Cost-effective screening of frontier acreage is an essential early part of any deepwater exploration strategy at continental margins.
Deepwater Exploration: The Importance of Crustal Structure
Screening of deepwater frontier acreage requires an understanding of crustal structure and crustal type within the ocean-continent transition of the associated rifted margin, with a view to addressing the question of whether the basement is likely to support a viable petroleum system within the overlying sedimentary basins.
Figure 1: Schematic outline of the OCTek gravity inversion method to determine Moho depth, crustal-basement thickness and lithosphere thinning factor, using gravity anomaly inversion incorporating a lithosphere thermal correction and decompression-melt prediction. Adapted from Roberts et al (2013) and Kusznir et al (2018).
Gravity Inversion
The gravity inversion method described by Alvey et al. (2008), Chappell and Kusznir (2008), and Kusznir et al. (2018) can be used to map and characterize the regional crustal structure of many of the world’s continental margins and their associated sedimentary basins. Key to this method are two corrections that must be made in order to account for the highly attenuated nature of the continental crust in deepwater basins and rifted margins. Firstly, an adjustment is made for the lithosphere thermal-gravity anomaly associated with the elevated geotherm produced by rifting and breakup. A prediction of magmatic addition to the crust at high stretching factors can then be made, following the decompression melting model (White and McKenzie, 1989).
These corrections are required in order to produce reliable estimates of the thickness of hot, thin crust at continental margins. Consultants Badley Geoscience, in collaboration with the University of Liverpool, have developed this method, known as OCTek, to characterize whether crust is continental, oceanic or transitional, and to quantify both total crustal thickness and radiogenic continental crustal thickness, along with the lithosphere stretching/thinning factor, across most of the world’s prospective margins.
Global Crustal Basement Thickness Map
Crustal thickness mapping has now been expanded to provide global coverage in a single map (Figure 2), extending the mapping onshore using only public domain data. Satellite free-air gravity data are freely available in the public domain with global coverage (Sandwell & Smith 2009, and subsequent updates), making global application of the OCTek method tractable. Alongside the free-air gravity data the method also requires (i) bathymetry/topography data, globally available from Smith & Sandwell (1997, and subsequent updates) and (ii) sediment thickness information. For the latter Divins (2003, and subsequent updates) offshore and Laske & Masters (1997, and subsequent updates) onshore were the primary sources.
Figure 2: Global map of total crustal basement thickness (continental and oceanic) from gravity inversion, overlain by a display of the shaded relief free-air gravity anomaly which enhances the underlying tectonic features (Robinson projection). Source: Badley Geoscience Ltd.
The use of public domain data is both a strength and weakness; its availability allows the global mapping to be performed, but its resolution and accuracy is an accepted compromise because of the scale of coverage. For this reason, local application of the method using optimized or proprietary sediment thickness data will always provide more accurate results than the global data.
Notwithstanding this, it is believed that the first-order results of the global mapping, in terms of identifying areas of thick or thin crust and the transition in between, will be correct in both onshore and offshore areas, although for the former, in particular, the scale (in kilometers) should be taken as a guide rather than an absolute value.
The global map resolves the majority of known oceanic areas with a crustal thickness of about 5–7 km (i.e. normal thickness oceanic crust). Exceptions are known volcanic plateaus such as Iceland, Kerguelen and Ontong-Java, hotspot trails like those in the Pacific and the Walvis Ridge in the Atlantic, and microcontinental blocks including Jan Mayen, the Seychelles-Mascarenes and several others.
Onshore crustal thickness has a ‘typical’ value of ~37.5 km, with thicker crust delineating the major orogenic belts and the thick cratonic crust of Africa. Thinner onshore crust typically delineates intra-continental rift basins, many of which also extend offshore and link into the thinned crust of continental margins.
Continental Lithosphere Thinning Factor
Figure 3 illustrates another outcome of the OCTek global mapping: the calculation of continental lithosphere thinning factor (= 1–1/β where β is the more traditionally used stretching factor). A thinning factor value of 0 represents no thinning, or even crustal thickening in the case of the orogenic belts, while 1 denotes complete thinning and removal of the continental crust and lithosphere and its replacement by oceanic crust and lithosphere, or in some localized cases by exhumed mantle. This map was produced by taking the total crustal thickness map of Figure 2, subtracting the predicted magmatic addition by decompression melting (including 7 km of oceanic crust in oceanic areas) and referencing the remaining thickness of continental crust to an initial thickness of 37.5 km.
