Carbon isotopic ratios of volatile hydrocarbon fractions of marine oils are diagnostic of organic facies and depositional environments of source rocks. For carbonate oils, low-molecular-weight volatile hydrocarbons (< C9) are isotopically lighter than high-molecular-weight volatile hydrocarbons (C9-C17). In contrast, for deltaic oils, low-molecular-weight volatile hydrocarbons are isotopically heavier than high-molecular-weight volatile hydrocarbons. Marine shale oils show patterns intermediate between carbonate and deltaic oils. This relative variation of carbon isotopic ratios among volatile hydrocarbons of oils is explained by earlier expulsion of marine oils derived from isotopically homogeneous (algal-bacterial) kerogens in rich so rce rocks, and secondary cracking of petroleum prior to expulsion for marine oils derived from isotopically heterogeneous (terrestrial) kerogens in lean source rocks. In basins with multiple source rocks, carbon isotopic ratios of volatile hydrocarbons are useful for determining oil-oil correlation and for inferring oil-source rock relationship.
A histogram of (isotope){13}C values of 621 post-Ordovician marine oils shows a trimodal distribution. Within this distribution, four groups of oils can be recognized on the basis of (isotope){13}C values, in conjunction with pristane/phytane ratios and sulfur contents. Oils with (isotope){13}C values of -32.0 to -28.0 o/oo are mainly marine shale oils older than Oligocene; oils with (isotope){13}C values of -28.0 to -23.5 o/oo are mainly deltaic oils of varying geologic age or Mesozoic carbonate oils; and oils with (isotope){13}C values heavier than -23.5 o/oo are mainly marine shale oils of Miocene age. The (isotope){13}C variation among oil groups is explained primarily by different factors that control the fractionation of carbon isotopes during primary production of rganic carbon. Miocene and younger marine shale oils are isotopically heavier than older marine shale oils because of decreased atmospheric CO[2] concentration since 25 Ma, which has resulted in decreased isotope fractionation by marine plankton during photosynthesis. Deltaic oils are mostly derived from terrestrial organic matter, and their (isotope){13}C values reflect the time-invariant average (isotope){13}C value of land plant carbon at about -25 o/oo. Mesozoic carbonate oils are isotopically heavy due mainly to salinity and temperature effects during primary production in restricted environments from which terrestrial organic matter is excluded. Processes that occur after oil generation generally have smaller effects on (isotope){13}C values of oils. As a result, (isotope){13}C val es of oils are useful for determining oil-oil and oil-source rock relationships and, in conjunction with other geochemical properties, can indicate the possible age and depositional environment of source rocks.
Systematic changes in gas-oil ratios in the OML 67–70 area (joint venture acreage) of the Niger Delta (Nigeria) illustrate the effects of hydrocarbon migration on the compositional changes observed in the accumulations over large depth intervals. One interpretation is that the reservoirs are currently both filling and leaking hydrocarbons, indicating that trap styles affect the migration pathways in this region of the Niger Delta. The accumulations are more liquid prone with depth, based on the bubble point of the fluids in the reservoir. Applying these results to regional exploration opportunities indicates that fluid contacts in the normally pressured section are controlled by leaking seals (class III traps). The potential exists for pressure-controlled filled-to-spill traps (class II traps) in the overpressured section, where a high-quality seal impedes the vertical flux of hydrocarbon fluids. Although these conclusions apply to a specific portion of the Niger Delta, combining the concept of trap style with compositional changes resulting from migration processes is useful in identifying new play concepts and opportunities in other regions of the world.
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