Origin and Diagenesis of Clay Minerals in Relation to Sandstone Paragenesis: An Example in Eolian Dune Reservoirs and Associated Rocks, Permian Upper Part of the Minnelusa Formation, Powder River Basin, Wyoming
POLLASTRO, RICHARD M., and CHRISTOPHER J. SCHENK, U.S. Geological Survey, Denver, CO
Eolian dune sandstones are the principal reservoir rocks in the Permian upper part of the Minnelusa Formation, Powder River basin, Wyoming. These sandstones formed as shorelines retreated and dunes
migrated across siliciclastic sabkhas. Sandstones are mainly quartzarenites; on average, clay minerals constitute about 5 wt.% of the whole rock. Although present in minor amounts, clay minerals play an important role in the diagenetic evolution of these sandstones.
Allogenic clay minerals are present in shaly rock fragments and laminae. Early infiltration of clays into porous sabkha sands commonly form characteristic menisei or bridges between framework grains or, when more extensive, form coatings or rims on grain surfaces. Authigenic clays include nearly pure smectite, mixed-layer illite/smectite (I/S), and late diagenetic illite and corrensite; these clay minerals are present as pore-lining cements. Smectite and I/S commonly line secondary pores following the dissolution of anhydrite cement. Kaolinite commonly fills pores in eolian sandstones at present burial depths less than 1200 m but is rare in core samples below 2100 m. Kaolinite formed locally from the dissolution or replacement of framework feldspars.
In addition to the deposition and neoformation of clay minerals throughout sandstone paragenesis, the conversion of smectite to illite occurred as temperatures increased with progressive burial. I/S ratios and ordering of I/S is variable. Random I/S is abundant in samples above 1200 m and present in samples to about 3100 m; only ordered I/S is found in samples at greater depths. In addition, the ratio of I/S to discrete illite decreases with increasing present-day burial depths.
A temperature of 103 degrees C is calculated at a present depth of 3200 m using a geothermal gradient of 30 degrees C/km and a mean annual surface temperature of 7 degrees C. After correction for uplift and erosion (250 m), the maximum calculated temperature for the conversion of all random I/S to ordered I/S is 110 degrees C. This calculated temperature is in excellent agreement with temperatures of 100-110 degrees C implied from I/S geothermometry.
AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)