During the Late Ordovician to the Early Silurian, graptolite organisms were flourishing in oceans. With a wide variety, wide distribution, and rapid evolution, graptolites have become the standard fossils for the study of biostratigraphy in the Sichuan Basin and its surrounding areas^[1]. In the last decades, systematic studies have been carried out on the division of graptolite zones^[2,3], distribution of different graptolite zones^[3], sedimentary environment and paleo-geographic setting for formation of the graptolite zones^[4], significance of graptolite for shale gas^[5], lithostratigraphic division and correlation^[6,7], and paleo-water oxidation-reduction conditions^[8] for formation of the graptolite zones in Ordovician Wufeng Formation-Silurian Longmaxi Formation in the Upper Yangtze area. Through the studies, researchers considered that the Wufeng Formation-Longmaxi Formation in the Upper Yangtze area can be divided into the Long-1 Member and Long-2 Member, of which the Long-1 Member can be further divided into 2 submembers and 4 production sublayers^[6]. There are mainly 13 graptolite zones in the Wufeng Formation-Longmaxi Formation, of which 4 graptolite zones are in the Wufeng Formation and 9 graptolite zones are in the Longmaxi Formation. The graptolite zones were formed in oxygen-deficient and anaerobic water environments, under relatively warm and humid climate^[8]. The graptolite shale is mainly distributed in areas of Chongqing-Sichuan and Yunnan, Mabian- Xichang, Dabashan foreland^[3], etc.

However, there are still three problems in the study of the biostratigraphy of the Wufeng Formation-Longmaxi Formation in the Sichuan Basin and its surrounding areas. The first one is the detailed graptolite composition in different graptolite zones, the lithology and electrical property of shale, and the distribution of shale in the Wufeng Formation-Longmaxi Formation. The second is the mineral composition, total organic carbon content (TOC) and lamina features in different graptolite zones of the Wufeng Formation-Longmaxi Formation. The third is the mechanism of quality difference of shale reservoirs in different graptolite zones of the Wufeng Formation- Longmaxi Formation. The solutions to these three problems are not only the keys to the division and correlation of chronostratigraphy in the whole region, but also the basis to determine the high-quality shale sections for shale gas exploration and development. In this paper, the graptolite composition, lithology and electrical property of shale, and the planar distribution of shale in graptolite zones of the Wufeng-Longmaxi Formation in the Sichuan Basin and its surrounding areas were examined. In addition, a preliminary discussion has been made on the formation settings, genesis and reasons of differences in reservoir properties of shale layers in different graptolite zones.

In the Sichuan Basin and its periphery, the Wufeng Formation-Longmaxi Formation is well developed (Fig. 1). The lower part of the Wufeng Formation is a large set of black shale, intercalated with multiple thin layers of volcanic ash deposits. The upper part of the Wufeng Formation is composed of limestone or marl, with rich Hirnantia Fauna fossils^[9]. The lower part of the Longmaxi Formation consists of black, grayish black thin-layered shale or massive shale, with abundant laminated structures and fractures^[10,11]. The upper part of the Longmaxi Formation is composed of grayish green, yellowish green shale and sandy shale, intercalated with siltstone or argillaceous limestone occasionally. The sandy content increases from bottom to top, forming upward coarsening sedimentary sequence in ascending order.

In the Sichuan Basin and surrounding areas, the Wufeng-Longmaxi Formation was formed during the collision stage between the Cathaysia Plate and the Yangtze Plate^[12]. After the Middle Ordovician, the Yangtze Plate entered into the tectonic evolution stage of foreland basin, and the Sichuan Basin and its surrounding areas were part of the post-uplift basin of the Yangtze foreland basin. In the Early Silurian, the compression in the southeast direction strengthened, the Sichuan Basin and its surrounding areas uplifted continuously, the paleo-uplift in the Central Sichuan Basin gradually expanded, the sea shrank in area and became shallower, and the sedimentary differentiation became stronger. During this stage, the Upper Yangtze area was sandwiched between the Central Sichuan paleo-uplift and the Central Guizhou- Xuefeng paleo-uplift, forming a semi-occluded stagnant sea basin.

