Major breakthroughs and significance of risk exploration for tight oil in Fuyu pay zone in the deep area of Gulong Sag, northern Songliao Basin, China

The concurrent exploration of shale oil wells in the Gulong Sag of the Songliao Basin has uncovered promising hydrocarbon shows in the Fuyu pay zone of the Lower Cretaceous Quantou Formation. To assess the hydrocarbon exploration potential of the Fuyu pay zone, this study systematically analyzes the main controlling factors for hydrocarbon accumulation, including source rock conditions, reservoir characteristics and migration capacity, in the deep area of the Gulong Sag, using seismic, drilling and core data, and reveals the hydrocarbon enrichment mechanism and accumulation model. The results indicate that the source rocks in the first member of Cretaceous Qingshankou Formation (Qing-1 Member) in the Gulong Sag are widely distributed, characterized by high quality, large area, high maturity and high hydrocarbon generation intensity, providing an ample oil source for the Fuyu pay zone. The Fuyu pay zone in the Gulong Sag features multi-phase channel sand bodies and beach-bar sands that are laterally superimposed and vertically stacked, forming large-scale sand-rich reservoir assemblages, which provide the storage space for tight oil enrichment. Influenced by overpressure pore preservation and dissolution-enhanced porosity, the porosity of the Fuyu pay zone can reach up to 13%, meeting the reservoir conditions necessary for large-scale tight oil enrichment. The episodic opening of hydrocarbon-source connected faults during the hydrocarbon expulsion period, combined with source-reservoir pressure differentials, drives the efficient charging and enrichment of hydrocarbons into the underlying tight reservoirs. The hydrocarbon accumulation model of the Fuyu pay zone is summarized as “source-reservoir juxtaposition, overpressure charging, lateral source-reservoir connection + vertical fault-directed bidirectional hydrocarbon supply, continuous sand body distribution, and large-scale enrichment in fault-horst belts”. A new insight for the deep area of the Gulong Sag is proposed as being sand-rich, having superior reservoirs, and being oil-rich. This insight guided the deployment of three risk exploration wells. The Well HT1H achieved a high-yield industrial oil flow rate of 35.27 t/d during testing, discovering light tight oil with low density and low viscosity. Through horizontal well volumetric fracturing treatment, the Well HT1H achieved the first high-yield breakthrough of tight oil in the deep area of the Gulong Sag, confirming the presence of geological conditions for large-scale hydrocarbon accumulation in this area. This expands the potential for hundred-million-ton tight oil resource additions in the Songliao Basin and deepens the theoretical understanding of continental tight oil accumulation.