Study On The Depositional System And Sequence Stratigraphy Of Upper Triassic-Jurassic In Sichuan Basin

Based on the theories of continental sequence stratigraphy and sedimentology, this paper examines the sedimentary system, sequence stratigraphy, lithofacies paleogeography and source-reservoir-cap assemblages of Xujiahe Formation to Jurassic in Sichuan basin by using logging, seismic and geologic methods.By firstly referring to the regional geotectonic setting and sedimentary setting data, and then investigating field sections, this paper divided the sedimentary system of upper Triassic to Jurassic in Sichuan basin into 3 types of sedimentary system groups, 6 types of sedimentary systems, 18 sedimentary subfacies and 34 sedimentary microfacies, based on outcrop, seismic and well drilling/logging data. It also determined 6 structural sequence, 11 third-order sequence or long cycles, and deliberately discussed the characteristics, distribution and controlling factor of sequence and system tract. The conclusions were made that Jurassic alluvial fan system was mainly developed in retrograding period, while delta system was mainly developed in prograding period.The structure-sequence lithofacies paleogeographical map of upper Triassic～Jurassic of Sichuan basin was compiled by adopting structural sequence system tract as basic mapping unit. Moreover, the lithofacies paleogeographical characteristics and evolution of different structural sequence and system tracts was demonstrated in details. By studying the source-reservoir-cap assemblage features in structural sequence settings, the paper determined 4 oil-bearing sub-systems and 5 source-reservoir-cap assemblages, and discussed the spatial and temporal distribution features of each assemblage. Then it concluded that sedimentary system developed in upper Triassic-Jurassic reservoir was dominantly delta sedimentary system. The reservoir sandstone was dominantly underwater distributary channel and debouch bar microfacies of delta front, located in basin extension system tract of structural sequence, late stage of basin contraction system tract and prograding system tract in secondary sequence setting.By combining previous exploration with the study of sedimentary system, sequence stratigraphy and their relationship with hydrocarbon distribution, favorable reservoir distribution in the study area was estimated in the paper.