Short-term base-level cycles and reservoir genesis on the delta front in the Changwu region, Shaanxi

The Chang-7 and Chang-8 delta front reservoir sandstones in the Changwu region, Shaanxi may be divided into:(1) subaqueous distributary channel sandstones including truncated distributary channel sandstones and uninterrupted distributary channel sandstones;(2) channel-mouth bar sandstones including isolated channel-mouth bar sandstones and distal bar sandstones, and (3) composite bar sandstones. The genesis of the above-mentioned types of sandstones are markedly controlled by base-level changes, accommodation space and sediment supply. In the course of the base-level rising, the truncated distributary channel sandstones are accentuated when the accommodation space/sediment supply ratios are less than 1, while the ratios are greater than 1, the uninterrupted distributary channel sandstones predominate.In the course of the base-level falling, the isolated channel-mouth bar sandstones are dominated when the ratios are less than 1, while the distal bar sandstones prevail when the ratios are greater than 1. The composite bar sandstones are developed at the depositional-shoreline break. The regular changes of the short-term base-level cycles in the course of base-level rising or falling are responsible for the types and distribution of the above-mentioned sandstones.