Oil-water inversion and its generation at top and bottom of the shallow sandstone reservoir in the Northern Chepaizi area, Junggar Basin, NW China

Based on fluid property and distribution, the reason for oil-water inversion at top and bottom of the shallow sandstone reservoir of the Neogene Shawan Formation in the Northern Chepaizi area, Junggar Basin was researched from the perspective of structure, lithology, carbonate cementation and oil thickening. The oil of Northern Chepaizi area is heavy crude oil, whose density and viscosity increase with the depth of oil layer. Carbonate cementation and oil thickening are the controlling factors of oil-water inversion at both top and bottom. When the top surface of oil layer in downthrown side is deeper than a definite depth, the density of crude oil, due to oil thickening, is closer to or higher than that of the formation water, and oil-water inversion at the bottom of reservoir occurs. In the structural high part, carbonate cementation deteriorates the reservoir physical property, the late migrated oil has no access to the high position reservoir, and oil-water inversion forms in the structural high part. In the middle to upper area, conventional heavy oil accumulates, because it's lighter than the formation water, oil and water differentiation is complete.