Sedimentation–diagenesis–tectonics coupling and its control on gas reservoir development in Sinian carbonates, western Deyang-Anyue Rift Trough, Sichuan Basin

Abstract

The Sinian Dengying Formation in the western Deyang-Anyue Rift Trough is a key target for deep carbonate gas exploration, yet its reservoir controls remain debated. Integrating core, thin-section, and geochemical data, this study clarifies the coupled mechanisms governing reservoir development. Results reveal that reservoirs predominantly occur in platform-margin and high-energy shoal facies, with pore systems comprising intercrystalline, dissolution, and fracture pores. Primary porosity is largely obliterated by deep-burial compaction and cementation, yielding an overall low-porosity, low-permeability matrix. However, structurally influenced platform-margin exposure zones and fracture-affected mound–shoal bodies outside strongly cemented fault cores locally contain higher-quality reservoirs because of dissolution and fracture-enhanced connectivity. Vertically, reservoirs are zoned: tight lower intervals transition upward into dissolution-enhanced, comparatively porous middle-upper sections. Critically, reservoir evolution is dictated by a “sedimentation–diagenesis–tectonics” coupling: sedimentary architecture and thickness define macroscopic reservoir distribution; compaction and cementation induce densification, while dissolution and fracturing create secondary pore-fracture networks. Hydrothermal activity superimposes a “dissolution-filling alternation,” drastically intensifying heterogeneity. This multi-scale, multi-stage coupling ultimately controls the pronounced spatial variability of the Dengying Formation reservoirs and provides a geological framework for screening favorable targets in deeply buried carbonate successions.

1 Introduction

Deeply buried Precambrian carbonate reservoirs represent a globally significant target for frontier petroleum exploration, yet their extreme age, complex diagenetic histories, and pronounced heterogeneity pose substantial challenges for reservoir characterization and prediction (Radwan, 2025; Hu et al., 2023). Internationally, commercially viable Precambrian petroleum systems have been documented in several key provinces: the latest Ediacaran–Early Cambrian Ara Group intrasalt carbonate ‘stringer’ play in the South Oman Salt Basin constitutes one of the world’s oldest producing reservoir systems, where dolomitic microbialite-dominated carbonates are encased in salt at depths of 3–7 km (Grotzinger and Al-Rawahi, 2014; Al-Siyabi, 2005); the Neoproterozoic carbonates of the Siberian Platform host significant hydrocarbon accumulations, with reservoirs strongly controlled by depositional facies, early dolomitization, and subsequent karstification (Lash, 2015; Kuznetsov and Suchy, 1992); and the Neoproterozoic Gillen Formation in the Amadeus Basin of central Australia represents an intra-salt petroleum system with comparable reservoir-seal configurations (Plummer, 2021). In North America, structurally controlled hydrothermal dolostone (HTD) reservoirs are major producers in Ordovician to Mississippian strata, where extensional and strike-slip faults serve as preferential pathways for dolomitizing brines; analogous fault-controlled HTD systems have also been documented along the rifted Atlantic margins in Jurassic to Cretaceous carbonate successions (Davies and Smith, 2006). Substantial international advances have been made in understanding (1) the role of microbialite-dominated depositional systems in constructing primary reservoir frameworks, with thrombolites and stromatolites exhibiting high initial porosities often exceeding 50% (Riding, 2000); (2) the geochemical signatures, fluid evolution pathways, and dolomitization mechanisms in structure-controlled hydrothermal systems (Davies and Smith, 2006; Lonnee et al., 2006); (3) the impact of multiphase tectonic events on diagenetic evolution and pore-system preservation in ancient carbonate successions (Dong et al., 2020; Bellizia et al., 2021); (4) the structural and diagenetic coupling that governs the spatial distribution of deep carbonate reservoirs, as exemplified in the Tarim Basin where strike-slip faulting and distinct diagenetic fluids jointly control reservoir development (Wang et al., 2025; Luo et al., 2011); and (5) the reformation mechanisms of high-quality carbonate reservoirs through the synergistic combination of facies-controlled distribution and tectonic-stress-enhanced dissolution (Luo et al., 2011; Li P. et al., 2025). However, the interplay between sedimentary architecture, multi-stage tectonism, and hydrothermal fluid flow in governing reservoir heterogeneity remains insufficiently constrained globally, particularly in rift-margin settings where these factors converge with pronounced intensity.

The Ediacaran Period represents a significant phase in the evolutionary history of the Yangtze Craton. Its sedimentary formations not only reflect large-scale carbonate deposition following the conclusion of the global glacial periods but also document geological events associated with the transition of tectonic regimes from the Precambrian to the Paleozoic (Hoffman et al., 1998; Li et al., 2008). In the Sichuan Basin, the thickness of the Sinian Dengying Formation can reach kilometer scale, with lithology dominated by dolostone and limestone. It serves as a significant carbonate reservoir system within the Lower Paleozoic of the basin (Zhu et al., 2015; Wei et al., 2016). In recent years, with the discovery of the Anyue Gas Field and its high and stable production, the exploration value of the Dengying Formation gas reservoirs has become increasingly prominent, gradually emerging as a key target formation for enhancing marine natural gas reserves and production in China (Zou et al., 2014; Ma et al., 2010).