Figure 3: Global map of continental-lithosphere thinning factor overlain by a display of the shaded relief free-air gravity anomaly to enhance underlying tectonic features. A thinning factor of 0 represents no thinning, in comparison to 1 which signifies complete thinning and removal of the continental crust and lithosphere (Robinson projection). Source: Badley Geoscience Ltd.
On a global scale the known oceanic areas ‘clean up’ nicely with a thinning factor of 1 (white). The anomalous oceanic areas of volcanic plateaus, hotspot trails and microcontinents all stand out with a lower thinning factor. Note that this map not only identifies the main ocean basins, it also recognizes smaller (often older and land-locked) ocean basins such as the Gulf of Mexico, Baffin Bay, the Mediterranean and the Black Sea.
The stretched and thinned continental margins, which are so important for frontier exploration, are delineated by the (colored) narrow zones of rapid thinning between unthinned continental crust onshore and oceanic crust offshore.
Within the onshore areas intra-continental rifts typically show up with a thinning factor of ~0.2–0.3 (e.g. the Amazon, Newark (USA), West Siberia and the North African Basins). Intra-continental rifts extending offshore may have a higher axial thinning factor of ~0.5 (e.g. the North Sea, Kara Sea and Canning (Australia) Basins).
Understanding the thinning/stretching factor within rifts and across margins is important, because this helps us quantify the transient thermal anomaly which results from lithosphere thinning, which in turn is a key input to any petroleum systems model in these basins.
Predicting Top-Basement Heat-Flow History
The use of these gravity inversion results as input to petroleum systems models can be extended still further by using them to make predictions of top-basement heat-flow history (Cowie and Kusznir, 2012; Roberts et al., 2018).
Figure 4: (a) Map of total crustal basement thickness (continental and oceanic) for the offshore Arctic, overlain by shaded relief free-air gravity anomaly; (b) Corresponding map of continental lithosphere thinning factor for the offshore Arctic, with oceanic areas picked out by a thinning factor of 1; (c) Three maps of present-day top-basement heat-flow for the offshore Arctic, derived from the results of the gravity inversion (without including the effects of sediment blanketing). Each is conditioned by a different value of initial crustal radiogenic heat-productivity: respectively 15, 30 (global average) and 45 mW/m2. Source: Badley Geoscience Ltd.
Top-basement heat-flow within a rift basin or margin results primarily from a combination of three heat sources: (i) radiogenic heat input from the continental basement, which following rifting is thinned from its initial pre-rift thickness and can be quantified from a map of crustal basement thickness; (ii) heat input from the transient elevated geotherm resulting from lithosphere stretching and thinning, which can be quantified from a map of continental-lithosphere thinning factor; and (iii) long term, steady-state heat loss from the mantle (~30mW/m2). Figure 4 shows the application of this technique to the offshore Arctic.
The principal uncertainty in the prediction of heat-flow is calibrating how radioactive the basement is. The global average value for radiogenic heat input from unstretched continental basement is ~30 mW/m2. The three examples illustrated in Figure 4c capture this value as the central case, with an uncertainty (in the absence of direct calibration) of +/- 50% on either side.
On all three heat-flow maps the active Atlantic/Eurasia spreading center stands out as a band of high heat-flow, while the old oceanic crust of the Canada Basin is cold. Top-basement heat-flow within the areas of thinned continental crust is seen to be dependent on the value of initial radiogenic heat used, reinforcing the importance of understanding the uncertainty within this parameter if no calibration is available.
The maps in Figure 4c all show a possible present-day case. A full heat-flow history, back through time to rifting/breakup, can also be produced using the same methodology (Roberts et al. 2018).
Understanding Crustal Structure
We hope this article has shown how a first-order understanding of crustal structure at a regional and global scale can be produced by using data which are freely available in the public domain. Quantified crustal structure, thinning factor and heat-flow history are all important contributions to any screening study of offshore frontier acreage, particularly in deep water. Maps of crustal thickness and thinning factor can also be used to provide constraints on plate reconstructions (Alvey et al., 2008; Kusznir et al., 2018).
The results presented here all contain some element of compromise as a consequence of the scale at which the mapping has been performed. The outcomes can, however, be refined and improved by focusing on smaller areas with bespoke parameterization of the gravity inversion and with the use of improved local sediment thickness information, where available.
References
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Chappell, A.R. & Kusznir, N.J. 2008. Three-dimensional gravity inversion for Moho depth at rifted continental margins incorporating a lithosphere thermal gravity anomaly correction. Geophysical Journal International, 174, 1–13.
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