Pore Development Characteristics of Marine–Continental Transitional Shale of Benxi Formation from the Ordos Basin, China

The mechanisms governing pore development, particularly of organic pores, in marine–continental transitional shales remain poorly constrained. This study investigated the primary controls on pore evolution within transitional shales of the Benxi Formation using low-pressure gas adsorption, field emission scanning electron microscopy, bulk mineralogy and maceral analyses, and other techniques. Total organic carbon (TOC) contents exhibit positive correlations with shale microporosity and negative correlations with macroporosity. On the basis of S/TOC ratios and sedimentological evidence, the Benxi shales accumulated across a range of marine, brackish, and freshwater depositional environments. In general, more negative δ13Ckerogenvalues correlate with higher S/TOC, consistent with a marine source for the most 13C-depleted kerogens. Kerogens with δ13C of < –25‰ reflect a marine origin, while those with δ13C of > –25‰ indicate varying salinity conditions. Both organic matter composition and pore development within type III kerogens proved highly sensitive to paleosalinity. Specifically, marine-sourced organic matter possesses substantially lower microporosity but higher meso- and macroporosities compared to material deposited in freshwater settings. Accordingly, thermally mature marine-influenced Benxi shales likely retain greater shale gas potential versus non-marine equivalents.