Characteristics and control factor of the relatively high permeability reservoir in the low permeability oil gas pool: A case study of Chang 101 oil layer of upper Triassic Yanchang Formation in Zhidan area, Ordos Basin

The relatively high permeability reservoir is the exploration and development direction of the low permeability oilfield. A new oil-bearing layer, i.e., Chang101 oil layer of Triassic Yanchang Formation, was found in Zhidan area of Ordos Basin in 2007. It has been proved by practice that the study area has a good exploration prospect. Based on core observation and all kinds of analytical means, such as cast thin sections, physical properties, X-ray diffraction, SEM and capillary pressure curve, the authors studied the characteristics of the relatively high permeability reservoir and its control factors. The results show that the relatively high permeability reservoirs are made of medium-coarse or medium feldspar sandstones which have relatively high compositional maturity. The cements are mainly chlorite film and laumontite, and the diagenesis mainly finds expression in secondary dissolution. Pore types are solution opening and inter-granular opening. Feldspar solution opening and laumontite solution opening constitute the main types of solution opening. The factors controlling the relatively high permeability reservoir are delta distributary channel microfacies which control the reservoir distribution and its primary material. The chlorite film is favorable for the preservation of primary inter-granular pore, whereas the secondary dissolution of feldspar and laumontite seems to be the key to improving the reservoir property.