault intersection, exhibited a high fracture linear density of 1.6 m−1. Furthermore, Well B19-8 features highly developed cataclastic zones with a distinct directionality, resulting from the combined effects of shear stress and tensile stress [65].
(3)
Weathering
The long geological evolution of the Bozhong Depression led to extended periods of weathering and erosion in the basement metamorphic rocks. Feldspar minerals, which are less resistant to weathering, underwent significant alteration, increasing rock fragmentation. Following physical weathering, chemical leaching and dissolution further modified the basement metamorphic rocks, influencing fracture formation and development.
On the basis of weathering intensity, the metamorphic basement reservoirs in the Bozhong Depression are vertically divided into a strong weathering fracture zone (SWFZ), a weak weathering fracture zone (WWFZ), and a basement zone (BZ). Image logs and core data from Well B19-15 (Figure 11) indicate that in the SWFZ, fractures intersect to form a network, with multiple dissolution pores and fractures observed. In the WWFZ, fracture development is lower than in the SWFZ, with fractures often occurring individually. In the BZ, image logs and core data show that pores and fractures are poorly developed. Overall, the degree of fracture development decreases as the weathering intensity decreases.
Table 3 presents the fracture linear density and fracture porosity characteristics across the strong and weak weathering fracture zones and the basement zone in the Bozhong Depression. As the weathering intensity decreases, which reduces the mechanical properties of reservoir rocks, the linear fracture density decreases progressively from the strong to the weak weathering zones and into the basement, indicating reduced fracture development. However, in some wells, the basement zone exhibits higher fracture porosity than the weathered zones due to clay layer formation from intense weathering [9,68]. Continued weathering intensifies mineral dissolution, causing increased precipitation of minerals such as clay and calcite, which can clog pores and fractures. This extensive fracture and pore filling explains why some wells have greater fracture porosity in the basement zone than in the weathering zones.
The duration and intensity of weathering and leaching in metamorphic basement reservoirs vary on the basis of paleogeomorphology, with higher paleogeomorphic positions typically experiencing longer exposures. Paleogeomorphic elevation, often indicated by the thickness of the overlying sedimentary layers, generally correlates positively with this thickness. Statistical analysis revealed a negative correlation between the thickness of these layers and the average fracture linear density and aperture in the strong weathering zone (Figure 12), suggesting greater fracture development at higher paleogeomorphic positions. However, the correlation is not strong, indicating that fracture development in metamorphic basement reservoirs is influenced by multiple factors beyond paleogeomorphology.

5.2. Vertical Stratification of Reservoirs

The metamorphic basement reservoirs in the Bozhong Depression show significant vertical heterogeneity, characterized by stratified hydrocarbon accumulation. This heterogeneity arises from variations in fracture development across rock types, leading to differences in porosity, permeability, and reservoir quality. High-quality reservoirs form in rocks with better physical properties and more fractures, whereas rocks with lower fracture development act as interlayers. These interlayers create multiple distinct pressure systems, contributing to the strong vertical heterogeneity observed in these reservoirs.
Core physical property tests revealed that rocks rich in brittle minerals exhibit better reservoir properties than those rich in plastic minerals. Statistical analysis revealed a positive correlation between the physical properties of different lithologies and the degree of tectonic fracture development across different lithologies (Figure 13, Table 1 and Table 3). Fractures are crucial for providing storage space and migration pathways for hydrocarbons, making them essential to effective reservoir formation. Thus, variations in fracture development are the primary cause of vertical stratification in metamorphic basement reservoirs.
In the Bozhong Depression, the lithologies of metamorphic basement reservoirs, ranked by fracture development from highest to lowest, are leptynite, migmatitic granite, gneiss, cataclasite, diorite-porphyrite, and diabase. As the plastic mineral content (e.g., biotite and amphibole) increases and the brittle mineral content (e.g., quartz and plagioclase) decreases, the likelihood of forming effective reservoirs diminishes under comparable geological conditions. Production data indicate that lithologies preceding diorite-porphyrite in this sequence generally bear hydrocarbons and serve as main reservoir rocks, with well-developed, unmineralized fractures. In contrast, diorite-porphyrite and diabase exhibit poorer physical properties but possess better sealing abilities and often act as interlayers within reservoirs.
On the basis of well test data, metamorphic basement reservoirs in the Bohai Bay Basin are classified into three levels: (1) Level I, with production above 1 × 104 m3/d; (2) Level II, with production between 0.1 × 104 and 1 × 104 m3/d; and (3) Level III, with production below 0.1 × 104 m3/d, often indicating dry layers or interlayers. In Well B19-15, leptynite is a Level I reservoir due to its high quality and better fracture development, while gneiss is classified Level II or III (Figure 14A). In Well B19-18, gneiss ranks as Level I and II, whereas diorite-porphyrite is Level III due to poorer fracture development (Figure 14B). In Well B19-14, diorite-porphyrite is classified as Level II and is confirmed as hydrocarbon-bearing (Figure 14C). Consistently, diabase in multiple wells (e.g., Well B19-20) is classified as Level III, functioning as an inner interlayer rather than an effective reservoir (Figure 14D). These examples indicate that lithologies preceding diabase may serve as reservoir rocks or interlayers, whereas diabase functions only as an interlayer.
In metamorphic basement reservoirs, the potential for rocks to become reservoir rocks is limited by their position in the lithological sequence. In the Bozhong Depression, diabase marks the lower limit. Lithologies preceding it can become reservoir rocks under suitable geological conditions, while diabase and those following serve as interlayers. However, the formation of reservoir rocks still depends on the extent of fracture development. This arrangement of reservoirs and interlayers establishes a favorable geological framework for hydrocarbon accumulation.

