Sedimentation Models and Development Mechanisms of Organic-Rich Shales of the Lower Carboniferous Dawuba Formation: A Case Study in the Yaziluo Rift Trough, South of Guizhou Province, Southern China.

China has made a breakthrough in shale gas production in the deepwater shelf shales of the Lower Cambrian Qiongzhusi Formation and the Upper Ordovician-Early Silurian Wufeng-Longmaxi Formation. In recent years, active shale oil and gas shows have also been found in the shale system of the Lower Carboniferous Dawuba Formation in the Yaziluo rift trough, south of Guizhou province in Southern China, which was formed in the tensional geotectonic setting of the Palaeo-Tethys Ocean from the Devonian through the Carboniferous to the Permian. This tectonic background makes the sedimentary environments and organic matter accumulation mechanisms of Dawuba shales vastly different from deepwater shales. To better understand the deposition and organic matter accumulation mechanisms of marine shale developed in the rift trough, we carried out detailed field surveys and drilling data interpretation to study the lithological assemblage, organic matter, and elemental geochemical characteristics of Dawuba shales. The results show the following: (1) The study area is located in a platform-slope-basin depositional model like the Florida-Bahama platform-trough system of the west Atlantic margin, with a rapidly geomorphologic variation from basin to bank, dominated by a coastal sandstone and mudstone system in the northwest, a marlite and mudstone slope system around the rift trench (Liupanshui county), and a deep water fine-grained-turbidite system in the southeast (Ziyun county). (2) Major element (ME), trace element (TE), and rare earth element (REE) data indicate significant terrestrial source material input [total organic carbon (TOC) correlates well with Ti/Al], high deposition rates [mean (La/Yb)N of 1.41], and complex oxic-dysoxic conditions (U/Th mainly between 0 and 0.5), which illustrate substantial terrigenous sedimentary input and changes in redox conditions in deep water. (3) The input of organic matter from terrestrial sources in the study area is predominant compared with internal basin-originated organic matter, and the organic matter type is mainly Type II2 or Type III. Stable carbon isotope (δ13Ccarb) data of carbonate rocks also indicates that the widely developed upwelling in this region brings abundant nutrients, which also contributes to organic matter enrichment. Organic-rich shales exist in the Yaziluo rift trough under the influence of strong tensile action. The results of the study are essential for understanding the sedimentology and hydrocarbon exploration in similar rift trough areas within the Paleo-Tethys Ocean.

Introduction

The report of The National Shale Gas Resource Potential Evaluation published by the Ministry of Natural Resources shows that the geological resources of shale gas in Guizhou Province are 10.48 × 1012 m3, of which the prospective resources of shale gas in the Lower Carboniferous Dawuba Formation is 1.1 × 1012 m3, accounting for 10% of the total resources, which has a huge potential for shale gas development.[1−3] The shales of the Carboniferous Dawuba Formation in Guizhou province are mainly developed in the south part of Guizhou province, located near the Earth’s equator during the Late Paleozoic, when the Paleo-Tethys Ocean was subducting and dying toward the Laurasia continent, and the tectonic evolution was extremely complex.[4−7]

When the shales of the Early Cambrian Qiongzhusi Formation and Late Ordovician–Early Silurian Wufeng–Longmaxi Formations were deposited, the tectonics were relatively stable, and the anoxic reduction state of the water body environment was dominant, along with high primary productivity.[8−10] The shale sedimentary environments in this region are quite different from the widely distributed deepwater shelf-facies shales like the Lower Cambrian Cripplesi Formation and the Upper Ordovician–Lower Silurian Wufeng–Longmaxi Formation.[11,12] The shales of the Lower Carboniferous Dawuba Formation in the study area were deposited in a rift trough and are important hydrocarbon source rocks of the Carboniferous in southern China.[5,13,14] Due to the different environments of sedimentation and tectonic evolution, the development mechanism of organic-rich shales in southern Guizhou is different from that of the Sichuan Basin, and therefore, the later exploration and exploitation patterns are also different.[15,16] In the past few decades, many scholars have demonstrated the applicability of using major elements, trace elements, and rare earth elements as effective indicators for studying sedimentary environments[17−19] and to determine mechanisms controlling the organic matter enrichment in black shales.[5,20,21] The sedimentary environments and lithological assemblage changed very rapidly vertically and horizontally, which caused disagreement about the sedimentary environments and patterns in the study area. For example, Tian believed that the region is developed in a gently sloping sedimentary environment,[22] whereas Lu and Xin thought that this region belongs to a deepwater shelf-facies sedimentary environment.[23,24] However, none of these studies can explain the rapid changes in the spatial distribution and properties of shales in the study area. As more wells have been drilled in recent years and more geophysical data, such as wide-field electromagnetic, was obtained in this region, reconstruction of the sedimentation models of Dawuba shales in this region becomes possible.

In this paper, we studied the black shales of the Dawuba Formation in southern Guizhou Province. We conducted elemental geochemical analyses on typical outcrops and drilling samples to reveal the sedimentary environments of organic-rich shales. The organic geochemical test, major element (ME), trace element (TE), rare earth element (REE), and stable carbon isotope (δ13C) analyses were used to determine the organic matter input and enrichment mechanism of the black shale and establish a sedimentation model of organic-matter-rich shale accumulation in the Yaziluo rift trough of south Guizhou province, which provides a basis for revealing the rapid deposition and development mechanism of organic-matter-rich shale in the context of the complex tectonic evolution of the Paleo-Tethys Ocean.

Geological Setting

Structure Background

The study area is located in the northwestern part of the southwestern margin of the Upper Yangtze Block (Figure a), and regionally in the Right River Basin at the eastern end of the Paleo-Tethys tectonic belt during the Devonian-Permian, a rift basin developed at the intersection of the Tethys and the Pacific Coastal tectonic domains (Figure d)[25] called the Yaziluo rift trough. The rift trough extends in the NW–SE direction, is about 400 km long and 10–80 km wide, and covers an area of about 15 000 km2 (Figure b). The northern boundary of the rift trough is the Central Guizhou Uplift. The eastern border is the Southern Guizhou Depression, the western border the Eastern Yunnan Uplift, and the southern border the southwest Guizhou Depression and the Nanpanjiang Depression.[26−29] According to structural changes to the Yaziluo fault zone during the strike, the study area can be divided into three sections from the northwest to the southeast. The northwest section of the study area refers to the area from Weining to Liupanshui, the development of four deep faults, and the transition from carbonate platforms to carbonate slopes. The middle section of the study of Liuzhi of Guanling section developed two deep faults; the southeast section from Ziyun to Luodian evolved into deep fault structure transformation characteristics.[30]

Figure 1

Main geological features of the Yaziluo rift trough in southwestern Guizhou Province. (a) Location and distribution of the Yaziluo rift trough. (b) Yaziluo Rift Trough structure geological map. (c) Lithologic stratigraphic section of the Yaziluo rift trough. (d) Global location map of the early Carboniferous of the Yaziluo rift trough. Adapted with permission from ref (37). Copyright 2013 Earth-Sci. Rev.

