Abstract
Keywords
Introduction
The carbonate-evaporite paragenesis system (hereinafter referred to as the paragenesis system) is a widespread phenomenon in the global geological record, spanning from the Precambrian to the Quaternary (Wen et al., 2021), and hosts significant hydrocarbon resources (Boschetti et al., 2020; Hu et al., 2023; Morad et al., 2019; Shi et al., 2021; Sun et al., 2021). More than half of the proven oil and gas fields worldwide are associated with the paragenesis system (Warren, 2016; Zhang et al., 2023b). Several oil and gas fields associated with the paragenesis system have been found in Longwangmiao Formation and Leikoupo Formation in Sichuan Basin (Du et al., 2016; Hu et al., 2019; Li et al., 2021b). However, persisting controversies regarding the developmental characteristics and depositional models of evaporites within these paragenesis system continue to constrain breakthroughs in hydrocarbon exploration.
The role of the paragenesis system in paleoclimate, paleoenvironment, and hydrocarbon resources was studied in previous studies (Allen, 2007; Mazumdar and Strauss, 2006; Prince et al., 2019; Warren, 2010). The gypsum-salt rock series of strata is suggested to have high-quality hydrocarbon-forming biological combination and great potential for hydrocarbon generation (Jarvie, 2014; Liu et al., 2013). Furthermore, the carbonate and gypsum-salt rocks in the paragenesis system can serve as significant hydrocarbon reservoirs (Jiang et al., 2018; Sarg 2001; Warren 2006; Vandeginste et al. 2009). Gypsum-salt rocks also function as essential caprocks due to their excellent sealing properties and plasticity (Amadi et al., 2012; Zhang et al., 2017). However, clarifying the distribution of evaporites, lithofacies association, and sedimentary evolution of the paragenesis system is a prerequisite for subsequent research. Schmid (2017) applied carbon isotope geochemical stratigraphy to investigate Neoproterozoic evaporite development in Australia's Amadeus Basin; Beigi et al. (2017) analyzed carbonate-evaporite assemblage sequences through sequence stratigraphy analysis; Zhang et al. (2023a) utilized seismic inversion to identify gypsum-salt rock distribution characteristics; Luo et al. (2024) employed petrography and log facies identification to characterize evaporite evolution. Previous studies indicate that the lithofacies association of the paragenesis system is notably complex (Amel et al., 2015; Hu et al., 2019; Strohmenger et al., 2010). Individual lithofacies cycles in different regions exhibit varying characteristics due to the variable sedimentary environment, climatic factors, paleosalinity, and sea level fluctuations (Grotzinger and Al-Rawahi, 2014; Hu et al., 2019; Jiang et al., 2018; Prasad et al., 2010; Schröder et al., 2003; Turner and Bekker, 2016; Wang et al., 2013).
The Sichuan Basin has undergone a complex tectonic-sedimentary evolution, with the lower Cambrian Qiongzhusi Formation containing high-quality hydrocarbon source rocks that provide a substantial gas sources for the paragenesis system (Zou et al., 2014). Additionally, the middle-lower Cambrian gypsum-salt rock layers serve as excellent caprocks, enhancing the potential for the formation of large-scale hydrocarbon reservoirs. Previous studies on the Cambrian paragenesis system have established foundational insights. Scholars propose that the stratigraphic sequence from the Longwangmiao to Xixiangchi formations develops two distinct depositional systems: evaporite-carbonate ramps and evaporite-mixed restricted platforms, characterized by localized intense evaporation and multi-lagoon evaporite development patterns (Ren et al., 2019; Shen et al., 2017; Wang et al., 2022, 2023, 2024). Reservoirs in these systems are predominantly composed of grain-dominated dolomites, where the presence of evaporites (gypsum-salt rocks) enhances dissolution intensity, facilitates dolomitization processes, and increases the likelihood of thermochemical sulfate reduction (TSR) (Liu et al., 2016; Zhou et al., 2015). Additionally, evaporites exhibit plasticity and mobility, with thick-bedded evaporites in the Gaotai Formation demonstrating significant sealing capacity (Lin et al., 2014; Liu et al., 2018a). Researchers have further synthesized the relationship between evaporite tectonic configurations and hydrocarbon accumulation mechanisms (Jin et al., 2006; Li et al., 2014). However, the findings of these studies have shown significant divergence, largely due to limited data and issues with precision.
To solve above question, this study is based on core, thin section, logging and seismic data, using high-resolution lithofacies paleogeographic mapping methods (Zhang et al., 2023a), try to finely dissect the lithofacies association and depositional evolution characteristics of the Cambrian Longwangmiao, Gaotai and Xixiangchi formations of the paragenesis system, and establishes the sedimentary model. The specific objectives include: (a) to identify the lithofacies association; (b) to analyze the sedimentary evolution and construct a depositional model; (c) to clarify of favorable hydrocarbon distribution zones. Furthermore, the findings of this study are expected to provide new insights for the future exploration of oil and gas in deep ancient carbonates.
Geological settings
The Sichuan Basin, located in the junction of the Yangtze Plate's western edge and the eastern edge of the Tibetan Plateau in southwestern China, covers an area of approximately 180,000 square kilometers. The basin has undergone multiple phases of tectonic activity, including those associated with the Longmenshan, Micangshan, Dabashan, and the Xuefengshan intracontinental orogens, resulting in a complex tectono-sedimentary history. This history includes the deposition of marine carbonate platform during the Sinian to Middle Triassic periods and the formation of a foreland basin during the Late Triassic (Liu et al., 2018b). These tectonic events have resulted in the current configuration of a foreland-craton composite basin. Since the late Early Cambrian, the Leshan-Longnvsi palaeo-uplift of the Sichuan Basin has been continuously undergoing synsedimentary uplift (Du et al., 2014). Additionally, influenced by the Caledonian orogeny, a series of syn-depositional faults occurred in the southeastern Sichuan Basin, such as the Huayingshan fault belts and Qiyueshan fault belts (Gou et al., 2024; He et al., 2017; Liu et al., 2021) (Figure 1(a)). The frequent tectonic activities created restricted marine environments conducive to the development of evaporites, leading to the presence of the paragenesis systems across multiple strata within the basin (Gu et al., 2019; Li et al., 2021b; Wang et al., 2013; Zheng et al., 2010).
