Reducing Uncertainty Using Full Tensor Gradiometry in Frontier Area: Case Study in Fold-Thrust Belt System in Eastern Indonesia

Abstract

Exploring for hydrocarbons in frontier areas of Eastern Indonesia is challenging, especially working in fold-thrust belt systems of Banda-arc. The severe deformations (thrusting) and thick Tertiary carbonates in the uppermost stratigraphy often lead to poor seismic data quality. Full Tensor Gradiometry (FTG) is a geophysical technology that measures the rate of change in the gravity field in all directions. The FTG data provides interpretation of subsurface features representing lateral density contrasts exhibited by rocks. Spectral analysis from FTG data is used to define gravity intervals from each wavelength that enable isolation of anomaly patterns from dominant geology. The FTG data is divided into 3 (three) major gradient trend of power spectral curve then extracted based on wavelength range 2 – 8 km, 8 – 30 km, and 30 – 116 km. The result estimates 3 (three) gravity intervals that relate to the tectonic setting in each interval. The data also allows investigation on the possibilities of both thin-skin and thick-skin structures developing in the area. The first gravity interval using short wavelength (2 – 8 km) is interpreted to correspond to very shallow geological structure at depth of approximately 1.5 km. The gravity anomaly evident on this interval is related to the fold-thrust belt system by the dominant occurrence of NE-SW trending gravity. The second gravity interval using intermediate wavelength (8 – 30 km) is interpreted to correspond to intermediate geological response at depth of approximately 3 km. The gravity anomaly evident in this interval is related to the fold-thrust belt system highlighted by the dominant occurrence of NE-SW trending gravity anomalies. The third interval shows a dissipation of NE-SW trending gravity anomalies indicating it is undisturbed by the fold thrust belt. Interestingly, N-S trending long wavelength gravity anomalies occupy the deeper interval which differs with the upper interval, hence thin skin structural configuration is likely. In Northern portion of study area, thick-skin configuration is more possible as the magnetic data are relatively higher in amplitude compared with the South. We interpret this magnetic anomaly caused by the occurrence of the Permian volcanics which emplaced into shallow level by the thrust sheets. The interpretation results give valuable subsurface information to evaluate the prospectivity of petroleum system in this frontier area which has not been explored previously.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018