Differences of Organic Matter Types of High Maturity Marine Organic-Rich Shale from Wufeng-Longmaxi Formation: Implication for Shale Gas "Sweet Spots" Prediction

Based on the identification of visual kerogen assessment, a subdivision method of organic matter types was proposed. Combined with GR curve, the distribution was further studied, the relationship between shale reservoir parameters and organic matter types was analyzed, and the significance was discussed. The results show that the kerogen types of organic rich shale in the lower part of Wufeng-Longmaxi Formation in the south of the Sichuan Basin are mainly of type I, II(1)a and II(1)b, while type II2 and type III are not developed. In Wufeng Formation, the shale organic matter is mainly of type II(1)a and II(1)b. From Long1(1)(1) to Long1(1)(4), the shale displays a trend of "Type I-II(1)b-II(1)a-II(1)b". Type I was characterized by higher TOC, porosity and gas bearing capacity of shale reservoirs than the other organic matter. The reservoir parameter of type II(1)a was better than type II(1)b, indicating that the organic matter type control on the quality of shale reservoirs. Type difference is closely related to the bio-precursors component, among which type I and type II(1)a are related to the content of planktonic algae, while type II(1)b is related to the content of benthic algae. This study provides a good reference for the subdivision of marine shale organic matter types and the evaluation and prediction of gas bearing properties.