In the late Early Devonian Sipai Formation, the rift system in the basin developed to the northwest, and the Yaziluo rift trough took shape, showing a NW–SE trending deepwater rift basin.[31] The Devonian entered the rifting stage in the middle and late Devonian, followed by the migration of the deepwater rifted basin to the southeast and the gradual reduction of its extent, forming a paleogeographic pattern of intertable basins.[30−32] At the end of the Devonian, under the influence of the Ziyun movement, except for Weining, Liupanshui, and other areas in the southwest of Guizhou Province, which still maintain continuous deposition of deepwater basins, uplift and denudation occurred to varying degrees in all other areas, and the Carboniferous inherited the paleogeographic pattern of the Devonian platform–basin interval.[33−35] The distribution of the shale of the Dawuba Formation is restricted by the northeastern and western uplift and the southern plateau or paleo-continent and was mainly deposited in the deepwater basin and slope areas. In the Late Carboniferous, due to the weakening activity of the Ziyun–Liupanshui Fault, a sea retreat occurred, and the seawater retreated to the southeast.[32] The study area is located in an area slightly distant from the dome uplift. The stratigraphic development is relatively complete,[36] and the shale of the Carboniferous Dawuba Formation, the target layer of this study, was deposited in the platform basin background.

Stratigraphy

The Yaziluo rift trough has been mainly formed in the Devonian–Middle Triassic. In the Early Carboniferous rift trough, the Muhua Formation, Dawuba Formation, and Nandan Formation were deposited from the bottom up (Figure c).

The Muhua Formation (C1mh) is a black–grayish-black siliceous limestone, mud crystal limestone, and granular limestone integrated over the Devonian Wuzhishan Formation. The granular marlite and muddy limestone are low-density flow-forming sediment. It contains mainly fossils of mesothelioma, odontoblasts, brachiopods, corals, and trilobites. It is mainly distributed in the deepwater sedimentation area around Pu’an a Mawei in southern Guizhou Province.[24,25]

The lithology of the Dawuba Formation (C1dw) is mainly clastic rock, with gray–black shale and siltstone shale in the lower part, interspersed with thin layers of siltstone. The central lithology is dominated by black thin-layered mudstone and shale, gray–dark-gray thin to medium-thick-layered marlite, and gray medium-thick-layered siltstone, interspersed with dark-gray thin-layered siliceous rock and siliceous shale. The upper lithology is dominated by siliceous rock, siliceous shale, and siltstone. Horizontal laminations can be seen in the shale. The siltstone is laminated and striated, often containing muddy-silt nodules, which are generally 12 × 18 cm in size and ellipsoidal, often distributed along with the layers. The Dawuba Formation is in integrated contact with the overlying Nandan Formation and the underlying Muhua Formation. The sedimentation environment is a platform, slope, and deepwater basin.[4−7,24,25]

The Nandan Formation (Cpn) is dominated by dark-gray mediums to thin-bedded Shelly limestone, mud crystal limestone, and conglomerate limestone, with a small amount of dolomite and flint strips and flint nodules, which are in integrated contact with the underlying Dawuba Formation. The biological fossils of this formation primarily include chrysolites, odontoclasts, and fusulinid fossils. The sedimentary environment is mainly a deepwater shelf and a preplatform slope.[24,25]

The study area covers about 15 000 km2, and the shale sediment thickness is 40–330 m. Shale cannot develop in the southwestern part of the study area. The shale of the Dawuba Formation gradually thickens from the northwest to the southeast, reaching 200 m in the Weining area, 250 m in the Liupanshui area, and 300 m in the Liuzhi area, with a shale thickness of 324 m in the Ziyun area (Figure ).

Figure 2

Distribution of lithological assemblages in the Yaziluo rift trough.

Sampling and Methods

The study conducted a route geological survey of the study area, and three outcrop profiles (ZN, LG, and MC) and five drilled wells (LC, QSD, QZY, DY, and CY) were observed from the northwest to the southeast of the study area, the locations of which are shown in Figure b. In this study, electronic microscopy, scanning electron microscopy observation, X-ray diffraction analysis, and elemental and geochemical analyses of 94 shale samples were completed.

Electronic Microscopy

The transmission light observation of thin rock sections was performed using a ZEISS Axioskop 40 optical microscope at the Energy Experiment Center of China University of Geosciences (Beijing). The microscopic characteristics of the shale were analyzed, and the sedimentary facies signatures were identified.

Geochemical Characteristics

Total organic carbon (TOC) and vitrinite reflectance (Ro) were tested on 94 shale core samples collected from the Dawuba Formation, QSD, QZY, DY, CY wells, and ZN profile. Following the Chinese Oil and Gas Industry Standard (COGIS) (GB/T4762008), samples were crushed and ground to less than 200 mesh and analyzed using a Leco CS-230 analyzer to obtain the TOC content. The specular reflectance tests were performed randomly on an Axio Imager Mlm microluminometer from ZEISS, following COGIS (SY/T5124-1995). Shale gas content was measured directly in the field using the shale gas content measurement system of the Key Laboratory of Shale Gas Resources Evaluation, China University of Geosciences (Beijing, China).

X-ray Diffraction (XRD) Analysis

XRD mineral content examination utilized a D8 Discover X-ray diffractometer in accordance with the oil and gas industry standard (SY/T5463-2010). Shale samples were crushed to less than 300 mesh, combined with ethanol, mashed into a mortar, and then coated and put on slides for XRD testing.

Major and Trace Elements and Rare Earth Elements (REEs)

X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS) were used for the elemental analysis of 30 outcrop samples from the ZN profile and 15 core samples from the QZY well. The oxide content of the major elements has been determined by XRF. U/Th and trace element determination was performed using an Element 6000 inductively coupled plasma mass spectrometer (ICP-MS) (GB/T14506.114-2010 and GB/T14506.30-2010). The powdered samples have been dissolved in an acid mixture (HNO3/HF = 1:2), immersed in a pressure-resistant sample jar for 24 h at 185 °C, dried, and then treated with HNO3. All samples were acid digested twice and then treated with HNO3 (5 mL) at 130 °C for 3 h. The trace element content of the samples was then determined using inductively coupled plasma mass spectrometry (ICP-MS) on the obtained liquids; the national standard reference material (GSR3) was used as the standard sample for quality control, and the testing procedure followed the national standard GB/T 14506.30-2010 with better than 5% accuracy.

