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Article

Distribution Pattern and Controlling Factors of Reef–Shoal Reservoirs on Both Sides of the Intra-Platform Depression in the Changxing Formation, Wolonghe-Yangduxi Area, Sichuan Basin

by
Yuhang Bi
1,2,
Zhonggui Hu
1,2,*,
Saijun Wu
3,
Jiuzhen Hu
1,2,
Weijie Tong
1,2 and
Min Yao
1,2
1
School of Geosciences, Yangtze University, Wuhan 430100, China
2
Hubei Key Laboratory of Complex Shale Oil and Gas Geology and Development in Southern China, Yangtze University, Wuhan 430100, China
3
Research Institute of Petroleum Exploration & Development, Beijing 100083, China
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(4), 2128; https://doi.org/10.3390/app15042128
Submission received: 9 January 2025 / Revised: 14 February 2025 / Accepted: 14 February 2025 / Published: 17 February 2025
(This article belongs to the Section Earth Sciences)
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Abstract

:
The development pattern of the high-quality reservoir in the Changxing Formation in the Wolonghe-Yangduxi area of southeastern Sichuan is complex. To clarify its evolution, genetic mechanisms, and distribution patterns, this study integrates data from profiles, cores, thin sections, and well logs. It reveals the distribution mechanisms of the bio-reef and shoal reservoirs in the Changxing Formation and discusses the controlling effects of tectonic, sedimentary, and diagenetic processes on reservoir development. The results show the following: (1) The Changxing Formation mainly develops open platform facies, platform margin facies, and slope-basin facies, which can be further subdivided into subfacies of platforms, intra-platform depressions, intra-platform depressions marginal reefs, and intra-platform depressions marginal shoals. The intra-platform margin reefs and the reefs at the edge of the platforms are favorable microfacies for reservoir development. (2) The high-quality reservoirs of the Changxing Formation are dominated by medium-thick-layered biogenic reef limestone and bioclastic limestone, with secondary porosity as the main reservoir space. (3) Sedimentary conditions, sea level fluctuations, and diagenesis are crucial factors for reservoir development. Paleogeomorphological conditions provide the foundation for reservoir development, while sea level fluctuations control the internal structure of the reef–shoal and the cyclical variations in the reservoir.

1. Introduction

The Sichuan Basin, with its widely developed marine carbonate reservoirs and abundant oil and gas resources, is an important natural gas supply area. According to the 13th Five-Year Resource Evaluation, PetroChina’s resources in the Changxing Formation in the Sichuan Basin amounted to 1100.34 billion cubic meters, and the resources to be discovered amounted to 963.557 billion cubic meters, with a proven rate of only 8.73% and a discovery rate of 12.43% [1,2]. Since the 1970s, when the Jian16 well was drilled and encountered the bio-reef reservoir of the Changxing Formation, the exploration of this formation has lasted for more than 50 years, and it is still one of the major gas-producing formations in the Sichuan Basin, which is a favorable prospect for exploration [3,4,5].
The marine stratigraphy of the Sichuan Basin has undergone multiple phases of tectonic uplift, developing reef reservoirs such as the Permian Changxing Formation and Triassic Feixianguan Formation [6]. As an important carbonate reservoir, the Changxing Formation has been extensively studied in terms of stratigraphy, sedimentation, and reservoirs, such as Xie Wuren et al. (2008) who took the lead in studying the Changxing Formation in the Kaijiang area from the perspective of high-resolution sequence stratigraphy based on core and logging data, and then divided the Changxing Formation into three medium-term cycles. Based on the petrological characteristics and geotectonic background of the Changxing Formation, Qin Peng et al. (2018) [7] and Hu Zhonggui et al. (2014) [8] concluded that in the early stage of the Changxing Formation in eastern Sichuan, a gently sloping phase was developed, and in the middle to late stage, an open platform phase and a platform edge phase were developed, and that the biogenic reefs and shoals were distributed either contiguously or intermittently along the plateau, the edge of the plateau depression, and the edge of the platform; Jing Xiaoyan et al. (2021) [9] concluded through physical and chemical analysis that the Changxing Formation in the Yuanba area mainly forms dolomite reservoirs, in which intergranular pores and fractures serve as the main types of reservoir space; Liu Yanting et al. (2018) [9,10,11,12,13,14] concluded that the biogenic reef–beach reservoirs of the Changxing Formation in the Heichiliang area, northeast Sichuan Province, are controlled by paleomorphology, diagenesis, and tectonics, in which paleomorphology controls the location of reservoir development, diagenesis improves the quality of the reservoir, and tectonics, on the basis of the former, optimize the conduction system and facilitate the modification of the prior pore layer with later acidic fluids.
Previous researchers [14,15,16] have achieved remarkable results in the field of reef flats, but the current research on the gas reservoirs of the Changxing Formation in the Sichuan Basin mainly focuses on the eastern Sichuan circum-Kaijiang-Liangping trough area, which has been studied to a higher degree and has better production capacity. The research on the Wusheng-Shizhu depression in southeast Sichuan, which also belongs to the Yangzi Plateau, is relatively weak [17,18,19]. Therefore, this study comprehensively uses core, thin section, and well log data to conduct detailed research on the reservoir development characteristics, evolutionary processes, and genetic mechanisms of the Changxing Formation. Based on sequence stratigraphy and sedimentary facies division, this study aims to clarify the distribution patterns of high-quality reservoirs and provide a basis for the selection of exploration targets in the region.

2. Regional Geological Setting

The Sichuan Basin, located in the western part of the South China Plate, is surrounded by a high mountain system, and is a large superposition basin formed through multiple phases of tectonic evolution. The basin’s tectonics are diamond-shaped with NE-SW and NW-SE oriented spreading (Figure 1a).
The study area is located in the southeastern part of the Sichuan Basin, with carbonate platform deposits separated by the Pengxi-Wusheng sag on the north and south sides, and is tectonically located in the East Sichuan arc fold belt (Figure 1b) [4,17].
The Changxing Formation in the study area is mainly composed of carbonate sediments, with the overlying Feixianguan Formation in a consolidated contact, and with the underlying Longtan Formation in an unconformable contact. Based on the division of lithological changes, the current mainstream viewpoints will be divided into three lithological segments [20]: The first member is predominantly composed of dark gray micritic limestone, argillaceous limestone and flint nodule limestone, with the argillaceous limestone marking the boundary with the Longtan Formation [21]. The second member is a set of micritic limestone formed in a low-energy environment, the content of bio-reef and bioclastic is increased in the middle and upper parts, and dolomite is often developed at the top. The third member primarily develops gray micritic limestone, with localized sections exhibiting bioclastic limestone and reef limestone. It is delineated from the top of the Feixianguan Formation by a boundary marked by relatively low-energy dark gray shale or muddy limestone [22].