Regional geological background and comprehensive stratigraphic column of the Sichuan Basin and its surrounding areas (modified from Shi et al., 2020; Xu et al., 2016).
The Cambrian within the basin is well-developed, sequentially consisting from bottom to top of the Maidiping Formation (∈1m), Qiongzhusi Formation (∈1q), Canglangpu Formation (∈1c), Longwangmiao Formation (∈1l), Gaotai Formation (∈2g), and Xixiangchi Formation (∈2 + 3x) (Li et al., 2019). The Cambrian the paragenesis system is primarily developed in the Longwangmiao and Gaotai formations, with a few occurrences in the Xixiangchi Formation (Xu et al., 2016) (Figure 1(c)). The Lower Cambrian Maidiping and Qiongzhusi Formations comprise deep-water continental shelf deposits, including siliceous and carbonaceous mudstones, which constitute a high-quality hydrocarbon source rock (Ren et al., 2016). The Canglangpu Formation developed coastal-continental shelf and mixed siliciclastic-carbonate platform deposits (Li et al., 2021a; Li et al., 2023; Tong et al., 2023; Yan et al., 2021). In contrast, the Longwangmiao, Gaotai, and Xixiangchi formations are characterized by carbonate ramp platform and restricted platform depositional environments (Jiang et al., 2023; Tang et al., 2024; Wang et al., 2022; Yan et al., 2021). The enclosed evaporitic environment, along with the arid and hot climatic conditions, has been instrumental in the deposition of gypsum-salt rocks (Gu et al., 2019; Xu et al., 2016).
The Longwangmiao and Gaotai formations in the eastern and southern regions are rich in gypsum-salt rocks, with minor development in the lower part of the Xixiangchi Formation (Shi et al., 2020; Wang et al., 2022, 2023, 2024; Xu et al., 2016). In the eastern Sichuan Basin, wells such as TH1 and LouT1 have gypsum-salt rocks total thicknesses exceeding 1000 m (Figure 1(b)).
Data and methodology
Petrographic observations
Lithological data were primarily derived from over 100 drilling wells, eight outcrop sections and more than 20 supportive sections within the study area, along with more than 1000 outcrop photos and over 500 rock samples. A total of over 400 thin sections were prepared, which were manufactured in the laboratory of Yangtze University. All of the thin sections were stained using Alizarin red-S to distinguish between dolomite and calcite (Dickson, 1966). The research-grade Leica DM4 P Upright Polarizing Microscope was used to observe thin sections.
Logging and seismic data
Conventional logging curve data were collected from more than 100 wells in the study area, such as GR, Acoustic (AC), and Density (DEN), with a sampling interval of 0.125 m. The logging curves have been fundamentally corrected and are primarily used for subsequent logging-lithology identification and lithology correction. More than 100 two-dimensional grid lines of seismic data and a key three-dimensional block area of 50,000 km2 were utilized, with the stratigraphic interpretation and seismic inversion completed using the Geoeast software, primarily to assist in lithofacies identification in areas lacking well data.
High-resolution palaeogeographic mapping process
With the development of technology, numerous geological databases have been established both domestically and internationally, playing a key role in paleogeographic studies (Fan et al., 2013a, 2013b; Zhang et al., 2023). The establishment of databases can be developed according to the needs of users or constructed by utilizing existing commercial software such as Gxplorer, Petrel, and Resform. These softwares play a significant role in storing basic data, constructing isochronic frameworks, and achieving high-resolution lithofacies paleogeographic mapping (Figure 2).
High-resolution lithofacies paleogeographic mapping process.
By importing a vast amount of data collected from drilling wells and outcrop sections into geological mapping software, the data importation and basic processing are completed. Concurrently, by integrating logging and seismic data analysis, a relatively complete comprehensive histogram of wells and profiles was established, which was used for subsequent lithofacies sequence analysis and single-factor analysis of different lithologies.
Lithology is an important basis for recognizing sedimentary facies; therefore, it is necessary to statistically analyze the types of rocks, accurately determine the content of different lithologies, and create corresponding single-factor maps for sedimentary facies division. This can be achieved through detailed section surveys, thin section observation and analysis, and well lithology interpretation. The auxiliary discrimination of seismic data is also important, typically employing two methods: seismic facies and seismic attribute extraction.
After the aforementioned operations, all well locations and profiles are integrated into a modern geographic map for the division of sedimentary facies. To address issues related to data absence and insufficient accuracy, quantitative interpolation methods are applied (Zhang et al., 2016). This technique is crucial for compiling paleogeographic maps, as it allows for a direct representation of lithological composition, relative content, structural characteristics, hydrodynamic conditions, and the depositional environment of specific facies units. By utilizing these foundational maps, foundational single-factor maps, comprehensive sequence, and dominant facies evolution maps, a more scientific approach to the division and study of high-resolution lithofacies paleogeographic units can be achieved.
Results
Petrographic results
Lithofacies of the carbonate rocks
Carbonate in the paragenesis system strata is predominantly composed of crystalline dolostone, dolo-grainstone, gypsum and gypsum-bearing dolostone, and argillaceous dolostone. Secondly, a small amount of gypsum-bearing mud-crystal calcarenite, argillaceous limestone, sandy limestone (Figure 3). Crystalline dolostone is further divided into micritic dolostone (Figure 3(a)), power crystalline dolostone (Figure 3(b)), and fine crystalline dolostone (Figure 3(c)) based on crystal sizes. Dolo-grainstone consists of dolarenite (Figure 3(d) to (g)), oolitic dolostone (Figure 3(h)), and dolorudite (Figure 3(i)). Grain types are dominated by intraclasts, with a spot of algal debris and ooids. The intergranular dissolved pores of dolarenite are well-developed, and partly filled with dolomite and bitumen (Figure 3(f) and (g)). Gypsum and gypsum-bearing dolostone show slight difference of gypsum contents of 25%‒50% (Figure 3(j)), and 10%‒25% (Figure 3(k)), respectively. Macroscopically, dolostone and gypsum feature interbedded; microscopically, fibrous and granular gypsum layers are intercalated between dolostone layers. The limestone is predominantly micritic, with minor sandy and calcarenitic (Figure 3(l)), interbedded with evaporites.
