Gas and water distribution of Ordovician Majiagou Formation in northwest of Ordos Basin, NW China

Based on the geologic background and reservoir characteristics, the characteristics and controlling factors of gas and water distribution of the carbonate reservoir in the 5th member of Ordovician Majiagou Formation in Northwest Ordos Basin have been examined from macro and micro aspects using data of structure, deposition, diagenesis, paleotopography, physical property, mercury injection, well logs and core observation. The 5th member of Majiagou Formation in the study area shows very complex gas and water distribution pattern: the gas layer is distributed uniformly across the whole area in 2D horizontal map, while the water layer is scattered and concentrates in small area, appearing in discontinuous distribution; there is no gas and water interbeds in vertical direction, making it difficult to develop the gas reservoir efficiently. The gas and water distribution is controlled by paleogeomorphology and diagenesis macroscopically, and is controlled by pore-throat structure, connectivity and the transportation relation between source rock and reservoir microscopically. The gas largely concentrates in and around the ancient grooves and low hill regions in the paleokast slope where the reservoirs have good physical properties and good pore – throat connectivity, and there is no bauxite between source rock and reservoir, making it easy for gas to charge and accumulate. Key words: carbonate gas reservoir, gas and water distribution, paleogeomorphology, diagenesis, pore structure, transportation relation, Ordos Basin, Ordovician Majiagou Formatio