The Upper Ordovician Wufeng Formation to the Lower Silurian Longmaxi Formation correspond to the Upper Ordovician Katian and Hirnantian stages and the Silurian Landolevitian Rhuddanian and Aeronian stages. In this unit, the organic-rich shale is large in thickness, with good quality, being the most explored and producible shale by far in China, and the most dominant target of shale gas exploration and development^[1,2,3,4]. At the initial stage of the resources and the reservoir evaluation and the prospect identification within the Wufeng-Longmaxi formations, the China National Petroleum Corporation (CNPC) constructed a scheme for stratification^[5]. The scheme was relatively mature at that time, and applied to many shale gas demonstration regions, such as Changning, Weiyuan and Zhaotong^[6,7,8,9]. Diachronism was noticed by China’s scholars when conducting well correlation across areas^[10]. However, no revision was made due to the lack of well data at the junction of the Changning, Weiyuan and Zhaotong regions. Great breakthrough has been made in deep-layer shale gas exploration in the Luzhou region, and large-scale shale gas has been produced rapidly^[11]. It is urgent to revise the stratification scheme for accurate prediction of favorable prospects.

Sequence stratification is an integrated stratification approach, which has been widely used in oil and gas exploration^[12,13]. Sequence stratification of fine-grained rock is important, but it is difficult to use in practice, particularly on the fourth- and fifth-order sequences^[14]. In the southern Sichuan Basin, in the Wufeng Formation to the Longmaxi Formation, there is a set of marine shale, which is serious in vertical and lateral heterogeneity, with abundant well log data, core data and detailed biozonation data as a time scale, being an ideal sample for making sequence stratification of fine-grained rocks. It is commonly accepted in China that, the whole Wufeng Formation can be considered as the sequence SQ1, corresponding to the Katian and Hirnantian stages (till the top of the Guanyinqiao Member). The Longmaxi Formation is divided into SQ2 (which corresponds to the Hirnantian, Rhuddanian and Aeronian stages, roughly equivalent to the first sub-member of the Long-1 Member proposed by CNPC) and SQ3 (partial Aeronian and Telychian stages, roughly equivalent to the second sub-member of the Long-1 Member and the Long-2 Member proposed by CNPC)^[15,16,17]. This study is focused mainly on SQ1 and SQ2, namely the Katian-Aeronian stages (not reach the Telychian stage).

On the other hand, so far, there is no unified lithofacies division scheme for this set of shale^[18]. Due to the diversified schemes, it is unfavorable for predicting the favorable prospects. Meanwhile, the previous studies were focused on restoring the sedimentary environment and lithofacies paleogeography for the third-order sequence framework due to the limitation of data and method^[19,20]. However, the target shale interval shows a great variation in the fourth-order sequences, as well as a large diversity in shale reservoirs of different fourth-order sequences. Conducting the division and correlation of the fourth- order sequences and determining their lithofacies paleogeographic evolution provide a guidance for identifying the shale prospects.

A fine division and correlation of the fourth-order sequences has been conducted for the Katian and Aeronian stages in the Southern Sichuan Basin using the sequence stratigraphic method, based on a combination of drilling, mud log and well log data collected from 110 wells, the quantitative evaluation of materials by scanning electron microscopy (QEMSCAN), the large-scale high-resolution imaging (MAPS Stitching) of samples collected from 20 wells, the petrographic studies and high-resolution biostratigraphic studies for 40 wells. Then, the efforts were made to restore the lithofacies paleogeography for each sequence, in order to lay a solid theoretical foundation for shale gas exploration and development in the Southern Sichuan Basin and provide a reference for the shale gas exploration and development.

