OIL SOURCE ROCK POTENTIAL OF THE LACUSTRINE JURASSIC SIM UUJU FORMATION, WEST KOREA BAY BASIN .1. OIL SOURCE ROCK CORRELATION AND ENVIRONMENT OF DEPOSITION

The offshore West Korea Bay (WKB) Basin is one of several NE-SW rift basins which formed over thin continental crust in Korea and eastern China during the Late Mesozoic and Early Cenozoic, and is characterised by a two-layered structure, with a Cenozoic basin superposed on a pre-Tertiary basin. Paraffinic oils recovered from Mesozoic and Tertiary sandstone intervals are thought to have been generated from different source beds. The Sim Uuju Formation consists mainly of fluvially-derived shales and sandstones deposited in a large, open, fresh-water, deep, tectonic lake, which occupied wide sub-basins formed by normal faults under a semi-humid climate, as a result of initial rifting during the Upper Jurassic. A probable source bed (400- to 500-m thick) has been geochemically located in the basinal offshore area of the Sim Uuju sequence penetrated by Well 606. The Jurassic bed could be considered as a potential source for the Mesozoic oil, provided that it contains the right type and amount of organic matter which has reached the stage of oil generation.