Stratigraphic Relationship of Alderson (Milk River) Strata Between the Hatton and Abbey-Lacadena Pools, Southwestern Saskatchewan – Preliminary Observations
Per Kent Pedersen
Pedersen, P.K. (2003): Stratigraphic relationship of Alderson (Milk River) strata between the Hatton and Abbey-Lacadena pools, southwestern Saskatchewan – preliminary observations; in Summary of Investigations 2003, Volume 1, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2003-4.1, CD-ROM, Paper A-11, 11p.
Abstract Large quantities of economically recoverable gas have recently been discovered within the Alderson Member of the Lea Park Formation in the Abbey-Lacadena pools. This occurrence, located approximately 50 km northeast of the prolific Hatton Pool, has sparked development within the area as well as exploration for similar plays elsewhere in southwestern Saskatchewan. Strata of the Alderson Member are commonly identified by industry and government geologists as belonging to the Milk River Formation. Recent research has indicated, however, that most of the Alderson strata are younger than strata of the Milk River Formation. Within the Abbey-Lacadena pools, the Alderson Member is composed of stacked, subtly upward-coarsening parasequences. Gas is hosted in thin, very fine-grained sandstone beds interbedded with silty mudstones. The sands likely were deposited in a storm- dominated, pro-delta setting below fair-weather wave base. The main productive sandstone is a composite lowstand unit, which both onlaps, and is overlain by, mudstones. Structural maps of the top of the Alderson Member (Milk River “shoulder”) show the Abbey-Lacadena area as a structural high. Thus, initial drilling indicates prolific gas production within the Abbey-Lacadena pools is closely related to the concurrence of a structural high and lowstand sandstones. Ongoing exploration in the surrounding area will reveal if gas is trapped within a conventional pool related to stratigraphy and structure, or if gas occurs in a more regionally extensive unconventional accumulation, possibly with the Abbey-Lacadena area being a prolific “sweet spot”. The latter would indicate the possible influence of other gas trapping mechanisms, such as within downdip fine-grained facies due to high capillary pressure.
Keywords: Alderson Member, Milk River Formation, southwestern Saskatchewan, biogenic gas, gas potential, shallow low-permeability sandstones, Cretaceous, Abbey-Lacadena pools, Hatton Pool.
1. Introduction Strata of the Santonian-Campanian Alderson Member (commonly referred to as Milk River by industry and government geologists; Figure 1) contain large quantities of biogenic gas hosted within shallow sandstone reservoirs in southwestern Saskatchewan and southeastern Alberta (Shurr and Ridgley, 2002). In Saskatchewan, Alderson production has mainly been within the large Hatton Pool, where gas is often co-produced from sandstones of the Medicine Hat Member (Niobrara Formation). The discovery in late 1999 of an estimated 14 billion m3 (0.5 TCF) producible gas within the Abbey-Lacadena pools, located approximately 50 km northeast of the Hatton Pool (Figure 2), has sparked exploitation in the area to delineate the pools and exploration for similar plays in southwestern Saskatchewan. The reservoir sandstones of the Abbey-Lacadena pools occur eastward of the previously mapped, northwest-southeast-trending, eastern depositional edge of Alderson sandstones (Meijer-Drees and Mhyr, 1981; Payenberg, 2002b). These sandstones are here interpreted as detached lowstand deposits. The basinward occurrence of economically productive gas from Alderson sandstones within the Abbey-Lacadena pools indicates that reservoir sandstones are present eastward of traditionally explored areas within southwestern Saskatchewan. The objective of this ongoing study is to achieve a better understanding of sedimentary facies, depositional environments, and sequence stratigraphic architecture of the Alderson Member and, therefore, of the distribution of reservoir sandstones containing economically productive gas. Integration of the stratigraphic architecture with structural maps will indicate whether the trapping mechanism is conventional or unconventional. Numerous cores of the First White Speckled Shale, Alderson Member, and Lea Park Formation within southwestern Saskatchewan have been described. Sedimentary facies and stratigraphically significant surfaces observed in cores were identified on core gamma-ray logs which were correlated with other well logs in order to map these features regionally.