However, the Dengying Formation reservoir demonstrates pronounced complexity and heterogeneity. On one side, its depositional environment exhibits significant variation, transitioning from high-energy grain shoals at platform margins to microbial mounds and shoals in restricted platforms, and further to slope–basin mudstone deposits, with rapid lateral facies shifts (Wang et al., 2018; Jiang et al., 2014). On the other side, it is shaped by the influence of multiple tectonic events (e.g., Tongwan Movement, Caledonian Movement, Hercynian Movement) alongside superimposed burial–diagenetic processes (including dolomitization, dissolution, and hydrothermal activities), leading to extreme heterogeneity in pore types and spatial distribution within the reservoir (He et al., 2017; Liu et al., 2020; Li et al., 2026; Li et al., 2020). Therefore, clarifying the development characteristics and primary controlling factors of the Dengying Formation reservoir has become a core issue in oil and gas exploration geology for this region.

Existing studies have made significant progress in sedimentary facies, reservoir classification, and diagenesis. For example, scholars have proposed the four-part sedimentary facies model of the Dengying Formation, highlighting the essential role of microbial mounds and grain shoals in reservoir formation (Yang et al., 2019; Zhang et al., 2020). Regarding diagenesis, the contributions of multi-stage dolomitization and exposed karstification to reservoir modification have been identified (Jin et al., 2018; Huang et al., 2021). In terms of regional tectonics, it is recognized that the wide exposure surfaces and unconformities caused by the Tongwan Movement serve as key nodes for reservoir development (Wang et al., 2015; He et al., 2021). However, several deficiencies persist: First, the coupling mechanisms between sedimentation and diagenesis on the western side of the rift trough, influenced by paleo-uplifts and fault systems, remain unclear; Second, the understanding of reservoir spatial distribution patterns is insufficient, which limits prediction accuracy; Third, comparative analyses of pore genesis and evolution mechanisms across different reservoir types are still lacking systematic approaches.

To address these issues, this study integrates conventional two- and three-dimensional seismic interpretation, drilling-core observations, stratigraphic correlation, petrographic analysis, petrophysical measurements, and geochronological constraints to investigate the following questions: (1) What are the stratigraphic-thickness and sedimentary-facies evolution characteristics of the Dengying Formation along the western margin of the rift trough? (2) How do petrological characteristics and pore types differ among the principal reservoir types? (3) How did the Tongwan exposure events and subsequent fault-mediated burial fluids jointly modify reservoir quality? and (4) What are the spatial distribution patterns and qualitative seismic-reflection expressions of favorable reservoirs?

Through this work, we establish a sedimentation–diagenesis–tectonics coupled reservoir-control model for the Sinian Dengying Formation along the western margin of the Deyang–Anyue Rift Trough. This model provides a geological basis for identifying favorable reservoir intervals and assessing exploration targets in the western trough and adjacent areas of the Sichuan Basin.

2 Geological setting

The Sichuan Basin is located in the western part of the South China Plate and constitutes an important component of the Yangtze Craton (Wang and Li, 2003; Mei et al., 2010). The Deyang-Anyue Rift Trough is the largest and most tectonically active rifting unit in the central Sichuan Basin, extending in a nearly northeast direction, bordered by the Central Sichuan paleo-uplift to the west and the Huayingshan-Dabashan fold belt to the east (Figure 1a) (Yang et al., 2017; Xu et al., 2019)

FIGURE 1

Since the Precambrian, the region has undergone multiple episodes of tectonic evolution, including Neoproterozoic rifting, Sinian-Cambrian marine transgressions and platform sedimentation, Ordovician-Silurian Caledonian Orogeny, Devonian-Carboniferous Hercynian Orogeny, and multiple phases of tectonic uplift during the Mesozoic-Cenozoic (Zhao et al., 2018; Liu et al., 2016; Li et al., 2022). The formation of the Deyang-Anyue Rift Trough is controlled by basement faulting and extensional tectonic activities within the Yangtze Craton, with its subsidence center located near the An’yue-Suining area. Differential subsidence within the trough has resulted in significant north-south and east-west variations in the thickness of the Dengying Formation. Basement faults not only controlled the subsidence and distribution of sedimentary systems during the depositional period but also served as fluid pathways during the burial period, driving hydrothermal fluids into carbonate reservoirs and resulting in multi-phase diagenetic alternations (Luo et al., 2015; Tian et al., 2020).

During the Ediacaran Period, the Sichuan Basin was predominantly situated within an intra-platform to platform-margin depositional system, characterized by the development of extensive carbonate platforms (Zhao et al., 2015; Sun et al., 2018). The western side of the rift trough is located along the platform margin, where diverse sedimentary facies are developed (Figure 1b). The platform interior is marked by microbial mounds, grain shoals, and restricted platform deposits; the margin areas are dominated by high-energy grain shoals and algal mounds; the slope zones feature thin-bedded limestone and turbidite deposits; while the trough interior predominantly contains deep-water argillaceous limestone and dolomitic limestone. The Tongwan Movement resulted in large-scale exposure and the formation of unconformities at the end of the second and fourth Members, leading to extensive karst development at the top of the platform. Brecciated dolostone and botryoidal dolostone frequently occur near exposure surfaces, serving as important stratigraphic markers for reservoir development. The prolonged uplift of the Central Sichuan paleohigh has significantly thinned or even entirely erased the Dengying Formation in the uplift region, reflecting the strong influence of tectonic–sedimentary coupling processes.

