Lithology and sedimentary heterogeneity of Longmaxi shale in the southern Sichuan Basin

Abstract

Producers have always been eager to know the reasons for the difference in the production of different shale gas wells. The Southern Sichuan Basin in China is one of the main production zones of Longmaxi shale gas, whereas the shale gas production is quite different in different shale gas wells. The Longmaxi Formation was deposited in a deepwater shelf that had poor circulation with the open ocean, and it is composed of a variety of facies that are dominated by fine-grained (clay to silt size) particles with varied organic matter distribution, causing heterogeneity of the shale gas concentration. According to the different mother debris and sedimentary environment, we recognized three general sedimentary subfacies and seven lithofacies on the basis of mineralogy, sedimentary texture and structures, biota, and the logging response: (1) There are graptolite-rich shale facies, siliceous shale facies, calcareous shale facies, and a small amount of argillaceous limestone facies in the deepwater shelf of the Weiyuan area and graptolite-rich shale facies and carbonaceous shale facies in the Changning area; (2) there are argillaceous shale facies and argillaceous limestone facies in the semi-deepwater continental shelf of the Weiyuan area and silty shale facies in the Changning area; and (3) argillaceous shale facies mainly developed in the shallow muddy continental shelf of the Weiyuan area, whereas silty shale facies mainly developed in the shallow shelf of the Changning area. Judging from the biostratigraphy of graptolite, the sedimentary environment was different in different stages.