Abstract: Age and Provenance of Massive Carbonates from Hydrocarbon Seeps in Deepwater Gulf of Mexico

AHARON, PAUL
Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA, 70803

Hydrocarbon seeps encompassing depths from 125 to >2000m were investigated on the northern Gulf of Mexico slope and deep basin. These seeps are associated with massive chemical carbonates and calcareous shells of chemosynthetic mytilids containing a detailed history of hydrocarbon seepage. In this study I use stable carbon, radiocarbon and strontium isotope compositions of the carbonates to provide insights on the source and nature of the hydrocarbon rich fluids from which the carbonates precipitated. U-series isotopes were used to determine the chronology of seepage.

The formation of the chemical carbonates through microbial degradation of fossil hydrocarbons is evidenced by the highly {13}C and {14}C-depleted values d{13}C = -9.5 o/oo to -53.3 o/oo and d{14}C = -898 o/oo to -992 o/oo for n=27). The precipitation of the carbonates from seawater-derived fluids, rather than from formation fluids advecting from deep aquifers, is supported by the {87}Sr/{86}Sr composition of the samples (mean 0.70915 plus or minus 1.9 X 10{-5}, n=14) that compares well with modern non-seep carbonates and the ambient Gulf of Mexico seawater (0.70917 plus or minus 0.8 X 10{-5}). This observation opens the possibility of precise dating of fossil hydrocarbon seep deposits by exploiting the chronometer property of the {87}Sr/{86}Sr evolution with time of the Phanerozoic oceans.

Radiometric ages based on {230}Th dating indicate that vigorous seepage from subsurface hydrocarbon reservoirs occurred uninterrupted in the Gulf of Mexico from at least the Pleistocene time to the present. The radiometric ages most likely reflect the time of sedimentary loading and associated salt diapirism that activated the fault conduits to the seafloor.

AAPG Search and Discovery Article #90932©1998 GCAGS/GCS-SEPM Meeting, Corpus Christi, Texas