Quantitative analysis of reservoir-cap rock assemblages of the first member of Sanduo Formation and prediction of favorable regions in Gaoyou Sag, North Jiangsu Basin

Overall, the reservoir forming conditions are good in the E₂s₁ (the first member of Sanduo Formation) of the Gaoyou Sag,North Jiangsu Basin. Sandstones were well developed. Reservoir-cap rock assemblage is one of the key factors controlling hydrocarbon accumulation. For the sake of evaluating reservoir-forming conditions and favorable exploration regions, it is important to analyze effective reservoir-cap rock assemblage and predict favorable regions. It was considered that there are multiple reservoir-cap rock assemblages, which are important factors controlling hydrocarbon accumulation and enrichment of each submember in the E₂s₁. Firstly, according to the degree of mudstone stability, the paper qualitatively divided the reservoir-cap rock assemblage into two sorts:regional and local. Secondly, in order to recognize effective reservoir-cap rock assemblage more accurately and based on the qualitative analysis, a quantitative oil reservoir analysis was completed. Through analyzing the relationship between sandstone ratio of the hydrocarbon and upper submembers with hydrocarbon accumulation, the boundary between reservoir rock and cap rock was confirmed, a quantitative evaluation threshold was built, and three types of effective reservoir-cap rock assemblage were identified (lower reservoir and upper cap rock assemblage, interior reservoir and cap rock assemblage, and mixed reservoir and cap rock assemblage). Thirdly, based on the quantitative analysis, the favorable regions of effective reservoir-cap rock assemblage were predicted. Several regions have great exploration potential, such as the eastern Huangjue to western Shaobo area in the field of both structural and subtle traps, which have achieved good results in exploration.