Sandstone Diagenesis and Source Rock Analysis of Cretaceous Blackleaf and Frontier Formations in Parts of Beaverhead, Madison, and Gallatin Counties, Montana

T. S. Dyman, W. J. Perry, Jr.

The mid-Cretaceous Blackleaf and Frontier Formations and equivalent strata in parts of Beaverhead, Madison, and Gallatin Counties, Montana, are composed of a marine and nonmarine sequence of clastic and carbonate rocks deposited along the western margin of the Western Interior seaway. At present, seven dry holes have penetrated this sequence in southern Beaverhead County. Sandstones in the formations are diagenetically diverse and exhibit calcite, silica, sericite, zeolite, and clay mineral alteration. Alteration is more pervasive in the upper Blackleaf and lower Frontier Formations because of compositional heterogeneity and the presence of volcanic detritus. Lower Blackleaf alteration is less complex and is mainly due to quartz overgrowths and calcite cements. Porosity i lower Blackleaf quartz arenites in eastern Beaverhead, Madison, and Gallatin Counties has been reduced significantly by quartz overgrowths. Lower Blackleaf sedlitharenites in Beaverhead County contain calcite pore-filling cement but exhibit the best reservoir rock potential of both formations.

Vitrinite reflectance ranges from a maximum of 3.62% for shales in the lower Blackleaf Formation in the Pioneer Mountains in northern Beaverhead County to a minimum of 0.43% for the lower Frontier Formation in the Lima Peaks area to the south. Maximum reflectance values are related to the proximity of Cretaceous and Tertiary intrusive bodies. Reflectance values for all other samples within the study area lie just below or at the base of the oil generation window. Total organic carbon (TOC) values for both formations are higher in the Gallatin Range (maximum value equals 8.1%), but Rock-Eval pyrolysis data indicate generally poor source rock potential. Black shales of the lower Blackleaf and lower Frontier Formations provide the best source rock for generation of hydrocarbons.

AAPG Search and Discovery Article #91040©1987 AAPG Rocky Mountain Section Meeting, Boise, Idaho, September 13-16, 1987.