Organic geochemistry and thermal maturity modeling of hydrocarbon generation in the northwest Java Basin, Indonesia

An integrated geological, geochemical, and basin modeling approach was used to investigate petroleum generation in the onshore portion of the Northwest Java Basin, Indonesia. A suite of 22 oils, ranging from heavy to very light oils and retrograde condensates (API gravities from {dollar}17.8spcirc{dollar} to {dollar}53spcirc),{dollar} were analyzed for bulk parameters, hydrocarbon distributions, biomarkers and stable carbon isotopic compositions.; Most oils in the NW Java Basin are interpreted to be derived from the fluvial-deltaic to nearshore marine Talangakar Formation. Principal Component Analysis of some of the biomarker and isotopic data distinguish two groups of oils. One group is derived from a more marine source interval, probably a delta front to prodelta depositional setting, while the second group is attributed to a more terrestrial source interval, probably a delta plain to delta front depositional environment. One oil does not fall within either of these two groups and appears to have been sourced by a carbonate rock. This oil is characterized by a slight excess of even-numbered n-alkanes, low diasterane/sterane, {dollar}rm Csb{lcub}23{rcub}{dollar} tricyclic {dollar}rm terpane > Csb{lcub}24{rcub}{dollar} tetracyclic terpane, {dollar}rm Csb{lcub}29{rcub}/Csb{lcub}30{rcub}{dollar} hopane and {dollar}rm Csb{lcub}35{rcub}/Csb{lcub}34{rcub} homohopane > 1,{dollar} high sterane/hopane, high sulfur content (1.2%), and relatively heavy carbon isotopic composition {dollar}rm(deltasp{lcub}13{rcub}C = {lcub}-{rcub}23perthous{dollar} PDB).; This is the first indication of the presence of a carbonate source rock in the onshore NW Java basin.; Post-generative alteration processes of oils (e.g., evaporative fractionation, biodegradation, and water washing) are widespread in this highly faulted region. Heavy oils, with API gravities less than {dollar}22spcirc,{dollar} all of which are in shallow reservoirs, are biodegraded. Compositional patterns and maturity estimates indicate that the light oils and retrograde condensates in this basin are due to migration and evaporative fractionation effects rather than to high thermal maturity. Pristane/phytane values are significantly affected by evaporative fractionation processes, with light oils and retrograde condensates having higher Pr/Ph values and residual oils having lower ratios than original pristine oils.; Thermal modeling shows that the Talangakar sections reached their maximum depth of burial during Pliocene time, and oil generation in this basin has been continuous for the past 1.6 to 6.0 Ma. Additional unexplored potential for oil generation from the Talangakar Formation and possibly also the Baturaja Formation exists within some of the deeper sub-basins, especially in the Tambun sub-basin.