Carbon Isotope

The sample was crushed to 80 mesh, weighed to 100 g, and extracted at 85 °C for more than 17 h until the fluorescence level in the extract was below grade 3. The extracted material was placed in an apparatus oxidizer and oxidized to CO2 and water at an oxygen flow rate of 30 mL/min and 1030 °C. CO2 gas was then injected into the instrument using a flow rate of 30 mL/min of He gas, and an ionization analysis was performed at a voltage greater than 3500 V and a pressure less than 1.33 × 10–3 Pa.

FE-SEM Imaging and EDS Analysis

Fifteen black shale blocks have been produced from the core of the QZY well, and the black shale samples were polished using argon ions (PECS II 685, Gatan). FE-SEM imaging and energy-dispersive X-ray spectroscopy (EDS) analysis images have been captured using a Carl Zeiss MERLIN Compact system and an EDS microprobe (Bruker Quantax 200 XFlash 6/30 detector).

Results

Characteristics and Regional Variations of Lithological Assemblages

The Lower Carboniferous Dawuba Formation in the study area is mainly distributed in Liupanshui, Puding, Ziyun, and Luodian. The lithology of the Dawuba Formation is primarily sandstone, limestone, marlite, shale, and siliceous rock. Longitudinally, it can divide into four sections. The first section is dominated by limestone and marlite with a small siltstone. The second section is dominated by marlite and mudstone. The third section is dominated by marlite. The fourth section is dominated by limestone and marlite (Figure ). The lithologic assemblage of sandstone and limestone is mainly developed in the platform facies, the lithologic assemblage of limestone and marlite in the slope facies, and the lithologic assemblage of shale and siliceous rock in the deepwater basin facies.

Northwest Section

In the northwest section of the study area, the sandstone and limestone assemblage was mainly developed in the Weining area, with frequent interbedded lithologies, storm breccia, and vein laminations, from Weining to the southeast into the area of Liupanshui, dominated by the slope facies (Figure ). Taking Well QSD as an example, the Dawuba Formation develops deepwater basin facies, slope facies, and platform facies sequentially from bottom to top, reflecting the gradual shallowing process of seawater, and the lithologic assemblage of limestone and marlite and the lithologic assemblage of siliceous rock and mud shale interbedded are mainly developed.

Figure 6

Distribution of sedimentary facies and lithological assemblages in the study area.

The lithology of the first section (1814.00–2400.00 m) is mainly gray–black thin-layered mudstone sandwiched by thin-layered marlite, formed in the deep-water basin facies. The first stage produces deepwater basin facies (1814.00–2400.00 m). The lithology is mainly gray–black thin-layered mudstone intercalated with thin-layered marlite. Horizontal bedding is developed, and banded pyrite or pyrite particles are often developed in the mudstone. The gamma value is higher in the lower part (1950–2400 m); the curve fluctuation range is not extensive, and the U/Th logging value is mainly between 0 and 1.25 (Figure ). The second and third sections (1463.00–1814.00 m) mainly develop lithological assemblages of limestone and marlite, formed in the slope facies. The continental slope is the steepest part of the ocean floor located between the outer continental shelf or the continental shelf–slope break and the continental uplift. It is generally 3–6°, with sufficient sunlight and sufficient oxygen, and the benthic organisms multiply. Bioclasts from the upper part of the slope are deposited here, and the organic matter is relatively more enriched. The primary lithology is gray–black calcareous mudstone and gray–black calcareous mudstone intercalated with carbonaceous mudstone, containing a little sandstone and siliceous rock. The lithology of the fourth section is dominated by limestone, formed in the platform facies (1247.00–1463.00 m). The carbonate platform refers to a shallow-water–carbonate depositional environment with flat terrain, where dark-gray, gray–black argillaceous limestone and silty shale are often developed.

Middle Section

From Liuzhi to Guanling belongs to the middle section of the Yaziluo fault zone, which connects the southeastern Guizhou depression in the northeast and the southwest depression in the southwest. The faults in the middle section are not well developed, and the tectonic conditions are relatively simple. The middle section of the Dawuba Formation is buried deeper, with a depth of over 6000 m in the rift trench and above 3500 m on the slope. At present, there is no drilling in the rift trough. Still, the interpretation results by the wide-field electromagnetic method show that there is a deepwater deposition center in the Dawuba Formation in the Guanling area of the rift trough, which is controlled by the direction of the NW fault (Figure a). The shale thickness is about 450 m. The Guanling area in the middle section mainly develops deepwater basin sedimentary systems (Figure ), and the lithology is dominated by mudstone and shale and deepwater fine-grained sediment (Figure ).

Figure 12

(a) Wide-field electromagnetic survey results, the extent of sedimentation in the Yaziluo rift trough of the Dawuba Formation. (b) Sedimentation pattern of the Lower Carboniferous Dawuba Formation in the Yaziluo rift trough, southern Guizhou Province.

In the middle section, taking the ZN section between Anshun and Ziyun as an example (Figure ), the third member of the Dawuba Formation is exposed, with dark-gray mudstone and shale intercalated with thin marlite, which belongs to the slope sedimentary system. The vertical lithologic changes of the ZN profile show a well-developed stratigraphic cyclicity, which consists mainly of two parts: the lower marlite/limestone and the upper shale/mudstone, reflecting the gradual deepening of the water column process. In the Dawuba Formation of the ZN section, the bottom of each stratigraphic cycle is mainly a limestone–marlite lithologic assemblage; the top is mudstone and shale, and four lithologic rotations were observed in the vertical direction (Figure ). As shown in Figure , at 10–55 m, the TOC content first increased, then decreased, and then increased, reaching a maximum of 2.83%. Ti/Al first increased, then decreased, and then increased. (La/Yb)N is mainly distributed around 1.0 with a mean value of 1.41.

Figure 7

Vertical variation of elements in black shale samples from the ZN Outcrop in the middle section of the study area. Fm. = Formation. M. = Member. Height is the distance from the start of the profile.

Southeast Section

The southeastern section of the study area is dominated by deepwater basin facies and slope facies. The slope facies is mainly located in the vicinity of Huishui, Daihua, and Luodian. The platform facies is mainly developed in the southwestern part of Zhenfeng and the northern part of Luodian (Figure ).

The QZY well is a typical well drilled in the southeast section of the study area. The deepwater basin facies is characterized near Ziyun by a lithologic combination of shale and siliceous rocks, weak hydrodynamics, dark color, the small particle size of sediments, and high total organic carbon content. According to the analysis of cores and outcrop profiles (Figure ), its main sedimentary features are silica-rich and carbon-rich, often developed with pyrite, radiolarians, sea lilies, and other finger facies fossils of the deepwater environment. The main lithology is black shale interspersed with dark-gray siliceous rock, a small amount of siltstone, and marlite. Shale thickness is usually 10–30 m (Figure ).

