Characteristics and formation mechanism of low porosity and extra-low permeability reservoirs in the Niujuanhu oil field, Santanghu Basin

The Xishanyao reservoir in the Niujuanhu oil field of Santanghu Basin, though shallow (usually less than 2 000 m), is a low porosity and ultra-low permeability reservoir.The reservoir characteristics and the causes for low porosity and ultra-low permeability were studied by using core and cast thin section observations, conventional high pressure mercury injection experiments, SEM, and combined with regional tectonic data. The Xishanyao Formation was mainly composed of feldspar lithic sandstones. The reservoir includes intergranular pores, dissolution pores and fractures. Sedimentation, diagenesis and tectonic effects resulted in low porosity and ultra-low permeability. During the early deposition period of Xishanyao Formation, carbonate cementation was weak due to the acidic environment of coal-bearing formations.A strong and rapid compaction occurred due to the lack of support between particles. At the same time, sedimentation also controlled the original physical properties of the reservoir from the size of particle and the content of capping agent. The compaction effect during diagenesis led to a porosity reduction of 18%, and the acidic environment also led to a large amount of kaolinite formation, quartz secondary growth and the development of late iron-rich carbonate cementation with a cementing porosity rate of about 5%. The cracks formed by tectonic stress provided channels for late stage dissolution. However, rocks have become consolidated due to early compaction. What's more, the coal seam is thin, and the acidic fluid is not enough to flow in large scale through small particles, leading to poor dissolution strength. As a result, coarse throats which have a great contribution to permeability generated little, and a reservoir with low porosity and ultra-low permeability was formed.