3. Materials and Methods

Based on the sea-level fluctuation patterns and previous research findings, the Permian Changxing Formation is divided in two complete tertiary sequences (Sq1 and Sq2), each containing both transgressive systems tract (TST) and highstand systems tract (HST), where Sq1 corresponds to the first member and the second member, and Sq2 corresponds to the third member [22,23,24]. Using the identification of sequence boundary characteristics, a chronostratigraphic framework was established, with third-order sequences as the basic units. A regional stratigraphic correlation analysis of the Changxing Formation was then carried out, with an emphasis on the sequence development characteristics in the study area [25]. Significant differences were observed between different regions in terms of sequence thickness, lithology, electrical properties, and reservoir characteristics [26]. This analysis helped summarize the overall development trends of the Changxing Formation in the study area [27,28,29,30].
Incorporating previous research, and by synthesizing field profiles, logging data, core samples, thin sections, and well log data (such as gamma-ray curves) from multiple wells in the Wusheng-Shizhu and surrounding areas, the variations in gamma-ray amplitude and morphology were analyzed. These variations were then correlated with sedimentary microfacies, and the corresponding gamma-ray characteristics for each facies were summarized. Additionally, by integrating the regional sedimentary background, the sedimentary facies types and characteristics of the Changxing period in the study area were determined. This helped clarify the distribution patterns of reef and shoal reservoir bodies in the Changxing Formation [8,31,32,33,34,35].

4. Results

4.1. Stratigraphic Divisions and Comparisons

The identification of sequence boundaries is key to defining sequence types. Three key sequence boundaries can be recognized in the Changxing Formation in the southeast Sichuan area (SB1, SB2, and SB3). The types of boundaries are primarily characterized as local exposure unconformities and lithological or lithofacial transition boundaries (Figure 1c).
SB1 is the interface between the black-gray mudstone of the Upper Permian Longtan Formation and the micritic limestone of the Changxing Formation. Below the SB2 interface exhibits crystal powder limestone, with some development of clastic limestones; above the SB2 interface is micritic limestone or marl, which exhibits low-energy phase deposition; SB3 is the interface between the bioclastic limestone of the Changxing Formation and the mud limestones of the Feixianguan Formation.

4.2. Sedimentary Facies Types and Signs

The Changxing Formation in the Wusheng-Shizhu area developed a carbonate slope and carbonate platform deposition system, in which the carbonate platform deposition can be further subdivided into open terrace phases, slope-basin phases, and others. The open platform facies are concentrated on the north and south sides of the area. In certain well sections located on paleogeographic highs, thin layers of micritic bioclastic limestone are observed.
(1)
Platform
This phase zone is located between the edge of the platform depression and the edge of the platform, with good water connectivity for most organisms and biogenic debris shoal deposits in localized geomorphic uplands (Figure 2a).
(2)
Intra-platform Depression
Genetically, the intra-platform depression is formed by negative structural movements within the platform, resulting from regional extensional tectonics. In terms of stratigraphic thickness, the thickness of the Changxing Formation within the Pengxi-Wusheng platform depression is only about 55 m, whereas the average thickness of the strata in the adjacent intra-platform reef–shoal belts is approximately 110 m. This significant thickness difference serves as a critical indicator for identifying platform depressions. The intra-platform depression facies is mainly characterized by the development of dark-colored marlstone, micritic limestone, and chert nodule-bearing limestone, interbedded with minor amounts of argillaceous limestone and micritic bioclastic limestone.
(3)
Platform Margin Depression
Compared to the intra-platform depression, the stratigraphic thickness in the platform margin area is significantly greater. Enhanced water circulation and higher energy levels create favorable conditions for the growth and development of reef and shoal bodies. Based on differences in sedimentary characteristics, the platform margin can be further subdivided into microfacies such as platform margin shoals and platform margin reefs. Platform margin reefs in intra-platform highs are often distributed in a patchy pattern and are primarily composed of sparry bioclastic limestone, residual bioclastic dolomite, and reef limestone. These reefs are generally smaller in scale compared to those at the platform edge (Figure 2b,f). Platform margin shoals, similarly located in high-energy zones, are commonly associated with platform margin reefs, forming reef–shoal complexes. The sediments are predominantly composed of grain shoals with good sorting. The lithology mainly consists of sparry bioclastic limestone. In some well sections, the tops of these deposits show evidence of dissolution and dolomitization, resulting in the development of solution-porous dolomite and dolomitized shoal deposits (Figure 2c,e).
(4)
Slope-Basin
Slope facies are transitional phase zones between carbonate terraces and basins. Their lithology is dominated by thin-bedded micritic limestone, and gravity-flow deposits such as landslide sediments from the platform margin zone can be seen. Basin facies are a deep-water depositional environment located below the redox interface, characterized by extremely low energy levels in the water column. The predominant lithologies include argillaceous limestone and siliceous rock. Biogenic fossils such as siliceous radiolarians and sponge spicules are commonly observed in these facies (Figure 2d).

4.3. Log Facies Characteristics

Combining the changes in the amplitude and morphology of the gamma curves of multiple wells in the area, along with the microscopic thin-section analysis, the gamma-ray curve characteristics corresponding to each sedimentary microfacies are summarized as follows [36,37,38,39,40,41].
(1) Finger-Shape: The natural gamma curve morphology of this type is highly variable and has the highest relative value, showing a rounded, sharp peak shape. The lithology is dominated by a large set of thickly bedded dark gray micritic limestone, which characterize the sedimentary response to low-energy microphases (Figure 3a–c).
(2) Box-Shape: The top and bottom of the GR value of this type of curve are in syncline contact, and the lithology is mostly medium-thick layer of clastic limestone, reef limestone, dolomitic limestone, and acicular dolomite, with particles containing coarse-grained calcite fillings, which indicates the high-energy microphase, and it is a symbol of the favorable development of reservoirs (Figure 3d–f).
(3) Bell-Shape: The GR value of this type of curves tends to increase from the bottom up, indicating that the hydrodynamic conditions are weakened, and the lithology is changed from crystalline bioclastic limestone to micritic limestone, suggesting a shift in the depositional environment from high-energy to low-energy conditions (Figure 3g–i).
(4) Funnel-Shape: The overall weakening trend of the GR value of this type of curve represents a gradual increase in the energy of the water body and a gradual change in lithology from mudstone or flint nodule limestone to bright crystalline clastic limestone, which is usually indicative of a transition from a low- to high-energy environment (Figure 3j–l).

