Abstract

Oil has been produced from Mississippian reservoirs in southeastern Saskatchewan since the early 1950s. Many papers have been written and talks presented over the years (Fuzesy, 1960; Kent, 1987; Nimegeers and Nickel, 2005; Marsh, 2006; Saskatchewan Ministry of Energy and Resources, 2006) regarding various aspects of the Mississippian strata (Figure 1) in this part of the province, often focusing on individual beds of the succession. There are also regional reports that discuss the entire Mississippian (Fuzesy, 1960; Gerhard et al., 1991); however, many of these publications were written using sparse well data, which was mostly a result of the limited number of wells that were drilled at the time the reports were written. With the current abundance of wells drilled through the Mississippian in southeastern Saskatchewan, it was possible to prepare updated regional- and pool-scale maps of these strata. This has resulted in the discovery of some interesting—but not surprising—relationships between paleotopography and hydrocarbon accumulation. This presentation will focus on the depositional and structural controls on the morphology of the reservoirs within the Midale Beds in the Pinto pool in southeast Saskatchewan (Figure 2).

The location, shape and orientation of Mississippian oil pools in southeastern Saskatchewan is directly correlated to the paleotopography of the strata in which the oil is found. Accurate knowledge of the paleotopography of Mississippian strata relies on the quality of the data available, as well as the density and distribution of the wells being used to produce paleotopographic maps and models. Detailed mapping of the paleogeography revealed a highly irregular sub-Mesozoic unconformity surface that dips, in a regional sense, to the south-southwest (Figure 3). Irregularities on the unconformity surface are caused by several subparallel and curvilinear, northeast-to-southwest-oriented, topographic highs that are related to deep-seated regional lineaments within this area of the Williston Basin (Holter, 1969; Thomas, 1974; Bell and Babcock, 1986; Brown and Brown, 1987; Bell et al., 1994; Wright et al., 1994; Kreis and Kent, 2000; Li and Morozov, 2007). The Mississippian strata that underlie the sub-Mesozoic unconformity surface generally mimic this paleotopographic surface.

The Midale Beds in the Pinto pool are a shallow-water sequence of carbonate rocks that were deposited on the northeastern flank of the Williston Basin. Hydrocarbon accumulations within the Midale Beds in the Pinto pool are directly related to both depositional facies and paleotopography. Higher oil-cuts and overall better well performance within the Pinto pool are typically found in areas where the Midale Beds are thickest and where they were deposited on the flanks of southwest-northeast-oriented paleotopographic highs (101/08-31-001-04W2/00 and 101/08-25-001-05W2/00), or within the associated lows (101/14-25-001-05W2/00) (Figures 4 and 5). Lowest oil-cuts and/or caprock within the Pinto Midale pool is commonly present atop structural lineaments where the Midale Beds often contain a large amount of anhydrite occluding the porosity (101/04-18-002-04W2/00).

Data from 310 non-horizontal wells in the Pinto pool of southeast Saskatchewan were used to prepare high-resolution maps of the sub-Mesozoic unconformity and the Midale Beds. Using maps produced by the analysis of production, well-log and core data (Figures 6, 7, 8 and 9), this presentation will show the relationship between structurally controlled paleotopography and Midale Beds oil production in the Pinto pool.