Laminae in multiple lithofacies and impact on pore structures in lacustrine shale: The cretaceous qingshankou formation, Songliao Basin

Abstract

Unconventional lacustrine shale oil reservoirs are characterized by strong heterogeneity of pore structures and intensive lithofacies variations in the vertical direction. This study systematically investigated the pore systems and factors influencing pore structures for the first member of the Cretaceous Qingshankou Formation (Qing 1 member) in southern Songliao Basin, employing geochemical, mineralogical, petrological, and petrophysical methods. Nine lithofacies types were identified based on total organic carbon (TOC) content-laminae structure-mineral composition. Organic-medium mixed lithofacies with silt-laminated structures were the most favorable in the Qing 1 shale, exhibiting dominant pore volume, exceptional connectivity, and significant movable oil content. Shale reservoirs were primarily controlled by interparticle pores and microfractures, with mesopores as the dominant reservoir space. Thick argillaceous laminae followed a "primary source and reservoir rock" model at the microscale, whereas "oil generation-migration-accumulation" in silt-laminated shale played a key role in pore system development. In the Qing 1 shale at the medium maturity stage, TOC content negatively impacted storage space due to asphalt blocking pores. Due to the in situ deposition of authigenic minerals and dissolved components, felsic minerals also negatively effected pore volume. Moreover, shales developed a significant number of pores associated with clay minerals to enhance reservoir space. Furthermore, an optimal balance of felsic minerals in the argillaceous laminae alleviated compaction. These findings enhance our understanding of pore formation mechanisms for lacustrine shale reservoir, which is crucial for accurately assessing shale oil resources.