Development Characteristics of Shale Lithofacies in the Longmaxi Formation and their Main Controlling Factors in the Changning Area, South Sichuan Basin, SW China

Based on core observations, thin sections, X-ray diffraction (XRD), and seismic data, the lithofacies types in the organic-rich Longmaxi shale (Lower Silurian) in the Changning area of the southern Sichuan Basin were identified. The factors controlling the spatial variations in the shale lithofacies and the influences of the shale lithofacies on shale gas development were also analyzed. Results indicate that there are seven main types of shale lithofacies in the Long11 sub-member of the Longmaxi Formation, including siliceous shale (S-1), mixed siliceous shale (S-2), carbonate-rich siliceous shale (S-3), clay-rich siliceous shale (S-4), carbonate/siliceous shale (M-1), mixed shale (M-2), and argillaceous/siliceous shale (M-4). A vertical transition from the carbonate shale association + mixed shale association at the bottom of the sub-member to a siliceous shale association and mixed shale association + siliceous shale at the top generally appears in the Long11 sub-member. The shale lithofacies of the Long11 sub-member also laterally change from the central depression (low-lying area) to the geomorphic highland in the east and west parts of the Changning area. The spatial variations in shale lithofacies in the Long11 sub-member of the Changning area were mainly controlled by palaeogeomorphology and relative sea level. The geomorphic highland area is dominated by carbonate-rich siliceous shale and mixed siliceous shale, but the depression (low-lying area) is mainly dominated by mixed siliceous shale and argillaceous/carbonate shale.


INTRODUCTION
Marine shales in China, which are characterized by a wide distribution, high thickness, high thermal maturity, high organic matter content, and strong hydrocarbon generation ability, show great potential for shale gas exploration and development Zou et al., 2010;Dong et al., 2015a;Zou et al., 2015;Dong et al., 2016;Wang et al., 2016;Zou et al., 2016;Dong et al., 2018;Ma, 2019;Qiu and Zou, 2020a;Qiu and Zou, 2020b;Jiang et al., 2020). The Lower Silurian Longmaxi Formation is one of the most important strata for shale gas exploration and development in South China, which has successfully produced commercial shale gas flows since 2012 and was appraised as one of the national marine shale gas demonstration zones in China (Dai et al., 2014;Guo and Zhang, 2014;Chen et al., 2020;Liu et al., 2021a).
As the most important shale gas exploration target strata, the Longmaxi Formation has attracted interest from numerous petroleum geologists. Increasing numbers of studies have confirmed that the Longmaxi shale exhibits strong heterogeneity, which is the key reason that led to different quality shale reservoirs formed in the Longmaxi Formation (Chen et al., 2015a;Liu et al., 2020a;Liu et al., 2020b;Liu et al., 2021b). Due to the strong heterogeneity of shale reservoirs from the macroscale to microscale, it is difficult to predict the effective shale gas reservoirs (Chen et al., 2015b;Tang et al., 2016). However, as the basic unit of shale, a shale lithofacies constitutes a shale reservoir, which reflects the geochemical, geological, and petrophysical information (Tang et al., 2016). Based on outcrops, drilling cores, thin sections, geochemical data, well logs, and seismic data, many scholars have researched the shale lithofacies of the Longmaxi Formation in the Sichuan Basin. However, most of these studies have focused on the classifications, descriptions, and comparisons of these shale lithofacies (Tang et al., 2016;Ou et al., 2018;Zhang et al., 2018;Hu et al., 2019;Wang et al., 2020b;Feng et al., 2020), and have rarely involved the temporal and spatial evolution of these shale lithofacies .
In this paper, the organic-rich shale section in the Long11 submember of the Longmaxi Formation was analyzed in the Changning area in the southern Sichuan Basin. Fine classifications were carried out for the shale lithofacies types by using core observations, thinsection analysis, and X-ray diffraction (XRD). The characteristics, vertical development patterns, and spatial distributions of shale lithofacies and main controlling factors of shale lithofacies development in the Changning area were analyzed.

