Architectural units and heterogeneity of channel reservoirs in the Karamay Formation, outcrop area of Karamay oil field, Junggar basin, northwest China

Abstract

Reservoirs of the Karamay Formation occur as oil sandstones in outcrop in the Karamay oil field. Two types of high-quality reservoirs are present in this field: low-sinuosity channel sandstones and distributary-channel sandstones, each having different sediments, bounding surfaces, and architectural units. The low-sinuosity channel has coarse-grained sediments, a smaller scale, and relatively simple architectural units composed of intrachannel units and lithofacies that occur in a vertically stacked pattern. The distributary channel has fine-grained sediments, a larger scale, and relatively complex architectural units, which are reflected in the following three ways. (1) The distributary channels have three multiorder architectural units: an intrachannel unit, a macroform accretion unit, and a lithofacies unit. (2) Isolated barrier beds, whose permeability is lower, occur near third-order and fifth-order surfaces. (3) The evolution of the distributary channels has two stages. In the flooding stage, the sediments were coarse grained, and the intrachannel units were in a vertically stacked pattern. In the waning stage, the sediments were fine grained, and the intrachannel units were in a laterally stacked pattern. Reservoir heterogeneity of distributary-channel sandstones is revealed in the following three scales. (1) In a microscale range, the growth of pores is associated directly with lithofacies. The pore diameter and the throat width both increase with the coarsening of the grain size in the sediments. The anisotropic pore structure is reflected in the variation of permeability. The horizontal permeability parallel to the paleocurrent is the largest; the horizontal permeability perpendicular to the paleocurrent is intermediate; and the vertical permeability is the smallest. (2) At a channel scale, the fluid-flow units are demarcated according to the isolated barrier bed and porosity and permeability inside the distributary channel. The fluid-flow unit, distributed mainly in the middle–lower interval of the distributary channel or the intrachannel unit, is associated closely with third-order or fifth-order surfaces. The porosity and permeability inside the fluid-flow unit decrease gradually from its center to the margin and upper interval, depending on the spatial distribution of lithofacies. In this kind of lithofacies, which is characterized by a low matrix content and by coarse-grained sediments with large trough cross-beds, high porosity and permeability are commonly found. (3) In a lacustrine delta system, the distributary-channel sandstone has the best porosity and permeability and is the best reservoir.