--> Analysis of Geological Effects on Methane Adsorption Capacity of Continental Shale: A Case Study of the Jurassic Shales in the Tarim Basin, Northwestern China

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Analysis of Geological Effects on Methane Adsorption Capacity of Continental Shale: A Case Study of the Jurassic Shales in the Tarim Basin, Northwestern China

Abstract

The geological effects on methane adsorption capacity for the Jurassic continental shales in the Tarim Basin, Northwestern China, have been investigated in this paper compared to the marine gas shales in the North Amarica and the South China. The methane adsorption capacity ranges from 0.58 cm3/g to 16.57 cm3/g and the total organic carbon (TOC) content is between 0.5 wt% and 13.5 wt%. The organic maturity measured by Tmax is between 410 °C (immature) and 499 °C (overmature). The methane adsorption capacity of the Jurassic continental shales in Tarim Basin is affected by many geological factors, including the TOC content, organic matter maturity, mineral composition, surface area, pore size distribution, etc. The TOC content is the most significant factor with a positive effect on the adsorption capacity of the Jurassic shales, and the influence varies piecewise according to the TOC content. The TOC content contributes much more to the methane adsorption capacity of organic-rich shale samples (TOC content > 0.7 wt%) than to the organic-lean samples (TOC content < 0.7 wt%). The mineral composition is a secondary factor, and the abundance of clay content has a positive effect on the methane adsorption capacity despite its relatively weaker adsorption ability compared to TOC. The pore size distribution has different effects on surface area and pore volume. Mesopores and micropores provide the major surface area and are mainly derives from TOC and illite, which has a positive influence on the adsorption capacity. Mesopores and macropores offer the major pore volume and are mainly formed by illite, which is the major contributor for pore volume rather than surface area. Besides, the TOC and illite contents of the Jurassic shales in Tarim Basin are closely related to the origin, maturity and diagenesis evolution of the shale: 1) both TOC and illite contents variations are related to the different provenances and depositional environments of shale; 2) the decrease of TOC content with increasing maturity is also partly attributed to hydrocarbon generation; and 3) the increase of illite content with increasing maturity is due to illitization in the diagenesis of shale.