Microfacies and Evolution of the Carbonate Factory During the Middle Permian in Northwest Sichuan Basin, China

Abstract

Located in the eastern Paleo-Tethys Ocean and near the equator, carbonate sedimentation widely developed in the Sichuan Basin in the Permian Guadalupian period. Although the growth and decline of carbonate particles are closely related to the surrounding sedimentary environment, the relationships between the grain composition and distribution of the northwest Sichuan Basin and the sedimentary environment are not clear. This study explored the particle type, particle content, and sedimentary structure of 300 thin sections from 19 wells and seven field profiles of the Guadalupian period in the northwest Sichuan Basin, identified seven microfacies and four microfacies associations, analyzed the sedimentary environment, and established a sedimentary evolution model. The results show that there was a warm-water Dasycladaceae-dominated and foraminifera-dominated open platform developed in the early Roadian era in the research area. As the climate cooled during the late Roadian era, the warm-water carbonate sedimentary environment was replaced by a cold-water bryozoan-dominated and echinoid-dominated marginal sedimentary environment. As the climate continued to cool and the sea level dropped, the platform margin grain beach sediment underwent further development in the early Wordian era. In the late Wordian era stage, the sedimentary environment was influenced by the Dongwu movement, resulting in sea level changes and acidification caused by hydrothermal activities, which reduced the degree of grain beach development. During the Capitanian era, the climate became warmer due to the eruption of the ELIP. The uplift caused by the Dongwu movement resulted in a relative shallowing of the platform margin region, and hence, the grain beach sediment only developed in the southwest, while the northeast was dominated by deepwater basin sediments. The sudden transition from a warm-water, autotroph-dominated carbonate open platform to a cold-water, heterozoan-dominated carbonate platform margin resulted from a combination of tectonic movements, rapid sea level changes, and sedimentary environment changes during the Guadalupian era.

1. Introduction

The Sichuan Basin in the upper Yangtze block is renowned for its substantial oil and gas reserves; the Maokou Formation of Guadalupian is considered a superior reservoir of oil and gas [1,2,3,4,5,6,7]. As a critical area for carbonate oil and gas, there are significant differences in the oil and gas production process across the study area [8,9,10,11].

Significant research has been conducted in this area, including work on structural activity [12,13], sequence stratigraphy [3,5,14], sedimentary facies [4,6,15,16], and carbonate reservoirs [4,8,9,11,17,18], but these studies primarily focused on traditional carbonate sedimentology, in which tectonic lifting and sea level changes were thought to control the platform deposit sequence relating to the thickness, stacking patterns (regression, progradation, or aggradation), frequency of changes, and spatiotemporal distribution of sedimentary facies [19,20,21]. These studies emphasized the physical aspects of the initial platform edge morphology and variations in the initial accommodation space, but they ignored the impact of biological types and quantities on carbonate production and the impact of biogenic buildup formation on the hydrodynamics of the depositional environment, which can directly affect the morphology of carbonate platforms. Marine environmental factors such as climate, temperature, salinity, and water nutrients influence carbonate production efficiency and characteristics by affecting biological assemblages and evolution [22,23,24].

In the early Guadalupian stage, carbonate deposition was widespread in subtropical and tropical regions [23,25,26]. However, the late Guadalupian stage witnessed the Permian Chert Event (PCE), which led to an increase in chert deposition and a reduction in carbonate deposition [27,28]. In regions such as the west of North America [27,28,29], the Russian Far East, and the Transcaucasus [30], Guadalupian carbonate platforms were covered by chert. The literature on shallow-water carbonate platforms indicates a long-term decline in the platform area from the Late Carboniferous stage to the Permian stage [25], while prolonged ocean acidification [31] or climatic cooling have been proposed as reasons for the decline in carbonate production [32]. However, the specific impacts of contemporaneous biotic and environmental changes on the extent of carbonate platform development remain unclear.

The South China Yangtze Carbonate Platform (YCP) is one of the tropical platforms that developed during the Guadalupian stage (Figure 1A), formed in a relatively simple and well-defined tectonic setting, despite the eruption of the Emeishan Large Igneous Province (ELIP) in the region during the late Guadalupian stage. The YCP is well preserved, including its surrounding marginal and basin facies, and compared to other marine profiles where the Guadalupian sequence records a major hiatus [33], South China has retained a relatively complete Guadalupian sequence, including the Global Stratotype Section and Point (GSSP) for the Guadalupian–Lopingian Boundary (GLB) [34]. Extensive investigations of fossil records within the Guadalupian strata have provided a high-resolution biostratigraphic framework, allowing for correlations across different platforms. Additionally, in the northwestern Sichuan region, tectonic activities led to faulting and hydrothermal upwelling, with research on these hydrothermal activities often focusing on the formation of siliceous rocks and their impact on dolomitization. However, the influence of these hydrothermal processes on carbonate development remains understudied.

