Sequence Stratigraphy in Bohal Rift Basin Lacustrine Environments

Lijun Mi, Cao Wenxian, and Percy Chen

Sequence stratigraphic studies of the Bohai rift basin have established seven (7) unconformity-bounded seismic sequences in Paleogene Iacustrine sediments. Studies of microfossils, sedimentary facies, and shoreline onlaps show these sequences can be further divided into twelve-eighteen transgression/regression events. Two of these events, marked by high contents of salinity and rises of lake level, can be correlated to the global eustatic rises at 48-44 and 35-30 MY. Others seem to be of mainly tectonic and paleoclimatic controlled and are time-diachronous with the global sea-level variation.

Distinct shelf breaks have not been observed on seismic, and the basin margins appear to have evolved from steep to gentle ramp geometry. Rapid lateral facies changes have been attributed to multi-delta inputs around basin. Systems tracts identified on seismic include fan deltas, deep lacustrine fans, and incised valleys in lowstands and regressive deltas in highstands. Contrary to open marine environments, the highstand systems tracts are represented by thick mudstone sections as a result of constantly ample supply of sediments.

Drilling results have demonstrated that reservoir rock facies are mainly in highstand and highstand systems tracts, and source rock facies are largely in transgressive systems tracts.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California