Reef-beach sedimentary model and favorable facies zones of new block drilled in eastern Sichuan Basin:A case study of Upper Permian Changxing Formation in south margin of Bandong area
Successive breakthroughs have been made in the exploration and development for reef-beach gas reservoirs of the Upper Permian Changxing Formation located in both sides of the Wusheng-Shizhu intra-platform depression of eastern Sichuan Basin in recent years.Three wells targeting organic reefs in the south margin of Wolonghe area have produced natural gas of 5×108 m3 approximately.In the adjacent south margin of Bandong area,a same reef-beach seismic anomalous body in large scale was found in the Changxing Formation,but its subparallel layered reflection characteristics are significantly different from the typical seismic response characteristics of organic reefs.It is still unclear whether the anomalous body belongs to reef or beach,since this area has been scarcely studied due to less drilling(only one well drilled,Well Bandong18).Therefore,it is urgent to establish the sedimentary model of the area,so as to provide a theoretical basis for sub-sequent exploration deployment.The sedimentary model and favorable facies zones of this area were investigated,based on the comparative analysis of thin sections,logging curves and seismic data from the south margin of Wolonghe area and the high zones of Guang'an,together with forward and paleogeomorphic analyses.The results show that,(i)bioclastic limestones are dominant in the second member of Changx-ing Formation,and organic-reef limestones can be seen in the third member,indicating that the south margin of Bandong area has the condi-tion for developing vertically superimposed organic reefs and bioclastic beaches;(ii)in the front edge of the platform depression of the study area,the top uplift,weakened amplitude,and internal discontinuous and blank reflections are the seismic response characteristics of organic reefs,while in the rear edge,the top uplift,weakened amplitude,and internal subparallel layered reflections are the response characteristics of superimposed reef-beach bodies;(iii)the paleogeomorphology of Changxing Formation was restored,and the favorable facies zones of reef-beach bodies predicted are 8.8 km2 in total;and(iv)the sedimentary model that organic reefs in the front edge and superimposed reef-beach bodies in the rear edge of the platform depression was established through the logging-seismic correlation with adjacent area and the forward analysis.The establishment of the reef-beach sedimentary model of Changxing Formation in the south margin of Bandong area is of great guiding significance for the exploration and development of reef-beach gas reservoirs in platform depression.
Wusheng-Shizhu platform depressionSouth margin of Bandong areaLate PermianChangxing FormationReef beachSedimentary model
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