Carbon Isotope Signals for Chemical Stratigraphy and Hydrocarbon Exploration in Northern Gulf of Mexico

Douglas F. Williams, Dwight Trainor

Stable carbon (C₁₃/C₁₂) isotope records derived from the carbonate contained in the fine-grained (< 63 µm) sediment of well cuttings exhibit coherent stratigraphic patterns. These patterns have potential importance for chemical stratigraphy and hydrocarbon exploration in Pliocene-Pleistocene deep-water tracts of the northern Gulf of Mexico. Preliminary x-ray diffraction results suggest that calcite is the dominant carbonate phase in the fine fraction. The fine fraction C₁₃ record in any given exploration well is characterized by a combination of events (or anomalies), shifts, and long-term trends with time. Wells from some offshore areas show definable negative "shifts" of nearly 7 per mil from the baseline C₁₃ signal fo a given stratigraphic interval. In addition to shifts, large negative C₁₃ "events," or anomalies, of > 10 ^pmil occur in some sections. Absolute isotope values of some parts of the C₁₃ record suggest that a portion of the fine-fraction calcite precipitated in the presence of isotopically light (negative) hydrocarbons. The observed isotopic anomalies may be due to the presence of (1) a particular facies with hydrocarbons, (2) lithostratigraphic units through which hydrocarbons migrated, and/or (3) facies from which hydrocarbons were expelled.

AAPG Search and Discovery Article #91042©1987 GCAGS and GC-SEPM Section Meeting, San Antonio, Texas, October 28-31, 1987.