In this study, cores of 17 wells and 7 outcrops were observed, and graptolite stratigraphy in 72 wells were divided and correlated. The layer in this study is the Wufeng-Longmaxi Formation. Systematic identification of graptolite fossils, division of graptolite zones, lithology description, TOC test, whole-rock X-ray diffraction analysis and major and trace element analysis were carried out on core samples from 17 typical wells and 4 outcrop profiles. Drilling and logging data, and graptolite stratigraphic division and correlation of 72 wells were analyzed. The drilling data were concentrated in the blocks of W202, Z205, L202, Changning, Y103, Fuling and Wuxi areas. The drilling data included logging data, core logging data and TOC analysis data. The outcrop profiles were mainly distributed in Changning, Wuxi and Xianfeng areas. The data mainly include lithology description data, graptolite fossil data, TOC, and major and trace element analysis data.

The study was carried out in 4 steps. First, the biostratigraphic framework of the study area was established through graptolite identification of the core samples and division of graptolite zones of 17 typical wells. Second, the logging and lithology identification templates for the graptolite zones were established through logging and lithology calibration of the graptolite zones in the 17 typical wells. Third, biostratigraphic frameworks of 72 wells were worked out by using the logging and lithology identification templates, and thickness contour map with the graptolite zone as unit was compiled. Fourth, petrological features, TOC, lamina and micro-fracture characteristics of the graptolite zones were examined to investigate the control of the graptolite zones on characteristics of the shale reservoirs.

There are 13 graptolite zones in the Wufeng-Longmaxi Formation in the Middle-Upper Yangtze area, 4 in Wufeng Formation and 9 in Longmaxi Formation. From bottom to top, the Katian Wufeng Formation of Ordovician has graptolite zones of Dicellograptus complanatus (WF1), Dicellograptus complexus (WF2) and Paraorthograptus pacificus (WF3)^[2,3]. The graptolite zone WF3 can be further subdivided into lower subzones, Tangyagraptus typicus and Diceratograptus mirus. The Hirnantian Wufeng Formation has one graptolite zone of Metabolograptus extraordinarius (WF4), and the Hirnantian Longmaxi Formation has one graptolite zone of Metabolograptus persculptus (LM1). From bottom to top, the Rhuddanian of Longmaxi Formation has 4 graptolite zones, Akidograptus ascensus (LM2), Parakidograptus acuminatus (LM3), Cystograptus vesiculosus (LM4) and Coronograptus cyphus (LM5). The Aeronian stage of Longmaxi Formation has 3 graptolite zones, Demirastrites triangulatus (LM6), Lituigraptus convolutus (LM7) and Stimulograptus sedgwickii (LM8). The Telychian stage of Longmaxi Formation has 1 graptolite zone, Spirograptus guerichi (LM9).

In most area of the Sichuan Basin and its surrounding areas, the Ordovician Wufeng Formation has graptolite zones WF1-WF3, while the graptolite zone WF1 is absent in southern Sichuan, and the Ordovician Longmaxi Formation has graptolite zone LM1. There are significant differences in the characteristic molecular composition of different graptolite zones (Fig. 2). The characteristic molecules of the graptolite zone Dicellograptus complexus (WF2) mainly include Appendispinograptus longispinus (Fig. 2a), Dicellograptus ornatus (Fig. 2b, 2c), Dicellograptus complexus and Amplexograptus latus (Fig. 2e), etc. The characteristic molecules of the graptolite zone Paraorthograptus pacificus (WF3) include Paraorthograptus pacificus (Fig. 2f, 2h), Rectograptus abbreviates, Dicellograptus minor (Fig. 2g) and Tangyagraptus typicus (Fig. 2d, 2i), etc. The characteristic molecules of graptolite zone Metabolograptus extraordinarius (WF4) are mainly Metabolograptus extraordinarius, etc. During this period, due to the cooling of global climate, a large number of graptolites died, while the Hirnantia fauna rich in brachiopods and crustaceans flourished ^{[13,14,15,16]} (Fig. 2j-2l). The characteristic molecules of graptolite zone Metabolograptus persculptus (LM1) mainly include Avtograptus ex gr. avitus (Fig. 3a) and Avtograptus avitus (Fig. 3b), etc. Core samples of 17 typical wells and 4 outcrop profiles in the Sichuan Basin and its surrounding areas all have complete graptolite zones WF4 and LM1, and there is no erosion or sedimentary hiatus seen between core samples, which is consistent with results of previous studies^[13], indicating that the Guanyinqiao Formation and Longmaxi Formation deposited continuously.

