Sedimentary Environment and Organic Matter Accumulation of Black Rock Series of Wufeng-Longmaxi Formations in Foreland Depression, Western Hunan Province: An Example from Well TD2 in Changde Area

As the main sediments across the Yangtze area during the Ordovician-Silurian (O-S) transition, the Upper Ordovician Wufeng Formation and Lower Longmaxi Formation are widely distributed in the Yangtze Sea, and they are also the main targets for marine shale gas exploration in the South China. However, little attention has been paid to the Wufeng-Longmaxi black rock series in the foreland basin, southeastern Yangtze Block, due to the lack of complete and fresh outcrops caused by intensive erosion and tectonic damage. In this paper, by utilizing the newly obtained drilling cores of the Well TD2 located in the Hunan Province, the sedimentary environment and organic matter accumulation of the Wufeng-Longmaxi black rock series in the foreland basin were investigated based on the systematically petrological and geochemical analysis. The organic-rich shale of the Wufeng-Longmaxi formations is dominated by siliceous shale and carbonaceous shale, in which plenty of terrigenous clast and even thin sandstone can be observed. The black shale with TOC > 2% is about 21 m in thickness. Four sedimentary cycles can be recognized from the O-S black series and they exhibit different lithological and geochemical variation characteristics. The organic shale generally developed in the Late Katian to the Early Rhuddanian, while the black rock series developed during the Late Rhuddanian to Early Aeronian have low TOC content and are discontinuous. Based on the analyses of lithology, mineral composition and geochemistry, tectonism, sea-level and terrigenous supply should be the primary controlling factors for the deposition of the O-S black shale in the foreland basin, Yangtze area. During the Late Katian to Early Rhuddanian, the study area experienced significantly tectonic subsidence and sea-level rise caused by regional tectonic compression and global sea-level rise, then strongly reducing conditions were formed and paleoproductivity was improved in the deep-depression area, which promoted the enrichment of organic matter in black rock series during this stage. Nevertheless, with intensifying tectonic collision between the Yangtze and Cathaysia blocks since Late Rhuddanian, the tectonism in the study area changed from tectonic subsidence to uplift. Persistent tectonic uplift and intensive weathering conditions led to the noticeable increase of terrigenous supply in the foreland depression, as well as gradual sea-level fall, and then, the development of black organic-rich shale in the foreland basin was terminated at the Early Aeronian. It should be noted that during the earliest Aeronian stage, global glacial sea-level rise promotes the development of black shale in the study area again. Finally, a sedimentary model for the O-S black shale in the foreland depression, western Hunan area was proposed. This study is not only beneficial for gaining more geological knowledge about the black shale formation in a marine foreland basin, but also helpful for understanding the sedimentary responses to major geological events occurred in Yangtze area during the O-S transition.