This study primarily investigates the types and abundances of carbonate producers across different depositional periods, along with the temporal changes in these producers. It explores the geological significance of the evolution of the Maokou Formation carbonate platform in northwestern Sichuan, considering regional tectonics, sea level fluctuations, glaciation events, and paleoceanographic conditions. The findings of this study will provide insights into the shifts in both deep-water and shallow-water carbonate factories by offering a reference for similar research in other regions and time periods and serving as a basis for future oil and gas exploration.

2. Geological Settings

The South China block is located in the equatorial eastern margin of the Paleo-Tethys Ocean (Figure 1A) [35], which mainly comprises the Yangtze Platform and the Cathaysia Old Land. The Sichuan Basin is located in the southwestern part of the South China Block and developed on the Upper Yangtze Craton. During the Middle Permian stage, the Upper Yangtze Craton formed a steady carbonate platform environment in the Kungurian and early Guadalupian (~273 Ma) stages. Afterwards, a major transgression occurred in the early Guadalupian (~273 Ma) stage, followed by a regression in the middle Guadalupian (~269 Ma) stage, with the main depositional environment remaining an open carbonate platform. The sedimentary differentiation occurred in the Sichuan Basin with strong tectonic movements affected by the Emeishan Large Igneous Province (ELIP) during the late Guadalupian (~260 Ma) stage and formed platform margin deposits and troughs.

Figure 1

Location and geological background of the Maokou Formation in the northwest Sichuan Basin. (A) Paleogeographic map of the South China block (modified from [36]),the red box is the location of the study area; (B) schematic diagram of the tectonic framework and tectonic zoning of the Sichuan Basin (modified from [11]),the red box is the location of the study area; (C) geological structure outline map of the study area (modified from [13]); (D) comprehensive histogram of the Maokou Formation in the northwest Sichuan Basin.

[Figure omitted. See PDF]

The study area is located in the northwest region of the Sichuan Basin (Figure 1B), which includes the northeast—southwest-trending Shuangyushi and Kuangshanliang–Hewanchang structural belts (Figure 1C). The Middle Permian strata in the northwestern region of Sichuan from bottom to top are the Liangshan Formation, Chihsia Formation, and Maokou Formation. The Liangshan Formation is mainly composed of grayish-yellow mudstone with coal; the Chihsia Formation is mainly composed of micritic limestone, bioclast limestone, and a certain quantity of dolomites; and the Maokou Formation is dominated by bioclast limestone and micrite with some cherts observed only in the middle and upper parts (Figure 1D).

The studied portion of the Maokou Formation spans the three stages of the Guadalupian Series, namely the Roadian, Wordian, and Capitanian. The biostratigraphic framework for the Guadalupian stage in the southwest Sichuan Basin was established and contains six conodont zones (from J. nankingensis to J. xuanhanensis) (Figure 1D), which provides a precise time frame for the Cisuralian–Guadalupian transition (CGT) explored in this study [36,37].

3. Samples and Methods

A total of 400 thin sections were uniformly sampled from 19 wells and nine field profiles in the Maokou Formation within the study area. In terms of selecting rock samples, due to diagenesis, which can damage the original structure of sedimentary rocks, samples should be chosen as much as possible that have not been affected by diagenesis. These thin sections encompass all sedimentary structures, grain types, and biological fossils present in the region and were identified and described using a polarizing microscope at the State Key Laboratory of Oil and Gas Resources and Exploration, China University of Petroleum (Beijing). All the thin sections were partially stained with alizarin red S and K-ferricyanide in a 0.2% HCl solution (cold) to distinguish between calcite and dolomite. The well log data, including natural gamma (GR), density (DEN), acoustic (AC), neutron (CN), true formation resistivity (RT), and flush zone formation resistivity (RXO), were also analyzed in this work.

The microfacies analysis method [38] was used to distinguish the microfacies (MF) types in the Maokou Formation. MF identification was based on the following criteria: (1) the sedimentary texture classification system proposed by Dunham (1962) [38]; (2) the visual comparison diagram of Baccelle and Bosellini (1965) [39], utilized to analyze the frequency of each microfacies type in a semiquantitative manner [39]; (3) the visual estimation of the circularity and sorting coefficients of various microfacies based on their comparison diagram, proposed by Flügel (2010) [38], and different matrix types such as micrite, microcrystalline structures, and silt; and (4) biological fossil characteristics such as biological types, biological assemblages, and burial characteristics. The study of foraminifera involves the systematic identification and quantitative analysis of benthic foraminifera at the genus level from thin rock sections. Based on the morphology and wall characteristics of foraminiferal shells, and with reference to Permian foraminiferal morphological group research, foraminifera in the study area are classified into morphological groups and illustrated. Dominant morphological groups for different stratigraphic levels are identified based on their relative abundance. The analysis explores how changes in dominant morphological groups and their vertical distribution reflect variations in marine environmental conditions. Following the microfacies classification, the identified microfacies (MFs) were compared with the standard microfacies (SMFs) in the carbonate platform model of Flügel (2010) [38] to determine the microfacies of the Maokou Formation. Microfacies are classified based on their lithology, bioclast composition, and texture, and detailed features of microfacies and their corresponding sedimentary environment are summarized in Table A1. Meanwhile, some geochemical indicators from previous research that reflect climate changes are quoted in this manuscript, such as the Chemical Index of Alteration (CIA), land surface temperature (LST), ⁸⁷Sr/⁸⁶Sr ratios and δ¹³Ccarb values [40], atmospheric CO₂ concentrations [41], ¹⁸O_apatite values [42,43], glacial events [44,45,46], and volcanic events [47,48,49,50].

