Analysis on distribution and main controlling factors of OM carbonization in marine shale in the Sichuan Basin of China and its periphery

Abstract

The distribution and main controlling factors of the organic matter carbonization points/zones of over-mature marine shale are important issues for shale gas exploration. This paper focuses on the Lower Cambrian and Lower Silurian shales in the Sichuan Basin and its periphery. Based on the detailed anatomy of key carbonization zones, predictions of shale organic matter carbonization zones and regional mapping of thermal maturity are carried out to explore and reveal the distribution and main controlling factors of the carbonization zones of organic matter in the two sets of shales. Four preliminary conclusions were obtained: (1) There are four organic matter carbonization zones in the Lower Silurian shale distribution area, namely eastern Sichuan-western Hubei, northwestern Hubei, west of southern Sichuan, and Renhuai-southwestern Chongqing, covering an area of nearly 40 × 10³ km². The main part of the western carbonization zone in the southern Sichuan is located in the Emei basalt distribution area. (2) The organic matter carbonization zone accounts for more than 80% of the Lower Cambrian shale distribution area, The non-carbonization zone is only distributed in Weiyuan-Ziyang, Moxi-Gaoshiti, Changyang, and Weng'an-Zhenyuan with an area of about 62.6 × 10³ km². (3) The Lower Silurian and Lower Cambrian shales in the west of southern Sichuan experienced an increase in thermal maturity R_O values of 0.2%–0.4% and 0.2%–1.0%, respectively, as a result of the Emei large igneous province, an extreme heat event in the Late Permian. The Lower Cambrian and Lower Silurian shales have the highest degree of carbonization of organic matter in Mabian-Ebian, which is also in the high-value area of the Emei basalt thickness. (4) The main controlling factors of organic matter carbonization of the Lower Cambrian and Lower Silurian shales have similarities and differences. The main controlling factor of the former is the long-term deep burial background, and the extreme thermal event in the Late Permian only exacerbated the degree of carbonization in some areas more seriously. The latter's main controlling factors are relatively complex and are controlled by deep burial background in the three regions of Renhuai-southwestern Chongqing, eastern Sichuan-western Hubei, and northwestern Hubei. It was governed by a deep burial background and a high geotemperature in the Late Permian.