Abstract:

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.

Key words: depositional system, depositional model, evolutional characteristics, Liushagang Fm, Eogene, Fushan Sag