Mississippian Madison Formation Low-Gravity Oilfields in Southwestern Saskatchewan: Exam Pies of Unconformity Diagenesis Controlling Reservoir Quality
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Mississippian Madison Formation Low-gravity Oilfields in Southwestern Saskatchewan: Exam pies of Unconformity Diagenesis Controlling Reservoir Quality D.M Kent I and L.K. Kreis Kent, D.M. and Kreis, L.K. (2001): Mississippian Madison Formation low-gravity oilfields in southwestern Saskatchewan: Examples of unconformity diagenesis controlling reservoir quality; in Summary of Investigations 200 I, Volume I, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 2001-4.1. 1. Introduction Saskatchewan. They provide additional information which gives new insights into the geology of the area. The Mississippian rocks of southwestern Saskatchewan, unlike their counterparts on the eastern The localities of interest in this paper all lie within flank of the Williston Basin, have yielded exclusively Townships 1 to 6 and Ranges 22 to 30 west of the low-gravity oil shows in cores, drill cuttings, and Third Meridian. The five producing localities, Battle drillstem tests since the early days of exploration in Creek, Battle Creek West, Divide, Rangeview, and both southeastern Alberta and southwestern Rangeview East have a total of 27 vertical and Saskatchewan. However, the potential of the rocks for directionally drilled wells and six horizontals that have commercial oil production has commonly been produced or are producing low-gravity oil from the overlooked because of the extremely low APJ ( 11 ° to Madison. Twenty-three of the wells are classified as 13°) in Mississippian core and drillstem tests. Twenty suspended or plugged back. The majority of these are four of these oil shows were identified from the area in the area encompassing Rangeview, Rangeview East, bounded by Range 20 west of the Third Meridian, the and Divide. Alberta/Saskatchewan boundary, and Townships l and 15, inclusive (Kent and MacEachem, 1990). The most The rocks of interest lie on the western flank of the impressive was from a dri llstem test of the Coop et al Sweetgrass Arch and have a general regional Battle Creek l-27-3-26W3 well that recovered 1262 m southeasterly dip of about 2.5 m/k.m toward the centre (4140 ft) of 11 ° API oil from Mississippian rocks of the Williston Basin. The Paleozoic strata underlying immediately beneath the sub-Mesozoic unconformity. the area are approximately 870 m thick, consisting of Subsequent tests of the same well yielded mainly 200 to 350 m of Cambrian siliciclastics belonging to water, leading to eventual abandonment of the well. the Deadwood and Earlie formations, and 520 to 670 m However, this discovery identified the Battle Creek of Ordovician, Devonian, and Mississippian rocks that area as having potential for low-gravity oil production, are mainly carbonates. The youngest rocks of the and, in 1966, Western Decalta offset the 1-27 well with carbonate succession belong to the Mississippian two boreholes at 1A-27-3-26W3 and 6-27-3-26W3. Madison Formation. They directly overlie the The former was abandoned shortly after completion, predominantly siliciclastic Devonian/Mississippian but the latter produced oil from the Madison Formation Three Forks Group which comprises (in ascending at the same stratigraphic level as the discovery well for order) the Torquay, Big Valley, and Bakken about one-and-a-half years (1966 to 1968) until a fire formations. at the treater caused abandonment. A third well, Canadian Reserve Decalta 15-22-3-26 W3, produced In much of the study area, the rocks immediately 11 695 m3 (73,562 barrels) of oil from 1972 to 1985 superjacent to the Madison belong to the Middle when it was plugged back to become a Jurassic Jurassic Gravelbourg Formation, a mixed unit of producer. In 1989, Cirque Energy revived Madison carbonates and fine siliciclastics. However, in places, production in the 6-27 well and drilled additional the basal part of the Gravelbourg includes brecciated vertical and horizontal wells at Battle Creek. Kent zones in which the clasts are in either chalky limestone (1995) reported that the horizontal wells significantly or mudrock matrix. These breccias and superjacent improved the productivity of the reservoir. More finely crystalline carbonates locally form a continuous recently, additional discoveries have been made at reservoir with the underlying Madison rocks. The Rangeview, Rangeview East, Divide, and an area remainder of the succession above the Gravelbourg is northwest of Battle Creek herein referred to as Battle dominated by siliciclastics of Jurassic, Cretaceous and Creek West (Figure I). Tertiary ages and is > 1200 m thick. Approximately I 00 new wells have penetrated Locally, the disconformable Mississippian/Jurassic Mississippian strata in the study area since Kent and contact is marked by a pronounced paleotopography Kreis ( 1995) outlined an exploration model for developed on incised or karsted Mississippian strata Mississippian reservoirs in southwestern (Kent and Kreis, 1995). Regionally, an angular I D.M. Kent Consulting Geologist Lid .. 