USGS assesses deep undiscovered gas resource

EMERGING U.S. GAS RESOURCES-2

Thaddeus S. Dyman, James W. Schmoker
U.S. Geological Survey
Denver
David H. Root
U.S. Geological Survey
Reston, Va.

The U.S. Geological Survey (USGS) estimated in 1995 that 1,412 tcf of technically recoverable natural gas remained to be discovered or developed in U.S. onshore areas. A significant part of that resource base, 114 tcf, is undiscovered gas in deep sedimentary basins assessed by the USGS in onshore areas and state waters.

These deep gas accumulations, at or below 15,000 ft, occur in numerous, widely differing geologic settings (Fig. 1 [138,549 bytes]). Much of the resource is concentrated in Gulf Coast, Alaska, Midcontinent, and Rocky Mountain basins. As deep drilling increases and our knowledge of deep sedimentary basins improves, it is likely that additional deep gas resources will be discovered.

The USGS 1995 National Petroleum Assessment¹ identified 162 deep conventional and 11 deep continuous-type (unconventional) gas plays.

One-hundred one of the conventional plays and six of the continuous-type plays were quantitatively assessed for undiscovered technically-recoverable deep natural gas. A detailed discussion of deep gas plays is provided in Dyman and others.²

This article contains:

1. Descriptions of the deep gas plays supplied by USGS province geologists;

2. Estimates of undiscovered technically-recoverable gas from these plays; and,

3. Comparisons of the USGS estimates with other recent deep gas assessments. For detailed discussions of the deep gas plays and maps illustrating the play outlines, refer to the 1995 USGS National Petroleum Assessment CD-ROM. Federal offshore waters were assessed by the U.S. Minerals Management Service (MMS),³ but these offshore plays were not subdivided by depth intervals.

Methodology

Conventional plays

The 1995 USGS assessment was based on a play analysis concept. ⁴ A play was considered either confirmed (known discoveries) or hypothetical (no known discovery of a significant field greater than 6 bcf).

Confirmed plays were analyzed using field-size distributions, numbers of known accumulations, depths to production, and other geologic and production factors. Hypothetical plays were analyzed using geologic analogs or simulation models based on geological, geophysical, and geochemical data. Hypothetical plays were risked as to the probability of discovering at least one significant accumulation.

Using play probability and other data, the estimated numbers and sizes of undiscovered fields were subjected to Monte Carlo simulation to establish the resource estimates. These estimates were presented as a range of values (F95, F50, and F5) plus a mean (arithmetic average), with F95 representing a 19 in 20 chance and F5 representing a 1 in 20 chance of the occurrence of at least the amount of resource estimated.

Undiscovered gas resources were subdivided into 5,000-ft depth intervals. The greater than 15,000 ft depth horizon was represented as a single interval.

Continuous-type plays

Continuous-type plays were assessed using a specialized methodology developed by the USGS. ⁵

These continuous-type plays are geologically diverse and include coalbed meth- ane, biogenic gas, fractured shale gas, and basin-centered gas accumulations. Continuous-type plays are large, potentially productive areas that cannot be defined as fields or discrete units with down-dip hydrocarbon and water contacts.

Continuous-type plays were divided into 5,000-ft depth intervals in the same way as conventional plays, with the deeper than 15,000 ft horizon represented by one interval.

Deep undiscovered gas plays, resources

The USGS estimated that about 114 tcf of technically-recoverable undiscovered natural gas remains to be discovered from deep sedimentary basins in the U.S. onshore and state waters2 ( Table 1 [18,196 bytes]). Associated gas from oil fields is only identified at the province level.

In addition to the 101 deep conventional plays listed (Table 1), 60 other conventional gas plays were identified but not quantitatively assessed and thus are not listed on Table 1 (for the full list see Dyman and others.2)

Table 2 [111,069 bytes] identifies the play name, depth range, and undiscovered deep gas resources for six continuous-type plays. Five additional plays are identified in Table 2 but were not assessed due to a lack of geologic information.

