Abstract:

This paper aims to conduct the first analysis and evaluation of the oil and gas geological conditions and exploration potential of the Lower Carboniferous strata in the Santanghu basin, which has the lowest level of exploration and the most uncertain resource prospects. Based on field outcrop observations, geochemical analysis, and reservoir thin section observations, the vertical sedimentary environment evolution, hydrocarbon source rocks, and reservoir geological characteristics of the target layer are revealed. The study indicates that the Santanghu basin in the Early Carboniferous was a residual back-arc basin with shallow marine sedimentary characteristics. The vertical sedimentary environment of the Lower Carboniferous strata experienced four stages of evolution: Island arc volcanic activity, closure of ancient oceans leading to semi-deep water environments, compression and uplift of continental-derived clastic sediment filling shallow water environments, and orogenic uplift basin contraction. Among them, hydrocarbon source rocks are widely distributed in the semi-deep water environment and shallow water sedimentary environment, and the organic matter type meets the standard of moderate hydrocarbon source rocks. The presence of abundant asphalt in rock thin sections also confirms that the hydrocarbon source rocks of the Lower Carboniferous not only have hydrocarbon generation ability but also have undergone a large amount of hydrocarbon expulsion processes. The Lower Carboniferous strata developed two major types of reservoirs: Igneous rocks and clastic rocks. Although the reservoirs are generally tight, the development of fractures and dissolution pores has improved the conditions for oil and gas accumulation, especially the development of granite fractures, which has a significant connectivity effect on dissolution pores. Comprehensive analysis suggests that around the hydrocarbon source rocks of the Lower Carboniferous, two types of oil and gas reservoirs can be developed: Self-generated and self-stored, and lower-generated and upper-stored. Among them, self-generated and self-stored oil and gas reservoirs are developed in structurally stable areas far from the oil source faults and have the characteristics of primary reserves. Lower-generated and upper-stored oil and gas reservoirs are developed near oil source faults within the Carboniferous system and are related to fault lithology oil and gas reservoirs.

Key words: Santanghu basin, Lower Carboniferous, oil and gas, reservoir, petroleum geologic conditions, reservoir forming model, exploration potential