Abstract:

Low oil-saturation reservoirs are widely developed in the Shuixigou group in the Taibei sag of the Tuha basin, which are laterally concentrated in the positive structures in the sag, and vertically in the Xishanyao formation and Sangonghe formation. The oil reservoirs in the study area have low oil saturation and poor physical properties, so the natural productivity is low. Although the production is high after reservoir stimulation, both oil and water are produced for a long time. In order to understand the genesis of such oil reservoirs, the relationships among reservoir, structure, reservoir evolution and oil saturation were studied. The results show that the reservoir and structure are primary factors resulting in the low oil saturation. The reasons are as follows: the first is that the oil displacement by water is not complete due to compaction and pore reduction, fine pores and micro throats; the second is the existence of interlayers or barriers, which separate the oil layers and block the vertical migration of oil and gas; the third is that the low-relief structure provides an insufficient reservoir forming dynamics which will bring impacts on oil saturation; and the fourth is that late tectonic activities destruct the ancient oil reservoirs. Comprehensive analysis shows that the low oil-saturation reservoirs in the Shuixigou group in the Taibei sag have a large amount of remaining hydrocarbon resources, which are important replaceable targets for oil and gas exploration in the Tuha basin. The overlapping areas of reservoir sweet spots and inherited positive structures are favorable for looking for the reservoirs with higher oil saturation.

Key words: Tuha basin, Taibei sag, Shuixigou group, low oil saturation, oil reservoir, physical property, structure, genetic analysis