4.4. Comparison of the Development and Distribution Patterns of the Intra-Platform Shoal

In order to clarify the horizontal and vertical development and distribution patterns of the Changxing Formation within the stratigraphic grid of the Pengxi-Shizhu Formation in the two areas of the Pengxi-Shizhu Formation, a comparative analysis was conducted on the profiles of three consecutive wells in the study area.

4.4.1. Vertical Distribution Pattern

The section of this continuous well goes through the depression and crosses the margins of the two sides of the depression. There is a significant lateral variation in the strata, with greater thickness at the edges of the platform depressions and a thinning trend towards the central part of the depression. Overall, the strata exhibit a ’north-high, south-low’ trend. Sedimentary differentiation is pronounced in the study area, with greater strata thickness in regions where biogenic reefs and shoals are developed along the edges of the platform depression, thinning towards the slope and central depression areas (Figure 4).

4.4.2. Horizontal Distribution Pattern

South Side of the Intra-Platform Depression

The study interval can be subdivided into two third-order sequences: Sq1 and Sq2. The lower boundary of Sq1 is a Type II sequence boundary, marking a lithological–lithofacies transition. The boundary between Sq2 and Sq1 is also a Type II sequence boundary, characterized by a local exposure and erosion surface, typically identified by a dolomite unit beneath the boundary. The upper boundary of Sq2 is the boundary between the Changxing Formation and the Feixianguan Formation, which is a Type II sequence boundary, representing a lithological–lithofacies transition.
From bottom to top, the Changxing Formation develops carbonate ramp, platform margin, and platform top subfacies. The platform margin subfacies includes the platform margin shoal and platform margin reef microfacies. The thickness of the platform margin subfacies is greater than that of the platform top subfacies. Additionally, burial dolomitization is more intense on the northern side of the platform margin compared to the southern side (Figure 5).
During the Sq1-TST sedimentary period, the sea level was relatively high, and hydrodynamic conditions were insufficient, so sedimentary differentiation had not yet developed in the study area. The sediments include deeper-water deposits such as siliceous nodule limestone, micritic limestone, and flint nodule limestone with interbedded bioclastic limestone. The whole area was in shallow-water gentle-slope deposition; only a small amount of low-energy clastic shoal development could be seen in the tectonic zone of Goujiachang (Well G3).
During the Sq1-HST depositional period, the sea level gradually declined, significant sedimentary differentiation occurred in the study area, and carbonate terrace deposition began to develop. Reef limestone and bioclastic limestone dominate the sediments, the reef complex is more developed, and the reef–shoal facies were further developed compared to the Sq1-TST period.
During the Sq2-TST depositional period, the stratigraphy was thinly deposited; micrite limestone, dolomitic limestone and siliceous limestones were developed. The size of the reef–shoal was drastically reduced due to the rise of the water body and an environment unfavorable to biological growth.
During the Sq2-HST depositional period, as the seawater slowly regressed, the energy of the sedimentary water body remained low. The study area was still in the open terrace depositional area, with the development of reef limestones, bioclastic limestones, and dolomitic limestones. Under the influence of intense dolomitization, these sediments transformed into calcareous dolomites and dolomites. A significant number of shoal bodies developed within the study area (Figure 6).

North Side of the Intra-Platform Depression

Horizontally, the stratigraphic thickness exhibits minimal variation, predominantly ranging between 200 and 300 m. Vertically, the types of depositional facies developed are similar to those observed on the southern side of the platform depression.
During the Sq1-TST sedimentation period, subject to the condition of sea erosion, the study area develops gentle slope sedimentation. The lithology is mainly micritic limestone, siliceous limestone and flint nodule limestone. The content of flint nodules increases compared with that on the south side of the platform depression, and reef flats are not developed.
During the Sq1-HST depositional period, the study area mainly developed the open terrace phase dominated by clastic limestones. A thin layer of reef limestone developed in the TL202 well area. Moreover, a large number of reef–shoals were developed in the margin of the intra-platform depression, C37, and high-energy clastic shoal deposition was developed in the W102, TL202, and TL2 well areas.
During the Sq2-TST depositional period, the study area experienced a short period of sea erosion, the energy of the water was weakened, and the high-energy phase zone was not developed. The thickness of the deposition was small; only a thin layer of low-energy clastic beach was developed around the well TL202.
During the depositional period of Sq2-HST, with the slow decline of the sea level, there is strong dolomitization in well TL202, and the development of clastic dolomite and dolomitic limestone.

