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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 72 (1988)

Issue: 6. (June)

First Page: 664

Last Page: 681

Title: Compaction and Porosity Evolution of Pliocene Sandstones, Ventura Basin, California

Author(s): John C. Wilson, Earle F. McBride (2)


Sandstones of the Pico and Repetto formations were studied to quantify the processes involved in compaction and to determine the porosity evolution of the beds during burial. The sandstones are mineralogically immature and, except for local carbonate-cemented beds, have insufficient cement to have prevented compaction. Stratigraphic studies and graphic porosity-depth plots suggest that the samples' maximum burial depths were approximately 10,000 ft (3,048 m) deeper than the present-day depths of 850-13,000 ft (259-3,962 m). Compaction produced closer grain packing as measured by the contact index (CI = average number of contacts per grain) and tight packing index (TPI = average number of long, sutured, and embayed contacts per grain). For samples with less than 10% cement the CI depth gradient is 0.21/1,000 ft (0.69/1,000 m) and TPI depth gradient is 0.27/1,000 ft (0.89/1,000 m). Assuming sands had 40% initial porosity, the sandstones that are at present most deeply buried lost a total absolute porosity of 26% by all compactional processes, which includes 14% by grain rearrangement and 6% each by pressure solution and ductile grain deformation. Porosity loss by nondeformational grain rearrangement was complete by a present-day depth of 5,000 ft (1,524 m), but in uncemented sandstones pressure solution (plus associated grain rearrangement) and ductile grain deformation continued to the maximum depths sampled. Compaction is most abrupt in sands with abundant micas and ductile rock fragments. The CI gradient is 0.14/1,000 ft (0.46/1,000 m) for samples with ess than 10% ductile grains, but 0.22/1,000 ft (0.72/1,000 m) for samples with more than 10% ductile grains. Compaction was almost entirely the result of overburden pressure and was not noticeably retarded in overpressured zones, suggesting that overpressures developed after maximum burial was achieved.

For samples with less than 10% cement, the CI is a better depth predictor than the TPI (r2 = 0.58 vs. 0.45), but the TPI is a better predictor of pre-cement porosity than the CI (r2 = 0.54 vs. 0.31).

Secondary porosity averages 20-40% of the total thin-section porosity. Secondary porosity is present at all depths, but makes up the greatest percentage of total porosity between present depths of 6,000 and 9,500 ft (1,829 and 2,896 m). Except for grain fracturing, secondary porosity formed only by dissolution of framework grains. Fractured-grain porosity contributes up to 29% of total porosity (5% absolute porosity) in some samples; fractures formed during Pleistocene tectonic episodes.

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