Saskatchewan Geological Survey 1 Summary of Investigations 2003, Volume 1 2. Stratigraphy In southwestern Saskatchewan and southeastern Alberta, distal, fine-grained strata located northeast of the pinchout of shoreface sandstones of the Virgelle Member of the Milk River Formation were defined as the Alderson Member of the Lea Park Formation by Meijer-Drees and Mhyr (1981). The Alderson Member includes strata previously referred to as Milk River equivalent, and which often continue to be referred to as Milk River by industry and government geologists. Silty shales and sandstones of the Alderson Member conformably overlie the white speckled, dark shales of the First White Speckled Shale Member of the Niobrara Formation (Nielsen, 2003). O’Connell et al. (1999) proposed the presence of a major unconformity within the lower portion of the Alderson Member, which truncates Milk River strata to the west and which is onlapped by strata of the middle and upper portion of the Alderson Member Figure 1 - Stratigraphic chart of Santonian-Campanian lithostratigraphic units in (Figure 1). Accordingly, except southwestern Saskatchewan and correlative stratigraphic units in nearby areas. for a thin lower shaly portion Stratigraphic position of an unconformity between the Milk River Formation and representing the distal deposit of Alderson Member is controversial, as indicated by the question marks; for detailed the eastward prograding Milk discussion see Shurr and Ridgley (2002). River clastic wedge, the main thickness of the fine-grained deposits of the Alderson Member in Saskatchewan are younger than strata of the Milk River Formation in Alberta. The interpretation of a diachronous relationship between the Milk River Formation and the middle and upper part of the Alderson Member is also supported by Ridgley (2000), O’Connell (2001), and Payenberg (2002a, 2002b). Shurr and Ridgley (2002), however, proposed that the lower Alderson unconformity might instead correlate with the boundary between the Virgelle and Deadhorse Coulee members of the Milk River Formation (Figure 1). Thus, more research is needed on the stratigraphic relationship between the Milk River Formation and the Alderson Member, and is beyond the scope of this paper.
The thickness of the Alderson Member increases toward the northeast from 130 m within the eastern Hatton Pool to more than 180 m within the Abbey-Lacadena pools (Figure 3). An interesting feature of the cross-section (Figure 3) is the near parallel arrangement of the top of the Carlile Formation and the top of the Alderson Member. This suggests that the northeastward increase in thickness of the Alderson Member, at least partly, reflects infilling of accommodation space that had not been filled by the eastward prograding clastic wedge of the Medicine Hat Member. The top of the Alderson Member is distinct on resistivity logs, and is referred to as the Milk River “shoulder” (Figure 3). In southwestern Saskatchewan, Alderson strata are unconformably overlain by a lag of chert pebbles in a matrix of Pakowki mudstones (observed in cores in wells 6-27-1-23W3 and 10-32-1-24W3). This relationship shows that the Pakowki transgression, and its geophysical log expression as the Milk River “shoulder”, reflect erosion and winnowing as far east as southwestern Saskatchewan. In southeastern Alberta, a similar transgressive pebble lag marks the boundary between the Alderson Member and shales of the Pakowki Formation (Meijer-Drees and Mhyr, 1981; O’Connell, 2001; Payenberg, 2002a, 2002b). In southeastern Saskatchewan, ongoing research suggests that the Milk River “shoulder” reflects a major unconformity related to substantial sea- level fall (Christopher and Yurkowski, this volume). Shales of the Pakowki Formation hence unconformably overlie the Alderson Member. The approximately two million year hiatus between the Milk River and Pakowki formations in southern Alberta (O’Connell, 2001; Payenberg, 2002a, 2002b) is thus, in Saskatchewan, encompassed by deposition of the middle and upper portions of the Alderson Member and the hiatus between the Alderson Member and Pakowki Formation (Figure 1).
Saskatchewan Geological Survey 2 Summary of Investigations 2003, Volume 1 Figure 2 - Structural map of the Milk River “shoulder” (top of the Alderson Member and Milk River Formation) in southwestern Saskatchewan. Note the concurrence of structural contours and pool boundaries. Location of the well-log cross- section of Figure 3 is shown. Pool boundaries are as defined as of January 2003. Contours in metres above mean sea level.
