GSST: Impacts on Hydrocarbon and Ore Exploration

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Distribution of proven (red) and unproven (blue) impact craters of the Earth (Reimold, 2014). Craters are clustered in several continental locations, and are showing good correlation with microplate and nanoplate boundaries, suggesting that at least one part of the craters is related to the escape of high-pressure mantle volatiles (Storetvedt, 1997).

After spending several years in the petroleum and mining industry it becomes obvious to anybody that structural geology is the α and Ω of exploration, because structural geology gives the platform where all the results of various analyses can be integrated. Besides local project details, structural geology has a large significance in predicting plate scale disposition of hydrocarbon and ore accumulations, delineating plays and structural trends. Structural geology is driving new venture acquisitions, exploration and forthcoming appraisal cycles, and even enhanced recovery projects.

Prospect generation is always based on preconceptions, and even object based geophysical processing is depending on structural modelling, thus building a robust regional and local structural model, without conflicts, mysteries and paradoxes is definitely important.

9.1. Hydrocarbon exploration

Global strike-slip tectonics (GSST) helps to better understand the cause of heat anomalies, explains the tectonic evolution of cool basins, like the Junggar, Transylvanian and Vienna basins. It gives a new approximation to the orogenesis process, and to the internal structuring of orogens.

The thermal history of basins is a basic input for source rock maturation studies; therefore it is critical to understand basin opening and later subsidence mechanisms. Heat transfer, for example it is more effective in convective systems, and a low stress (open) fracture network may provide effective conduits for fluid circulation.

Local stress field prediction is useful from many purposes. In order to identify active migration pathways, reservoir charging conditions we need to spot the low-stress regions, and in contrast, in case we are interested in the location of sealing faults, we need to search for high-stress zones. When working with tight reservoirs, identification of natural fracture zones not only provides sweet spot targets, i.e. higher hydrocarbon yields, but it is also reducing fracking costs and minimizing environmental risk. In addition, predicting the location of deep crustal faults helps with a better CO2 risk assessment.

There are several ways of characterizing, measuring local stress fields, but prediction for wildcut wells is more efficient approaching from a regional perspective. The nanoplate and pikoplate concepts can be very useful in delivering hydrocarbon play assessments.

9.2. Ore exploration

Channelways of primary dispersion and concentration of elements are indisputably provided by crustal faults and fractures, which are enabling the rising of mantle and lower crustal origin magmas or hydrothermal fluids to create magmatic and hydrothermal deposits. Therefore, it is not surprising that largest ore accumulations are related to strike-slip tectonics, after the so called impact craters, the origin of which seems disputable, and needs further research, however they are commonly related to cosmic origin.

Deep crustal faults are needed also to maintain the element budget of oceanic basins, and subsequently for charging marine sedimentary ore traps, like in the case of the Urgonian limestone hosted Zn-Pb deposit from Reocin, Basque Country (Velasco et al., 2003).

Delineation of second order structures related to crustal scale structures also involve structural geology techniques, because 1) breccia pipe deposits, for example, are related to fault intersections, which usually hold copper and uranium mineralization, or 2) anastomosing shear zones may host quartz–carbonate–sulphide veins and disseminated stockwork deposits (e.g. copper & gold in porphyry associated deposits), 3) ore plunge frequently might correlate with fold axis, and finally 4) extensional structures host order of magnitude variation in gold grade, for example, and therefore the evolution of paleo-stress fields is also important.

Fault systems are accumulating ore mineralization usually in low stress sites of brittle structures or cavities, in 1st, 2nd, 3rd order vein systems (Siddorn, 2010), which enable the circulation of hydrothermal fluids.

Presence of CO2 can induce immiscibility both within the magmatic volatile phase and in the hydrothermal systems, therefore CO2 may indirectly aid the process of metallogenesis by inducing phase separation (Lowenstern, 2001). Because orogenic gold deposits need CO2-rich H2S bearing low salinity fluids, mapping deep crustal CO2 conduits might help in prospecting gold.

Impact structures – further research direction

Considering that impact structures are holding the largest ore deposits on the Earth, it worth to take a glance at their origin, particularly to their disposition against microplate boundaries.

Apart from the widely known and largest impact craters Chicxulub (Mexico), Sudbury (Canada) and Vredefort (South Africa), there were identified dozens of impact craters across the world (Fig. 34), Reimold has identified 49 sites in Africa alone. From the 49 impact crater sites proposed in Africa, 28 already have been proved not to be of meteorite impact origin (Reimold and Koeberl, 2014); however they are concentrating in the same geographic area, e.g. South Africa.

An alternative explanation to the origin of these crater morphologies is provided by Storetvedt, who argues that the Chicxulub crater in Mexico (off Yucatan Peninsula) is a major blow-out crater of high-pressure mantle volatiles (Storetvedt, 1997), that kicked up billions of tonnes of sulfur (Ainsworth, 1994). The Chicxulub impact predates the K–T boundary by about 300 kyr (Keller et al., 2007).

Seismic images of Moho at Chicxulub are showing an upwarped mantle (Christeson et al., 2009), which come in the support of the blow-out model.

Similar structures in Sudbury, Canada also interpreted as cosmic impact product, are showing more than one generation breccia (Grieve et al., 2008), suggesting a repetitive event, which is indeed possible by mantle volatile activity. This crater is situated north of the Huron Lake, and shows an elongated crater, with pre-impact magnetic dykes, in radial disposition.

The associated breccia of Vredefort crater, South Africa is showing post impact deformation (Grieve et al., 2008).

In conclusion, considering the various pre-impact, post-impact features, mantle upwarp below the crater, the presence of high sulphur volumes, and the very high correlation with major crustal fragment boundaries, we find that the impact crater mystery might be solved with further research on GSST microplate/ nanoplate boundaries.

Published in: Kovács, J.Sz., 2015 (in press), Elements of Global Strike-Slip Tectonics: a Quasi-Neotectonic Analysis, Journal of Global Strike-Slip Tectonics, v1., Szekler Academic Press, Sfintu Gheorghe.