Assessment of Shallow Biogenic Gas Resources In Montana

Transcription

1 Page No Assessment of Shallow Biogenic Gas Resources In Montana J.L. Ridgley *, T.C. Hester, S.M. Condon, T. Cook, L.O. Anna, P.G. Lillis, E.L. Rowan U.S. Geological Survey, Denver, CO 8225 and G.T. Snyder University of Rochester, Rochester, NY ABSTRACT Large accumulations of biogenic gas are known to occur in shallow (<1 m) Cretaceous reservoirs of the Northern Great Plains (mainly Montana), USA and southeastern Alberta and southwestern Saskatchewan, Canada. Figure 1 shows the distribution of producing shallow gas accumulations in Canada and Montana. Production is from the Belle Fourche, Medicine Hat, and Milk (Alderson member of the Lea Park Formation) formations in Canada and from the Belle Fourche, Greenhorn, Carlile, Niobrara, Eagle, and Judith formations in Montana. The U.S. Geological Survey (USGS) in the 1995 National Assessment of United States Oil and Gas Resources (Gautier and others, 1996) made a mean estimate of 1.18 x 1 12 m 3 (42 trillion cubic feet) of technically recoverable resources of continuous-type (unconventional) gas in Cretaceous shallow biogenic gas plays of northern and central Montana (Rice and Spencer, 1996). About 9 percent of this 1995 gas estimate was in hypothetical plays, and of this percentage, over one-half was based primarily on similarity of the potentially productive facies in Montana to the facies that host large shallow biogenic gas resources in southeastern Alberta and southwestern Saskatchewan. The controls on the hypothetical play boundaries were poorly understood in From , the USGS conducted a multidisciplinary project (Ridgley and others, 1999) to determine the controls on Upper Cretaceous shallow biogenic gas accumulations in the Northern Great Plains, primarily in northern Montana, and Canada, in order to provide more geologic information for reassessment of resource estimates of shallow gas in the region. Using the information generated from these (Condon, 2; Condon and others, 2; Ridgley, 2; Ridgley and others, 2; Ridgley and others, 21) and other studies, the USGS has recently completed its reassessment of shallow biogenic gas resources in Montana. Unlike the 1995 assessment, which estimated technically recoverable resources and had an unlimited forecast span, this assessment estimated those resources that have the potential to be added to reserves in the next 3 years. Therefore, it does not represent total gas in place or total technically recoverable gas. Rock the Foundation Convention, June 18-22, 21

2 Page No An understanding of the geologic nature of the shallow gas accumulations determines the methodology used in generating assessment forcasts. If the gas is hosted in conventional (discrete) reservoirs, the methodology employed by the USGS estimates the sizes and numbers of undiscovered fields and prepares a probability distribution of each of these parameters. Conventional gas accumulations typically have a downdip water contact (float on water), occur in structural or stratigraphic traps, and have a defined spatial extent. However, where large areas, such as shallow gas in Cretaceous rocks of Montana and Canada or basin-centered gas found in Rocky Mountain basins, are seemingly everywhere charged with gas, lack a water drive, and occur in multiple stacked intervals, a different methodology for assessment is required. Fields/pools lack the well-defined boundaries required to employ the conventional methodology based on estimating sizes and numbers of undiscovered fields. Rather, in continuous accumulations, fields/pools simply represent "sweet spot" occurrences within an overall gas-charged area. The methodology using in assessing continuous types of accumulations uses a cell-based grid system that attaches some probability to the estimated ultimate recovery (EUR) of gas in each untested cell, based on our knowledge of the geologic controls and production characteristics in the area being assessed or in an analog area (Schmoker, 1996; 1999). For the assessment process, we adopted a petroleum system approach (Magoon and Dow, 1994). We used one petroleum system for the entire Northern Great Plains, although we recognized the variability in total organic content and kerogen types in the different formations assessed. We identified seven assessment units within the total petroleum system; the cell-based methodology for continuous accumulations was used for each. The units assessed included 1) Judith Formation, 2) Eagle Sandstone west, which encompasses areas where the formation is dominated by nonmarine and marginal marine sandstone (includes the Tiger Ridge area), 3) Eagle Sandstone east, which includes dominantly fine-grained marine facies, such as those found on Cedar Creek Anticline, 4) combined Niobrara Formation and Carlile Shale, 5) combined Greenhorn Limestone and upper Belle Fourche Formation (Mosby or Phillips sandstones), 6) lower Belle Fourche Formation (commonly called Blackleaf A),and 7) Bowdoin Dome, all producing formations. We assessed each unit separately, incorporating available geologic, hydrologic, and gas geochemistry data and results of new log analysis techniques (Hester, 1999). Our recent deuterium isotopic studies of methane and co-produced waters from the Belle Fourche and Eagle Sandstone, suggest that the gas and water are in equilibrium. Preliminary, uncorrected ratios of 129 I/I of the waters from four samples (three from the Belle Fourche and one from the Eagle) indicate a minimum age range of 35.6 to Ma, which suggests that the gas is at least this old. Rock the Foundation Convention, June 18-22, 21

