The Gobe Field, PNG: Influence of Basement Architecture on Fold and Thrust Belt Structural Style

Abstract

The 25 km long Gobe Anticline in the SE Papuan Fold Belt is sinuous in map view, being divided into at least three structural compartments along strike, believed to be in part controlled by Early Jurassic rift architecture in the Permo-Triassic basement. The main hydrocarbon-bearing reservoir is Upper Jurassic Iagifu sandstone. The overlying 1 km thick Cretaceous mudstone is relatively weak, and has detached the reservoir sequence from the overlying 1 km thick more competent Miocene limestone that formed the Pliocene to recent thin-skinned structures seen at the surface. In contrast to the thin-skinned deformation, a few km to the south is the basement-cored Iehi anticline that appears to converge with and intersect the Gobe Anticline to the NW. Data at Gobe are spatially limited relative to this structural complexity. Hence, the geological model has uncertainty concerning reservoir presence, geometry, and compartmentalisation. To minimise risk for ongoing field development it is key to understand the relative role and timing of thick and thin-skinned tectonics at reservoir level. A new 3D structural model was constructed honouring regional trends and incorporating all data including surface maps, well tops, true stratigraphic thicknesses (TSTs) and dips in ~40 wells, and twelve 2D seismic lines of poor to moderate quality acquired under challenging surface conditions. 3D data integration and sequential structural restoration allowed testing of concepts and scenarios and led to balanced, valid and internally consistent sections, and an improved understanding of the structural style and history. The Gobe Anticline resulted from a progressive interplay of two conjugate contractional fault sets, both spatially related to the sides of an inverted basement horst. The fault set on the NE side comprises the NE-dipping Gobe Thrust, displacement on which resulted in development of the majority of the Gobe anticline by fault-bend folding. Inversion of faults on the SW side of the horst created the Iehi-trend. Inversion occurred both before and after the Gobe anticline was formed. The present-day first order geometry of the reservoir as well as compartmentalisation has resulted from the interplay of the two fault sets, with the relative importance of each dictated by the width and extent of the underlying basement horst. A 3D model of the basement horsts, grabens and transfer faults has guided interpolation at reservoir level.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015