Prediction of Oil and Gas Distribution and Favorable Hydrocarbon Accumulation Zones in the Shallow Layer of Sag Zones in Offshore Bohai Bay Basin, Eastern China
In the shallow layer of fault-depressed basin, oil and gas is mainly found in uplift zones, rarely in sag zones. There is a lack of research on regularity of hydrocarbon accumulation in sag zones. Based on comprehensive research on oil fields found in the shallow layer of sag zones in offshore Bohai Bay basin in recent years, this article summarizes three factors including development degree of oil source faults, development of structural ridge of Guantao Formation and contact relationship between faults and sand body, which control the enrichment of oil and gas in shallow layer of sag zones. Firstly, deep faults, which spread to source rock and move intensely in the later period, are the primary condition of forming shallow oil and gas reservoirs. Minghuazhen Formation and Guantao Formation in the shallow layer of sag zones have no hydrocarbon potential and lack lateral transport layer, so source faults used for vertical migration are necessary for hydrocarbon accumulation in shallow layer. Furthermore, hydrocarbon accumulation in the shallow layer of Bohai basin mainly happen in the neotectonic movement period from 5.1Ma ago, so source faults are not only required but also need to have strong activity in the later period. Secondly, structural ridge of Guantao Formation is a secondary transfer station, which can redistributes oil and gas to the shallow layer away from deep oil source. Regional gravel layers develop in Guantao Formation of Bohai area, which make oil and gas migrate to low potential area like structural ridge of Guantao Formation speedily, and continue to migrate to the shallow layer. The actual drilling also confirms that oil and gas in the shallow layer is richer in the area where structural ridge of Guantao formation develops. Furthermore, good contact relation between sand body and fault guarantees hydrocarbon accumulation in the shallow layer. It must have good contact relation between sand body and fault that oil and gas migrate to the shallow layer through fault and accumulate in sand body. In general, the sand body whose low part have large contact area with faults, can form effective trap more possibly and have more oil and gas accumulation. In recent years, under the guidance of this cognition, we clear about the direction of choosing exploration areas and discover 200 million tons of reserves in shallow layer of sag zones, realize the important breakthrough from uplift to sag in shallow layer of offshore Bohai Bay basin.
AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014