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.
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.
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.
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).
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.
Alvey, A., Gaina, C., Kusznir, N.J. & Torsvik, T.H. 2008. Integrated crustal thickness mapping and plate reconstructions for the high Arctic. Earth and Planetary Science Letters, 274, 310-321, doi:10.1016/j.epsl.2008.07.036.
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.
Cowie, L. & Kusznir, N.J. 2012b. Mapping crustal thickness and oceanic lithosphere distribution in the Eastern Mediterranean using gravity inversion. Petroleum Geoscience, 18, 373–380, doi:10.1144/petgeo2011-071.
Divins, D.L., 2003. Total Sediment Thickness of the World's Oceans & Marginal Seas, NOAA National Geophysical Data Center, Boulder, CO. updates http://www.ngdc.noaa.gov/mgg/sedthick/sedthick.html
Kusznir, N.J., Roberts, A.M. & Alvey, A.D. 2018. Crustal structure of the conjugate Equatorial Atlantic Margins, derived by gravity anomaly inversion. In: McClay, K.R. (ed.) Passive Margins: Tectonics, Sedimentation and Magmatism, Geological Society, London, Special Publications, 476.
Laske, G. & Masters, G. 1997. A Global Digital Map of Sediment Thickness, EOS Trans, AGU, 78, F483. updates http://igppweb.ucsd.edu/~gabi/sediment.html
Roberts, A.M., Kusznir, N.J., Corfield, R.I., Thompson, M. & Woodfine, R. 2013. Integrated tectonic basin modelling as an aid to understanding deep-water rifted continental margin structure and location, Petroleum Geoscience, 19, 65-88, doi:10.114/petgeo2011-046.
Roberts, A.M., Alvey, A.D. & Kusznir, N.J. 2018. Crustal structure and heat-flow history in the UK Rockall Basin, derived from backstripping and gravity-inversion analysis, Petroleum Geoscience (https://doi.org/10.1144/petgeo2017-063)
Sandwell, D. T. & Smith, W.H.F. 2009 Global marine gravity from retracked Geosat and ERS-1 altimetry: Ridge Segmentation versus spreading rate, Journal of Geophysical Research, 114, B01411, doi:10.1029/2008JB006008. updates http://topex.ucsd.edu/WWW_html/mar_grav.html
Smith, W. H. F. & Sandwell, D.T. 1997. Global seafloor topography from satellite altimetry and ship depth soundings, Science, 277, 1957-196. updates http://topex.ucsd.edu/marine_topo/
White, R.S. & McKenzie, D.P. 1989. Magmatism at rift zones: the generation of volcanic continental margins and flood basalts, Journal of Geophysical Research, 94, 7685–7729.