ScienceWeek

EARTH SCIENCE: ON HYDROCARBON RESERVOIRS

The following points are made by N. White et al (Nature 2003 426:334):

1) Whatever the controversies surrounding our dependency upon fossil fuels, one issue is very clear. There is a finite amount of hydrocarbon left to discover. On the basis of current reserves, liquid hydrocarbon production will peak at 30 billion barrels per year in 15-20 years (1 barrel contains 0.16 m^(3) oil), declining to 5-10 billion barrels per year by the end of the 21st century(1,2). At present, the hydrocarbon industry depends for its economic survival upon its ability to locate and extract hydrocarbons. During the first half of the twentieth century, sites of hydrocarbon production were predominantly located onshore. In the late twentieth century, exploration moved offshore, where drilling costs and exploration risks are potentially much higher. The bulk of major hydrocarbon fields located in shallow-water depths (that is, up to 200 m) have probably been located, if one excludes fields at depths greater than 5 km and the unexplored margins of Antarctica and the Arctic Ocean.

2) The hydrocarbon industry is now pursuing two different strategies. The first is to efficiently extract greater amounts of hydrocarbons from existing reserves, from which typically only 30-40% of oil is recovered. This strategy is an obvious one because modest efficiencies in a region with substantial undeveloped reserves will surpass the fruits of exploration elsewhere. The major technical problem concerns the connectivity of reservoir rocks. Its solution relies on a combination of sophisticated engineering (for example, drilling deviated wells to target hydrocarbon "sweet-spots") and time-lapse imaging (that is, repeated seismic monitoring of producing fields where changes in acoustic response enable the movements of subsurface fluids to be tracked). In essence, it is an intimate combination of geophysics, reservoir engineering and economic practicality.

3) The second strategy is to explore virgin areas, an inherently risky and expensive enterprise. On Earth, the largest piece of unexplored continent consists of submerged continental margins that form a "fringe" around the deep ocean basins. Over the past ten years, there has been a relentless drive to explore ever-increasing water depths. This drive has been stimulated by an engineering technology that has allowed us to drill, in water depths of nearly 3 km, 20 cm diameter holes into rocks that lie 4 km beneath the seabed. From a strictly economic perspective, this strategy is irrational: a single deep-water exploration well costs up to US$50 million, which is about two orders of magnitude greater than a typical onshore well in the Middle East. However, political and economic realities conspire to make deep-water exploration a commercially viable proposition.

4) In summary: Areas of exploration for new hydrocarbons are changing as the hydrocarbon industry seeks new resources for economic and political reasons. Attention has turned from easily accessible onshore regions such as the Middle East to offshore continental shelves. Over the past ten years, there has been a marked shift towards deep-water continental margins (500 to 2500 m below sea level). In these more hostile regions, the risk and cost of exploration is higher, but the prize is potentially enormous. The key to these endeavors is a quantitative understanding of the structure and evolution of the thinned crust and lithosphere that underlie these margins.(3-5)

References (abridged):

1. C. Kuykendall. M. K. Hubbert and his Heirs: A Hubbert Peak Half-Biography at http://www.oilcrisis.com/library/hubheir.pdf (2003)

2. The Association for the Study of Peak Oil & Gas http://www.peakoil.net

3. Parrish, J. T. Palaeo-upwelling and the Distribution of Organic-Rich Rocks. 199-205 (Spec. Publ. 26, Geol. Soc. Lond., 1987)

4. Milliman, J. D. & Syvitski, J. P. M. Geomorphic tectonic control of sediment discharge to the oceanó the importance of small mountainous rivers. J. Geol. 100, 525-544 (1992)

5. Kuenen, P. H. & Migliorini, C. Tubidity currents as a cause of graded bedding. J. Geol. 58, 91-127 (1950)

Nature http://www.nature.com/nature

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EARTH SCIENCE: ON PETROLEUM DEPOSITS

The following points are made by Jeffrey S. Seewald (Nature 2003 426:327):

1) Petroleum consists of liquid (oil) and volatile (natural gas) organic alteration products, and is generated during the conversion of metastable macromolecular kerogen to thermodynamically favored lower molecular mass compounds. The chemical reactions responsible for this transformation occur in response to the removal of kinetic barriers as temperatures increase with progressive burial in sedimentary basins.

2) Formation of an economic petroleum deposit requires, in addition to suitable source rocks containing sufficient organic matter, a sequence of geological events that leads to the expulsion, migration and trapping of generated hydrocarbons. Despite great success in finding and producing petroleum reserves during the past century, there is still extensive debate regarding the chemical and physical processes that result in the generation and accumulation of oil and natural gas.

3) Traditional models for the formation of oil and natural gas typically involve a catagenic process dominated by thermal cracking reactions that release low-molecular-mass hydrocarbon fragments (1,2). These reactions are viewed as unidirectional, kinetically controlled processes that are influenced solely by time, temperature and the composition and structural characteristics of the source kerogen. In recent years, however, geochemists have increasingly recognized that subsurface chemical environments play an active part in the formation and compositional evolution of petroleum. In particular, inorganic compounds such as water and minerals may participate as reactants or catalysts during organic matter maturation. Moreover, organic inorganic interactions in sedimentary basins have direct implications for petroleum migration and trapping, because many organic alteration products participate in processes that create or destroy sediment porosity and permeability.

4) A variety of field, laboratory and theoretical approaches have been adopted in studies that identify a broad spectrum of interactions involving water, minerals and hydrocarbons during the formation of a petroleum deposit. Many of these studies are controversial, and few have gained general acceptance. Nonetheless, they represent new paradigms that may substantially enhance our ability to predict the occurrence of petroleum.

5) In summary: Petroleum deposits form as a consequence of the increased temperatures that accompany progressive burial of organic matter deep within sedimentary basins. Recent advances in petroleum geochemistry suggest that inorganic sedimentary components participate in organic transformations associated with this process. Water is particularly important because it facilitates reaction mechanisms not available in dry environments, and may contribute hydrogen and oxygen for the formation of hydrocarbons and oxygenated alteration products. These findings suggest that petroleum generation and stability is influenced by subsurface chemical environments, and is a simple function of time, temperature and the composition of sedimentary organic matter.(3-5)

References (abridged):

1. Tissot, B. P. & Welte, D. H. Petroleum Formation and Occurrence (Springer, New York, 1984)

2. Hunt, J. M. Petroleum Geochemistry and Geology (W.H. Freeman, San Francisco, 1996)

3. Miknis, F. P., Turner, T. F., Berdan, G. L. & Conan, P. J. Formation of soluble products from thermal decomposition of Colorado and Kentucky oil shales. Energy Fuels 1, 477-483 (1987)

4. Lewan, M. D. Experiments on the role of water in petroleum formation. Geochim. Cosmochim. Acta 61, 3691-3723 (1997)

5. Ziegel, E. R. & Gorman, J. W. Kinetic modeling with mutltiresponse data. Technometrics 22, 139-151 (1980)

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