Potential Field Analysis
For exploration to be successful in many of the frontier areas of the globe that lack detailed seismic and well data, a robust set of methodologies and technical advances in technology have to be employed to identify and evaluate the petroleum systems of a basin.
Although GIS technology provides the following core activities, Integration, Management, Analysis and Presentation, the usefulness of using such powerful state-of-the-art software for geospatial analyses, is totally dependent on the quality and quantity of the information in the database and on the explorationist’s knowledge of the tectonostratigraphic history of the region. It is therefore essential, that we start to understand the structural framework (‘basic building blocks’) of the region in question and, placed in a plate tectonic context, use this information to develop dynamic geological model that can be used for predictive purposes.
One of the ways to achieve this is to use potential field data (gravity and magnetics), in combination with seismic and well data, to produce a detailed structural/geological coverage for the entire region of interest.
The integration of potential field data with various geological datasets can be used to define:
- Principal structural basement fabrics that characterise the region
- Distribution of different crustal types (continental, oceanic and heterogeneous) and their boundaries
- Carbonate distribution, and the extent of igneous intrusives (e.g., granite) and volcanics
- Total sediment thickness (depth-to-basement) and overall basin geometry
We specialise in…
- Seismic survey planning
- Identifying unrecognised rift basins and their geometry
- 2D gravity and magnetic modelling to determine crustal compositional changes and depth-to-basement
- The mapping of fractured igneous intrusive and volcanic plays
- Detailed mapping of basement fabrics and the structural configuration of depocentes
The role of potential field data
Gravity and magnetic data have been traditionally thought of as regional screening tools capable of providing basin definitions and basement mapping (Jacques et al. 2003). However, in recent years, the application of potential field data has been greatly expanded to include global palaeotectonic modelling through to modelling of prospect-level targets (see also Jacques et al. 2004). As discussed by both these papers, one of the most important phases of any exploration screening programme, particularly, in areas that lack seismic and well data, is the integration of potential field data with various geological datasets to define structural elements, continental block outlines, and crustal types, with the aim of producing a detailed, digital structural and geological coverage.
‘Traditional’ qualitative techniques. Used as a regional and/or basin screening tool, potential field data has traditionally been used as a cost-effective way to perform tectonic, structural and geological interpretations; particularly, prior to undertaking an expensive seismic programme in a frontier area or difficult exploration terrain. This type of analysis is not confined to the initial stages of an exploration programme, but is often performed during seismic acquisition, albeit using more enhanced imaging techniques, to aid the orientation of the seismic survey. Once the seismic has been shot, trends observed in the seismic are often evaluated and extended into surrounding areas using both gravity and magnetics.
The availability of gravity and magnetic interpretation over an exploration area also commonly provides:
- Vital interpolation and extrapolation of structure in areas of irregular seismic coverage
- Resolution of seismic section ambiguity caused by data incoherence
- Detection of, for example, strike-slip faults and unfaulted contacts, to which the seismic method is largely insensitive.
‘Innovative’ quantitative techniques and imaging methods. Quantitative interpretation takes the process one step further by modelling along profiles and applying algorithms to the grids to provide estimates of the location and depth of specific horizons. For example, these may relate to crystalline basement from an analysis of magnetic data, or the depths of basins from 3-D inversion of residual gravity.
(a) Gravity and magnetic data enhancements, modelling and inversion techniques. These are generally used to draw attention to particular features in the gravity or magnetics by filtering and enhancement. The end result is often a series of shaded relief images, which are particularly useful for defining tectonic and structural trends, and differentiating deep crustal lineaments from faults confined entirely to the sedimentary cover. Enhancement techniques are employed to essentially enhance features of interest by using certain wavelengths to filter the gravity and magnetic data. For instance, Wavelength Filtering is often used to band-pass filter the data at a given depth range to enhance features of interest – an essential requirement for depth structure maps, particularly for identifying structural decoupling horizons for, example, growth fault systems that sole out into a mobile substrate horizon such as salt or shale, and do not continue as throughgoing fault systems that interact directly with the basement.
Spatial derivatives are designed to enhance the edges of an anomaly producing body. Several enhancement techniques are used, often in conjunction – First and Second Vertical Derivatives, Horizontal Derivatives and Total Horizontal Derivatives – to define, in particular, fault location and deformational style and, in turn, the position and geometry of horst and graben. Another enhancement technique is Dip-azimuth Visualisation, which is based on calculating the direction of the maximum slope/gradient of the anomaly field to reveal the relationship of complex geological surfaces.