Typical carbonate lithofacies and thin-section images of carbonate rocks. (a) Zuo3, 6010 m, Longwangmiao Formation, mud crystalline dolostone, PPL; (b) Taiyuan section, Gaotai Formation, powder crystal dolostone, PPL; (c) MX12, 4651.56 m, Longwangmiao Formation, gypsum-bearing mud-crystal calcarenite, PPL; (d) Houtan section, Xixiangchi Formation, residual sandstone dolostone, well developed residual intergranular pores and intergranular dissolved pores (blue), PPL; (e) Houtan section, Xixiangchi Formation, dolarenite, intergranular pores and dissolved pores (blue), PPL; (f) MX19, Longwangmiao Formation, dolarenite, intergranular pores (blue) partly filled with dolostone and bitumen, PPL; (g) GS7, Longwangmiao Formation, residual dolarenite fine-crystalline dolostone partly filled with bitumen, PPL; (h) Li1, Gaotai Formation, oolitic dolostone, PPL; (i) WT1, 6895.02 m, Longwangmiao Formation, doloarenite-dolorudite dolostone, PPL; (j) WT1, Gaotai Formation, gypsum micritic dolostone and gypsum, PPL; (k) WT1, 6609 m, gypsum dolostone, PPL; (l) LouT1, 6491 m, Longwangmiao Formation, gypsum-bearing mud-crystal calcarenite, PPL.
Lithofacies of the gypsum-salt rocks
Evaporites mainly include gypsum, dolomitic and dolomitic-bearing gypsum, gypsum-dissolved breccia, and salt rocks (Figure 4). Pure gypsum rock is mostly gray to gray-white, dense and massive, with gypsum and anhydrite comprising more than 90% of its composition, along with minor dolostone, clay minerals, and quartz, and occasionally chalcedony, celestite, and pyrite. Gypsum shows crystalline and radial structures (Figure 4(a) and (b)). Dolomitic and dolomitic-bearing gypsum are classified based on the values of dolostone content: the content of dolostone in dolomitic-bearing gypsum rock is 10%‒25% (Figure 4(c) and (d)); while the content of dolostone in dolomitic gypsum is 25%‒50% (Figure 4(e) and (f)). Gypsum in both types of rocks is often dissolved and not easily recognized. Under the microscope, interwoven sheet-like dolostone crystals and gypsum crystals can be observed. The breccia varies in size and shape, displaying subrounded, triangular, and angular forms, and the content of breccia is about 70% (Figure 4(g) and (h), image reference from Wang et al., 2023). The color is mostly gray‒yellow, gray‒green, often coexisting with purplish‒red mudstone or with mud crystal dolomite, common breccia structure and rhyolitic banding structure, and the composition of the paste-solution breccia is mostly mudstone type or mud crystal dolomite. Salt rock, primarily made of potassium salt, is generally coarse-grained crystalline structure, dense block, can also form salt crystal clastic structure. It is lenticular or laminated, often interbedded with gypsum (Figure 4(i), image reference from Wang et al., 2023).
Typical evaporite lithofacies and thin-section images of evaporite rocks. (a) GongS1, Longwangmiao Formation, gypsum, radial structure, PPL; (b) Li1, Gaotai Formation, gypsum, crystalline structure, PPL; (c) WT1, 6846 m, Gaotai Formation, dolomitic-bearing gypsum, PPL; (d) WT1, 6852 m, Gaotai Formation, dolomitic-bearing gypsum; (e) TH1, Gaotai Formation, dolomitic gypsum, PPL; (f) Li1, Gaotai Formation, dolomitic gypsum, PPL; (g) LouT1, 6050 m, Gaotai Formation, gypsum-dissolved breccia, PPL; (h) LouT1, 5565 m, Gaotai Formation, gypsum-dissolved breccia; (i) LouT1, 5565 m, Gaotai Formation, salt rock (potassium salt), PPL.
Types and distribution of lithofacies association
Gypsum dolomitic flat-grain shoal (F1)
From bottom to top, lithology changes from gray gypsum dolostone, gypsum-bearing dolostone, and argillaceous dolostone, to carbonate dominated by dolo-grainstone and fine (power) crystalline dolostone (Figure 5(a)). The single-layer thickness of carbonate ranges from 1 to 3 m, and the thickness of evaporite layers ranges from 1.5 to 4 m with a shallowing-upward sedimentary sequence. In terms of the proportion of rock types, evaporite-bearing strata constitute 40%‒60% of the total thickness, while carbonate comprises slightly less.
Types of Cambrian carbonatite-evaporite lithofacies sequence assemblages in the Sichuan Basin.
Dolomitic-argillaceous lagoon-grain shoal (F2)
The lithology is characterized by medium- to thick-bedded dolo-grainstone intercalated with thin micritic and argillaceous dolostone (Figure 5(b)). The thickness of the single layer of dolo-grainstone varies greatly, ranging from 0.5 to 8 m, and the thickness of the micritic and argillaceous dolomite is similarly variable, ranging from 1 to 8 m. In this lithofacies sequence, evaporites are essentially absent, occurring only as interbedded layers within the carbonate.
Gypsum lagoon-grain shoal (F3)
This type of combination exhibits two patterns from bottom to top: (a) thin-bedded dolo-grainstone, medium-thick bedded transparent salt rock interbedded with argillaceous gypsum, to thin-bedded dolo-grainstone. For example, the thickness of the Gaotai Formation saline rocks in Wells Lin7 and Zou3 can reach 5‒10 m; (b) a distinct structural hierarchy from bottom to top, showing thin-bedded dark gray grain limestone and micritic dolostone, medium-thick bedded gypsum, to thin-bedded micritic dolostone or sandy dolostone interbedded with thin-bedded gypsum (Figure 5(c)). In terms of the ratio of lithology, the thickness of the evaporite-bearing stratum accounts for about 75%, while the carbonate accounts for about 25%.
Calcareous-dolomitic(argillaceous-dolomitic) lagoon-evaporation lagoon (F4)
This lithology is characterized by thickly bedded massive limestone or dolomitic limestone in the middle part, with the bottom and top section mainly consisting of blocky gypsum dolostone and dolomitic gypsum (and pure gypsum), respectively (Figure 5(d)). The blocky gypsum dolostone and dolomitic gypsum are interbedded with argillaceous and micritic dolostone, with the single-layer thickness of gypsum dolostone and gypsum around 2 m and single-layer thickness of micritic dolostone ranging from 0.5 to 1 m. In terms of the proportion of rock types, it is classified as the evaporite interbedded carbonate, with the thickness of the evaporite stratum accounting for more than 60%, mainly consisting of gypsum and salt-bearing gypsum.