The Sinian Dengying Formation constitutes a significant carbonate unit within the Lower Paleozoic of the Sichuan Basin and is subdivided into four members (Figure 1c). The first Member (hereafter Deng 1) primarily comprises micritic limestone and dolomitic limestone, deposited in a platform to restricted-platform environment, with a thickness of 0–400 m (Wang et al., 2012; Zhang et al., 2024). The second Member (hereafter Deng 2) is characterized by microbial mounds and granular dolostones, with a thickness of 400–700 m across the western trough, representing the most critical interval for reservoir development (Zhu et al., 2017; He et al., 2019). The third Member (hereafter Deng 3) has a thickness of 0–200 m and is mainly composed of mixed shelf deposits interbedded with black argillaceous rock layers, locally absent in some areas (Liang et al., 2018). The fourth Member (hereafter Deng 4) is 0–300 m thick, primarily composed of micritic dolostone, algal limestone, and siliceous bands, with its top frequently forming pseudoconformities or unconformities with the Cambrian system (Zhang et al., 2018). The overall thickness of the Dengying Formation ranges from several hundred meters to over a thousand meters (Jiang et al., 2011), and its distribution is jointly controlled by rift trough subsidence and paleohigh uplift. A thick succession is developed along the western trough margin (Chongqing–Hongya–Dayi line), where the Deng 1–Deng 2 Members can reach 900–1100 m; in contrast, the Deng 3–Deng 4 Members exceed 300 m in thickness in the Emei–Jinkouhe area (Liu et al., 2019; Yang et al., 2022). Significant stratigraphic absence is observed in the core of the Central Sichuan paleohigh, indicating strong erosion and exposure caused by the Tongwan Movement.

The reservoir formation conditions of the Dengying Formation depend not only on the reservoir space but also on the compatibility between the source rocks and the cap rocks. The black shales of the Cambrian Qiongzhusi Formation and Maidiping Formation developed within the rift trough are the most critical source rocks in the region, characterized by high hydrocarbon generation intensity and forming a vertically stacked source-reservoir system with the weathering crust-type karst reservoir of the Dengying Formation. Moreover, the Cambrian shales and marls act as regional cap rocks with excellent sealing properties. This assemblage relationship explains the widespread enrichment of natural gas within the Dengying Formation in the Anyue Gas Field and the western trough region.

3 Samples and methods

3.1 Geological and geophysical datasets

This study integrates conventional 2D and 3D seismic profiles covering approximately 1,200 km² with well, core, and outcrop data from the western margin of the Deyang–Anyue Rift Trough. Well-to-seismic calibration and conventional reflection interpretation were used to identify platform-margin geometry, stratigraphic terminations, and reflection packages potentially associated with vertically stacked mound–shoal deposits. The seismic data are therefore used qualitatively to constrain depositional architecture and reservoir distribution, rather than as a standalone impedance- or attribute-based reservoir predictor.

Several representative wells, including MX8, PT103, PS6, and PS8, provided more than 4,500 m of cumulative core observations. These subsurface datasets were complemented by outcrop investigations in the Emeishan, Hongya, and Qionglai areas to characterize sedimentary-facies architecture and karstification features. Additional core and thin-section observations from wells JS1, HS1, DT1, Z1, and Datan 1 were used as regional analogues to characterize representative lithofacies, pore types, and exposure-related karst features of the Dengying Formation.

3.2 Petrological and geochemical analyses

More than 800 thin sections and rock samples were examined for integrated petrological and reservoir characterization. Polarized-light microscopy, scanning electron microscopy, and cathodoluminescence imaging were used to identify lithofacies, pore types, cement generations, recrystallization features, and cross-cutting relationships among diagenetic minerals. Representative saddle-dolomite cements and associated fine-grained dolerite were selected for in situ U–Pb dating to constrain the timing of major burial-stage geological and fluid-related events. Reservoir physical properties were evaluated using routine helium-porosity and steady-state or unsteady-state permeability measurements.

Fluid-inclusion microthermometric data were not available for the analyzed cement generations. Consequently, homogenization temperatures and salinities are not quantitatively reported, and the interpretation of burial-fluid evolution is based primarily on petrographic relationships, cement generations, mineral assemblages, cathodoluminescence characteristics, and U–Pb ages (Li et al., 2025b; Li H. et al., 2024; Fan et al., 2024).

3.3 Analytical strategy

The analytical procedure proceeds from sedimentary-facies and stratigraphic analysis to petrological and petrophysical characterization, followed by geochronological constraint of major burial-stage events and seismic-constrained interpretation of reservoir distribution. This multidisciplinary approach enables the depositional framework, diagenetic sequence, pore-system evolution, and spatial distribution of favorable reservoirs to be evaluated at complementary scales.

4 Results

4.1 Sedimentary characteristics

The Deng 1–Deng 2 succession displays systematic lateral thickness and facies variations across the western margin of the Deyang–Anyue Rift Trough. It is generally 600–900 m thick along the western trough margin and locally exceeds 1,100 m, where relatively continuous platform-margin microbial mound–shoal and grain-shoal deposits are developed. Toward the core of the Central Sichuan paleo-uplift, the succession thins rapidly and is locally truncated because of uplift-related erosion. Toward the trough interior, platform-margin mound–shoal facies progressively pass into slope and deeper-water deposits, accompanied by a reduction in reservoir continuity.

