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Dynamic View of Sediment Sources and Its Significance for Exploration - Case Studies in Bohai Basin Offshore Area, China

Zhu, Weilin 1; Lai, Weicheng 2; Zhou, Xinhuai 2; Li, Jianping 2
1 CNOOC, Beijing, China.
2 CNOOC, Tianjin, China.

For the last decade, exploration activities in Bohai, a paleogene faulted lacustrine basin, have been shifted to the central part of the basin and consequently stratigraphic traps become more favorable. Therefore, two subjects need to be understood for prospecting: 1) the occurrence of sedimentary sources in the basin center that is far from significant uplifts, and 2) control of sand deposits by such occurrence if it existed.

Traditional source analysis was based on the erosion extent of formations or members, which works well along basin margins where prominent uplifts occur. According to such methodology, areas with only low-relief uplifts in the middle of a basin, such as Bohai, could be considered to be far away from source and be sand-starved there.

The author here proposed the dynamic sedimentary source concept which helps geologists approach the study of sand deposits with the integration of erosion, sequence stratigraphy, and paleogeomorphology. The regional palaeogeomorphology and extent of outcrop under erosion varies according to the change of lacustrine surface in the sequence, which affects the supply of sediments, especially for low-relief uplifts as the only sediment supplies in the middle of the basin; during a low-stand, more area, including low-relief uplifts, will be exposed for erosion, sediment supply will be abundant and sand accumulations could be formed even in the center of a basin. However, a high-stand could stop any erosion.

Sediment supply is controlled by water level which is dynamically changing. Therefore, geologists need a dynamic view in the prediction of reservoirs. Cases of Chengbei low-relief uplift and JZ25-1 oil field in Liaoxi low-relief uplift presented in this article prove the significance of the dynamic view for exploration with both failed and successful reservoir prediction experiences.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009