Co⁃evolution characteristics of organic matter and reservoir in continental shale: a case study of Shahezi Formation in Changling Faulted Depression, Songliao Basin

Abstract

In the Songnan Faulted Depression Group of the Songliao Basin, continental shale gas has huge exploration potential. However, evolution regularities of both organic geochemistry and pore characteristics in organic shale containing type Ⅲ kerogen in this area are not made clear. According to thermal evolution simulation experiments on hydrocarbon generation and expulsion and the tests and analysis on a series of reservoir characteristics in organic shale samples containing type Ⅲ kerogen from the Lower Cretaceous Shahezi Formation, Changling Faulted Depression, it was found that there were two key nodes of thermal evolution in the shale, namely, equivalent vitrinite reflectance (Ro) of 1.5% and 2.0%, respectively. In the stage of equivalent Ro from 0.7% to 1.5%, it was the stage of fast oil generation and expulsion, in which pore shapes were mainly ink bottle and partially plate, few pores and caverns were developed gradually in organic matters, and mineral dissolution pores occurred. In the stage of equivalent Ro from 1.5% to 2.0%, it was the stage of rapid gas generation, in which a large amount of organic pores and inter⁃mineral pores were developed with gradually increasing pore size by organic matter decomposition, and the pores were mainly platy or wedge⁃like and a few pores were ink bottle shaped. In the stage of equivalent Ro higher than 2.0%, only platy or wedge⁃like pores were developed with further increasing size. The stage with Ro higher than 1.5% is favorable for co⁃evolution of source rocks and reservoir of shale gas accumulation in the Songnan Faulted Depression Group.