Origin, chemistry and flow of formation waters in the Mississippian–Jurassic sedimentary succession in the west-central part of the Alberta Basin, Canada

Abstract

The flow and chemistry of formation waters was analyzed in the Mississippian–Jurassic succession in the west-central part of the Alberta Basin, using publicly available standard chemical analyses of formation waters and drillstem test data collected by the petroleum industry in Alberta. The interpretation of the thoroughly culled hydrogeological data show that a transition between a southern and a northern basin-scale flow system takes place in the study area. In the southeastern half of the study area, the Mississippian–Jurassic hydrostratigraphic group represents a single continuous carbonate–sandstone aquifer system, consisting of the Lower Jurassic Nordegg Member, the Triassic Montney Formation, the Permian Belloy Formation and the Mississippian Stoddart and Rundle groups. The confining units are the competent Fernie and Exshaw–Banff aquitards at the top and base, respectively. The salinity of formation waters increases northwestwards from ∼50 to ∼150g/l, while the bicarbonate content decreases in similar fashion from >3 to ∼0.5g/l. The flow pattern, based on hydraulic heads, and chemical composition of formation waters indicate decreasing mixing of meteoric water, carried northward in the southern basin-scale long-range system, with heavier connate water present in the northern part of the study area. In the center and in the northeast, the Permian and Mississippian carbonates, and the Triassic Montney and Charlie Lake–Halfway sandstones form individual aquifers separated by the intervening Doig–Montney aquitard. The flow in the Charlie Lake–Halfway and the Permo-Mississippian aquifers is directed east–northeastward from the deformation front into the overlying aquitards, or discharging into the southern basin-scale flow system, respectively. Only in the Montney aquifer, the flow is directed inward, south–southwestward, drawn into a local hydrocarbon-saturated sink, which is created by a higher rate of gas escape than of gas generation through a low-permeability and capillary-seal region. The high salinity, in the 130–150g/l range, and low bicarbonate content (∼0.5g/l) of formation waters in the northern part of the study area indicate a connate origin. The relation of Na–Ca–Cl–Br concentrations in the formation waters suggest that these waters initially originated as seawater that was altered to some degree by subaerial evaporation and dolomitization, but halite dissolution probably is the main source of salinity. The driving mechanism for the flow of these waters could be either tectonic expulsion from deep strata in the deformed part of the basin, as suggested by the high salinity of these waters, or topography-driven meteoric recharge in the Rocky Mountains through a tortuous path, unidentified as yet. The integrated interpretation of flow and chemistry of formation water shows that Mississippian–Jurassic formation waters in the deep, west-central parts of the Alberta Basin have been influenced less by mixing with meteoric water than was identified previously in the central, southern and northern parts of the basin. The main reasons for the isolation of ‘relict’ Mississippian–Jurassic formation waters in the deep Alberta Basin are effective confining aquitards and the fact that this area is influenced only marginally by mixing with meteoric water carried by a gravity-driven flow system.