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Innovative Geochemical Approaches to Chronostratigraphy and Hydrocarbon Detection in Subsurface: Neogene Exploration Wells, Northern Gulf of Mexico

Dwight M. Trainor, Douglas F. Williams

Application of isotope chronostratigrahy to numerous exploration wells in the northern Gulf of Mexico provides a large data base for making stratigraphic correlations with resolutions of 20,000 years in the Pleistocene and 50,000 years in the Pliocene-Miocene. A composite record of the O-18 and C-13 of foraminifera for the last 4.5 m.y. enables the recognition of 131 O-18 stages and a similar number of C-13 stages independent of the O-18 stages. The integrated approach incorporating these complimentary stages and all available biostratigraphic information is used to derive a chronostratigraphy of unparalleled resolution in exploration wells. Within this absolute time framework, we have determined the timing and extent of more than five regional unconformities. These uncon ormities can be distinguished from those produced by more localized salt tectonics or growth faulting.

The carbon isotopic composition of fine-grained sediments in exploration wells provides a combination of stratigraphic and geochemical information of importance to hydrocarbon exploration. The range in isotopic values seen in the fine-grained sediments is greater than 20 ^pmil, which is extremely different than that observed in the biogenic carbonate. The C-13 record is characterized by a series of 2 to 6 ^pmil shifts in mean isotopic value in both the positive and negative direction. Superimposed over these shifts are numerous events, mainly negative excursions, on the order of -2 to -10 ^pmil. The chronology of the C-13 fine fraction records is based on the chronostratigraphy derived from the foraminiferal-based O-18 record. C-13 events and shifts can be used to correlate wells with n exploration trends like the Flexure Trend. Some of the negative C-13 events appear to reflect the occurrence of calcite formed by the microbial biodegradation of crude oil and gas.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.