Regional characteristics of strontium, carbon, and oxygen isotopes in salt dome cap rocks of the western Gulf Coast

Thesis/Dissertation · Wed Jan 01 04:00:00 EST 1986

OSTI ID:5077115

Posey, H H

Isotopic analyses of salt dome cap rocks, diapiric anhydrite, and oil field brines are combined with petrographic analyses to discern patterns of chemical evolution in subsiding sedimentary basins and to develop a comprehensive model for salt dome cap rock formation. Several tenets of existing cap rock formation models are re-evaluated. First, radiogenic strontium in diapiric anhydrite suggests that salt transmits fluids during diapirism. Second, anhydrite compositions are altered by re-crystallization and sulfate reduction during cap rock formation. Third, calcite cap rock forms in cycles that advance from the anhydrite/sediment interface toward the center of anhydrite cap rock. Fourth, barite and sulfides form from a second source or from late-arriving parts of the calcite-generating fluid. As organics mature CO{sub 2} is generated followed by liquid hydrocarbons and methane. During upward migration in a fluid column CO{sub 2} dissolves limestone lowering {delta}{sup 13}O of the fluid. Late fluids of the column encounter less limestone so retain {delta}{sup 13}C of the starting fluid. In cap rock {delta}{sup 13}C decreases down section owing to high methane carbon and low dissolved-limestone carbon. Strontium isotopes show similar relationships; shallow calcite has comparatively low {sup 87}Sr/{sup 87}Sr owing to dilution of radiogenic brines with dissolved limestone. Gulf Coast basin cap rocks have heavier average {delta}{sup 13}C than domes farther inland because they formed from deeper, enriched source fluids. These domes also have higher {sup 87}S/{sup 86}Sr and lower strontium concentrations owing to exchange with radiogenic strontium and more recrystallization during vertical transport and deformation. Saline, hydrocarbon-bearing fluids in Gulf Coast sediments are presently being drive from geopressured zones into shallow hydropressured zones along salt dome flanks.

• Research Organization: North Carolina Univ., Chapel Hill, NC (USA)
• OSTI ID: 5077115
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400202 -- Isotope Effects
Isotope Exchange
& Isotope Separation
58 GEOSCIENCES
580000* -- Geosciences
ALKALINE EARTH ISOTOPES
CALCULATION METHODS
CAP ROCK
CARBON ISOTOPES
DATA
DATA ANALYSIS
EXPERIMENTAL DATA
GEOLOGIC MODELS
GULF COAST
INFORMATION
ISOTOPES
MEASURING INSTRUMENTS
MEASURING METHODS
NORTH AMERICA
NUMERICAL DATA
OXYGEN ISOTOPES
STRONTIUM ISOTOPES
USA