GEOLOGICAL SETTING
The Sichuan Basin, which is located on the southwestern margin of the Yangtze Platform, is a complex marine and terrestrial superposed basin with stratigraphical sequence of Presinian metamorphic rocks as the basement, marine carbonate rocks from the Sinian to Middle Triassic, and continental clastic rocks from the Late Triassic to Eocene (Wang et al., 2002). From the Late Ordovician to Early Silurian, the Yangtze Plate collided with Cathysia, which resulted with the uplift of the Yangtze Plate in the northwest and downward deformation in the southeastern margin, leading to a deep-water sedimentary environment formed in the Upper Yangtze Platform (referred to as the Yangtze Sea by Li et al., 2021) (Figure 1A). Bounded by the Chuanzhong uplift, Qianzhong uplift, and Xuefeng uplift and separated by the interior submarine high of the Hunan-Hubei Arch, the Yangtze Sea evolved from a shallow carbonate platform into a semi-isolated and siliciclastic-dominated cratonic basin from the Late Ordovician to Early Silurian . During the Early Silurian, due to the restricted exchange of water masses with the open ocean, the Yangtze Sea developed anoxic, stagnant, low-energy bottom waters, especially within local depressions such as the Chuandong and Chuannan depocenters (Wang et al., 1993;Liang et al., 2009;Zou et al., 2018), which led to the organicrich shales of Longmaxi Formation being widely deposited in the Yangtze Platform, especially in the Chuandong and Chuannan depocenters ( Figure 1C) (Guo et al., 2004;Mou et al., 2011). The black shale of the Longmaxi Formation varies from 20 to 100 m in thickness in the Sichuan Basin ( Figure 1C).

Petrographic Observations
The targeted research area is the Changning area in the southern Sichuan Basin with 81 core samples from the Longmaxi shale collected from three wells (e.g. N11, N15, and Y2). Petrographic observations, including detailed core descriptions and thinsection observations, were conducted for this study.

Geochemical Analyses
Seven hundred ninety-two XRD data points for the Longmaxi shale from 10 wells were collected for use in this study. In addition, a total of 81 core samples were also collected from Wells N11, N15, and Y2 for geochemical analyses, including total organic carbon (TOC) content tests, XRD, major element tests, and trace element tests.
For the TOC measurements, all collected samples were leached with excess ∼20% hydrochloric acid to remove the carbonate phases and were washed in distilled water prior to analysis. The dried residues and non-acidified samples were measured with a LECO CS230 carbon sulfur analyzer at the Natural Gas Geology Key Laboratory of Sichuan Province at Southwest Petroleum University.
The major element and trace element tests and quantitative analysis of whole-rock minerals by using XRD were conducted at the State Key Laboratory Oil and Gas Reservoir Geology and Exploration at Southwest Petroleum University. For the major element and trace element measurements, dilithium tetraborate was used to melt the samples, ammonium nitrate was used as the oxidant, lithium oxide and a small amount of lithium bromide were used as the flux and mold release agent, an automatic fusion machine was used for fusion at a temperature range of 1,150°C-1,250°C, glass sample wafers were made and measured with an X-ray fluorescence spectrophotometer, and the contents of the major and trace elements were calculated separately according to their fluorescence intensities . In addition, a Rigaku SmartLab9 X-ray diffractometer was used for the quantitative analysis of wholerock minerals.

Siliceous Shale (S-1)
The siliceous shale (S-1) is mainly composed of silicate with contents of 75%-90%, and the clay minerals and carbonate minerals content are both less than 25% (Figure 2). This shale lithofacies is characterized by well-developed quartz lamination (detrital quartz), abundant quartz particles recrystallized from biogenic silica ( Figures 3A, C), and abundant graptolites with the random arrangement ( Figure 3B). This lithofacies is mainly developed in the Long11-1 section, Long11-2 section, and Long11-3 section in the Changning area.