The Longmaxi Formation has graptolite zones LM2- LM9 with obvious differences in the characteristic molecular composition (Fig. 3). The characteristic molecules of the graptolite zone Akidograptus ascensus (LM2) include A.ascensus, P.praematurus, N.anjiensis, N.bicaudatus, Neodiplograptus modestus, and Atavograptus atavus, etc. The characteristic molecules of the graptolite zone Parakidograptus acuminatus (LM3) are Parakidograptus acuminatus, Cystograptus ancestralis, Hirsutograptus sinitzini, Agetograptus primus and Hirsutograptus comanits, etc. The characteristic molecules of graptolite zone Cystograptus vesiculosus (LM4) include Dimorphograptus nankingensis (Fig. 3c, 3e), Pseudorthograptus sp. (Fig. 3d), and Cystograptus vesiculosus (Fig. 3f, 3g), etc. The characteristic molecules of the graptolite zone Coronograptus cyphus (LM5) consist of Coronograptus cf. cyphus (Fig. 3h), Coronograptus cyphus (Fig. 3i), etc. The characteristic molecules of the graptolite zone Demirastrites triangulatus (LM6) include Rastrites guizhouensis (Fig. 3j), Pernerograptus difformis, Coronograptus cf. gregarius (Fig. 3k, 3m), Coronograptus gregarius, Glyptograptus cf. tamaricus (Fig. 3l), Campograptus communis, Falcatograptus falcatus and Demirastrite triangulatus, etc. The characteristic molecules of the graptolite zone Lituigraptus convolutus (LM7) include Lituigraptus convolutus (Fig. 4b, 4d, 4e), Cephalograptus cometa (Fig. 4a) and Torquigraptus decipiens (Fig. 4f), etc. The characteristic molecules of the graptolite zone Stimulograptus sedgwickii (LM8) include Clinoclimacograptus retroversus, Pseudoretiolites daironi and Stimulograptus cf. sedgwickii (Fig. 4c), etc. The characteristic molecules of the graptolite zone Spirograptus guerichi (LM9) are Oktavites sp. (Fig. 4g), Stimulograptus sedgwickii (Fig. 4h), Spirograputs guerichi (Fig. 4i, 4j) and Torquigraptus planus (Fig. 4k), etc.

The characteristics of lithology and electrical property of different graptolite zones in the Wufeng-Longmaxi Formation in the Sichuan Basin and its surrounding areas can be compared across the whole area (Table 1). Among them, the graptolite zones WF1-WF3 correspond to the graptolite shale sections in the lower part of the Wufeng Formation, and WF4 corresponds to the Guanyinqiao Member. The graptolite zones LM1-LM5 correspond to the first submember of the Long-1 Member of Longmaxi Formation (hereinafter referred to as Long-1₁). The graptolite zone LM1 corresponds to the Long-1₁¹ sublayer, the graptolite zones LM2-LM3 correspond to the Long-1₁² sublayer, the graptolite zone LM4 corresponds to the Long-1₁³ sublayer, the graptolite zone LM5 corresponds to the Long-1₁⁴ sublayer, the graptolite zones LM6-LM8 correspond to the Long-1₂ submember, and the graptolite zone LM9 corresponds to the Long-2 Member.

During the depositional period of the Wufeng-Longmaxi Formation, the Sichuan Basin and its surrounding areas was bounded by the Central Yunnan-Xuefeng paleo-uplift in the southeast, and Central Sichuan paleo-uplift in the northwest; and black shale was distributed between the two paleo-uplifts. During this period, there were two major depositional areas in the southwest and northeast. The graptolite zones in the southwestern depositional area were thinner, and the graptolite zones in the northeastern depositional area were thicker. Inside the two major depositional areas, the strata thicken first and then thin from southeast to northwest. The Wufeng Formation is thinner, while Longmaxi Formation is thicker. The depocenters are mainly distributed inside the basin, with obvious vertical inheritance (Fig. 5).