The southern Sichuan Basin is located at the southwestern margin of the Upper Yangtze platform in South China. It is bordered to the east by the Daliang Mountain, to the south by the denudation line of the Longmaxi Formation at the Leshan-Longnüsi paleo-uplift, to the west by the Huaying Mountain and to the north by the Qianbei sag, covering an overall area of 4×10⁴ km². This area is situated within the Southern Sichuan low-steep dome belt^[21] (Fig. 1a) and contains a widespread tectonic stable area. The burial depths of Wufeng Formation ranges from 2000 m to 6000 m. According to the shale gas field distribution map and mineral right partition of CNPC, this area can be divided into the Changning, Luzhou, Weiyuan and Yuxi regions (Fig. 1b).

The target layer was deposited approximately 438.49 Ma to 447.62 Ma ago, and extended broadly across the Yangtze region. This stratum is continuous and conformable vertically. The lithology consists mainly of shale, with subordinate silty mudstone, argillaceous siltstone, marl, and shell limestone (Fig. 1c). Major sedimentary structures include lamina and massive bedding. Graptolite, brachiopoda, gastropods, siphonopods and siliceous pelagic plankton^[22] are present. In China Petrochemical Corporation’s meeting hold in Fuling in 2014, Chen et al. proposed a graptolite succession covering the Katian, Aeronian and Telychian stages in the Yangtze region, along with the codes and isotopic ages at the geological time scale published by International Commission on Stratigraphy (ICS)^[23]. Henceforward, this succession was often applied to stratification of the Katian and Aeronian stages by explorationists as a time scale. In this study, this graptolite succession is used as one of the bases for sequence division.

The definition of sequence order introduced by Mitchum in 1991^[13] was used in this study to name the sequence as SQ, and the system tract or system tract set as the fourth-order sequence or parasequence set (Pss).

Sequence division is based on the following indexes: i.e., the core-observed lithology, sedimentary structures, graptolite zone, inorganic geochemical characteristics, and electric features. Electric features involve an integration of multiple logs, such as gamma ray (GR) log, uranium-free gamma ray log, sonic log, density log, resistivity log and mass fraction ratio of uranium to thorium (w(U)/w(Th) log). The value of w(U)/w(Th) smaller than 0.75 is indicative of an aerobic environment, 0.75-1.25 denotes the oxygen-lean environment, and larger than 1.25 indicates the anaerobic environment^[24].

For the target layer, we have identified three continuous conformable surfaces as sequence boundaries (i.e., SB1, SB2 and SB3), and two sequences (i.e., SQ1 and SQ2) have been divided.

SB1 is a continually regressive transfer surface of sedimentary build-up, which is underlain by pyrite- bearing nodular limestone of the Early Katian Upper Ordovician Linxiang Formation, and overlain by clayey shale and siliceous shale (SQ1) of the Middle Katian Wufeng Formation that contains calcareous aggregates or lens. At the Ordovician Sandbian stage, the paleogeographic framework of the Cambrian time was inherited in the entire South China, consisting of “two lands, three or two platforms, one basin and one slope”. At the Late Sandbian and the Early Katian stages, the Kangtien land was expanded, and the interplatform Jiangnan deep-sea shale basin vanished^[25,26]. SQ1 is a “regressive shale”, which was surrounded by the Qianzhong old land (the eastern segment of the southern end of the Kangtien land) and the Leshan-Longnüsi subaqueous structural high, with shallower water than the Sandbian-Early Katian carbonate platform^[26].

SB2 is a transfer surface of the sedimentary build-up formed after the melting of glacial period at glacial period and the paleo-sea status. A momentous extinction event occurred at the Hirnantian stage^[27], which was academically divided into two episodes^[28]. Episode I (0.5-1.0 Ma) commenced, with the abrupt drop in global temperature by 5 °C to 8 °C. In consequence, a large ice sheet that covered an overall area of 1500×10⁴ km² was formed in the South Pole, thermophilic organisms became extinct, and psychrophilic organisms occupied the world. The global sea level dropped by 80 m to 120 m. The Wufeng Formation Guanyinqiao Member was deposited in the southern Sichuan Basin, consisting mainly of shell limestone, with interbedded calcareous and argillaceous siltstone and shell limestone, and manganese-rich algal reef. The Guanyinqiao Member is too thin to be identified on well log, and thus is assigned to SQ1. At Episode II, temperature increased, glaciers melted, and sea level rose rapidly. As a result, psychrophilic organisms became extinct, and thermophilic organisms emerged. In the southern Sichuan Basin, carbonate was transformed into shale, leading to deposition of the Longmaxi Formation black shale.

