Sedimentary characteristics of the upper part of the fourth member of Leikoupo Formation of Middle Triassic in western Sichuan Basin

Abstract

Based on the data of outcrop, drilling core, logging, rock thin section and carbon and oxygen isotope test, this paper carried out systematic analysis for characteristics and model of sedimentary of the upper part of the fourth member of Leikoupo Formation in western Sichuan Basin, favorable sedimentary facies for the development of reservoir were proposed. Results showed that in the sedimentary period of the upper part of the fourth member of the Middle Triassic Leikoupo Formation, the western Sichuan Basin was in the sedimentary environment of tidal flat within restricted and evaporative platform. The sedimentary microfacies were gypseous-dolomite flat, (algae)-dolomite flat, (algae)-dolomite-lime flat, (algae)-lime-dolomite flat, (algae) intraclast beach, (algae)-lime flat, intraclast beach etc. According to the establishment of the sedimentary model, due to the barrier of beach/island and the influence of arid climate conditions, the seawater supply was limited during the deposition of the upper submember of the fourth member of Leikoupo Formation in the western Sichuan Basin, which formed a high salinity sedimentary environment by strong evaporation and was good for large-scale dolomization. Flat terrain, shallow water, low energy, and the frequent exposure of multiple periods caused atmospheric water leaching and dissolution in the western Sichuan region. Algae were widely developed, which formed diverse carbonatite by algal activities, including algal stromatolite dolomite, algal clot dolomite, algal lamina dolomite and algal arene dolomite. By comprehensive analysis, for most part of the western Sichuan Basin, it was in the intertidal zone (algae)-dolomite flat sedimentary area, which was favorable for the penecontemporaneous exposure to atmospheric water leaching and dissolution, and was a favorable developing zone for porous-type reservoir.

Keywords

• tidal flat subfacies
• sedimentary characteristics
• the upper part of the fourth member of leikoupo formation
• triassic
• western sichuan basin