4.4.3. Plane Distribution Pattern of Reef Shoals in the Changxing Formation

In the early stage of Sq1-TST deposition of the Changxing Formation, the sea level continued to rise. At this time, the whole area was still dominated by gentle slopes, the reefs and shoals were undeveloped, with only a small number of low-energy bioclastic shoals distributed in the south of the terrace in the shape of dots. Only a few low-energy bioclastic shoals are dotted in the southern part of the Wolonghe and the Goujiachang tectonic zone, and the thickness of the stratigraphic deposits in the northern part of the intra-platform margin is large.
During the Sq1-HST period, with the gradual lowering of the sea level, the extent of the terrace decreased. Due to the presence of elevated geomorphological belts on both sides of the platform depression, biogenic reef and shoal facies developed more significantly compared to the Sq1-TST period. Biogenic reefs are mainly distributed on the southern side of the platform depression, around the W061-1 well area and the Taiyun profile. On the northern side of the depression, they occur as isolated patches in the WL102 wells and TL202 wells. The biogenic shoals were spread in a band on the edges of the depression on the north and south sides in the Wolonghe tectonic zone and other tectonic zones.
During the Sq2-TST period, only the TL202 well area on the north side of the depression shows a small-scale development of clastic beaches; on the south side of the depression, there is a small-scale localized bio-reef deposition in the Taiyun profile, and clastic shoals are developed in the well SF1 area.
During the Sq2-HST depositional period, a new phase of reef flats redeveloped. On the whole, the morphology of the fringe zone around Wolonghe and Goujiachang on the south side of depression varies greatly. The Dachigan and Tailai blocks on the north side of the depression are mainly characterized by single-row bio-reef deposits. The thickness of bio-reef deposits on the north side of the depression is relatively small, while the thickness of bio-reef deposits on the south side of the depression is greater, which has been modified to a high degree by dolomitization.
The bio-reef was developed in the late depositional stage of the Changxing Formation, and patch reefs were developed along the edge of the intra-platform depressions, and the scale of individual reefs was smaller than that of the platform margin.
The shoal of the intra-platform margin was developed in the middle and late stage of the Changxing Formation, with a small sedimentary thickness, wide lateral distribution (Figure 7).

4.4.4. Differential Sedimentary Facies Patterns on Both Sides of the Depression

Based on the previous analysis of typical single-well and multi-well sedimentary facies, as well as the planar facies distribution in the study area, a depositional model for the Changxing Formation was established as follows:
At the first member (Sq1-TST), the study area as a whole is characterized by gently sloping carbonate deposits.
At the second member (Sq1-HST), in the Late Permian tensile background, tensile depressions appeared in the interior of the terrace, and after tectonically differentiated sedimentation, the depositional pattern of “three rises and three depressions” was formed. At this time, there was a transition from shallow-water gently sloping sedimentation to carbonate terrace mode. On the northern side of the platform depression, the development of shallow shoals along the edge of the platform depression was influenced by multiple factors, including the water body of the Kaijiang–Liangping shelf and the edge facies of the platform depression. No large-scale dolomitization occurred in this area. On the southern side of the depression, the development of shallow shoals along the edge of the platform depression and biogenic reefs was primarily controlled by micropaleogeomorphological changes within the platform. The biogenic reefs, which emerged above sea level, underwent selective dissolution and dolomitization at the surface, with dolomitization occurring at the reef top.
At the third member (Sq2), the study area undergoes the process of sea retreat–sea invasion, and the depositional environment changes from low energy to high energy, the north and south sides of the shoal are exposed to the water surface, and different degrees of burial dolomitization and diagenetic dolomitization modification occur.

4.5. Reservoir Characteristics and Distribution Pattern

4.5.1. Reservoir Diagenesis

Based on the staged evolution of hydrocarbons, combined with diagenetic petrology, characteristic processes, and types of secondary pores, the diagenetic processes of carbonate reservoirs can be classified into different stages.
(1)
Syndepositional Stage—Submarine Diagenetic Environment
The submarine diagenetic environment represents the earliest diagenetic setting experienced by marine sediments. The primary diagenetic processes occurring in this environment include mechanical compaction, submarine cementation, and penecontemporaneous dolomitization. Syndepositional diagenesis is closely linked to the sedimentary environment and depositional processes; therefore, different sedimentary environments exhibit distinct syndepositional diagenetic characteristics.
(2)
Early Diagenetic Stage—Shallow to Intermediate Burial Environment
The early diagenetic stage corresponds to a shallow to intermediate burial environment, where the major diagenetic processes include the following: ① The earliest burial dissolution and cementation, occurring after early meteoric freshwater leaching and dissolution during subaerial exposure. ② Seepage-reflux dolomitization, following the penecontemporaneous evaporative concentration of seawater. In lagoonal or tidal flat settings behind reefal and shoal complexes, concentrated brines percolate downward through early dissolution channels formed during reef–shoal exposure, leading to dolomitization of the carbonate deposits.
(3)
Middle Diagenetic Stage—Intermediate to Deep Burial Environment
The middle diagenetic stage corresponds to an intermediate to deep burial environment, where the dominant diagenetic processes include tectonic fracturing and dissolution induced by organic acids generated during hydrocarbon generation and migration. The key diagenetic processes in this stage are as follows: ① Re-equilibration or adjustment of early-formed dolomite in response to evolving diagenetic fluids in a progressively closed system. ② Recrystallization, which enhances the pressure resistance of carbonate grains and interstitial fluids. As burial depth increases, early-stage pressure dissolution intensifies, disrupting the initial equilibrium, leading to the formation of stylolites and pressure-solution seams. This process facilitates later-stage dolomitization, forming intercrystalline pores, inter-crystalline dissolution pores, and dissolution fractures in grainy dolostones developed within back-reef shoals.
(4)
Late Diagenetic Stage—Deep Burial Environment
The late diagenetic stage corresponds to a deep burial environment, where the dominant diagenetic processes include the infilling of reservoirs with pyrobitumen due to hydrocarbon thermal cracking and burial dissolution caused by the generation of CO2 from overmature organic matter. Additionally, pressure solution, dolomitization adjustment, and recrystallization also occur during this stage.
Overall, as burial depth increases, the diagenetic environment progressively becomes more closed, transitioning into an alkaline and reducing system. However, within subsurface diagenetic settings, variations in temperature, pressure, and organic matter maturation over time lead to spatial and temporal differences in pore water chemistry. For the Changxing Formation reservoir, dolomitization and dissolution are the key diagenetic processes that have significantly modified the reservoir quality.
The process of reservoir formation in the Changxing Formation of the study area is as follows:
Synsedimentary period: The rock structure is loose with high pore water content, and compaction is weak. Primary porosity is well developed, including intragranular pores, intergranular pores, and framework pores. Synsedimentary dolomitization forms a small amount of inter-crystalline pores.
Early diagenesis: Compaction increases, and pore water is rapidly expelled, leading to a significant reduction in primary porosity. As burial depth increases, compaction becomes stronger, and early cementation occurs. Primary porosity drastically decreases or even disappears. During this period, intergranular fluids dissolve along particle contact surfaces, forming some compaction fractures. Mineral transformation and recrystallization generate some inter-crystalline pores.
Middle diagenesis: Dolomitization develops, and dissolution enhances, forming a large number of inter-crystalline pores and various types of secondary dissolution pores, including intergranular dissolution pores, intragranular dissolution pores, crystal dissolution pores, and sponge-morph cavity dissolution pores, thus creating good reservoir space.
Late diagenesis: On the basis of secondary porosity developed during the early and middle stages, further dissolution enlarges, forming some moldic pores and large pores. Simultaneously, multiple stages of tectonic fractures develop, extending in a net-like pattern, ultimately forming high-porosity reef dolomite or bioclastic dolomite.

