Abstract

Organic geochemistry has been used to determine the source facies within the stratigraphic sequence of the Eastern java Sea and to delineate the liquid window using data from available wells. A combination of the geochemical data with analyses of the discovered oil has placed constraints upon the migration routes and timing so that source-trap relationships have been clarified. Source facies are not necessarily coincident with lithofacies so extrapolation beyond the drilled areas is difficult.

Visual analysis shows the kerogen to consist primarily of structural plant debris, while minimal amounts of amorphous kerogen have been observed. Maximum generation of liquid hydrocarbons from the thermal degradation of this structural kerogen occurs at ratings of 2+ to 3+ on the thermal alteration index. Within our contract area, this correlates to temperatures of 250° to 360°F. The alteration has been improved by a higher thermal gradient, which also results in a rather narrow oil window.

Organic carbon content ranges from low to high, but is generally higher in the regressive sequences, Two formations, the "Lower OK" and "Unit III", were found to uniformly contain rocks with organic carbon values over 1%. The "Unit III" generally occurs within the liquid window so it is, therefore, adjudged to be the best oil source.

Long distance migration was apparently not effective in this area. Much of the exploration thus far has been for Miocene reefs which appear to have received their oil by vertical migration from Oligocene source shales. Certain facies proved to have good source potential leading to some new plays. Future exploration will concentrate on reservoirs within the oil window or with a direct connection to the oil window.