Origin and enrichment factors of natural gas from the Lower Silurian Songkan Formation in northern Guizhou province, south China

Abstract

The northern Guizhou area, located near the southwestern margin of the Yangtze Block and south of the Sichuan Basin, is a promising area for shale gas exploration and development. The discovery of a natural gas accumulation in the Lower Silurian Songkan Formation in the study area is the first discovery of natural gas in this stratum in China, marking an exciting breakthrough in natural gas exploration in northern Guizhou area. This paper presents an investigation of this accumulation using gas geochemistry, including gas composition, carbon and hydrogen isotopes, in context with geological information available for the study area.Our results indicate that the natural gas accumulation in Songkan Formation is probably sourced from the Lower Silurian Wufeng-Longmaxi shale. The δ13C(CH4) values range from −33.9‰ to −33.2‰, the δ13C(C2H6) values range from −37.0‰ to −36.2‰, and the δ2H(CH4) values range from −157‰ to −144‰. These results indicate that the gases are of thermogenic origin and are oil-derived. Furthermore, isotopic rollovers seen in the natural gas is similar to that seen in the natural gas from the local Wufeng-Longmaxi shale, which is identified as carbon exchange at high temperature. The δ13C(CO2) values range from −20.8 to −17.1‰, suggesting that carbon dioxide was mainly generated by thermogenic processes. The fracture investigation indicates that the large development of horizontal fractures in calcareous mudstone is one of the reasons for the high yield of natural gas in Songkan Formation. The petrophysical parameters suggest that limestone with low porosity and permeability effectively prevents natural gas escaping and seals natural gas into calcareous mudstone with horizontal fractures.