86 Metcalfe Road. Regina. SK S4V OH8 46 Summary ofInvestigations 2001. f'o/ume J 6 + ... 5 •+ • + ,a + 4 ~ CD .Q .. - 3 ct Divide . +++ 2 1 30 29 28 27 26 25 24 23 22 Montana • Well Location Figure 1 - Map ofthe study area showing the distribution ofthe low-gravity oil-producing areas. relationship between Mississippian and Jurassic rocks 2. Descriptive Geology of the Oil-bearing also exists. Kent and Kreis (1995) show a well Strata developed dissected upland on the unconfonnity surface in the southwestern comer of Saskatchewan, a) Madison Formation extending from at least Range 20 west of the Third Meridian to the Alberta boundary. The upland is Kent (1974) correlated the Mississippian rocks of on lapped by progressively younger Jurassic strata. southwestern Saskatchewan with the Madison Group Both Battle Creek and the two Rangeview fields lie on of Montana (Figure 4), but as the contacts between the promontories on the northeast-facing slope of the fonnations comprising that group could not be easily upland (Figure 2). In addition to paleotopographic identified, he suggested that the entire Mississippian relief on the su b-Mesozoic unconfonnity, a succession in southwestern Saskatchewan be monadnock-like feature on the Precambrian erosion recognized as one formation, the Madison, but surface appears to have been rejuvenated during the subdivided it into two marker-defined units: the Laramide Orogeny. It underlies the Battle Creek younger Killdeer Beds and the older Strathallen Beds. region, its influence apparently extending into the The fonner is correlated with the Ratcliffe Beds and Rangeview area. The feature appears as a broad the latter with the Souris Valley, Tilston, Frobisher eastward-plunging anticline (Figure 3), part of which is Alida, and Midale beds of southeastern Saskatchewan identified as the Battle Creek Structure (Sawatzky et (Figure 4). The Killdeer Beds are present only in an at. , 1960; Kent and MacEachem, 1990; Kreis et al ., area east of Range 13 west of the Third Meridian and 2000). The combination of sub-Mesozoic therefore are not present in the study area. paleotopography and structure appears to have played an important role in creating traps for the hydrocarbons The Strathallen Beds extend from the base of the at the Battle Creek, Battle Creek West, Rangeview, Madison Formation to th e sub-Mesozoic unconformity Rangeview East, and Divide fields. and attain a maximum thickness of225 m. This marker-defined unit includes three lithotypes: I) unit A: a basal slightly argillaceous, burrow-mottled, glauconitic, fossi l-bearing lime mudstone; 2) unit B: a laminated lime mudstone with partings of finely Saskatchewan Geological Survey 4 7 6 5 ,a ~ 4 cu .a -<( 3 2 1 30 29 28 27 26 25 24 23 22 Montana Contour Interval=2m + Well Location Figure 2 - lsopach map of Upper Watrous (?) and lower Gravelbourg strata overlying the Madiso11 Formation. The map largely reflects the paleotopography ofthe sub-Mesozoic surface ofun conformity prior to deposition ofthe se strata. disseminated organic carbon emphasizing the outcrops in the Sweetgrass Hills, some 150 km to the interlaminar surfaces; and 3) unit C: a succession of southwest of the study area, where crossbedded crinoidal packstones and grainstones interbedded with crinoidal grainstones vary from 1.25 m thick foreset wackestones and lime mudstones. This latter unit tabular bodies exposed across several metres of subcrops at the sub-Mesozoic unconformity and is host outcrop face, to beds 0. I 5 to 0.3 m thick which are to the low-gravity oil reservoirs at Battle Creek, Battle interbedded with repeated successions of lime Creek West, Rangeview, Rangeview East, and Divide. mudstone and stromatolites. The thickness of unit C is largely a function of the The scarcity of cores in the study area makes difficult depth to which erosion has extended below the sub the determination of the facies relationships within the Mesozoic unconformity and varies from 30 to 160 m, Madison. Evidence from the few cores in the Battle but averages about 130 m. The unit comprises lime Creek area suggests that oil shows are in highly mudstones, skeletal wackestones, and crinoidal micritic rocks, ranging fro m lime mudstones to packstones and grainstones. In places, the former two wackestones and wacke-packstones, immediately lithologies alternate with thin beds of the latter two. In subjacent to the unconformity. other places, the packstones and grainstones of unit C are as much as 30 m thick. Although dominated by The micrite-rich rocks are, for the most part, crinoidal debris, typically disarticulated columnals and unstratified to poorly stratified, but locally include plates, they also contain brachiopod valve fragments, skeletal packstone to grainstone lenses and layers that particles ofbryozoan zooaria, and solitary corals. By range from 3 to 30 cm thick. They are intercalated with contrast, the wackestones and lime mudstones concentrations of thin partings of greyish green and, incorporate a wide variety of organisms including rarely, reddish brown, fissile claystone. Skeletal crinoids, bryozoans, brachiopods, pelecypods, remains are dominantly crinoidal and brachiopodal, ostracods, and corals.