Through composite method of geological

log

seismic and core analyses

depositional system of Eogene Liushagang Fm in Fushan sag has been studied. Braided delta and lacustrine facies are two major depositional systems developed in Liushagang Fm. The braided delta facies can be divided into three sub-facies: delta plain sub-facies

delta front sub-facies and pro-delta sub-facies based on depositional environment and sedimental features. Among which

the delta front sub-facies constitutes the main body of sediments characterized by abundant sedimentary structures

such as the lateral-accretionary cross beddings. The lacustrine depositional system can also be divided into four sub-facies

i.e. shallow lacustrine

moderate lacustrine

deep lacustrine and sublacustrine fan sub-facies.The sublacustrine fan deposit is relatively well developed and characterized by the interbeddings of sand bodies

glutinites

and mudstone and shales.It is vertically characterized by progading and composite stacking

and thickening and coursening upward sequences. The lateral evolution of Liushagang Fm shows the evolutional patterns of sediments from basin center to basin margin to be sublacustrine fan

deep lacustrine

moderate lacustrine

delta front

and delta plain.The sediments have mainly sourced from the southern part of Fushan sag. Very thick black mudstone occurring in the 2nd member of Liushagang Fm constitutes the main oil source rock and caprock;while the thick sandbodies in braided delta front sub-facie belt

developed in the 1st member and 3rd member of Liushagang Fm

would be good reservoirs

both of them would constitute a good combination of source

reservoir and cap rocks.The Liushagang Fm in Fushan sag would

therefore

be the target of further exploration.