3 Page No Production data were tabulated for each formation examined; where production is historically combined or where there was insufficient production data available for a particular formation, formations were combined into one assessment unit as defined above. By using production data from wells in each assessment unit and generating estimated ultimate recoveries (EUR) from that production data, a distribution of well EURs was created for each assessment unit. EUR distributions for various fields and formations inside an assessment unit were then examined to see if any similarities or differences were readily apparent. The resulting EUR distributions form a foundation for estimating EUR distributions for untested cells. The curves of the EUR distributions (Figure 2a) are similar for all the assessment units, except for Eagle Sandstone west. The steeper slope of the EUR distribution for the Eagle Sandstone west assessment unit (Figure 2 b) is mostly likely controlled by the better quality of the sandstone reservoir. Preliminary results indicate that resource estimates will be be less than 1 trillion cubic feet of gas, using a 3-year forecast. This is a downward revision from the 1995 assessment. References Condon, S.M, 2, Stratigraphic framework of Lower and Upper Cretaceous rocks in central and eastern Montana: USGS Data Digital series DDS-57. Condon, S.M., J.L. Ridgley, E.L. Rowan, L.O. Anna, P.G. Lillis, T.C. Hester, and Troy Cook, 2, Re-evaluation of Controls on Cretaceous Shallow Biogenic Gas Accumulations, Northern Great Plains, USA, and Southeast Alberta and Southwest Saskatchewan, Canada: CSPG Annual Meeting Abstract 592. Gautier, D. L., G.L. Dolton, K.I. Takahashi, and K.L. Varnes, (eds.), 1996, 1995 National Assessment of United States Oil and Gas Resources - Results, Methodology, and Supporting Data: U.S. Geological Survey Digital Data Series DDS-3. Release 2. Hester, T.C., 1999, Prediction of gas production using well logs, Cretaceous of north-central Montana: Mountain Geologist, v. 36, no. 2, p Magoon, L.B., and W.G. Dow, 1994, The petroleum system, in Magoon, L.B. and W.G. Dow, The petroleum system from source to trap: American Association of Petroleum Geologists Memoir 6, Chapter 1, p Rice, D.D. and C.W. Spencer, 1996, Northern Great Plains shallow biogenic gas; in Gautier, D.L., G.L. Dolton, K.I. Takahashi, and K.L. Varnes, (eds.), 1995 National Assessment of United States Oil and Gas Resources - Results, Methodology, and Supporting Data: U.S. Geological Survey Digital Data Series DDS-3. Release 2. Ridgley, J.L., 2, Lithofacies architecture of the Milk Formation (Alderson Member of the Lea Park Formation), southwestern Saskatchewan and southeastern Alberta its relation to gas accumulation, in Summary of Rock the Foundation Convention, June 18-22, 21

4 Page No Investigations 2, Volume 1, Saskatchewan Geological Survey, Saskatchewan Energy Mines, Misc. Rep , p Ridgley, J.L., T.C. Hester, S.M. Condon, L.O. Anna, E.L. Rowan, T. Cook, and P.G. Lillis, 1999, Re-evaluation of the shallow biogenic gas accumulation, northern Great Plains, USA - Is the similar gas accumulation in southeastern Alberta and southwestern Saskatchewan a good analog?; in Summary of Investigations 1999, v. 1, Saskatchewan Geological Survey, Saskatchewan Energy Mines, Miscellaneous Report , p Ridgley, J.L, D.H. McNeil, C.F. Gilboy, C.G. Fisher, S.M. Condon, N.S. Fishman, and J.D. Obradovich, 2, Interplay of structure, facies, and unconformities on sites of biogenic gas accumulation in the Upper Cretaceous Belle Fourche and Second White Specks formations Alberta, Saskatchewan, and Montana: CSPG Annual Meeting Abstract 313. Ridgley, J.L., D.H. McNeil, C.F. Gilboy, S.M. Condon, and J.D. Obradovich, 21, Structural and stratigraphic controls on sites of shallow biogenic gas accumulations in the Upper Cretaceous Belle Fourche and Second White Specks Greenhorn formations of southern Alberta, Saskatchewan and northern Montana; in Anderson, D.S., Robinson, J.W., Estes-Jackson, J.E., and Coalson E.B. (eds.), Gas in the Rockies: Rocky Mtn. Assoc. Geol., Chap. 16, 29p. Schmoker, J.W., 1996, Methodology for assessing continuous-type (unconventional) hydrocarbon accumulations, in. Gautier, D.L., G.L. Dolton, K.I. Takahashi, and K.L. Varnes, eds., 1995 National Assessment of United States Oil and Gas Resources Results, Methodology, and Supporting Data: USGS Digital Data Series DDS-3, release 2. Schmoker, J.W., 1999, U.S. geological Survey assessment model for continuous (unconventional) oil and gas accumulations the "FORSPAN" model: U.S. Geological Survey Bulletin 2168, 9 p. (available on-line at Rock the Foundation Convention, June 18-22, 21

5 Page No Southeast ALBERTA SASKATCHEWAN Alberta gas field 51 1 Bow Red Brooks Deer Saska tchewan Oldman Taber Sweetgrass arch Cut Bank South Medicine Hat Lake Pakowki Milk Havre 1 Tiger Ridge Maple Creek Battle Creek 1 Wymark Pool Moose Jaw syncline Malta Bowdoin dome 1 Williston Basin MONTANA 47 Thrust front boundary Contour Interval = 2 Ft 5 km 5 miles Great Falls Cedar Creek Anticline 2 Figure 1. Map showing distribution of currently producing shallow biogenic gas accumulations in Montana and Canada. Gas production in the Southeast Alberta gas field (magenta) is from the Belle Fourche, Medicine Hat, and Milk formations; in the Wymark pool and scattered wells, it is from the Belle Fourche Formation (blue); in the Bowdoin Dome field (purple), it is from the Carlile, Greenhorn, and Belle Fourche formations; and in the Tiger Ridge and Cedar Creek Anticline fields (green), it is mostly in from the Eagle Sandstone with minor production from the Judith Formation. Rock the Foundation Convention, June 18-22, 21

6 Page No Figure 2. Estimated ultimate recovery distributions, A) Bowdoin Dome, includes production data from the Carlile, Greenhorn, and upper Belle Fourche formations, and B) Eagle Sandstone west (Tiger Ridge area), includes production only from the Eagle Sandstone. Rock the Foundation Convention, June 18-22, 21