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Natural Irradiation – The New Approach to Hydrocarbon Source Rock Evaluation

George Mirkin, Quanto International Co., Ltd., Toronto, ON M4V 2T2, Canada

Hydrocarbon source rock evaluation is a very important tool for hydrocarbon exploration and production development. Despite methodical differences of present methods, estimation of the kerogen maturity is based on thermodynamic principles. Practically, only geothermal heat is taken into accounts.

At least three major types of evaluation methods are applied:

Methods based on the Arrhenius equation (Tisso’ et al. method);

Methods based on determination of paleoheat flux (Lopatin’ method);

Methods based on lab paralysis of rock formation samples.

Historical application of these methods for the same objects quite often provided significantly different results. Methodological problems are also aggravated by reconstruction uncertainness of many applied geological parameters such paleodepth, paleotemperature, and others.

It is known that many hydrocarbon source rocks have high levels of natural radioactivity due to their enrichment by radioactive elements such as Uranium. Thus, radioactive decay is also an important cause of kerogen chemical alterations. For example, highly radioactive Upper Jurassic Bazhenov shales are the recognized as major hydrocarbon source rock in Western Siberia (Russia), particularly for such giant oil fields as Samotlor, Mamontovsk, Salym, Krasnoleninsk and others.

Bazhenov shale presently contains 23.5 g/ton of Uranium (98% of the initial content). Our quantitative evaluation showed that energy produced by decayed Uranium disseminated within the formation is more than sufficient for a high degree maturation of the formation kerogen.

Taking into account organic rich rock radioactivity is also a base of paleo organic geochemistry reconstruction that may be quite different from interpretations based on thermodynamic principles. The formation mechanism of active hydrocarbon particles (ions and radicals) from kerogen under irradiation is very different from the formation mechanism due to heating. Results of irradiation, in contrast to geothermal heating, are not dependant directly on formation temperature but on irradiation dose rate and timing. Therefore, one can expect a different quantitative and qualitative hydrocarbon output from a source rock. However, because natural clay catalysts such as montmorillonite and smectite play an important role in ion and radical accumulation, a source rock geothermal and irradiation heating has to also be taken into account to evaluate chain reactions of accumulated active particles resulting hydrocarbons.

AAPG Search and Discovery Article #90059©2006 AAPG Eastern Section Meeting, Buffalo, New York