In comparison, the Deng 3–Deng 4 succession is generally 0–300 m thick and displays substantially poorer lateral continuity. It is commonly absent over the Central Sichuan paleo-uplift but thickens moderately toward the Emei–Jinkouhe area. The lateral termination mechanisms of the two successions are therefore different. The Deng 1–Deng 2 Members are mainly reduced by erosional truncation toward the paleo-uplift and by depositional thinning and facies transition toward the trough interior, whereas the Deng 3–Deng 4 Members underwent more extensive stratigraphic omission and erosion. Consequently, the Deng 1–Deng 2 Members constitute the principal regional reservoir fairway, whereas the Deng 4 Member forms thinner and less continuous, but locally high-quality, exposure-related karst reservoir intervals.

The sedimentary-facies distribution comprises platform-margin mound–shoal, intra-platform shoal, restricted-platform, slope, and basin environments. Stromatolitic, thrombolitic, and clotted microbialites are mainly developed in platform-margin and mound–shoal transition zones. Grain shoals are composed predominantly of oolitic, peloidal, and algal-fragment dolostones. The Deng 3 Member records temporary platform drowning and increased terrigenous input, whereas the Deng 4 Member is dominated by microbial and micritic dolostones locally containing siliceous bands. Its upper boundary was extensively modified during Tongwan Stage II exposure. The vertical stacking and lateral migration of these facies jointly established the primary depositional framework for subsequent reservoir modification.

4.2 Petrological characteristics

Reservoir rock types can be classified into six categories. Stromatolitic dolostone (LF1; Figure 2a) and thrombolitic dolostone (LF2; Figure 2b) are predominantly distributed at the top of microbial mounds and in transition zones between mound margins and inter-mound areas. The porosity is well developed, primarily characterized by dissolution vugs and intercrystalline pores. Brecciated dolostone (LF3) includes two types: tectonic breccia (Figure 2c) and karst breccia (Figure 2d), which are commonly associated with exposure-karst processes. The reservoir spaces are primarily composed of irregular dissolution cavities and fractures. Grainy dolostone retains significant primary porosity and exhibits relatively good reservoir properties (LF4; Figure 2e). Crystalline dolostone is influenced by recrystallization, with porosity depending on the extent of intercrystalline pore development (LF5). Micritic dolostone features a dense texture and poorly developed porosity, making it mostly a non-reservoir or low-efficiency reservoir (LF6; Figure 2f). Overall, LF1–LF4 types constitute the most important reservoir units, typically characterized by vuggy or composite porosity.

FIGURE 2

The main pore types include intercrystalline pores, dissolution pores, and fracture pores (Shi et al., 2026; Li J et al., 2025; Cen et al., 2026; Liu et al., 2026). Intercrystalline pores are commonly found in rocks with significant early dolomitization. These pores are evenly distributed but have relatively small pore sizes. Dissolution pores were formed during the exposure period of the Tongwan Movement or during the hydrothermal transformation process in the burial period, characterized by large pore diameters, irregular shapes, and frequent connectivity with dissolution cavities. Fracture pores are closely associated with faulting activities and can serve as both storage spaces and significant permeability enhancers for the reservoir. Different pore types often appear superimposed in the same reservoir, resulting in a general characteristic of high porosity and low permeability.

4.3 Reservoir characteristics

It can be observed from the figure that, compared to the Deng 2 Member, the rocks developed in the Deng 4 Member are extremely heterogeneous. The average porosity of various carbonate rocks generally ranges between 2% and 3%, with some dissolution cavity samples reaching about 8% (Figure 3a,b). The porosity of the Dengying Formation reservoirs mostly ranges from 2% to 4%, with the permeability generally below 0.1 mD, classifying them as low-porosity and low-permeability reservoirs. Among these, the porosity characteristics of the Deng 2 and Deng 4 reservoirs are similar; however, Deng 4 Member exhibits relatively higher permeability and contains more medium-permeability reservoirs.

FIGURE 3

Additionally, core and thin-section observations show that reservoir quality varies strongly among different depositional and diagenetic facies. Platform-margin mound–shoal and grain-shoal dolostones generally have porosities of 4%–8%, and locally exceed 10% where dissolution vugs and fractures are well developed. In contrast, inter-shoal, back-shoal, and platform-interior micritic or argillaceous dolostones commonly have porosities lower than 2%, with pore systems dominated by isolated intercrystalline micropores. These data indicate that high-energy mound–shoal facies provide the primary material foundation for reservoir development, whereas subsequent dissolution and fracturing determine the effective pore connectivity.

4.4 Diagenetic characteristics

Diagenesis plays a crucial role in controlling the evolution of reservoirs. Dissolution is the chemical process whereby pore fluids, such as meteoric freshwater and hydrothermal fluids, dissolve carbonate minerals like calcite and dolomite, forming a secondary pore system (Li J. et al., 2025; Li J. et al., 2024). Multistage dissolution processes have profoundly impacted reservoir development in the study area, including syndiagenetic dissolution and supergenetic dissolution.

