The Role of Meteoric Water in Diagenesis of Shallow Sandstones: Stable Isotope Studies of the Milk River Aquifer and Gas Pool, Southeastern Alberta: Part 1. Concepts and Principles

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 18O and 13C values: clay minerals (<2 µm), dominated by authigenic kaolinite, 18O = +11.3 to +14.2 (SMOW); authigenic calcite, 18O = +15.3 to +18.5 (SMOW), 13C = -9.9 to -2.6 (PDB). The uthigenic minerals with the lowest 18O 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 13C values for the calcite indicate incorporation of organically derived CO2, 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 18O 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 18O-poor meteoric water no longer present in the system. This water was displaced by 18O-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 18O-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 18O to other Cretaceous oil and gas pools that occur in Alberta. The low-13C 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 18O 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 18O (+24.4 to +28.3, SMOW) and 13C (-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.