Diagenesis in Shallow Conventional and Low-Permeability Biogenic Methane Reservoirs of Eagle Sandstone, Montana: ABSTRACT

Abstract

The Upper Cretaceous Eagle Sandstone and equivalent rocks in north-central and eastern Montana provide an excellent opportunity to investigate postdepositional effects in gas reservoirs that have never attained thermal conditions sufficient for the generation of petroleum. Investigations reveal that these reservoirs display inorganic diagenetic features similar to those of rocks having a burial history suitable for the formation of oil or thermogenic gas. These features are observed in both high-permeability and low-permeability reservoirs. Complex paragenetic sequences such as the following are common: (1) authigenic clay formation, (2) quartz overgrowth, (3) calcite cement and replacement, (4) carbonate dissolution, and (5) additional authigenic clay formation. Exotic phases, such as authigenic tourmaline, are observed locally. More importantly, there is widespread evidence of former calcite cementation and replacement, especially of plagioclase, followed by carbonate dissolution. The distribution of these features suggests that calcite has at various times occupied virtually every pore in many of the Eagle reservoirs. However, several lines of evidence, including the timing of gas generation and entrapment and the distribution of calcite in the Eagle Sandstone, suggest that the reservoirs have never been completely sealed by carbonate rock. Thermal maturation of organic matter is not a prerequisite for the development of secondary porosity, nor for the development of minerals potentially hazardous to well-completion and treatment procedures. In addition, unequivocal evidence demonstrates that dissolution porosity can be produced without a previous period of total destruction of reservoir quality. End_of_Article - Last_Page 712------------