Abstract: Widespread Occurrence of Laumontite in Late Mesozoic and Tertiary Basins of Pacific Margin

Richard J. Stewart, Thane H. McCulloch

Laumontite is widespread as an alteration product in feldspathic sandstones and tuffaceous rocks that have undergone "burial metamorphism" at moderate depths. It is present in many late Mesozoic and Tertiary sedimentary basins of the Pacific margin as pore-filling cement, partial replacements of detrital plagioclase grains, rare detrital-carbonate clasts, and volcanic glass, and in veins. Volcanic detritus apparently is not required for laumontite formation because some marine arkosic sandstones that totally lack volcanic detritus contain abundant laumontite. However, as many workers have reported, volcaniclastic and tuffaceous strata are especially susceptible to laumontitization.

Observations of sandstones in three California oil fields, presently at maximum burial depths, suggest that threshold conditions for formation of laumontite in feldspathic marine sandstones without volcanic detritus are 120.5 ± 0.5°C at ~300 bars. A further requirement appears to be abundantly available hyposaline water (17,000 to 20,000 ppm) dominated by Na⁺ and Cl^-, at a pH near 7 in sandstones, initially of reservoir quality (average porosity of 20% and average permeability of 25 md).

Published data about laumontite occurrences suggest that laumontite formation is suppressed by pore fluids of high salinity, by high partial pressure of CO₂, by early diagenetic conversion of smectite to pore-lining chlorite cement, and by low-permeability regions where supplies of pore fluids are limited. Laumontite forms most readily in the presence of abundant vitric debris, in high-permeability regions, where geothermal gradients are high, and in fluvial deposits. The crystallized threshold for laumontite apparently may have been depressed to temperatures as low as 75°C in some nonmarine sequences where geothermal gradients are high. Our observations indicate that laumontite does not occur--even where present temperatures substantially exceed 120°C--in rocks t at were buried rapidly and in which compaction in temporarily cool environments produced intensive early reduction of sandstone permeability.

Laumontitization can be either regional or highly localized, both vertically and horizontally. Variable permeabilities and lithologies control localization to a greater extent than do burial depth or temperature. Probably because of their superior permeabilities, petroleum-reservoir feldspathic sandstones appear to be selective targets for laumontite formation where other threshold conditions are satisfied. Destruction or serious degradation of reservoir quality is a result. Furthermore, the threshold temperature for laumontite formation is within the range probably needed for petroleum generation from source rocks. For these reasons, criteria should be established for distinguishing between regional and localized occurrences of laumontized sandstones.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado