Lithofacies Architecture of the Milk River Formation
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Lithofacies Architecture of the Milk River Formation (Alderson Member of the Lea Park Formation), Southwestern Saskatchewan and Southeastern Alberta - Its Relation to Gas Accumulation Jennie l. Ridgley I Ridgley, J.L. (2000): Lithofacies architecture of the Milk River Formation (Alderson Member of the Lea Park _For!11ation), southwestern Saskatchewan and southeastern Alberta~ its relation to gas accumulation; in Summary of lnvest1gat1ons 2000, Volume I, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 2000-4.1. l. Introduction estimate was in hypothetical plays, and of this percentage, over one-half was based primarily on Large accumulations ofbiogenic gas are known to similarity of the potentially productive facies in occur in shallow (< 1000 m) Cretaceous reservoirs of Montana to the facies that host large shallow biogenic the Northern Great Plains (mainly Montana) and gas resources in southeastern Alberta and southwestern southeastern Alberta and southwestern Saskatchewan, Saskatchewan. The controls on the hypothetical play Canada. The 1995 U.S. Geological Survey national oil boundaries were poorly understood in 1995. and gas assessment made a mean estimate of 1.15 to Sedimentologic, structural, and hydrologic studies of 12 3 1.18 x 10 m (41 to 42 trillion cubic feet) of potential the important shallow gas-producing formations in additions to reserves of continuous-type southwestern Saskatchewan and southeastern Alberta (unconventional) gas in Cretaceous shallow biogenic have been undertaken in order to better understand the gas plays of northern and central Montana (Rice and relative importance of each of these parameters as a Spencer, 1995). About 90 percent of the 1995 gas control on the site of the shallow gas accumulation. The objective of this research is to apply the Canadian model to 0 0 11~3°_____ 1...,.12_ _____11,...1 °_____ 1.,..1 0_ ____1....,0_9 · _____10,..a_ " -~ Montana where gas might be expected to occur. W.4 M W.JM R.20 R.15 R.10 R.5 R.1 R.28 R.25 The Milk River Formation is one R.20 of the principal formations that yields shallow gas in the large . T25 southeast Alberta gas field 51 • (Alberta and Saskatchewan) (Figure I). Gas in the Milk River occurs in interbedded, very fine T.20 grained sandstone and shale reservoirs. In contrast, shallow gas production in the correlative Eagle Sandstone in Montana is T.15 from fine- to medium-grained sandstone reservoirs. Gas in both 50" the Milk River and Eagle T.1 0 Sandstone is of biogenic origin. Whereas gas in the Eagle Sandstone reservoirs has been assessed as a conventional gas T5 accumulation, gas in the Milk 0 10 20 MILES River Formation would be 20 40 KM considered a type of Tl unconventional gas accumulation 49° that fits the "continuous-type" gas accumulation category first defined and used in the 1995 Figure I - Map showing location of core holes and cross-section A-A' and B-B ', U.S.G.S. national assessment depositional extent of Virgelle Sandstone (hachured line), Writing-on-Stone Provincial Park, and location of southeast Alberta gas field (Alberta and Saskatchewan). Type (Schmoker, 1995). Continuous Alderson section ARCO Alderson wel/ 10-J.f-15-IOW.f is shown as X. type accumulations are 'US Geological Survey, Denver. CO 80225. /06 Summary of Investigations 2000. Volume 1 considered to be large, low-grade, non-conventionally 2. Regional Stratigraphy reservoired (i.e. not bounded by a downdip water contact) gas concentrations that crosscut lithologic The Santonian-Campanian Milk River Formation in boundaries on a regional scale. Where they occur, Canada and its regionally correlative Eagle Sandstone continuous-type gas accumulations are thought to be in Montana (Figure 2) form the basal part of the mostly gas-charged everywhere in the gas-bearing Montana Group. The Milk River is composed of three interval, although the gas may not always be economic. members, from base to top, the Telegraph Creek Other characteristics of continuous-type gas Member, the Virgelle Member, and the Deadhorse accumulations include an updip water contact, the Coulee Member. The Virgelle is often informally insignificance of conventional traps and seals, a close subdivided into a lower and upper part; the boundary is association of reservoir with source rock, and low placed where a pronounced change in depositional reservoir permeability in facies that have large areal environment is evident. In Montana, the correlative extent. Application of the concepts of continuous-type Eagle Sandstone is subdivided into the Virgelle gas accumulations to the area of the Northern Great Member and unnamed rock units of alluvial origin. Plains (Montana) and southeastern Alberta and The Telegraph Creek is placed stratigraphically below southwestern Saskatchewan indicates that gas the Eagle. In both Canada and north-central Montana, resources should be present over an area greater than the base of the Telegraph Creek is placed at the top of that in which this commodity