Abstract: Structure, Stratigraphy and Sedimentation in the Nanpu Block of the Bohai Basin, People's Republic of China

Dag Nummedal

The Bohai Basin is China's second largest petroleum producing province. The basin was formed by dip-slip rifting in Eocene and Oligocene time. A right-step offset of about 150 km has produced a rift zone some 350 km wide beneath the coastal plain east of Beijing and beneath the Quaternary Bohai Bay. Transfer zones between offset-parallel faults, and between rift-parallel and offset-parallel ones, became loci of major, coarse-grained, lacustrine deltas throughout the Oligocene, creating the Liuzan, Gaoshanpu and Beipu "fans." These sandstones and conglomerates are major reservoirs.

Seismic stratigraphy and facies analysis are difficult to perform in the critical areas of the Nanpu block, both because of intense local faulting in the transfer zones and the inferred, extensive slumping on the sublacustrine slopes. Nevertheless, in the Luizan "fan" one can recognize steeply inclined, parallel reflectors (of the 4th Member of the Shahajie Formation) changing upward into distinct clinoforms (of the 3rd Member of the Shahajie Formation). This pattern is interpreted as an evolution from an alluvial cone to a less coarse-grained delta, probably reflecting the establishment of a lower-gradient, integrated drainage basin during the Oligocene phase of gradual reduction in the rate of rift subsidence.

Core control reveals that high-density turbidites and frictional and cohesive debris flows were the dominant sedimentation mechanisms. Dilute turbidites and "pelagic" sedimentation characterized basin locations more than 5-10 km from the fan apices. The observed lithofacies are consistent with observations from modern, coarse-grained delta slopes in such rift-basin settings as Lake Baikal and the East African lakes. Also, many steep, narrow fjord-head (marine) deltas have similar facies.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994