Abstract: Hydrocarbon Prospect Mapping Using Balanced Cross Sections and Gravity Modeling, Onin and Kumawa Peninsulas, Irian Jaya, Indonesia

M. Untung, Sardjono, I. Budiman, J. Nasution, E. Mirnanda, E. G. Sirodj, L. F. Henage

Exploration field geology mapping and acquisition of gravity data has been conducted on 26 surveyed traverse lines totaling approximately 650 line km across the Onin and Kumawa peninsulas of western Irian Jaya. The predominant surface outcrop is karstified New Guinea Limestone with a maximum thickness of 2150 m, which precludes the use of seismic data. The integrated use of field geology data, balanced cross sections, and gravity modeling has, however, enabled us to identify two giant hydrocarbon prospects.

The Onin and Kumawa peninsulas lie at the margin of Jurassic age rift faulting associated with the Australian northwest shelf. Jurassic rift sands of the Lower Kembelangan Formation are the primary reservoir target. During the Pliocene-Pleistocene collision of the Australian plate and the Banda are 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 (1992) and that collected by Shell Oil in the 1950s in the structurally less-deformed Bomberai region east of Onin and Kumawa. Bouguer reduction was carried out using a density of 2.4 gm/cc and GRS67 and IGSN71 formulas. Spectral analyses indicate that basement is at a depth of about 3 km in the Onin area and at about 6 km in the Bomberai area. The Bomberai and Onin-Kumawa regions are separated from one another by a steep southeast-southwest-striking gravity gradient, which may mark a change of lithology.

Prospects were defined by evaluating traverse profile data. Balanced cross sections were constructed using detailed biostratigraphy to determine formation thicknesses and fault offsets. Where possible, onshore balanced reduction 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 included in the model were held constant.

Constraining the variables to one (Lower Kembelangan thickness) in the gravity profile modeling maintains credibility of the technique. Errors inherent in the structural maps derived from the cross sections are likely to be a 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 modeling offshore, seismic data across the western plunge of the Onin peninsula were obtained. The structural style demonstrated in the modeling and the seismic data are comparable. This enhances our confidence in the modeling.

AAPG Search and Discovery Article #90982©1994 AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994