Gas Generation Potential and Characteristics of Oil-Prone Shale in the Saline Lacustrine Rifting Basins: A Case Study of the Dongpu Depression, Bohai Bay Basin

摘要

In comparison to the continuous breakthrough in shale gas of marine basins, the pace of study of lacustrine basins is relatively slow. The latest studies have shown the saline lacustrine rifting basins (SLRBs) are one of the favorable exploration areas from the view of shale pore, mineral composition, and organic matter accumulation. However, the study on the gas generation potential and characteristics is still lacking because the traditional view is that lacustrine source rocks are oil-prone. Therefore, to identify gas generation potential and characteristics of oil-prone shale in SLRBs, the samples from the saline area (SA) and non-saline area (NA) of the Dongpu Depression are selected for the organic geochemical experiments and closed gold cube thermal simulation experiments. Geochemical results show that the SA samples are definitely oil-prone source rocks with high quality, and their main type is II kerogen. However, the results from thermal simulation experiments indicate that the gas generation potential of oil-prone shale in SLRBs is enough for the accumulation of shale gas with the peak gas yield of 469.11 mg/g of TOC at the heating rate of 20 °C/h TOC and 466.75 mg/g of TOC at the heating rate of 2 °C/h and TOC. Moreover, according to the dynamic hydrocarbon yields and carbon isotope (ln C_(2)/C_(3), ln C_(1)/C_(2), and δ~(13)C_(1)–δ~(13)C_(2)) during the increasing temperature, the gas generation process for SA and NA samples can be divided into three stages: kerogen cracking, oil cracking, and wet gas cracking. In comparison to other types of source rocks (marine shale, coal, and coal mudstone), the SA oil-prone shale can experience longer intervals in kerogen cracking and oil cracking, which may be the main stage of gas generation. The model based on kinetic parameters shows that the main gas generation period for SA and NA samples is the late Es_(1) (the first member of Shahejie Formation) to Ed (the Dongying Formation) of the Early Cenozoic, which can be consist of the depression stage with the deepest depth and the largest subsidence amplitude in the Dongpu Depression. Such information can be a reference for other SLRBs.