Figure 3

Sedimentary facies signatures in the Yaziluo rift trough of the Lower Carboniferous Dawuba Formation. (a) Sea lily fossils in siliceous rocks, single polarized light. (b) Sea lily fossils in siliceous rocks, orthogonal polarized light. (c) Inclusion of coiled laminae, siltstone shale of the Dawuba Formation, LG profile. (d) Slip formation, muddy siltstone of the Dawuba Formation, LG profile. (e) Marlite of the Dawuba Formation with striated pyrite, QZY well, Core. (f) Mud shale of the Dawuba Formation with silt nodules, MC profile.

The lithology of the first section of the QZY well is dominated by limestone and formed in the platform facies. The second and fourth sections of the QZY well (Figure ) are mainly dark-gray siliceous rocks and black, gray shales interspersed with siliceous shales, and the sea lily fossils in siliceous rocks indicate a deepwater anoxic environment (Figure a,b). The shale is rich in pyrite particles (Figures e and 5a,b,f) and biological fossils, such as sea lily fossils (Figure a–d) and radiolarian fossils (Figure c,d). The lithology is mainly siltstone and fine sandstone, with very little conglomerate. The third section (2765.00–2845.00 m) is mainly a lithological combination of limestone and marlite, formed in the slope facies. The upper part is dominated by middle limestone and mudstone, with horizontal laminations, and the mud shale laminations are often interspersed with gravity flow deposits, such as thin layers of turbidite sand, and slip deformation structures (Figure c,d). More scattered and banded pyrite can be seen within the layers (Figure e), indicating that it was deposited in a deeper reduction environment of the water body.

Figure 8

Vertical variation of elements in black shale samples from the QZY Well in the southeast section of the study area.

Figure 5

FE-SEM image of black shale of QZY well Dawuba Formation. (a) QZY-4, striped and massive organic matter, diverse pyrite morphology, developing microfractures. (b) QZY-12, massive organic matter, strawberry pyrite, developing organic matter pores, intragranular pores and microfractures. (c) QZY-6, massive organic matter, developing microfractures. (d) QSD well, dissolution holes developed inside the mineral, developing microfractures. (e) CY well with microfractures developed within the clay mineral. (f) QZY-14, pyrite intergranular pores are filled with organic matter and clay minerals, preserving the development of a few intergranular pores. Abbreviations used: F-Py = strawberry pyrite, OM = organic matter, Q = quartz, Py = pyrite, CM = clay minerals.

Figure 4

Electron microscopic observation of the Dawuba group. (a) Siliciclastic rocks of the Dawuba Formation with sea lily fossils (red arrow), MC profile, single polarized. (b) Orthogonal polarization of panel a. (c) Shale of the Dawuba Formation, sea lily fossil (red arrow), radiolarian-bearing fossil (yellow arrow), MC profile, single polarized light. (d) Orthogonal polarization of panel c. (e) Single polarized light, marlite, calcite, or organic matter bands, MC profile, 71 m in the second section of the Dawuba Formation. (f) Orthogonal polarization of panel e. (g) Single polarized light, siliceous rock with sponge bone needles, MC profile, section 4 of the Dawuba Formation, 15 m. (h) Orthogonal polarization of panel g.

Mineral Composition

The XRD mineral composition of the black shales of the Dawuba Formation (Table and Figure S1) shows that the marlite in the study area is high in carbonate minerals and low in quartz and clay minerals. The black shale minerals are dominated by quartz (4–63%, average of 30.09%) and clay minerals (2–76%, average of 37.29%). The calcite and dolomite contents were 0–89% and 0–37%, with an average value of 18.24% and 6.73%, respectively. Pyrite content is higher, at 0–48%, with an average of 5.44%. The content of feldspar is low (average of 2.79%).