Argillaceous dolomitic flat-gypsum dolomitic flat (F5)
The assemblage is primarily characterized by the interbedding or coexistence of argillaceous dolostone, dolomitic mudstone, and gypsum dolostone, with minor development of muddy siltstone. Lithology changes from gypsum-bearing micritic dolostone, blocky gypsum dolostone, to argillaceous and micritic dolostone (Figure 5(e)). In terms of the proportion of lithology, the rock series containing evaporites accounts for about 30%, single-layer thickness ranging from 0.5 to 1 m, enveloped by dolostone and argillaceous dolostone.
Gypsum lagoon-salt lagoon (F6)
The rock types are primarily characterized by laminated gypsum dolostone, salt, and salt-bearing gypsum, with rapid transitions to blocky salt rocks. The salt minerals consist mainly of potassium and sodium salts (Figure 5(f)), the gypsum-salt rock or salt rock single layer thickness ranging from 5 to 50 m. In terms of lithology proportions, the proportion of evaporite-bearing stratigraphic series accounts for more than 90%, with minor amounts of carbonate and argillaceous-bearing rocks.
Geophysical response characteristics
Logging characteristics
For wells lacking core samples, effectively identifying different types of the paragenesis systems requires determining the logging response characteristics of each type (Al-Mudhafar, 2020; Carrasquilla and Lima, 2020; Marzan et al., 2021). Therefore, based on drilling and sections geological data, core and thin-section photographs, and conventional logging curves (Tian et al., 2019; Zhu et al., 2024), the logging response characteristics corresponding to the six types of the paragenesis system lithofacies association were analyzed, and recognition templates for different combination types were established.
In the F1 (Figure 6(a)), the Gamma Ray (GR) values are low, ranging from 10–20 API, with no significant anomalies in Acoustic (AC) and Density logging (DEN).
Logging response characteristics of lithofacies sequence combination.
In the F2 (Figure 6(b)), the GR values of dolo-grainstone range from 30 to 40 API, with an average value of around 33 API; the GR values of mudstone dolostone and mud-fine crystalline dolostone range from 30 to 50 API, with an average value of around 40 API. There are no significant anomalies in AC and DEN.
In the F3, both of them have the same value of GR, but in assemblage (a), the AC and DEN show significant anomalies in the gypsum-salt rock layers, with AC showing abnormally high values and DEN low values. The RLLD and RLLS curves also show significant differentiation; in assemblage (b), the gypsum lagoon section, due to the development of multiple lithological interbeds, GR shows high values, ranging from 30 to 80 API, with an average value of about 50 API, and there are no significant anomalies in the AC and DEN curves.
In the F4 (Figure 6(e) and (f)), the GR values in the evaporation lagoon section are higher than those in the calcareous and argillaceous dolomitic lagoon, and the mudstone interbedded in the gypsum section shows abnormally high GR values. The DEN curve shows relatively high values in the gypsum rock section and relatively low values in mudstone, argillaceous dolostone. The AC curve shows no significant anomalies.
In the F5 (Figure 6(g) and (h)), the GR values in the argillaceous dolomitic flat and gypsum dolomitic flat sections are similar, showing a serrated pattern. The DEN curve also shows a serrated pattern.
In the F6 (Figure 6(i) and (j)), the GR values in the salt rock section are less than those in the gypsum. Moreover, the AC curve in the salt rock section shows obviously high values, while the DEN curve shows noticeably low value sections.
Seismic characteristics
Based on the variations in thickness and content of evaporites, four types of seismic facies models have been delineated (Figure 7). In areas lacking gypsum-salt rock distribution, lithologies mainly consist of dolostone and siliciclastic rocks, with lithofacies association including F1 and F2, which develop in mixed tidal flat and restricted platform facies belts; the seismic response in these areas is characterized by weak amplitudes, mainly distributed to the west of Huayingshan fault zones.
Seismic facies model and interpretation of depositional environments of the Cambrian paragenesis system in the Sichuan Basin.
In the type where gypsum-salt rock and carbonate interbed or alternate as interlayers, the lithofacies association is mainly F3, F4, and F5, with relatively thin evaporites, mainly appearing in tidal flat or lagoon; the seismic response is characterized by a single strong wave peak or multiple medium-strong reflections within the Gaotai Formation.
In the thick evaporite rock, this pattern develops thick layers of gypsum rock interbedded with thin carbonate, with a total evaporite thickness of more than 200 m, and the lithofacies association is mainly F6. The seismic characteristics are manifested as multiple short-axis strong reflections, chaotic reflections, and blank reflection combinations.
In the type of compound with multiple rock, it reflects the transition zone between platform, salt lagoon, and tidal flat, with lithofacies association including F1 and F3. The seismic response is characterized by the transitions of multiple medium-strong reflections or short-axis strong reflections, chaotic reflections, and blank reflections.
Single-factor analysis of lithology
Quantification of the content of each type of lithology (Table S1) by combining drilling lithology with core, thin section, and logging lithology corrections. A single-factor analysis of different lithologies was carried out to determine the types and distribution boundaries of different sedimentary phases through the principle of dominant facies delineation, which provided support for the subsequent petrographic paleogeographic mapping.
Single factor of Longwangmiao formation
The regional distribution maps of grain rock content (Figure 8(a)), limestone content (Figure 8(b)), dolostone content (Figure 8(c)), and the thickness of gypsum-salt rocks (Figure 8(d)) are shown here. The content of grain rocks ranges from 0% to 55%, with the highest content in the central part of the basin and the northeastern area outside the basin, gradually decreasing towards the east; the content of limestone ranges from 0% to 75%, with the highest content in eastern Sichuan, gradually decreasing towards western Sichuan; the content of dolostone ranges from 0% to 85%, with the highest content in central Sichuan, followed by northwestern Sichuan, gradually decreasing towards eastern Sichuan; the cumulative thickness of gypsum-salt rocks ranges from 0 to 50 m, with the maximum thickness at the well JianS1, rapidly decreasing to about 5–10 m in the vicinity of well JianS1 to Liangping-Kaixian-Lichuan, while some areas in southern Sichuan exhibit a greater thickness of about 15‒25 m.