Mixed Siliceous Shale (S-2)
The mixed siliceous shale (S-2) is rich in silicate with contents ranging from 50% to 75%, and poor in clay and carbonate minerals with contents both less than 25% ( Figure 2). This lithofacies is massive with quartz, dolomite, calcite, and a small amount of uniformly distributed flaky minerals ( Figure 4D). Faint lamination can be occasionally observed in this lithofacies ( Figure 4D). Most of the quartz particles are authigenic quartz that recrystallized from biogenic silica. Mixed siliceous shale (S-2) is the lithofacies type with the highest developmental frequency in the study area and is commonly developed in the Long11 sub-member, especially concentrated in the Long11-1 to Long11-3 sections.

Carbonate/Siliceous Shale (M-1)
The carbonate/siliceous shale (M-1) is characterized by low contents of clay minerals (<25%), moderate contents of carbonate minerals, and silicate (25%-50%) ( Figure 2). This lithofacies, which is characterized by well-developed horizontal lamination, occurs in the form of thin beds that are interbedded with other lithofacies ( Figure 4B). Bentonite layers can be found in this lithofacies. Carbonate/siliceous shale (M-1) is not common in the Longmaxi Formation and is only developed at the top of the Long11 sub-member.

Mixed Shale (M-2)
The mixed shale (M-2) displays a massive structure with intricately distributed quartz, calcite, and clay minerals ( Figures 4C, I). This lithofacies contains approximately equal amounts of silicate, clay, and carbonate minerals, which range from 25% to 50% (Figure 2). Mixed shale (M-2) shows a relatively higher development frequency in the Longmaxi Formation and mainly developed in the upper part of the Long11 sub-member in the form of thin beds.

Argillaceous/Siliceous Shale (M-4)
The argillaceous/siliceous shale (M-4) is poor in carbonate minerals with a content below 25%. This lithofacies displays a massive structure with siltstone beds developed. The clay minerals and silicate contents both range from 25% to 50% in argillaceous/siliceous shale (M-4), which lead to the abundant quartz and clay minerals observed under the microscope ( Figure 4G). Argillaceous/siliceous shale (M-4) is mainly developed in the upper part of the Long11 submember.

Vertical Variations of Shale Lithofacies
Vertically, the shale lithofacies exhibit a clear development pattern in the Long11 sub-member in the study area. The carbonate shale association + mixed shale association dominate the Long11-1 section. The siliceous shale association is mainly developed in Long11-2 section, Long11-3 section, and the lower part of Long11-4 section, and the mixed shale association + siliceous shale association is  Figure 5 shows the vertical distributions of the shale lithofacies in the Long11 sub-member of well N15. Mixed carbonate shale (C-2) is mainly developed in the Long11-1 section and accounts for 70% of all developed lithofacies in this section. The siliceous shale association (S) is dominant in the Long11-2 section, in which the mixed siliceous shale (S-2) accounts for more than 80% of all developed lithofacies in this section. Mixed siliceous shale (S-2) and argillaceous/siliceous shale (M-4) are the main lithofacies in the Long11-3 section, which account for 71% and 29%, respectively. In the Long11-4 section, the siliceous shale association (S) is mainly developed in the lower part, and the mixed shale association (M) is mainly developed in the upper part, which account for 53% and 47%, respectively ( Figure 5).

