Geochemical Evaluation of Petroleum Source Rock in Bonaparte Gulf-Timor Sea Region, Northwestern Australia

Abstract

The source-rock potential and maturation history of sediments are critical parameters to the generation, migration, accumulation, and ultimate degradation of hydrocarbons. These parameters, when considered within the constraints of regional tectonics, assist in explaining the absence of commercial reserves in wells drilled to date in the Bonaparte Gulf-Timor Sea region and may subsequently be useful in delineating areas for future exploration. The area now occupied by the Timor Sea is a structural complex of intersecting Phanerozoic tensional features overlain by a thick, Tertiary carbonate platform. Pertinent geochemical data were integrated from analyses performed on cuttings and cores from 25 exploratory wells penetrating the complex. The Bonaparte Gulf basin originated in response to an early Paleozoic rifting phase. Geochemical data from sediments deposited in the basin from the Early Cambrian through Early Permian are very limited. Sedimentary rocks of Late Permian to Middle Jurassic age selectively exhibit source-rock characteristics and maturation levels adequate for the generation of hydrocarbons. The Permian section has yielded significant gas on test. A second rifting phase transpired in the Middle Jurassic and was responsible for creating the prominent tectonic elements that are apparent today in the offshore region. Clastic sediments of Middle Jurassic through Late Cretaceous age which occupied the newly formed depocenters exhibit variable source-rock potential and are sufficiently matured for hydrocarbon generation over an extensive area. Significant hydrocarbons have been noted in wells penetrating this section. The overlying Tertiary carbonate rocks are mainly immature and, except locally, are considered to have poor source-rock potential. Several factors have been detrimental to the accumulation and preservation of hydrocarbons on the relatively positive tectonic features in the Bonaparte Gulf-Timor Sea region. The factors include unfavorable phase relations among structural development, hydrocarbon generation, and subsequent erosion, silicification of sandstone reservoirs, and flushing by meteoric waters. Sediments within major depocenters, however, have had a more continuous maturation history and are believed to have generated significant hydrocarbon reserves.