Characteristics and causes of Mesozoic reservoirs with extra-low permeability and high water cut in northern Shaanxi

Considering actual oil field development and based on comprehensive analysis on the oil production and oil test data, this paper summarizes the characteristics of the Chang 6 reservoir of extra-low permeability and high water cut in the Weijialou Oilfield in northern Shaanxi and the distribution rule of high water cut zones. The results show that the Chang 6 reservoir in the Weijialou Oilfield is a typical reservoir of extra-low permeability and high water cut, its water cut contour shows irregular annular distribution around local structure highs in plane. The water cut distribution within the reservoir shows high heterogeneity; along the NE-SW direction, water cut change is relatively stable, water cut contour extending in band shape; along the NW-SE direction, water cut fluctuates greatly, contours showing an alternating distribution of high and low water cut zones; there exists high water cut zone out-thrusting locally. Lack of effective hydrocarbon source rocks and poor resource conditions are the underlying causes of high water cut in the Chang 6 extra-low permeability reservoir. High crude oil viscosity, great difference of fluid properties, high fluidity ratio, and well developed natural fractures are the main internal cause of the high water cut.