Distribution of different lithological contents (thickness) in Longwangmiao Formation: (a) grain rocks content; (b) limestone content; (c) dolostone content; and (f) thickness of gypsum-salt rock.
Single factor of Gaotai formation
The distribution maps of grain rock content (Figure 9(a)), limestone content (Figure 9(b)), dolostone content (Figure 9(c)), and the thickness of gypsum-salt rocks (Figure 9(d)) the Gaotai Formation were generated using the mapping software. The content of grain rocks ranges from 0% to 45%, with the highest content in eastern Sichuan; the content of limestone ranges from 0% to 70%, with the maximum content in the southeastern part of the basin periphery and 0–15% within the basin; the content of dolostone ranges from 0% to 70%, with the highest content in the southeastern part of the basin periphery, followed by the northwestern part, and a thickness of 0%‒30% within the basin; the cumulative thickness of evaporite rocks ranges from 0 to 100 m, with the maximum thickness of 100 m near the JianS1 well in the northeastern part of the basin, gradually decreasing to the west, and a greater thickness of about 70‒80 m near the well YD4 outside the basin.
Distribution of different lithological contents (thickness) in Gaotai Formation: (a) grain rocks content; (b) limestone content; (c) dolostone content; and (d) thickness of gypsum-salt rock.
Discussion
Lithofacies paleogeographic distribution of the paragenesis system
Lithofacies paleogeographic of Longwangmiao formation
Based on prior data, by analyzing in detail the lithofacies association and evolutionary patterns of the paragenesis system, combined with the interpretation of logging and seismic facies patterns, the lithofacies paleogeographic map of the Cambrian evaporite-related strata in the Sichuan Basin has been modified and improved (Figures 10 and 11).
Distribution of sedimentary facies in the Cambrian Longwangmiao formation paragenesis system in the Sichuan Basin.
Distribution of sedimentary facies in the Cambrian Gaotai formation paragenesis system in the Sichuan Basin.
During the Early Cambrian Longwangmiao Formation, an evaporite-carbonate ramp depositional model was primarily developed, generally consistent with Xu et al., (2016) and Yang et al., (2022b) views, featuring restricted lagoons in the eastern and southern Sichuan regions, and extensive distributions of grain shoals within the platform (Figure 10). The study further identified two major centers of evaporite deposition, located near the zones of well TH1-JianS1 and GongS1-DengT1-Zuo 3, respectively (Shi et al., 2020; Xu et al., 2016). In addition, several lagoons can be seen near Chongqing-Luzhou areas is similar to the multi-lagoon model proposed by Gu et al., (2019). The salt rocks are developed in well Zuo3 and DeS1, with a thickness of about 40 m, indicating a strong evaporation environment. Since the deposition of the early Cambrian Canglangpu Formation, the Sichuan Basin has completed the filling and leveling that began with the Dengying Formation, and the Leshan-Longnvsi palaeo-uplift gradually began to rise syndepositionally (Song et al., 2016; Yan et al., 2021), high-energy grain shoals were distribution along the core and slope areas of the palaeo-uplift. The type of lithofacies association is F2, and the thickness of the shoal units is mostly between 1 and 2.5 m, with a total thickness between 50 and 110 m. At least three stages of regressive upward shallowing sedimentary rhythmic structures were included, dominated by medium-thick bedded dolo-grainstone, residual grain-bearing fine- to medium-crystalline dolostone interbedded with fine to powder crystalline dolostone in alternation or superposition. This lithofacies sequence primarily reflects a depositional environment characterized by frequent oscillations between deeper and shallower water conditions, indicative of an inner ramp facies. It is distributed in wells NC1 and GS17, as well as in the majority of wells within the Longwangmiao Formation in the Gaoshiti-Moxi area. The grain shoals from different periods exhibited two types of combination models: progradational and aggradational. Seismically, these are characterized by onlap, with a gradual onlapping from the slope area towards the palaeo-uplift area. Gypsum-salt rock from well JianS1 and Lin1 can be identified on seismic profiles (Figure 12). The gypsum rock of JianS1 well and Lin1 well can be identified on the seismic profile, which is mainly interbedded and intercalated. The lithology is primarily characterized by argillaceous dolostone in mid-ramp facies, with dolo-grainstone at palaeogeomorphic highs in some areas. Gypsum-salt rock is not developed in the mid-ramp.
The seismic response characteristics of gypsum-salt rock. (line 1 across Well JianS1; line 2 across Well Lin1).
Lithofacies paleogeographic of Gaotai formation
During the depositional period of the Middle Cambrian Gaotai Formation, the ancient land and palaeo-uplift experienced significant uplift and expansion, with the Leshan-Longnvsi palaeo-uplift partially exposed above the sea surface. The Wanzhou-Yibin depression underwent increased subsidence, leading to a rise in the amount of terrestrial siliciclastic material in the basin increased. An evaporite-mixed restricted platform depositional model was mainly developed (Figure 11). To the west of the Huayingshan fault, above and around the Leshan-Longnvsi palaeo-uplift, mixed tidal flat and mixed evaporite restricted platform facies were predominantly developed. The strata thickness ranged from 50 to 120 m, with common interbedded thick layers of sand-mudstone and thin layers of dolo-grainstone, thin layers of gypsum sand-mudstone, gypsum dolostone, and dolomitic gypsum-salt rocks. Shoal were not developed, and only marginal platform shoals were deposited in wells LaoL1 and WS1. The thick and interbedded (and intercalated) gypsum-salt rock of Gaotai Formation is widely distributed in southern and eastern Sichuan. Seismically, the thick gypsum-salt rock shows blank reflection, and the interbedded or intercalated of gypsum-salt rock on both sides show multiple strong amplitude reflections. Under strong stress extrusion, plastic deformation occurs, forming a convex anticline in the upper part of the gypsum-salt rock layer, accompanied by large slip fracture (Figure 12). The lithofacies association of gypsum dolomitic flat is F3 and F4, while the gypsum lagoon is F5. Salt rocks appear in the more intense evaporation areas of gypsum lagoon facies, such as wells DeS1, Zuo3, JianS1, and LouT1 (Figure 6(i) and (j)).
