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  • Neil Pendock

Thermal remote sensing for mineral and hydrocarbon exploration

The news late last year that Mineral Resources Minister Gwede Mantashe plans to spend R20-billion over 10 years on an integrated multi-disciplinary mapping programme to boost global investment in local exploration has generated a fair amount of heat in local exploration circles. Whether this will divert attention from issues in the mining charter, the more likely reason SA attracts less than 1% of global exploration spend, remains to be seen. But before Tito Mboweni pulls out the national cheque book and mobilizes the drill rigs, it’s worth considering what can be achieved with free data. 


Such as using signal-to-noise ratio measurements recorded by Global Navigation Satellite Systems to map soil moisture for hard pressed farmers [1]. Or satellite synthetic aperture radar to monitor leaks in municipal water reticulation systems [2]. Or using satellite thermal imagery to explore for minerals under vegetation or transported cover [3]. In the case of hydrocarbons, even deposits buried hundreds of metres beneath the surface are detectable.


Free imagery for thermal mapping is collected by the Japanese Aster satellite. It was launched in December 1999 and is still operating well beyond design specifications. It is a wonderful piece of engineering. Thermal reflectance is measured in five wavebands at 90 m spatial resolution. There is an online database of many millions of images, so there are cloudfree scenes galore over most areas with the exception of Guyana and Jamaica.

So how can a satellite orbiting at 705 km above the earth detect buried hydrocarbons? There are four main factors.


1) A proxy for gravity


A decade ago, the author spent several months in Carajas in the heart of the Amazon jungle, processing airborne gravity gradient data for that Brazilian mining company Anglo American very nearly bought. The idea was that compact hematite would have a gravity response so different to other rocks that changes in the gravity field could be measured from an aircraft bouncing around in the thermals above trees many tens of metres tall. Of course aircraft turbulence is a gravity acceleration several orders of magnitude larger than rock density responses, so perhaps a better mousetrap would have been to measure temperature differences over the jungle.


At night, dense rocks would cool down more slowly than lighter ones while during the day, they would heat up more slowly. So measuring thermal inertia by subtracting daytime and nighttime temperature measurements sounds like a plan. Aster collects images by day and night so by fitting a black body to each 90 m pixel, high resolution thermal inertia images are possible. Which may be of interest to explorers in the Northern Cape. For renewable resources, it can also map aquifers in West Australia as you will find out if you download a pair of Aster images over Kalgoorlie. Oilmen use gravity to find a likely source of hydrocarbons, called the kitchen. And as they say in the movies, if you can’t stand the heat, get out of the kitchen.


2) Mineral alteration


The rocks above a hydrocarbon deposit may be altered by a so called “chimney effect”. No natural reservoir has a perfect seal and migration of hydrocarbons towards the surface can lead to the development of oxidation-reduction zones and mineralogic changes such as the formation of calcite, pyrite, uranium, elemental sulphur, and magnetic iron oxides and sulphides as well as the alteration of clays. This alteration typically forms mineral halos which can be mapped using the five bands of Aster emittance data which can map particular minerals present, even beneath moderate vegetation and transported cover.


Multispectral thermal imagery can be used to map suitable lithologies for hydrocarbon exploration. Such as the Otavi Group carbonates in the Owambo basin of northern Namibia and southern Angola. The closest match to the observed Aster thermal signature in a library of mineral spectra measured in a laboratory is aragonite and the spatial correlation of these carbonate abundances to a soil gas map from 1993 are very convincing [4].


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Otavi Group in blue, squares, triangles and circles denote hydrocarbons


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Seismic surveys are the dotted black lines


3) Direct detection of methane seeps


Fugitive methane emissions can be directly detected as methane gas has distinctive absorption features which can be detected by an airborne or spaceborne thermal camera. Using a thermal spectrum of methane measured in a laboratory, the Hwange coalfield in Zimbabwe is neatly mapped. There is obvious application to the coalfields of the highveld for mapping coalbed methane potential.


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Aster visible image of Hwange with methane on a temperature scale


4) Oil as a thermal insulator


American oilman Lloyd Fons was awarded a patent in June 1996 that relied on the physical observation that a charged reservoir acts as a thermal insulator to heat flow from the mantle and thus temperature differences are a hydrocarbon target generator. 

