Mississippian

An Integrated Geological Geophysical and Hydrodynamic Evaluation of the Mississippian in Northeastern British Columbia

---

Summary:
Location: Blocks S.E.: 87-20W6, N.W.: 94-J-12
Strata: Mississippian (Banff to Stoddart)
Year of Study: 1995

---

Introduction

Three major depositional phases characterize Mississippian sedimentation in northeast British Columbia. The initial phase, following the Antler Orogeny, is a period of progradational basin fill, in which dominantly argillaceous sediments clinoformally fill the developing basin and mask all but the more prominent Antler structures. By the close of Shunda time, an extensive carbonate ramp was established across the basin, initiating the second depositional phase. The Fort Simpson High and the Hay River Fault System are continent-scale basement domains characterized by distinct tectonic grains that are reflected through the overlying Phanerozoic sediments.

The close of the Lower Debolt occurs with a renewal of tectonic activity which causes widespread erosion of the uppermost unit of the Lower Debolt. The Upper Debolt unconformably onlaps the Lower Debolt.

The Upper Debolt has been mapped into parasequences which reflect an initial transgression of the ramp by dominantly subtidal sediments of the distal and medial ramp facies, followed by gradual shallowing into proximal facies. The two youngest sequences contain the major commercial hydrocarbon reservoirs in the area, which were generated where higher-energy shoaling conditions removed the mud fraction to yield predominantly grain-supported sediments.

Two styles of dolomitization occur in the area. The first is a low temperature alteration of limestone by progressive, fabric selective, matrix dolomitization, and a final stage of dissolution, resulting in leached biomoldic vuggy porosity. This dolomitization probably occurred at relatively shallow depths as a result of the Permo-Pennsylvanian truncation of Mississippian sediments. It is one of the principal factors in reservoir development at the Blueberry Field. The second type of dolomitization occurred during burial, and is episodic and multiphase. It was initiated at moderate temperatures, but was progressively modified as fluid temperature and pressure increased. This trend culminated with a hot hydrothermal diagenetic event which cross-cut earlier phases of dolomite and limestone; it may have a negative effect on earlier matrix porosity in older dolomites.

Extensional tectonics initiated a third depositional phase, causing lowered sea level on the ramp, and a deepening basin westward across growth fault scarps. As a result, Stoddart deposition begins with argillaceous slope sediments, into which are injected thin turbidite sheets of reworked carbonate sand and gravel derived from the outer proximal facies of the ramp. As the basin deepened, carbonate tongues decreased in frequency, and eventually were overwhelmed by shale deposition. The basal unit of the Stoddart, therefore, represents the transition from deposition on the Debolt carbonate ramp to the growth faulted sequences of the Stoddart. The Stoddart is dominated by argillaceous sediments with varying proportions of carbonate and more rarely siltstone; sands are not a significant element in this area. Parasequences were be mapped and interpreted in the Stoddart.

In Late Cretaceous time, the contraction of the Laramide Orogeny resulted in propagation of a series of inversion structures of complex geometry, which are typical of thrust styles in inverted extensional basins. Within the main structural culmination, extensional faults accommodating up to 500 metres of sediment are inverted by as much as 900 metres. Fluid expulsion along fault planes as faults reverses their throw is a likely source of dolomitizing fluids. This fits with the episodic and increasingly hot pattern of diagenesis within the Sikanni Culmination.

There are three underexplored play trends in the area, each with a reasonable probability of hosting high-gravity oil.

1. Significant shows of oil occur in the Lower Debolt from Clarke Lake in the north to Halfway River in the south. Most significant is the indication of thick pay sections and drill stem test recoveries of light oil at Adsett, substantiating the role of older basement structures in establishing suitable reservoir facies. A total of 15 gas and 9 oil prospects have been delineated.
2. There is a stratigraphic separation between an Upper Debolt and a basal Stoddart reservoirs. These reservoirs pinch out eastward independently and, therefore, each unit presents an independent series of traps.
3. Oil would have migrated into the updip pinchouts of the Upper Debolt and Stoddart. Continued faulting and uplift separated the reservoirs, either isolating the updip oil column, or creating a pressured system inaccessible to the full impact of the hydrothermal invasion. By integrating structural, stratigraphic and reservoir quality data, twelve Upper Debolt and seven Stoddart prospects have been identified.