2-D (or 2.5D or 2.75D) gravity and magnetic profile modelling involves the refinement of fracture and body locations on a map basis (GIS) and the calculation of likely source body depths. It is frequently used to determine the thickness of different crustal units, evaluate the position of their boundaries, and to model the surface of the mantle. It provides a means to test alternative models for lithospheric extension (e.g., pure shear vs. simple shear) which, ultimately, has a direct bearing on understanding palaeoheat flow for source rock maturation considerations. The ultimate success of this technique is dependant upon whether other constraints are available, such as seismic, well control and outcrop data.
Typical modelling results – gravity example, using GmSys modelling software
As 2-D (or 2.5D or 2.75D) gravity and magnetic profile modelling is a rather time consuming process, other methods are often brought into play enabling a broader coverage in a considerably shorter period of time e.g. magnetic slope measurement methods such as Koefoed (50-75 method), Bott-Smith depth rules for gravity, and, analytic methods such as Euler and Werner deconvolution. 2-D and 3-D inversion modelling techniques – Spectral Methods, Euler and Werner Deconvolution, and Local Wavenumber Methods, and Curie Point Analysis, too name a few – invert the gravity and magnetics, and can be used to help determine the location, depth and nature of an anomaly source;
(b) Pseudolithology maps derived from the Poisson relation equating magnetics and calculated gravity gradients (see Dransfield et al. 1994) are particularly adapt at defining the distribution of different crustal types, and basement highs and lows. In the Gulf of Mexico, where disagreement surrounds defining the geographical extent of oceanic crust which, as a result, has resulted in numerous alternative tectonic models being proposed, this technique – pseudolithology mapping – has proved to be very useful, with the relict spreading axis being defined across the Abyssal Plain, for over 1200 km (see Jacques et al. 2004). On both sides of the spreading axes, we feel that we can convincingly support a model of counterclockwise rotational opening, with syn-rift generated volcanics symmetrically opposed at the continental-oceanic transitional boundaries;
(c) Plate tectonic modelling – A continental-scale analysis of potential field data. An essential requirement, before any plate tectonic modelling can take place, is to rigorously define continental block outlines and the distribution of different crustal types, so that the basic tectonic and structural framework can be defined and then used in the modelling process to create a pre-tectonic fit that will depict the ‘original’ configuration of the blocks. However, due to current software limitations, in which the progressive internal deformation of the continental blocks is not taken into account through time (as most models to-date can only perform rigid plate rotations), the resultant pre-tectonic fit for most conjugate margins are plagued with overlap and underlap problems. To create a more “geometrically pleasing” fit, two quick fixes are often used: (a) the removal of part(s) of the overlapping margin(s), thus creating the problem of removing potentially very useful data (e.g., well, magnetic and gravity) from the palaeotectonic reconstruction; and/or (b) changing the shape of the continental margin by creating offsets on a series of translatory structures, sub-dividing the plate into a number of smaller plates/blocks (microplates). Of course the latter approach is geologically more acceptable, but the position, continuity and kinematics of these crustal fractures must be evaluated and thoroughly tested. The ideal solution is to palinspastically restore the continental margins back through time and then focus on block movements along important intraplate structures in areas where there are still pre-tectonic fit problems. This technique is currently being investigated for the passive margins of the South Atlantic and Gulf of Mexico. If achievable, the non-rigid modelling technique will allow data (including gravity, magnetic and well) to be retained for key parts of the margin, of often significant economic importance.
The highs and lows of gravity data – SE Asia as an example
Traditionally, gravity data (Free Air, Bouguer and Isostatic Gravity anomaly fields) is used to define basin geometry. Combined with well and seismic data, this provides a very powerful technique for defining depth-to-basement, particularly, as an extrapolation method for areas of the basin with sparse or no well and/or seismic control. Magnetic data, on-the-other-hand, is often used to define the spatial extent and depth of basement structures. Collectively, the magnetics and gravity data provide an invaluable insight into the overall shape of the basin, and the identification of predominant basement fabrics, and more subtle structural trends, the type(s) of crust that floors the basin (i.e. continental, oceanic or transitional) and the extent of geological units such as carbonate, salt and igneous rocks.
Basin gravity responses
Offshore SE Asia is dominated by basins (rift systems) of all sizes that have been typically generated in a compositionally heterogeneous basement, often dominated by granitic intrusions and/or volcanic material, that is often expressed by gravity data (e.g., East Java Sea). Gravity responses of extensional basins vary according to the particular basin development stage, and will also be significantly affected by the presence of dense syn-rift volcanics (as demonstrated by the 2D model above).
Distinct gravity zones of the Sunda Shelf and East Java Sea
Typical major observed gravity responses for extensional sedimentary basins are:
- Regional gravity lows, reflecting low density sedimentary infill
- Regional less well developed/subdued gravity lows reflecting a significant positive gravity component arising from mantle uplift caused by crustal attenuation.
Therefore, understanding the evolutionary cycle of the basin is very important if we are going to understand the gravity response, and is often overlooked, because we are concerned with defining what we see present day.