As the patent records “the earth's surface temperature at selected points within a location is compared to the earth's surface temperature at a plurality of points in the surrounding geographic area to determine locations having anomalously low surface temperatures which are indicative of the presence of oil or gas deposits. Representative earth surface temperatures for all locations under consideration and reference temperatures in the geographic area are obtained, under similar ambient conditions, at points having similar topography, vegetative cover and surface features for minimizing extraneous factors which affect earth surface temperature, such that the earth surface temperatures may all be compared with each other to determine locations which have low earth surface temperatures, and therefore are more likely to have oil or gas deposits beneath them.”


Comparison of a daytime and nighttime Aster temperature image over the Kern River oil field in the San Joaquin Valley, California, confirms a temperature difference of 1℃ is measurable. 


Somalia is an underexplored oil play


Coriole-Afgoye Basin, Somalia 


As an example of using satellite thermal imagery to prospect for oil, we consider the Coriole-Afgoye Basin, the closest hydrocarbon deposit to Mogadishu. Somalia is very prospective for hydrocarbons but is relatively underexplored on account of the turbulent political situation over the past several decades. Italy, the former colonial power, conducted extensive exploration programs in the 1950s and 60s and nearly 70 wells have been drilled over the past six decades.


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Some of the oil wells drilled in Somalia


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Focusing on particular wells confirms the Fons local temperature minimum as well as the presence of methane gas.


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Two wells are temperature lows and methane emission maxima


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Methane still seeping out of wells drilled in the 1960s


So why are we not compiling a thermal database for the Karoo?


Certainly the dolerites have a distinctive thermal signature which can be mapped [3] and potential oil bearing shales can be inferred too. But thermal battles to get traction as a serious hydrocarbon exploration tool and once again, four factors can be identified.


1) Seismic envy

Seismic surveys have been very successful in identifying hydrocarbon deposits and other techniques struggle to compete. Of course the suggestion is not to replace seismics but to rather focus expensive seismic surveys on more attractive lithologies. Besides Aster does not scare the sheep.


2) Too cheap

At around US$1/Km2, thermal interpretations are simply too cheap to the exploration mindset that you get what you pay for. Ambitious exploration managers enjoy the corporate power big budgets bring and trips to Schlumberger in Paris to interpret the data are always welcome.


3) Not taught at school

Yet. While successful thermal exploration case studies are rarely published. In fact our largest client in Australia prevents us confirming they use thermal remote sensing for exploration.


4) Remote sensing has been oversold

As any mining company who drilled shortwave infrared crosstalk anomalies (a nasty noise problem in early Aster imagery) in Mongolia will confirm, remote sensing is not a magic bullet.


But perhaps the question exploration managers should ask themselves is if thermal maturity is essential for hydrocarbon formation, surely thermal imaging makes sense for exploration? Ditto those epithermal gold deposits and kimberlites which made a fiery entrance from the mantle some time ago. Or those millions of tons of copper oxidizing quietly in the DRC, Zambia and the Northern Cape.


References

[1] Nicolas Roussel, Frédéric Frapart, Guillaume Ramillien, José Darrozes, Frédéric Baup and Vincent Bustillo (2014). Detection of changes in soil moisture content using GNSS SNR signals. https://www.researchgate.net/publication/263163813_Detection_of_changes_in_soil_moisture_content_using_GNSS_SNR_signals

[2] 2016 Could satellites be the secret to deecting water leaks? https://esriaustralia.com.au/esri-australia-blog/could-satellites-be-the-secret-to-detecting-water-leaks-blg-37

[3] Neil Pendock. Hot spots for G-spots, Proceedings of the International Geological Congress, Cape Town, September 2016.

[4] Thomas E. Hoak, Alan L. Klawitter, Charles F. Dommer, and Pasquale V. Scaturro, Integrated Exploration of the Owambo Basin, Onshore Namibia: Hydrocarbon Exploration and Implications for a Modern Frontier Basin. http://www.searchanddiscovery.com/documents/2014/10609hoak/ndx_hoak.pdf

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