Episodic Rifting and the Control on Cenozoic Source Rock in the Pearl River Mouth Basin, South China Sea

Li, Qi ¹; Zhang, Yingzhao ²
(1)School of Marine Sciences, China University of Geosciences, Beijing, China. (2) Faculty of Earth Resources, China University of Geosciences, Wuhan, China.

These basins, with great potential of large oil/gas reserves, include the Qiongdongnan, Pearl River Mouth and Beibuwan basins in which the Qiongdongnan and the Pearl River Mouth basins are characterized as divergent margin basins and were formed by extension. The basin show a typical double-layer configuration, that is, grabens, half grabens and horsts in their lower parts overlain by broad basin subsidence in their upper parts resulting from thermal cooling of lithosphere in a postrift phase.Detailed seismic interpretation and fossil zones near breakup unconformity have shown that the cessation of rifting and the commencement of the thermal subsidence did not occur synchronously along the margin. The rifting phase of the Pearl River Mouth basin ended early and the breakup unconformity, T70, is located within Oligocene strata, whereas the transitional boundary (T60) from synrift phase to postrift phase in the Qiongdongnan and Yinggerhai basins is located between Oligocene and Miocene strata. In contrast to many other passive margins, episodic rifting characterizes the northern margin of South China Sea. Based on seismic and drilling data and research on the structures, filling sequences and subsidence history of these basins, three rifting episodes have been identified.

The second rift episode occurred in Middle Eocene (49.5 Ma) to Early Oligocene time (30 Ma).This rift episode can be subdivided into two secondary episodes. The first resulting in a new generation of NE-NEE-trending fault basins filled with a mainly lacustrine strata, dominated by dark shales with sandstone interbeds. These strata are the principal source for hydrocarbon in the area. The second, during the Late Eocene to Early Oligcene was a relative stable period of subsidence. Pre-existing fault basins continued to subside and were filled by continental, coarse-grained clastics with coal beds.

This rift episode was the most favorable stage for the development of source rocks, the bay stage developed reservoirs when the lake regressed and there was a marine transgression, and the open-marine stage deposited thick shale that acted as regional seal rock, except for the sandstone reservoir. They formed an excellent match of source rock-reservoir-seal rocks, which is the foundation of petroleum systems.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.