5.3. Fracture Patterns in Metamorphic Rocks

During the early Indosinian period, the metamorphic basement reservoirs of the Bozhong Depression in the Bohai Bay Basin underwent intense NNW–SSE compressional orogeny. According to the Coulomb fracture criterion, an increase in the maximum principal stress expands the Mohr stress circle, causing it to intersect with the fracture envelope and resulting in rock fracturing. This process primarily produced medium- to low-angle shear fractures trending NEE–WSW, along with high-angle shear, high-angle longitudinal extension, and transverse extension fractures (Figure 15A).
During the late Indosinian period, intense NE–SE compressional thrusting further affected the metamorphic basement reservoirs in the study area, leading to the formation of numerous medium- to low-angle NW–SE shear fractures. Additionally, several high-angle shear, high-angle longitudinal extension, and transverse extension fractures also developed (Figure 15B).
During the Yanshanian period, the study area experienced sustained NW–SE compressional forces due to the low-angle subduction of the ancient Pacific Plate beneath the eastern Eurasian margin, combined with the oblique subduction of the Pacific Plate and the distant effects of the Indian Plate’s convergence. These forces caused NE-oriented sinistral strike-slip motion. The resulting stress conditions expanded or shifted the Mohr stress circle, increasing the likelihood of fracturing. This led to numerous medium- to low-angle NE–SW shear fractures, along with some nearly S–N high-angle shear fractures, as well as a few high-angle longitudinal and transverse extension fractures (Figure 15C).
During the Himalayan period, the stress field in the Bozhong Depression shifted to a primarily S–N-oriented extensional stress, reactivating preexisting fractures. This reactivation, along with the formation of a few nearly E–W-oriented high-angle tensile fractures, highlights the Himalayan period as a crucial phase in the development of effective fractures within the metamorphic basement reservoirs of the Bozhong Depression (Figure 15D).

