Dominant lithofacies and factors controlling reservoir formation of the shale sequence in the upper member of the Paleogene Lower Ganchaigou Formation,Ganchaigou area, Qaidam Basin

In recent years, major breakthroughs in shale oil exploration have been achieved in the Ganchaigou area of the Qaidam Basin, highlighting the significance of investigating the lithofacies and reservoir characteristics of the plateau saline lacustrine basin for sweet spot identification. Focusing on shale oil reservoirs in the Ganchaigou area, we identify the lithofacies types and systematically examine the differences in the reservoir and oil-bearing properties of shales with varying lithofacies. Furthermore, we determine dominant lithofacies and primary factors controlling the reservoir formation. The methods employed in this study include core and thin section identification, whole-rock mineralogy and in-situ trace-element distribution testing, large-field splicing scanning electron microscopy (MAPS), high pressure mercury injection (HPMI) and nitrogen adsorption experiments, along with one-dimensional nuclear magnetic resonance (NMR) measurements. The results indicate that the reservoirs in the study area exhibit four lithofacies: massive limy dolomite (carbonate mineral content: above 70 %), laminated limy dolomite (carbonate mineral content: 50 %-70 %), lamellar mixed siliciclastic-carbonate rock (carbonate mineral content: 10 %-50 %), and mixed siliciclastic-carbonate rock with intermittent horizontal beddings (carbonate mineral content: 10 %-50 %). The massive and laminated limy dolomites, among others, exhibit the predominance of intercrystalline pores, large reservoir spaces with high connectivity, and pores with large oil saturation index (OSI), thus serving as dominant lithofacies; The lamellar mixed siliciclastic-carbonate rock manifest poorly developed matrix pores but high permeability due to intensively developed laminae; The mixed siliciclastic-carbonate rock with intermittent horizontal beddings, lacking pores and fractures, display poor reservoir properties. The reservoir formation process of the dominant lithofacies is governed by three factors: (1) carbonate fabrics, which determine the degree of macropore development; (2) the contents of terrigenous felsic fine-grained sediments and clay minerals, which dictate the specific surface adsorptivity; and (3) the developmental degree of laminae, which can ultimately enhance the reservoir permeability.