Fine-Grained Lithofacies Types and Sedimentary Model of the Upper Permian Longtan Formation Coal Measures in the Western Guizhou Region, South China

The upper Permian Longtan Formation is widely distributedin southwesternChina and is well known for multilayer coal and high organic shale,with significant shale gas potential that has yet to be fully exploredand developed. The Longtan coal-bearing strata are composed of complexlithological assemblages of fine-grained sedimentary rocks such assandstone, coal, shale, and limestone, which exhibit significant differencesfrom marine shale. To better understand the organic-rich lithofacies,their distribution, and their controlling factors, this study carriedout a detailed survey of the outcrop and drill cores in the westernGuizhou region and examined the fine-grained lithofacies, their assemblages,and their geochemical characteristics. The results showed that (1)the total organic carbon of the Longtan Formation shale in westernGuizhou ranged from 1.44 to 14.79%, with an average of 6.41%, andthe organic matter was mainly composed of vitrinite. The mineral compositionwas mainly clay minerals and brittle minerals; the clay minerals weremainly composed of kaolinite (average 11.13%) and illite/smectitemixed layers (average 26.69%) and the brittle minerals were mainlycomposed of quartz (average 31.63%) and feldspar (average 12.88%).(2) Eight types of lithofacies were identified, including silty mudstone,muddy siltstone, carbonaceous mudstone, carbonaceous shale, bioclastic-bearingmudstone, bioclastic-bearing sandstone, fine sandstone, and coal seam.(3) The six typical lithofacies assemblages were developed in theLongtan Formation, which represented different sedimentary environmentsof the marine-continental transitional facies in the studyarea. The lithofacies assemblages A and C represent sedimentationin the lagoon environment. The lithofacies assemblage B representspeat swamp facies. The lithofacies assemblage D represents a tidalflat facies peat flat-mixed flat-sand flat sedimentary environment.The lithofacies assemblage E and F represent the delta sedimentaryenvironment. (4) The sedimentary model of the Longtan Formation inwestern Guizhou was predominantly deltaic and tidal flat sedimentarysystems. Lithological and lithofacies studies of Longtan fine-grainedrocks were used to provide a geological framework for examining thefine grain deposition distribution and shale gas resource evaluation.This study is highly important for understanding the sedimentologyand oil and gas exploration in the region, providing a basis for identifyingand exploring coal-bearing shale gas potential and a reference forthe analysis of shale in the world's continental transitionalareas.

Automated summary

This study conducted a detailed survey of the outcrop and drill cores in the western Guizhou region to examine the distribution and geochemical characteristics of the Longtan Formation. The results showed that the shale in the Longtan Formation has a significant amount of total organic carbon and is mainly composed of vitrinite, clay minerals, and brittle minerals. The study identified eight types of lithofacies and six lithofacies assemblages, representing different sedimentary environments. The sedimentary model of the Longtan Formation in western Guizhou was predominantly deltaic and tidal flat systems.