SB3 is the surface that records the abrupt change in lithology and sedimentary structures resulted from the sharp fall in sea level and the heterogeneous input of terrigenous sediments during the continuous regression. In the southern Sichuan Basin, residual Silurian strata exhibit vertically the progradational facies evolution. This tendency is consistent with the global continuous regression at that time^[29]. The surface varies across different areas. In areas outside Changning (including the West Changning region), SB3 is underlain by the SQ2 clayey shale, which is rich in laminas and barite nodules, with relatively high GR, low uranium-free GR, high sonic, low density, low resistivity, and high w(U)/w(Th), and overlain by the SQ3 rhythmic interbeds of silty mudstone/argillaceous siltstone and pelitic stringer, with relatively low GR. In the Changning region (excluding the West Changning region), the same differential electric features are also noted. However, the close proximity to the Qianzhong old land allowed this region to receive sufficient sediments. As a result of this effect, the SB3 is underlain by silty mudstone or argillaceous siltstone, with occasional very fine barite nodules, and overlain by rhythmic interbeds of calciferous argillaceous siltstone and limestone stringer.

2.2.1. Division of system tracts and their electric features

As a case study, Well LU205 was analyzed (Fig. 2):

SQ1 can be divided into two system tracts. The lower section is an inherited highstand system tract (HST), inherited from the Linxiang Formation regression (the interval of 4036.70-4042.74 m in Fig. 2). The upper section is a transgressive system tract (TST) (the interval of 4032.29-4036.70 m in Fig. 2). The boundary between them is readily identified. TST has lower GR, lower uranium-free GR, lower density, higher resistivity and higher sonic than HST. Particularly, uranium-free GR and density logs have the most prominent differences in logging response, and can act as good markers for stratification based on electric features. In addition, a thin-bedded Guanyinqiao Member was deposited at the glacial period, with no conclusive time duration. Thus, the TST that contains Guanyinqiao Member is tentatively considered as an entire cycle of thick transgressive bed and thin regressive bed. Since the continuous transgression terminated at the glacial period, the base of the Guanyinqiao Member can be acted as the first maximum flooding surface (SQ1-MFS, at 4033.69 m in Fig. 2), which is the only maximum flooding surface in SQ1.

SQ2 emerged under the backgrounds of “large-scale transgression resulted from the melting of ice sheet at Episode II” and “continuous elastic regression at the Silurian time”. Four MFSs (with GR peaks) were identified, as shown in Fig. 2, at 4031.76 m (SQ2-MFS1), 4024.65 m (SQ2-MFS2), 4004.88 m (SQ2-MFS3) and 3982.28 m (SQ2-MFS4), respectively. In addition, five surfaces that represent either the gradual change or abrupt change in lithology were defined within this sequence, as shown in Fig. 2: (1) the first surface that is marked by gradual change in lithology is at 4031.33 m, above which the GR, uranium-free GR and sonic logs decrease, and the density and resistivity logs increase; (2) the second surface that is marked by gradual change in lithology is at 4026.50 m, above which the GR, uranium-free GR, sonic and density logs increase, and the resistivity log decreases; (3) the third surface that is marked by abrupt change in lithology is at 4024.13 m, above which the clay mineral content increases remarkably, the GR and sonic logs decrease significantly, and the uranium-free GR, density and resistivity logs increase greatly; (4) the fourth surface that is marked by gradual change in lithology is at 4009.54 m, above which the GR, uranium-free GR, sonic and density logs increase significantly, the resistivity log decreases slightly, and the density and sonic logs become scattered; and (5) the fifth surface that is marked by gradual change in lithology is at 3995.60 m, above which the GR, uranium-free GR, sonic, density and resistivity logs increase significantly. Within this surface, multiple low GR peaks emerge. The most prominent low GR peak occurs at the middle part (at 3982.90-3984.86 m, as shown in Fig. 2). Above this peak, a high GR peak is defined as SQ2-MFS4. Accordingly, system tracts starting from the base to the top of SQ2 include: ①TST—②HST—③HST—④TST—⑤HST—⑥HST—⑦TST—⑧HST—⑨TST—⑩HST.

