Geochemical Properties and Gas-Bearing Analysis of Lower Cambrian Black Shale in Western Hunan Province

Abstract

Western Hunan province and its surrounding areas are significant targets for shale gas exploration and development in southern China, where the black shale of the lower Cambrian Niutitang Formation and Wunitang Formation is extensively distributed. Geochemical analysis was conducted on the lower Cambrian black shale from a new exploration well of XAD1 located at the southeast margin of the Yangtze paraplatform, followed by a discussion on gas-bearing properties using molecular dynamics simulation. The geochemical characteristics indicate that the black shale in well XAD1 was primarily deposited in a strongly reducing marine environment, with organic matter predominantly composed of type I kerogen derived from algae. Currently, it has reached a stage of high to over maturity with limited potential for liquid hydrocarbon generation. The recovery of the original hydrocarbon generation potential shows that they are excellent source rocks and have completed the main hydrocarbon generation evolution. Despite the favorable conditions for shale gas formation observed in well XAD1, the low measured gas content within the Niutitang Formation suggests that other geological factors may have contributed to a substantial loss of shale gas. Gas adsorption simulation reveals that the maximum methane adsorption capacity (15.77 m3/t) was achieved by Niutitang shale during the late Silurian period when there was an abundant source of natural gas without any influence from CO2, H2O or other molecules. However, due to a lack of natural gas replenishment and subsequent tectonic uplift and subsidence causing variations in temperature and pressure, the methane adsorption capacity gradually decreased (to 6.56 m3/t). Furthermore, water occurrence within the shale reservoir further reduced the methane adsorption capacity (below 2 m3/t), while tectonic activities exacerbated the loss of shale gas potential within this study area. The findings indicate that the dynamic alteration of gas-bearing properties in shale reservoirs due to tectonic movements is a crucial factor influencing the success rate of shale gas exploration in the study area, provided that there are sufficient gas resources and superior reservoir conditions.