4. Results

4.1. Facies Descriptions

The Maokou formation in the northwest Sichuan Basin consists of seven microfacies.

4.1.1. Platform-Margin Shoals

Platform-margin shoals include microfacies MF1 and MF2. Foraminifera-bearing intraclastic limestone (MF1) is composed of intraclasts cemented by clear calcite and contains (Figure 2A,B) minor amounts of ostracods and foraminifera (Globivalvulina) (Figure 2C). The bioclast packstone (MF2) consists of a rich mix of bryozoans, serpulids, and ostracods (Figure 2D–F). These bioclasts are unevenly oriented, with some showing high degrees of fragmentation. The gamma-ray log characteristics are relatively low (5–30 in API), with the gamma-ray curve displaying a boxy shape and relatively smooth edges along the profile (Figure 5A).

4.1.2. Platform-Interior Grain Shoals

The platform-interior grain shoals include microfacies MF3 and MF4. The Dasycladaceae–foraminifera packstone (MF3) is primarily composed of Dasycladaceae (Figure 3A,B) and benthic foraminifera (Pachyphloia, Sumatrinaand and Globivalvulina), with minor amounts of bryozoan and ostracod bioclasts (Figure 3C–E). The wackestone with brachiopod bioclasts (MF4) features well-preserved, relatively large brachiopod fragments, typically a few millimeters in size (Figure 3F). Additionally, this microfacies includes minor amounts of crinoid stems, foraminiferal skeletal fragments, and re-crystalized matrix. The gamma-ray log characteristics is relatively low (10–40 in API), with the gamma-ray curve displaying a boxy or slightly serrated shape (Figure 5B).

4.1.3. Platform-Interior Inter-Shoal

The platform-interior inter-shoal includes microfacies MF5 and MF7. The fine-grained bioclast wackestone (MF5) is composed mainly of bioclasts with sizes less than 50 μm, which are too small to link to a specific species (Figure 4A–C). The limy–mudstone (MF7) consists primarily of micrite, making up over 90% of the facies. This microfacies lacks significant bioclasts, with some samples showing microfractures filled with organic material (Figure 4E,F). MF7 contains minor amounts of benthic foraminifera, like Globivalvulina and Nodosaria. (Figure 4E). The gamma-ray log characteristics are relatively high (30–85 in API), exhibiting a wide funnel-shaped morphology (Figure 5C).

4.1.4. Slope and Toe-of-Slope

The slope and toe-of-slope include microfacies MF5, MF6, and MF7, a combination located in the northeastern part of the study area. The spiculate wackestone (MF6) is composed of micritic matrix with matrix-supported fabric (Figure 4D). Many sponge spicules are observed floating within the “micritic matrix”, with spicule sizes ranging from a few hundred micrometers to several millimeters. The spicules are predominantly monaxial, with a few being polyaxial. Additionally, bryozoan bioclasts are rare and highly fragmented. The gamma-ray log characteristics is relatively high (50–120 in API), displaying a broad funnel-shaped morphology (Figure 5D).

4.2. Change in Sedimentary Facies in the Study Area

Through analyzing individual wells, this study examines the lateral variations in sedimentary facies through the construction of representative cross-well sections (Figure 6). The comparative profiles extend from Tongkou in the west to Wangjiagou in the northeast. In the early Roadian stage, shoals were confined to the Shuangyushi area, with sedimentation occurring between intra-shoal and marginal shoal environments. By the late Roadian stage, marginal shoals developed around the Shuangyushi area. During the early Wordian stage, marginal shoal sedimentation expanded into the Kuangshanliang area. By the mid-Wordian stage, marginal shoals thickened and extended throughout the Kuangshanliang area, concurrently with the development of intra-shoals in the Jiulongshan area. In the late Wordian stage, marginal shoal areas shrank to the Shuangyushi area, while intra-shoal areas in the Jiulongshan area also diminished. The Capitanian stage exhibited pronounced sedimentary heterogeneity, characterized by the slope and toe-of-slope developing toward the northern Kuangshanliang and Jiulongshan areas, while broad platform and marginal shoal sedimentation continued in the southwest.