Lateral Variations of Shale Lithofacies
The west-east cross section (Well Y2-Well Nx2-Well N11-Well N17-Well N16) was used to analyze the lateral distribution characteristics of the shale lithofacies in the Long11 submember of the Longmaxi Formation in the Changning area.
The results show that the lithofacies exhibit significant variations in their regional distributions ( Figure 6). In the Long11-1 section, the shales are thicker in the west and thinner in the east. The western and eastern areas are dominated by mixed siliceous shale (S-2), while the middle part of the study area is mainly occupied by carbonate-rich siliceous shale (S-3). The shale thicknesses do not change much across the entire area in the Long11-2 section. The siliceous shale association (S) dominates the whole area in this section. From west to east, the shale lithofacies change from mixed siliceous shale (S-2) to siliceous shale (S-1) and then Frontiers in Earth Science | www.frontiersin.org November 2021 | Volume 9 | Article 775657 change to carbonate-rich siliceous shale (S-3). The shale thicknesses do not vary significantly across the whole area in the Long11-3 section. The shale lithofacies gradually change from a siliceous shale association (S) to siliceous shale association (S) + mixed shale association (M) from west to east. Mixed siliceous shale (S-2) and clay-rich siliceous shale (S-4) dominate the western area and then change to siliceous shale (S-1) in the middle and change to clay-rich siliceous shale (C-1) interbedded with clay-rich siliceous shale (S-4) in well N17. In the east, it is dominated by mixed siliceous shale (S-2) and clay-rich siliceous shale (S-4) in the Long11-3 section. The shale thicknesses exhibit a thinning trend from west to east in the Long11-4 section. Clayrich siliceous shale (S-4) dominates the whole area except for carbonate/siliceous shale (M-1) and argillaceous/siliceous shale (M-4), which are well developed in the west ( Figure 6).

Spatial Distributions of Shale Lithofacies
The spatial distribution of the shale lithofacies shows obvious variations in different sections of the Long11 sub-member (Figure 7). During the period of the Long11-1 section, mixed siliceous shale (S-2) dominated most areas of the Changning area, except that carbonate-rich siliceous shale (S-3) occupied the east area and carbonate/siliceous shale (M-1) were concentrated in the middle area ( Figure 7A). During the period of the Long11-2 section, siliceous shale association (S) was developed in the whole area. Mixed siliceous shale Frontiers in Earth Science | www.frontiersin.org November 2021 | Volume 9 | Article 775657 (S-2) occupied the most area. Siliceous shale (S-1) was developed in the northwest area, and carbonate-rich siliceous shale (S-3) was widely distributed in the east area ( Figure 7B). During the period of the Long11-3 section, the whole area was dominated by mixed siliceous shale (S-2) with siliceous shale (S-1) locally developed in the northwest and by argillaceous/siliceous shale (M-4) that developed in the middle area ( Figure 7C). During the period of the Long11-4 section, clay-rich siliceous shale (S-4) and mixed siliceous shale (S-2) occupied the whole area ( Figure 7D).

Main Controlling Factors of Spatial and Temporal Variations in Shale Lithofacies
Previous research has focused on the main controlling factors of how shale lithofacies formed in the Longmaxi Formation in the Sichuan Basin and its surrounding areas (Zhao et al., 2016a;Tang et al., 2016;Wang et al., 2016;Wu et al., 2016;Lu et al., 2017;Liu et al., 2018;Wang et al., 2020b;Wang et al., 2021b). It is generally believed that volcanic activities, terrigenous supplies, sea-level changes, and redox conditions played an important role in controlling the development of the shale lithofacies types (Zhao et al., 2016a;Wang et al., 2018). However, the main controlling factors of the spatial and temporal distributions of the shale lithofacies were not involved in previous studies. This section focuses on the main geological factors that controlled the spatial and temporal distributions of the shale lithofacies in the Longmaxi Formation in the Changning area.

