Sedimentary evolution of a shallow-water delta in a long-term semi-cycle of base-level falling and its application to oilfield development—taking Oilfield A of Baohai Basin as an example

The Lower Minghuazhen Member of Middle Neogene (N₁m^L) in the Bohai Basin is dominated by shallow-water deltaic deposits. High-Resolution sequence stratigraphy is applied to reveal the lateral distribution and vertical evolution of the shallow water delta. The deltaic deposits of N₁m^L are composed of a semi-cycle formed in a long-term base-level falling cycle. During the semi-cycle, sand bodies in blocky, lobate and dendritic shapes are successively observed from bottom to top. Inter-well correlation suggests that the shallow water delta-front sands developed in the early stage are relatively isolated with poor connection and high porosities and permeabilities, indicating a lake of high water level with strong wave action. Later in the middle stage, a swinging distributary channel systems are developed, which resulted in lateral superimposition of deltaic sandbodies. In the final stage, owing to the falling of base level, strong incision occurred and sands were deposited in a pattern of vertical superimposition. Such a distribution pattern of sand bodies was mainly controlled by the relative change in sediment supply and accommodation space. Plane distribution and vertical superimposition relationship of shallow water delta also affect the sand drilling success rate of horizontal wells. Statistics of drilling data reveal that the rate of drilling success is 84.0% for blocky type sand, 81.5% for dendritic type of sand and 72.6%, the lowest, for of lobate type of sand.