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A set of high-quality marine facies organic-rich shales developed in the Lower Carboniferous Dawuba Formation, which is considered to be the main target of shale gas exploration and development in Guizhou Province. In this paper, 53 samples from Well ZY1 are selected, and the core observation data, field-emission scanning electron microscopy (FE-SEM) images, and geochemical data of these samples are analyzed. On the basis of these data, the main influencing factors of organic matter enrichment in the Dawuba Formation shale were identified and an organic matter accumulation model was established. The results show that total organic carbon (TOC) values of the Dawuba Formation in the ZY1 well vary between 1.97 and 4.11%, with high values appearing at the depths of 2796−2814 m (3.00−4.11) and 2877−2894 m (1.97−3.49). The redox-sensitive element enrichments are generally low, indicating that these samples were deposited under oxic−suboxic conditions. The micronutrients (Zn, Cu, and Ni), biological Ba (Ba XS ), and P/Al also show low values, indicating low primary productivity. The chemical index of alteration (CIA) and terrigenous clastic input index (Ti/Al) showed two obvious high-value zones, indicating that shale in the study area was affected by terrigenous inputs. Similarly, the calculation results show that Fe/Mn and Rb/K values have two abnormal data segments at the same depth. The anomaly of these data at the same depth section further suggests that the shale was affected by terrigenous input during deposition. Moreover, the terrigenous input reaches the maximum in the above TOC high-value region, and it is inferred by combining with the core observation results that the gravity flow occurs in this depth. The carbon isotope of kerogen (δ 13 C org ) ranges from −26.84 to −24.36%, indicating that the source of organic matter is likely to be terrestrial plants. This is further supported by the widespread presence of filamentous organic matter using FE-SEM, despite the low productivity and poor preservation conditions during deposition of the Dawuba Formation; the enhanced terrigenous input may have provided additional sources of organic matter for the Dawuba shale.

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A series of qualitative descriptions and quantitative analyses was used to determine the lithofacies characteristics and recognize the favorable shale intervals of Dawuba Formation in the Ziyun area of South Qian Depression. The qualitative descriptions include core description and scanning electron microscope (SEM) observation. The quantitative analyses include X-ray diffraction, total organic content analysis, vitrinite reflectance, maceral composition, porosity, and permeability, as well as gas and element composition. The Dawuba Formation could be divided into four members. In general, the shale in the first and third members showed similarly high organic matter content with most samples in range of 2–2.5% and higher brittle mineral content with a content of quartz 40.53% and 33.21%, respectively, compared with the other two members, as well as high gas content. However, the first member shale samples exhibited much higher porosity and permeability than the third member shale samples. Furthermore, the shale gas in the first member was chiefly composed of methane (average: 83.63%), while that in the third member mainly consisted of nitrogen (average: 79.92%). Hence, the first member should be regarded as the most favorable target.

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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 II 2 or Type III. Stable carbon isotope (δ 13 C carb ) 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.