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

Abstract

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 ¹³C _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.