Lithofacies paleogeographic of Xixiangchi formation
During the depositional period of the Middle-Late Cambrian Xixiangchi Formation, local sedimentary hiatuses occurred on the Leshan-Longnvsi palaeo-uplift, influenced by the Yunnan Movement and paleoclimate. The Xixiangchi Formation developed dolomitic lagoons in the Zigong-Muchuan-Suijiang area. In the eastern-southern lagoon development area, the low-lying deposition of landform inheritance was developed, with grain shoals developed on both sides (Gu et al., 2020). Previous studies (Li et al., 2019; Xu et al., 2016) revealed that only a small amount of evaporites was found at the base of the Xixiangchi Formation in the southern Sichuan Basin. The overall sedimentary framework and depositional model remain consistent with the established carbonate-restricted platform to open-platform depositional patterns documented in prior research. Therefore, we will not discuss the petrographic paleogeographic changes in detail. However, the Xixiangchi dolomites constitute significant reservoir units that have developed various post-salt hydrocarbon accumulation models, demonstrating substantial petroleum resource potential.
Sedimentary evolution of Longwangmiao and Gaotai formations
Sedimentary evolution of Longwangmiao formation
The depositional facies model of the Longwangmiao Formation in the Sichuan Basin has long been debated between the “ramp model” and “platform model.” Some scholars propose that the Dayong-Huayuan-Songtao-Majiang area exhibits distinct platform-margin shoal facies, characterized by debris flow deposits (e.g., intraclastic limestone and calciturbidites), syndepositional deformation, and slump structures observed in the Yaqiao (Huayuan), Dayong, and Fenghuang sections, suggesting a weakly rimmed platform depositional system (Wang et al., 2025). This divergence primarily stems from differing interpretations of basinal tectonic differentiation: the northwestern region retained low-angle ramp features, while the eastern area developed platform margins controlled by syndepositional faults, forming a “ramp-to-platform transitional model.” Vertically, the Cambrian paragenesis system in the Sichuan Basin is primarily distributed in the Longwangmiao and Gaotai formations, with depositional facies evolving from evaporite-carbonate ramp to evaporite-mixed restricted platform systems from bottom to top. The Longwangmiao Formation commonly contains dolarenite, micritic dolostone, fine- to medium-crystalline dolostone, and gypsum dolostone. The Gaotai Formation predominantly consists of thin-bedded micritic dolostone, fine- to medium-crystalline dolostone, dolo-grainstone, and gypsum dolostone or gypsum-bearing sand-mudstone interbeds, with rare thin-bedded transparent pure gypsum (anhydrite) and halite rocks, such as in wells Zuo3 and Lin7 where the evaporite sequence thickness exceeds 500 m. The Early Cambrian Longwangmiao period was characterized by an evaporation ramp platform, with a gypsum-salt lagoon developing in the Liangping-Chongqing area within the platform, indicating regression and seawater salinization (Gu et al., 2020).
The Sichuan Basin and its vicinity have a nearly northwest-southeast oriented continuous well depositional facies profile, overall exhibiting a uniform slope-ramp model that steepens distally. The Longwangmiao Formation of well MX32 displays relatively stable thicknesses in both the upper and lower member, primarily composed of dolostone with grain shoals, and is geologically positioned to the west of Huaying Mountain fault, within the inner-ramp dolostone lithofacies region; wells LG1, DingS1, and Miaoziwan section belong to the mid-ramp, with gradually increasing depositional thicknesses, located between the Huayingshan fault and Qiyue Mountain fault, representing an evaporitic gypsum lagoon area where gypsum layers developed in the upper member of the Longwangmiao Formation. The lower member of the Longwangmiao Formation represents a carbonate ramp depositional model, potentially influenced by the combined effects of Paleogeomorphology, and did not form an evaporation lagoon depositional environment over a wide area in eastern-southern Sichuan, with the upper section successive developing a distally steepened pattern. During this period, a closed environment gradually formed, creating a regionally restricted setting in eastern-southern Sichuan where some gypsum dolostone was deposited. As the seawater further restricted and the crust uplifted, it transitioned into the Gaotai Formation depositional period, combined with further aridification of the paleoclimate, forming a marginated carbonate platform and gypsum dolomitic lagoon deposition in eastern-southern Sichuan, with salt lagoon deposition in local areas. After the deposition of the lower section of the Gaotai Formation, the changes may be associated with the humid climate and the position of the Yangtze Plate, which successive formed dolostone lagoon deposition in eastern-southern Sichuan during this period (Figure 13).
Sedimentary evolution of Cambrian sub-salt Longwangmiao formation the Sichuan Basin (see figure 10 for profile location).
Sedimentary evolution of Gaotai formation
During the Middle Cambrian Gaotai Formation depositional period, the basin retained a depositional pattern characterized by higher elevations in the northwest and lower in the southeast. As regression progressed, the ancient land expanded, and the terrestrial siliciclastic material increased in the central and western Basin, leading to the development of mixed tidal flat depositions. The restricted platform continued to develop to the east, forming a barrier in the Yongshun area, marking the development of a rimmed carbonate platform. Within the evaporation platform, the Liangping-Chongqing area saw further expansion of the salt lagoon due to the shallowing and concentration of seawater. During the Middle to Late Cambrian Xixiangchi Formation depositional period, the paleogeomorphological pattern of being higher in the northwest and lower in the southeast was inherited, with evaporite rocks developing in the basin's interior. In the early stages of its deposition, a rapid large-scale transgression occurred, covering the Sichuan Basin with extensive restricted platform areas. The enclosed depositional environment of evaporite concentration transitioned into an open to semi-open environment, with the Yibin-Kaijiang area shifting from an evaporation lagoon to a platform depression. Controlled by the Paleogeomorphology, high-energy grain shoal belts developed on both sides of the platform depression.
