四川盆地蓬莱—南充地区茅二段高频层序划分及滩体分布特征OA

The Middle Permian Maokou Formation is a critical gas-bearing interval in the Sichuan Basin.It mainly develops dolomite shoals,which are thin,highly heterogeneous,and significantly controlled by sequences.High-frequency sequences can reflect short-period seal level fluctuations,thereby controlling sedimentary facies migration of shallow-water carbonates.So far,the high-frequency sequences and their controls on shoal evolution and distribution in the Penglai-Nanchong area of the Sichuan Basin have been rarely studied.Based on seismic data,core observations,thin section identification,and logging data,the high-frequency sequence stratigraphic framework of the Maokou Formation in the study area was established.The microfacies were identified and classified,and the responding relationship between shoal evolution,migration and high-frequency sequences was defined.Moreover,the planar distribution of shoals under the control of high-frequency sequences was analyzed.Results show that(i)the Maokou Formation can be divided into 3 third-order sequences(SQ1-SQ3)and 11 fourth-order sequences(SSQ1-SSQ11).Each third-order sequence consists of a transgressive system tract and a highstand system tract;(ii)the sedimentary cycles present multiple assemblage sequences,including intershoal sea-bioclastic shoal-psammitic shoal,bioclastic shoal-psammitic shoal,intershoal sea-bioclastic shoal,psammitic shoal-psammitic shoal,and bioclastic shoal-bioclastic shoal.The types of sedimentary cycles are jointly governed by fourth-order sea-level fluctuations and sedimentary microgeomorphology.Psammitic shoals are susceptible to dolomitization and constitute favorable shoal reservoirs;and(iii)favorable shoals are mainly developed in the upper part of Mao-2 Member,with a thickness ranging from 5 to 16 meters.At platform margins,favorable shoals are mainly distributed in SSQ7 of the Well PY10 area as well as in SSQ8 of the Well PY10 and Well PS2 areas.Also,within open platforms,favorable shoals are concentrated in SSQ9 of the Well PS12-NC2 area.It is concluded that the exploration of the Mao-2 Member dolomite shoals in the Penglai-Nanchong area should focus on fine characterization of micro-geomorphology constrained by fourth-order sequences,especially the micro-geomorphic highlands.