5.4. Development Models of Reservoir Fracture Networks and the Role of Natural Fractures in Reservoirs

On the basis of a clear understanding of the fracture characteristics, main controlling factors, mechanical properties, and formation stages of the metamorphic basement reservoirs in the Bozhong Depression, a development model of the fracture network system has been constructed. The complex lithology of these reservoirs has a significant effect on the distribution of fracture networks. Additionally, a distinct vertical development pattern is observed in the metamorphic basement reservoirs, represented by a two-layer model consisting of a weathering fracture zone and a basement zone. In the weathering fracture zone, fracture development is controlled by both weathering and lithology, resulting in an increased degree of fracture development and a wide range of lithologies, forming a three-dimensional spatial fracture network composed of multiple cataclastic zones.
Fractures in the basement zone are predominantly influenced by fault development caused by tectonic movements. Under similar tectonic stress conditions, layers with a high content of brittle minerals and a low content of plastic minerals are more conducive to the formation of cataclastic zones. Since the Cenozoic, the study area has undergone several phases of tectonic activity, resulting in the formation of a complex fault system and stress field distribution. Fracture development zones are typically located within compressional anticline structures that formed during the Indosinian period as a result of a compressional tectonic setting. Drilling data indicate that rocks such as leptynite and migmatitic granite, which have low plastic mineral content, are well developed and prone to fracturing, thereby making them favorable zones for fracture development within the basement zone.
The metamorphic basement reservoirs in the study area exhibit distinct vertical distribution characteristics. The accumulation of multiple phases of tectonic stress has led to the formation of a three-dimensional network reservoir system, comprising a weathering fracture zone and a basement zone (Figure 16). The weathering fracture zone, located at the top, is influenced by weathering and leaching processes along faults, which facilitate the formation of effective fracture-type reservoirs. Moreover, the deeper basement zone, which is primarily controlled by lithology and multiple phases of tectonic activity, has promoted the development of larger-scale fracture-type reservoirs. This three-dimensional network reservoir system is essential for the formation of extensive, high-quality fracture-type reservoirs within the metamorphic basement of the Bozhong Depression.
Following their formation, the metamorphic basement reservoirs in the Bozhong Depression underwent multiple phases of superimposed tectonic and weathering processes over a prolonged geological evolution, resulting in highly heterogeneous reservoir properties [69]. The storage space in these metamorphic basement reservoirs comprises a dual system of pores and open-mode fractures. The primary porosity is insufficiently developed; thus, the predominant storage spaces are secondary pores. This secondary porosity includes intergranular dissolution pores, intragranular dissolution pores, and pores formed along fractures through dissolution. Additionally, the open-mode fractures consist of both tectonic fractures and certain types of dissolution fractures.
Microscopic observations of thin sections indicate that the apertures of microscale open-mode fractures in metamorphic basement reservoirs are comparable in size to matrix pores. This finding suggests that open-mode fractures are integrated into the pore system and can serve as effective storage spaces. Statistical analysis of 109 thin sections shows that the surface porosity of open-mode fractures in the metamorphic basement reservoirs of the Bozhong Depression ranges from 1.1% to 9.3%, with an average value of 3.1%, accounting for more than 60% of the total surface porosity. Therefore, these open-mode fractures provide effective storage space for oil and gas in metamorphic basement reservoirs.
The widespread development of secondary pores in metamorphic rocks significantly improves the reservoir performance, even in otherwise dense rocks. However, these secondary pores are typically distributed in an isolated manner, which poses challenges for achieving effective connectivity. The multiple phases of open-mode fractures developed in metamorphic basement reservoirs intersect and interconnect, providing favorable pathways for pore connectivity. These results indicate the critical role of fractures in improving the physical properties of metamorphic basement reservoirs and in enhancing fluid flow capabilities. Moreover, the formation of fractures creates advantageous channels for the infiltration of organic acids and meteoric freshwater, establishing a foundation for subsequent dissolution processes [70]. In some cases, fractures underwent intense dissolution along the fracture surfaces after their formation, leading to the development of secondary pores along the fractures. Although some pores in thin sections have experienced a certain degree of filling in later stages, they generally retain effective properties.
The results of core and thin section fluorescence scans revealed prominent hydrocarbons within the fractures (Figure 17), indicating that fractures of various scales provide effective pathways for hydrocarbon migration and significantly improve the quality of metamorphic basement reservoirs, particularly their flow capacity. Additionally, the Bozhong Depression features smaller-scale fractures and pores. Scanning electron microscopy (SEM) observations reveal that these microscale reservoir spaces are more widely distributed and more numerous than macroscopic fractures and pores, playing a similarly critical role in the formation of metamorphic basement reservoirs.

