The Lower Cretaceous Chinkeh Formation: A Frontier-Type Play in the Liard Basin of Western Canada (1)

Abstract

The Lower Cretaceous Liard basin in western Canada covers an area of 9500 sq km (3668 sq mi) but is relatively unexplored despite its size. The present-day expression of the basin, which formed during the latest Cretaceous to early Tertiary, trends north-south and is delineated by the outcrop of the coarse-clastic Upper Cretaceous Dunvegan Formation. The lowermost Cretaceous unit, herein named the Chinkeh Formation, is up to 32 m (105 ft) thick and unconformably overlies older Paleozoic strata. The Chinkeh Formation contains four major lithotypes: (1) conglomeratic breccia interpreted as debris-flow or talus deposits, (2) interbedded coal, carbonaceous shale, rooted sandstone, and conglomerate interpreted as nonmarine valley fill or channel deposits, (3) conglomeratic lag related to marine transgression, and (4) upward-coarsening sandstone interpreted as abandoned shoreline deposits. The lower part of the Chinkeh Formation consists locally of an angular-chert breccia overlain by medium- to coarse-grained sandstone and conglomerate with coal and rooted beds. The uppermost Chinkeh Formation consists of a basal conglomeratic layer overlain by an upward-coarsening, fine- to medium-grained, well-sorted marine sandstone, with bioturbated shale. Sediment was recycled repeatedly with the dominant sediment source being from the east with perhaps some western sources. Cretaceous strata in the Liard basin have good petroleum source-rock and reservoir potential, and hydrocarbons may be present in sandstone of the Chinkeh Formation. Potential play types include stratigraphic traps formed by incised-valley deposits and shallow-marine sandstone pinching out laterally into marine shales of the Garbutt Formation. A potential structural play may occur along the Bovie fault zone where reservoirs may abut against a shale seal on the eastern side of the fault. Most of Lower Cretaceous strata falls within the hydrocarbon generation window, with increasing maturity to the southwest. Pyrobitumen is locally abundant and constitutes 30% of the bulk rock volume in one sandstone. Potential source rocks include the lowermost Garbutt Formation and underlying Triassic oad and Grayling formations. The Chinkeh Formation sandstone has porosity values of 8-18%.