Abstract

Sequence stratigraphic analysis within the West Natuna Basin provides an integrated approach for understanding the temporal and spatial evolution of petroleum systems. Tertiary strata in this area are subdivided into three megasequences based on tectonic stages of basin evolution: syn-rift, post-rift, and syn-inversion. These megasequences are further subdivided into third-order depositional sequences that define a sequence stratigraphic framework for the basin. Within this framework sequence-keyed facies analysis integrated with structural and geochemical modeling is used for generation of basin-specific predictive exploration models.

This integrated exploration framework for Conoco's West Block "B" PSC area, S. Natuna Sea, has documented the occurrence of two major Tertiary petroleum systems. These systems occur within strata of the Late Oligocene to Early Miocene syn-rift megasequence and the Early to Middle Miocene syn-inversion megasequence. Two distinct source rock intervals exist within the syn-rift petroleum system of the larger rift sub-basins (half-grabens). These are: (1) an early syn-rift open lacustrine source rock characterized by oil-prone, algal organic matter, and (2) a late syn-rift shallow lacustrine/shoreline source rock characterized by mixed algal and terrestrial organic matter. The early syn-rift source is inferred from oil chemistry data using geochemical inversion, and interpreted seismically based on stratal geometries and reflection character. The late syn-rift source comprises carbonaceous and coaly shales interbedded with reservoir-quality progradational shoreline sandstones documented by well data tied to 2D regional seismic data.

Source rocks of the syn-inversion petroleum systems consist of coals and coaly shales deposited as part of a widespread lacustrine plain developed during conditions of rising base level associated with a regional increase in accommodation space.

An overview of the regional sequence stratigraphic framework and integrated interpretations for the synrift megasequence are given to illustrate the approach used for basin analysis and implications for new play concepts developed for stratigraphic and combination traps in Oligo-Miocene syn-tectonic strata in this mature, structurally complex basin.