Quasi-contemporaneous dissolution refers to the selective dissolution by meteoric freshwater along permeable layers, such as microbial mound-shoal complexes, due to high-frequency sea-level fluctuations during periods of short-term exposure when sediments are not yet fully consolidated. A large number of penecontemporaneous dissolution features have been identified in the Deng 2 Member: beneath the exposure surface, near-in-situ brecciation caused by karst gully cutting, with breccia fragments exhibiting strong reassemblability (Figure 4a-f). The microbial mound-shoal complexes in the study area are positive relief units on the platform. When sea level drops, the tops of these mounds and shoals, as the highest points on the platform, are the first to be exposed above sea level and subjected to leaching by meteoric freshwater. Microbial mounds exhibit relatively high original porosity and permeability, which facilitates the infiltration of meteoric freshwater into the sediment, thereby promoting dissolution processes that enhance porosity and improve the reservoir.

FIGURE 4

Epigenic dissolution refers to the process where tectonic uplift exposes strata to the surface for extended periods, leading to regional dissolution by atmospheric freshwater infiltrating along fractures and unconformity surfaces. The Deng 2 Member is minimally affected by epigenic karstification during Tongwan Stage I, displaying only small dissolution pores below the weathering crust, with no large-scale infilling sections. Its development is limited in scale and occurs exclusively in elevated geomorphological areas. In contrast, large-scale epigenic dissolution within the study area predominantly occurs in the Deng 4 Member, driven by tectonic uplift during Tongwan Stage II. This prolonged surface exposure of the Deng 4 Member’s dolostones under atmospheric freshwater leaching resulted in the development of a mature karst system marked by caves and weathering crusts. After the exposure and uplift phase of Tongwan Stage II ended, debris from the weathering crust backfilled and mechanically infilled large karst caves, leading to the development of cave segments and porosity-permeability layers beneath the weathering crust, with a high degree of infill observed in the large karst caves (Figure 5a,b). Compared with the highly infilled large karst caves, the small-scale pore system formed by epigenic karst in the Deng 4 Member exhibits stratiform development characteristics with good facies control. The absence of early dolomite cementation allowed the pores to remain well-preserved, making it the highest-quality reservoir type within the Dengying Formation.

FIGURE 5

Dolomitization occurred in at least two stages: early microbial-mediated dolomitization in shallow restricted-platform environments, followed by burial-stage recrystallization and locally fault-related hydrothermal dolomitization. The Deng 2 and Deng 4 reservoirs generally underwent early microbial-mediated dolomitization. Spherulitic, dumbbell-shaped, nanoscale, and needle-like dolomite textures are locally preserved and indicate mineral precipitation associated with microbial activity (Figure 6a-d). These early dolomite fabrics contributed to the preservation of intercrystalline and framework-related pores in some microbial mound–shoal deposits.

FIGURE 6

The preservation of microbial fabrics and the limited development of late coarse-crystalline cement in some samples suggest that parts of the early-formed pore system escaped complete burial-stage occlusion. In contrast, early marine cements locally occupied framework and intergranular pores, but some of these cements were subsequently dissolved during exposure-related meteoric alteration.

During burial, basement-fault activity facilitated multiphase fluid migration and diagenetic alteration of the Dengying Formation reservoirs. The U–Pb ages presented in Figure 7 record several burial-stage geological events. The fine-grained dolerite yields older ages that constrain associated igneous activity, whereas two generations of saddle-dolomite cement yield ages of 415 ± 16 Ma and 259.4 ± 3 Ma, respectively (Figure 7). Only the latter two ages are used here to constrain the principal carbonate-cementation events relevant to reservoir modification.

FIGURE 7

The earlier saddle-dolomite generation, broadly corresponding to the late Caledonian tectonic stage, is associated with recrystallization, dissolution seams, fracture-related dissolution pores, and locally enlarged vugs (Figure 7a,b). These petrographic relationships indicate a dissolution-dominated fluid–rock interaction stage that locally improved pore connectivity (Figure 7c,d). However, because fluid-inclusion microthermometric data are unavailable, the absolute temperature, salinity, and detailed chemical composition of the responsible fluid cannot be quantitatively determined.

The later saddle-dolomite generation yields an age of 259.4 ± 3 Ma and broadly corresponds to the Hercynian tectonic stage (Figure 7e-h). It is associated with carbonate and locally quartz cementation within fractures, breccia-related pores, and earlier dissolution vugs. Cross-cutting relationships and cathodoluminescence zoning indicate successive cement growth and changes in fluid conditions rather than a single continuous precipitation event. This later stage was predominantly filling-dominated and resulted in partial to complete occlusion of previously formed pore–fracture systems.

The two principal carbonate-cementation stages are therefore distinguished by their U–Pb ages, cement generations, petrographic relationships, and cross-cutting relationships. The available evidence supports an earlier dissolution-dominated stage followed by a later filling-dominated stage, but does not permit direct quantitative reconstruction of fluid temperature or salinity.

In summary, early microbial-mediated dolomitization in shallow restricted environments contributed to the preservation of primary depositional fabrics and locally generated intercrystalline pores. Tongwan exposure at the tops of the Deng 2 and Deng 4 Members promoted meteoric dissolution and the formation of dissolution pores, vugs, fractures, and breccias. During burial, basement faults provided migration pathways for multiple fluid pulses. Earlier fluid–rock interaction locally enlarged pores and fractures, whereas later carbonate and quartz cementation partially or completely occluded the previously formed pore systems.

Botryoidal dolostone and karst breccia are important indicators of exposure-related and multiphase diagenetic modification. Their occurrence records a complex sequence of exposure, dissolution, filling, and local redissolution. Reservoir quality therefore reflects the net result of constructive dissolution and destructive cementation rather than the effect of a single diagenetic process.

