Paleokarst reservoir and the formation factors of the Loushanguan group, the Cambrian system in west Hunan and Hubei

The study area is located in the west Hunan and Hubei, belong to the Yangtze platform. There is a tidal facies carbonate formation from the middle Cambrian to the early Ordovician sedimentaries, namely the Loushanguan group. Because of the sea-level lowering event at the end of the Cambrian, the carbonate platform was exposed. It created conditions for the development of the ancient karst reservoir. In order to master the characteristics of paleokarst reservoir and the formation factors of the Loushanguan group, the Loushanguan group is studied by means of original geological data analysis, geology survey, karst fillings test, thin section of rock identify and physical property test. It is concluded as follows,（1）the Loushanguan group dolomite reservoir is holes-cracks reservoir, and the reservoir space includes solution pores, intergranular pore and intergranular dissolved pore and crack soluble seam, etc;（2）dissolution is not controlled by the rock fabric, and the reservoir belongs to non-fabric selective dissolution. Solution pores distribute along the bedrock plane. Its distribution range is mostly within about 20 m below the unconformity plane, and the solution pores become less with depth; (3)the ancient weathering crust karst is not only a good channel for oil and gas migration, but also a favorable space for hydrocarbon generation of organic matter accumulation;(4)karst reservoir is mainly controlled by the geological structure, climate, environment and lithology. The unconformity plane between the Cambrian and Ordovician systems and a large number of structural fractures provided convenient conditions for fluid migration and storing. Because the climate is hot and dry during the sedimentary hiatus, the surface water is not very rich, and reservoir lithology is mainly the powder and fine crystal dolomite which is not conducive to water-rock effect, the karst form is mainly intergranular pore and solution pore, and the big ancient karst cave system didn’t form; (5)although the big ancient karst cave system didn’t form, the Loushanguan group still has nice reserving space for oil and gas, so it will be one of the important reservoirs for oil and gas exploration under the marine faces sediments of the middle and lower Yangtze formations. The exploration efforts should be strengthened in the next stage to seek new breakthrough in the field of oil and gas exploration in China.