6. Conclusions

The metamorphic basement reservoirs in the Bozhong Depression, Bohai Bay Basin, feature well-developed natural fractures of various scales, predominantly tectonic fractures, with relatively few dissolution fractures. The degree of fracture development is primarily influenced by lithology, fault activity, and weathering. Tectonic fractures occur more often in metamorphic rocks with higher brittle and lower plastic mineral contents. Consequently, leptynite exhibits the highest average fracture linear density, followed by migmatitic granite and gneiss, while diorite porphyrite and diabase show lower average fracture linear densities. Fractures are more developed at elevated paleogeomorphic positions, with faults significantly controlling fracture orientation.
Owing to the varying extent of fracture development in different lithologies, metamorphic basement reservoirs exhibit distinct vertical zonation. On the basis of the degree of fracture development, the rocks in metamorphic basement reservoirs can be ranked from high to low as follows: leptynite, migmatitic granite, gneiss, cataclasite, diorite porphyrite, and diabase. Rocks ranked higher are more likely to become reservoir rocks in this lithological sequence, whereas those ranked lower typically serve as internal interlayers. In metamorphic basement reservoirs, rocks capable of becoming reservoir rocks generally exhibit a lower limit within this lithological sequence. Diabase represents the lower limit and cannot act as a reservoir rock, whereas rocks preceding diabase have the potential to become reservoir rocks under certain geological conditions.
The compressional effects of several phases of tectonic activity laid the foundation for the formation of tectonic fractures in the metamorphic basement reservoirs of the Bozhong Depression. The large-scale compressional orogeny during the Indosinian period marked the primary phase of tectonic fracture formation. Subsequent uplift and inversion during the Yanshanian period further modified and overlaid the Indosinian structures. These structures are characterized by strong strike-slip strain, resulting in a series of conjugate shear fractures. The Himalayan period primarily reactivated preexisting fractures, which significantly influenced fracture effectiveness.
The fracture network system in the studied metamorphic basement reservoirs is shaped by a coupling mechanism between dominant lithology and multiphase fracturing. Weathering and fault systems promote fracture development in the weathering zone, while the superposition of multistage tectonic stresses and lithological heterogeneity forms the basis for the development of large fracture zones in the basement zone. This three-dimensional network, consisting of weathering and basement zones, is essential for creating large-scale reservoirs. The important storage spaces are open-mode fractures and dissolution pores along fractures, which increase connectivity and improve reservoir quality. Multiscale natural fractures exert a controlling influence on hydrocarbon migration and accumulation in the metamorphic basement of the Bozhong Depression.

Author Contributions

Methodology, G.Z.; Software, N.S.; Validation, J.L.; Resources, J.L. and Q.C.; Data curation, Q.C.; Writing—original draft, G.Z.; Writing—review & editing, G.Z., L.Z., E.A., Q.C. and Y.L.; Project administration, J.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the National Natural Science Foundation of China (No. 42102156), and the “CUG Scholar” Scientific Research Funds at China University of Geosciences (Wuhan) (Project No. 2022046).

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Acknowledgments

This study would not have been possible without the assistance of the Tianjin Branch of the China National Offshore Oil Corporation, who provided cores, thin sections, scanning electron microscopy (SEM) images, and image logs for this study.

Conflicts of Interest

Authors Lei Zhang and Qi Cheng were employed by the company Tianjin Branch of China National Offshore Oil Corporation. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential con-flict of interest.