Thickness distribution and sedimentary facies evolution determine the macroscopic distribution patterns of reservoirs; rock types and pore structures control the mesoscopic characteristics of reservoirs; and diagenetic modifications, especially dolomitization, karstification, and hydrothermal activity, shape the microscopic pore evolution sequences of reservoirs. This multi-level coupling mechanism is the root cause of the complexity and heterogeneity of the Dengying Formation reservoirs in the western trough region. The constructive and destructive effects of different types of diagenesis in the study area are classified and summarized (Table 1).

5 Discussion

5.1 Tectonic framework and sequence control on reservoirs

The Deyang–Anyue rift trough has long exhibited differential subsidence across the region, with the western edge adjoining the Central Sichuan paleo-uplift. Controlled by this factor, the Dengying Formation forms a stable thickness zone at the western margin of the trough, where the Deng 1–Deng 2 Members are generally thick and well-connected; whereas in the uplift core and adjacent areas, the Deng 3–Deng 4 Members are noticeably thinner or completely absent. This thickness pattern directly constrains the “main reservoir intervals”: thicker segments provide a long-term “environment” conducive to the continuous deposition of platform carbonates, shoal reformation, and early cementation, also leaving ample soluble medium for later alteration; thinner or missing segments more readily form exposure surfaces at the top, and upon dissolution and incision, develop layer-controlled karst caves and fractures, though they generally exhibit poor lateral continuity.

The multi-phase uplifts during the Tongwan period led to extensive exposure at the end of the Deng 2 and Deng 4 Members. Brecciation, infilled veins, and top dissolution surfaces commonly observed in cores and outcrops indicate intense near-surface dissolution occurred during this period. Following exposure, the overlying layers are predominantly composed of muddy or siliceous sediments, whose sealing effects facilitate the preservation of the underlying pores. The stratigraphic assemblages characterized by “exposure-dissolution-cover” frequently reappear near platform margins or slope breaks, serving as spatiotemporal nodes for optimal reservoirs.

Basement faults traverse the entire process of sedimentation and burial. During the depositional period, they define subsidence centers and shape the morphology of isopach lines; during the burial period, faults function as upward conduits for deep fluids, driving repeated episodes of dissolution and cementation within near-well zones or across areal extents. Areas in close proximity to faults are more prone to intensified late-stage cementation, whereas the hilltop shoals relatively farther from faults are more likely to retain pores formed during the exposure period. Overall, the spatial overlap of thickness zones, exposure zones, and fault zones determines “where mobilizable pore systems are more likely to develop.” For deployment purposes, the following sequence can be used for selection: (1) platform margin—shoal ridge lines within the thickness zone; (2) exposed surface positioning zones corresponding to specific stratigraphic intervals; (3) segments where secondary faults “intersect but are not overly close.” This sequence considers the reservoir body size, opportunities for pore formation, and late-stage sealing risks.

Within the sequence framework, thick intervals often correspond to relatively complete upward-shallowing cycles, including multiple smaller-scale exposures and subsequent re-covering; thin intervals are more commonly associated with cycle omissions and unconformity truncation. The multi-cycle stacking of thick sections increases the probability of shoal body rejuvenation and the number of exposure surfaces, thereby enhancing the reproducibility and traceability of the reservoir; although single or few cycles in thin sections can form high porosity, they are not easily connected as continuous intervals. By jointly constraining thickness, cycles, and exposure, we can separately discuss “having porosity” and “connectivity,” thereby reducing the bias caused by using porosity-only selection criteria.

5.2 Effects of depositional systems and shoal superposition on reservoirs

The Dengying Formation reservoirs exhibit significant differences in spatial distribution and physical properties, which are closely related to the sedimentary system configurations and the vertical stacking of shoal bodies. On the western margin of the rift trough, both drilling and seismic data reveal a high degree of consistency between reservoir distribution and the spatial extent of platform margin mound-shoal deposits. For example, in the drilling cores from the Anyue-Guanghan area, large-scale algal mounds and oolitic beaches are developed in the Deng 2 to Deng 3 Members, where numerous secondary dissolution pores and incompletely filled dissolution fractures can be observed in the cores. Porosity generally ranges from 4% to 8%, with some areas exceeding 10%. In contrast, in low-energy depositional zones such as intershoal and back-shoal areas, the cores are predominantly composed of argillaceous dolomite and siliceous dolomite, which are typically dense, with porosity below 2%, mainly consisting of isolated intercrystalline pores. It is evident that sedimentary facies zones directly control the distribution range and pore types of reservoirs (Shan et al., 2021).

This difference can be explained by the energy conditions of the depositional systems. The platform margin and shoal ridge zones are situated in relatively high-energy hydrodynamic environments, characterized by abundant granular material and thriving biological communities, which facilitate the formation of thick-layered grainy beaches or biogenic mounds. This type of rock experiences limited porosity loss during its early burial stage, enabling partial preservation of primary porosity. More significantly, when sea level falls or tectonic uplift causes exposure, the top of the shoal body comes into direct contact with atmospheric freshwater environments. Acidic water infiltrates through intergranular spaces and microfractures, causing extensive dissolution and resulting in secondary porosity primarily composed of dissolution pores and vugs. Conversely, in low-energy environments of intershoal or intra-platform settings, sediments tend to have higher argillaceous and siliceous components, with sedimentary particles tightly bonded together. Early compaction and cementation are more pronounced, hindering later dissolution processes, which collectively result in generally poorer reservoir quality.