5. Discussion

5.1. Microfacies Association Interpretation

5.1.1. Platform-Margin Shoals

Platform-margin shoals are characterized by high-energy deposition occurring above normal wave bases. MF2 represents a medium degree of energy due to its lower degree of sorting and rounding of bioclast particles, whereas MF1 represents a high degree of energy because of the higher degree of sorting and rounding of particles and the presence of bright crystalline cement between the particles. The vertical stratigraphic succession is MF2~MF1, with changes in foraminifera types within MF1 reflecting variations in depositional periods and the environment. The common foraminifera include Globivalvulina. The presence of these foraminifera in bioclast limestones signifies an oxygen-rich and nutrient-abundant environment favorable for their growth and reproduction [51]. The vertical microfacies characteristics of this microfacies assemblage indicate a shallowing upward trend with increasing energy from the base upwards (Figure 3A).

5.1.2. Platform-Interior Grain Shoals

Platform-interior grain shoals are distributed in the high-energy zones of open platforms. They develop within the photic zones and in relatively shallow water. Dasycladalean algae in MF3 are autotrophic organisms that thrive in photic zones. Benthic foraminifera, including Pachyphloia, Sumatrinaand, and Globivalvulina, also inhabit well-lit, warm-water environments, indicating suitable water conditions with high amounts of dissolved oxygen. These foraminifera are predominantly detritivorous or sediment-feeding types. The proliferation of green algae provides an abundant food source, reflecting good light conditions and warm waters. In the brachiopod bioclast facies (MF4), the bioclasts are predominantly from benthic heterotrophic organisms, suggesting development in deeper waters. The vertical microfacies characteristics of this microfacies assemblage are interpreted as a shallowing upward trend with increasing energy. Concurrently, the biotic assemblage indicates warm living conditions.

5.1.3. Platform-Interior Inter-Shoal

The platform-interior inter-shoal facies belt is the sedimentary zone between the grain shoals in the platform interior. The bioclast group in MF5 is mainly composed of benthic heterotrophs, while there is a lack of bioclasts in MF7. MF7 can occur in various carbonate sedimentary environments ranging from shoals to deep-water basins, with the specific sedimentary environment determined by bioclast types. These characteristics indicate its forms in relatively deeper water. This microfacies association is therefore inferred to have formed in a deep-water environment.

5.1.4. Slope and Toe-of-Slope

The slope and toe-of-slope facies distributed in the deep shelf and the sponge spicule in MF6 reflect the deep-water sedimentary environment. The absence of ostracods indicates a low oxygen level in the sedimentary environment.

5.2. The Influence of Climate and Environment on the Sedimentary Evolution of Maokou Formation Carbonate Rocks

5.2.1. Volcanic Activity

Volcanic activity frequently occurred during the Permian stage, with the formation of the Emei volcanic province in South China being considered the most significant volcanic event impacting the entire South China region [52,53]. The volcanic activity of the Maokou Formation can be divided into two stages. One is the continuation of the Kungurian volcanic activity, during which there was a surge in volcanic activity, including the Tarim LIP in northwestern China (approximately 300–280 Ma) [48], the Zaduo LIP on the North Qiangtang Block (approximately 283–276 Ma) [49], and the Taigonos volcanic arc in Okhotsk (approximately 278.8 ± 3.0 Ma) [47]. This LIP volcanic activity essentially ceased during the Roadian stage, with the pause coinciding with a significant reduction in atmospheric pCO₂ (from 500 ppm to 300 ppm) and a decrease in the chemical weathering index, which contributed to triggering the P3 glaciation. This led to changes in the carbonate sedimentary environment and the alteration of carbonate producers [41]. Before P3 glaciation, the atmospheric temperature during the Roadian stage was relatively high and the environment was warmer. During this time, the carbonate rock producers were dominated by coarse algae and foraminifera. After P3 glaciation, due to the decrease in temperature, the levels of carbon dioxide, the chemical weathering index, and the surface temperatures in the study area significantly dropped. At this point, the carbonate rock producers shifted to bryozoans and bivalves, and the sedimentation rate of carbonate rocks also decreased [40,54].

The second stage pertains to the Emeishan Large Igneous Province (ELIP), the eruption of which occurred toward the end of the entire Guadalupian epoch. According to conodont biostratigraphy, Sun et al. (2022) preliminarily indicated that the eruption of ELIP began in the J. altudaensis zone and peaked in the J. xuanhanensis zone [54]. A series of high-precision CATIMS U-Pb dating results accurately constrained the peak time of ELIP to approximately 260 Ma [55]. The temporal consistency between the ELIP eruption and warming suggests that the massive injection of carbon dioxide into the atmosphere from volcanic eruptions led to a rapid rise in temperature [40]. Additionally, the release of significant greenhouse gases due to contact metamorphism induced by the ELIP mantle plume [56] and the destabilization of methane hydrates in permafrost might have contributed to additional greenhouse gases, thus exacerbating climate warming [57]. The warming climate ended the P3 glaciation, which favored the development of carbonate rock producers. This is evidenced by the increase in the size of the foraminifera in the strata.