The shale layers of graptolite zones WF1-WF4 are 1.5-13.0 m in thickness, and 6-12 m in depocenter around Well N201. The shale layer in graptolite zone LM1 is 1-4 m in thickness, and the boundary between the southwest and northeast depositional areas is located in the area from Well WJ1-Well R201-Jiangjin-Qijiang (Fig. 5a). In the depocenter in the southwest located in Changning area, the shale layer of graptolite zone LM1 is greater than 3 m thick. In the depocenter in the northeast located in the Zhongxian-Wuxi area, the shale layer of graptolite zone LM1 is greater than 4 m thick, and the Fuling area is located on the slope of the depocenter. The shale layers in graptolite zones LM2-LM3 are 2-10 m thick. The boundary of the southwest and northeast regions is located in the area of Well H12-Chongqing-NY1 (Fig. 5b). The depocenter in the southwest is located in the Changning-R201 well area, and the shale is 4-8 m in thickness. The depocenter in the northeast is located in the Zhongxian-Wuxi area, where the shale layer in graptolite zones LM2-LM3 is more than 8 m thick, and the Fuling area is located on the slope of the depocenter. The shale layer in graptolite zone LM4 is 4-12 m thick, and the boundary of southwest and northeast regions is along Well He 12-Chongqing-Well Nanye 1 (Fig. 5c). The depocenter in the southwest is located in Changning area, the shale layer in graptolite zone LM4 is more than 6 m thick, and the Luzhou area is located on the slope. The depocenter in the northeast is located in the Zhongxian-Wuxi area, the shale in graptolite zone LM4 is greater than 8 m in thickness, and the Fuling area is still located on the slope of the depocenter. The shale layer in graptolite zone LM5 is 4-24 m thick, and the boundary of the southwest and northeast regions is along Well H12-Chongqing-Well NY1 (Fig. 5d). The depocenter in the southwest is located in the Changning-Chishui area, the shale layer in graptolite zone LM5 is 10-16 m thick. The second depocenter is located in the Zhongxian-Wuxi area, where the shale in graptolite zone LM5 is 12-24 m thick, and the Fuling area is located on the slope of the depocenter.The shale layer in graptolite zones LM6-LM8 is 20-160 m thick. There are two major depositional areas in the southwest and northeast of the basin, with the boundary located along Well WJ1-Qijiang (Fig. 5e). The depocenter in the southwest is located in Changning area, where the shale is 40-100 m thick. The depocenter in the northeast is located in Zhongxian-Wuxi area, where the shale layer in the graptolite zones LM6-LM8 is 80-160 m thick. The shale layer in the graptolite zone LM9 is 50-500 m thick and the boundary of the southwest and northeast depositional areas is located along Daxian-Well Fuling (Fig. 5f). The depocenter if the southwest is located in the Z202 well area-YS1 well area-Jiangjin area, which can be further subdivided into three secondary depocenters, namely Z202 well block, YS1 well block and Jiangjin area, with a thickness greater than 400 m. The depocenter inthe northeast is located in the JS1 well area, with a thickness greater than 250 m.

The whole-rock X-ray diffraction test results of samples from Wufeng-Longmaxi Formation in the Sichuan Basin and its surrounding areas show that the black shale samples from Well L203 have quartz (average content of 48.8%), carbonate (with an average content of 20.3%) and clay (with an average content of 22.8%) as primary mineral components, and plagioclase (with an average content of 2.5%) and pyrite (with an average content of 2.7%) as secondary minerals (Table 2). Black shale layers in different graptolite zones differ widely in mineral composition. In general, the quartz content of the shale in the graptolite zones LM1-LM4 is greater than 50%. Similarly, the TOC test results of samples from Well L203 show that the TOC value of the black shale ranges from 0.6% to 5.8%, with an average value of 2.3%. In general, the TOC value of the shale in the graptolite zones LM1-LM4 is greater than 2%.

Quartz, carbonate and clay minerals are used as three end-members to classify the black shale samples. When the mass fraction of one mineral component is greater than 50%, this component name is taken as the lithology name; if the mass fractions of the three primary components are all less than 50%, the sample is called hybrid shale^[17]. The mass fraction of quartz in siliceous shale is more than 50%, the mass fraction of carbonate in calcareous shale is more than 50%, the mass fraction of clay in clayey shale is more than 50%, and the mass fractions of the three minerals in hybrid shale is less than 50%. With TOC values of 4% and 2% as dividing points^[18], the black shale samples can be further divided into organic-rich shale with TOC of greater than 4%, organic-containing shale with TOC of 2% to 4%, and organic-lean shale with TOC less than 2%. The results show that the black shale samples of the Wufeng-Longmaxi Formation in the Sichuan Basin and its surrounding areas are mainly siliceous shale and hybrid shale. Among them, the graptolite zones LM1-LM5 have mainly organic-rich siliceous shale and organic-containing siliceous shale, while the other graptolite zones have largely organic-containing or organic-lean hybird shale.

