Inversion of Gravity to Estimate Depth to Basement in the East Papuan Fold Belt: Implications for Development of Isolated Carbonate Platforms

Abstract

Nucleation of Isolated Carbonate Platforms (ICPs) on basement ridges and uplifted basement-involved fault blocks is the observed mechanism for carbonate deposition in the Gulf of Papua at the Pasca and Pandora gas fields and onshore in the East Papuan Fold Belt (EPFB) at the Antelope gas field. Constraining basement architecture is therefore fundamental to understanding the presence and quality of carbonate reservoir across the region. Whilst seismic imaging in the EPFB has considerably better resolution than elsewhere in Papua New Guinea, the resolution depth of imaging is insufficient to consistently image basement. The sparsity of 2D seismic across much of the EPFB further limits attempts to define basement. In this study, an integrated, regionally extensive geological interpretation of basement architecture of the EPFB was completed using gravity-inversion techniques. Inversion of gravity data involves estimating the subsurface distribution of mass from gravity observations. By applying a series of fixed subsurface parameters, it is possible to use gravity data to model the depth and relief of the basement. Across the EPFB, basement relief can be calculated on the basis of the density contrast between the basin fill and pre-Mesozoic basement rock. Using the Sander airborne gravity, Satellite Free Air gravity V23.1 from SIO, NOAA, NGA, and ground gravity data from the Getech GCMGS, a grid of 2D gravity sections was spaced across the EPFB. The basement was modelled across each section, and then all sections gridded into a single regional surface. Seismic, well and outcrop data was subsequently integrated with the modelled basement grid. This onshore grid was then merged with a high-quality offshore Gulf of Papua basement grid, developed using both satellite gravity and high-resolution 2D seismic data. Through the gravity modelling process, five key observations were made regarding the gravity response across the region: 1. Negative gravity anomalies offshore and in regions of low topographic relief (<1000 m) onshore correspond to a thick Cenozoic-Mesozoic sedimentary sequence overlying pre-Mesozoic basement. 2. The increasingly negative gravity anomaly across the Eastern Highlands likely results from the isostactic response of the Moho in regions of high topographic relief (>1000 m). To mitigate the influence of isostatic compensation on the gravity data, a grid of the anomaly from the density contrast across an Airy-derived Moho was calculated. The resulting grid was subtracted from each of the gravity data sets to produce an isostatic-corrected map. 3. Moderate positive anomalies in the isostatic-corrected gravity grid are observed across the Darai-shelf foreland carbonates. 4. Significant positive gravity anomalies observed offshore correspond to shallow pre-Mesozoic basement overlain by ICPs. 5. Significant density contrasts across the sedimentary section are required to fit the gravity gradient across the EPFB. This is consistent with the density contrast derived from well data between strata throughout the region. These results support the suggestion that nucleation of ICPs is, in part, facilitated by the presence of basement highs. The study demonstrates how gravity data can be used in areas of limited seismic coverage to produce a regional-scale model of basin architecture.

AAPG Datapages/Search and Discovery Article #90371 © 2020 AAPG Asia Pacific Region, The 1st AAPG/EAGE PNG Geosciences Conference, PNG’s Oil and Gas Industry: Maturing Through Exploration and Production, Port Moresby, Papua New Guinea, February 25-27, 2020