Biostratigraphy and reservoir characteristics of the Ordovician Wufeng Formation—Silurian Longmaxi Formation shale in the Sichuan Basin and its surrounding areas, China

Abstract

Through graptolite identification in profiles, graptolite zone division, contour map compilation, and analysis of mineral composition, TOC content, lamina distribution features of shale samples, the biostratigraphic and reservoir characteristics of Ordovician Wufeng Formation—Silurian Longmaxi Formation in the Sichuan Basin and its peripheral are sorted out. There are 4 graptolite zones (WF1 to WF4) in Wufeng Formation and 9 (LM1 to LM9) in Longmaxi Formation, and the different graptolite zones can be calibrated by lithology and electrical property. The shale layers of these graptolite zones have two depocenters in the southwest and northeast, and differ in mineral composition, TOC, and lamina types. Among them, the graptolite zones of lower WF2 and WF4 are organic matter-poor massive hybrid shale, the upper part of WF1–WF2 and WF3 have horizontal bedding hybrid shale with organic matter, the LM1–LM4 mainly consist of organic-rich siliceous shale with horizontal bedding, and the LM5–LM9 graptolite zones consist of organic-lean hybrid shale with horizontal bedding. The mineral composition, TOC and lamina types of shale depend on the paleo-climate, paleo-water oxidation-reduction conditions, and paleo-sedimentation rate during its deposition. Deposited in oxygen-rich warm water, the lower parts of WF1 and WF2 graptolite zones have massive bedding, low TOC and silicon content. Deposited in cooler and oxygen-rich water, the WF4 has massive bedding, high calcium content and low TOC. Deposited in anoxic water with low rate, the upper part of WF2, WF3, and LM1–LM4 are composed of organic rich siliceous shale with horizontal bedding and high proportion of silt laminae. Deposited in oxygen rich water at a high rate, the graptolite zones LM5–LM9 have low contents of organic matter and siliceous content and high proportions of silt lamina.