Deep conventional gas

The USGS estimated a mean 55.3 tcf of undiscovered technically-recoverable gas for 101 deep conventional onshore plays in the lower-48 states, Alaska, and state waters. The bulk of the deep conventional gas resource, 45 tcf, is in the Gulf Coast and Alaska.

Fig. 2 [67,914 bytes] shows cumulative undiscovered conventional gas resources as a percent of the total for the 101 deep plays containing assessed resources.

The most significant deep gas play is the North Alaska Eastern thrust belt play, with 9.1 tcf of undiscovered deep gas. The Tuscaloosa deep sandstone and Norphlet Mobile Bay plays in the Gulf Coast contain 5.3 tcf and 4.8 tcf of undiscovered deep undiscovered gas, respectively. These three plays account for 35% of the deep undiscovered conventional gas resource.

The 13 most significant plays contribute nearly three-quarters of the deep undiscovered conventional gas resource.

Of the 101 conventional deep gas plays, 20 have maximum depths ranging from 20,000-25,000 ft, five plays have maximum depths of 25,000-30,000+ ft, and three plays exceed 30,000 ft in depth. Seventy deep gas plays are confirmed (those having known production), whereas 31 plays are hypothetical. Fifty-nine of the confirmed plays have more than half of their estimated resources in the form of nonassociated gas.

Deep unconventional gas

The USGS assessment estimated 58.4 tcf of technically recoverable deep gas in the onshore U.S. and state waters (Table 2).

Of the 11 identified deep unconventional gas plays, six were quantitatively assessed by the USGS. Four of these quantitatively assessed plays are in Cretaceous and Tertiary low-permeability sandstone reservoirs in the Greater Green River basin of Wyoming and account for the great bulk of the undiscovered unconventional deep gas resource.

The two largest deep unconventional gas plays, the Mesaverde and Cloverly-Frontier in the Green River basin, account for 26 tcf and 25 tcf respectively (Fig. 3 [24,314 bytes]).

As the understanding of "basin-centered" gas accumulations improves, it is likely that additional deep unconventional gas plays will be defined. For example, deep unconventional gas plays may exist in the basins of interior Alaska and coastal California and in the Rocky Mountain basins such as Crazy Mountains, Raton, Powder River, and Wind River.

Assessments compared

Four recent national petroleum assessments provide estimates for undiscovered deep gas, subdivided by region and depth interval ( Table 3 [70,447 bytes]).

Each estimate is based on a distinct assessment method and procedure and uses somewhat different geographic areas, definitions, and sources of data. For example, the Gas Research Institute (GRI) total does not include Alaska deep gas because GRI did not subdivide Alaska data by depth. The National Petroleum Council (NPC) did not assess the deep gas in Alaska, and the Potential Gas Committee (PGC) estimated only negligible deep gas in Alaska basins.

A summary comparison of these four studies shows:

Overall, GRI has the highest estimate for undiscovered deep gas, 184 tcf. GRI places the largest portion of its deep gas resources in the Gulf Coast.
All four of the deep gas studies look to the Gulf Coast and Rocky Mountain region to contain large volumes of deep gas, although when combined, these two regions exhibit a wide range of estimates (Table 4 [45,121 bytes]).
Differences in deep gas resource estimates also exist for the Midcontinent and Permian basin, which contain fewer estimated resources than the Rocky Mountain and Gulf Coast regions (Table 4).
The USGS Midcontinent and Permian basin estimates for deep gas are noticeably low when compared to the other three estimates. Conversely, the USGS Rocky Mountain Region estimate is noticeably high in part because of the inclusion of unconventional continuous-type plays.

