Natural recession of the eastern margin of the Leofnard salt in western Canada

Abstract

The Lloydminster area (T35-65, R15W3M-10W4M) of east‐central Alberta and west‐central Saskatchewan, Canada, is dissected by the north‐northwest trending updip active dissolution margin, of the Devonian Leofnard Member rock salt. West of this margin, up to 150 m of rock salt is preserved; updip and to the east, the salt has mostly been leached from the rock record. The margin is up to 40 km wide and characterized by extreme local variations in net salt thickness. The dissolution of the Leofnard rock salt in the Lloydminster area has resulted in the entrapment of significant hydrocarbon accumulation. Stratigraphic traps, for example, have formed where reservoir facies were either preferentially deposited or preserved in salt‐dissolution lows. Structural traps, in contrast, have formed where reservoir facies are draped across residual salt or collapse features. It has been estimated that three trillion barrels of oil (mostly of high viscosity and unrecoverable) are entrapped along the eastern dissolution margin of the Leofnard rock salt in western Canada. A record of the westward progression of the dissolutional edge of the Leofnard salt is locked in the stratigraphic column. This progression is recorded as localized interval thickening in areas where dissolution and deposition were contemporaneous. The horizontal positioning of these interval thicks as a function of their geologic age provides a time record for the positioning of the salt edge. To further explain the process of salt dissolution in the Lloydminster area, we present a suite of contour maps, geologic cross‐sections, and seismic profiles. These data depict the present‐day distribution of the Leofnard salt in the Lloydminster study area. They support the theses that: (1) the dissolution margin of the Leofnard rock salt originated along the Elk Point outcrop to the east of the study area during the pre‐Cretaceous; and (2) the margin receded into the northeastern part of the Lloydminster study area during earliest Cretaceous or pre‐Cretaceous time and migrated progressively thereafter into its current position. From the perspective of the explorationist, such information is important because it identifies prospective play areas with high potential for the formation of salt‐related stratigraphic traps and/or structural traps.