Distinct evolution trends of nanometer-scale pores displayed by the pyrolysis of organic matter-rich lacustrine shales: Implications for the pore development mechanisms

Abstract

Organic matter (OM) compositions greatly affect hydrocarbon generation and expulsion processes, which are critical for the organic porosity development in shale. Lacustrine shale samples of low thermal maturity were pyrolyzed using two pyrolysis systems (closed and semi-closed systems). Pore development was measured by low-pressure gas adsorption and field emission-scanning electron microscopy (FE-SEM), and bulk porosity was modeled utilizing organic geochemical data. The gas adsorption analysis indicated that shale samples of different OM compositions differed in pore volume evolution with thermal maturity, mainly related to the different amounts of hydrocarbon generated and expelled. Residual bitumen significantly reduced the pore volume of OM-rich oil generative shale samples, and restricted the micro-, meso-, and macro-pore volumes to different extents. Calculations and experiments both showed that OM-rich and oil generative shale experienced a greater increase in pore volume after the oil peak. In addition, it was observed that variations in the main pore types were associated both with shale compositions and with exerted overburden pressure. Increase in overburden pressure were found to greatly facilitate the development of nanometer-size spongy and complex OM pores in shales containing type Ⅱ/Ⅲ kerogens, possibly as a result of the expulsion of gaseous hydrocarbons. By contrast, the shale with abundant type Ⅰ kerogen tended mainly to develop relatively large pores following oil expulsion regardless of overburden pressure. The modeled organic porosity of pyrolyzed samples of type III OM was similar to the porosity of geological shale, but the porosity of OM-rich oil generative shale samples with high expulsion efficiency at the oil generation stage was two to three times the measured porosity of the geological shale. In some cases, the higher modeled shale porosity might be related to the higher expulsion efficiency. For geological shales of expulsion efficiency comparable to the pyrolyzed samples, geological processes (e.g., compaction and cementation) may have greatly reduced the OM-associated pore volume.