is currently being the Niobrara Formation of the Colorado Group. The exploited. This report presents preliminary correlations Niobrara Formation is a calcareous, coccolithic-bearing of lithofacies along two strike-oriented transects shale and thus, in core, the top of the formation is an (Figure 1) within the Milk River depositional system excellent marker horizon separating rocks of the and examines the relative control of lithofacies on sites Montana Group from those of the Colorado Group. of gas accumulation. This summary represents work in The Milk River is overlain by the Pakowki Formation progress on the sequence stratigraphic characterization and the Eagle Sandstone is overlain by the Claggett of the Milk River Formation. Shale (Figure 2). The Ardmore bentonite beds, found in the lower part of the Pakowki and Claggett, provide an excellent upper marker unit. Thus, the two lithologic markers can be used to provide boundaries to the Milk River Formation depositional system. ln Montana, rocks assigned to the Virgel\e Member are, in part, North-central Southeastern Alberta and older than those assigned to the Montana Southwestern Saskatchewan Virgelle in Canada. The Milk River and Eagle have generally Claggett Shale Pakowki Formation been considered to represent a mainly progradational, wave x x Ardmore xx x x x Bentonite x x Beds x dominated depositional system within the Cretaceous Interior seaway of North America. The a. Dead horse Virgelle in Canada represents the ::i Coulee later part of the main seaward 0 Q) ..... c: Member stepping progradational event and {.!} 0 c: ... c: is correlative to the wedge of I'll vi 0 Q) 0 c: "O .0 -~ marine sandstone in the middle I'll c: -~ Virgelle I'll E part of the Eagle (Gill and c: Q) ...__E - CfJ § Member 0 :E 0 Cobban, 1973, p 16). :E Cl) LL LL O> ...__ c: I'll Cl) 0 ~ LU > ~ I'll Cl) a.. a: "O 3. Previous Studies ~ I'll <( Cl) Telegraph ....J The principal study area for the Creek current investigation is between Member longitude I 08° and 113° and Telegraph Creek latitude 49° and 51 ° in Formation southwestern Saskatchewan and southeastern Alberta. Within this area, only a few detailed facies Niobrara Niobrara architecture studies of the Milk Formation Formation River Formation have been made (limited to Alberta). Outcrops of Figure~ - Stratigraphic chart showing correlation ofmembers ofthe Milk River the Milk River Formation are Formation, Altlen·on Member ofthe Lea Park Formation, Saskatchewan am/ Alberta, confined to the vicinity of the Canada, and members of the Eagle Sandstone, Montana, U.S.A. Writing-on-Stone Provincial Saskatchewan GeuloKicuf Survey /07 Park, Alberta, (Figure J) where the Milk River has contrast response on wireline logs due to the shaly dissected outcrops of this rock unit, providing excellent nature of the rocks throughout much of the three-dimensional exposures for stratigraphic and depositional system. Detailed observations of selected sedimentologic analysis. Three detailed studies of these cores, logged at a scale of0.3 m (one ft), from exposures have resulted in three slightly different Saskatchewan and Alberta have been used to calibrate interpretations of the origins of the formation wire line log responses to depositional facies. Principal (McCrory and Walker, 1986; Chee! and Leckie, 1990; observations included sedimentary structures, Meyer et al., 1998). Each of the studies interpreted the lithologies, trace fossil and macrofossil assemblages, Telegraph Creek to have been deposited in the and major diagenetic changes (mostly the presence of offshore-transition zone below shore face sands of the siderite and glauconite). Preliminary results of the lower part of the Virgelle Member. All three studies detailed description of the Milk River are presented in subdivided the Virgelle Member into an upper and two strike-oriented cross-sections through the main lower part based on a distinct paleodepositional part of the Milk River gas field (Figures 3 and 4 ). The environment break. The lower part of the Virgel le was detailed observations permit lateral correlation of interpreted to have been deposited primarily as lower lithofacies and of significant erosion surfaces between shoreface to middle shoreface sands along a wave wells, and serve as a basis for evaluation of gas dominated coastline. The upper part was interpreted to productive facies. The datum used in the cross-sections have been deposited in a complex tidal setting. The is a prominent bentonite bed in the lower part of the models for the tidal processes vary somewhat between Pakowki Formation. the three reports. The tidal complex models will not be reviewed here because the link between these outcrop facies and the subsurface facies in the distal offshore 5. Lithofacies Architecture has not been completed. All three studies of the Milk River outcrops interpret the Deadhorse Coulee Member Seven distinct lithofacies and their preliminary inferred (Figure 2) to have been deposited in fluvial-alluvial depositional environments are summarized below and plain environments.