Supergiant hydrocarbon field just leaked away

The largest producing hydrocarbon field, which is unlikely to be bettered, is the Gharwar oil field of Saudi Arabia.  It extends for about 3500 km2 and still contains 80 billion barrels of oil.  Anything comparable in size, or bigger, would have been tripped over decades ago, because of the sheer size of the geological trap structures.  That is one of the reasons to believe that hydrocarbon resources are unlikely to last until the 22nd century, unless other kinds of accumulation can be exploited economically.  There are vast onshore reserves of tar sands from which the more volatile hydrocarbons have leaked away, but for them to become generally useable requires very large rises in oil price.  The same conditions will have to prevail before oil shales, the source rocks for conventional hydrocarbons, become viable..  Had tectonics not induced the Colorado Plateau to rise and be eroded, oil would be far cheaper and more secure, and the USA would have even more economic and political clout than it already has.  The recognition of unroofed hydrocarbon fields in that region of western North America may therefore come as a relief to many people (Beitler, B., Chan, M.A. & Parry, W.T. 2003.  Bleaching of Jurassic Navajo Sandstone on Colorado Plateau Laramide highs: Evidence of exhumed hydrocarbon supergiants.  Geology, v. 31, p. 1041-1044).

The desert dune sandstones of the North American Jurassic form some of the world’s most spectacular scenery, because of their vast outcrops in Utah national parks, such as Monument Valley.  Their attraction lies in the colours of the sandstones as well their deep incision.  Discovery of what was once a series of supergiant hydrocarbon fields lies in variations of that coloration.  When laid down, the sandstones were reddened by precipitation of ferric (Fe3+) oxides from water that seeped through them during diagenesis under oxidising conditions.  However, large tracts now show signs of variable bleaching, which gives the variegation that tourists flock to see.  Iron has been removed in places, and for that to happen, the insoluble Fe3+ has been reduced to the more soluble Fe2+, or ferrous form.  That can occur when conditions in the rock change to highly reducing, as in the case of hydrocarbons migrating in along with water.  Most wind-blown sands have good porosity and their uniform grain size induces excellent permeability as well, so they are near-ideal reservoirs.  However, for them to become permeated by hydrocarbons that migrated from source rocks (usually shales) requires pathways and structures in which the hydrocarbons can be trapped.  The Jurassic of the western USA has alternations of these sandstones with less permeable rocks, and was deformed into huge open anticlines during the Laramide orogeny, that originally might have created such traps on a regional scale.  Brenda Beitler and her colleagues from the University of Utah have mapped the zones of bleaching using Landsat-7 Enhanced Thematic Mapper data.  Sure enough, the most bleached areas coincide with the crests of the large upfolds, and with reverse faults that link them to basins with source rocks and may have acted as fluid migration pathways.  The pore volume that could have been available for hydrocarbon trapping would have been 2200 km3, equivalent to 18.5 trillion barrels, about 6 times larger than estimates of the modern world’s recoverable oil.  Since the Cretaceous, the Colorado Plateau has undergone more than 2 km of uplift and every single upfold has been breached and deeply incised.  Sorry George, the oil leaked out long ago!  The inevitable leakage of the gas fraction, perhaps as much as 2 billion tonnes, could have warmed the Tertiary climate, if a significant fraction were released quickly.  The main incision of the Colorado Plateau was probably in the late Miocene (around 6 Ma), when ocean-floor data suggest global warming of the order of 0.5 to 1ºC.

Background to globalisation of water resources

“The second provision of any civilised society after a system of laws, is that of a safe water supply” is anonymously attributed in the repeated warnings about the parlous state of water provision for about two thirds of the world’s population.  Many of the private companies that took over the public water authorities in Britain now stride the planet organising that provision.  In South Africa, the resulting increases in water pricing are the main source of anger throughout the poorer sections of its population, especially in the townships.  In Cochabamba, Bolivia there have been mass protests about similar price hikes that came years ahead of any improvement in supplies.  A consortium of national and transnational companies needed the extra cash to finance a major dam project, instead of looking to global investors in the project.  Science carried a lengthy article that provides a context for this new trend in globalisation (Gleick, P.H. 2003.  Global freshwater resources: soft-path solutions for the 21st century.  Science, v. 302, p. 1524-1528)

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s