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Tan-Lu Fault Zone and Hydrocarbon Accumulations in Offshore Bohai Bay Basin, Eastern China


The NE-trending Tan-Lu fault zone is a crustal-scale strike-slip fault zone on the eastern continental margin of China. The fault zone traverses the eastern side of the offshore Bohai Bay basin, and has close relationship to hydrocarbon accumulation. The Tan-Lu fault zone usually acts as boundary faults of uplifts and depressions in the eastern offshore Bohai Bay basin. The new high-quality 3D seismic data show that the Tan-Lu fault zone gradually widens to the south and its along-strike segmentation is very obvious. The northern segment, which is located in the Liaodongwan subbasin in the northernmost part of the offshore Bohai Bay basin, comprises of 2-3 major faults with relatively straight and throughgoing traces. The dextral movement of the northern segment is the most prominent. There are many typical strike-slip structures, including flower structures, releasing and restraining bends, horsetail splays and en echelon folds and faults. In addition, the Tan-Lu fault zone has also controlled the formation of the Liaodong uplifts. The middle segment in the Bozhong subbasin is composed of several branch faults with poor continuity, which is characterized mainly by superimposed deformation of early extensional and lately strike-slip systems. In the southern basin, the branch faults of the southern segment are nearly vertical in cross sections and overlap in plan view. Furthermore, the NNE-trending Tan-Lu fault zone interplays with the EW-trending extensional faults at right angles, which has also altered the basin architecture. Up to date, many large- and medium-scale oilfields have been discovered within the Tan-Lu fault zone. Generally, the fault zone has significance on formation of traps, distribution and evolution of source rocks and hydrocarbon migration. The important traps involve faulted anticlines and faulted blocks, which are especially developed well in the fault bends. Several main hydrocarbon-generation sags are distributed along the Tan-Lu fault zone which has also influenced the evolution and maturity of some key source rocks. The combination of the main faults in deep strata and branch faults in shallow strata is also favorable pathways for hydrocarbon migration.