Fault control on depositional systems and sequence stratigraphic architecture in a multiphase, rifted, lacustrine basin: A case study from the paleogene of the central Bohai Bay Basin, northeast China

Abstract The stratigraphic evolution of rift basins is complicated by out-of-phase displacement on boundary and intrabasinal faults and this analysis of Paleogene strata in the central Bohai Bay Basin of eastern China exemplifies this conundrum. The spatial and temporal distribution of depositional systems and sequences in the central Bohai Bay Basin are evaluated by using seismic reflection data in conjunction with drill cores and well logs. Three second-order composite sequence boundaries are identified on seismic profiles on the basis of truncations below and onlap relationships above and by abrupt changes abrupt changes in gamma-ray values on well logs. Composite sequences are comprised of third-order sequences identified as distinctive seismic packages. Variations in thickness and distribution of third-order sequences are consistent with out-of-phase displacement of boundary and intrabasinal faults which resulted in migration of depocenters in space and time. A range of depositional systems are recognized including fan delta, braid delta, river delta, sublacustrine fan, and shallow and deep lacustrine. During rift-climax stages, lowstand fan deltas developed on the hanging wall adjacent boundary faults to the north, braid deltas on the footwall ramps to the south and sublacustrine fans in the central basin. Following transgression, highstand braid deltas prograded from the north and south whereas river deltas during the late stages of basin fill. Both tectonic activity and climate were important controls on accommodation and the balance between sediment supply and accommodation determined the sedimentary architectures. Faulting, uplift and erosion as well as fault-block rotation controlled the formation of sequence boundaries and sequence geometries whereas variations in sediment flux can be attributed to climate changes. High-quality reservoirs are associated with deltaic channel-fill and sublacustrine fan sandstones. Intrabasinal faults provided pathways for oil migration whereas prodeltaic, lacustrine mudstones are source rocks as well as seals. This study demonstrates that the spatial and temporal distribution of deposystems in rift basins, in general, requires an understanding of the history of boundary and intrabasinal faults within these compartmentalized basins.