Chin Coulee

Mannville and Jurassic Petroleum Geology of the Chin Coulee Area, South-Central Alberta

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Summary:
Location: T1-10, R10-19W4
Strata: Mannville/Sawtooth
Year of Study: 1998

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Introduction

Jurassic and lower Mannville strata of the Chin Coulee study area have been pursued as exploration targets since the early days of the Canadian petroleum industry, yet still offer significant exploration and development potential for both oil and gas. Existing fields offer excellent play analogues, while a growing infrastructure and abundant land opportunities provide numerous fairways for economically attractive plays. Petrel Robertson’s Chin Coulee study completes our Jurassic and lower Mannville coverage of extreme southern Alberta.

Petrel’s regional study creates a stratigraphic framework for the Chin Coulee area, using well logs, cores, petrography, hydrogeology and seismic data. We recognize a more complex regional stratigraphy than has previously been documented. Shallow marine to shoreline sands of the Sawtooth Formation are cleanest and thickest in the north, on the flank of the Sweetgrass Arch, but extend several townships southward. The overlying Rierdon and Swift formations do not contain reservoir rocks, but they provide an excellent basis for mapping incision on the overlying pre-Mannville unconformity. We have recognized a new unit, the X sand, that caps thick Jurassic successions and forms cuestas on the pre-Mannville surface. Lower Mannville strata, including the basal Cut Bank sandstone, filled valleys on the pre-Mannville unconformity as base level rose in the later stages of the Early Cretaceous. At the peak of the lower Mannville transgression, during Ostracod time, marginal marine to lacustrine conditions engulfed the western portion of the study area. The Moulton sandstone is the product of Ostracod-related shoreline deposition in the Coutts area and into northern Montana. Episodic relative sea level fall and rise saw the incision of two major sets of valleys, which were filled by estuarine (quartzose Glauconitic) and fluvial (lithic Glauconitic) strata.

Although stratigraphic factors are critical to hydrocarbon entrapment in the Chin Coulee area, structural elements played a role syn- and post-depositionally. Reactivation of older block faults along the Sweetgrass Arch, driven largely by western orogenic activity, radically affected depositional and preserved thicknesses of the Sawtooth, and set the structural configurations for traps in several different zones. Small, sharply-bounded grabens with up to 100 metres of relief formed as the result of Mississippian karst solution and/or Devonian salt solution, and are locally important factors in structural trapping of oil and gas.

Hydrocarbon occurrences and potential are classified in terms of nine play types built on the stratigraphic framework summarized above. Stratigraphic mapping and hydrodynamic interpretations demonstrate significant exploration and development opportunities in:

1. Sawtooth stratigraphic plays
2. Sawtooth structural plays
3. X sand stratigraphic plays
4. Lower Mannville Cut Bank Valley edge stratigraphic plays
5. Lower Mannville structural plays
6. Lower Mannville eastern valley plays
7. Moulton stratigraphic plays
8. Quartzose Glauconitic stratigraphic plays
9. Lithic Glauconitic stratigraphic plays

Careful stratigraphic mapping is essential to developing prospects in each of these plays. Seismic is a useful tool in detailing individual prospects, although its applicability is limited in certain play types. Economically attractive prospects and play fairways can be generated incorporating multiple Cretaceous objectives. Explorationists should also consider incorporating deeper (Mississippian and Devonian) objectives into their programs.