Abstract

Highly significant oil and gas accumulations have been discovered recently in the East Java Basin, turning the area into the "hottest spot" for hydrocarbon exploration in Indonesia. These discoveries, fortunately, are commensurate with the fast growing fuel consumption in the East Java Province. Accordingly, hydrocarbon exploration in the basin has been aggressively enhanced.

Despite the recent significant discoveries, there are still many uncertainties concerning aspects of petroleum geochemistry in the basin. This paper summarizes a recent regional study of the geochemistry and habitat of oil and gas in the East Java Basin. It was envisioned that a better understanding of the basin's oil and gas geochemistry would aid exploration activity. During the study, geochemical data of oil, gas, and source samples comprising physical to molecular properties from around 100 wells and seeps of the onshore and offshore areas of the East Java Basin were interpreted using various geochemical crossplots and methods. The habitats of oil and gas are examined by synthesizing the results of geochemical evaluation with geologic framework of the basin.

It is shown from the oil geochemistry study that most oils were derived from sub-oxic to oxic terrestrial to marginal marine source facies (class "D"). The Paleogene Ngimbang, Lower Kujung, and Lower Tuban shales and coals are the inferred source rocks for the oils. Offshore oils have a more terrestrial signature than those of onshore. The source of offshore oils is thought to have sourced by older sources than those of onshore.

Based on gas geochemistry study, three genetic types of natural gases have been identified in the East Java Basin, they are: thermogenic, biogenic, and mixed thermo-biogenic gases. The thermogenic gases were sourced by the Paleogene sources, whereas the biogenic gases were sourced by the Miocene to Pleistocene Tawun to Lidah shales and coals. The study also evaluated the amount of CO₂ gas pollutant, identified areas with high CO₂ gas content, and investigated the origin of CO₂.

Four trends of habitats are recognized: the Ngimbang, Kujung, Ngrayong, and Tawun-Mundu Trends. It is within these trends that hydrocarbon prospectivity of the East Java Province is situated.