Exploration breakthroughs and geological characteristics of continental shale oil: A case study of the Kongdian Formation in the Cangdong Sag, China

Petroleum geology and deposition features of the fine grain phase region in the second member of the Kongdian Formation were investigated using traditional petroleum geology and sedimentary petrology. New theories for shale oil and gas formation, based on data from 500 m continuous coring, systematic analysis of over ten thousand core samples, and logging data in the center of the lake basin in the Cangdong Sag (Bohai Bay Basin, China) were developed to determine the distribution of shale oil and provide a basis for shale oil exploration deployment. Results showed that a wide distribution of semi-deep to deep lake sub-facies fine-grained sediment (shale) was deposited in the study area during the largest lake flooding period. This sediment consists of felsic shale, mixed shale, and dolomites, but no clay shale. The shale features multiple rock components with few dominant minerals; high clastic mineral content and low clay mineral content; rich analcite and lean pyrite; rich brittle minerals with lean quartz; diverse rock types with limited shale oil; more tight reservoirs and fewer reservoirs with no fractures; more high-quality source rock with less non-hydrocarbon source rock; and more high frequency cycles with fewer monolithic deposits. Results indicate that dolomites are located in strips on the plane and are locally enriched, contradicting the understanding that only hydrocarbon source rock, and no reservoirs, develop in the middle of a continental closed lake basin. The study region demonstrates good conditions for shale oil formation, a favorable configuration of source rock and reservoirs, and 260 km2 of sweet spots, indicating high exploration value. Several prospecting wells have obtained industrial oil flow with guidance based on geological knowledge and engineering technology support.