Hierarchy Analysis for Flow Units Division of Low-Permeability Reservoir: A Case Study in Xifeng Oilfield, Ordos Basin

The Chang 8 Member of Yanchang Formation in Xifeng Oilfield is deposited by delta front, and the mircofacies contains distributary channel, mouth bar, sheet sands and interdistributary bay. The reservoirs within the Chang 8 Member are characterized by strong heterogeneity, great lateral change of facies and low permeability with a value of 0.1-10mD. It is significant for the effective development of these low-permeability reservoirs to divide the flow units accurately. Base on the principle that flow units are controlled by macroscopic sedimentation and microscopic petrophysics, a hierarchy analysis for flow units division was proposed. The reservoir unit can be subdivided into three hierarchies which comprises genetic unit, architectural unit and flow unit. Macroscopically, genetic unit is designated by sequence stratigraphic interpretation, and architectural unit is delineated within genetic unit by reservoir architecture analysis. Microscopically, flow unit is defined within architectural unit by petrophysical parameters measurement. The method was applied to divide flow units of low permeability reservoirs within the Chang 8 Member of Xifeng Oilfield. It was shown that the flow unit of type A and B with good development potential dominantly locate in the main part of distributary channel and mouth bar, and the flow unit of type C with poor development potential mainly locates in sheet sands, edges of mouth bar and distributary channel. The study indicates that the flow units are identified by the method which reflects macroscopic characteristics of sedimentation, as well as microscopic details of petrophysical properties.