The Characteristics Of Sedimentary Reservoirs Of Ordovician Carbonate Rocks In Northern Jizhong Area And Their Influential Factors

In northern Jizhong area, Ordovician carbonate rocks are mainly distributed in the north and the east of Langgu sag and Baxian sag, and vary greatly in thickness from10m to about800m. They are carbonate rocks typically deposited in neritic platform facies, including Yili Formation, Liangjia Formation, lower Majiagou Formation, upper Majiagou Formation and Fengfeng Formation from the bottom to the top. They are one of the important reservoirs of the lower Palaeozoic buried-hill oil and gas accumulations in the area of Jizhong.The carbonate reservoirs of all formations in Ordovician are similar in rock type and they are a set of purer limestones or dolomites varied gradually from argillaceous carbonate rocks from the bottom to the top. The mineral compositions are mainly composed of calcite, secondarily dolomite, and a small amount of anhydrite, silica and clay mineral. The rock textures are mainly grain, crystal grain, porphyritic and breccia. The main types of reservoir spaces are pore, cave and fracture. The reservoir spaces are mainly combined in the following two patterns, namely macrocave-macrofracture and pore-microfracture. Fengfeng Formation and upper and lower Majiagou Formation are the main hydrocarbon producing pays.The Ordovician carbonate reservoirs are mainly influenced by sedimentary facies, tectonic stress, karstification and diagenesis. The distribution of pores, caves and fractures are controlled by sedimeantry facies belts, the development and distribution of structural fractures are controlled by tectonic stress and the development and distribution of dissolved pores, caves and fractures are controlled by karstification.