3. Facies and Depositional Environments Previous studies of the Alderson Member in southwestern Saskatchewan and southeastern Alberta interpreted the fine-grained deposits as having a marine, shoreface, shelf to pro-delta origin (Meijer-Dress and Mhyr, 1981; O’Connell et al., 1999; Ridgley, 2000; O’Connell, 2001; Payenberg, 2002a, 2002b). Initial examination of Alderson cores within the eastern portion of the Hatton Pool and Abbey-Lacadena pools reveals a succession of silty shales interbedded with a vertically and laterally variable content of very fine-grained and minor fine-grained sandstone beds. A high content of swelling clay, montmorillonite (Meijer-Dress and Mhyr, 1981), gives the rocks a muddy impression when wet, and hampers observation of sedimentary structures.
The Alderson succession consists of 2 to 20 m thick parasequences characterized by upward increase of very fine- grained sandstone beds (Figure 4). Three sedimentary facies (A, B, and C) are recognized within these
Saskatchewan Geological Survey 3 Summary of Investigations 2003, Volume 1 FWS, First White d Abbey-Lacadena pools. Within the pools. d Abbey-Lacadena uction from Husky Abbey 11-24-21-19W3 ection is shown in Figure 2. ection ic units between the Hatton an the Hatton between ic units “shoulder”). Location of cross-s “shoulder”). Location of is a lowstand systems tract. Initial prod Initial tract. lowstand systems is a showing correlation of stratigraph showing correlation of of the main reservoir unit, indicating it indicating reservoir unit, the main of neutron; and SGMA, short space sigma log. neutron; and SGMA, short space sigma d (558 Mcfd). Datum is the top of the Alderson Member (Milk River Member (Milk River the Alderson top of the is Datum (558 Mcfd). d 3 Lacadena Pool, note the southeastward onlap the note Pool, Lacadena m 271 averaged 15 Figure 3 - Southwest-northeast oriented well-log cross-section Specks; GR, gamma ray; Res, resistivity; N,
Saskatchewan Geological Survey 4 Summary of Investigations 2003, Volume 1 rface bounding the lowstand systems rface bounding the Abbey-Lacadena pools. The main Abbey-Lacadena undary and flooding su undary and flooding een the eastern Hatton Pool and Pool eastern Hatton the een ss sandytract. lowstand systems FWS, First White Specks; SB, sequence g of the sequence bo ting the facies relationship betw relationship facies ting the 6-5-18-26W3. Note also the younger, le systemsthe tract. Note westward mergin t space sigma log; and Res, resistivity. log; t space sigma overlying this surface in this surface overlying sandstone unit in 11-24-21-19W3 is a lowstand sandstone unit in 11-24-21-19W3 is a Figure 4 - Core and geophysical well-log cross-section illustra cross-section well-log and geophysical Figure 4 - Core tract, and the pebble lag tract, and the pebble boundary; FS, flooding surface; SGMA, shor
Saskatchewan Geological Survey 5 Summary of Investigations 2003, Volume 1 parasequences. Within all three facies, dispersed plant and wood fragments are abundant, locally concentrated in thin beds, and siderite concretions and calcite-cemented intervals are common. Abundant ammonites, inoceramids, gastropods, and foraminifera together with a diverse trace fossil assemblage indicate deposition in an open marine environment. Facies A is dominated by mudstones and consists of silty shales interbedded with 1 to 10 mm thick siltstone and very fine-grained sandstone beds. Siltstone and sandstone beds are usually structureless, but several are normally graded. Bioturbation is moderate to intense and dominated by Helminthopsis, Pilichnus dichotomus, and small horizontal and sub-horizontal simple tubes filled with black mud or pyrite. Facies B consists of silty shale interbedded with 2 to 20 mm thick siltstone and very fine-grained sandstone beds (Figure 5B). The very fine-grained sandstone beds are well sorted, have fair to good porosity (Figure 5F), and comprise 20 to 50 percent of the succession. The sandstone beds are horizontally and undulatory parallel bedded, and locally cross-laminated (Figures 5C, 5D, and 5E). Bioturbation is moderate to rare and dominated by sand- and mud-filled horizontal and subhorizontal burrows as in Facies A, but with some vertical Skolithos and locally abundant Chondrites. Facies C is sandstone dominated, with mudstones constituting less than 30 percent of the facies. Primary sedimentary structures are commonly obscured by intense bioturbation (Figure 5A). Observable sedimentary structures consist of ripple lamination, small-scale hummocky cross-stratification, combined flow ripples, planar lamination, wave ripples, and graded beds. The trace fossil assemblage is diverse and includes both horizontal and vertical traces of Planolites, Skolithos, Rosselia, Terebellina, Teichichnus, Zoophycos, Pilichnus dichotomus, and Helminthopsis. Locally, this facies has a moderate to high content of glauconite (Figure 6C), in places concentrated within Skolithos burrows.