4.5.2. Reservoir Lithology Characterization

Based on the analysis of profiles, cores, and thin-section identification, it is found that micrite limestone, bio-reef limestone, bioclastic limestone, calcareous dolomite, detrital dolomite, and dolomite and other rock types are developed in the study area. Among them, crystalline dolomite, residual bioclastic dolomite, and biogenic reef limestone form favorable reservoir rocks, while fractures in some micritic limestone, modified by tectonic fracturing, give rise to fracture-type reservoirs with excellent permeability, which can also serve as favorable reservoirs to some extent (Figure 8a,b). Crystalline dolomite: Mainly distributed in the top of Changxing Formation, mostly composed of fine-grained dolomite, with a large number of inter-crystalline pores and inter-crystalline solution pores developed, which is the most important type of reservoir rock in the area (Figure 8c,d). Residual bioclast dolomite: This type of reservoir is the second most favorable type of reservoir in the region, mostly formed after strong dolomitization, with high dolomite content (>85%), and the original internal bio-morphology and structure have been damaged and are difficult to identify (Figure 8e,f). Bio-reef limestone: The reservoir rocks are rich in biotopes, including foraminifera, corals, sponges, algae, and echinoderms (Figure 8g–i). Biological contours are more clearly defined and the water column is more energetic.

4.5.3. Reservoir Space Types and Characteristics

The carbonate reservoirs in the study area exhibit diverse types of reservoir space, which can be categorized into porosity-type and fracture-type reservoirs based on their origin and morphology [42,43,44,45,46,47]. The majority of reservoirs belong to the porosity-type, with secondary porosity mainly developed in inter-crystalline pores, intergranular pores, intragranular dissolution pores, and biogenic voids. Fractures primarily manifest as tectonic fractures, with a smaller development of diagenetic and dissolution fractures. Inter-crystalline pores mainly develop in dolomites that have undergone dolomitization, which exhibit a crystalline texture. The pore size is typically small, mostly medium to fine, with poor connectivity. The pores may be filled with minerals such as calcite and organic matter. Inter-crystalline dissolution pores are formed when the inter-crystalline pores undergo further dissolution during burial diagenesis. The extent of their development depends on the degree of dolomitization and the intensity of subsequent dissolution processes (Figure 9a,b). These types of pores are widely developed in the dolomite reservoirs of the Changxing Formation within the study area. Biogenic Voids: Biogenic organisms, such as foraminifera, create cavities through dissolution during the process of sedimentation and diagenesis. These voids are not filled, (Figure 9e) resulting in high porosity, and can serve as effective reservoirs. Intergranular pores: Developed between particles such as sand, mud, and ooliths, interparticle dissolution pores can form when these voids are enlarged by dissolution. The two types of voids are often symbiotic, generally developed in the sand, raw debris, oolites, and remnants of granular support of the sand, raw debris, oolites, and residual particles of dolomite (Figure 9c,d). Intragranular Pores: Often associated with intergranular pores (Figure 9g), the study area is dominated by bioclastic endosols and sand clastic endosols. Fractures are mainly comprised diagenetic fractures, tectonic fractures, dissolution fractures (Figure 9h).

4.5.4. Reservoir Petrophysical Characteristics

An analysis of the petrophysical data from 10 wells in the study area shows that the porosity of the Changxing Formation ranges from 0.16% to 22.05%, with an average value of 2.37%. The permeability varies mainly from less than 0.01 × 10⁻3 μm2, with an average of 1.6 × 10⁻3 μm2. Overall, the reservoirs are classified as low porosity, low permeability (Table 1).
Although the Changxing Formation is generally considered to be a low-porosity, low-permeability reservoir, significant variations in reservoir properties are observed between wells in different locations. Reservoir porosity and permeability are notably higher in wells located in reef-flat development areas compared to non-reef-flat wells. Further analysis of reservoir properties in different well locations within the region, based on petrophysical data, reveals that the properties in low-energy environments are poor. In contrast, the high-energy platform edge zone, influenced by exposure, dissolution, and dolomitization, shows well-developed secondary porosity and better reservoir performance. This zone represents the most favorable facies for reservoir development in the Changxing Formation and can form high-quality reservoir units.
The shoal facies reservoirs in the study area are predominantly scattered in a spotty distribution across the study area. Laterally, the reef facies dolomite reservoirs develop in the middle and upper parts of the sequence and are discontinuously distributed with poor continuity. The southern end of the Wolonghe structural belt and the Shuanglong area also show well-developed shoal facies gas reservoirs, with the best reservoir properties. The Goujiacun structural belt exhibits slightly inferior reservoir properties, while the Yandongxi, Dachigan, and other structural belts may develop fracture-pore reservoirs. Shoal facies reservoirs are rare in the region, and their reservoir quality is poor. There is a clear lateral and planar distribution difference in reservoirs across the region, showing pronounced heterogeneity. Vertically, the reservoir properties in the southern part of the platform depression are slightly better than those in the northern part.

5. Discussion

Although there are certain differences in the specific mechanisms of the formation of high-quality carbonate reservoirs, the main controlling factors are generally similar [48,49]. These include the dual control of sedimentary processes and diagenesis, with tectonic forces and fluid dynamics also playing varying roles in the formation of high-quality carbonate reservoirs. Sedimentary processes provide the foundation for the formation and evolution of carbonate reservoirs [50], influencing the general distribution range of major reservoirs and the types and intensity of subsequent diagenesis. Diagenesis controls the final distribution of the reservoirs and determines the internal pore structure of the reservoir.

