Potential Field

Key Takeaways

• Gravity gradiometry, which measures the spatial rate of change of the gravitational acceleration vector (the gravity gradient tensor), is increasingly used in petroleum exploration as a complement to or replacement for conventional gravimetry because it provides better resolution of the lateral extent and geometry of subsurface density anomalies: conventional gravity meters measure a single component of the gravitational field (the vertical component gz), while gravity gradiometers measure multiple components of the gradient tensor (the second derivatives of the gravitational potential in different spatial directions), providing directional information about the orientation and shape of causative bodies that single-component measurements cannot resolve; full tensor gravity gradiometry (FTG), commercialized by Bell Geospace and originally developed for US Navy submarine navigation, measures all five independent components of the symmetric gravity gradient tensor and provides significantly improved lateral resolution compared to conventional gravity (typically 100-300 meter anomaly resolution for airborne FTG versus 500-2,000 meter resolution for conventional airborne gravity); FTG surveys are widely used in the Gulf of Mexico and offshore West Africa to map salt body geometry for depth migration velocity model building, to detect shallow geohazards (shallow water flows, shallow gas) that threaten deepwater drilling operations, and to calibrate 3D density models that constrain seismic velocity models.
• Salt body geometry mapping using gravity data exploits the large density contrast between salt (2.16 g/cc) and the surrounding sediments (typically 2.3-2.7 g/cc for Cenozoic clastic sequences), which creates a persistent negative gravity anomaly over allochthonous salt bodies that can be detected even when seismic imaging of the salt base and flanks is poor; the combination of gravity inversion (computing the 3D salt geometry that best reproduces the observed gravity anomaly) with seismic interpretation (providing the geometry where the seismic image is clear) provides a constrained salt model that reduces depth uncertainty in sub-salt reservoir targets; in the Gulf of Mexico, where the salt canopy creates massive imaging challenges for sub-salt exploration, gravity data has been used routinely since the 1990s to define the sub-salt structural geometry below the seismic resolution limit and to guide the velocity model building needed for pre-stack depth migration of deepwater exploration data; the Sigsbee Escarpment, a prominent bathymetric feature marking the southern edge of the allochthonous salt canopy, is clearly expressed in the regional gravity field and was mapped decades before deepwater seismic became routinely available in that area.
• Magnetic data interpretation in petroleum exploration focuses primarily on mapping the depth to magnetic basement (the top of the crystalline basement rocks that typically have high magnetic susceptibility), which provides constraints on the thickness of the overlying sedimentary column and therefore the potential for petroleum generation and accumulation: a thick sedimentary section (deep basement) is necessary but not sufficient for a petroleum system, and the basement depth map derived from magnetic inversion provides one of the earliest quantitative constraints on basin architecture in frontier exploration areas where seismic data coverage is sparse or absent; magnetic data also maps igneous intrusions (sills, dikes, laccoliths) that can act as barriers to fluid flow or as heat sources for maturation of organic matter in adjacent source rocks; the detection of oil seeps and hydrocarbon microseeps by magnetic surveys relies on the authigenic magnetite and greigite (iron sulfide minerals) that form in soils and shallow sediments where ascending hydrocarbon seepage creates reducing conditions that diagenetically alter iron-bearing minerals — this hydrocarbon-induced magnetic anomaly technique (also called magneto-hydrocarbon survey) is used in reconnaissance exploration to prioritize areas for more detailed seismic surveys.
• Regional potential field data interpretation requires careful separation of the observed anomaly into contributions from different geological sources at different depths: long-wavelength gravity anomalies (wavelengths of tens to hundreds of kilometers) reflect deep crustal and lithospheric density variations (crustal thickness changes at passive margins, isostatic root variations beneath mountain belts) that are not directly relevant to petroleum exploration; intermediate-wavelength anomalies (5-50 km wavelength) reflect basement relief and variations in the density of major sedimentary units; short-wavelength anomalies (less than 5 km) reflect shallow density contrasts including salt bodies, carbonate reefs, and dense igneous intrusions that are the direct targets of petroleum exploration; wavelength filtering (using upward continuation, bandpass filtering, or regional-residual separation) isolates the wavelength range of interest and removes the contributions of other sources; the residual anomaly (obtained by subtracting the regional field from the total observed field) highlights the local anomalies that correspond to exploration targets and is the primary deliverable of the potential field interpretation for play fairway analysis.
• 3D inversion of gravity and magnetic data produces quantitative models of the subsurface density or magnetic susceptibility distribution that can be compared with seismic velocity models and geological maps to provide integrated subsurface characterization: modern inversion algorithms (using minimum structure or compact body constraints) find the simplest density or susceptibility model that fits the observed potential field data within measurement uncertainty, producing a result that can be displayed as a 3D property volume consistent with the seismic data; joint inversion of gravity and seismic data (using rock physics relationships between density and seismic velocity to couple the two datasets) provides improved resolution compared to either method alone and is an active area of research for sub-salt and basement characterization; the uncertainty in potential field inversion is fundamentally non-unique (many different subsurface models produce the same surface potential field) and must be managed by incorporating geological constraints (known basement depths from wells, salt geometry from seismic, formation densities from core measurements) that reduce the solution space to geologically reasonable models consistent with all available data.