Abstract:

 Based on the theory of sedimentology and sequence stratigraphy, and by analysis of seismic response characteristics, well logs, core sedimentary structures, the regional geological conditions and distribution of heavy minerals ,it is concluded that two complete sublacustrine fans developed in the third Member of Shahejie Formation of Gangzhong Oilfield, which cover 30 km² to 50 km² and each exhibit a positive cyclic sedimentary sequence.  The sublacustrine fans mentioned above could be further divided into four subsedimentary facies and seven microfacies, of which the middlefans are main parts with the coverage of  15 km² to 20 km². Turbidity could not be the only cause for the formation of sublacustrine fans. In the early phase of deposition, the fan is mainly influenced by gravity, but in the late one, by traction together with gravity. The provenances of sublacustrine fans mainly come from the uplift of Gangxi. Sandstone distribution takes the flowershape with multisand stacks. Braided channel of the middlefan with best physical properties and surrounded by mudstone, can provide an excellent space for hydrocarbon accumulation. There are two regional erosional interfaces for stratigraphic reservoirs at the ends of middle and upper oil groups, the third Member of Shahejie Formation. The sublacustrine fans with quite large area and braided channel developed on it, where the sandstone pinches out in the mudstone, is easy to form lens and wedge outs of sandstone lithologic reservoirs.

Key words: Huanghua Depression, Gangzhong Oilfield, the third Member of Shahejie Formation, sublacustrine fan