The profile stretches from the northwest to the southeast (Figure 14). Well JT1 is within a mixed tidal flat development area, featuring dolomitic siltstone, and silty dolostone interbedded with gypsum dolostone; well GT2 develops gypsum dolomitic flat, with the main sedimentary rocks being gypsum dolostone, dolarenite, and sandy dolostone; the well JiaoS1 is developed in an evaporation lagoon, with primary rock types including gypsum, salt rock, gypsum dolostone, micritic limestone, and dolomitic limestone, and the upper part of the Gaotai Formation depositional stage exhibits thicker single-layer and cumulative thicknesses of evaporite rocks; well CS1 has similar depositional environment to the GT2; Gaodongmiao section is on the margin of the platform, with the development of superimposed grain shoals, and the thickness of a single shoal can be more than 20 m; Xiunao section is the platform margin slope zone; Dingjiaping and Dashuitian sections are the basin facies, with the main rock types being mudstone and siliceous shale (Figure 14).
Sedimentary evolution law of Cambrian inter-salt Gaotai formation in Sichuan Basin (see figure 10 for profile location).
In terms of lithofacies sequence evolution, grain shoals are widely developed within the restricted platform, mainly comprising sand-clastic and oolitic grain shoals. The development of these grain shoal bodies is primarily controlled by the Leshan-Longnvsi palaeo-uplift, with individual layer thickness generally around 10 m. Especially, the influence of the palaeo-uplift on the Longwangmiao Formation is more pronounced, with two or three sets of shoal bodies ranging from 10 to 70 m in thickness in the central Sichuan Gaoshiti-Moxi region, and more developed in the Moxi area compared to the Gaoshiti. In the southwestern part of the basin, lagoon deposits are also well developed. The Longwangmiao Formation developed gypsum dolostone lagoons in the Longchang-Zigong-Muchuan area, and gypsum lagoons in the Yongchuan-Changshou area. The Gaotai Formation developed gypsum lagoons in the Naxi-Yongchuan area and dolostone lagoons in the Muchuan-Yibin-Chishui area.
Sedimentary facies model
From a sedimentary evolution and paleogeographic distribution perspective, the Cambrian paragenesis system in the Sichuan Basin is primarily distributed in the Longwangmiao and Gaotai formations, with only minor development of gypsum dolostone at the base of the Xixiangchi Formation. The sedimentary facies model for the Longwangmiao and Gaotai formations has been established (Figure 15). Overall, there is some similarity to previous research, and the depositional evolution characteristics from bottom to top include evaporite-carbonate ramp, evaporite-mixed restricted platform, and restricted-fringe platform (Yang et al., 2022a). The specific features are as follows: (1) The Cambrian depositional paleogeographic pattern in the Sichuan Basin superimposes a “three uplifts and two depressions” paleogeomorphological pattern on a northwest-southeast large background, with highlands developing grain shoals in inner-platform and depressions developing evaporite or low-energy micritic limestone or micritic dolostone lagoon facies deposits; (2) The stratigraphic filling shows a trend of being thinner in the northwest and thicker in the southeast, with a “bottom overriding, top cutting” characteristic on seismic profiles; (3) The paragenesis system includes four third-order sequence cycles, containing multiple upward-shallowing secondary cycles (Figures 13 and 14). Furthermore, influenced by the differences in paleogeographic pattern, paleoclimate, paleo-ocean environment, and the variability of sediment sources and sea-level changes, there are significant differences in the depositional environment, grain shoal distribution, and the scale of mixed tidal flat, evaporite tidal flat, or evaporation lagoon of the Longwangmiao and Gaotai formations. Regional regression at the end of the Longwangmiao stage intensified environmental restriction, while continued aggradation of carbonate shoals in the Dayong-Huayuan-Songtao-Majiang belt gradually established true platform margins. By the Gaotai period, the system transitioned to an evaporitic platform depositional system. From west to east, the depositional environment is distributed as a mixed tidal flat-restricted platform (evaporite platform)-open platform-platform margin-slope-basin depositional facies, with grain shoals extensively distributed both within the platform and on the platform margin. Early Caledonian NE-trending uplift-sag structures (influenced by the Huayingshan and Qiyaoshan syndepositional faults) coupled with aridification during the Miaolingian Epoch enhanced terrigenous sediment influx in the western basin, jointly shaping the Gaotai Formation's mixed tidal flat-evaporitic platform architecture. From bottom to top, the scale of evaporation lagoons and evaporite tidal flats gradually increases (Figure 15).
Sedimentation model of the Cambrian paragenesis system in the Sichuan Basin.
Implications for hydrocarbon exploration
Carbonate reservoirs related to evaporation rocks in Cambrian of Sichuan Basin include post-salt Xixiangchi Formation, inter-salt Gaotai Formation, sub-salt Longwangmiao and Canglangpu Formation. From the perspective of sedimentary environments, the platform margin grain shoals and intra-platform grain shoals, distributed along the lagoonal margins and controlled by evaporative platform, represent favorable depositional environments for the development of carbonate reservoirs. From the perspective of source rocks, the Qiongzhusi Formation source rocks are widely distributed within the basin, with an average thickness of approximately 100 m, representing a regionally high-quality source rock, providing sufficient source rock conditions for hydrocarbon accumulation in the Cambrian strata (Zou et al., 2014). However, the thickness of the source rocks exhibits significant lateral variation, with multiple sedimentary centers. In the Luzhou area, the source rock thickness reaches 300–400 m, while in the Nanjiang area, it is around 250 m. The source rock thickness is relatively thin in most regions, ranging from 50 to 100 m. For the reservoir, the influence of sedimentary facies, high-frequency short-term exposure and weathering crust karstification are important mechanisms for reservoir formation in the Sichuan Basin, and gypsum-salt rocks are often dissolved by atmospheric water, forming excellent reservoirs (Liu et al., 2020). Microscopically, a large number of dissolution intergranular pores can be seen from the thin sections (Figure 3(d) to (g)), which can be used as high-quality reservoirs. Regarding traps, influenced by thick layers of gypsum-salt rocks, multiple late-stage concealed structures exist in the southern and eastern regions of the Sichuan Basin, providing favorable conditions for hydrocarbon accumulation and supporting various types of post-salt and sub-salt broad gentle anticline-type hydrocarbon accumulation models. Therefore, considering the distribution of various types of reservoirs in the Cambrian, along with conditions such as source rocks, migration systems, cap rocks, and traps, the Canglangpu Formation on the northern slope of the Leshan-Longnvsi palaeo-uplift, and the Xixiangchi Formation in Weiyuan-Hechuan and Guang'an-Yilong-Yingshan areas, can be selected as the most favorable exploration zones (Figure 16).
