The Knoll Limestone “bench” of the Three Forks Formation, West-Central Montana: An Exposed Unconventional Reservoir
Recent field studies in support of PRISEM's ongoing Sappington-Bakken project in west-central Montana provide a new understanding of the stratigraphy and sedimentology of unconventional carbonate reservoirs in the Three Forks Formation and their relationship to coeval strata in the subsurface of Montana, Alberta and North Dakota. The Lower to Middle Famennian Three Forks Formation occurs between the Birdbear/Nisku and overlying Sappington/Bakken formations. The Three Forks was deposited across western North America during a global lowstand and subsequent transgression. At Logan Gulch, Montana, the Three Forks is divisible into a lower restricted Logan Gulch Member (Stettler/Torquay) and an upper restricted to open marine Trident Member (Big Valley). The Logan Gulch Member is comprised of restricted sabkha lithologies with interbedded evaporites which are leached into collapse breccias in outcrop; this facies is 100s of feet thick in the subsurface (Potlatch Member). The overlying Trident Formation consists of restricted and subaerially exposed shales and limestones, overlain by green fossiliferous marine shales and limestones that record the incursion of open marine waters onto the craton. The boundary between the two members of the Three Forks Formation is marked by a regionally persistent 5 to 40 ft thick limestone herein named the Knoll Limestone after a prominent knoll at Logan Gulch. The Knoll Limestone was deposited above a sharp sequence boundary incised into the underlying Logan Gulch Formation. The Knoll Limestone is a petroliferous bioturbated lime mudstone, with interbeds of cross-bedded pelloidal and intraclastic grainstone, stromatolites, and rare brachiopods. The Knoll Limestone varies in thickness over short distances due to brecciation of the underlying Logan Gulch Member, although field relationships also reveal syndepositional salt movement. Secondary diagenetic fabrics like brecciation, dissolution, and recrystallization are associated with subaerial exposure before deposition of the overlying Trident Member. Oil and gas shows are common in the subsurface of Montana and the Knoll Limestone should be considered a tight, unconventional carbonate reservoir with low primary porosity that has been enhanced by secondary diagenesis. The Knoll Limestone records regional flooding of the exposed Logan Gulch platform across most of Montana, Idaho and Alberta (correlating with carbonates in the Stettler or Wabamun formations). It also extends eastward into the Williston Basin where it correlates with the producing “benches” of the Three Forks Formation. Continuing studies of the Knoll Limestone in outcrop will assist in defining areas with higher primary porosity (coarser facies) and contribute to our understanding of these unconventional evaporite-associated carbonate reservoirs.
AAPG Datapages/Search and Discovery Article #90193 © 2014 Rocky Mountain Section AAPG Annual Meeting, Denver, Colorado, July 20-22, 2014