Areas for future study

A look at USGS and other deep gas assessments reveals several areas for future study:

Maintain a timely update of the reservoir characteristics and production histories of existing and potential deep gas plays as new information becomes available. A high priority would be re-evaluating the plays that were too poorly defined to quantitatively assess in 1995. Of particular value would be establishing the geologic conditions that help preserve or improve reservoir properties in deep formations.
Undertake geologic studies to determine the spatial relationships that exist between deep conventional and continuous-type plays in some deep basins. Conventionally-trapped deep gas plays may grade into basin-centered, continuous-type gas plays down-dip in the Appalachian, Anadarko, Powder River and other basins.
Quantitatively assess the continuous-type (basin centered) plays that were identified but not assessed previously as soon as adequate geologic data become available. In the 1995 USGS assessment, 61 deep conventional and five deep continuous-type plays were defined but not assessed because of a lack of basic geologic information.
Undertake an economic analysis of deep gas accumulations. An economic analysis would provide information for subdividing the deep gas resources into short, intermediate, and long-term supply categories.
Determine the existence of continuous-type (basin-centered) gas plays in basins for which they were not previously assessed. These basin-centered plays may contain significant volumes of undiscovered technically-recoverable deep gas not previously assessed or inappropriately assessed as conventional gas.

Acknowledgments

This work was funded in part by the U.S. Geological Survey's Energy Resources Program and a Cooperative Research & Development Agreement (Crada) with Advanced Resources International Inc., Arlington, Va. Funding for the Crada originated from a contract with the Gas Research Institute, Chicago.

References

Gautier, D.L., Dolton, G.L., Takahashi, K.I., and Varnes, K.L. (eds.), 1995 national assessment of the U.S. oil and gas resources, results, methodology, and supporting data, USGS Digital Data Series DDS-30, 1995, one CD-ROM.
Dyman, T.S., Schmoker, J.W., and Root, D.H., Assessment of deep conventional and continuous-type (unconventional) natural gas plays in the U.S., USGS Open-File Report 96-529, 1996, 30 p.
Minerals Management Service, An assessment of the undiscovered hydrocarbon potential of the nation's Outer Continental Shelf, U.S. Department of Interior, MMS, OCS Report MMS 96-0034, 1996, 40 p.
Gautier, D.L., and Dolton, G.L., Methodology for assessment of undiscovered conventional accumulations, in Gautier, D.L., Dolton, G.L., Takahashi, K.I., and Varnes, K.L., eds., 1995 national petroleum assessment of U.S. oil and gas resources on CD-ROM, USGS DDS-30, 1995.
Schmoker, J.W., Method for assessing continuous-type (unconventional) hydrocarbon accumulations, in Gautier, D.L., Dolton, G.L., Takahashi, K.I., and Varnes, K.L., eds., 1995 national petroleum assessment of U.S. oil and gas resources on CD-ROM, U.S. Geological Survey DDS-30, 1995.

The series

Part 1-Kuuskraa, Vello A., Outlook bright for U.S. natural gas resources, OGJ, Apr. 13, 1998, p. 92.

End Part 2 of 6

The Authors

Ted Dyman is a petroleum geologist with the U.S. Geological Survey Central Region Energy Team in Denver, where he has been employed since 1976. His areas of research include geologic controls and resource potential of natural gas resources, stratigraphy, and petroleum potential of Cretaceous rocks in the Western Interior basin and quantitative applications to petroleum geology. He holds BS and MS degrees in geology from Northern Illinois University and a PhD in geology from Washington State University. Jim Schmoker is a geophysicist with the USGS Central Region Energy Team in Denver, where he has been employed since 1974. His areas of research include petroleum resource assessment, studies of reservoir quality (in particular the evolution of porosity with burial), and the petroleum potential of organic-rich black shales. He holds BS and MS degrees in physics from the University of Minnesota and a PhD in geophysics from Virginia Polytechnic Institute and State University.

David H. Root joined the USGS as a petroleum analyst in 1974. He was involved in a wide range of resource assessment activities until his retirement in 1997. He received a BS degree in mathematics from Massachusetts Institute of Technology and a PhD in mathematics from the University of Washington in 1967. He taught mathematics and statistics at Purdue University until joining USGS in 1974.