MULTIPLE PETROLEUM SYSTEMS IN TERTIARY EXTENSIONAL BASINS, EAST CHINA: A CASE STUDY OF THE GUNAN‐FULIN BASIN

The Gunan‐Fulin Basin, a small, extensional basin of Tertiary age, is located within the Shenglipetroleum “province” which structurally corresponds to the Jiyan megabasin in East China. The Lower Tertiary succession in the Gunan‐Fulin Basin is dominated by the Eocene‐lower Oligocene Shahejie Formation, which is divided into four members. Major source rocks are present in the Es3 Member (Eocene), and the Es 1 Member (Oligocene). Both units were deposited during lacustrine transgressions in the Early Tertiary. They are composed of dark, organic‐rich mudstones and oil shales of lacustrine origin, and contain good quality Type I‐II kerogen. The Es 1 source rock was deposited in a saline lake occasionally invaded by the sea, while the Es3 unit was laid down in a fresh‐water lake. The maturity and characteristics of these source rocks and the related crude oils can be distinguished on the basis of their biomarker contents. Es 1 source rocks and related oils are of relatively low maturity, while those of Es3 derivation are mature to highly‐mature. There are therefore two separate petroleum systems in the Gunan‐Fulin Basin — Eocene and Oligocene. These petroleum systems are isolated hydraulically, and have independent migration pathways in which fault planes and unconformity surfaces play important roles. The distribution of oil accumulations in the two systems is different as a result of faulting. Oil from the Eocene source rocks is trapped in reservoirs which are distributed stratigraphically from the basement to the Tertiary, while oil from the Oligocene source rock is confined to Oligocene and Miocene reservoir rocks. The existence of multiple petroleum systems is an important feature of Tertiary extensional basins in East China, and results from multiple phases of block faulting and a high geothermal gradient.