All facies indicate deposition occurred within an open-marine, storm-influenced environment below fair-weather wave base. The change in bedforms from Facies A, through Facies B to Facies C reflects an upward increase in depositional energy at the seafloor. Facies A is dominated by suspension sedimentation, whereas Facies C is dominated by traction deposition from both oscillatory and unidirectional currents. The three facies are interpreted as deposited in upward-shallowing cycles; mudstones of Facies A deposited in a shelf setting, Facies B in a lower offshore setting, and the sandy deposits of Facies C in an upper offshore setting. The change in ichnological assemblage from a Zoophycos ichnofacies in Facies A to a mainly Cruziana ichnofacies within Facies C also indicates a change from a shelf to upper offshore depositional environment. The diversity of ichnofacies and macrofossils indicate that the bottom waters were well oxygenated and had normal salinity.
Payenberg (2002a) suggested that the silty character of the mudstones indicates proximity to a deltaic or tidal coastline, also consistent with the high content of plant and wood debris, and presence of contorted beds. Both tidal and deltaic shorelines are commonly muddy, which might explain the absence of sandy shoreline deposits associated with the apparent prodeltaic or subtidal deposits of the Alderson Member. Correlative deposits of the Alderson Member in northern Montana, the Eagle Sandstone, have been interpreted as deposited within a large delta (Rice, 1980; Payenberg, 2002b).
Within strata of the Alderson Member in southwestern Saskatchewan, pebble lags centimetres to tens of centimetres thick occur, mainly composed of intra-formational pebbles of sub-rounded, concretion fragments, but locally also with chert and quartz pebbles (Figure 6D). The majority of these pebble lags unconformably overlie upward- coarsening cycles and are overlain by shaly deposits, although locally they are overlain by more sandy deposits (450.70 m in 6-5-18-26W3; Figure 4). Several of the surfaces overlain by pebble lags are also demarcated by Glossifungites ichnofacies (Figures 6B and 6C). These pebble lags are interpreted as transgressive lags, indicating significant erosion of previous deposited strata and concentration of early formed concretions in lags. However, lags containing chert and quartz pebbles suggest these beds were genetically associated with sea-level falls and basinward shift of facies (e.g. lag overlying sequence boundary at 450.70 m in 6-5-18-26W3; Figure 4 and Figure 6D).
4. Sequence Stratigraphic Framework The sedimentary facies described above form numerous 2 to 20 m thick upward-coarsening successions or parasequences. Two or more of these parasequences form progradational, aggradational, and retrogradational parasequence sets bounded by major flooding surfaces. These major flooding surfaces can generally be correlated regionally, whereas the flooding surfaces bounding individual parasequences commonly can be correlated only locally (Figure 3). Preliminary correlations of flooding surfaces, major flooding surfaces, and log markers (mainly bentonite beds) identified in cores and interpreted on well-log cross-sections between the Hatton and Abbey- Lacadena pools reveal a distinct vertical change in stratigraphic architecture within the Alderson Member.
Saskatchewan Geological Survey 6 Summary of Investigations 2003, Volume 1 Figure 5 - Core photographs of Alderson Member sedimentary facies. A) Intensively bioturbated muddy sandstones of Facies C, mainly Pilichnus dichotomus and Helminthopsis (294 m in 11-24- 21-19W3). B) Reservoir interval of interbedded, graded, very fine-grained sandstone and silty shale beds of Facies B (451.30 m in 6-5- 18-26W3). C) Ripple-laminated very fine-grained sandstone bed of Facies B (332.75 m in 11-24-21-19W3). D) Small-scale hummocky cross-stratified sandstone bed; note abundance of plant debris and Terebellina within the sandstone bed (415 m in 6-5-18-26W3). E) Lenticular, current-rippled sandstone bed of Facies B (333.75 m in 11-24-21-19W3). F) Thin-section of ripple-laminated sandstone beds (Facies B) within lowstand reservoir interval (lowstand systems tract I); note good intergranular porosity, abundant plant debris, and intense bioturbation of the mudstones (335.95 m in 11-24-21- 19W3).
