Sequence-based lithofacies and paleogeography of Lower Triassic Feixianguan Formation in southeastern Sichuan Basin

Abstract：Guided by the sequence stratigraphy and sedimentology and aimed at oil-gas exploration, the authors recognized two third-order sequences (SQ1, SQ2) in the Feixianguan Formation, both of which belong to the typeⅡsequence. Sequence-based lithofacies and paleogeographic maps of Feixianguan Formation were compiled by means of condensation and instantaneity with mapping units of sedimentary system tracts. The result shows that, from the phase of SQ1TST to that of SQ2LHST, the deposition evolution changed from the mixing shelf facies to the platform facies. That is to say, during the phase of SQ1TST, the mixing shelf constituted the setting of the study area due to the rising of the sea level. During the phase of SQ1HST, based on previous deposition, the depth of the basin water gradually turned shallow and the environment changed form the mixing shelf to the open platform; at that time, the oolite beach was developed. During the phases of SQ2TST and SQ2EHST, the setting of the study area didn't change, but the oolite beaches experienced something unusual. During the phase of SQ2EHST, the number and the sizes of the oolite beaches became optimal. During the phase of SQ2LHST, however, the relative falling of the sea level and the deposition of the sediments resulted in the change of the environment of the study area from the open platform to the restricted platform, with the sedimentary rocks being mainly limestone, dolomite and gypsum rocks. It is thus concluded that the oolite beaches were mainly developed during the phase of SQ2EHST, and these beaches hence deserve much attention in future exploration.