In the Wufeng-Longmaxi Formation of Sichuan Basin and its surrounding areas, there are two main types of bedding, massive bedding and horizontal bedding^[10,19], and two types of fractures, bedding fractures and non-bedding fractures^[11]. All the graptolite zones have differences in bedding type and fracture density. From bottom to top, the Wufeng Formation has bioturbated massive bedding, transition horizontal bedding and homogeneous massive bedding (Fig. 6). The bioturbated homogeneous bedding turns up in the lower part of graptolite zone WF2, with intensity of bioturbation weakening from bottom to top. This graptolite zone is characterized by low fracture density and a small number of bedding fractures. The transition horizontal bedding occurs in WF3 and the upper part of WF2. The transition beds increase gradually in thickness from bottom to top. Compared with the shale in the lower part of WF2, the shale layers in the upper part of graptolite zone WF2 and WF3 have much higher fracture density and bedding and non- bedding fractures well developed. The homogeneous massive bedding appears in the shale of graptolite zone WF4. The shale with homogeneous massive bedding contains rich bioclastics such as bivalves which represent the typical Hirnantian biotype assemblage. The shale layer of graptolite zone WF4 has low fracture density, and no bedding fractures and non-bedding fractures. The Longmaxi Formation has banded siltstone horizontal bedding, siltstone-mudstone transition horizontal bedding, and siltstone and mudstone interlaminated horizontal bedding from bottom to top. The shale of graptolite zone LM1 has banded siltstone horizontal bedding, a large number of bedding and non-bedding fractures interweaving into networks. The shale of the graptolite zone LM2 has siltstone-mudstone transition horizontal bedding, bedding fractures with higher density, and non-bedding fractures with lower density. The shale of graptolite zones LM3 and above has siltstone and mudstone interlaminated horizontal bedding, fractures with lower density, and only a few bedding fractures. The different graptolite zones have differences in siltstone and mudstone interlaminated horizontal bedding. In the shale of graptolite zone LM3, the siltstone and mudstone interlaminated horizontal beddings turn up as thin siltstone-mudstone interbeds, with the thickness ratios of silt lamina to mud lamina of 1/3-1/2. In the shale of graptolite zone LM4, the siltstone and mudstone interlaminated horizontal beddings are siltstone-mudstone interbeds with equal thickness, with thickness ratio of wilt lamina to mud lamina of 1/2-1. In the shale of graptolite zones of LM5 and above, the siltstone-mudstone interbedded horizontal beddings feature thick siltstone and thin mudstone, with a thickness ratio of silt lamina to mud lamina of greater than 1.

There are organic pores, inorganic pores and micro- fractures in the black shale of the Wufeng-Longmaxi Formation in the Sichuan Basin and its surrounding areas^[19]. The inorganic pores are mainly quartz intercrystalline pores, and dissolution pores formed by the dissolution of carbonate minerals and a small amount of feldspar. The shale in graptolite zones WF2-WF3 of the Wufeng Formation has a surface porosity of 1.3%-2.1%, and the shale in the graptolite zone WF4 has a surface porosity of 0.7%. The shale layers in different graptolite zones of the Longmaxi Formation decrease in surface porosity gradually from bottom to top. Specifically, the shales in the graptolite zones LM1, LM2-LM3, LM4, LM5, LM6-LM8 and LM9 have surface porosities of 1.7%-3.3%, 0.3%-1.2%, 0.2%-0.7%, 0.1%-0.5%, and less than 0.1% respectively. With the decrease of total surface porosity, the surface porosity values of organic pore, inorganic pore and micro-fracture decrease accordingly.