Main Controlling Factors of Reservoir Formation

The development of reef–shoal facies is closely related to the formation of high-quality reservoirs in the Changxing Formation. However, not all reef–shoal facies can develop into high-quality reservoirs. The types of diagenesis that occur in different sedimentary environments and cycles lead to diverse reef–shoal reservoir types. The development of reef–shoal reservoirs in the Changxing Formation is primarily controlled by factors such as sedimentary facies [51], sea-level oscillations, and diagenesis. Sedimentary facies control the location of development, sea-level oscillations control the timing of development, and diagenesis controls the quality of the developed reservoirs [52,53,54,55,56].
(1)
Sea-Level Fluctuations
The growth and development of reef–shoal facies require a relatively high-energy sedimentary environment, with periodic sea-level oscillations controlling water energy conditions. During a sea-level rise, the water depth increases, and hydrodynamic conditions become unsuitable for reef–shoal development. At this time, a large amount of micrite fills the spaces between the skeletons and particles, and even some early-developed intra-grain pores are cemented and filled. On the other hand, during sea-level fall, water depth decreases, and under strong hydrodynamic conditions, reef–shoal facies develop more extensively. When the sea level reaches its lowest point, the tops of the reef–shoal facies are exposed, undergoing leaching by atmospheric freshwater, resulting in selective dissolution and the formation of different types of porosity. From the perspective of reservoir development patterns, their formation also exhibits distinct cyclic characteristics. The Changxing Formation’s platform margin reefs and shoals primarily develop in the high-order system tract of the sequence. Dolomite reservoirs display similar characteristics, most of which also develop in the high-order system tract, located at the top of the reef–shoal reservoirs.
(2)
Moderate Dolomitization
The process of dolomitization, where dolomite replaces calcite (or aragonite and high-magnesium calcite), is known as dolomitization. In the study area, the best reservoir spaces and hydrocarbon-bearing layers within the Changxing Formation carbonate rocks are predominantly dolostones formed by dolomitization. Among these, dolostones with dissolution pores exhibit the best porosity and permeability properties, indicating that the development of reservoirs is closely related to the extent of dolomitization. The Changxing Formation reservoirs on the northern side of the platform depression have been less affected by dolomitization, predominantly consisting of limestone-type reservoirs. In contrast, dolomitized reservoirs are more common in the southern side of the platform depression, particularly in the areas of wells W061-1, W117-, W118-, W118-1, and S18. The southern section of the region shows a significantly higher dolomite development compared to the northern section, where dolomite thickness is relatively thin, and dolomitization is primarily developed in the second segment (Figure 10).
(3)
Dissolution-alteration
During the burial process, acidic fluids initially enter the reservoir’s pore spaces and cause alteration/dissolution. At this stage, the main body of the reservoir has already formed, so the primary role of dissolution during the burial period is to further dissolve and rework the existing reservoir, significantly improving its quality. However, the contribution to the initial formation of the reservoir is limited. The dissolution mainly affects the surrounding impermeable rock, which undergoes localized dissolution to become a reservoir, thereby expanding the overall reservoir scale.
Dissolution may have occurred in multiple stages in the study area. During the early stages of diagenesis, the dissolution in the atmospheric water diagenetic environment and the submarine diagenetic environment was relatively weak in the Changxing Formation. However, selective dissolution of pores does occur in the Changxing Formation. Both synsedimentary freshwater dissolution and burial dissolution played important roles in the development of the reservoir.

6. Conclusions

(1) Based on the analysis of sedimentary facies, logging facies, seismic facies, and the biogenic development characteristics during the Changxing period, the sedimentary system types of the Changxing Formation in the eastern Sichuan region have been clearly defined. During the early Changxing period, the overall sedimentary environment was dominated by carbonate deep–shallow ramp deposition. In the middle to late Changxing period, carbonate platform deposition developed, featuring open platform facies, platform margin facies, and slope-basin facies. These facies can be further subdivided into subfacies such as platform flat, intra-platform depressions, platform margin reefs, and platform margin shoals.
(2) The paleo-topography of the Wolong River-Yangdu platform inner depression shows that the paleogeomorphology of the northern margin of the platform was higher than that of the southern margin, the bio-reef and shoal assemblage on the northern edge primarily develops in the lower part of the Changxing Formation, sections 2 and 3, while on the southern edge, it mainly develops in the upper part of section 3 of the Changxing Formation.
(3) The Changxing Formation in the eastern Sichuan region exhibits significant heterogeneity. The platform reef and shoal facies represent the most favorable depositional environment for reservoir development. Reef limestone (dolomitic limestone) and grain dolomite are the primary lithologies of the reservoir. Reservoirs are primarily developed during periods of relative sea-level fall. The distribution of the dominant reservoirs also exhibits distinct characteristics. On the northern side of the platform depression, the Changxing Formation reservoirs have undergone relatively low levels of dolomitization, predominantly consisting of limestone reservoirs. In contrast, the southern side of the platform depression is primarily composed of crystalline dolomite and residual biosparite dolomite reservoirs. The main types of reservoir space are inter-crystalline (dissolution) pores, moldic pores, and intraparticle dissolution pores. Some well sections also show the development of fractures.
(4) Through the analysis of petrophysical data from multiple well locations, it was found that the reservoirs in this region generally exhibit low porosity and low permeability characteristics. This study indicates that the reservoirs primarily develop during periods of relative sea-level fall. The diagenetic processes that favor reservoir development include dolomitization, dissolution, and fracturing. Dissolution promotes the further development and enlargement of pores, while fracturing plays a key role in improving the petrophysical properties of the reservoir. Dolomitization, particularly burial dolomitization, is the most widespread and significant diagenetic process in the Changxing Formation reservoir. It plays a crucial role in enhancing the reservoir properties by increasing porosity and permeability in specific intervals, thus promoting the development of favorable reservoirs.
(5) This study also indicates that basin subsidence has a significant impact on compaction and cementation, which in turn influences the development of primary porosity and secondary pores. Tectonic subsidence, changes in bathymetry, and diagenetic processes (such as early cementation, dissolution, and dolomitization) collectively control the heterogeneity and distribution of the Changxing Formation reservoirs.
(6) The development of dominant reservoirs is often controlled by factors such as depositional conditions, sea-level fluctuations, and diagenetic processes. Paleogeomorphological conditions determine the distribution of relatively high-energy facies belts, which provide the material foundation for reservoir development. The periodic fluctuations in sea level control the internal structure of the bio-reef and shoal facies as well as the vertical stacking distribution of the reservoirs. Dolomitization plays a significant role in enhancing the reservoir properties. Additionally, this study shows that the subsidence of the basin has a crucial impact on compaction and cementation, which in turn affects the development of primary porosity and secondary pore networks. Tectonic subsidence, changes in bathymetry, and diagenetic processes (such as early cementation, dissolution, and dolomitization) together govern the heterogeneity and distribution of the Changxing Formation reservoirs.

