Systems Tract Analysis of a Lacustrine Rift Basin--The Shahejie Formation of the Liaodong Bay, Bohai Basin, China

Fuping Zhu, Dag Nummedal, Robert R. Remy, Ming Pang, and Yong Mo

Sequence stratigraphic analyses indicate that the Shahejie Formation (Eocene and Oligocene syn-rift deposits) in the Liaodong Bay consists of two and a half depositional sequences. The lower two sequences are interpreted as representing the rift climax. The upper half sequence represents the reduction of rift activity. It is the LST and TST of another sequence, which includes part of the overlying Dongying Formation.

LST, TST and HST are all identified on log curves. The falling stage systems tract (FSST) is also locally identified based on offlap patterns in seismic profiles, while other systems tracts all on lap onto the sequence boundary. Condensed sections are easily recognized with low resistivity and high GR values. They can sometimes be identified in seismic data by relatively continuous reflectors with downlap termination above.

The lower two sequences are dominated by the TSTs, characterized by hangingwall and footwall sublacustrine fans (sediment-gravity flow) at the margins, and distal turbidites and hemipelagic mudstone in the centers of half-grabens. While all other systems tracts consist mainly of siliciclastic rocks, the LST and TST of the upper sequence contain numerous thin beds of carbonates and organic shales. These are interpreted as sedimentary responses to high-frequency climatic signals. They are superimposed on a long-term tectonic subsidence trend due to the decrease of subsidence rate and climatic changes.

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