摘要
Stratigraphic division and correlation are crucial for the identification of sweet spots and drilling design of shale gas.In this study,a stratigraphic division and correlation was carried out for the Wufeng-Longmaxi Formations in southern China from the prospective of lithostratigraphy,sea level changes,and biostratigraphy using data from seismic investigation,wells,and outcrops.The Wufeng and Longmaxi Formations were respectively divided into four members,Wu 1 and Wu2 for the former and Long 1 and Long 2 for the latter.Of the members,Long 1 was subdivided and its first subdivision(Long 11)was further divided into 4 layers(Long 1^(1)_(1),Long 1^(2)_(1),Long 1^(3)_(1),and Long 1^(4)_(1)).Three eustatic cycles were recognized in the Wufeng-Longmaxi Formations.Cycle I corresponds to the Wufeng Formation with the maximum flooding surface at the top ofWu 1.Cycle II corresponds to Long 1,with the maximum flooding surface at the top of Long 1^(3)_(1).CycleⅢⅢcorresponds to Long 2.Furthermore,4 graptolite biozones(WF1 to WF4)were identified in the Wufeng Formation and 9 graptolite biozones(LM1 to LM9)in the Longmaxi Formation.WF1-2 and WF3-4 correspond to Wu 1 and Wu 2,respectively;and LM1,LM2-4,LM5,LM6,and LM7-9 correspond to Long 1^(1)_(1),Long 1^(2)_(1),Long 1^(3)_(1),Long 1^(4)_(1),and Long 12 and Long 2,respectively.Highquality shales mainly occur in the Wufeng Formation and Long 11.The major intervals that should be investigated with regards to shale gas production include LM1eLM5(10m thick)in the Weiyuan Block and WF1eLM5(20e35m thick)in the Changning Block.Long 1^(1)_(1)is believed to be an optimal target for drilling due to its high TOC content,siliceous content,porosity,microfracture density,and horizontal/vertical permeability ratio.
基金
Preparation of this manuscript was supported by the National Natural Science Foundation of China(No.41572079)
National Major Science and Technology Project(No.2017ZX05035).