Author Contributions

Writing—original draft, Methodology, Software, Validation, Y.B.; Investigation, Writing—review and editing, Z.H.; Data curation, Formal analysis, Methodology, S.W.; Formal analysis, J.H.; Methodology, W.T.; Formal analysis, M.Y. All authors have read and agreed to the published version of the manuscript.

Funding

The authors of this research work are grateful for the funding support from The National Science and Technology Major Project, Grant No. 2016ZX05007-002.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All data and materials are available on request from the corresponding author. The data are not publicly available due to ongoing research using a part of the data.

Acknowledgments

I would like to thank my teachers and classmates for their help in this article, and thank all the editors and reviewers for their hard work.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Geologic map of Changxing Formation in the study area. (a) Sedimentary background of the Sichuan Basin; (b) key elements of the sedimentary pattern distribution map; (c) stratigraphic column for Well W118.
Figure 1. Geologic map of Changxing Formation in the study area. (a) Sedimentary background of the Sichuan Basin; (b) key elements of the sedimentary pattern distribution map; (c) stratigraphic column for Well W118.
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Figure 2. Photographs of the main lithofacies cores and thin sections of the Changxing Formation in the southeastern Sichuan area. Optical microscopy (a–d) and macroscopic photographs (e,f). (a) Micritic bioclastic limestone, well GX2, 3559 m, (-); (b) sparry bioclastic limestone well W118, 3828.91 m, (-); (c) fine crystalline gray dolomite well W117, 3979.06 m; (d) spongy bone-bearing needle-mud crystal limestone well W118, 4029 m; (e) bio-reef limestone well TL202, 4978.54 m; (f) bio-reef limestone.
Figure 2. Photographs of the main lithofacies cores and thin sections of the Changxing Formation in the southeastern Sichuan area. Optical microscopy (a–d) and macroscopic photographs (e,f). (a) Micritic bioclastic limestone, well GX2, 3559 m, (-); (b) sparry bioclastic limestone well W118, 3828.91 m, (-); (c) fine crystalline gray dolomite well W117, 3979.06 m; (d) spongy bone-bearing needle-mud crystal limestone well W118, 4029 m; (e) bio-reef limestone well TL202, 4978.54 m; (f) bio-reef limestone.
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Figure 3. Characterization of logging phases of Changxing Formation in the study area. (a) SF1; (b) micrite limestone, well SF1, 4486.72 m, (-); (c) micrite limestone, well SF1, 4492.00 m, (-); (d) well W118; (e) algal limestone, well W118, 3911 m, (-); (f) finely crystalline dolomite, well W118, 3829.99 m, (-); (g) well TL2; (h) finely crystalline reef dolomite, well TL 2, 5264.64 m, (-); (i) finely crystalline reef dolomite, well TL 2, 5266.45 m, (-); (j) well TL2; (k) algal siliceous rock, well TL2, 5437.88 m, (-); (l) siliceous rock, well TL2, 5401.25 m, (-).
Figure 3. Characterization of logging phases of Changxing Formation in the study area. (a) SF1; (b) micrite limestone, well SF1, 4486.72 m, (-); (c) micrite limestone, well SF1, 4492.00 m, (-); (d) well W118; (e) algal limestone, well W118, 3911 m, (-); (f) finely crystalline dolomite, well W118, 3829.99 m, (-); (g) well TL2; (h) finely crystalline reef dolomite, well TL 2, 5264.64 m, (-); (i) finely crystalline reef dolomite, well TL 2, 5266.45 m, (-); (j) well TL2; (k) algal siliceous rock, well TL2, 5437.88 m, (-); (l) siliceous rock, well TL2, 5401.25 m, (-).
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Figure 4. Comparison of sedimentary phase profiles of consecutive wells of Changxing Formation in the study area.
Figure 4. Comparison of sedimentary phase profiles of consecutive wells of Changxing Formation in the study area.
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Figure 5. Stratigraphy of NW-SE continuous wells in the southern study area.
Figure 5. Stratigraphy of NW-SE continuous wells in the southern study area.
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Figure 6. Stratigraphy of NW–SE continuous wells in the northern study area.
Figure 6. Stratigraphy of NW–SE continuous wells in the northern study area.
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Figure 7. Distribution of sedimentary facies in the Changxing Formation in the Wolonghe-Yangduxi zone, east Sichuan.
Figure 7. Distribution of sedimentary facies in the Changxing Formation in the Wolonghe-Yangduxi zone, east Sichuan.
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Figure 8. Major rock types of the Changxing Formation in the Wolonghe-Yangduxi zone, East Sichuan. (a) Grayish-white micritic limestone with visible fractures infilled by calcite veins, 1-13/52, well GX2; (b) light gray micritic limestone, predominantly inclined cracks, which are open and partially filled with calcite, 3322.34~3322.48 m, well SF1; (c) grayish-white biotitic powdery crystalline dolomite with limited solution pores sporadically distributed, 3965.38~3965.59m, well W118; (d) gray lysoconitic powdery fine-crystalline dolomite, lysoconitic, 3981.42~ 3981.62m, well W118; (e) grayish-white residual biotite powder crystal dolomite, high-angle microfracture development, solution hole cave development, filled or semi-filled 3967.54~3967.76 m, well W118; (f) gray residual bioclastic powdery crystalline dolomite with solifluction development 3538.91~3539.01 m, well W102; (g) light gray sponge-bonded stone, micritic structure, calcite-filled biotite cavities, 3535.18 ~ 3535.27 m, well W102; (h) light gray sponge reef dolomite with extremely developed sponges and a few cracks, 3339.93~3340.03 m, well C12; (i) gray biogenic reef dolomite, with large number of individual organisms, 3-158/174, well C24.
Figure 8. Major rock types of the Changxing Formation in the Wolonghe-Yangduxi zone, East Sichuan. (a) Grayish-white micritic limestone with visible fractures infilled by calcite veins, 1-13/52, well GX2; (b) light gray micritic limestone, predominantly inclined cracks, which are open and partially filled with calcite, 3322.34~3322.48 m, well SF1; (c) grayish-white biotitic powdery crystalline dolomite with limited solution pores sporadically distributed, 3965.38~3965.59m, well W118; (d) gray lysoconitic powdery fine-crystalline dolomite, lysoconitic, 3981.42~ 3981.62m, well W118; (e) grayish-white residual biotite powder crystal dolomite, high-angle microfracture development, solution hole cave development, filled or semi-filled 3967.54~3967.76 m, well W118; (f) gray residual bioclastic powdery crystalline dolomite with solifluction development 3538.91~3539.01 m, well W102; (g) light gray sponge-bonded stone, micritic structure, calcite-filled biotite cavities, 3535.18 ~ 3535.27 m, well W102; (h) light gray sponge reef dolomite with extremely developed sponges and a few cracks, 3339.93~3340.03 m, well C12; (i) gray biogenic reef dolomite, with large number of individual organisms, 3-158/174, well C24.
Applsci 15 02128 g008
Applsci 15 02128 g009
Figure 9. Spatial characteristics of the Changxing Formation reservoir in the Wolonghe-Yangduoxi zone, southeast Sichuan. (a) Powder-fine crystalline dolomite, dolomite grains are large, the surface is not clean, triangular pores are present between the crystals, and organic material is locally visible, well GX2, (-); (b) powder fine-grained dolomite, with a cloudy surface, indicative of a replacement origin, inter-crystalline dissolution pores are well developed, well C24, (-); (c) powder fine-grained dolomite, inter-crystalline pores development, well C24, (-); (d) powdered crystalline gray dolomite with two calcite semi-filled tectonic cracks, well C24, (-); (e) residual bioclastic powdery crystalline dolomite, with cavernous porosity, well GX4, 4313.75 m~4313.82 m, (-); (f) residual bioclastic fine-grained dolostone with dissolution pores, with well-developed intercrystalline pores between dolomite crystals, well W117, block 439, ×370 (SEM secondary electron image); (g) intragranular solution pores with residual structural micro-powdery crystalline dolomite, well GX4, 4313.75 m~4313.82 m; (h) dissolution joints, micritic clastic graywacke, well GX2, 4300.13 m~4300.23 m.
Figure 9. Spatial characteristics of the Changxing Formation reservoir in the Wolonghe-Yangduoxi zone, southeast Sichuan. (a) Powder-fine crystalline dolomite, dolomite grains are large, the surface is not clean, triangular pores are present between the crystals, and organic material is locally visible, well GX2, (-); (b) powder fine-grained dolomite, with a cloudy surface, indicative of a replacement origin, inter-crystalline dissolution pores are well developed, well C24, (-); (c) powder fine-grained dolomite, inter-crystalline pores development, well C24, (-); (d) powdered crystalline gray dolomite with two calcite semi-filled tectonic cracks, well C24, (-); (e) residual bioclastic powdery crystalline dolomite, with cavernous porosity, well GX4, 4313.75 m~4313.82 m, (-); (f) residual bioclastic fine-grained dolostone with dissolution pores, with well-developed intercrystalline pores between dolomite crystals, well W117, block 439, ×370 (SEM secondary electron image); (g) intragranular solution pores with residual structural micro-powdery crystalline dolomite, well GX4, 4313.75 m~4313.82 m; (h) dissolution joints, micritic clastic graywacke, well GX2, 4300.13 m~4300.23 m.
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Applsci 15 02128 g010
Figure 10. Relationship between the degree of dolomitization and sedimentary microfacies in the core section of the Changxing Formation at well W118.
Figure 10. Relationship between the degree of dolomitization and sedimentary microfacies in the core section of the Changxing Formation at well W118.
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Table 1. Petrophysical properties of the Changxing Formation reservoir at different well locations in the study area.
Table 1. Petrophysical properties of the Changxing Formation reservoir at different well locations in the study area.
WellPorosity (%)Permeability (×10−3 μm2)
Minimum ValueMaximum ValueAverage ValueMinimum ValueMaximum ValueAverage Value
C370.314.650.780.0180.503.54
GX40.820.870.84
S150.2022.054.270.01746.0027.27
S181.1921.996.740.01204.0014.86
W1020.677.402.430.014.360.23
W1170.3114.254.600.01342.003.45
W1180.657.921.730.016.910.21
W490.232.190.880.0057.901.09
W750.241.600.590.004.710.10
W840.164.120.540.0112.850.35
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Bi, Y.; Hu, Z.; Wu, S.; Hu, J.; Tong, W.; Yao, M. Distribution Pattern and Controlling Factors of Reef–Shoal Reservoirs on Both Sides of the Intra-Platform Depression in the Changxing Formation, Wolonghe-Yangduxi Area, Sichuan Basin. Appl. Sci. 2025, 15, 2128. https://doi.org/10.3390/app15042128

