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Depositional and Facies Models in Evaporitic Sediments: A Case Study of the Prairie Evaporite and Lotsberg Formations in South-Central Alberta

Abstract

The Central Alberta Basin (CAB) is host to a wide range of evaporite deposits, which are often associated with hydrocarbon exploration (e.g., as seals, sources). However, evaporite deposits, in particular salt bodies, are understudied due to difficulties associated with deformation, extensive diagenesis, and a lack of modern analogs. Further study of these deposits permits a better conceptualization of structural stability of salt bodies during solution mining and may provide insights into coeval hydrocarbon source rocks. We present high 3D facies models of the Prairie Evaporite and Lotsberg Formations in the CAB which can integrated into hydrocarbon development models.

The Prairie Evaporite and Lotsberg Formations in the Elk Point Group (EPG) are Givetian to Frasnian sedimentary strata comprising hypersaline lagoon and salt pan deposits. Deposition of these strata was controlled by the extent of evaporation, water input, and proximity to emergent topography. To better constrain the depositional facies of the CAB, we studied six cores and mapped 550 individual wells with well log data that penetrate the Prairie Evaporite and Lotsberg Formations. Both formations are laterally extensive, but the Prairie Evaporite is nearly continuous across the basin while the Lotsberg is confined to the NE of the CAB.

Logged cores provide petrographic evidence of primary depositional features (e.g. chevron crystals) and secondary dissolution truncation surfaces that mark flooding events, resulting from influxes of fresh water. Flooding can be explained by sea level rising above the Presqu’ile barrier or increased rainfall. To define the transition from salt pan to saline lagoon facies we correlate major flooding events that freshen the depositional waters, dissolve previously emplaced halite, and deposit insoluble layers. Within the mapped lithologies, these mixed clay layers are easily correlated over long distances (100’s of kilometers). The Prairie Evaporite Formation has many centimeter-thick, meter-wide lenses of anhydrite and clay partings deposited in a lagoon environment, whereas the Lotsberg Formation has a thick anhydrite and clay unit in the center of the deposit, which has been associated with post burial concentration of insolubles and redistribution of halite. We apply these observations to established facies models and develop a new 3D model for the Prairie Evaporite and Lotsberg formations. Our new model will better inform salt cavern development in the EPG.