Shale layers in different graptolite zones were developed under different paleo-climate, paleo-water salinity, paleo-water oxidation-reduction conditions, and sedimentation rate. In terms of paleo-climate, during the deposition of Wufeng-Longmaxi Formation, the Sichuan Basin and the surrounding areas experienced changes from warm and humid to cold and dry, warm and humid, and cold and dry climates^{[15, 20-22]}. During the depositional period of the graptolite zones WF2-WF3, the paleo-climate was warm and humid. During the depositional period of the graptolite zone WF4 (Guanyinqiao Member), the paleo-climate was cold and dry. During the depositional period of the graptolite zone LM1, the paleo-climate became warm and humid again. During the depositional period of the graptolite zones LM2-LM3, the paleo-temperature was the highest, and then decreased gradually. The paleo-water salinity during the depositional period of Wufeng-Longmaxi Formation was relatively low, but changed in different periods. During the depositional period of graptolite zones WF2-WF3, the paleo-salinity was relatively high, and then decreased gradually and then increased again. It reached the maximum in the depositional period of graptolite zones WF2-WF3, and decreased to the lowest in the depositional period of graptolite zone LM1, and then gradually increased. In terms of paleo-oxidation-reduction conditions, the water body during the depositional period of graptolite zones WF2-WF3 was oxidation-weak oxidation environment, and then changed gradually to oxygen-deficient/sulfidation environment. During the depositional period of graptolite zone WF4, the water body turned into oxygen enriched-oxygen deficient again. During the depositional period of graptolite zone LM1, the paleo-water was oxygen deficient, and then changed gradually to be oxygen-poor and oxidation^{[8, 23-25]}. In terms of paleo-sedimentation rate, the sedimentation rate of Wufeng-Longmaxi Formation increased gradually from bottom to top. The deposition of Wufeng Formation of 1.5-13.0 m thick took 3.19 Ma at the sedimentation rate of 0.5-4.1 m/Ma. The deposition of the graptolite zone LM1 of 1.0-4.5 m thick took 0.6 Ma at the sedimentation rate of 1.7-7.5 m/Ma. The deposition of the graptolite zones LM2-LM3 of 2-10 m thick took 1.36 Ma at the sedimentation rate of 1.5-7.4 m/Ma. The deposition of the graptolite zone LM4 of 2.0-14.5 m took 0.9 Ma at the sedimentation rate of 2.2-16.1 m/Ma. The deposition of 4-25 m thick graptolite zone LM5 took 0.8 Ma at the sedimentation rate of 5.0-31.2 m/Ma. The deposition of 20-160 m thick graptolite zones LM6-LM8 took 2.28 Ma at the sedimentation rate of 8.8-70.2 m/Ma. The deposition of 50-510 m thick graptolite zone LM9 took 0.36 Ma at the sedimentation rate of 138.9-1416.7 m/Ma.

From the Late Ordovician to the Early Silurian, the Sichuan Basin and its surrounding areas were a semi-closed epicontinental sea located near the ancient equator^[4]. The changes of paleo-climate caused the changes of rainfall, and led to changes of paleo-water salinity^[16]. During the depositional period of graptolite zones WF2-WF3, due to the warm and humid climate, rainfall increased, and the water body was gradually desalinated. During the depositional period of graptolite zone WF4, the climate became cold and dry and the rainfall decreased, consequently, the salinity of the water body increased. During the depositional period of graptolite zone LM1, the climate turned warm and humid again, and rainfall increased, causing the paleo-water salinity to decrease again. During the depositional period of the graptolite zone LM2 and later, as the paleo-climate gradually became cold and dry, rainfall decreased, and the paleo-water salinity increased gradually. The changes of paleo-climate also affected the paleo-water oxidation-reduction conditions. With warm and humid paleo-climate, a large amount of low-density warm fresh water fed into the basin, the convection initiation between surface water and bottom water was difficult, and the reducing condition of bottom water enhanced. The cold and dry climate condition caused the increase of surface water density, the up-down convection of water body strengthened, and the oxygen content of the bottom water body increased^[8].

The change in the sedimentation rate of the Wufeng- Longmaxi Formation in the Sichuan Basin and its surrounding areas is closely related to the tectonic events of the peripheral plates in this area. At the end of the Middle Ordovician, the Cathaysia Plate and the Yangtze Plate converged, and a foreland basin formed in the Middle-Upper Yangtze area. During the early stage of Silurian, the Middle-Upper Yangtze area had been in a tectonic setting of compression and contraction. In the Early Silurian, with the enhancement of compression from the south-east direction, the Sichuan Basin and its surrounding areas, as a part of the post-uplift basin of the Yangtze foreland basin, had been uplifting continuously, the central Sichuan paleo-uplift extended gradually, the sea shrank in area and became shallower, and the sedimentary differentiation intensified. In the Late Ordovician Katian to the Rhuddanian, the tectonic events of the peripheral plates were relatively weak with less terrigenous clastics, the basin deposits were dominated by internal sources, and the graptolite zones WF2-WF3 to LM5 deposited at low sedimentation rate. After the Early Silurian Aeronian, due to the enhanced tectonic events of the peripheral plates, the supply of terrigenous clastics increased, resulting in a significant increase in the content of clay minerals in the graptolite zones LM6-LM8 and LM9, and an increase in sedimentation rate.

