Workflow for improvement of 3D anisotropic CSEM resistivity inversion and integration with seismic using cross-gradient constraint to reduce exploration risk in a complex fold-thrust belt in offshore northwest Borneo

Abstract

The focus of hydrocarbon exploration has now moved into frontier regions where structural complexity, heterogeneous overburden, and hydrocarbon system fundamentals are significant challenges requiring an integrated exploration approach. Three-dimensional controlled-source electromagnetic (CSEM) anisotropic resistivity imaging is emerging as a technique to combine with seismic imaging in such regions. However, the typically reconstructed horizontal resistivity [Formula: see text] and vertical resistivity [Formula: see text] models often have conflicting depth structures that are difficult to explain in terms of subsurface geology. It is highly desirable to reduce ambiguity or subjectivity in depth interpretation of [Formula: see text] and [Formula: see text] models and also achieve comparability with other coincidentally located subsurface models. We have developed a workflow for integrating information from seismic well-based inversion, interpreted seismic horizons, and resistivity well logs in a cross-gradient-guided simultaneous 3D CSEM inversion for geologically realistic [Formula: see text] and [Formula: see text] models whose parameter estimates for a selected reservoir interval can then be better optimized to aid reservoir characterization. We developed our workflow using exploration data from a complex fold-thrust belt. We found that the integrated cross-gradient approach led to [Formula: see text] and [Formula: see text] models that have a common depth structure, are consistent with seismic and resistivity logs, and are hence less ambiguous for geologic interpretation and reservoir parameter estimation.

Using Large-Size Three-Dimensional Marine Electromagnetic Data for the Efficient Combined Investigation of Natural Hydrogen and Hydrocarbon Gas Reservoirs: A Geologically Consistent and Process-Oriented Approach with Implications for Carbon Footprint Reduction

“…Industry benchmarking in 2017 revealed inconsistencies in horizontal and vertical resistivity distributions from anisotropic 3D CSEM inversion models produced by the major service providers for a given field dataset used for the test; invited academic practitioners abstained from the test [13]. This provided the motivation for the development of a crossgradientbased anisotropic-resistivity-imaging algorithm that directly enforces structural similarity between the horizontal and vertical resistivity models without control from seismic or well data [4,14,15]. The seismic, image-guided anisotropic 3D inversion of CSEM and MT data is an adaption of this crossgradient philosophy, in which the structural controls from seismic or other images are infused in the anisotropic resistivity inversion process [7,[16][17][18].…”

“…It is emerging that integrating 3D anisotropic CSEM-MT data imaging into seismic or structural constraints leads to geologically robust models [4,7,15]. Two methods of geologically consistent 3D anisotropic CSEM and MT resistivity inversion (using crossgradient constraints) have recently been developed [7,15,18] and are briefly described below.…”

“…An important consideration in any future investigations in this area will be the possibility of re-using the hydrocarbon reservoirs or saline aquifers for carbon sequestration. Figure 10 shows the result of the crossgradient 3D CSEM inversion for structurally consistent vertical and horizontal resistivity distributions for one of the three selected sweet spots (sweet-spot 3 in Figure 8b) [4]. The key messages from this figure are the following:…”

“…The seismic, controlled-source electromagnetic (CSEM), magnetotelluric (MT), gravity, and magnetic methods of geophysics play major roles in marine exploration for hydrocarbon and/or native hydrogen reservoirs. On their own, individual geophysical methods provide non-unique models of subsurface properties and the fluid type present, but integrating them [2,3] together with geological models maximizes accuracy, minimizes uncertainty in a "Shared Earth", and leads to a consistent and possible indication of a working resource or surveillance system [1,4,5]. Optimizing this process has huge implications for time and cost savings, and there are opportunities for carbon footprint reductions at all spatial scales, as advocated for in this paper.…”

“…The zones of relatively high anisotropy in the crust and mantle shown in Figure 11c may be multi-level detachment zones, suggesting active deformation involving a combination of tectonic-and gravity-driven processes. Step 3: Drilling the right well, as demonstrated for sweet-spot 3 [4]. Shown here is the result of prospect-scale 3D CSEM resistivity inversion integrated with the seismic PSDM data for improved decision-making [4].…”