Summary Three-dimensional (3D) imaging of the lithosphere in the Mexican Ridges fold belt is important for understanding how the crustal deformations in this basin relate to deep tectonic processes and structures inherited from extinct Jurassic seafloor spreading. Here, we use broadband (0.0001–0.4642 Hz) marine magnetotelluric data from the basin to reconstruct the 3D anisotropic resistivities of the lithosphere and their spatial gradients. The resistivity gradients maxima enabled independent definition of important geological boundaries (seen on collocated seismic reflection data) and estimation of crustal thickness. We found anomalous layered zones of low resistivity and high electrical anisotropy at 5-8 km depth (coinciding with the regional detachment zone in Eocene shales in 3D seismic data) and in the upper mantle which we interpret as indicating intense deformation and/or recent magmatic influence. We also found a banded crystalline basement structure across the fossil spreading centre comprising WSW-ENE trending, 6-10 km wide, electrically resistive sub-vertical sheets with conductive and anisotropic borders, which merge into a basal resistive stock-like body at 15-20 km depth. These are cut or bounded by later NNW trending major faults. These WSW and NNW structural trends correlate with the previously interpreted transform and normal faults that formed during the Late Jurassic opening of the Gulf of Mexico only if rotated clockwise by 25–30 degrees. Surprisingly, the rugged thrust-related seabed is offset at the projected positions of the steep resistive-conductive basement sheets (which also have spatially coincident high magnetic intensity and seismicity) enabling us to infer they represent magmatic intrusions facilitated by pre-existing faults. Their conductive borders spatially coincide with possibly fluid-filled vertical fracture-sets in the overlying sediments seen in seismic data which we interpret as hydrothermal fluid pathways. We infer that a magmatic body recently intruded our study area, its ascent controlled by pre-existing basement structures, and influenced the deformation of the Neogene sequences and the seafloor topography.

Fractured and/or karstic carbonates (FKC) in the subsurface are commonly associated with significant drilling risk, or reservoir effectiveness issues. Understanding the distribution of karsts will help avoid drilling into possible voids and also re-assess risks associated with reservoir effectiveness. A 2020 3D seismic acquisition in offshore northwest Borneo provided data for identifying new carbonate prospects and an opportunity to revisit a 2010 marine controlled source electromagnetic (CSEM) survey that was not designed specifically for FKC mapping but can serve to test new inversion capability for imaging karsted carbonate bodies. 3D CSEM data was used to detect and characterize the nature (i.e., fractured and/or karsted) of carbonate prospects mapped using 3D seismic data. In one approach, structure-tensors computed from these seismic data were used to constrain the 3D CSEM inversion on the Azure cloud platform. In another approach, cross-gradients between vertical and horizontal resistivities were used as a structural guide obviating the need for seismic data. The resulting anisotropic resistivity models were validated using resistivity logs from a nearby well; the resistivity results matched the well logs satisfactorily, providing geological justification to interpret the presence of karst features in the resistivity volumes. Lateral boundary of the carbonate prospects was mapped based on resistivity characteristic, both in seismic- and non-seismic guided structure-coupled inversion. Resistivity-depth cross-section of the area revealed the presence of resistivity discontinuity at the seismic-inferred boundary of the main carbonate prospect. Also, resistivity depth slices of the models clearly demonstrated that the areal distribution of low and high resistive zones can be mapped and useful deductions such as the presence of FKC and basement characteristics can be made using the 3D inversion of the CSEM data.

Accurate mapping of crustal thickness variations and the boundary relationships between sedimentary cover rocks and the crystalline basement is very important for heat-flow prediction and petroleum system modeling of a basin. Using legacy industry 3D data sets, we investigated the potential of 3D joint inversion of marine controlled-source electromagnetic (CSEM) and magnetotelluric (MT) data incorporating resistivity anisotropy to map these parameters across subbasins 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 have reconstructed 3D horizontal and vertical resistivity models from the seabed down to >40 km depth for a 35×42 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 approximately 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 approximately 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.

No abstract

No abstract