The change in the sedimentation rate of the Wufeng- Longmaxi Formation in the Sichuan Basin and its surrounding areas not only leads to differences in the thickness and mineral composition of shale in different graptolite zones, but also a special stratigraphic distribution pattern. During the depositional period of the graptolite zones WF2-WF3 to LM5, the shale layers were thinner, and dominated by siliceous and carbonate minerals in composition, with low contents of clay minerals. The depocenter was located in the semi-deep-deep water shelf environment, and there was no sign of passing without deposition or short-term exposure in relatively shallow water area. The shale layers deposited during the depositional period of the graptolite zones LM6-LM9 are thicker, and although dominated by siliceous and carbonate minerals, increase in content of clay minerals significantly. During this period, the depocenter area was still located in the semi-deep-deep water shelf sedimentary environment, but there are some signs of passing without deposition in the relatively shallow water area. This distribution pattern is closely related to the genesis of sediments. In the shale of the Wufeng-Longmaxi Formation, siliceous minerals are mainly biogenic^[26,27], carbonate minerals are mainly bio-chemogenic or chemogenic^[28], and clay minerals are mainly of terrigenous clastic origin^[29]. During the depositional period of the graptolite zones WF2-WF3 to LM5, the fine-grained deposits were in-source origin. The shale layers in the semi-deep-deep shelf facies were thicker due to the clean water body; while the shale layers in the relatively shallow water areas were thinner due to the high content of terrigenous clastics. During the depositional period of the graptolite zones LM6-LM9, the fine-grained deposits was still dominated by in-source origin, but the supply from outside of the source increased. The semi-deep-deep water shelf facies had a relatively large accommodation space, so the shale layers were thicker. In comparison, the shallow water area had smaller accommodation space, so the shale layers in this area were thinner and had signs of passing without deposition or short-term exposure.

The Wufeng-Longmaxi Formation in the Sichuan Basin and its surrounding areas is dominated by quartz, carbonate, and clay in mineral composition. The shale in graptolite zones LM1-LM4 has quartz contents of greater than 50%, while shale in the other graptolite zones has quartz contents of less than 50%. The high siliceous content of the shale in graptolite zones LM1-LM4 is closely related to the extremely low supply rate of terrigenous clastics and warm and humid climate conditions during this period. The siliceous content in shale of graptolite zones LM1-LM4 is mostly biogenic origin^[26,27], and the siliceous organisms were mostly siliceous sponges and radiolarians^[30], etc. Previous studies have shown that organisms such as siliceous sponges and radiolarians mostly lived in clean water environment, and the supply of large amounts of terrestrial clastics might cause their suffocation and death^[31,32]. In the relatively warm and humid season, the large amounts of nutrients came from terrestrial fresh water or upwelling would likely lead to the bloom of siliceous organisms^[33], resulting in periodic silicon deposition^[34]. During the depositional period of graptolite zones LM1-LM4, the sedimentation rate was very low (only 1.7-16.1 m/Ma), and the amount of terrigenous clastics supplied was small, which was conducive to the growth of siliceous organisms. During the depositional periods of shale in other graptolite zones, the excessive supply of terrigenous clastics was not favorable for the survival of siliceous organisms, so the siliceous content of the shale layers decreased.

The shale in the graptolite zones LM1-LM4 of the Wufeng-Longmaxi Formation in the Sichuan Basin and its surrounding areas have high TOC values of greater than 2%, while shale in the other graptolite zones have TOC values of less than 2% generally. During the depositional period of the graptolite zones LM1-LM4, the climate was warm and humid, favorable for the growth and development of plankton such as phytoplankton and acritarch, so the primary productivity greatly improved, meanwhile the anaerobic-anoxic water body was conducive to the preservation of organic matter in large quantities^[35]. Meanwhile, at extremely low sedimentation rate during this period, the limited organic matter would not be diluted. Therefore, the shale in graptolite zones LM1-LM4 has higher TOC values. In contrast, during the deposition of the other graptolite zones, the water bodies were in the oxidized condition, and the climate was relatively cold and dry, which were not conducive to the generation and preservation of organic matter. In addition, the high sedimentation rate reduced further the abundance of organic matter. Therefore, the shale in these graptolite zones has lower TOC.

