Suppression of thermal maturity indicators in lacustrine source rocks: A case study of Dongying Depression, eastern China

Abstract

Abstract The Paleogene Shahejie formation of the Dongying Depression in eastern China is a unique region and can offer insights into the suppression of thermal maturity in a large set of successive lacustrine shales. This study focuses on the Paleogene Shahejie formation of N-38 Well in the Dongying Depression. The unusual distribution of thermal maturity indicators, such as vitrinite reflectance (VRO) and C29 sterane thermal parameters was investigated through geochemical analysis. The research interval (2770–3376 m) was divided into three parts: the upper part of Es32 (2770–2990 m), the lower part of Es32 (2990–3190 m), and the Es31 (3190–3376 m). The Es31 mainly contained organic matter type I and some type II, which included mostly good and excellent source rocks. Meanwhile, the upper Es32 mainly contained organic matter type III, which included mostly poor and moderate source rocks. The parameters G/C30H, C35H/C31-35H, C31M/C31H, C29-20S/(20S + 20R), C31-22S/(22S + 22R), and C32-22S/(22S + 22R) showed a different distribution corresponding to the different lake sedimentary environments. Various organic facies were developed at the different evolution stages of the lake basin, which showed a noticeable influence on the parameters. The thermal maturity indicator VRO and molecular thermal parameters, such as C29-20S/(20S + 20R), C31-22S/(22S + 22R), C32-22S/(22S + 22R), and C31M/C31H, presented abnormal distributions in the large set of lacustrine shales, especially in the Es31. This observation suggested suppression of several thermal maturity indicators. VRO suppression positively correlated well with the corrected hydrogen index and alginite content in maceral composition. The high content of alginite group was considered to be the primary reason behind the evident negative VRO anomaly.