Saskatchewan Geological Survey 7 Summary of Investigations 2003, Volume 1 Figure 6 - Core photographs of stratigraphically significant surfaces within the Alderson Member. A) Calcite-cemented, fine-grained sandstone bed overlying a flooding surface within lowstand systems tract I in the Abbey- Lacadena pools; rounded black fragment is possibly a bone (331.65 m in 11-24-21-19W3). B) Glossifungites demarcated transgressive surface; note Zoophycos and abundant Chondrites (295.55 m in 11-24-21-19W3). C) Glossifungites demarcated transgressive surface; note siderite rim of the deep, mud-filled burrow, and the glauconitic character of the sandstone (Facies C, 404 m in 6-5-18-26W3). D) Transgressively reworked sequence boundary overlain by lag of large, sub-rounded clasts of intra-formational siderite concretions; note millimetre-sized sub-rounded to angular concretion fragments, chert grains, and large glauconite pellets (450.70 m in 6-5-18-26W3).
Saskatchewan Geological Survey 8 Summary of Investigations 2003, Volume 1 The lowermost part of the Alderson Member is characterized by gently northeastward-dipping flooding surfaces, with individual parasequences characterized by a gradual northeastward decrease in thickness and sandstone content (Figure 3). a) Lowstand Systems Tract I The overlying stratigraphic unit, forming the main sandstone reservoir interval (322 to 333 m below KB) within the Abbey-Lacadena pools, thins and pinches out to the southwest (Figure 3 and Figure 4). In the vicinity of the pinchout, well logs show this stratigraphic unit to have a sharp base (e.g. 10-36-19-23W3; Figure 3 and Figure 4), whereas the lower boundary is gradational farther to the northeast (e.g. 11-24-21-19W3; Figure 3 and Figure 4). The stratigraphic unit is interpreted as a lowstand systems tract that overlies an erosional sequence boundary which grades eastward into a correlative conformity (Figure 3 and Figure 4). In the 11-24-21-19W3 well, a transgressive lag of 1 to 2 cm diameter, sub-angular clasts of intraformational concretion fragments in a matrix of glauconitic, fine-grained sandstone overlies the flooding surface capping the lowstand reservoir sandstones (Figure 3 and Figure 4). This shows the transgression was associated with erosion and winnowing of the uppermost portion of the lowstand deposits. Southwestward of the pinchout, within the Hatton Pool, the correlative flooding surface is overlain by a 10 cm thick transgressive pebble lag of subangular concretion clasts and chert pebbles in a matrix of glauconitic, muddy fine- grained sandstone (e.g. 6-5-18-26W3; Figure 4 and Figure 6D). This surface is, therefore, interpreted as a transgressively reworked sequence boundary, which was characterized by sedimentary by-pass during sea-level lowstand, and subsequently subjected to erosion and winnowing during the following transgression. This might explain the absence of lowstand coastal deposits, bearing in mind that the lowstand deposits in the Abbey-Lacadena pools consist of interbedded thin sandstones and mudstones deposited below fair-weather wave base (Figure 5F).
The lowstand reservoir sandstones are encased by older highstand mudstones below, and younger transgressive and highstand mudstones above (Figure 3 and Figure 4). The lowstand systems tract is composed of at least three parasequences (Figure 4), showing that it is a composite lowstand systems tract. This lowstand event might be related to an erosion-based succession of coastal and possibly fluvial sandstones in the Cypress Hills area of southwestern Saskatchewan described by Ridgley (2000). Further research will address the nature of these surfaces and the compartmentalization of reservoir sandstones within the Alderson Member. b) Lowstand Systems Tract II Within the overlying succession, parasequences are characterized by a gradual northeastward decrease in sandstone content. Within the Abbey-Lacadena pools, a younger composite lowstand systems tract is indicated by westward onlap of several parasequences and by westward pinchout of the lowstand unit (Figure 3 and Figure 4). Within the Hatton Pool, the flooding surface forming the upper boundary of this lowstand unit is correlative with a transgressive lag of fine-grained sandstone and claystone clasts overlying a transgressively reworked sequence boundary (Figure 4). Significant amounts of economically producible gas have been tested within lowstand systems tract II, although the total sandstone thickness is not as thick as in lowstand systems tract I. The upper third of the Alderson Member is dominated by mudstones, and no reservoir sandstones have yet been discovered within the Abbey-Lacadena area.
