The organic geochemistry, pore structure and methane adsorption/storage capacity of lacustrine shales from the Cretaceous Madongshan Formation, Liupanshan Basin, China

Abstract

Lacustrine shale from Madongshan Formation in Liupanshan basin has been the important target strata for shale gas exploration in northwest China. However, the system cognition in generation, reservoir conditions and geological gas-in-place (GIP) estimation of Madongshan shale is quite limited. In this research, the shale strata are evaluated in terms of shale quality, pore structure and methane adsorption/storage capacity. The results show that Madongshan shales mainly belong to mixed mudstone and siliceous mudstone and have the relatively high brittle minerals content and low clay minerals contents, indicating a favorable condition for hydraulic fracturing. The Rock-Eval pyrolysis reveals that the shales can be classified as good to very good to source rock with relatively high TOC over 2% and contain type Ⅰ and Ⅱ kerogen in the early maturation stage with a fixed amount of gas can be produced. In terms of pore structure, the Type IV isotherms and Type H3 hysteresis loops of shale samples show that mesopores and macropores with slit-like pore types are the dominated space in Madongshan shales. The maxima of methane adsorption capacity (n0) ranged from 2.32 ml/g to 5.96 ml/g are significantly by organic matters and clays. Despite the relative importance of organic carbon to methane adsorption capacity, influence of TOC on pore surface area and volume is negative interaction, perhaps reflecting the dissolution of methane in the residual bitumen. Thus, an assessment on lacustrine shale gas with low maturity needs to consider the influences of solute methane in residual bitumen, apart from adsorption methane, so as to obtain a reliable shale gas resource. The clays show a complex relationship of influence on n0 and it may be related to the pore structure and the difference of methane capacity between clays and organic matters. At last, a modified and simpler equation which can eliminate the errors is adopted to calculate the methane storage capacity of Madongshan shales. And the methane storage capacities of Madongshan shales range from 2.06 ml/g to 2.16 ml/g.