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Figure 1. (A) The location of the Bozhong Depression in China (modified after [33]). (B) Locations of the study wells in the Bozhong Depression. (C) Typical geological section through the Bozhong Depression. The position of profile AA’ is shown in (C).
Figure 1. (A) The location of the Bozhong Depression in China (modified after [33]). (B) Locations of the study wells in the Bozhong Depression. (C) Typical geological section through the Bozhong Depression. The position of profile AA’ is shown in (C).
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Figure 2. Stratigraphic framework of the Bozhong Depression, Bohai Bay Basin (after [42]). Light green indicates the main target layers.
Figure 2. Stratigraphic framework of the Bozhong Depression, Bohai Bay Basin (after [42]). Light green indicates the main target layers.
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Figure 3. Rose diagrams showing the dominant orientations of tectonic fractures in metamorphic rocks of the Bozhong Depression, Bohai Bay Basin, identified based on image logs (N = 6066).
Figure 3. Rose diagrams showing the dominant orientations of tectonic fractures in metamorphic rocks of the Bozhong Depression, Bohai Bay Basin, identified based on image logs (N = 6066).
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Figure 4. Tectonic fracture characteristics observed in image logs of metamorphic rocks from the Bozhong Depression, Bohai Bay Basin. (A) Unfilled, grouped tectonic fractures with similar occurrence from Well B13-4. (B) Unfilled mesh tectonic fractures from Well B13-4. (C) Filled tectonic fractures from Well B13-2. (The red lines represent the unfilled tectonic fractures; the yellow lines represent the filled tectonic fractures).
Figure 4. Tectonic fracture characteristics observed in image logs of metamorphic rocks from the Bozhong Depression, Bohai Bay Basin. (A) Unfilled, grouped tectonic fractures with similar occurrence from Well B13-4. (B) Unfilled mesh tectonic fractures from Well B13-4. (C) Filled tectonic fractures from Well B13-2. (The red lines represent the unfilled tectonic fractures; the yellow lines represent the filled tectonic fractures).
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Figure 5. Dissolution fracture characteristics observed in image logs of metamorphic rocks from the Bozhong Depression, Bohai Bay Basin. (A) Dissolution fractures from Well B19-9. (B) Dissolution fractures from Well B19-8. (C) Dissolution fractures from Well B19-8. (Green indicates that the fractures have been dissolutioned).
Figure 5. Dissolution fracture characteristics observed in image logs of metamorphic rocks from the Bozhong Depression, Bohai Bay Basin. (A) Dissolution fractures from Well B19-9. (B) Dissolution fractures from Well B19-8. (C) Dissolution fractures from Well B19-8. (Green indicates that the fractures have been dissolutioned).
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Figure 6. Tectonic fracture characteristics in cores of metamorphic rocks from the Bozhong depression, Bohai Bay Basin. (A) High-angle fractures from Well B19-7 at a depth of 4683.21 m; fracture surfaces are uneven. (B) Fractures from Well B19-14 at a depth of 4362.46 cm; tectonic fractures intersect to form a network of fracture systems. (C) High-angle fractures from Well B19-7 at a depth of 4625.56 m; fracture surfaces are uneven. (D) Tectonic fractures from Well B19-7 at a depth of 4682.22 m; later fractures in Group B terminate earlier fractures in Group A.
Figure 6. Tectonic fracture characteristics in cores of metamorphic rocks from the Bozhong depression, Bohai Bay Basin. (A) High-angle fractures from Well B19-7 at a depth of 4683.21 m; fracture surfaces are uneven. (B) Fractures from Well B19-14 at a depth of 4362.46 cm; tectonic fractures intersect to form a network of fracture systems. (C) High-angle fractures from Well B19-7 at a depth of 4625.56 m; fracture surfaces are uneven. (D) Tectonic fractures from Well B19-7 at a depth of 4682.22 m; later fractures in Group B terminate earlier fractures in Group A.
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Figure 7. Filling characteristics of tectonic fractures in thin sections of metamorphic rocks from the Bozhong Depression, Bohai Bay Basin. (A) Calcite-filled fractures in migmatitic granite from Well B19-9 at a depth of 5265 m. (B) Dolomite-filled fractures in gneiss from Well B19-2 at a depth of 4225 m. (C) Quartz-filled fractures in cataclasite from Well B19-9 at a depth of 5139 m. (D) Three-stage fracture filling in gneiss from Well B19-12 at a depth of 3878 m. Arrows labeled A, B, and C indicate: A, the first stage of filling with microcrystalline felsic material; B, the second stage of filling with limonite; and C, the third stage of filling with iron dolomite after the early cracks reopened. Qz: quartz, Bi: biotite, Pl: plagioclase, Kfs: K-feldspar.
Figure 7. Filling characteristics of tectonic fractures in thin sections of metamorphic rocks from the Bozhong Depression, Bohai Bay Basin. (A) Calcite-filled fractures in migmatitic granite from Well B19-9 at a depth of 5265 m. (B) Dolomite-filled fractures in gneiss from Well B19-2 at a depth of 4225 m. (C) Quartz-filled fractures in cataclasite from Well B19-9 at a depth of 5139 m. (D) Three-stage fracture filling in gneiss from Well B19-12 at a depth of 3878 m. Arrows labeled A, B, and C indicate: A, the first stage of filling with microcrystalline felsic material; B, the second stage of filling with limonite; and C, the third stage of filling with iron dolomite after the early cracks reopened. Qz: quartz, Bi: biotite, Pl: plagioclase, Kfs: K-feldspar.
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Figure 8. Dissolution fracture characteristics in thin sections of metamorphic rocks from the Bozhong Depression, Bohai Bay Basin. (A) Dissolution fractures in diorite-porphyrite from Well B19-13 at a depth of 4490 m; early tectonic fractures have been transformed by dissolution. (B) Dissolution fractures in diorite-porphyrite from B19-8 at a depth of 4731 m; partial dissolution of dolomite filling has reactivated the fracture. Qz: quartz, Pl: plagioclase, Dol: dolomite, Bi: biotite.
