Reservoirs pore space types and evolution in M55 to M51 submembers of Majiagou Formation of Middle Ordovician in central gasfield area of Ordos basin.
鄂尔多斯盆地中部气田区中奥陶统马家沟组马五5-马五1亚段储层孔隙类型和演化

The upperest part M55-M51 submembers of Middle Ordovician Majiagou Formation is one of mainly reservoir and producting horizon in central gasfield area of Ordos Basin, the thickness is 70~80M, and area about 4.8×104 Km2. The main primary lithological characters of reservoirs are containing anhydrite crystal and nodules silt-sized crystal dolostone and coarser silt-crystal dolostone contain more or less intercrystal pores. Subaerial exposure and kastfication more than 140Ma from late Ordovician to middle Carboniferous, the phenomenon of palaeokasting was very strong, especially in silt-sized crystal dolostone with anhydrite crystal and nodules, formed various reservoir space types and widely apperence in whale central gas field area. The secondary pores developed from burial diagenesis stage always stacking at the early pore spaces, were mainly take shaped in palaeokastic rocks. Such as the anhydrite nodule cast pores enlargement resolution, resolution or partly resolution of the sediments subfilled in the palaeokastic space, and so on, In direction of 80° structure fracture produced in Yinzhi age and 30°~60° conjugate fracture produced in Yanshan age also obviously developed in palaeokarsting rocks that bearing secondary pore spaces. Twenty one forms reservoir space, such as pores, solution slit cave and structure fracture had been recognized in well core and casting thin section, include three forms origine pores proceed from depositional (penecontemporaneous)stage, nine forms secondary solutinal pore space procced from poleoweathering crust karsting hyperdiagenesis stage, ive forms secondary pore space procced from buried diagensis stage, and four forms structure fracture and fault. So that often seen several pore spaces appeared in same core or casting thin section. The development degree, distribution range and contributing value for reserivor of each pore space in central gas field are various, lead to the reservoir anisotropic.