Prediction of organic matter carbonization zones of the Lower Silurian Longmaxi Formation in the Middle-Upper Yangtze region, China

Abstract

Selecting and evaluating marine shale gas areas in South China to determine the distribution range of organic matter carbonization zones and exclude the high-risk zones of shale gas exploration are significant tasks. Based on a large number of drilling and outcrop data, this study has carried out the identification and prediction of the organic matter carbonization areas of the Lower Silurian Longmaxi Formation in the Middle-Upper Yangtze region. This study compiled the industrial maps of the distribution of the organic matter carbonization areas in the Longmaxi Formation, as well as preliminarily obtained four achievements and understandings. Firstly, the criteria for identification of organic matter carbonization in high-over mature marine shales were put forward, in which the response characteristic of thin neck type from the logging resistivity is exclusively correlated with that of the carbonization of organic matter in shales. The above is the perfect standard utilized as it is reliable to apply this method and criterion to judge organic matter carbonization of high-over mature marine shales. Secondly, the distribution of organic matter carbonization for the Longmaxi Formation in the Middle-Upper Yangtze region has certain regularity, indicating that carbonization points are mainly distributed in four exploration areas (i.e., East Sichuan-West Hubei, Northwest Hubei, the west of South Sichuan, and the east of the Changning structure) covering an area of more than 35,000 km2. Thirdly, the organic matter carbonization of the Longmaxi Formation shows a trend of continuous distribution in Eastern Sichuan-Western Hubei and the west of Southern Sichuan, with characteristics of a wide area as well as weak-severe carbonization. Finally, regions of organic matter carbonization are relatively small, showing the characteristics of serious and weak carbonization in Northwest Hubei and the east of the Changning structure, respectively. Consequently, these four carbonization areas are restricted zones for the shale gas exploration of the Longmaxi Formation, and high-risk belts for the shale gas resource evaluation and strategically selected areas.