Paleogeomorphology
To study how the paleogeomorphology controlled the shale lithofacies distributions, we compared the shale lithofacies in different paleogeomorphology units during the period of the Long11-1 section and Long11-2 section. During the period of the Long11-1 section, the relatively high geomorphic area, which was located in the area of Well Y1 in the west, was dominated by mixed siliceous shale (S-2). The lowlying areas, which were located in the middle of the research area, were dominated by mixed siliceous shale (S-2) and carbonate/siliceous shale (M-1). The geomorphic highland in the east was dominated by carbonate-rich siliceous shale (S-3) and mixed siliceous shale (S-2) (Figure 8). During the period of the Long11-2 section, the relatively high geomorphic area in the west was dominated by siliceous shale (S-1) and mixed siliceous shale (S-2); the low-lying areas in the middle were dominated by mixed siliceous shale (S-2), and the geomorphic highland in the east was dominated by carbonate-rich siliceous shale (S-3) and mixed siliceous shale (S-2) (Figure 9). It is obvious that the shale lithofacies in the geomorphic highlands had high silicate contents, while these were low in the low-lying areas. According to previous research, the silica in the shales of the Longmaxi Formation mainly were derived from silica radiolarians, which mainly live in deep-water areas (Zhao et al., 2016b;Lu et al., 2018;Yang et al., 2018;Dong et al., 2019;Li et al., 2019;Liu et al., 2019;Yang et al., 2019; Guan et al., 2021;Xu et al., 2021). According to this conclusion, the shale lithofacies should be rich in siliceous content in the depressions (low-lying areas) but be lacking siliceous content in geomorphic highlands. However, the results of this research are obviously contrary. This result indicates that there may have been multiple silica sources in the shales of the Longmaxi Formation in the research area. The paleogeomorphology has a certain controlling effect on the distribution of the shale lithofacies. The control by the paleogeomorphology on the shale lithofacies may be related to the provenance system. The southeast and northwest areas are close to the provenance (Rong et al., 2003;Wang et al., 2020c). This led to large amounts of terrigenous siliceous materials coming from these two provenances, which formed high-silica shale together with biological silicon in the geomorphic highlands in the western and eastern areas. However, due to the small terrigenous influence, the contents of carbonate minerals and clay minerals increased in the low-lying areas, which eventually led to the development of shale lithofacies with relatively higher content of carbonate and clay minerals.

Relative Sea-Level Change
Surrounded by the Qianzhong Uplift, Chuanzhong Uplift, and Xuefeng Uplift, a semi-restricted sedimentary environment was formed and occupied most areas of the Sichuan Basin during the period of the Longmaxi Formation . Sea level changes directly affected the degrees of restriction and retention of the water mass and further controlled the development of the shale lithofacies. The Mo-TOC covariant relationship can effectively indicate the degree of restriction of a water mass in a sedimentary environment (Algeo and Lyons, 2006). Shales with high TOC and low Mo contents are easily deposited in restricted environments, while those deposited in open environments are generally characterized by low TOC and high Mo contents (Algeo and Lyons, 2006). The Mo-TOC covariant relationship in the Longmaxi shale shows that the degree of restriction of the sedimentary environment gradually improved from the Long11-1 section to the Long11-4 section in the Changning area ( Figure 10). Shale samples with TOC greater than 2% and less than 4% are distributed in weakly restricted and moderately restricted environments, while nearly all shale samples with TOC greater  Transgression occurred during the time of the Long11-1 section and Long11-2 section, with sea level rising rapidly in the Changning area (Figure 11), which led to an improvement in the restriction degree. However, the sedimentary environment gradually changed from a carbonate platform to shelf environment from the period of the Guanyinqiao section of the Upper Ordovician Wufeng Formation to the period of the Long11-1 section. Therefore, lithofacies rich in carbonate (e.g., carbonate-rich siliceous shale and carbonate/ siliceous shales) were widely distributed in the research area during the period of the Long11-1 section and then changed to mixed siliceous shale (S-2) and siliceous shale (S-1) during the period of the Long11-2 section. During the period of the Long11-3 section, the sea level was relatively higher (Figure 11), and the restricted degree of the water mass was further improved, which caused the shale lithofacies to be more stable than that of the Long11-2 section. Mixed siliceous shale (S-2) dominated the study area during the Long11-3 section. Sea levels began to fall during the period of the Long11-4 section (Figure 11), which caused the terrestrial inputs to increase, and resulted in the domination of mixed siliceous shale (S-2) + argillaceous/siliceous shale (M-4) in the Long11-4 section compared with the Long11-3 section.

