The Role of Deep Geofluids in the Enrichment of Sedimentary Organic Matter: A Case Study of the Late Ordovician-Early Silurian in the Upper Yangtze Region and Early Cambrian in the Lower Yangtze Region, South China

Abstract

Organic matter is the material basis for shales to generate hydrocarbon, as well as the main reservoir space and seepage channel for shale gas. When the thermal evolution degree is consistent, the organic carbon content in present shales is subject to the abundance of primitive sedimentary organic matter. Deep geofluids significantly influence the sedimentary organic matter’s enrichment, but the mechanism remains unclear. This paper is aimed at determining how hydrothermal and volcanic activities affected the enrichment of sedimentary organic matter by studying lower Cambrian shales in the lower Yangtze region and upper Ordovician-lower Silurian shales. Oxidation-reduction and biological productivity are used as indicators in the study. The result shows that hydrothermal or volcanic activities affected the enrichment of sedimentary organic matter by influencing climate changes and the nutrients’ sources on the waterbody’s surface and reducing water at the bottom. In the lower Cambrian shales of the Wangyinpu Formation in the lower Yangtze region, hydrothermal origin caused excess silicon. During the sedimentary period of the lower and middle-upper Wangyinpu Formation, vigorous hydrothermal activities increased the biological productivity on the waterbody’s surface and intensified the reducibility at the bottom of the waterbody, which enabled the rich sedimentary organic matter to be well preserved. During the sedimentary period of the lower upper Ordovician Wufeng Formation and the lower Silurian Longmaxi Formation in the upper Yangtze region, frequent volcanic activities caused high biological productivity on the waterbody surface and strong reducibility at the bottom of the waterbody. As a result, the abundant organic matter deposited from the water surface can be well preserved. During the sedimentary period of the upper Longmaxi Formation, volcanic activities died down gradually then disappeared, causing the biological productivity on the water surface to decrease. Besides, the small amount of organic matter deposited from the water surface was destroyed due to oxidation.

Geochemical Characteristics of the Middle Devonian Dacaozi-Tanshanping Shale Strata in the Yanyuan Basin, Southwest China: Implications for Organic Matter Accumulation and Preservation

“…In particular, the initial productivity of waterbody can be characterized by the 14 Geofluids nutrient richness and organic carbon flux of surface water, and the change of initial productivity plays a key role in the accumulation of organic matter. The Ba content is an indicator widely used to reflect the paleoocean bioproductivity [56][57][58][59][60]. There is a high concentration of SO 4 2-ions reoxidized by H 2 S on the surface of decayed organic matter.…”

A Study on Astronomical Cycle Identification and Environmental Response Characteristics of Lacustrine Deep-Water Fine-Grained Sedimentary Rocks: A Case Study of the Lower Submember of Member 3 of Shahejie Formation in Well Fanye-1 of Dongying Sag, Bohai Bay Basin, China

“…Previous studies on lacustrine fine-grained rocks have shown that the sedimentation process of lacustrine finegrained rocks with frequent lithological changes is closely related to the changes in the paleoenvironment. The evolution of the paleoenvironment tends to lead to the changes in the chemical composition of rocks, mineral composition, content of organic matter, rock type, and other aspects [50][51][52][53][54][55][56][57][58], so indicators such as geochemistry and mineral composition can be used to study the paleoclimate, paleowater depth, paleoredox conditions, paleosalinity, paleoproductivity, and other aspects [56][57][58][59][60]. In this paper, various indicators for the changes in the sedimentary environment were selected to further explore and confirm that the sedimentary environment of lacustrine fine-grained rocks in the lower Es3 submember of the Paleogene Shahejie Formation in Well Fanye-1 of the Dongying sag was affected by the Milankovitch cycle.…”

Slope Belts of Paleouplifts Control the Pore Structure of Organic Matter of Marine Shale: A Comparative Study of Lower Cambrian Rocks in the Sichuan Basin

“…However, from the perspective of the Niutitang shale in Chongqing, its thermal maturity is much higher than that in the Jingyan, Yichang, and Guizhou regions, while there are no OM pores of the Niutitang shale in Chongqing. However, other parameters such as TOC content, mineral composition, and kerogen type of the Niutitang shale reservoirs in the four regions (Jingyan, Chongqing, Yichang, and Guizhou) have similar characteristics [2,6,7,40,42,[48][49][50][51]. Therefore, thermal maturity is the key factor to control the OM pore development.…”