Crustal structure and basement-cover relationship in the dangerous grounds, offshore NW Borneo, from 3D joint CSEM and MT imaging

Abstract

Accurate mapping of crustal thickness variations and the boundary relationships between sedimentary cover rocks and the crystalline basement are very important for heat-flow prediction and petroleum systems modeling of a basin. Using legacy industry 3D datasets, we investigated the potential of three-dimensional (3D) joint inversion of marine controlled-source electromagnetic (CSEM) and magnetotelluric (MT) data incorporating resistivity anisotropy to map these parameters across sub-basins in the Dangerous Grounds in the southwestern rifted margin of the South China Sea, where limited previous seismic and potential field basement interpretations are available for comparison. We reconstructed 3D horizontal and vertical resistivity models from seabed down to over 40 km depth for a 35 by 42 km2 area. The resistivity-versus-depth profile extracted from our 3D joint inversion models satisfactorily matched the resistivity and lithologic well logs at a wildcat exploration well location chosen for model validation. We found that the maximum resistivity gradients in the computed first derivative of the 3D resistivity volumes predict a depth to basement that matches the acoustic basement. The models predict the presence of 2 to ~5 km thick electrically conductive (<20 Ωm) sedimentary cover atop an electrically resistive (>100 Ωm) crystalline crust that is underlain by an electrically conductive (<100 Ωm) upper mantle at depths that vary laterally from about 25 to 30 km below sea level in our study area. Our resistivity variation with depth is found to be remarkably consistent with the density distribution at Moho depth from recent independent 3D gravity/gradiometry inversion studies in this region. We suggest that 3D joint inversion of CSEM-MT, seismic and potential field data is the way forward for understanding the deep structure of such rifted margins.