Table 1

Mineralogical Compositions of the Black Rock Series Samples in This Study

sample No.	depth (m)	quartz (%)	clay (%)	f.a (%)	cal.a (%)	dol.a (%)	pyrite (%)
QSD-1	1656.10	13.40	11.50	2.30	66.00	5.00	1.80
QSD-2	1660.00	10.00	8.00	0.00	75.00	6.00	1.00
QSD-3	1661.60	18.00	10.20	0.00	59.70	6.30	5.80
QSD-4	1682.25	16.00	15.00	0.00	37.00	30.00	2.00
QSD-5	1683.80	15.50	20.10	0.00	38.00	24.70	1.70
QSD-6	1687.40	19.00	21.00	0.00	27.00	31.00	2.00
QSD-7	1690.10	14.60	10.50	0.00	48.40	24.90	1.60
QSD-8	1930.90	28.80	55.90	0.00	0.00	14.40	0.90
QSD-9	1931.25	8.00	40.00	0.00	12.00	37.00	3.00
QZY-1	2773.59	34.00	62.00	0.00	0.00	0.00	4.00
QZY-2	2776.32	34.00	35.00	5.00	13.00	0.00	13.00
QZY-3	2778.00	44.00	38.00	0.00	11.00	4.00	3.00
QZY-4	2801.40	44.00	47.00	0.00	3.00	2.00	4.00
QZY-5	2804.70	36.00	52.00	3.00	1.00	0.00	8.00
QZY-6	2810.50	35.00	44.00	10.00	3.00	3.00	5.00
QZY-7	2812.95	63.00	0.00	0.00	34.00	2.00	1.00
QZY-8	2813.40	23.00	48.00	0.00	19.00	5.00	5.00
QZY-9	2884.23	18.00	60.00	0.00	10.00	5.00	7.00
QZY-10	2892.23	33.00	31.00	9.00	17.00	3.00	7.00
QZY-11	2933.48	33.00	42.00	4.00	15.00	0.00	6.00
QZY-12	2941.65	25.00	51.00	6.00	9.00	0.00	9.00
QZY-13	2944.10	4.00	2.00	1.00	89.00	3.00	1.00
QZY-14	2950.91	27.00	51.00	6.00	8.00	0.00	8.00
QZY-15	2981.90	40.00	44.00	1.00	12.00	3.00	0.00
CY-1	712	5.00	20.00	5.00	38.00	31.00	1.00
CY-2	721	22.00	65.00	0.00	7.00	1.00	5.00
CY-3	733	29.00	63.00	2.00	6.00	0.00	0.00
CY-4	749	23.00	76.00	0.00	1.00	0.00	0.00
CY-5	753	24.00	14.00	7.00	0.00	7.00	48.00
CY-6	765	18.00	63.00	10.00	7.00	0.00	2.00
CY-7	771	33.00	49.00	5.00	8.00	3.00	2.00
CY-8	780	11.00	60.00	14.00	12.00	0.00	3.00
CY-9	789	19.00	48.00	3.00	20.00	9.00	1.00
CY-10	798	21.00	53.00	0.00	10.00	11.00	5.00
CY-11	805	41.00	54.00	0.00	2.00	0.00	3.00
CY-12	816	21.00	55.00	0.00	10.00	11.00	3.00
CY-13	825	25.00	65.00	1.00	3.00	3.00	3.00
CY-14	833	23.00	43.00	0.00	15.00	15.00	4.00
CY-15	841	14.00	22.00	35.00	27.00	0.00	2.00
CY-16	852	17.00	3.00	0.00	74.00	5.00	1.00
CY-17	861	21.00	53.00	0.00	19.00	3.00	4.00
CY-18	874	15.00	48.00	0.00	4.00	5.00	28.00
CY-19	879	23.00	68.00	0.00	3.00	3.00	3.00
CY-20	891	39.00	60.00	0.00	0.00	0.00	1.00
CY-21	901	45.00	18.00	0.00	34.00	1.00	2.00
CY-22	906	45.00	19.00	0.00	29.00	5.00	2.00
CY-23	911	40.00	18.00	0.00	39.00	1.00	2.00
CY-24	918	40.00	22.00	0.00	35.00	2.00	1.00
DY-1	468	50.20	33.61	5.03	6.79	0.83	3.53
DY-2	490	33.25	10.31	0.87	38.28	15.85	1.44
DY-3	509.87	22.25	20.57	0.00	46.35	9.85	0.98
DY-4	519.11	33.19	31.15	0.00	15.38	16.41	3.87
DY-5	530.45	39.75	37.74	3.06	2.72	5.28	11.44
DY-6	542.76	36.99	48.23	3.10	0.00	1.22	10.45
DY-7	545.74	30.48	39.72	3.66	1.58	6.84	17.72
DY-8	562.23	49.26	32.71	4.49	0.00	4.79	8.75
DY-9	567.04	48.69	29.35	4.84	1.77	7.48	7.86
DY-10	568.14	48.57	22.73	3.34	3.67	10.91	10.78
DY-11	569.8	45.45	30.46	5.17	4.05	6.11	8.78
DY-12	578.15	45.10	37.88	3.57	4.58	5.84	3.03
DY-13	595.21	46.92	41.30	2.86	3.41	2.66	2.85
DY-14	604.59	50.30	24.74	3.84	7.07	5.11	8.94
DY-15	615.65	48.97	25.22	3.36	6.58	2.42	13.44
DY-16	623.16	46.00	25.38	2.06	15.20	3.55	7.81
ZN-1	1.2	21.00	29.00	4.00	33.00	11.00	2.00
ZN-2	3	28.00	26.00	5.00	35.00	14.00	2.00
ZN-3	6	24.00	23.00	2.00	37.00	12.00	2.00
ZN-4	8.5	24.00	20.00	0.00	41.00	13.00	2.00
ZN-5	10.2	42.00	54.00	2.00	0.00	0.00	2.00
ZN-6	12.9	48.00	47.00	3.00	0.00	0.00	2.00
ZN-7	14.8	40.00	55.00	2.00	0.00	0.00	3.00
ZN-8	17.2	23.00	32.00	2.00	35.00	8.00	3.00
ZN-9	20	56.00	42.00	0.00	0.00	0.00	2.00
ZN-10	24.5	58.00	39.00	0.00	0.00	0.00	3.00
ZN-11	27	38.00	59.00	0.00	0.00	0.00	3.00
ZN-12	29.5	40.00	58.00	0.00	0.00	0.00	2.00
ZN-13	31.5	35.00	60.00	2.00	0.00	0.00	3.00
ZN-14	33.5	25.00	66.00	6.00	0.00	0.00	3.00
ZN-15	35.5	33.00	60.00	5.00	0.00	0.00	2.00
ZN-16	37	31.00	63.00	4.00	0.00	0.00	2.00
ZN-17	38.7	29.00	63.00	3.00	0.00	0.00	5.00
ZN-18	40.2	26.00	66.00	6.00	0.00	0.00	2.00
ZN-19	46.5	44.00	53.00	2.00	0.00	0.00	1.00
ZN-20	49.8	35.00	61.00	2.00	0.00	0.00	2.00
ZN-21	52	25.00	23.00	0.00	36.00	14.00	2.00
ZN-22	54.5	25.00	71.00	1.00	1.00	1.00	1.00
ZN-23	56	19.00	76.00	0.00	2.00	2.00	1.00
ZN-24	58	23.00	64.00	0.00	6.00	4.00	3.00
ZN-25	63.2	23.00	58.00	0.00	12.00	5.00	2.00
ZN-26	65.5	32.00	46.00	0.00	18.00	0.00	4.00
ZN-27	68	27.00	49.00	5.00	13.00	3.00	3.00
ZN-28	72.8	26.00	57.00	0.00	10.00	6.00	1.00
ZN-29	75.6	31.00	52.00	4.00	9.00	2.00	2.00
ZN-30	78.4	25.00	56.00	0.00	11.00	5.00	3.00

f., feldspar; cal., calcite; dol., dolomite.

The QSD well is a typical well drilled in the Liupanshui area of the northwest section of the Yaziluo Rift Trough through the Dawuba Formation, which can be divided into four sections. From the XRD test results, the mineral composition of the shale is mainly composed of quartz (4–63%, average of 25.93%) and clay minerals (2–62%, average of 34.33%), with higher calcite content (0–75%, average of 40.34%). The content of quartz and clay gradually decreases, and calcite gradually increases from the fourth section to the first section.

The mineral composition of the middle section is dominated by quartz (19–58%, average of 31.87%) and clay minerals (20–76%, average of 50.93%), with calcite content (0–41%, average of 9.97%) being low. From the bottom to the top, the quartz content gradually decreases, and the clay mineral content gradually increases.

The mineral composition of the southeast section is dominated by quartz (4–63%, average of 32.87%) and clay minerals (0–62%, average of 43.36%), while the calcite content (0–89%, average of 14.63%) is low.

Organic Geochemical Characteristics of Shale

Total organic carbon (TOC) data are presented in Table . The results of the TOC frequency distribution are shown in Figure S2. TOC concentrations ranged from 0.56% to 4.51% (average of 1.73%).