5. Controls on Gas Accumulation Evaluation of productivity of sandstone reservoirs containing economically recoverable gas, and hence of controls on gas accumulations, is hampered by factors including: perforation of several sandstone intervals, commingled production of gas from Medicine Hat and Alderson strata within the Hatton Pool, and difficulty in detecting gas on well logs. The trapping mechanism of gas in the shallow, tight sandstones of the Milk River/Alderson gas pools is still controversial. Based on a study of Milk River and Alderson strata mainly in southeastern Alberta, O’Connell et al. (1999) interpreted a stratigraphic trapping mechanism for the gas in the Alderson Member. They related trapping of gas to onlap of Alderson sandstones onto an unconformity separating Milk River and Alderson strata. Ridgley (2000) interpreted the Alderson gas pools as an unconventional, continuous-type gas accumulation. Unconventional gas accumulations are typically laterally extensive, occur downdip from water, and are not bounded by a downdip water contact. Conventional traps and seals are considered unimportant for unconventional gas accumulation, although structures might create sweet spots within larger accumulations (Schmoker, 1995; Ridgley, 2000; Shurr and Ridgley, 2002). Evaluation of trapping mechanisms is further complicated by the fact that the biogenic gas was generated shortly after deposition and during the following tens of million years (Fishman et al., 2001). Consequently, the current gas distribution and potential trapping mechanisms need to be understood in relation to the structural and hydrodynamic history of the region.
Saskatchewan Geological Survey 9 Summary of Investigations 2003, Volume 1 Based on drilling to date in the Abbey-Lacadena area, some preliminary interpretations and conclusions can be drawn, although they will likely be somewhat modified as more information becomes available. The presence of economically recoverable gas within the Abbey-Lacadena pools is dependent on the presence of lowstand reservoir sandstone beds, the millimetre- to centimetre-thick siltstone and very fine-grained sandstone beds described above. These lowstand sandstone units are encased in mudstones, suggesting that gas might be trapped stratigraphically. Presently defined pool boundaries of the Abbey-Lacadena pools seem to follow structural contours (Figure 2), however, but are likely to be modified as the margins of economically producible gas become better known. Likewise, structural contours and pool boundaries of the Hatton Pool also seem to have similar orientations, especially along the eastern margin, and, to a lesser degree, along the northern and southeastern margins (Figure 2). The southern boundary of the Hatton Pool might be partly controlled by small-offset faults related to the Medicine Hat Syncline (Nielsen, 2003). Thus, structures may be interpreted to influence the distribution of economically viable gas accumulations. Stratigraphic data from test holes, listed in Saskatchewan Energy and Mines (1990), will be integrated to improve structural control where data from oil and gas wells are poor. Trapping of gas within the Hatton Field has alternatively been attributed to updip water (Berkenpas, 1991) or to gas trapped within downdip fine-grained facies as a result of high capillary pressure (Shurr and Ridgley, 2002). Areas of current prolific gas production within the Abbey-Lacadena pools clearly indicate a strong stratigraphic and structural influence. Ongoing exploration and improved definition of the boundaries of economically producible gas will reveal if the area of current economic production simply represents a prolific “sweet spot” within a larger unconventional gas pool.
