Carbonate cementation and dissolution in miocene sandstones from Corsair Trend, offshore, Texas
Middle Miocene sandstones occur in Picaroon field (Corsair trend; offshore Texas Gulf Coast) at depths of approximately 13,000-17,000 ft (3.9-5.1 km). These deltaic sandstones contain evidence of the following sequence of diagenetic events: (a) precipitation of chlorite coatings on detrital grains; (b) partial dissolution of feldspar; (c) quartz cementation; (d) calcite cementation; (e) dissolution of calcite cement; (f) ankerite cementation. The reservoir quality of the sandstones is largely a function of porosity enhancement due to calcite dissolution. Calcite cement was emplaced at depths of 3,000 to 8,500 ft. The calcite has /sup 87/86/Sr values of 0.70833 to 0.70865, eliminating coeval (--15 m.y.) seawater and marine carbonate (0.70873-0.70885) as a potential source. The /sup 87/86/Sr composition of calcite cement is consistent with mass transfer of calcite from older marine sources to younger sediments. Fluid inclusion measurements indicate that ankerite cement formed at temperatures of approximately 120/sup 0/-145/sup 0/C (11,000 to 14,000 ft). Oxygen isotope modeling predicts that at these depths shales would expel waters with /delta//sup 18/O/sub SMOW/ of +5 to +8 during smectite-illite conversion. Ankerite (/delta//sup 18/O/sub PDB/ = -7.8) would be in isotopic equilibrium with the predicted waters at the temperatures derived from fluid inclusions. Ankerite cements have relatively radiogenic /sup 87/86/Sr ratios (--0.7097) which are consistent with their formation from shale-derived fluids. Calcite dissolution occurs between the precipitation of calcite and ankerite. It is therefore concluded that calcite cement dissolution occurred at burial depths of 8,500 to 11,000 ft (95/sup 0/-120/sup 0/C). Although the cause of calcite dissolution is unknown, these temperatures overlap with the range in which high concentrations of dissolved organic acids have been reported in Gulf Coast formation waters.
- OSTI ID:
- 5952071
- Report Number(s):
- CONF-880301-
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
020200* -- Petroleum-- Reserves
Geology
& Exploration
ALKALINE EARTH METAL COMPOUNDS
ATLANTIC OCEAN
CALCITE
CALCIUM CARBONATES
CALCIUM COMPOUNDS
CARBON COMPOUNDS
CARBONATE MINERALS
CARBONATE ROCKS
CARBONATES
CARIBBEAN SEA
CEMENTING
CENOZOIC ERA
CHEMISTRY
DISSOLUTION
EXPLORATION
GEOCHEMISTRY
GEOLOGIC AGES
GEOLOGIC DEPOSITS
GEOLOGIC HISTORY
GEOLOGY
GULF OF MEXICO
MINERAL RESOURCES
MINERALS
MIOCENE EPOCH
NATURAL GAS DEPOSITS
ORGANIC ACIDS
ORGANIC COMPOUNDS
OXYGEN COMPOUNDS
PETROLEUM DEPOSITS
PETROLEUM GEOLOGY
POROSITY
RESERVOIR ROCK
RESOURCES
ROCKS
SANDSTONES
SEAS
SEDIMENTARY ROCKS
SURFACE WATERS
TERTIARY PERIOD