Table 2

TOC and Ro Data for the Shale of the Dawuba Formation

section	sample No.	depth (m)	TOC (%)	Ro (%)	section	sample No.	depth (m)	TOC (%)	Ro (%)
Northwest section	QSD-1	1656.10	1.94	2.13	Southeast section	QZY-9	2884.23	1.45	4.52
	QSD-2	1660.00	0.62	1.95		QZY-10	2892.23	0.95	4.69
	QSD-3	1661.60	0.80	1.97		QZY-11	2933.48	0.61	3.26
	QSD-4	1682.25	0.99	2.32		QZY-12	2941.65	2.18	5.11
	QSD-5	1683.80	0.7	2.24		QZY-13	2944.10	2.27	–a
	QSD-6	1687.40	0.96	2.19		QZY-14	2950.91	2.40	5.04
	QSD-7	1690.10	0.9	2.13		QZY-15	2981.90	2.23	3.46
	QSD-8	1930.90	1.13	2.57		CY-1	712	1.26	2.70
	QSD-9	1931.25	1.08	2.29		CY-2	721	1.02	2.46
Middle section	ZN-1	1.2	0.97	–		CY-3	733	2.1	2.49
	ZN-2	3	0.91	–		CY-4	749	1.91	3.00
	ZN-3	6	0.9	–		CY-5	753	0.56	–
	ZN-4	8.5	1.06	–		CY-6	765	2.14	3.03
	ZN-5	10.2	1.36	–		CY-7	771	0.63	–
	ZN-6	12.9	1.69	–		CY-8	780	1.85	–
	ZN-7	14.8	1.51	–		CY-9	789	4.51	–
	ZN-8	17.2	1.05	–		CY-10	798	2.12	–
	ZN-9	20	1.64	–		CY-11	805	1.63	2.77
	ZN-10	24.5	2.83	–		CY-12	816	2.13	–
	ZN-11	27	2.29	–		CY-13	825	2.11	–
	ZN-12	29.5	1.27	–		CY-14	833	2	2.31
	ZN-13	31.5	1.42	–		CY-15	841	0.82	2.52
	ZN-14	33.5	1.82	–		CY-16	852	2.2	2.15
	ZN-15	35.5	1.72	–		CY-17	861	3.12	2.13
	ZN-16	37	1.47	–		CY-18	874	2.06	2.53
	ZN-17	38.7	1.93	–		CY-19	879	1.28	–
	ZN-18	40.2	2.12	–		CY-20	891	1.82	2.78
	ZN-19	46.5	1.19	–		CY-21	901	2.74	3.13
	ZN-20	49.8	1.27	–		CY-22	906	1.65	3.05
	ZN-21	52	1.08	–		CY-23	911	1.55	3.27
	ZN-22	54.5	1.46	–		CY-24	918	1.63	2.53
	ZN-23	56	1.42	–		DY-1	468	1.45	–
	ZN-24	58	1.43	–		DY-2	490	1.54	–
	ZN-25	63.2	1.14	–		DY-3	509.87	1.16	–
	ZN-26	65.5	1.18	–		DY-4	519.11	1.78	2.4
	ZN-27	68	1.38	–		DY-5	530.45	0.75	–
	ZN-28	72.8	1.24	–		DY-6	542.76	2.25	2.25
	ZN-29	75.6	1.29	–		DY-7	545.74	2.92	–
	ZN-30	78.4	1.43	–		DY-8	562.23	3.05	2.19
Southeast section	QZY-1	2773.59	2.92	5.05		DY-9	567.04	–	2.47
	QZY-2	2776.32	2.54	5.03		DY-10	568.14	2.87	–
	QZY-3	2778.00	0.90	–		DY-11	569.8	2.46	2.65
	QZY-4	2801.40	3.93	4.66		DY-12	578.15	1.93	2.48
	QZY-5	2804.70	2.3	–		DY-13	595.21	1.37	2.66
	QZY-6	2810.50	1.47	4.81		DY-14	604.59	2.32	–
	QZY-7	2812.95	1.97	4.77		DY-15	615.65	3.31	2.65
	QZY-8	2813.40	1.38	4.93		DY-16	623.16	4.36	2.15

– indicates that no experimental testing was performed. TOC = total organic carbon (weight percent wt % of the whole rock). Ro = vitrinite reflectance. Samples are from profile and drilling.

The TOC content of the shales in the northwest section of the Dawuba Formation ranges from 0.62% to 1.94%, with an average of 1.01%. The shale in the second section is more enriched in organic matter, with a maximum TOC of 1.94% and a maximum Ro of 2.57%.

The TOC contents of the 30 shale samples from the ZN profile are shown in Table . The lower marlite and limestone have lower TOC content values, while the upper mudstone and shale have higher TOC content values. The TOC content of shales in the middle section of the Dawuba Formation ranges from 0.9% to 2.83%, with an average of 1.45%. The TOC content increases from the bottom to the top first, reaching a maximum of 2.83%, and then decreasing to a minimum of 1.08%.

The TOC content of the shales in the southeastern section of the Dawuba Formation ranges from 0.61% to 3.93%, with an average of 1.97%. The TOC content increases from the bottom to the top and decreases, and the organic matter is relatively more enriched in the second and third sections.

Characteristics of Major and Trace Elements

The ratio of U/Th to V/Cr is often used to characterize the redox conditions of the sedimentary environment.[38−40] The ratio of U/Th in the middle section of the study area is 0.15–0.71, with an average of 0.25, and the ratio of V/Cr is 0.37–1.28, with an average of 1.06 (Table , Figure ). The detailed test results are shown in Table S1. The Ti/Al ratio is often used to characterize the influence of terrestrial sourced material input, with higher Ti/Al ratios indicating more significant terrestrial sourced input during the period.[41,42] The ratio of Ti/Al ranged from 0.03 to 0.09, with an average of 0.06 (Table , Figure ). (La/Yb)N values are often used to characterize the sedimentation rate of shales, with the ratio closer to 1.0, the higher the sedimentation rate.[43−45] The (La/Yb)N ratios ranged from 0.89 to 2.83, and the (La/Yb)N ratios were mainly concentrated around 1.0, with an average of 1.45 (Table , Figure ).

Table 3

Concentrations and Ratios of Trace Elements in Black Rocks of Dawuba Formation in the ZN Outcrop

