Hirantian lithofacies palaeogeography and their geological significance for shale gas in Micangshan-Dabashan area

A unique sedimentary unit with Hirantina-Dalmanitina fauna develops in the upper Yangtze block. Its sedimentary environment is controversial and the detailed study of its space-time changes of sedimentary facies is deficient. In this paper, by the ways of systematic data collection, detailed observation of outcrops, detailed microscopic studies and analyses of thin sections, and the "point-line-plane" analytical method of lithofacies palaeogeography, the Hirantian lithofacies and palaeogeography in Micangshan-Dabashan area have been rebuilt. Analyses of sedimentary facies show that the Guanyinqiao Member in the study area developed in different sedimentary environments with different lithofacies. It was the product of Hirantian sealevel fall, and more than one sea level fluctuation happened during Hirantian period. The sedimentary records of the marine regression are relatively obvious, with the transgression records responding only at the edge or the underwater highlands of the Yangtze basin. The recovered paleoeography shows a sedimentary paleogeographical pattern high in the north, low in the north and the west, and a number of underwater highlands in the middle, which is conducive to the formation of high-quality organic-rich shales in the Yangtze craton basin. Sedimentary facies of Guanyinqiao Member in the study area are dominated by continental shelf facies, especially the shallow shelf sub-facies, with some local isolated platforms and carbonate ramps, and the littoral facies are only distributed in Yangtze craton basin along the northwestern margin of the old land in the region at that time. The continental shelf facies developing silty mudstones and carbonificious mudstones, which is far away from sedimentary provenances and submarine highs, are the high-quality reservoir rocks for shale gas. It is suggested that in order to promote the production of shale gas in the isolated platforms and carbonate ramps, dolostone and muddy limestone layers of the Wufeng-Longmaxi Formation should be fractured throughout.