2.2.2. Relation between the fourth-order sequence and the graptolite zone

Graptolite zone is one of the key markers for sequence division and of great “clock scale” significance to stratigraphic division and correlation. In Well Y101H2-7 with clear boundary for the first appearance of graptolite zone (Fig. 3), for example, SQ1-HST corresponds to the WF2 graptolite zone (the WF1 graptolite zone is rare in the southern Sichuan Basin), with the time duration of approximately 0.6 Ma, and is assigned to the Pss1, which corresponds to the Early-Middle Katian Stage; SQ1-TST corresponds to the WF3-WF4 graptolite zone and Hirnantia Fauna, with the time duration of approximately 0.6 Ma, and is assigned to the Pss2, which corresponds to the Middle-Late Katian Stage and the Episode I of the Hirnantian Stage; SQ2-①TST—②HST correspond to the LM1 graptolite zone (the first appearance of LM2 remains unreached), with the time duration of approximately 0.6 Ma, and is assigned to the Pss3, which corresponds to the Episode II of the Hirnantian Stage; SQ2-③HST corresponds to the LM2-LM3 (including a small portion of LM4), with the time duration of approximately 1.4 Ma, and is assigned to the Pss4, which corresponds to the Early-Middle Rhuddanian Stage; SQ2-④TST—⑤HST correspond to partial LM4 (LM5 remains unreached), with the time duration of approximately 0.9 Ma, and is assigned to the Pss5, which corresponds to the Middle-Late Rhuddanian Stage; SQ2-⑥HST corresponds to the local part of LM4, LM5 and a small portion of LM6, with the time duration of approximately 0.8 Ma, and is assigned to the Pss6, which corresponds to the Late Rhuddanian Stage and the Early Aeronian Stage; SQ2-⑦TST—⑧HST corresponds to partial LM6 (LM7 remains unreached), with the time duration of approximately 1.6 Ma, and is assigned to the Pss7, which corresponds to the Middle Aeronian Stage; SQ2-⑨TST—⑩HST corresponds to LM7 and partial LM8 (including a small portion of LM6, but LM 9 remains unreached), with the time duration of approximately 0.7 Ma, and is assigned to the Pss8, which corresponds to the Late Aeronian stage.

2.2.3. Lithological characteristics

So far, there is no academically commonly agreed scheme to name the shale lithology worldwide. Thus, dividing sequence with lithological markers is one of the biggest challenges. QEMSCAN and MAPS Stitching were utilized in this study to conduct the micron- to nano-scale semi-quantitative analysis. As a result of this analysis, the common lithologies of the target layer are preliminarily divided into four classes and ten sub-classes that are listed in Table 1. In Well LU205, for example, Pss1 is siliceous clayey shale, composed mainly of terrigenous quartz; Pss2 is calcite-bearing siliceous shale, composed mainly of authigenic quartz with Hexagonal bicone shape; Pss3 is dolomite-bearing siliceous shale, composed mainly of authigenic quartz; pss4 is dolomite-bearing siliceous shale, dominated mainly by authigenic quartz, with lesser dolomite content than Pss3; Pss5 is siliceous shale, composed mainly of authigenic quartz, with high abundance of silica (XRD shows increased clay content, as in many wells, and QSEMSCAN, however, shows extremely high quartz content and extremely low clay content); Pss6 and Pss7 are siliceous clayey shale composed mainly of terrigenous quartz grains; and Pss8 is calcareous clayey shale composed mainly of terrigenous quartz grains, with coarsening-upward grain size. These sequences produce a good matching of XRD and QEMSCAN results, except for Pss5.

