Lacustrine Sequences in the Nanpu Depression of the Bohai Rift Basin, China

Dag Nummedal, Changlin Wu, Robert R. Remy

Eocene to Oligocene rifting along the eastern margin of China produced that country's second largest oil province, the Bohai Basin. Parts of this rift basin are filled with up to 5000 meters of lacustrine, deltaic and fluvial strata. A detailed study of the Nanpu depression has identified lacustrine subenvironments ranging from turbidite fans and channels, through fan delta deposits to subaerial alluvial fans. Conventional seismic reflection profiles tied to well logs and core data allow the identification of several depositional sequences that were probably driven by high-amplitude lake level changes.

The stratal stacking pattern appears to support the sequence stratigraphic model proposed for the Quaternary East African lakes, namely that most sediment influx into the lake occurred during times of late transgression or highstand of lake levels. These highstand systems tracts form large-scale (100s of m thick) clinoform packages downlapping onto lacustrine source rock shales. The well logs indicate that the highstand clinoforms represent prograding sets of smaller sequences. Core data suggest that they were emplaced by sediment gravity flow mechanisms. Distal turbidite fans and channels may be present in the deepest parts of the basin. Lowstand deposits show much more evidence of traction transport, both by currents and lake waves. Moreover, lowstand deposits appear as thin (one or two reflector thick) deltas, sometimes onlapped by muddy healing phase deposits.

Caution is required in mapping rift basin stratal geometries because of the common tendency for closely-spaced, synthetic normal faults in the footwall block to offset strata such as to create the appearance of depositional clinoforms or oblique shingles.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995