Airborne Gravity Gradiometer Survey over the Pelarang Anticline, Onshore Kutai Basin, Indonesia

Abstract

The Pelarang Anticline is part of the NNE-SSW oriented Samarinda Anticlinorium, a detached thrust-and-fold belt in the Tertiary Kutai Basin. Results from an airborne gravity gradiometer survey over the Pelarang Anticline are presented herein. The Pelarang Anticline is interpreted as a detachment fold ~30km long with steeply dipping (70o-80o) flanks. However, seismic imaging on existing 2D data is poor. In October 2016 Cue Energy acquired airborne gravity gradiometer survey data over the anticline. The survey revealed a large (~10mGal) gravity signal range, and that the anticline is associated with a strong, positive gravity anomaly. Subsequent application of potential field enhancement filters clearly delineated the crest and the flanks of the feature. 2D modelling of selected profiles across the anticline suggests that it can be modelled as a 1,500m-2,000m wide, by ~2,000m high shale body that is close to breaching the surface in places. This is in alignment with an interpretation that the feature is cored by high-pressure shales, resulting in un-prospective areas. However, 3D modelling has revealed significant along-strike variations in the depths to the crest of the anticline, suggesting the presence of several anomalous structural lows. Further investigation suggests these features are pull-apart mini-grabens, formed in response to localized shear movements. At least two commercial hydrocarbon accumulations, Sambutan and Mutiara, appear to be genetically related to the newly recognized structural anomalies. This survey has led to the recognition of a new exploration play in the region, and provided a tool to pursue it.