2.2.4. Sedimentary structure

At the target layer, there are abundant sedimentary structures. They provide the most direct and important markers for dividing the fourth-order sequences, since their vertical distribution is good matched with these sequences (as shown in Fig. 4, digital core scanning image).

At the bottom of Pss1, zonal or lenticular calcareous aggregates are observed, with possible bioturbation, spotted pyrite and less lamina. At the top of Pss1, two sets of potassium bentonite stringer^[30] are observed, which are readily identified on cores (Fig. 4a). Pss2 contains numerous very thin to thin laminas, changing gradually and decreasing upwards, with occasional thin biosilica stringers, containing radiolarian and sponge spicule, and abundant bentonites. These bentonites contain numerous authigenic pyrites, with relatively small crystal. Some wavy calcite veins extending in parallel to the bedding plane (Fig. 4b) can act as the structural marker for stratification. It is assumed that, a small-scale tectonic action occurred soon after the deposition of the Hirnantian stage, and the pre-existing Guanyinqiao Member shell limestone provided a stress barrier. Since the overlying sediments were limited in amount and remained unconsolidated, the influence of this tectonic action was limited to the Middle-Late Katian stage sediments and the rock bed in the proximity of the Guanyinqiao Member. Pss3 is rich in massive bedding and discontinuous, with very thin silt and pyrite laminas. Biosilica stringers or laminas can be observed (Fig. 4c). Within this sequence, pyrite nodules are common, with no trace of bentonite. Thus, bentonite provides a marker to distinguishing between Pss2 and Pss3. Pss4 contains abundant laminas that consist mainly of concentrated pyrites or silts (Fig. 4d), and rich biosilica stringers or laminas. Pss5 exhibits a superimposition of massive bedding interval and lamina interval. The laminas consist mainly of pyrite or silt and became dispersed and gradually changed (Fig. 4e). Pss6 contains abundant laminas that are thick-bedded and closely spaced, predominately thin to moderate laminas, with lesser pyrite lamina. Biosilica stringer is present. A thick bed of bentonite emerges at the base of that sequence (Fig. 4f). Moreover, wavy calcite veins parallel to bedding plane are common, similar to but thinner than those in Pss2. Pss7 also contains the bentonite stringers of varying thickness, in which coarse-grained authigenic pyrite cubes are visible. In areas outside Changning, this sequence exhibits the superposed laminas and massive beddings. Laminas are thick-bedded and gradually changed, with occasional syn-sedimentary crumpling of moderate-thick laminas of pyrite (Fig. 4g). In the Changning region, laminas become thicker and oblique, with occasional small-scale Bouma sequence (Fig. 4h). Pss8 also contains the bentonite stringers of varying thickness, with occasional biosilica stringer, in which coarse-grained authigenic pyrite cubes are visible. In areas outside Changning, this sequence is rich in generously spaced laminas, with numerous along-lamina barite nodules (Fig. 4i). These nodules are mostly ring-shaped, with pyrite at the inner ring, barite at the outer ring, and occasional uralite at the outermost ring. Their diameters tend to decrease and then vanish from north to south. The widespread barite nodules constitute a “front belt” that is often indicative of an alternating aerobic, oxygen-lean, and anaerobic environment^[31]. In the Changning region, laminas become thicker-bedded with finer barite nodules, and sedimentary structures representative of strong hydrodynamic conditions, such as gravity flow or fluidized sediment flow sedimentation, are commonly present (Fig. 4j). Furthermore, barite nodules are absent in several zones with low GR, low sonic and high density in Pss8, which consist mainly of calcareous siltstone, limy mudstone and marl, with abrupt contact with the overlying and underlying rocks (Fig. 4k). They are therefore considered the auxiliary marker for stratification. Pss8 is overlain by the strata rich in rhythmic sandstone-mudstone beddings.