The lamina types of the shale in different graptolite zones of Wufeng-Longmaxi Formation in the Sichuan Basin and its periphery is obviously different, and their formation is closely related to the water oxidation-reduction conditions and sedimentation rates in different periods. During the formation period of the shale in the lower part of graptolite zone WF2, the water body of the basin was in a low-energy and oxygen-rich state^[8], and the sedimentation rate was extremely low, so a large number of organisms colonized for a long time^[36], forming strongly bioturbated massive bedding. During the formation period of the shale in graptolite zone WF4, the global climate became cooler, and the water body increased in oxygen content and hydrodynamic force, so the organisms such as shells grew in large numbers. The strong hydrodynamic force and biological activities strongly transformed the bottom sediments, giving rise to homogeneous massive bedding. During the formation period of the shale in the upper part of graptolite zone WF2 and WF3, due to the occlusion of water^[27], the supply of terrigenous clastics was severely insufficient, and seasonal changes in climate resulted in normal grading layers or reverse grading layers, and thus formed horizontal beddings. During the formation period of shale in graptolite zones LM1 and above, the marine environment was relatively open, and the water body was dominated by advection, bringing about different parallel beddings. At the same time, with the increase of sediment supply, banded siltstone horizontal bedding, siltstone-mudstone transition horizontal bedding, and siltstone and mudstone interlaminated horizontal bedding were formed successively, with sand/mud ratio and thickness of single sand layer increasing gradually^[37]. Due to high TOC, high siliceous content, high gas content and high porosity, the shale layer in graptolite zones LM1-LM4 of the Wufeng- Longmaxi Formation in the Sichuan Basin is a “sweet spot” for shale gas exploration and development^[38]. Its formation is attributed to the joint work of stable tectonic settings, low terrestrial clastic material supply, and warm and humid climate. Widely distributed black shale layers in the Lower Cambrian Qiongzhusi Formation of the Sichuan Basin and surrounding areas and the Wufeng- Longmaxi Formation of Middle-Lower Yangtze area^[3] will become important domains for shale gas exploration in the future. The formation and distribution of high-quality shale intervals may also be controlled by the tectonic settings, paleo-climate and supply of terrestrial clastics.

The graptolite zones WF1-WF3, WF4 and LM1-LM9 developed in the Wufeng-Longmaxi Formation of Sichuan Basin and its surrounding areas. With differences in lithological and electrical properties, the shales in different graptolite zones can be identified by lithological and electrical property characteristics. The shale in different graptolite zones is characterized by two major depocenters in the southwest and northeast, and the shale is concentrated between the Central Yunnan-Xuefeng paleo-uplift and the Central Sichuan paleo-uplift.

In different graptolite zones, the mineral composition, TOC and lamina types of shales are different. In the lower part of graptolite zones WF2 and WF4, organic-poor hybrid shale with massive bedding is well developed. In the upper part of graptolite zones WF2 and WF3, organic hybrid shale with horizontal bedding is well developed. In the graptolite zones LM1-LM4, organic-rich siliceous shale is well developed, with horizontal bedding. In the graptolite zones LM5-LM9, organic-poor hybrid shale with horizontal bedding is well developed.

The mineral composition, TOC and lamina types of shale are controlled by the paleo-climate, paleo-water oxidation-reduction conditions, and paleo-sedimentation rate. Depositing in oxygen-rich and high temperature water, the lower part of graptolite zone WF2 shale has massive bedding, low siliceous content and low TOC value. Depositing in cooler and oxygen-rich water, the WF4 shale has massive bedding, high calcareous content and low TOC value. Depositing in anoxic water at low sedimentation rate, organic-rich siliceous shale with horizontal bedding and high proportion of silt lamina developed in the upper part of WF2, WF3, and LM1-LM4. Depositing in oxygen-rich water at high sedimentation rate, the shale of graptolite zones LM5-LM9 has low TOC and siliceous content, and high proportion of silt lamina.

The names of the graptolite genus and species involved in this paper were all identified by Academician Chen Xu of the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences. The graptolite photos were provided by Guo Wei, Zhao Qun and Liang Feng of the PetroChina Research Institute of Petroleum Exploration and Development. During this study, we received careful guidance from Academicians Zou Caineng and Chen Xu. We express our gratitude to them for their support.