Sequence stratigraphy and tectonics of the Guantao and Minghuazhen Formations, Zhao Dong Field, Bohai Bay, Eastern China

The Cenozoic tectonic evolution of the Bohai Basin in eastern China has been documented through the interpretation and integration of 8,000 km of offshore 2D seismic data, a 3D seismic volume, and 38 well logs that are tied to these reflection data. The Cenozoic stratigraphy of this basin records: 1) a late Paleocene-late Oligocene basin opening across a diffuse set of half-grabens; the widespread distribution of rifts in the Bohai Basin and adjacent areas supports a regional extension possibly related to a previously proposed rollback of the subducted Pacific plate beneath the Asian continent; 2) a late Oligocene uplift and regional erosional event marking the end of major rifting; 3) early Miocene widespread thermal subsidence manifested by a large, relatively unfaulted sag basin; and 4) middle Miocene to recent strike-slip faulting; the regional-scale “lazy-Z” map pattern of the Bohai Basin depocenter indicates the importance of right-stepping pull-apart control on the younger sag section. Initiation of strike-slip deformation in the basin is attributed to a change in the direction of the convergence vector from WNW-ESE to E-W between the Eurasian plate and the subducting Pacific plate during early to late Miocene time. The Miocene Guantao and Pliocene Minghuazhen Formations in Zhao Dong field, western Bohai Bay, represent a 1.7-km-thick fully continental succession deposited in a large sag basin formed during the post-rift, early Miocene and younger thermal subsidence stage of the Bohai Basin. Correlation of 38 closely spaced wells and mapping of 2D and 3D seismic data in the 280 km2 Zhao Dong oil field allowed correlation and chronostratigraphic subdivision of these fluvial deposits into ten fieldwide and correlatable accommodation/supply cycles. Seismic mapping, well-log correlations and total sand thickness maps estimated from impedance volumes revealed two styles of sand distribution in Zhao Dong field: channel-fill facies in the Minghuazhen Formation are intermittently and locally deposited within a background matrix of high-impedance floodplain facies. Channel-fill facies in the Guantao Formation are highly amalgamated and form areally extensive fluvial sandstone bodies that extend several hundred meters along dip and strike directions.