AAPG Annual Convention and Exhibition

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Gas-Source Correlation in Petroleum Systems

Abstract

The definition of a multi-source petroleum system requires techniques that allow the identification of the source rock of an oil. Because oils inherit the molecular and isotopic character, the geochemical DNA, of the source, modern isotope and biomarker analyses on the molecular level allow readily the identification and separation of multiple sources of oils in a petroleum system. In contrast to oils, natural gases do not inherit their molecular or isotopic properties from their parent sources kerogen and/or oil. The bond-breaking processes that produce C1 to C4 hydrocarbon gases involve temperature-dependent isotope fractionations that vary with i) number of carbons in the molecule ii) temperature and iii) the type of precursor (type of kerogen or oil). Therefore, an identification of the source of a gas requires a reconstruction and correction of the fractionations that occur during gas formation. This can only be accomplished with a simulation of the gas forming processes in laboratory experiments. We found in many projects that each gas source rock can be characterized by a specific correlation of gas isotopes that, in turn, allows the identification of those gases that only fit to a specific source rock. We will present results from two major North European Petroleum Systems:

  1. 1)Haltenbanken/Norwegian Sea: We conducted analyses on several facies of the Upper Jurassic Spekk shale and various facies of the Lower Jurassic Åre Coal as well as oil cracking experiments of a Norne oil. We find that the gases in the Haltenbanken area are predominantly Åre coal gases. The Speck shale as the principal source of the oils is only a minor contributor to the gases.
  2. 2)Barents Sea: A comparison of 41 Barents Sea gases from 13 Blocks with 4 source rock isotope calibrations suggests that the post-mature deep Permian is the principal source of the gases and not the oil source rocks (M. Triassic, U. Jurassic Hekkingen). These results have fundamental implications for both petroleum systems: Because oil and gas are of different origins, GOR and API gravities are not controlled by the maturation of the oil-producing kerogens, rather migration of gases from the deeper gas source rocks control API and GOR.