Abstract

Geochemical techniques have been used in many parts of the world for evaluating hydrocarbon potential of areas and stratigraphic sequences. These techniques are not new, much has been written on the subject. However, few areas demonstrate more graphically than the Java Sea, the value of these techniques in guiding exploration in large, sparcely known geologic provinces. The N.W. Java Basin exhibits a variety of paleogeochemical environments which have a full suite of hydrocarbon potentials ranging from excellent to poor. Geochemistry has aided greatly in defining the limits of prospective and non prospective areas.

As an aid to preliminary evaluation of sparsely drilled parts of the N.W. Java Basin, paleo temperatures were used to determine whether rocks had been subjected to adequate temperature and pressure to form hydrocarbons. Prior to this, well cuttings and cores were examined to determine which sediments exhibited source rock potential. The application of these geochemical techniques resulted in a generalized vertical and lateral zonation of the area into favourable and unfavourable areas for producible hydrocarbons. This zonation has aided regional evaluation of hydrocarbon potential in large parts of the N.W. Java Basin.

The geochemical picture of the portion of the N.W. Java Basin studied indicates that source rock exists in the interbedded sandstone-shale sequences of Upper Cibulakan and Talang Akar Formations. However, only questionable indications of source rocks can be inferred for carbonate sequences such as the Batu Raja and Parigi, even though they do contain commercial accumulations of oil and gas. Individual facies in each of these sequences have different source rock potential. In general, the "deltaic facies" of each of these sequences appears to be richer in organic material than outer shelf or coastal plain facies.

As an overprint to source rock analyses, to determine where producible hydrocarbons might be abundant, geothermal gradient and temperature maps were constructed. It appears that present day temperatures are historical maximums, Therefore, subsurface temperatures derived from Electric Log and DST data can be used to estimate the degree of thermal maturation. The combination of geothermal temperatures of 180°F or greater with good source rocks apparently formed producible hydrocarbons throughout the N.W. Java Basin. All commercial accumulations of oil discovered to date in the Java Sea fall within favourable areas as outlined by geochemical parameters.