oa Using airborne gravity data to better define the 3D limestone distribution at the Bwata Gas Field, Papua New Guinea

As part of an appraisal program by InterOil of the Bwata gas resource and prior to undertaking further 2D seismic surveying, a 3D geology model of the project was rapidly built using the 3D GeoModeller software.

The software implements a methodology developed in the BRGM to jointly interpolate geological contact data and dips of geology formations. The method uses the chronostratigraphic order of geological formations, and their rock-relationships. The model is calculated using an implicit 3D potential function as the interpolator for each component part of the geological history. The order and relationships recorded in the stratigraphic column are used to automatically resolve the intersections between component parts, and produce volume reconstructions. The methodology allows the geologist to focus on geological issues and consider alternative interpretations.

The 3D structural geology model was built using a single 2D seismic line, well data from the Bwata-1 and Triceratops-1 wells, surface geological data and airborne gravity data. Eight cross sections across the Bwata Anticline were created from surface geology, seismic and well data in 2D Move. These sections were imported into 3D GeoModeller. A 3D model was then created and the forward gravity response computed. Density variations from general background of 2.24 t/m³ are provided by the Cretaceous Ieru Formation at 2.40 ± 0.05 t/m³ and the Puri and Mendi Limestones at 2.70 ± 0.05 t/m³. The computed response was compared to observed data derived from an airborne gravity survey. On the basis of such comparisons several iterations of geologic revision were proposed to improve the fit between the computed and observed data.

The outcome of this study was the prediction of the geological setting and the extent and thickness of the limestone beds. The model incorporated c. 30 degree dipping thrusts and a steeper backthrust and introduced two NE/SW near vertical faults which exhibit a sinistral strike slip and east-side-up displacement. A substantial increase in the size of the field was interpreted at its western end. Using the 3D model will enable InterOil to design a follow-up 2D seismic survey with greater confidence that the survey will meet program objectives.