Petrology of the Eagle Sandstone, Bearpaw Mountains area, north-central Montana [report]

1981 unpublished
The Eagle Sandstone, in the Bearpaw Mountains area of Montana, is an important conventional reservoir for shallow gas. It represents shoreline sedimentation during a major regression of the Late Cretaceous sea. Hydrocarbons in the Eagle Sandstone mainly are isotopically light methane, which was probably generated in associated source rocks early in their burial history. The formation is composed of sandstone and lesser amounts of mudstone. The mudstones are of several varieties and contain
more » ... amounts of highly expansible mixed-layer clay and coarser grained minerals similar to those of the associated sandstones. Quartz of diverse origins makes up 30 to 50 percent of the sandstone; intermediate plagioclase (An 30 -An 40 ), as much as 25 percent; K-feldspars, 5 to 10 percent; sedimentary, metasedimentary, and volcanic rock fragments, 10 to 30 percent; and transported dolomite grains, 5 to 10 percent. Glauconite is common, as are small amounts of biotite, muscovite, and detrital chlorite. Accessory heavy minerals are mainly apatite, zircon, garnet, tourmaline, and opaque grains. The source area for the Eagle Sandstone lay to the west and was a mixed terrane of Proterozoic Belt Supergroup metasediments, Paleozoic sedimentary rocks, and volcanic rocks of probable Cretaceous age. Clay mineralogy, carbon isotopes in gases, vitrinite reflectance, and stratigraphic reconstruction indicate the rocks have never been buried deeper than about 1,400 m and have not undergone thermal conditions sufficient for oil generation. Diagenesis was important in determining the reservoir properties of the Eagle Sandstone. Localized calcite precipitation began shortly after deposition. Compaction and minor quartz cementation occurred as burial depths increased. Then, extensive precipitation of sparry calcite tightly sealed intergranular pores and selectively replaced plagioclase. This calcite contains isotopically light (0'3{;= -7.5 permil) carbon. Siderite developed locally, usually because of increased iron activities associated with altering biotite. Dissolution of calcite cement and especially dissolution of calcite replacements of plagioclase significantly enhanced reservoir porosity. These processes provided space for precipitation of authigenic clays. The present distribution of calcite-cemented layers limits vertical permeability and compartmentalizes the reser-1 2 EAGLE SANDSTONE, BEARPAW MOUNTAINS AREA, MONTANA voirs. Early-formed carbonates may have acted as gas-trapping seals, particularly in lower permeability reservoirs to the east. Fluid-sensitive authigenic days and iron carbonates may cause formation damage if not taken into account during the development of gas recovery methods. The Eagle Sandstone displays many diagenetic features similar to those of rocks that have undergone thermal maturation sufficient for the generation of oil or gas. It is concluded that thermal maturation of organic matter is not required for the development of significant secondary porosity, nor for the occurrence of a complex diagenetic history.
doi:10.3133/b1521 fatcat:iofxbssswnbennlxvnmane24uy