Basically, the features of laminas are consistent across these fourth-order sequences. It is inferred that laminas are more influenced by availability of source supply than simply a characterization of the variation in hydrodynamic condition. In other words, under a shelf setting, laminas were formed in the Changning region, meaning laminas of varying magnitudes would also occur in the Luzhou, Weiyuan and Yuxi regions at that time. Thus, the characteristics of lamina provide an optimal marker for stratification.

In recent years, w(U)/w(Th) has been determined by many inorganic geochemists as the most optimal indicator to redox condition of target layer. It is clearly correlated to TOC^[4]. In Well LU205 (Fig. 2), for example, Pss1 shows a generally low w(U)/w(Th) that ranges from 0.07 to 0.23, averaging 0.13, which has a stable range of value and indicates an aerobic environment, and an average TOC of 1.03%, with a high value at the top of the sequence; Pss2 shows a w(U)/w(Th) that ranges from 0.15 to 2.36, averaging 0.78, which is significantly higher than that of Pss1 and tends to increase gradually from the base to the top, indicating that the redox environment transitioned from aerobic at the base to anaerobic at the top, and an average TOC of 3.36%; Pss3 shows a relatively high w(U)/w(Th) that ranges from 1.37 to 2.38, averaging 1.90, which has a stable range of value and indicates an anaerobic environment, and an average TOC of 4.43%; Pss4 shows a w(U)/w(Th) that ranges from 1.27 to 1.57, averaging 1.41, which has a stable range of value, is slightly lower than that of Pss3 but still indicates an anaerobic environment, and an average TOC of 3.88%; Pss5 shows a w(U)/w(Th) that ranges from 0.98 to 1.67, averaging 1.34, which has a stable range of value, is slightly lower than that of Pss4 and indicates an environment dominated by anaerobic with occasional oxygen-lean, and an average TOC of 3.77%; Pss6 shows a w(U)/w(Th) that ranges from 0.33 to 0.86, averaging 0.46, which has a stable range of value, is clearly lower than that of Pss5 and indicates an aerobic environment with oxygen-lean base, and an average TOC of 2.59%; Pss7 shows a w(U)/w(Th) that ranges from 0.27 to 0.74, averaging 0.45, which has a stable range of value, is lower than that of Pss6 and indicates an aerobic environment, and an average TOC of 2.40%; Pss8 shows a w(U)/w(Th) that ranges from 0.29 to 1.25, averaging 0.64, which is significantly higher than that of Pss7 but exhibits fairly unstable range of value (the maxima occurs at the base of the sequence) and indicates the aerobic environment with occasional oxygen-lean and anaerobic, and an average TOC of 2.28; and the strata that overly Pss8 show a w(U)/w(Th) less than 0.75, with no high value observed, and a TOC of less than 2%. In general, high value of TOC emerges where MFS occurs (Fig. 2). High values of TOC across Pss3 and Pss5 are resulted mainly from redox condition, and across Pss6 and Pss8 are possibly controlled jointly by redox condition and paleo-productivity.

Advances in petrographic studies allowed a fine characterization of shale lithofacies in the vertical direction. In practice, the most prominent shale-siliceous shale lithofacies (high TOC and high brittleness) and siliceous clayey shale lithofacies (high TOC and low brittleness) have been defined^[32]. In this study, we try to conduct the restoration of the period and scale of the prominent shale lithofacies of the Katian-Aeronian stages in the southern Sichuan Basin.

Pss1 is relatively thin and emerged as bioturbated calcareous clayey shale deposited in an aerobic environment across the whole area. This shale appears to be of poor quality with low TOC and poor brittleness. Pss2 presents laterally highly heterogeneous lithofacies, which emerged as siliceous shale in the Luzhou and Yuxi regions, and varied across wells in the other regions, including argillaceous siltstone, clayey shale and even marl/dolomite. Samples collected may not be nonrepresentative. Thus, the focus of lithofacies paleogeographic restoration was placed on Pss3-Pss8 (Fig. 5).

