Controlling Factors of Shale Oil Enrichment in the Paleogene Funing Formation, Gaoyou Sag, Subei Basin, China

Abstract

In light of significant advancements in shale oil exploration within China’s principal oil and gas bearing basins, small to medium-sized basins such as the Subei Basin have also demonstrated considerable potential. However, the complex geological conditions within the Subei Basin present substantial challenges for the identification of favorable strata, thereby hindering large-scale shale oil development. This study utilizes practical exploration data from wells H1 and H3 within the Gaoyou Sag of the Subei Basin, integrating core observations with a suite of sophisticated analytical techniques, including X-ray diffraction, organic geochemistry analysis, argon ion polishing-scanning electron microscopy, nitrogen adsorption-mercury injection capillary pressure, and laser confocal analysis. The primary objective is to elucidate the geological characteristics and controlling factors influencing shale oil enrichment in the second member of the Funing Formation (E₁f₂) shale. The E₁f₂ member shale in the Gaoyou Sag exhibits an average total organic carbon content of 1.24% and a mean hydrocarbon generation potential of 5.38 mg/g, with a strong positive correlation observed between these parameters. The mineral composition is markedly heterogeneous, predominantly comprising mixed shale, followed by felsic shale and calcareous/dolomitic shale. The identified pore types include intergranular pores in clay minerals, carbonate intragranular dissolution pores, and a minor proportion of organic matter pores. The shale demonstrates significant vertical variability in oil enrichment, characterized by three source-reservoir couplings: “self-generating and self-reserving”, “mud generating and felsic reserving”, and “mud generating and carbonate reserving”. Formations enriched in carbonates and felsic materials exhibit superior reservoir properties, identified as “sweet spots” due to their enhanced hydrocarbon saturation and mobility. Despite the segmentation resulting from tectonic activities, the impact on preservation conditions remains limited, primarily influencing areas adjacent to sag-controlling faults. Effective preservation conditions are critical for shale oil accumulation and retention. Consequently, the deep sag regions of major depressions in the Subei Basin emerge as prime targets for shale oil exploration, which is pivotal for the efficient development of the E₁f₂ member shale. This study provides valuable insights into the exploration and development of shale oil within lacustrine faulted basins.