Print this pageAbstract: 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