5.2.2. Ice Age

The Late Paleozoic Ice Age spanned from the Carboniferous to Permian stages and consisted of eight glaciation periods and the interglacials between them, making it the longest-lasting ice age since the Phanerozoic Eon [44]. During the Middle to Late Permian period, the last two of the eight glaciations—P3 and P4—occurred, with the Guadalupian period primarily characterized by P3 glaciation. Regarding the onset of this glaciation, based on the distribution of glacial facies in the Permian strata of the Sydney and Bowen Basins in eastern Australia and the high-precision U-Pb CATIMS dating of glacial sequences [45], combined with the compilation of δ¹⁸Oseawater [58], the onset of P3 glaciation occurred in the early Roadian stage. Support for the early Roadian global cooling and the beginning of P3 glaciation includes the appearance of glacial deposits in the Atkan Formation in Russia’s Northern Hemisphere and trends interpreted from clay mineralogy in the low-latitude epeirogenic section’s foraminiferal biostratigraphy [59]. The glaciation led to a decrease in water temperatures, causing warm-water carbonate producers to disappear and giving way to cold-water carbonate factories characterized by the non-cohesive bioclasat debris of sponges, bryozoans, echinoderms, and brachiopods [60,61,62]. Additionally, the glacial and interglacial periods influenced variations in thermohaline circulation in the oceans, which in turn affected the development of carbonate platforms [28,40,42,43].

5.2.3. Climate Fluctuations and Chemical Weathering Processes

The direct manifestation of climate fluctuations is the change in temperature. During the Permian stage, variations in the concentration of carbon dioxide (CO₂) in the atmosphere also influenced changes in the depositional environment, which is reflected in the atmospheric temperature and seawater pH levels. An increase in CO₂ levels led to a rise in the atmospheric temperature, resulting in climate warming; simultaneously, the increase in atmospheric CO₂ caused a significant increase in water acidification. During the Permian stage, the elevated CO₂ levels in the atmosphere were primarily due to volcanic eruptions, including volcanic activity during the Kungurian stage and the eruptions of the Emeishan Large Igneous Province. These volcanic activities produced large amounts of CO₂, affecting the atmospheric temperature.

However, an increased influx of CO₂ does not equate to a rise in atmospheric CO₂ levels, as these levels also depend on the chemical weathering of silicates. A schematic diagram of the long-term changes in marine and CO₂ buffering capacity in the Sverdrup Basin, established by Beauchamp et al. [31], indicates that intensified silicate weathering led to the flow of high bicarbonate levels into the oceans, thus buffering the ocean pH. This resulted in lower atmospheric pCO₂ levels and global temperatures, facilitating widespread Gondwanan glaciation [63], conditions which were also conducive to the development of carbonate rocks. Conversely, when silicate weathering decreased, pCO₂ levels increased, leading to a rise in atmospheric temperature and seawater acidification.

5.2.4. Upwelling and Hydrothermal Fluids

The Permian stage was marked by strong oceanic upwelling [15,64,65,66]. During this time, the supercontinent Pangea connected the two polar regions and influenced ocean circulation [28]. During the Late Paleozoic Ice Age (LPIA) in the late Middle Permian period, the intensification of Gondwana glaciation and oceanic thermohaline circulation driven by the cold climate may have led to the upwelling of nutrient-rich deep water to the surface, fostering the development of some typical coastal upwelling systems [67]. This process displaced nutrients and silica-rich water, pushing them along the oceanic margins through upwelling. Additionally, lighter meltwater was transported southward along northwest Pangea due to Coriolis forces, which led to the contraction of warm-water carbonate production [28]. During the Permian period, South China was located near the equatorial eastern margin of the Paleo-Tethys Ocean, placed within a tropical upwelling zone. Previous studies have indicated that significant upwelling during the Permian period primarily occurred in the Guadalupian stage. Episodic seawater anoxia in the basins surrounding the Yangtze carbonate platform likely affected carbonate production in the platform-margin areas [68,69]. Concurrently, in the Middle Permian period, siliceous organisms thrived in environments with enhanced upwelling due to strong thermohaline circulation; this inhibits the development of carbonate producers [28,70,71]. In the Middle Permian period, deep-seated fault zones in the northwest Sichuan Basin, generated by the Dongwu Movement, allowed hydrothermal fluids to ascend through fault channels into the ocean. This hydrothermal fluid was delivered to the surface by circulation and upwelling, which led to seawater acidification and locally elevated seawater temperatures [72,73]. From a geological perspective, the role of carbonate sedimentary evolution in the entire Guadeloupe hydrothermal study area is relatively minor, accounting for no more than 10% of the overall process [74]. It had a more significant impact on the rise in seawater temperatures, thereby reducing the suppressive effects of upwelling on carbonate rocks.