The lithofacies of Pss3 is dominated by the dolomite (ferrodolomite)-/calcite-bearing siliceous shale in the central Yuxi, the northern Luzhou and the northwestern Changning regions, and transitions into siliceous shale towards both sides, with occasional clayey siliceous shale at the northern side of the Luzhou region. Its thickness thins from southeast to northwest, laying a foundation for variation in thickness and lithofacies of subsequently deposited fourth-order sequences (Fig. 5a). It is noteworthy that, the currently deepest water level should be within the Luzhou region, as evidenced by thickness, sedimentary structures, redox condition and pre-Pss3 sedimentary environment. A vast number of samples, however, highlight the presence of dolomite-bearing siliceous shale, with occasional calcite-bearing siliceous shale. Thus, it is believed that a carbonate mineral deposit zone exists at the deepest water zone of the deep- water shelf. The production performance data revealed that, dolomite/calcite-bearing siliceous shale is assigned to the high-quality lithofacies. At that time, the southern Sichuan Basin was covered by high-quality lithofacies.

The lithofacies of Pss4 is dolomite-bearing siliceous shale and siliceous shale in the western Changning, the northern Luzhou and the southern Yuxi regions, clay-bearing siliceous shale in the northern Weiyuan-Yuxi and the southern Luzhou regions, siliceous shale at the eastern side of the Changning region, and clay-bearing siliceous shale in the Changning region (Fig. 5b). In general, Pss4 thins gradually from southeast to northwest.

The lithofacies of Pss5 shows no trace of dolomite-/ calcite-bearing siliceous shale in the southern Sichuan Basin, and becomes siliceous shale at the western side of the Luzhou and Changning regions, and clay-bearing siliceous shale in other areas (Fig. 5c). Pss5 thins gradually from southeast to northeast, with the depocenter being situated along the Naxi-Jiangan-Shuifu-Gaoxian.

Pss6 shows dramatically increased clay mineral content relative to Pss5. It emerges as siliceous clayey shale in the central and eastern Changning regions, with clay-bearing and feldspar-bearing siliceous shale being present in localized areas, argillaceous siltstone in the western part and at the southwestern side of the Changning region, feldspar-bearing siliceous shale in the central Yuxi, the northern Luzhou and the northwestern Changning regions, with clay-bearing siliceous shale being present in localized areas, feldspathic clayey shale in the Weiyuan and the northern Yuxi regions, and siliceous clayey shale in the rest areas (Fig. 5d). Pss6 recorded a higher sedimentation ratio than Pss3-Pss5. Its thickness also thins from southeast to northwest, with the depocenter being situated in the Changning region.

The lithofacies of Pss7 changes greatly across regions. It emerges as calcareous clayey shale in the central and southwestern Weiyuan regions, feldspathic clayey shale and siliceous clayey shale in the eastern Weiyuan and the northern Yuxi regions, feldspathic clayey shale in the central Yuxi, the central Luzhou and the northwestern Changning regions, siliceous clayey shale in the northern Luzhou region, and calcareous clayey shale and argillaceous siltstone in the southern Luzhou and the Changning regions (Fig. 5e). Sedimentation ratio further increased in Pss7. Its thickness also thins from southeast to northwest, with the depocenter being situated in the Changning region.

The lithofacies of Pss8 changes across areas (i.e., the southeastern side of the SE-NW trending Changning region, Changning region, Luzhou region, middle and eastern parts of the Weiyuan region, and Yuxi region). Typical lithofacies include marl, limy marl, argillaceous siltstone, calcareous clayey shale, feldspathic clayey shale, and siliceous clayey shale. Feldspathic clayey shale is present in the western Weiyuan region (Fig. 5f). This sequence exhibits higher magnitude of sedimentation ratio relative to Pss3-Pss7. Its thickness also thins from southeast to northwest, with the depocenter or lateral aggradation center being situated in the Changning region.

