Braded river delta—shallow lacustrine depositional system of Guantao Formation in Jiyang Depression

Being lack of systematical research, for a long time, it has been held that the lower member of Guantao Formation in Jiyang Depression is fluvial facies and it lacks good cap rocks, which evidently fetters the exploration of it. Through the recent exploration practice and the research of it, it is found that there exists a braided river delta depositional system on the basis of shallow lacustrine deposit in the lower member of Guantao Formation in the northern region of Jiyang Depression. The authors provide the evidences for the existence of this depositional system from six aspects such as palaeo-geomorphology, paleontology, and sedimentary structure etc. The depositional system is characterized by shallow water with a great coverage of braided river delta front sand bodies. The braided river delta is composed of braided delta plain, braided delta front and braided prodelta. The character shows that the sand from subaqueous distributary channel composes its main body; and the river mouth bars and infralittoral bars are less developed and pro-delta facies belts are thin. Cooperating with oil-source faults, lacustrine shale stone can serve as lateral and horizontal barrier beds, which can enclose oil and gas. As a result, subtle traps are formed on a large scale in a certain area. The discovery of the braided river delta depositional system obviously breaks through the traditional understanding of lower member of Guantao Formation in Jiyang Depression, which will bring great impact on the shallow formation exploration in this area. Great efforts should be made in such work as the readjustment of exploration projects, the reevaluation of petroleum resources and the rearrangement of well pattern. These efforts will turn those old fields into new ones.