Shale Gas Accumulation Conditions and Gas Content Calculation: A Case Study of Sichuan Basin and Its Periphery in the Lower Paleozoic

Abstract

Based on theory and condition of shale gas accumulation, shale gas accumulation factors in this study are classified into internal and external ones so as to better study accumulation conditions and calculation methods of gas content. The internal factors include geochemical parameters, mineral compositions and petrophysical parameters of shale, while the external factors include reservoir depth, temperature and pressure, etc. Geochemical analysis of 41 black shale samples from Lower Cambrian and Upper Ordovician and Lower Silurian strata in Sichuan basin and its surrounding area shows that mineral compositions, physical features, depth, temperature and pressure affect the shale gas contents and also reflect eight internal parameters (organic carbon, total hydrocarbon content, quartz content, clay minerals, porosity, density, pyrite and carbonate contents) which have important influence over shale gas accumulation. A formula of calculation for shale gas contents was obtained using the multivariate linear regression analysis between two parameters, six parameters, or eight parameters and gas content, and results based on this formula is in good agreement to in site measurement. The more parameters, the more precision. The study recommends that if conditions permitting, it is better to use more parameters to predict the shale gas content in order to achieve better results. It is also suggested that at the early stage of exploration, two leading parameters (organic carbon content and porosity) should be given priority to calculate the shale gas content.