Reservoir Characteristics Of The Weyburn Field, Southeastern Saskatchewan

Abstract

Abstract The Weyburn Field is located at the western periphery of the prolific Midale-Steelman oilfield trend along the northeastern flank of the Williston Basin. Hydrocarbon production is from the Mississippian Midale Beds. These beds constitute a small portion of a thick succession of Mississippian cyclic sediments. The Midale Beds dip southward toward the centre of the Basin. Post-Mississippian erosion has truncated the entire Midale section immediately north of the field. Oil is trapped by a combination of structural, diagenetic and stratigraphic elements. Production data indicate a preferential fluid migration that is caused by the presence of a northeast- to southwest-trending fracture system. Variation in the types of diagenesis affecting lithostratigraphic units has led to the formation of different porosity types. As a result of the varying types and areal distribution of the porosity, a significant difference in reservoir characteristics between zones is apparent. Consequently an understanding of reservoir quality and distribution is essential to optimize oil recovery, whether through recompletion, infill drilling or enhanced recovery. Introduction The Weyburn Field of southeastern Saskatchewan, as outlined by Saskatchewan Energy and Mines, covers portions of Twps. 05 to 07, Rges. 11 to 14, W2M. The field is found near the present northwestern edge of the Midale-Steelman trend (Fig. 1)Oil accumulation is in the Midale Beds of the lowermost Mississippian Charles Formation (Madison Group). The discovery well, Central Del Rio No. 14-06-007-13 W2M, was drilled to a dep[h of 1562 m and completed as a Midale Beds oil producer in 1955. Within the designated field boundary, approximately 950 wells have been drilled to date. The greater part of the pool was unitized in 1963 and an inverted nine-spot waterflood scheme was implemented in 1964. At the end of 1986, the Weyburn Unit included 547 producing wells, 117 injectors, 42 miscellaneous wells (water source, pressure observation, temporarily shut in, etc.) and 2B abandoned wells. Geology Regional Setting The Mississippian Madison Group of the Williston Basin contains a 400 m to 700 m thick section of carbonate and evaporite sediments. The sedimentation pattern is typical or a major upward shoaling sequence, deposited as a thick basin ward migrating wedge of sediments. The Madison Group in southeast Saskatchewan has been subdivided into a series of marker-defined stratigraphic units termed "Beds ", as illustrated in figure 2(2,3). Continued subsidence resulted in the southward dipping strata observed at the present. Subsequent post-Mississippian erosion truncated progressively older Mississippian strata northward across southern Saskatchewan, creating the present-day Mississippian subcrop pattern. Weyburn Area The Upper Madison Group stratigraphy and typical log response in the Weyburn area is shown in Figure 3. The Midale Beds comprise the Frobisher Evaporite and the Midale Carbonate. Within the Weyburn Field boundary, the depositional edge of the Frobisher Evaporite is indicated by a lateral change from massive dense anhydrite in the northeast to dolomitic, often argillaceous lime mudstones and wackestones in the southwest(4). The Midale Carbonate in the Weyburn area is subdivided into two distinct lithostratigraphic units, referred to as Vuggy (lower) and Marly (upper) (Fig. 3).