Abstract: Taking the braided river deltas of the Q6 Block in the Kunbei Oilfield in the Qaidam Basin as an example, this study investigated methods to model the architecture of thin interbedded reservoir sandstones. Traditional methods are based on the relatively maximum values of natural gamma measurements and (or) positive spontaneous potential abnormalities to determine the boundaries of sub-layers. Meanwhile, the sub-layer division based on the tangent points of reflected and diffracted wavefronts can more accurately depict the thickness and depth of reservoir layers, insulating layers, and interlayers. The interlayers in the study area are dominated by localized mudstones. Hence, two parameters, the thickness of low-permeability layers and permeability return level, were adopted to identify the interlayers. This enhanced the model accuracy. Because the braided river deltas were under the hydrodynamic control of both river and lake waves and the strata are characterized by thin interbedded low-inclination sand-mudstones, this study proposes that strict data control, hierarchical modeling and reservoir layer/interlayer nesting should be included to construct an architectural model of the thin interbedded deposits. Subsequently, an architectural model for the braided river deltas of the Q6 Block in the Kunbei region was established. Analysis of the relationships between the architectural units of the model shows that there are four types of architecture in the target layers. They are lateral stitching, lateral separation, progradation, and accretion. Among them, the lateral stitching and vertical accretion modes of the sand bodies are the most developed. Compared to the traditional modeling method, the proposed method is more advantageous and sensible for thin interbedded deposits.

Key words: Qaidam Basin, Kunbei Oilfield, thin interbedded deposit, braided river delta, reservoir architecture, modeling