Abstract Oxygen- and carbon-isotope compositions have been determined for clay and carbonate minerals from the Upper Cretaceous clastic rocks of the Milk River and Lea Park Formations. These units contain the Milk River aquifer and the southeastern Alberta Milk River Gas Pool, respectively. The stable isotope data provide important information concerning the diagenesis and paleohydrology of the study area. Authigenic minerals from sandstones in the Milk River aquifer are characterized by low δ 18 O and δ5 13 C values: clay minerals (<2 μm), dominated by authigenic kaolinite, δ 18 O = +11.3 to + 14.2 (SMOW); authigenic calcite, δ5 18 O = +15.3 to +18.5 (SMOW), δ 13 C = −9.9 to −2.6 (PDB). The authigenic minerals with the lowest δ5 18 O values occur within a zone of local recharge in the aquifer. Here the authigenic clay minerals and calcite closely approach isotopic equilibrium with existing meteoric water at low temperatures (<+15°C). Low δ 13 C values forthe calcite indicate incorporation of organically derived CO 2 , probably from decaying plant material in the overlying soil and till. Close agreement between actual formation temperatures and those calculated from isotopic data disappears in downdip portions of the aquifer, mostly because of the drastic enrichment in 18 O of the formation water in this direction (−20 to −6, SMOW; Schwartz et al, 1981). Authigenic minerals from these locations have retained isotopic signatures characteristic of 18 O-poor meteoric water no longer present in the system. This water was displaced by 18 O-rich formation fluids that are themselves now being flushed from the aquifer by modern-day ground water. Authigenic minerals from sandstones in the southeastern Alberta Milk River Gas Pool are more 18 O-rich than those from the aquifer: clay minerals, dominated by illite, +14.7 to +15.8, SMOW; calcite, +19.3, SMOW. Such compositions are compatible with mineral crystallization at low temperatures (+15 to +20° C) from formation fluids similar in δ5 18 O to other Cretaceous oil and gas pools that occur in Alberta. The low- 13 C nature of authigenic calcite and some dolomite (−7.6 to −3.0, PDB) from the Milk River Gas Pool may be related to the production of biogenic methane in this reservoir. Of all clay minerals analyzed, the illite-dominated mixtures from argillaceous rocks of the Milk River aquifer and the Milk River Gas Pool have the highest δ 18 O values (+16.0 to +19.0, SMOW). Such compositions reflect a detrital origin rather than diagenetic processes. Isotopic exchange between these clay minerals and formation water is insignificant. Most dolomite from the sandstones and the argillaceous rocks is not in equilibrium with the authigenic calcite. The dolomite is much richer in 18 O (+24.4 to +28.3, SMOW) and 13 C (−2.7 to +1.0, PDB); such values are typical of platform carbonate rocks. No evidence for extensive isotopic exchange between formation water and the dolomite can be demonstrated.

Abstract The Upper Cretaceous Milk River Formation in southeastern Alberta and southwestern Saskatchewan has produced more than 2 tcf of dry (>99% methane) microbial gas (δ 13 C PDB –65 to –71‰) that was internally sourced. Production is from underpressured fine-grained sandstone and siltstone reservoirs, whereas the gas was generated in interbedded organic-bearing mudstones with low organic carbon contents (0.5–1.50%). The formation experienced a shallow burial history (maximum burial, <1.3 km [<0.8 mi]) and cool formation temperatures (<50°C [<122°F]). Petrologic and isotopic studies suggest that methanogenesis began shortly after deposition and continued for at least 20 to 25 m.y. Mercury injection capillary pressure data from the Milk River Formation and the overlying Upper Cretaceous Pakowki Formation, which contains numerous regionally extensive bentonitic claystones, reveal a strong lithologic control on pore apertures and calculated permeabilities. Pore apertures and calculated permeabilities in Milk River mudstones range from 0.0255 to 0.169 μm and less than 0.002 to 0.414 md, respectively, and claystones from the overlying Pakowki Formation have pore apertures from 0.011 to 0.0338 μm and calculated permeabilities of 0.0017 to 0.0065 md. The small pore apertures and low permeabilities indicate that claystones and mudstones served as seals for microbial Milk River gas, thereby permitting gas to accumulate in economic quantities and be preserved for millions of years. Based on the timing of gas generation, the gas system of the Milk River Formation can be considered an ancient microbial gas system, which is one of several ways it differs from that of the Devonian Antrim Shale, Michigan Basin, where microbial gas generation is a geologically young (Pleistocene and younger) phenomenon. The difference in timing of gas generation between the Milk River and Antrim systems implies that gases in the two formations represent end members of a spectrum of microbial gas accumulations in fine-grained rocks, with the Milk River Formation being an excellent example on which to base a paradigm for an ancient microbial gas system.

Abstract Low-permeability reservoirs in which gas is the regionally continuous phase (gasifers) occur over large areas in the Alberta Basin in Canada and the Rocky Mountain basins in the United States. These tight-gas reservoirs have also been called “deep basin” and “basin-centered” gas systems and contain very large resources of natural gas. Observation and theory show that a gasifer, or a regional low-permeability gas system, can be developed in a four-stage process: genesis, transition, steady state, and imbibition. This process involves the generation, migration, and leakage of gas, accompanied by the regional dewatering of the system, even in the siltstones and shales.The genesis stage contains both conventional gas pools, early in the development of the gasifer, and unconventional gas pools later. Late genesis gas pools are characterized by tall gas columns, with normal downdip apparent gas-water contacts. These tall gas columns generate enough capillary pressure to drain very low-permeability reservoirs and establish gas as the continuous fluid over a very large part of the basin. At this point, the gasifer is developed. The transition stage has normal and underpressured gas, with tall columns that crosscut waterlines on pressure-versus-elevation plots. In the steady-state stage, gas is underpressured, and the tall gas columns have updip gas-water contacts. The imbibition stage marks the decline of the gasifer and is characterized by shorter, underpressured gas columns and underpressured waterlines. Laboratory experiments based on a simple capillary tube model support the four-stage development of the regional low-permeability gas system and defined both a normally pressured and overpressured gas-water system in the genesis stage. These experiments demonstrated that the mechanism for the underpressuring of the gasifer in the transition and steady-state stages was gas leakage, which confirmed the conclusions based on capillary theory. The combination of the empirical approach using pressure-versus-elevation plots and the capillary theory with the laboratory experiments leads to several interesting concepts. For example, a regional low-permeability gas system can be viewed as a source rock undergoing primary migration. Gas generation may be thermal or biogenic. Therefore, this four-stage process would also apply to the shallow biogenic gasifers in the Milk River and Horseshoe Canyon formations in southern and central Alberta. The genesis stage will contain some moveable water, and there will be an overprint of structural and stratigraphic traps with higher water production downdip. Reservoirs in the genesis stage will have variable water saturations, and therefore, relative permeability should be a concern. A basin may have any one of these stages well developed, or all four may be present at various levels of development, as is the case for the Alberta deep basin. By knowing the stages, the gasifer can be defined, and an effective exploration strategy can be developed.