sample No.	depth (m)	U/Th	V/Cr	Ti/Al	(La/Yb)N
ZN-1	1.2	0.15	1.18	0.08	1.86
ZN-2	3	0.16	1.2	0.08	1.87
ZN-3	6	0.17	1.28	0.09	1.77
ZN-4	8.5	0.37	1.18	0.03	0.81
ZN-5	10.2	0.21	1.1	0.06	1.55
ZN-6	12.9	0.2	1.03	0.06	1.98
ZN-7	14.8	0.24	1.05	0.06	1.7
ZN-8	17.2	0.15	1.1	0.07	0.85
ZN-9	20	0.23	0.98	0.05	0.89
ZN-10	24.5	0.38	1.01	0.04	1.6
ZN-11	27	0.25	1.04	0.06	1.55
ZN-12	29.5	0.23	1	0.07	1.32
ZN-13	31.5	0.22	1.04	0.07	1.94
ZN-14	33.5	0.24	1.2	0.08	0.88
ZN-15	35.5	0.24	1.13	0.08	0.82
ZN-16	37	0.23	1.1	0.08	1.09
ZN-17	38.7	0.24	1.14	0.09	1.63
ZN-18	40.2	0.23	1.1	0.08	1.48
ZN-19	46.5	0.18	1.09	0.07	1.18
ZN-20	49.8	0.18	1.09	0.07	1.65
ZN-21	52	0.17	1.18	0.08	1.12
ZN-22	54.5	0.18	1.02	0.08	1.73
ZN-23	56	0.17	1.14	0.08	1.72
ZN-24	58	0.18	1.17	0.07	1.64
ZN-25	63.2	0.18	1.2	0.05	1.51
ZN-26	65.5	0.2	0.61	0.05	1.38
ZN-27	68	0.71	0.37	0.03	1.47
ZN-28	72.8	0.35	1.04	0.04	1
ZN-29	75.6	0.29	1.06	0.04	1.1
ZN-30	78.4	0.45	0.93	0.04	1.19

The ratio of U/Th in the shale of the Dawuba Formation in the southeast section of the study area ranges from 0.20 to 1.40, with an average of 0.42, and the ratio of V/Cr ranges from 0.31 to 2.27, with an average of 1.07 (Table ). The Ti/Al value is relatively high from the upper part of the first section to the upper part of the third section (2892.25–2770.00 m) (Table , Figure ). The detailed test results are shown in Table S2.

Table 4

Composition of Major Trace Elements and δ13C in Shale of the Dawuba Formation in the QZY Well

sample No.	depth (m)	U/Th	V/Cr	Ti/Al	δ13C (%)
QZY-1	2773.63	0.21	1.84	0.041	–25.534
QZY-2	2776.38	0.25	1.44	0.043	–25.466
QZY-3	2778	0.26	1.18	0.044	–25.418
QZY-4	2801.38	0.39	0.41	0.047	–25.024
QZY-5	2804.75	0.27	0.41	0.044	–24.928
QZY-6	2810.5	0.39	0.46	0.041	–24.909
QZY-7	2813	0.36	0.43	0.042	–24.842
QZY-8	2813.38	0.37	0.45	0.049	–24.851
QZY-9	2884.25	0.2	0.35	0.04	–25.308
QZY-10	2892.25	0.27	0.32	0.047	–25.777
QZY-11	2933.5	0.34	1.8	0.038	–24.625
QZY-12	2941.63	0.51	1.76	0.022	–24.356
QZY-13	2944.13	0.36	1.32	0.034	–24.586
QZY-14	2951	0.72	1.65	0.041	–25.113
QZY-15	2980	1.4	2.27	0.035	–26.866

The change of δ13Corg is closely related to the rise and fall of the sea level. When the sediment–water body becomes shallow, the dissolved oxygen content in water increases; the oxidation of organic matter is enhanced, and the carbon isotope of organic matter is heavy. When the sediment–water body becomes deeper, the dissolved oxygen content in water decreases; the oxidation of organic matter is weakened, and the carbon isotope of organic matter is light. The range of δ13Corg in the southeast section of the study area is 26.866–24.356%, with a mean value of −25.174%, and relatively large δ13Corg values in the upper part of the first section to the upper part of the third section (Table , Figure ).

Discussion

Redox Environment of Water Bodies

U/Th less than 0.75 reflects an oxidized water environment, a U/Th ratio between 0.75 and 1.25 a dysoxic water environment, and U/Th more than 1.25 an anoxic water environment. V/Cr less than 2 is an oxidized water environment, V/Cr between 2 and 4.25 a dysoxic water environment, and V/Cr more than 4.25 an anoxic water environment.[46−48] Trace element analysis (Figure ) of the organic-rich shale facies in the QZY well and ZN section in the study area, with U/Th mainly in the range 0–0.5 and V/Cr mainly in the range 0.3–2, indicates that the shales of the Dawuba Formation in the study area belong to an oxic and dysoxic depositional environment. The particle size distribution of strawberry pyrite in the study area ranges from 0.5 to 10 μm, reflecting that the water body at the time of deposition was an oxygen-rich–oxygen-poor depositional environment (Figure ), which is consistent with previous studies on the redox environment of water bodies in the study area and its surrounding areas.[47,48] The U/Th and V/Cr values of the shales of the Dawuba Formation are lower than those of the deepwater shelf shales of the Wufeng–Longmaxi Formation (Figure ),[16] probably because the Dawuba Formation’s organic-rich shales formed mostly on the slope. In the background of rift trap tectonics, the oxygen content of water is high, which is an oxic and dysoxic depositional environmental condition. The U/Th and V/Cr distributions of the shales of the Dawuba Formation in the study area are very similar to those of the Permian Longtan Formation in the western part of Guizhou Province,[29] indicating that the study area has also been characterized by marine–continental transitional face shales.

Figure 9

Trace element composition relationship chart. The data from the Longtan Formation are from Liu’s study. Adapted with permission from ref (29). Copyright 2018 J. Nat. Gas Sci. Eng.

Organic Matter Sources

Organic matter enrichment may be controlled mainly by multiple factors such as primary productivity, terrestrial inputs, and sedimentation rates.

The primary productivity is controlled by the input of organic matter and the water environment. The inorganic carbon isotope δ13Ccarb fluctuates strongly in the first section of the Dawuba Formation (Figure ). It gradually becomes positively skewed toward the fourth section, indicating the development of upwelling in the study area during the sedimentation of the Dawuba Formation, which is consistent with previous knowledge on the development of upwelling in the Early Carboniferous Palaeoequatorial.[49−55] Upwelling brings rich nutrients and an extraordinary flourishing of organic matter. The vertical variation of δ13Corg in the study area is similar to the trend of vertical fluctuation of TOC and gamma curves (Figure ). From the lower part of the first section to the upper part of the first section, δ13Corg gradually increases, and TOC is relatively larger. From the upper part of the first section to the third section, δ13Corg first slowly decreases and then gradually increases. The corresponding GR values, δ13Corg, and TOC contents have a good trend of similar changes in this depth range (Figures and 10), indicating that the primary productivity under the influence of upwelling has a significant effect on the organic matter enrichment in shale. The TOC of the shales of the Dawuba Formation in the study area tends to increase gradually with increasing U/Th and V/Cr values (Figure a,b), which is consistent with previous studies on the influence of the depositional environment of marine shales on organic carbon content;[56] this is because as the U/Th and V/Cr values increase, the oxygen content in the sedimented water gradually decreases, which is conducive to the preservation of organic matter. Organic matter in the marine-continental transitional face is more influenced by the input from land sources than in the marine environment. For example, the organic matter in the shale of the Longtan Formation belongs to this category,[29] when it is not accurate to study the influence of the water environment on organic carbon enrichment by using trace elements like U/Th and V/Cr values.

