Sequence Stratigraphy and Lithofacies Paleogeography of the Lower Permian Qixia Stage in Southwestern Sichuan Basin
[Objective]The Lower Permian Qixia Stage in southwest Sichuan Basin has shown significant exploration potential in recent years,but the views on the sedimentary filling pattern and paleogeographic framework in this area are not unified,which seriously restricts the further oil and gas exploration and well location deployment in the basin.[Methods]This study of the Lower Permian Liangshan Formation and Qixia Formation in southwestern Sichuan Ba-sin was based on the comprehensive use of field profiles,drilling cores and logging data,and analysis of rock types and sedimentary facies.The application of the principles and methods of marine carbonate stratigraphical sequencing led to the identification of third-order sequence interfaces in the Qixia Stage.Single-factor analysis and multi-factor comprehensive mapping were adopted,taking the three-level sequence as the mapping unit.Combining these with the dominant phase,maps of the SQ0 + SQ1 and SQ2 sequence paleogeographic lithofacies were drawn for the Qixia Stage in the study area.[Results]Four third-order sequence interfaces were identified in the Qixia Stage,from bot-tom to top:lower interface of Liangshan Formation(SB Ⅰ),inner interface of Qixia Formation Group 1 section(SB Ⅱ),interface of Qixia Formation Groups 1 and 2(SB Ⅱ)and interface of the Qixia Formation and Maokou Formation(SB Ⅰ).These occurred in three tertiary sequences,each consisting of a transgressive domain and a high-order do-main.Comparative analysis of the sequential stratigraphic framework suggested that the SQ0 sequence of the lower Qixia Stage corresponds roughly to the Liangshan Formation.In the lower Qixia Formation Group 1 section,SQ0 is ev-ident only in the paleogeomorphic lowland before the deposition of Qixia Stage in the study area,with a filling se-quence overlapping a paleogeomorphic highland.Sequence SQ1 in the middle region and SQ2 sequence in the upper region are found throughout the area.The sequence in the southwestern Sichuan Basin consists of a marine carbonate platform bordering the Kang-Dian ancient land in the west,with obvious differentiation of paleogeomorphology and sedimentary facies.From west to east,the Kang-Dian land and its eastern margin consisted of a tidal flat,intraplat-form depression,intraplatform mound-shoal,and an open semi-confined platform.In addition,an intraplatform mound-shoal beach developed along the intraplatform slope break zone,tending to surround the intraplatform depres-sion.[Conclusions]Analysis of the influence of the slope break zone on carbonate reservoir formation highlighted that the SQ2 sequence is the most favorable reservoir facies belt in the study area,with favorable exploration areas around the intraplatform mound-shoal facies belt.A new understanding of the filling pattern and paleogeographic litho-facies characteristics of the Qixia Stage,as determined from the sequence stratigraphy,provides a new framework for future reservoir studies and the prediction of favorable exploration areas in the Qixia Formation.
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