Abstract:

By using of cores, core slices, logging and geochemical analysis of test data, researchers studied the characteristics, stages and the control mechanism for reservoir distribution of buried karstification in the deep carbonate rocks of Ordovician Yingshan Formation in Tazhong area. Researches showed that three periods of different development scales, degrees and karst-fracture characteristics of buried karstification in Ordovician Yingshan Formation have been identified. The first period which occurred in the Late Caledonian to the Early Hercynian period distributed in the whole region, but its corrosion is weak; the second occurred in the Late Hercynian to the Early Indosinian period is obviously controlled by tectonic action, previous pore layers and favorable sedimentary facies belts, and may be associated with the magmatic hydrothermalism activity of the Late Hercynian period, thus fracture and pore forming are filled with calcite and fluorite; the third happened in the Himalayan period, controlled by strike-slip faults and fracture development, then fractures and holes are filled by light crude oil and natural gas. Overall, the deep burial dissolution usually associated with acidic hydrothermal fluid and dissolution of carbonate minerals which are related to the thermal evolution of organic matter, and often the early dissolution occured along the previous pore and fracture cracks with the characteristics of multi-stages, unequal sizes, proximal dissolution and distal cementation, solution cave forming on the inheritance of original fracture-cave system, which is a key factor in the reconstruction of reservoir formation and optimization.

Key words: karstification period, dissolution and filling, buried karstification, Yingshan Formation, Tazhong area