6. Recommendations Independent of trapping mechanisms, the prolific gas production from thin sandstone beds (less than 1 cm thick) in the Abbey-Lacadena pools warrants a re-evaluation of which facies can produce economic amounts of gas. This thin-bedded sandstone reservoir facies (Facies B) also exists within the Hatton Pool, where perforations are commonly limited to the sandiest intervals in the upper offshore deposits of Facies C and in lower shoreface deposits. Re-evaluation of productive intervals may result in the use of higher gamma-ray and lower resistivity cut- off values to define pay zones and perforation intervals. Within existing pools, gas reserves are probably higher than present estimates as thicknesses of already perforated reservoir sandstone intervals may have been underestimated due to the likely existence of additional reservoir sandstone intervals. Outside existing pools, re-evaluation of cut- off values warrants the remapping of potential reservoir sandstones and the calculation of total sandstone reservoir thickness within southwestern Saskatchewan.
7. Acknowledgments Early drafts of this paper were greatly improved by comments on an earlier version by Jennie Ridgley, Karsten S. Nielsen, and Steve Whittaker.
8. References Berkenpas, P.G. (1991): The Milk River shallow gas pool: Role of updip water trap and connate water in gas production from the pool; Proceedings of the 1991 Society of Petroleum Engineers Annual Technical Conference and Exhibition v66, p371-380. Fishman, N.F., Ridgley, J.L., and Hall, D.L. (2001): Timing of gas generation in the Cretaceous Milk River Formation, southeastern Alberta and southwestern Saskatchewan – evidence from authigenic carbonates; in Summary of Investigations 2001, Volume 1, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 2001-4.1, p125-136. Meijer-Drees, N.C. and Mhyr, D.W. (1981): The Upper Cretaceous Milk River and Lea Park formations in southeastern Alberta; Bull. Can. Petrol. Geol., v29, p42-74. Nielsen, K.S. (2003): Lithostratigraphy, sequence stratigraphy and paleoenvironments of the Upper Colorado Group in southern Alberta and southwestern Saskatchewan: Definition of the Carlile and Niobrara formations (Upper Turonian and Upper Santonian); unpubl. PhD thesis, Carleton Univ., 517p.
Saskatchewan Geological Survey 10 Summary of Investigations 2003, Volume 1 O’Connell, S. (2001): The Second White Specks, Medicine Hat and Milk River formations – a shallow gas workshop, consult. rep. O’Connell, S.C., Campbell, R., and Bhattacharya, J. (1999): The stratigraphy, sedimentology and reservoir geology of the giant Milk River Gas Field in southeastern Alberta and Saskatchewan; Can. Soc. Petrol. Geol., Annual Meeting, Abstr. 99-83 O. Payenberg, T.H.D. (2002a): Integration of the Alderson Member in southwestern Saskatchewan into a litho- and chronostratigraphic framework for the Milk River/Eagle shoreline in southern Alberta and north-central Montana; in Summary of Investigations 2002, Volume 1, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2002-4.1, CD-ROM, p134-142. ______(2002b): Litho, chrono- and allostratigraphy of the Santonian to Campanian Milk River and Eagle formations in southern Alberta and north-central Montana: Implications for differential subsidence in the Western Interior Foreland Basin; unpubl. PhD thesis, Univ. Toronto, 221p. Rice, D.D. (1980): Coastal and deltaic sedimentation of Upper Cretaceous Eagle Sandstone: Relation to shallow gas accumulations, north-central Montana; Amer. Assoc. Petrol. Geol. Bull., v64, p316-338. Ridgley, J.L. (2000): Lithofacies architecture of the Milk River Formation (Alderson Member of the Lea Park Formation), southwestern Saskatchewan and southeastern Alberta – its relation to gas accumulation; in Summary of Investigations 2000, Volume 1, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 2000-4.1, p106-120.
Schmoker, J.W. (1995): Methodology for assessing continuous-type (unconventional) hydrocarbon accumulations; in Gautier, D.L., Dolton, G.L., Takahashi, K.I., and Varnes, K.L. (eds.), 1995 National Assessment of United States Oil and Gas Resources - Results, Methodology, and Supporting Data; U.S. Geol. Surv., Digital Data Series DDS-30, Release 2.
Saskatchewan Energy and Mines (1990): Catalogue of structure test hole locations; Sask. Energy Mines, Sed. Geodata Branch, Misc. Rep. 90-6, 231p.
Shurr, G.W. and Ridgley, J.L. (2002): Unconventional shallow biogenic gas systems; Amer. Assoc. Petrol. Geol. Bull., v86, p1939-1969.
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