Stratigraphic framework of the Wufeng-Longmaxi shale in and around the Sichuan Basin, China: Implications for targeting shale gas

Abstract

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 111, Long 112, Long 113, and Long 114). 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 of Wu 1. Cycle II corresponds to Long 1, with the maximum flooding surface at the top of Long 113. Cycle III corresponds 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 111, Long 112, Long 113, Long 114, and Long 12 and Long 2, respectively. High-quality shales mainly occur in the Wufeng Formation and Long 11. The major intervals that should be investigated with regards to shale gas production include LM1–LM5 (10 m thick) in the Weiyuan Block and WF1–LM5 (20–35 m thick) in the Changning Block. Long 111 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.