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The San Joaquin basin of California is a young, rapidly buried basin in which feldspar alteration has major control over calcite cementation and porosity forming reactions in hydrocarbon reservoirs. The eastern part of the basin includes up to 3,658 m (12,000 ft) of Miocene and younger arkosic marine sand and shale. The Miocene sand is cemented by numerous thin bands of dolomite and calcite, which, as oxygen isotopic studies indicate, formed at burial temperatures between 20° and 90°C (68° and 194°F).
Strontium isotopic studies show that the calcium source for the calcite cements is plagioclase feldspar. Calcium in calcite cements forming at shallow burial depths and low temperatures (40°-70°C or 104°-158°F) was derived from plagioclase alteration reactions at deeper levels and higher temperatures (> 80°C or 176°F) in the basin. Time-temperature burial plots indicate that minimum required flow rates from deep to shallow basin levels would have to be about 1 cm/yr (0.4 in./yr). In contrast to the low temperature cements, the calcium source for cements forming at deep burial depths and at high temperatures (70°-90°C or 158°-194°F) was plagioclase dissolving within the reservoir. In addition, aluminum released from the dissolving fe dspar was precipitated as kaolinite in adjacent pore spaces. These diagenetic trends reflect increasingly restricted pore-water mobility during the basin history.
Strontium isotopic values in modern pore waters of the basin record increasing feldspar alteration with depth and suggest that present mixing of basin waters is restricted to subregions (10-15 mi or 16-24 km horizontally by 2 mi or 3 km vertically). Helium isotopes also suggest limited mobility of this gas on a scale similar to that of strontium.
Plagioclase dissolution (up to 5% of the rock volume) occurred just prior to the incoming of hydrocarbons, which suggests that acids associated with and perhaps moving in front of the hydrocarbons effectively created a substantial part of the reservoir porosity. Modern pore waters in the basin are rich in organic acids that may be responsible for the dissolution reactions.
The last calcite cements formed about 2-3 Ma, based on time-temperature burial plots. Thus, hydrocarbons filled the reservoirs (up to 500 million bbl) within less than 3 m.y. This short time for oil emplacement is surprising considering the low permeability of the reservoir rocks (1-10 md).
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