On the whole, Pss1-Pss8 recorded six regression and five transgression events. SQ2-MFS1, the first transgression emerged after the glacial period, is the maximum flooding surface (Fig. 6). At the Pss1 period, the whole area was within an aerobic environment, with relatively low sea level, high paleo-productivity, and shallow water level (Fig. 6a). At the Pss2 period, a shallow water retention oxygen-lean to anaerobic environment became dominant at the non-glacial period, where the sea was closed, with continuous transgression and moderate paleo-productivity (Fig. 6b), and then the environment became aerobic with increased paleo-productivity when entering the glacial period (Fig. 6c). At the Pss3 period, along with the melting of ice sheets, sea level rose dramatically, the water level over the whole area was relatively deep, and the environment became aerobic, with semi-retention of sea and decreased paleo-productivity (Fig. 6d). At the Pss4 period, the sea was elastically recovered and the sea level remained deep despite the sea level fall to some extent, and the environment became anaerobic across the Weiyuan, Yuxi, Luzhou regions, and transitioned into oxygen-lean in the Changning region, with semi-retention of sea and relatively low paleo-productivity (Fig. 6e). At the Pss5 period, the sea level rose first and then dropped slowly, and the environment became anaerobic to oxygen-lean across the Weiyuan, Yuxi and Luzhou regions resulted from the weakened retention level of sea and even oxygen-lean to aerobic in the Changning region, but with increased paleo-productivity (Fig. 6f). At the Pss6 period, sea level dropped rapidly to increase the retention level to some extent, and the environment became aerobic with occasional oxygen-lean across the Weiyuan, Yuxi and Luzhou regions and aerobic in the Changning region, with rapidly increased paleo-productivity (Fig. 6g). At the Pss7 period, the sea level rose for a short period of time and then dropped continuously, with decreased retention level of sea, and the environment became oxygen-lean with occasional aerobic across the Weiyuan and Yuxi regions and aerobic across the Luzhou and Changning regions, with continuously increased paleo-productivity (Fig. 6h). At the Pss8 period, the sea level varied frequently at early stage and then continuously dropped at the later stage, with decreased retention level of sea, and the Weiyuan, Yuxi and Luzhou regions were covered by the Ba-rich sea to form an alternating aerobic, oxygen-lean, and anaerobic environment, which became aerobic in the Changning region, with the maximum paleo-productivity recorded across all the sequences stated above (Fig. 6i).

The scheme for division of the sequence stratigraphy in the Katian-Aeronian stages has been established in the southern Sichuan Basin. Two sequences, three sequence boundaries, five MSFs and twelve system tracts have been identified. Relying on the system tracts and their combination pattern, eight fourth-order sequences have been further divided, namely Pss1 to Pss8 from older to younger.

The prominent shale lithofacies of the Katian-Aeronian stages in the southern Sichuan Basin have been studied to determine their development period and scale. Dolomite and calcite-bearing siliceous shale, siliceous shale, clay-bearing siliceous shale and feldspar-bearing siliceous shale are developed mainly within Pss3-Pss5 in the Weiyuan, Yuxi and Luzhou regions, Pss6 in the western Changning, the northern Luzhou and the central Yuxi regions, and Pss3-Pss4 in the Changning region. Siliceous clayey shale is developed mainly within Pss6 in the southern Luzhou and Changning regions (excluding the western Changning region), Pss7 in the eastern Weiyuan, the northern Yuxi and the southern Luzhou regions, and Pss8 in the northern Luzhou, Weiyuan and Yuxi regions.

The evolution model of the fourth-order sequences has been built for the Katian-Aeronian stages in the southern Sichuan Basin. From Pss1 to Pss8, six regression and five transgression events occurred. SQ2-MFS1, the first transgression after the glacial period, is the maximum flooding surface.