Sedimentary Model of Shale Lithofacies Evolution
The paleogeomorphology exhibits depressions (low-lying areas) and geomorphic highlands that developed during the period of the Longmaxi Formation in the study area. The geomorphic highland areas were located in the west and east. The depression (low-lying area) was located in the middle of the study area. After the Gondwana Glaciation in the Hirnantian (Late Ordovician), temperatures began to rise and glaciers began to melt, leading to a large-scale transgression occurring during the Early Silurian on the Upper Yangtze Platform, which resulted in the sedimentary environment changing to a restricted anoxic environment (Brenchley et al., 2001;Yan et al., 2021;Chen et al., 2015b;Chen et al., 2021;Wei et al., 2021). During the period of the Long11-1 section, the transgression began with relatively low sea levels, and the sedimentary water mass was weakly restricted (Yan et al., 2021). The siliceous shale association (S) + mixed shale association (M) were mainly developed during this period. Mixed siliceous shale (S-2) and carbonate-rich siliceous shale (S-3) were mainly distributed in the geomorphic highland areas, and the carbonate/siliceous shale (M-1) was mainly developed in the depression (low-lying area) in the middle of the study area during the period of the Long11-1 section. As the transgression continued and sea levels rose, the sedimentary environment became weakly restricted-moderately restricted during the period of the Long11-2 section. The siliceous shale association (S) dominated the study area, with siliceous shale (S-1) mainly  Frontiers in Earth Science | www.frontiersin.org November 2021 | Volume 9 | Article 775657 distributed in the geomorphic highland areas in the west, carbonaterich siliceous shale (S-3) developed in the geomorphic highland areas in the east (higher carbonate content), and mixed siliceous shale (S-2) developed in the depression (low-lying area) in the middle area. During the period of the Long11-3 section, the transgression reached its peak and resulted in a moderately restricted sedimentary environment that dominated the study area. The mixed siliceous shale (S-2) occupied the largest area, with siliceous shales (S-1) distributed sporadically in the geomorphic highland area in the west, and argillaceous/carbonate shales (M-3) developed in the depression (low-lying area) in the middle area. During the early stage of the Long11-4 section, the sedimentary environment was still moderately restricted in the research area. Regression occurred at the late stage of the Long11-4 section. The whole area was dominated by clay-rich siliceous shale (S-4), except that the eastern and middle areas were mainly dominated by mixed siliceous shale (S-2) ( Figure 12).
CONCLUSION 1) Seven main shale lithofacies were identified in the Long11 sub-member of the Longmaxi Formation in the Changning area, i.e., siliceous shale (S-1), mixed siliceous shale (S-2), carbonate-rich siliceous shale (S-3), clay-rich siliceous shale (S-4), carbonate/siliceous shale (M-1), mixed shale (M-2), and argillaceous/siliceous shale (M-4). 2) Vertically, the carbonate shale association + mixed shale association, siliceous shale association, and mixed shale association + siliceous shale association are developed from the bottom to top of the Long11 sub-member of the Longmaxi Formation in the Changning area. The spatial distribution of the shale lithofacies shows clear variations in the Long11 sub-member. Compared to the shale lithofacies in the central area, the shale lithofacies changes rapidly in the eastern and western parts of the study area. The lithofacies are rich in silicate in the eastern and western areas while rich in carbonate and clay in the middle area.
3) The spatial and temporal distributions of the shale lithofacies are mainly controlled by paleogeomorphology and relative sea-level changes. The geomorphic highland area is dominated by siliceous shale associations, e.g., siliceous shale (S-1), mixed siliceous shale (S-2), and carbonate-rich siliceous shale (S-3). The depression (low-lying area) is mainly dominated by carbonate/siliceous shale (M-1) and mixed siliceous shale (S-2) in the Changning area.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this article will be made available by the authors without undue reservation.

AUTHOR CONTRIBUTIONS
QT contributed as the major author of the article. LZ and LC conceived the project. XT collected the samples. GW analyzed the samples. All authors contributed to the article and approved the submitted version. Frontiers in Earth Science | www.frontiersin.org November 2021 | Volume 9 | Article 775657