The Characteristics And Main Controlling Factors Of The Fourth Member Of Middle Triassic Leikoupo Formation Microbialite Reservoir In The Western Sichuan Basin

The Middle Triassic was between the two mass extinction events in Earth's history.It was the slow recovery period of earth creatures after the end-Permian mass extinction is herein.During this stage,the oxygen content in the atmosphere was relatively low,and the Sichuan Basin in the South China plate was located in a high-temperature and dry environment near the equator or in the northern latitudes at low latitudes.At the same time,in the western Sichuan region,due to the surrounding paleo-uplift confinement,the high-salinity,high-temperature and drought-and extreme-anoxic extreme environments,microbial rocks are widely developed.This article takes 10 key drillings and 3 field sections in the western Sichuan area as the research object.Combined previous studies on sedimentary reservoirs in the Sichuan Basin,and based on the sedimentary environment research,macro-micro features analysis and trace element geochemical characteristics analysis,the study found that the Leikoupo Formation of the Middle Triassic mainly developed three kinds of microbial rocks: thrombolites,stromatolites,and thrombolitic stromatolites.The type,morphology,and distribution of microbialites are clearly controlled by the depositional environment.Microbialites mainly develops in high salinity and anoxic weak hydrodynamic environment under arid and hot climate,in which the long-term and stable anoxic sulfide environment is more conducive to the development of the thrombolite.Compared with stromatolites,the development environment of thrombolites is weaker and deeper.Microbial dolomites mainly captures organic colloids rich in light rare earth elements(LREEs)due to initial complexation on organic ligands in the biofilm.It was found that the constructive diagenesis of microbialites in the study area are dolomitization and dissolution.The dolomitization fluids in the Leikoupo Formation of the Middle Triassic are close to seawater,and microbial dolomites are mainly formed in the contemporaneous-penecontemporaneous phase.There are three types of microbialite reservoirs developed in the fourth member of the Leikoupo Formation in the study area: thrombolitic dolomite reservoirs,stromatolitic dolomite reservoirs,and thrombolite-stromatolitic dolomite reservoirs.The thrombolitic dolomite reservoir spaces are mainly composed of unfilled irregular lattice pores between the clot grids,dissolved pores related to the microsphere structure,intergranular dissolved pores and intercrystalline pores.These irregular pore distributions are obvious fabric selective and mostly distributed in the bright layer.The space of stromatolite dolomite reservoirs is mainly composed of stratified microbial-rich dark-layer and inter-frame dissolved pores.The pore distribution in stromatolite dolomite reservoirs has obvious structure selectivity and directionality,and is mainly distributed in the bright strata in the stromatolite frame and spread along the development direction of the layer.The thrombolite-stromatolitic dolomite reservoir space is mainly composed of framework pore,intercrystalline pore and intergranular pore.The distribution of thrombolite-stromatolitic dolomite reservoir space is non-directional and layered.The microbial carbonate reservoir space is obviously controlled by the microbialite structure.Microbialite reservoirs are controlled by sedimentary environment and diagenesis.The high salinity and anoxic weak hydrodynamic environment under the arid and hot palaeoclimatic climate in the study area provide a favorable environment for the extensive development of thrombolites and stromatolites.The dolomitization and burial dissolution a key role in reservoirs' modification.