Figure 10

Carbon isotope study of the Dawuba Formation. The shaded part is the δ13Ccarb distribution range of modern normal seawater, and the gray curve represents the global carbon and oxygen isotope background values. Ord. = Order. Adapted with permission from ref (50). Copyright 2011 Palaeogeography, Palaeoclimatology, Palaeoecology.

Figure 11

Environments and sources of organic matter deposition. (a) U/Th and TOC relationship graph. (b) V/Cr and TOC relationship graph. (c) w(ΣREE) vs w(La)/w(Yb) diagram of black shale in the study area. (d) Ti/Al and TOC relationship graph. The bottom image of part c is in accord with Allegre’s study. Adapted with permission from ref (64). Copyright 1978 Earth Planet. Sc. Lett.

The input of terrestrial sources in this region is evidenced in various ways. The types of kerogen microcomponents in the shales of the Lower Carboniferous Dawuba Formation in southern Guizhou Province are mainly the vitrinite and chitinite, followed by the inert group. The organic matter types are main types II2 or III,[15,57] showing the contribution of a large number of terrestrial higher plants to the organic matter.[51,52,58,59] The mineral composition of the shale is high in clay minerals, with an average content of 37.29% (Table ), and the high clay mineral content indicates a strong terrestrial source input. The rare-earth element composition of the shale shows that the source in this region is a mixture of sedimentary rocks and granites (Figure c), and previous work has demonstrated that these granite and sedimentary rock hosts are likely to have originated from the adjacent Eastern Yunnan Uplift and Central Guizhou Uplift.[60,61] Shale trace element evidence also shows a contribution from terrestrial sources of input. For example, the Ti/Al ratio is an indicator to distinguish the source, and the shale Ti/Al values in this region are relatively low and characteristic of marine sediments (Figure d); however, the high TOC area is also the area of high Ti/Al values, which implies that the terrestrial source input has a dual complementary effect of organic matter and sediment (Figures and 8). The extensive development of gravity flow deposits in deep water (Figure c,d) provides sedimentological evidence for this strong terrestrial source input.

The average value of (La/Yb)N for the shales of the Dawuba Formation in the study area is 1.41 (Figure , Table ), indicating a high deposition rate. On one hand, higher deposition rates dilute organic matter. On the other hand, under oxic and dysoxic conditions, higher deposition rates can shorten the exposure time of organic matter to the oxic environment, thus reducing the degradation of organic matter.

Even in an oxic environment, sufficiently rich organic matter can form high TOC marine sediments, and the rich organic matter can consume oxygen as it decomposes.[62,63] Meanwhile, the organic matter input from terrestrial sources during the deposition of the Dawuba Formation compensated for the unfavorable conditions in the oxidation environment during the sedimentation period. It enhanced the organic matter content in the mudstone and shales of the Dawuba Formation.

Sedimentary Facies Model

Unlike the more stable deepwater shelf environment of the Wufeng–Longmaxi Formation shales, the shales of the Dawuba Formation in the study area were formed in a geotectonic setting of tensional rifting, and the Bahamas Plateau in the modern Caribbean Sea was developed in this geotectonic setting.[65] The interior of the Bahama Plateau mainly deposits granular limestone and marlite, and the slopes develop biotite limestone and marlite. While the surrounding deepwater basin is dominated by marlite and mudstones with thin sandstones, the transition between the Bahama Plateau and the surrounding deepwater basin is a carbonate slope with extremely developed gravity flow.[65] The early Carboniferous geographic position of the study area near the equator and the formation of isolated carbonate platforms are similar to the modern Bahamian platforms (Figure d). The upwelling developed near the equator brings rich nutrients to promote the production of organic matter, which is conducive to the reproduction of microorganisms. The decomposition of organic matter by microorganisms consumes a lot of oxygen, which may cause the oxygen content of the bottom water to decrease, which is conducive to the preservation of organic matter. The higher primary productivity plays a decisive role in enriching organic matter. The input of abundant terrestrial material facilitates the accumulation of organic matter. The faster sedimentation rate effectively reduces the degradation of organic matter under oxic and dysoxic conditions, eventually forming organic-matter-rich shale.

On the basis of the above studies, combined with the data from wide-field electromagnetic exploration (Figure a), an interpretative model of the organic-rich shales of the Lower Carboniferous Dawuba Formation of the Yaziluo rift trough in southern Guizhou Province was developed, similar to the platform–slope–basin depositional model of the Florida–Bahamas platform–trough system of the western Atlantic margin (Figure b). The distribution of the organic-rich shales of the Dawuba Formation is controlled by the Yaziluo rift troughs, where the seawater gradually deepens from northwest to southeast; the water body gradually deepens from both sides of the rift trough to the interior, and the paleogeography of the rift trough changes from platform/bayside and slope to the deepwater basin. The two sedimentation centers are located in the Liupanshui and Ziyun areas, which have deposited a large amount of marine organic matter and the input of terrestrial material from the north and northeast.

Conclusions

Based on the mineral composition, microstructure, organic geochemical characteristics, etc., three lithological combination types were identified: the lithological combination of siltstone and limestone (Platform Facies), the lithological combination of marlite and limestone (Slope Facies), and the lithological combination of shale and siliceous rock (Deepwater Basin Facies). According to the sedimentary facies markers, typical outcrops, and drilling data, the distribution range of the sedimentary facies zones is divided considering tectonic paleogeography. The characteristics of different facies zones are compared. The northwest section of the Yaziluo rift trough in southern Guizhou Province is dominated by platform–slope facies sedimentation, the middle section by deepwater sedimentation, and the southeast section by slope–deepwater basin facies sedimentation.

The Yaziluo rift trough in southern Guizhou Province belonged to a platform basin sedimentary system during the deposition of the Lower Carboniferous Dawuba Formation, with an oxic–dysoxic water environment. Upwelling of the seafloor brings abundant nutrients, resulting in high primary productivity, and microorganisms multiply aerobically and consume large amounts of oxygen, which is conducive to preserving organic matter. Higher primary productivity, abundant input from terrestrial sources, and faster deposition rates control the formation of organic-rich shales in deepwater basins within rifted troughs. Higher primary productivity is the main controlling factor.

The organic-rich shales of the Lower Carboniferous Dawuba Formation in the study area are symmetrically distributed around the Yaziluo rift trough, and the seawater gradually deepens from the northwest to the southeast and the water body from both sides of the rift trough to the interior, changing from the platform and slope facies to the deepwater basin facies. A large amount of marine organic matter was deposited in the rift trough, and there are also terrestrial sources from the north and northeast input.