Distribution of grain shoals and thickness of source rocks in Cambrian, Sichuan basin. (thickness of source rock data cited from Zou et al., 2014; some grain shoal data cited from Shi et al., 2020 and Zhou et al., 2015).
The Weiyuan-Hechuan area is located within the superimposed region of palaeo-uplifts and ancient rifts, in close proximity to the primary hydrocarbon kitchen of the Cambrian Qiongzhusi Formation. The inherited palaeo-uplift serves as a stable zone for hydrocarbon accumulation, and is also a large-scale development area for high-quality grain shoal reservoirs in the Longwangmiao and Xixiangchi Formation, along with favorable preservation conditions. The target layer is relatively shallow, making it a favorable exploration area for both post-salt and sub-salt accumulation systems. In the central Sichuan region, the Guang'an-Yilong-Yingshan area is situated on the slope of the palaeo-uplift, where grain shoal reservoirs of the Xixiangchi Formation are developed. The inherited ancient structure provides favorable conditions for the development of ancient oil reservoirs, and the presence of a well-developed fault system offers good conditions for post-salt accumulation. The Canglangpu Formation on the northern slope of the Leshan-Longnvsi palaeo-uplift features well-developed grain shoals, positioned adjacent to high-quality source rocks, making it favorable for sub-salt accumulation. Here, the grain shoal reservoirs of the Xixiangchi Formation, located in the Pingchang-Dianjiang-Liangping, Kaixian-Datianchi, and well DongS1-YS1 regions, are developed near the Silurian Longmaxi Formation hydrocarbon kitchen. The presence of high-steep fault traps facilitates the lateral connection between the Silurian Longmaxi Formation source rocks and high-quality shoal reservoirs, potentially forming lateral post-salt hydrocarbon reservoirs. However, early traps have been significantly modified by later tectonic movements, and the development of later faults poses risks to preservation conditions.
Overall, the distribution of the “rift trough” primary hydrocarbon kitchens, high-quality grain shoal reservoirs, inherited palaeo-uplifts, evaporite cap rocks, and the degree of later tectonic modifications control hydrocarbon accumulation and favorable exploration zones in the Cambrian strata of the Sichuan Basin. In recent years, the Southwest Oil and Gas Field Company has made significant exploration progress in the lower section of the sub-salt Canglangpu Formation, however, no large-scale reserves have been identified, and other strata have yet to achieve new breakthroughs (Yan et al., 2021). Comprehensive analysis suggests that the post-salt accumulation combination of the Xixiangchi Formation may possess considerable exploration potential, particularly in favorable zones such as the northern slope of the Leshan-Longnvsi palaeo-uplift, Weiyuan-Hechuan, and Guang'an-Yilong-Yingshan areas. The exploration area of traps exceeds 1500 km². The sub-salt accumulation of the Longwangmiao Formation also has conditions for further exploration expansion and is worthy of attention in the southern Sichuan region, especially as regional large structures may become the next breakthrough area. Favorable zones also exist on the southern and northern slopes of the Leshan-Longnvsi palaeo-uplift, with favorable exploration trap areas exceeding 900 km². Additionally, the post-salt Gaotai Formation has developed the gypsum dolomitic flats along the Huayingshan fault belt, which presents good reservoir development conditions. It may be an important area for Cambrian exploration in the future, with favorable zones located in the northern Sichuan region, such as the Pingchang-Guang'an area, and in the southeastern Sichuan region, including the Tiangongtang-Luoguanshan area.
Conclusions
The lithology in the Cambrian paragenesis system in the Sichuan Basin mainly includes gypsum, and dolomitic and dolomitic-bearing gypsum, salt rock, gypsum and gypsum-bearing dolostone, dolo-grainstone, and crystalline dolostone. Six types of lithofacies association were established, including gypsum dolomitic flat-grain shoal, dolomitic-argillaceous lagoon-grain shoal, gypsum lagoon-grain shoal, calcareous-dolomitic (argillaceous-dolomitic) lagoon-evaporation lagoon, argillaceous dolomitic flat-gypsum dolomitic flat, gypsum lagoon-salt lagoon. Further, four seismic response models of lacking gypsum-salt rock, carbonate and gypsum-salt rock interbedded, thick gypsum-salt rock and compound type are identified.
The high-resolution lithofacies paleogeography of the paragenesis system in Sichuan Basin is established. The tidal flat, multi-lagoon, carbonate platform sedimentary system is mainly developed, showing a paleogeographic pattern of “two uplifts and one depression.” During the depositional period of the Longwangmiao Formation in the southern Sichuan region, near wells TT1, Lin7, and JianS1, gypsum lagoons were developed. The grain shoals on the inner ramp are distributed in a large area, and the mid ramp is scattered; In the Gaotai Formation, two large salt lagoons developed in the eastern and southern Sichuan regions, with grain shoals developing on both sides of the lagoon margins.
The northern areas of the Leshan-Longnvsi palaeo-uplift, Weiyuan-Hechuan and Guang'an-Yilong-Yingshan regions in the post-salt Xixiangchi Formation of the Cambrian in the Sichuan Basin are favorable zones. The sub-salt Longwangmiao Formation in the south Sichuan Basin is worth paying attention to, particularly the regional large structures may become new areas for breakthroughs, and there are favorable zones on the south and north slopes of the Leshan-Longnvsi palaeo-uplift as well. The inter-salt Gaotai Formation should focus on two areas: the gypsum dolomitic flats and lagoon margin shoals, and there are favorable zones such as the Pingchang-Guang'an area in northern Sichuan and the Tiangongtang-Luoguanshan area in southeastern Sichuan.
Supplemental Material
sj-xlsx-1-eea-10.1177_01445987251361939 - Supplemental material for Sedimentary evolution and model of Cambrian carbonate-evaporite paragenesis system in Sichuan Basin, SW China: Implications for hydrocarbon exploration
Supplemental material, sj-xlsx-1-eea-10.1177_01445987251361939 for Sedimentary evolution and model of Cambrian carbonate-evaporite paragenesis system in Sichuan Basin, SW China: Implications for hydrocarbon exploration by Zhuangzhuang Bai, Shuyuan Shi, Wei Yang, Yongjie Hu, Wuren Xie, Gang Zhou, Hua Jiang, Xuan Zhang, Kui Ma and Yan Zhang in Energy Exploration & Exploitation
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