This difference is clearly observable through core observations. For example, the core sample from the platform-margin shoal body in the Deng 3 Member of an exploratory well shows grain-supported dolomite, with dissolution-enlarged pores commonly observed between the grains. Some of these pores are partially filled with secondary calcite but remain incompletely sealed. Conversely, core samples from the inter-shoal area of the same well display high argillaceous content and dense microcrystalline structures, with only sporadic intercrystalline pores and extremely poor connectivity. Thin-section observations further corroborate these findings: rocks in the shoal margin typically exhibit subhedral dolomite crystal structures, with intercrystalline pores, dissolution pores, and fractures interconnected. In contrast, rocks from the inter-shoal areas are predominantly composed of fine-crystalline dolomite and siliceous cement, with pores nearly completely occluded.

The vertical stacking of shoals is also an important phenomenon in the development of Dengying Formation reservoirs. Multiple shoals can be observed in profiles as stacked layers or lenses. After deposition, each shoal experiences varying degrees of exposure and dissolution, forming unconformity surfaces and secondary dissolution zones (Figure 8). When a new shoal body overlies an existing one, the pores in the underlying shoal body are not only preserved but may also be further enlarged due to superimposed dissolution. This results in the reservoir exhibiting multilayered stacking vertically, often forming multiple sets of effective reservoirs within the same well section. For example, drilling in the Guanghan area reveals that in the Deng 2 Member, three sets of superimposed shoals have developed. Each shoal set has distinct dissolution layers and fracture systems at the top, and the overall porosity is higher than that of the surrounding single-layer shoal distribution areas.

FIGURE 8

On the available well-calibrated seismic profiles, vertically stacked mound–shoal bodies are locally expressed as lens-shaped reflection packages with relatively strong and laterally variable amplitudes along the platform margin (Abudeif et al., 2025; Dzhangirov et al., 2024). Their distribution is broadly consistent with the positions of reservoir-bearing intervals identified by drilling. In contrast, areas interpreted as single-layer or poorly stacked shoals generally display less distinctive and less continuous reflection packages (Reda et al., 2024; Reda et al., 2025).

Because seismic-attribute slices and impedance-inversion results are not presented in this study, these reflection characteristics are treated as qualitative supporting evidence. They should not be regarded as independent quantitative criteria for reservoir prediction. Reliable target evaluation requires their integration with well calibration, stratigraphic interpretation, lithofacies information, and petrophysical evidence.

This series of phenomena indicates that sedimentary facies control not only affects the basic lithology and pore types of reservoirs but also determines their planar distribution patterns and vertical connectivity through the spatial distribution and stacking patterns of shoal bodies. From the genetic mechanism perspective, particle accumulation in high-energy platform-margin areas provides the material basis for reservoir formation, while multi-stage exposure and dissolution, resulting from sea-level fluctuations, continuously superimpose and transform the original pores, diversifying the pore system and enhancing connectivity. The shoal body stacking zones, due to multi-phase superposition effects, form the most optimal reservoir units, whereas the intra-platform and inter-shoal areas are generally depleted in reservoirs due to unfavorable conditions of material composition and diagenesis.

In conclusion, the depositional system and the superposition of shoal bodies are the fundamental causes of heterogeneity and distribution variations in the Dengying Formation’s reservoirs. High-quality reservoirs predominantly develop in platform-margin and shoal body stacking zones, characterized by the composite appearances of intercrystalline pores, dissolution pores, and cavities, with porosity and permeability significantly superior to those in the inter-shoal to intra-platform zones (Elgendy et al., 2025). Understanding this pattern not only aids in uncovering the geological context of reservoir formation but also provides a reliable basis for predicting the distribution of high-quality reservoirs and planning exploration targets.

5.3 Diagenesis and burial-fluid modification of reservoirs

Diagenesis profoundly modified the pore systems of the Dengying Formation reservoirs. Early-formed framework and intergranular pores were progressively reduced by mechanical compaction, recrystallization, and carbonate cementation during burial (El-Gendy et al., 2023; Qin et al., 2026). These destructive effects are most evident in low-energy platform-interior deposits and mud-rich intervals, where fine-crystalline and argillaceous dolostones commonly contain poorly connected micropores.

Exposure-related dissolution produced a contrasting effect. During the Tongwan events, meteoric water entered mound–shoal deposits through bedding surfaces, intergranular pathways, and fractures, selectively dissolving carbonate minerals and generating secondary pores and vugs. Dissolution was concentrated in high-energy mound–shoal facies and beneath regional exposure surfaces rather than being uniformly distributed throughout the succession. Consequently, favorable depositional facies provided the material framework, whereas exposure and dissolution controlled the local enhancement of reservoir quality.

Cement petrography, cross-cutting relationships, cathodoluminescence characteristics, and U–Pb ages further indicate that fault-mediated burial fluids exerted both constructive and destructive effects. Earlier fluid–rock interaction locally enlarged pre-existing pores and fractures. Later carbonate and quartz precipitation partially or completely filled the same pore–fracture systems. The coexistence of mineral-filled fractures and peripheral dissolution pores demonstrates that dissolution and cementation occurred repeatedly during burial.

