Abstract:

Buried hills in the Jizhong Depression are important exploration targets for petroleum production/reserve enhancement. Ordovician buried hill, however, hinders petroleum discovery due to its strong reservoir heterogeneity and complex reservoir-control factors. Here, based on abundance of core, thin section, logging, seismic and geochemical data, and on the basis of geological analysis, the reservoir-forming conditions/modes of the Yangshuiwu and Wen’an-slope buried hills are systematically analyzed to guide explorations for major breakthroughs. The study shows that three sets of source rocks of the 3 rd and 4 th members of the Paleogene Shahejie Formation and upper Paleozoic Carboniferous-Permian are developed in the Jizhong Depression, providing sufficient material basis for the formation of buried hill oil/gas reservoir. The reservoir-control mechanism including three main factors—carbonate dolomitization/karst/fault—is ascertained, and three reservoir models of karst-pore type, fault-pore fracture type and dolomite-pore type—are established, expanding the exploration space. Controlled by multi-stage tectonism in Indosinian, Yanshanian and Himalayan periods, two types of buried hills were formed: one was formed from depressed strata after uplift and the other from uplifted strata after depression. Based on the analysis of reservoir-forming factors of Ordovician buried hills, three models of buried hills are concluded: deep structural and stratigraphic buried hills with layered reservoirs, median buried hills trapped by Mesozoic and Upper Paleozoic mudstones, and shallow buried hills sealed by Paleogene mudstones. In conclusion, the deep structural and stratigraphic buried hills with layered reservoirs in northern Jizhong Depression have the most favorable reservoir-forming conditions, and the Sicundian and Xinzhen buried hills are the favorable exploration targets.

Key words: Jizhong Depression, Yangshuiwu buried hill, Ordovician, type of reservoir, reservoir mode