Research on the Occurrence State of Methane Molecules in Postmature Marine shales—A Case Analysis of the Lower Silurian Longmaxi Formation Shales of the Upper Yangtze Region in Southern China

Kun Zhang,Yao Zeng,Pei Liu,Liwen Zhang,Liangyi Tang,Zhenxue Jiang,Fengli Han,Xuecheng Chen,Xuejiao Yuan,Yiming Yang,Zehao Zheng,Xueying Wang,Yan Song

doi:10.3389/feart.2022.864279

Abstract

The Yangtze region in southern China is endowed with abundant marine shale gas. Methane molecules exist in either adsorption state or free state in postmature marine shales, depending on the components of shales. In this study, the core samples of the selected well in the Lower Silurian Longmaxi Fm. shales from the Sichuan Basin, upper Yangtze region, southern China, were taken as study objects. We carried out TOC content, organic matter maturity, mineral component, and core gas content analyses and isothermal adsorption, FIB-SEM, and FIB-HIM experiments to analyze the occurrence state of methane molecules in postmature marine shales. The conclusions are as follows: most methane molecules exist in the organic matter pores of the postmature marine shales, and only a small amount of them exist in clay mineral pores. The organic matter pores in organic-rich shales are large in number with excellent roundness and are well connected, with large pores covering small ones. Thus, abundant free gas can be stored in the organic matter pores and pore throats, making it possible to densely and continuously adsorb methane molecules with a relatively large adsorption space. The flake-shaped clay minerals have a small number of pores with low roundness. Among the three clay minerals in postmature marine shales of the Longmaxi Fm., the I/S mixed layer offers certain reservoir spaces for adsorbed and free gases and chlorite stores a little adsorbed gas and little free gas, while illite hardly stores the adsorbed gas but contains a little free gas.

Highlights

In recent years, thanks to the advancement of unconventional geological theory, horizontal drilling technology, and hydraulic fracturing technology, shale gas exploration has scored a complete success in North America, changing the world energy use structure (Święch et al, 2017)
Langmuir volume refers to the maximum adsorbing capacity, whose physical denotation is the adsorbed gas content when the shales are saturated with methane at a given temperature, and the unit is m3/t
Among the three clay minerals in postmature marine shales in the Longmaxi Fm., the I/S mixed layer has the largest mineral layer spacing with powerful adsorption capacity, offering reservoir spaces for free gas and adsorbed methane molecules

Summary

INTRODUCTION

Thanks to the advancement of unconventional geological theory, horizontal drilling technology, and hydraulic fracturing technology, shale gas exploration has scored a complete success in North America, changing the world energy use structure (Święch et al, 2017). Shale gas can be divided into free gas and adsorbed gas based on the occurrence state The former exists in the reservoir space of shales, and the latter is adsorbed onto the inner surface of the organic matter and clay minerals. The primary cause of massive variance in the adsorption capacity of organic matter, clay minerals, and clastic minerals lies in the adsorption sites’ gas characteristics on the mineral surface In these years, massive exploration and advanced experiments on shale gas provided more data on the analysis of the adsorption state of methane in postmature marine shales with different agents. The research object in this study is the shales of the Lower Silurian Longmaxi Fm. of the upper Yangtze region in southern China. Some of the experimental data in this article came from the referred works (Guo et al, 2019; He et al, 2021; Jiang et al, 2022)

RESULTS AND DISCUSSION

DATA AVAILABILITY STATEMENT

CONCLUSION