The reconstructed paragenetic sequence indicates that reservoir evolution was strongly stage dependent. During early burial, compaction and cementation progressively reduced primary pore volume. Subsequent uplift and exposure promoted meteoric dissolution and locally improved reservoir quality. During deeper burial, fault-mediated fluids caused alternating dissolution and mineral precipitation, with late cementation reducing the connectivity of some earlier pore–fracture systems. Reservoir quality therefore evolved through repeated episodes of pore destruction and secondary improvement rather than through a simple linear trend.

Fault-associated fluid flow produced spatially variable effects. Fracture corridors locally enhanced dissolution and pore connectivity, whereas fault cores and zones affected by pervasive late-stage cementation experienced substantial pore occlusion. Favorable reservoirs are therefore more likely to occur in mound–shoal bodies influenced by fracture-assisted dissolution but located outside strongly deformed and cemented fault-core zones.

5.4 Petroleum geological implications for exploration

The reservoir control model established in this study carries direct implications for exploration targeting and well placement in the study area, and provides a geological analogue for similar deeply buried carbonate systems.

5.4.1 Implications for the study area

High-quality reservoirs are preferentially developed in platform-margin mound–shoal complexes within the Deng 1–Deng 2 Members, where cumulative thicknesses locally reach 900–1,100 m along the western trough margin. This thickness belt defines a favorable fairway in which mound–shoal deposition, repeated shoal stacking, and exposure-related dissolution are superimposed. In contrast, the interior of the Central Sichuan paleo-uplift, where the Dengying Formation was extensively thinned or removed by Tongwan erosion, has a higher risk of reservoir absence.

On the available well-calibrated seismic profiles, some platform-margin mound–shoal complexes are expressed as lens-shaped reflection packages with relatively strong amplitudes. These features may assist in tracing favorable depositional architecture when integrated with well calibration, stratigraphic interpretation, and geological evidence, but they should not be used alone to define drilling targets.

Regional unconformities at the tops of the Deng 2 and Deng 4 Members mark intervals affected by meteoric karstification. The most favorable targets occur where these exposure surfaces coincide with platform-margin mound–shoal buildups. Brecciated dolostone, botryoidal dolostone, dissolution vugs, and weathering-related fabrics provide useful indicators for identifying these intervals in cores and cuttings.

Basement faults served as conduits for both dissolution-enhancing and porosity-occluding fluids, resulting in spatially variable reservoir quality near fault zones. However, the available dataset does not provide a statistically calibrated relationship among fault distance, seismic attributes, reservoir properties, and well productivity. Consequently, a universal metre-scale fault-standoff distance or fixed impedance or coherence threshold cannot be reliably proposed.

Relative operational screening criteria are therefore recommended. First, the interpreted main-fault core and its lateral uncertainty zone should be excluded because intense deformation, drilling risk, and late-stage cementation may occur within these zones. The width of the uncertainty zone should be determined from local seismic resolution and interpretation confidence rather than from a universal fixed distance. Second, highly discontinuous or chaotic reflection corridors should be treated cautiously, particularly at intersections of major faults. Third, priority should be given to mound–shoal crests where favorable reflection packages remain laterally continuous and are supported by well calibration, favorable lithofacies, and exposure-related karst indicators. Horizontal-well trajectories should remain outside the interpreted main-fault core and its uncertainty zone and should be adjusted using local seismic resolution and real-time geological information.

Where coherence, discontinuity, or impedance products are available during subsequent field deployment, locally calibrated attribute anomalies may be used as supplementary screening parameters. However, their numerical cutoffs should be established separately for each seismic survey and processing workflow and should not be transferred directly between blocks.

Vertically, the dissolution-enhanced middle–upper parts of individual reservoir cycles should be prioritized over dense lower intervals. These middle–upper sections commonly contain composite pore–fracture systems that provide both matrix storage and fracture-enhanced connectivity and are therefore the most favorable intervals for completion and stimulation.

5.4.2 Comparison with representative precambrian carbonate petroleum systems

The sedimentary and diagenetic controls identified in the Dengying Formation can be compared with several representative Precambrian carbonate petroleum systems worldwide. Although microbialite development and dolomitization are common features, the relative importance of exposure, karstification, burial alteration, and evaporite preservation differs substantially among the basins (Table 2) (Adams and Mackenzie, 1998; Allen and Allen, 2013; Frolov et al., 2015; Plummer, 2021).

The comparison demonstrates that microbialite development and dolomitization constitute common foundations for Precambrian carbonate reservoirs, whereas their secondary modification and preservation mechanisms differ. The Ara Group is characterized by isolated carbonate bodies enclosed within thick evaporites, and reservoir preservation is strongly influenced by intra-salt compartmentalization. The Riphean reservoirs of the Siberian Platform are closely associated with pre-Vendian weathering crusts and fractured–vuggy carbonate systems. The Gillen Formation represents a less extensively drilled intra-salt exploration target dominated by stromatolitic dolostones.

In comparison, the Dengying Formation records the superposition of platform-margin mound–shoal deposition, repeated Tongwan exposure and karstification, and multiphase fault-mediated burial-fluid alteration. Its distinguishing characteristic is therefore not microbialite development alone, but the repeated coupling of favorable depositional architecture, subaerial dissolution, and burial-stage dissolution–cementation.

6 Conclusion

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