5.3. Establishment of Sedimentary Evolution Models for the Maokou Formation

Based on studies of sedimentary microfacies and the evolution of sedimentary environments, and combined with previous research on the climate and environment during the Guadalupian period, a sedimentary evolution model for the Maokou Formation in the northwest Sichuan Basin is proposed.

In the early Roadian era, volcanic activity during the late Kungurian stage led to increased pCO₂ levels, and the Chemical Index of Alteration (CIA) remained high. This is supported by elevated strontium isotope values [47,48,52]. During the early Roadian stage, a significant reduction in forest areas resulted in increased exposure of bedrock, which raised the global rate of silicate weathering and lowered atmospheric pCO₂ levels [31,75]. The reduction in atmospheric CO₂ led to an increase in CIA and a rise in seawater pH. Additionally, the volcanic activity during the Kungurian stage caused the early Roadian period to be relatively warm, with both surface and seawater temperatures being higher, environmental factors which thus created conditions favorable for carbonate rock deposition. The study area had relatively shallow seawater, as previously discussed. A microfacies analysis shows that the MF3 coarse algae–foraminifera assemblage was dominant and widely developed within the platform, indicating a shallow and warm marine environment. Foraminifera types suggest that the bottom water conditions were favorable, with good light conditions and high dissolved oxygen levels. Consequently, the study area developed a broad platform characterized by warm-water carbonate rock formations. The paleogeographic pattern inherited from the previous Kungurian Chixia Formation showed that within the platform, the Shuangyushi area had grainy shoals, while other areas had inter-shoal flats (Figure 7A). In the late Roadian era, widespread weathering of surface silicates led to significant decreases in the CO₂ levels, chemical weathering indices, and surface temperatures in the study area. This triggered P3 glaciation and the sedimentary environment gradually cooled, which was inconsistent with warm-water carbonate deposition [49]. Log curve variations indicate a reduction in the area of grainy shoals within the platform and an increase in the area of inter-shoal flats. By the end of the Roadian stage, as the sedimentary environment continued to cool, the types of carbonate producers shifted from warm-water taxa (such as Dasycladaceae) to cold-water taxa (such as bryozoans and Bivalvia). The gradual increase in bryozoan proportion reflects a potential cooling of the water [51]. Meanwhile, the Emei mantle plume (EMP) began to uplift from the southwest without volcanic eruptions, causing changes in the paleogeography. The cooling climate and changes in biological composition and mantle plume tectonic activity led to a transition in microfacies from platform-interior grainy shoals and inter-shoal flats to platform-margin shoals (Figure 7B).

In the early Wordian era, P3 glaciation caused a drop in atmospheric temperatures. Additionally, the reduction in Roadian tropical forests led to increased bedrock exposure, while the increase in CIA further lowered atmospheric CO₂ levels [40]. During this period, as temperatures declined, the rate of carbonate rock accumulation decreased, and carbon isotope data show several slight negative excursions, likely attributed to multiple episodes of seawater anoxia caused by upwelling [51]. Upwelling also impacted carbonate production and exacerbated the effects of anoxia on platform growth [51]. During this time, the study area developed platform-margin shoals along the paleogeographic high points from the Roadian stage, but the area of these shoals diminished in the cooler sedimentary environment (Figure 7C).

In the mid-Wordian era, atmospheric temperatures, CO₂ levels, and CIA values remained relatively low. Microfacies assemblages and gamma-ray log characteristics indicate that the sea level during this period was at the lowest point of the entire Guadalupian series in the northwest Sichuan Basin, with the platform-margin grainy shoal area reaching its maximum. It is inferred that mantle plume activity played a major role in the development of the platform-margin shoals during this phase [76], causing further uplift of the paleogeography across the study area (Figure 7D).

In the late Wordian era, the paleoclimatic conditions were similar to those of the previous period. Microfacies assemblages and gamma-ray log features reveal a period of sea level rise and fluctuations. A decrease in strontium isotope values suggests that the Dongwu movement accelerated seafloor spreading, which intensified hydrothermal activity along seafloor fractures [47,48,52]. The increase in seawater temperature in the region reduced the intensity of upwelling and weakened its impact on shallow marine carbonate deposition [51]. Additionally, the Dongwu movement led to noticeable sedimentary differentiation in the study area. Platform-margin grainy shoals appeared only on the southwestern side of the study area, while the northeastern side experienced tectonic subsidence, with sediment predominantly consisting of wackestones and limy mudstones (Figure 7E).

