Abstract

Calcite cements occur as thin bands (up to 1 m thick by 200 m long) in a Stevens turbiditic sandstone (upper Miocene) reservoir at North Coles Levee. These cements formed from evolved marine power water and show up to 10 mole % of mainly Fe + Mg. Stable isotopic analysis and estimated isotopic compositions of pore fluid indicate the cements formed from 40/sup 0/ to 80/sup 0/C and that delta/sup 13/C ratios range from +1.2 to -12.66. A burial history curve indicates the cements formed from 7.5 to 3.5 Ma. Dissolution of calcite, widespread dissolution of plagioclase (up to one-third total porosity), precipitation of kaolinite, and migration of hydrocarbon (500 million bbl) occurred after this time. Hence considerable mass transfer is implied over a relatively short period of time, and oil migration closely followed the dissolution event. Increased concentrations of acids during kerogen maturation as observed by Carothers and Kharaka in 1978 for organic acid species at 80/sup 0/ to 100/sup 0/C in present-day pore waters of the basin may have caused the dissolution process. Very recent, possible post-oil migration processes in the field include albitization of plagioclase and dolomite rhombs growing in crushed biotites. These stress-related processes may bemore » the indirect result of low fluid pressures (1000 psi at 8500 ft) from release of the reservoir gas cap in 1965. At present, calcite with relatively high magnesium contents (to 8 mole %) precipitate as scales in the tubing string, presumably a result of CO/sub 2/ degassing where pressure drop occurs in the flow line.« less

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