ABSTRACT: Source Rock Potential and Hydrocarbon Composition of Sediments and Crude Oil from the Visayan Basin, Philippines

BROOKS, LEIGH J., Australian Worldwide Exploration, North Sydney, New South Wales, Australia, ANDREW P. MURRAY and ROGER E. SUMMONS, Bureau of Mineral Resources, Canberra, Australian Capital Territory, Australia

The hydrocarbon source characteristics of a suite of sediment samples from the Visayan basin, northwest Leyte, Philippines, have been examined using Rock-Eval pyrolysis and GCMS techniques. Shales and claystones from the late Oligocene-early Miocene Taog Formation have organic carbon contents in the range 0-16% with many samples in the range 1-4% having HI values of 100 to 700. These represent good to excellent petroleum source rocks. A range of sample maturities was also encountered in the sampling locality although igneous activity rather than burial depth could be main control on maturation in the outcrop.

Hydrocarbons extracted from a selection of the sediment samples generally show immature patterns of biomarkers with the presence of waxy n-alkanes of odd-carbon-number preference, oleanane, and bicadinanes betraying a predominant contribution of organic matter from vascular plants. Widespread oil seeps and residual bitumens from the same area tend to be heavily biodegraded but have hydrocarbon signatures dominated by oleananes and bicadinanes. This suggests that the source organic matter for the oils comprised a predominant input from resin and other lipid of tropical angiosperms, especially plants of the family Dipterocarpaceae. The hydrocarbon patterns of the oils have close similarities with other oils from southeast Asia suggesting that analogous considerations will be important f r assessments of source rock quality and maturation.

As with many productive southeast Asia basins, the Visayan basin source rocks are contained in predominantly non-marine early graben fill. In the Visayan basin they are succeeded by marine sediments with poor source quality but which show extensive patch reef development. This stratigraphic relationship is evident on seismic data immediately offshore from Leyte.

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)