Chin Coulee Field--a Stratigraphic Trap in Lower Cretaceous Fluvial Valley-Fill Sandstones: ABSTRACT

Abstract

Hydrocarbons, stratigraphically trapped in fluvial valley-fill sandstones, constitute some of the more important oil fields in North America. The Chin Coulee field in southeastern Alberta, Canada, has produced over 4.4 million bbl of oil since its discovery in 1960, with an estimated 1 million bbl still to be produced via secondary recovery. The field covers approximately 80 km2 with an average net pay thickness of 3 m. The hydrocarbons are stratigraphically trapped in the fluvial Sunburst Sandstone, which records the Early Cretaceous transgression of the Boreal Sea. These lower Mannville aggradational sequences accumulated in an incised drainage system in Jurassic marine shales, which provide the bottom seal for the reservoir sandstones. Following deposition of the Sunburst Sandstone, an accelerated rise in base level led to the accumulation of argillaceous limestones and calcareous shales commonly referred to as the ostracod zone. These fine-grained strata provide an effective seal above the porous sandstones. Postdepositional tilting of the strata to the west, due to subsidence in the foreland basin east of a rising orogene (Nelson uplift), resulted in migration of hydrocarbons in an easterly updip direction. Stratigraphic trapping of these hydrocarbons occurred in sandstones pinching out in the upper reaches of tributary valleys to the east of the main drainage system (Chin Coulee and Taber Southeast fields). Regional paleoenvironmental reconstructions suggest that the Sunburst Sandstone is equivalent to the Cut Bank Sandstone of the Cut Bank field area in northwestern Montana. The Cut Bank field lies along the same Late Jurassic drainage system (Fox Creek escarpment), and similar tributary valleys situated east of the escarpment may exist between these two producing field areas. This suggests the presence of additional untested hydrocarbon reservoirs exhibiting stratigraphic characteristics similar to those described in the Chin Coulee field. End_of_Article - Last_Page 300------------