In the Capitanian era, there was a noticeable positive excursion in Carbon isotopes and low CIA value. Previous studies demonstrated that a high positive plateau of δ13Ccarb values (>+5‰) was initially recognized in a mid-Panthalassan paleo-atoll limestone in the Kamura area, Japan, which is termed the Kamura cooling event [77,78,79]. Thus, the low CIA value in the Early Capitanian stage may imply the presence of the Kamura cooling event, which has been interpreted as a three-million-year-long period of high productivity and enhanced burial of organic carbon during the Capitanian stage. Meanwhile, the increase in the size of benthic foraminifera also indicates that the sedimentary environment of this period was rich in oxygen and contained sufficient food. At the end of the Capitanian stage, the carbon isotopes decreased and the CO₂ emissions and seawater temperature increased. These data indicate that carbon dioxide levels increased after the eruptions from the Emeishan Large Igneous Provinces, leading to rising temperatures and resulting in the initiation of melting during P3 glaciation. Further uplift of the Emeishan mantle plume (EMP) intensified sedimentary differentiation [76], leading to a relative sea level drop on the southwest side of the study area. The warming environment and relative sea level decrease further decreased the development of platform-margin grain shoals in the southwest of the study area. Meanwhile, changes in paleogeomorphology hindered the upwelling into the southwest region, hence reducing the adverse factors hindering the carbonate development and slope and toe-of-slope developed on the northeast side (Figure 7F).

6. Conclusions

This detailed microfacies analysis, combined with a petrological and sedimentary environment study, reveals a shift in the carbonate platform in the Permian stage in the Sichuan Basin.

During the early Roadian stage, the sedimentary environment was influenced by volcanic activity from the Kungurian stage, resulting in a relatively warmer climate. This led to the formation of a broad-platform facies in the study area, characterized by Dasycladaceae and foraminifera. In the late Roadian stage, an increase in CIA triggered P3 glaciation, causing a decrease in seawater temperature. This shift resulted in a transition from a green algae–foraminifera microfacies to a community dominated by bryozoans, brachiopods, and echinoderms. Additionally, the uplift of the paleogeography caused by the Emei mantle plume contributed to the change from broad-platform sedimentation to platform margin sedimentation.

The entire Wordian stage experienced lower temperatures due to the influence of P3 glaciation. In the early Wordian stage, an increase in CIA further reduced atmospheric CO₂ levels. Concurrently, multiple episodes of seawater anoxia caused by upwelling led to a decreased rate of carbonate rock accumulation. During this period, the platform margin shoals developed on the paleogeographic foundation of the Roadian stage, with a reduction in area. In the mid-Wordian stage, mantle plume activity played a major role in the development of the platform. This tectonic activity led to further uplift of the paleogeography of the study area, with the platform margin grainy shoal area reaching its maximum during this time. In the late Wordian stage, a period of sea level rise and fluctuations occurred. The Dongwu movement accelerated seafloor spreading, intensified hydrothermal activity, and reduced the influence of upwelling on the development of shallow marine carbonates. The study area began to show significant sedimentary differentiation.

During the Capitanian stage, evidence from Capitanian LST and CIA values, as well as the increase in benthic foraminifera volume, indicates that the sedimentary environment was oxygen rich and nutrient abundant, favoring the development of shallow marine carbonates. At the end of the Capitanian stage, volcanic eruptions from the Emei large igneous province led to an increase in CO₂ levels and higher temperatures, marking the end of P3 glaciation. Additionally, the uplift of the Emei mantle plume and warming of the environment further diminished the impact of upwelling on the development of shallow marine carbonates.

Author Contributions

Conceptualization, S.Z. and H.S.; methodology, D.Z.; software, S.Z.; supervision, H.S.; visualization, X.H.; validation, X.L.; formal analysis, H.S.; investigation, S.Z., X.H. and C.W.; data curation, S.Z.; writing—original draft preparation, S.Z.; writing—review and editing, H.S. and D.Z.; funding acquisition, H.S. and D.Z. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors are grateful to the anonymous reviewers for their critical and constructive reviews, which greatly improved this manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures

Figure 2. The types of microfacies identified in the Maokou Formation in the study area. All thin sections are under Parallel Nicols. (A) WJG section, MF1 (foraminifera-bearing intraclastic limestone), with crystals of calcite filling between the grains. Casting thin section. (B) ST 9 well, 7440 m, MF1 (foraminifera-bearing intraclastic limestone). Casting thin section. (C) WMT Section, MF1 (foraminifera-bearing intraclastic limestone), with Globivalvulina (red arrow) and crystals of calcite filling between the grains. Casting thin section. (D) S1 well, 5291 m, MF2 (bioclast packstone), with closely packed bioclasts. Some complete bryozoan fossils can be seen. Casting thin section. (E) K1 well, 4213.14 m, MF2 (bioclast packstone), with closely packed bioclasts. Casting thin section. (F) GDB section. MF2 (bioclast packstone), with closely packed bioclasts. Casting thin section.

