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Exploration field geology mapping and acquisition of gravity data has been conducted on approximately 650 line kilometres of26 surveyed traverse lines across the Onin and Kumawa Peninsulas of western Irian Jaya. Karstified New Guinea Limestone with a maximum thickness of 2,150 m is the predominant surface outcrop, and precludes use of seismic. However, integrated use offield geology data, balanced cross-sections, and gravity modelling has enabled us to identify two giant hydrocarbon prospects. The Onin and Kumawa Peninsulas lie at the margin of Jurassic age faulting associated with the Australian Northwest Shelf. Jurassic rift sands of the Lower Kembelangan Formation are the primary reservoir target. During the Plio-Pleistocene, collision of the Australian plate. and the Banda Arc inverted sections of the rift system, including the Onin and Kumawa Peninsulas. A better understanding of the regional structure was gained by integrating the Mobil Oil gravity data (of 1992) and that collected by Shell Oil (in the 1950's) in the structurally less deformed Bomberai region east ofOnin and Kumawa. Bouguer reduction was carried out using 2.4 glcm3 density, GRS 1967 and IGSN 1971. A sequence of gravity maps were generated, including Bouguer, regional, residual, downward continued, and second derivative. Spectral analyses indicate that basement is about 3 km depth at Onin and about 6 km in the Bomberai area. The Bomberai and Onin-Kumawa regions are separated from one another by a steep gravity gradient which has a SE-NW strike direction. This gravity gradient may represent a change of lithology. Prospect definition was obtained by evaluating traverse profile data. Balanced cross-sections were constructed, using detailed biostratigraphy to determine formation thicknesses and amount of fault offsets. Where possible, onshore balanced section profiles were tied to offshore seismic profiles and wells. Two dimensional forward gravity models were calculated, using formation densities from well data. The calculated profiles were then compared to profile observed values. Differences between calculated and observed profiles were resolved by adhering to the exploration model, which required that the main thickness changes would be in the Lower Kembelangan Formation rift section. Density values for the seven formations were held constant. Constraining the variables to one (Lower Kembelangan thickness) in the gravity profile modelling maintains credibility of the technique. Errors inherent in the structural maps derived from the crosssections are likely to be vertical shift which would be approximately constant across the prospects. The interactive work between geologists and geophysicists was effective in producing a logical representation of subsurface geology, which in turn allowed selection of drill sites with some degree of confidence. Subsequent to completing the modelling, offshore seismic data across the western plunge of the Onin Peninsula was obtained. The structural style demonstrated in the modelling and the seismic data are comparable. This enhances our confidence in the modelling results.

Hydrocarbon prospect mapping using balanced cross-sections and gravity modelling, Onin and Kumawa Peninsulas, Irian Jaya, Indonesia