Sedimentary facies analysis of Maokou formation of middle Permian in Sichuan basin

Based on the detailed study of field profile and drillingl sections,combined with well logging and seismic data and previous research results,we analyzed the sedimentary facies of the Maokou formation of middle Permian in Sichuan Basin.The Maokou formation of middle Permian can be defined as carbonate platform sedimentary systems including four major sedimentary facies such as shallow open platform,deeper open platform,platform edge shallow facies and platform margin slope facies.The shallow open platform consists of inner bar and subtidal intrafacies;the platform edge shallow facies includes organic bank and interbank intrafacies;the platform margin slope facies can be divided into the upper slope and lower slope intrafacies.In the Maokou stage of middle Permian,Sichuan basin experienced a largescale and three small-scale transgressive-regressive cyclic deposition.We divided the Maokou formation into four sections such as Mao Ⅰ,Mao Ⅱ,Mao Ⅲ and Mao Ⅳ,in which the lower Mao Ⅰ-the upper Mao Ⅰ sections compose the first sedimentary cycle,the lower Mao Ⅱ-the upper Mao Ⅱ sections compose the second sedimentary cycle and the Mao Ⅲ-Mao Ⅳ sections compose the third sedimentary cycle.Comprehensive study indicates that the platform edge shallow facies with high energy organic bank is the most favorable facies belt for the evolution of the reservoirs.It develops in Mao Ⅱ,Ⅲ and Ⅳ and mainly distributes in the northern margin-southwestern margin of Sichuan basin with large thickness and wide spread.In the study area,deeper open platform and platform margin slope facies mainly distributes in Mao Ⅲ and Mao Ⅳ in vertical and northernpart of the basin in horizontal,so it is more favorable to hy drocarbon generation and evolution.