AMA Style

Bi Y, Hu Z, Wu S, Hu J, Tong W, Yao M. Distribution Pattern and Controlling Factors of Reef–Shoal Reservoirs on Both Sides of the Intra-Platform Depression in the Changxing Formation, Wolonghe-Yangduxi Area, Sichuan Basin. Applied Sciences. 2025; 15(4):2128. https://doi.org/10.3390/app15042128

Chicago/Turabian Style

Bi, Yuhang, Zhonggui Hu, Saijun Wu, Jiuzhen Hu, Weijie Tong, and Min Yao. 2025. "Distribution Pattern and Controlling Factors of Reef–Shoal Reservoirs on Both Sides of the Intra-Platform Depression in the Changxing Formation, Wolonghe-Yangduxi Area, Sichuan Basin" Applied Sciences 15, no. 4: 2128. https://doi.org/10.3390/app15042128

APA Style

Bi, Y., Hu, Z., Wu, S., Hu, J., Tong, W., & Yao, M. (2025). Distribution Pattern and Controlling Factors of Reef–Shoal Reservoirs on Both Sides of the Intra-Platform Depression in the Changxing Formation, Wolonghe-Yangduxi Area, Sichuan Basin. Applied Sciences, 15(4), 2128. https://doi.org/10.3390/app15042128

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