Figure 8. Dissolution fracture characteristics in thin sections of metamorphic rocks from the Bozhong Depression, Bohai Bay Basin. (A) Dissolution fractures in diorite-porphyrite from Well B19-13 at a depth of 4490 m; early tectonic fractures have been transformed by dissolution. (B) Dissolution fractures in diorite-porphyrite from B19-8 at a depth of 4731 m; partial dissolution of dolomite filling has reactivated the fracture. Qz: quartz, Pl: plagioclase, Dol: dolomite, Bi: biotite.
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Figure 9. Tectonic fracture characteristics observed in SEM images of metamorphic rocks from the Bozhong Depression, Bohai Bay Basin. (A) Intergranular fracture development in quartz particles in gneiss from Well B19-10 at a depth of 4478 m. (B) Crack fractures within quartz particles in gneiss from Well B19-15 at a depth of 5038 m. (C) Regularly developed mica cleavage fractures in migmatitic granite from Well B19-7 at a depth of 4757 m. (D) Two sets of regularly developed cleavage fractures in potassium feldspar in migmatitic granite from Well B19-7 at a depth of 4753 m. Qz: quartz, Kfs: K-feldspars, Bi: biotite.
Figure 9. Tectonic fracture characteristics observed in SEM images of metamorphic rocks from the Bozhong Depression, Bohai Bay Basin. (A) Intergranular fracture development in quartz particles in gneiss from Well B19-10 at a depth of 4478 m. (B) Crack fractures within quartz particles in gneiss from Well B19-15 at a depth of 5038 m. (C) Regularly developed mica cleavage fractures in migmatitic granite from Well B19-7 at a depth of 4757 m. (D) Two sets of regularly developed cleavage fractures in potassium feldspar in migmatitic granite from Well B19-7 at a depth of 4753 m. Qz: quartz, Kfs: K-feldspars, Bi: biotite.
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Figure 10. Major fault and tectonic fracture orientations of metamorphic basement reservoirs in the Bozhong Depression, Bohai Bay Basin. (Tianjin Branch of China National Offshore Oil Corporation).
Figure 10. Major fault and tectonic fracture orientations of metamorphic basement reservoirs in the Bozhong Depression, Bohai Bay Basin. (Tianjin Branch of China National Offshore Oil Corporation).
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Figure 11. Vertical stratification of metamorphic rock reservoirs in Well B19-15, Bozhong Depression, Bohai Bay Basin. The degree of fracture development gradually decreases from the strong weathering fracture zone (SWFZ), through the weak weathering fracture zone (WWFZ), to the basement zone (BZ). DF: dissolution fracture; MF: microfracture. (A) Cores at a depth of 4633 m; (B) Cores at a depth of 4868 m; (C) Cores at a depth of 5212 m.
Figure 11. Vertical stratification of metamorphic rock reservoirs in Well B19-15, Bozhong Depression, Bohai Bay Basin. The degree of fracture development gradually decreases from the strong weathering fracture zone (SWFZ), through the weak weathering fracture zone (WWFZ), to the basement zone (BZ). DF: dissolution fracture; MF: microfracture. (A) Cores at a depth of 4633 m; (B) Cores at a depth of 4868 m; (C) Cores at a depth of 5212 m.
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Figure 12. (A) Relationship between the average fracture linear density of the strong weathering fracture zone (SWFZ) and the thickness of the overlying sedimentary layer. (B) Relationship between the average fracture aperture of the SWFZ and the thickness of the overlying sedimentary layer.
Figure 12. (A) Relationship between the average fracture linear density of the strong weathering fracture zone (SWFZ) and the thickness of the overlying sedimentary layer. (B) Relationship between the average fracture aperture of the SWFZ and the thickness of the overlying sedimentary layer.
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Figure 13. Relationship between permeability and porosity based on physical property analysis of basement reservoirs in the Bozhong Depression, Bohai Bay Basin (N = 51).
Figure 13. Relationship between permeability and porosity based on physical property analysis of basement reservoirs in the Bozhong Depression, Bohai Bay Basin (N = 51).
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Figure 14. Metamorphic basement reservoir types in the Bozhong Depression, Bohai Bay Basin. (A) Well B19-15: Leptynite is classified as a Level I reservoir, and gneiss as Level II and III reservoirs. (B) Well B19-18: Gneiss is classified as Level I and II reservoirs, and diorite-porphyrite as a Level III reservoir. (C) Well B19-14: Gneiss is classified as Level I and II reservoirs, and diorite-porphyrite as Level II and III reservoirs. (D) Well B19-20: Gneiss is classified as Level I and II reservoirs, and diabase as a Level III reservoir.
Figure 14. Metamorphic basement reservoir types in the Bozhong Depression, Bohai Bay Basin. (A) Well B19-15: Leptynite is classified as a Level I reservoir, and gneiss as Level II and III reservoirs. (B) Well B19-18: Gneiss is classified as Level I and II reservoirs, and diorite-porphyrite as a Level III reservoir. (C) Well B19-14: Gneiss is classified as Level I and II reservoirs, and diorite-porphyrite as Level II and III reservoirs. (D) Well B19-20: Gneiss is classified as Level I and II reservoirs, and diabase as a Level III reservoir.
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Figure 15. Fracture patterns in metamorphic rocks of the metamorphic basement reservoirs in the Bozhong Depression and Bohai Bay Basin, across different tectonic periods. (A) Rose diagram of early Indosinian fault activity and fracture patterns in metamorphic rocks. (B) Rose diagram of late Indosinian fault activity and fracture patterns in metamorphic rocks. (C) Rose diagram of Yanshanian fault activity and fracture patterns in metamorphic rocks. (D) Rose diagram of Himalayan fault activity and fracture patterns in metamorphic rocks.
Figure 15. Fracture patterns in metamorphic rocks of the metamorphic basement reservoirs in the Bozhong Depression and Bohai Bay Basin, across different tectonic periods. (A) Rose diagram of early Indosinian fault activity and fracture patterns in metamorphic rocks. (B) Rose diagram of late Indosinian fault activity and fracture patterns in metamorphic rocks. (C) Rose diagram of Yanshanian fault activity and fracture patterns in metamorphic rocks. (D) Rose diagram of Himalayan fault activity and fracture patterns in metamorphic rocks.