Figure 3. The types of microfacies identified in the Maokou Formation in the study area, under Parallel Nicols. (A) ST3 well, 7114 m, MF3 (Dasycladaceae–foraminifera packstone), showing transverse and longitudinal sections of Dasycladaceae. Casting thin section. (B) H12 well, 3524 m, MF3 (Dasycladaceae–foraminifera packstone), with filling cracks. Casting thin section. (C) XBX section, MF3 (Dasycladaceae–foraminifera packstone) with Pachyphloia (red arrow). Casting thin section. (D) YQ section l, MF3 (Dasycladaceae–foraminifera packstone) with Sumatrina (red arrow). Casting thin section. (E) TK section, MF3 (Dasycladaceae–foraminifera packstone) with Pachyphloia (red arrow)and Globivalvulina (red arrow). Casting thin section. (F) METsection, MF4 (wackestone with brachiopod bioclast), of which the fossils are complete and floating in the matrix. Some of the matrix has undergone recrystallization. Casting thin section.

Figure 4. The types of microfacies identified in the Maokou Formation in the study area, under Parallel Nicols. (A) JZC Section, MF5 (fine-grained bioclast wackestone), with bioclasts whose morphology cannot be accurately identified, floating in the matrix. Casting thin section; (B) XBX Section, MF5 (wackestone with fine-grained bioclast), with a small quantity of bioclasts floating in the matrix. Casting thin section. (C) XBX Section, MF5 (fine-grained bioclast wackestone), with bioclasts whose morphology cannot be accurately identified, floating in the matrix. Casting thin section. (D) WJP section, MF6 (spiculate wackestone); most of the skeletal types are unidirectional. Casting thin section. (E) MET Section, MF7 (limy mudstone), see Globivalvulina and Nodosaria. Casting thin section; (F) K2 Well, 2228 m, MF7 (limy mudstone), basically no detrital particles. Casting thin section.

Figure 5. Different types of sedimentary microfacies associations: (A) platform-margin shoals; (B) platform-interior grain shoals; (C) platform-interior inter-shoal; (D) Slope and Toe-of-Slope.

Figure 6. Well correlation profile of the Maokou Formation facies in the northwestern Sichuan Basin.

Figure 7. Sedimentary patterns and control factors of carbonate platform deposits in the Guadalupian era of the northwest Sichuan Basin. Key geochemical indicators include Chemical Index of Alteration (CIA), land surface temperature (LST), 87Sr/86Sr ratio and δ13Ccarb values [40], atmospheric CO2 concentrations [41], 18Oapatite values [42,43], glacial events [46], and volcanic events [47,48,49,50]. (A) Sedimentary patterns in the early Roadian era. (B) Sedimentary patterns in the late Roadian era. (C) Sedimentary patterns in the early Wordian era. (D) Sedimentary patterns in the middle Wordian era. (E) Sedimentary patterns in the late Wordian era. (F) Sedimentary patterns in the Capitanian era.

Appendix A

Table A1

Microfacies classification and description.

Microfacies	Particle Types and Contents (%)		Matrix Types and Contents (%)		Biogenic Content and Texture	Standard Microfacies (Flügel, 2010) [38]	Description of the Sedimentary Environment
	Bioclasts	Intraclasts	Micrite	Cement
MF1: Foraminifera-bearing intraclastic limestone	3	73	—	24	Abundant well-rounded intraclasts with rare ostracods and foraminifera; grain-supported with sparry cement	SMF13	Platform-margin sand shoals
MF2: bioclast packstone	81	—	19	—	Common Bryozoan, echinoderm ossicles and ostracod bioclasts; grain-supported with few micritic limestone	—	Platform-margin sand shoals
MF3: Dasycladaceae–foraminifera packstone	69	—	31	—	Common Dasycladalean and benthic foraminifera with few ostracods and bryozoan; grain-supported with few micritic limestone	SMF18	Platform-interior grain beach subfacies
MF4: Wackestone with brachiopod bioclast	59	—	41	—	Well-protected brachiopod and few Crinoid and ostracod bioclasts; micritic matrix-supported	SMF8	Terrace intertidal subfacies
MF5: Fine-grained bioclast wackstone	34	—	66	—	Minor undefined fine-grained bioclast, micritic matrix-supported	SMF2	Toe of the slope
MF6: The spiculate wackestone	46	—	54	—	Abundant sponge spicule with few Bryozoan bioclast, micritic matrix-supported	SMF1	Deep shelf
MF7: limy-mudstone	2.5	—	97.5	—	Pure micritic mudstone with few bioclast	SMF23	Low-energy environment

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