Location: T50-60, R8W5-26W5
Year of Study: 1991
In the Edson region of west-central Alberta, the Viking Formation contains lenticular, areally small, but highly prolific gas reservoirs. A 1300 well analysis of the stratigraphy, depositional facies and environments, petrography, hydrodynamics and geophysics of the Viking illustrates the controls on present production, as well as the development of new prospect fairways.
The Viking Formation is an eastward-thickening wedge which reflects shelf to lower shoreface deposition under progradational conditions. However, sea level variations have complicated the internal stratigraphy and at least one major intra-Viking unconformity is present, which formed during a relative sea fall. Subsequently, base level rise triggered sedimentation within estuarine and shoreface environments, and aggradation within major estuarine valleys occurred. Continued transgression caused the erosion of all proximal deposits except those found within the deepest topographic lows.
Estuarine valley fills are the most significant reservoirs in the study area. They contain the coarsest Viking sediments and, although presently buried to depths of approximately 2800 metres, retain significant volumes of primary or modified primary porosity, commonly over 12 percent. In the deep, supernormally pressured portions of the study area, reservoirs exhibit significant volumes of fracture-induced porosity. The major detrital component is quartz and the dominant cement is quartz overgrowths. In shallow, fine-grained marine units located to the northeast, porosity is poorer, and clays and carbonate cements are present. These strata also possess a greater abundance of rock fragments and feldspars.
The Viking Formation displays a progressive increase in pressure to the west and supernormally pressured, gas-charged estuarine channel/valley fill reservoirs are located in the deepest, westernmost portion of the study area. The lenticularity of the estuarine channelized bodies, their encasement within very impermeable rocks, and the high subsurface temperatures in west-central Alberta have all combined to form localized reservoirs capable of sustained high deliverabilities. For example, 6-25-54-21W5M in the Sundance Field has produced 54 BCF of gas since 1978. Further east, formation pressures and potentiometric values decrease until a potentiometric sink is encountered east of Edson. This sink marks the meeting place of fluids escaping from the superpressured areas of the west, and westerly-moving meteoric waters which originated updip to the northeast.
Several new prospects and extensions of existing pools are outlined, mainly in the supernormally- and normally-pressured area, where lenticular estuarine reservoirs are common. Most Viking production in the region is gas with a high liquids component. An interesting oil show is located at 32-57-18W5M, bordering the large potentiometric low found in the east-central portion of the study area. This area is also characterized by shoreface sandstones which may be truncated along depositional strike by channelized bodies. Areally small prolific oil pools hosted in channel fills similar to those found at Crystal may be encountered in the vicinity of the oil show at 32-57-18W5.
The key to future Viking exploration efforts will be the explorer's ability to resolve prospective zones by geophysical means. At present, seismic resolution of productive Viking units has been hampered by the general thinness of reservoirs, combined with an emphasis on deeper targets. However, analysis of template lines across the Sundance and Edson pools indicates that true amplitude processing allows for distinct resolution of gas-charged channel/valley fill sandstones.For more information contact:
Petrel Robertson Consulting Ltd.
500, 736- 8th Avenue S.W.
Phone: (403) 218-1618
Fax: (403) 262-9135