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Figure 16. Development model of the fracture network system in metamorphic basement reservoirs of the Bozhong Depression, Bohai Bay Basin. The intense compressional collision during the Indosinian orogeny, combined with left-lateral strike-slip faulting under compressive-torsional forces during the Yanshanian period, resulted in the formation of numerous structural fractures within the metamorphic basement reservoirs. However, fractures were less prevalent in areas located farther from faults or where gneiss formations dominated. Differential uplift during the Indosinian and Yanshanian orogenies led to significant weathering and erosion of the higher regions of the metamorphic basement, where early-stage structural fractures at the reservoir’s upper surface were widened by atmospheric freshwater dissolution, forming a weathering fracture zone. Subsequent tectonic activity during the Himalayan orogeny reactivated previously closed and filled faults and fractures, transforming preexisting faults into key pathways for later fluid migration. This process further enlarged the original fractures near the faults through dissolution, leading to the development of internal dissolution fractures.
Figure 16. Development model of the fracture network system in metamorphic basement reservoirs of the Bozhong Depression, Bohai Bay Basin. The intense compressional collision during the Indosinian orogeny, combined with left-lateral strike-slip faulting under compressive-torsional forces during the Yanshanian period, resulted in the formation of numerous structural fractures within the metamorphic basement reservoirs. However, fractures were less prevalent in areas located farther from faults or where gneiss formations dominated. Differential uplift during the Indosinian and Yanshanian orogenies led to significant weathering and erosion of the higher regions of the metamorphic basement, where early-stage structural fractures at the reservoir’s upper surface were widened by atmospheric freshwater dissolution, forming a weathering fracture zone. Subsequent tectonic activity during the Himalayan orogeny reactivated previously closed and filled faults and fractures, transforming preexisting faults into key pathways for later fluid migration. This process further enlarged the original fractures near the faults through dissolution, leading to the development of internal dissolution fractures.
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Figure 17. Fluorescence scanning photographs of fractures in cores and thin sections of metamorphic rocks in the Bozhong Depression, Bohai Bay Basin. (A) Fractures containing hydrocarbons in gneiss from Well B19-11 at depths of 5127.6–5128.1 m. (B) Fractures containing hydrocarbons in gneiss from Well B19-12 at depths of 5522.7–5523.3 m. (C) Fractures containing hydrocarbons in gneiss from Well B19-3 at a depth of 3853 m. (D) Interlaced fractures forming a complex fracture network with hydrocarbons present in gneiss from Well B19-3 at a depth of 3850 m.
Figure 17. Fluorescence scanning photographs of fractures in cores and thin sections of metamorphic rocks in the Bozhong Depression, Bohai Bay Basin. (A) Fractures containing hydrocarbons in gneiss from Well B19-11 at depths of 5127.6–5128.1 m. (B) Fractures containing hydrocarbons in gneiss from Well B19-12 at depths of 5522.7–5523.3 m. (C) Fractures containing hydrocarbons in gneiss from Well B19-3 at a depth of 3853 m. (D) Interlaced fractures forming a complex fracture network with hydrocarbons present in gneiss from Well B19-3 at a depth of 3850 m.
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Table 1. Semi-quantitative amounts of minerals in the different rock units in the metamorphic basement reservoir of the Bozhong Depression, Bohai Bay Basin (average weight percent, %).
Table 1. Semi-quantitative amounts of minerals in the different rock units in the metamorphic basement reservoir of the Bozhong Depression, Bohai Bay Basin (average weight percent, %).
PetrologyQuartz (wt.%)Alkaline Feldspar (wt.%)Plagioclase (wt.%)Biotite (wt.%)Amphibole (wt.%)Pyroxene (wt.%)Others (wt.%)
Leptynite49.622.321.50.91.204.5
Migmatitic Granite31.328.330.54.33.801.8
Gneiss9.339.631.87.98.303.1
Cataclasite11.332.234.59.39.503.2
Diorite-Porphyrite8.120.345.110.114.302.1
Diabase7.519.532.32.30.821.216.4
Table 2. Porosity and permeability of rocks with different lithologies in metamorphic basement reservoirs of the Bozhong depression, Bohai Bay Basin.
Table 2. Porosity and permeability of rocks with different lithologies in metamorphic basement reservoirs of the Bozhong depression, Bohai Bay Basin.
PetrologyNumber of SamplesPorosity (%)Permeability (×10−3 μm2)
MinimumMaximumAverageMinimumMaximumAverage
Leptynite42.48.84.60.1640.8810.578
Migmatitic Granite140.76.13.20.0530.7280.142
Gneiss181.28.72.80.0220.2590.068
Cataclasite60.76.22.60.0210.2210.067
Diorite-Porphyrite50.85.82.10.0240.0460.041
Diabase41.32.01.70.0190.0630.039
Table 3. Characteristics of the fracture density and aperture in the strong weathering fracture zone (SWFZ), weak weathering fracture zone (WWFZ), and basement zone (BZ).
Table 3. Characteristics of the fracture density and aperture in the strong weathering fracture zone (SWFZ), weak weathering fracture zone (WWFZ), and basement zone (BZ).
WellFracture Density of Strong Weathering Fracture Zone (m−1)Fracture Density of Weak Weathering Fracture Zone (m−1)Fracture Density of Basement Zone (m−1)Fracture Porosity of Strong Weathering Fracture Zone (%)Fracture Porosity of Weak Weathering Fracture Zone (%)Fracture Porosity of Basement Zone (%)
B19-20.520.450.394.03.84.2
B19-71.211.131.023.22.82.7
B19-100.910.620.454.33.83.4
B19-110.280.110.043.23.13.6
B19-121.571.231.163.52.92.6
B19-142.011.630.755.85.45.0
B19-150.740.520.413.02.73.3
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Zhang, G.; Liu, J.; Zhang, L.; Ahmed, E.; Cheng, Q.; Shi, N.; Luo, Y. Development Characteristics of Natural Fractures in Metamorphic Basement Reservoirs and Their Impacts on Reservoir Performance: A Case Study from the Bozhong Depression, Bohai Sea Area, Eastern China. J. Mar. Sci. Eng. 2025, 13, 816. https://doi.org/10.3390/jmse13040816

