A new breakthrough in shale oil exploration in the 4th member of the Paleogene Funing Formation, Gaoyou Sag, Subei Basin and its implications

A deviated well Y106 and a horizontal well Y108C were drilled in the 4th member of the Paleogene Funing Formation (also referred to as the Fu 4 Member) in the Gaoyou Sag, Subei Basin. Post-fracturing tests recorded peak daily oil and gas equivalents reaching up to 32.9 t and 54.6 t, respectively, marking a new breakthrough in shale oil exploration in a new sequence, i.e., the Fu 4 Member, following the previous breakthrough in shale oil exploration in the Fu 2 Member. This study aims to develop a comprehensive understanding of the shale oil enrichment pattern in the Fu 4 Member. By integrating data from seismic surveys, well logging, core observations, and laboratory tests, we systematically analyze the geological characteristics of the shales and summarize the major factors controlling the shale oil enrichment in the Fu 4 Member. These will provide a basis for further expanding the exploration scope and evaluating the resource potential. The results indicate that laminated mixed shales occur in the Fu 4 Member in the deep-lying sag zone of the Gaoyou Sag. The major factors controlling shale oil enrichment are summarized as follows. (1) Favorable hydrocarbon generation conditions as the shales exhibit great thicknesses, moderate organic matter abundance, high-quality hydrocarbon-generating parent materials, and appropriate thermal maturity; (2) efficient source rock-reservoir combinations, as the frequent interbedding of carbonate and clay laminae results in the formation of efficient source rock-reservoir combinations; (3) multi-scale pore-fracture systems, facilitated by laminated structures, and composed of fractures and inorganic pores of diverse types and scales, providing favorable storage and seepage spaces for shale oil enrichment; and (4) favorable hydrocarbon preservation conditions, with the high sealing capacities of the roof and floor strata, coupled with underdeveloped faults and anomalously high pressure, jointly creating favorable compartments for shale oil preservation and enrichment. The breakthrough in shale oil exploration in the Fu 4 Member marks a strategic expansion of shale oil exploration in the Gaoyou Sag, from the Fu 2 Member in the complex fault-block area on the slope to the Fu 4 Member in the tectonically stable deep-lying sag area. Accordingly, a new target for large-scale reserve growth has been identified. This advancement will contribute to the improvement of the technical system for the exploration and exploitation of shale oil with medium to low total organic carbon (TOC) content from a complex fault-block area on a slope to a tectonically stable deep-lying sag area within a continental faulted basin.