Architecture and formation of mouth bars in a shoal-water delta: a case study from the Zhahaquan oilfield, Qaidam basin, China

Distinguishing between shoal-water deltas and shore–shallow lake beach-bars is challenging, creating a widespread problem for bar-scale reservoir architecture analysis and ultimately affecting oilfield development. Using the Miocene Upper Ganchaigou Formation in the Z7 and Z401 wellblocks of the Zhahaquan oilfield in the Qaidam Basin as an example, this study examines the differences between mouth bars and beach-bars and establishes an architectural model for mouth bars in a shallow water environment. The main methods used in this study include grain size analysis, architecture analysis, and well-tied single sandbody correlation. Passega’s C-M diagram and G.M. Friedman’s skewness-standard deviation plot provide reliable criteria for distinguishing mouth bars from beach bars. Architectural analysis is applied to identify architectural units and to establish their relationships with corresponding microfacies. A connected single sandbody—characterized by a single, unified oil-water contact—serves as an effective basis for well-to-well correlation and for interpreting 4th-order architectural units. Two conclusions are drawn. First, grain size analysis shows that rivers dominated the bars in the Z7 and Z401 wellblocks, with lesser influence from waves, indicating that they are mouth bars. Second, architectural analysis shows that there are three levels of architectural units: a 5th-order unit represented by a compound mouth bar formed through the superimposition of multiple single mouth bars; 4th-order units consisting of individual mouth bars, including those partially truncated by distributary channels; and 3rd-order units comprising mouth bar accretion bodies and individual distributary channels. Well-tied sand body correlation, architectural analysis, and microfacies interpretation of III-5-2-2 indicate that six single mouth bars are stacked in an imbricated, lamellar pattern. These bars prograded lakeward sequentially and were intermittently incised by distributary channels as the shoreline migrated toward the lake under an arid climate, shoal water, and a gentle slope. The results of this case study are helpful to distinguish mouth bars from beach-bars and for conducting architectural analysis. They offer valuable guidance not only for Zahaquan, Gasikule, and other oilfields in the Qaidam Basin but also for those that contain shoal-water delta systems in other clastic basins. • A method to distinguish the deltaic mouth bar from the beach-bar. • Establishment of the architecture patterns of mouth bars. • Settings for the formation of mouth bars are proposed.