AMA Style

Zhang G, Liu J, Zhang L, Ahmed E, Cheng Q, Shi N, Luo Y. Development Characteristics of Natural Fractures in Metamorphic Basement Reservoirs and Their Impacts on Reservoir Performance: A Case Study from the Bozhong Depression, Bohai Sea Area, Eastern China. Journal of Marine Science and Engineering. 2025; 13(4):816. https://doi.org/10.3390/jmse13040816

Chicago/Turabian Style

Zhang, Guanjie, Jingshou Liu, Lei Zhang, Elsheikh Ahmed, Qi Cheng, Ning Shi, and Yang Luo. 2025. "Development Characteristics of Natural Fractures in Metamorphic Basement Reservoirs and Their Impacts on Reservoir Performance: A Case Study from the Bozhong Depression, Bohai Sea Area, Eastern China" Journal of Marine Science and Engineering 13, no. 4: 816. https://doi.org/10.3390/jmse13040816

APA Style

Zhang, G., Liu, J., Zhang, L., Ahmed, E., Cheng, Q., Shi, N., & Luo, Y. (2025). Development Characteristics of Natural Fractures in Metamorphic Basement Reservoirs and Their Impacts on Reservoir Performance: A Case Study from the Bozhong Depression, Bohai Sea Area, Eastern China. Journal of Marine Science and Engineering, 13(4), 816. https://doi.org/10.3390/jmse13040816

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