Reservoir characteristics and exploration potential of Paleogene shale oil in the Ganchaigou area, Western Qaidam Basin

The Well C902 achieved a major breakthrough in the exploration of Paleogene shale oil by obtaining high production oil flow in the Ganchaigou area, Western Qaidam Basin. However, problems such as unclear evaluation criteria and resource potential of Paleogene shale oil restrict the efficient development of shale oil in the Ganchaigou area. Based on exploration results of Well C902 and other wells, the exploration potential of Paleogene shale oil in the Ganchaigou area is evaluated comprehensively by using the lithology, physical property, pore structure and hydrocarbon generation potential of shale oil. The results show that lithofacies types of shale oil reservoirs include lamellar limestone, lamellar limestone, lamellar limestone, lamellar clay shale and lamellar mudstone. Carbonate rock has obvious mixed characteristics, and the proportion of brittle minerals ranges of 60 ~ 90%, showing good compressibility. The average porosity and permeability are 5.1% and 0.24mD, respectively, indicating the low-porosity and ultra-low permeability reservoirs. The reservoir space is dominated by intergranular pores, solution pores, laminar fractures and microfractures, and the proportion of intergranular pores and laminar fractures is up to 85%. The average value of TOC is more than 0.9%. The chloroform asphalt "A" and S1+S2 jointly indicate that the hydrocarbon generation potential is high. The organic matter type is mainly Ⅰ-Ⅱ1 type. Combined with Ro values, Paleogene shale oil is in the peak period of oil generation. The Ⅳ ~ Ⅵ layers of upper member of Xiaganchaigou Formation are widely developed and have the characteristics of good source rock quality, porosity and fracture development and integration of source and reservoir, which is conducive to the nearby preservation of shale oil after formation. Second, the overlying salt rock is also widely developed and has good sealing, contributing to generating overpressure and improving oil and gas recovery after fracturing. Therefore, the Paleogene shale oil in the Ganchaigou area has great exploration potential.