Gladys Rundle Pool: Reservoir Description With Well Test Confirmation

Introduction The Gladys Rundle Pool is located approximately 25 miles southeast of Calgary (Figure 1). The discovery well, Sabine et al Blackie lO-l6-20-27-W4M, was drilled in August 1977 and recovered 1100 metres of 18 ° API oil on a drill stem test, from a depth of 2065 metres, of a zone immediately below the pre Cretaceous unconformity. The well was completed as a Mississippian Rundle oilwell and five additional wells capable of Rundle production have been drilled. (Figure 2). The Mississippian trap mechanism at Gladys is similar to a host of other Mississippian fields on trend to the northwest. Closure against the erosionalunconformity is set up by deep down cutting of the erosional surface in the updip and flank positions with initial deposits overlying lower Cretaceous rock being impermeable shales. The lower Cretaceous sandstone accumulations in the erosional channels postdate the Basal shale deposits resulting in achannel fill deposit encased in imperneable shale. This paper is concerned only with the Rundle (Elkton) producers. Wells completed in this interval are capableof production rates of up to 50 m3/day of sweet 36 to 38 ° API gravity oil. Although there is no evidence of a gas cap accumulation. it is believed that the oil is saturated to a GOR of 150 m3/m3. Water is present in the base of the structure but it is difficult to define a common oil/water contact due to the extreme variability of the reservoir rock properties. Geology General Geological Setting The Gladys Rundle reservoir formed in the updip limits of an outlier or peninsula connected to the regional Mississippian to the west. The area was initially drilled for the Crossfield formation of Devonian Age with the Rundle (Elkton) as a secondary objective. Contours on the Shunda horizon indicate a minor positive feature under the Gladys field, however, the only significant exists in the pool and allows for its division into the ‘reef exterior’ and ‘reef interior’ areas (Figure 2). structural feature is the regional dip of approximately 100 feet/mile to the southwest. Porosity Development The Rundle consists of a series ofshallow water bioclastic carbonates that have been dolomitized. Sediments consisted primarily of lime muds and calcarenites with minor interbedded shales. Loading of porous lime muds resulted in differential movement within the formation. This soft sediment deformation r which may in part have been a dewatering phenomena, created small fractures, dikes and intraformational breccias. Following consolidation, a reduction in volume through pressure solution formed stylolites and freed quartz which was re-deposited as chert. Formation of these features early and their distribution within the section influenced the percolation of leaching solutions at preCretaceous time. The maximum effect of the leaching occurred close to the pre-Cretaceous unconformity and any local barriers to flow. The effect diminished towards the centre and base of the outlier and the lower Elkton shows less permeability enhancement as do thin Elkton sections as encountered in 16-10-20-27. The interplay between early intraformational movement, pressure solution and leaching is summarized in Figure J.