Play disaggregation reveals key Texas gas growth trends

Eugene M. Kim, William L. Fisher
Bureau of Economic Geology
Austin

Play disaggregation has revealed important ultimate recovery growth trends for the major natural gas fields in Texas Railroad Commission Dist. 4 (RRC-4).

Although significant growth and future potential were observed for the major fields, important ultimate recovery growth trends were masked by total, aggregated analysis based on a broad geological province.

When disaggregated by plays, significant growth and future potential were displayed for plays that were associated with relatively recently discovered fields, deeper reservoir depths, high structural complexities due to fault compartmentalization, reservoirs designated as tight gas/low-permeability, and high initial reservoir pressures.

Ultimate recovery growth

Estimates of ultimate recovery, the sum of the proved reserves and cumulative production up to a specific time, are initially conservative due to the lack of understanding of the geological, engineering, and production characteristics of the reservoir or field.

Ultimate recovery tends on average to increase substantially over time and drilling due to better understanding of the reservoir or field and application of advanced technologies. In recent years, natural gas ultimate recovery growth has become a major component of total U.S. annual natural gas reserve additions.^{1 2}

Although there is a wide range in ultimate recovery growth potential by play and that potential is a function of the drilling and technology applied, previous ultimate recovery growth studies are gross, averaging wide ranges, aggregated by broad geological provinces. An assessment on a play basis is the key to understanding the future role of natural gas ultimate recovery growth.

Dist. 4 play disaggregation

The play concept is a basic tool for organizing a vast amount of data available from natural gas reservoirs of RRC-4.

A play is defined as a conceptual geologic unit having one or more reservoirs that can be genetically related on the basis of depositional origin of the reservoir, structural or trap style, source rocks and hydrocarbon generation, migration mechanism, seals for entrapment, and type of hydrocarbon produced.

Plays are the geologically homogeneous subdivision of the universe of petroleum pools within a basin.³ Therefore, individual plays have unique geological features that can be used as a conceptual model that incorporates geologic processes and depositional environments to explain the distribution of petroleum.

The Bureau of Economic Geology's Atlas of Major Texas Gas Reservoirs⁴ was used as the primary guide to play delineation. The U.S. Energy Information Administration's 1993 Oil and Gas Integrated Field File (OGIFF)⁵ for RRC-4 was reduced into 520 major fields with significant ultimate natural gas recovery. Field-level ultimate recovery growth data were disaggregated into 11 geologically delineated plays (Fig. 1 [128,634 bytes]).

When taken as a total, the 520 major fields of RRC-4 are currently experiencing significant ultimate recovery growth activity. A total of 14.9 tcf of natural gas ultimate recovery growth is attributable for the period from 1977-93 (Fig. 2 [118,910 bytes]). However, important natural gas ultimate recovery growth activity is obscured or masked by this total, aggregated analysis. Certain plays in RRC-4 are experiencing significant natural gas ultimate recovery growth while others show little to no future growth potential.

Historical trends from 1977 to 1993 in terms of annual natural gas ultimate recovery, production, and proved reserves revealed several distinctive trends. Natural gas ultimate recovery for the following plays display significant growth trends (Fig. 3 [51,422 bytes]):

• Lower Wilcox Lobo Trend (WX-2);
• Wilcox deltaic sandstone in the Rio Grande embayment (WX-4);
• Vicksburg deltaic sandstone in the Rio Grande embayment (VK-1); and
• Frio fluvial/deltaic sandstone along the Vicksburg Fault Zone (FR-4).

The tripling and doubling of natural gas ultimate recovery for plays WX-2 and WX-4 in the 17-year time frame is particularly noteworthy. Although plays VK-1 and FR-4 show less noticeable growth trends, they are important due to their significance as being the two largest natural gas plays in RRC-4 in terms of 1993 natural gas ultimate recovery. However, in terms of annual natural gas production and proved reserves, play FR-4 has displayed a significant decline trend, unlike plays WX-4, WX-2, and VK-1 Fig. 4 [50,687 bytes] and Fig 5 [57,441 bytes].

Out of the 11 selected plays in which ultimate recovery growth analysis based as a factor of time was performed, plays WX-4, WX-2, and VK-1 experienced the greatest amount of ultimate recovery growth (Fig. 6 [56,198 bytes]).

Cumulative growth factors (CGF) represent the ratio of the size of a field n years after discovery to the initial estimate of its size in the year of its discovery. The CGF curve rises very rapidly in the early years after initial discovery. When the CGF curve is rising, ultimate recoveries from those initial discovery years are currently being revised upward. Where the CGF curve is level, upward and downward revisions are about equal with no appreciable ultimate recovery growth.⁶

Excluding these three plays, the remaining eight selected plays of RRC-4 have experienced lower natural gas ultimate growth trends than that projected for the total 520 major fields of RRC-4. Plays MC-1 and MC-2 are even experiencing little to no natural gas ultimate recovery growth.

For the total 520 major fields of RRC-4, field discoveries peaked in 1955 and showed a steady decline. Plays WX-4, WX-2, and VK-1 were comprised of relatively recently discovered fields. Plays with less ultimate recovery growth than these plays tend to be comprised by older fields. Play MC-2, which reveals little to no growth, is comprised mainly of older fields discovered in the 1920s-40s. This contradicts the prevalent theory held by some researchers that ultimate recovery growth is largely from older fields. Field depths of the total 520 major fields of RRC-4 generally fall within a range of 10,000-14,000 ft. Noticeable trends toward deeper fields were revealed in plays WX-2, WX-4, and VK-1.

Plays WX-4, WX-2, and VK-1 display greater structural complexities, initial reservoir pressures, and reservoirs designated as tight gas/low-permeability. A probable important control on these representative play characteristics is reservoir depths. Deeper reservoirs tend to exhibit more complex structures, cementation, and high volumes of natural gas. Although it was assumed that these three plays would have vertical stacking and multiple sands associated with them, the available data set revealed no distinctive correlation.²

Future research directions

Undoubtedly, ultimate recovery growth is a large and crucial component in the future natural gas supply of RRC-4. However, several future research directions must be addressed to achieve a more complete understanding of this important component of future natural gas supply.

Impacts of technology on ultimate recovery growth most be addressed in more detail. Specific technologies applied and amenable on a play basis must be determined. Moreover, cost-benefit analysis of the application of various technologies in achieving natural gas ultimate recovery growth should be addressed.

The methodology of natural gas ultimate recovery growth modeling by plays can be verified and applied more broadly to other natural gas resource areas with significant natural gas ultimate recovery growth potential. Using the modeling methodology developed, other Gulf Coast basin areas in Texas, specifically RRC Dists. 1, 2, and 3, can be initially analyzed.

Acknowledgments

Thanks to the U.S. Department of Energy for funding this project and to the U.S. Energy Information Administration for the data.

References

1. Fisher, W.L., Future supply potential of U.S. oil and natural gas, The Leading Edge of Exploration, Vol. 10, No. 12, 1991, pp. 15-21.
2. Kim, E.M., 1998, Natural gas ultimate recovery growth modeling by plays in the Gulf Coast basin, The University of Texas at Austin, 1998, 288 p.
3. White D.A., Assessing oil and gas plays in facies-cycle wedges, AAPG Bull., Vol. 64, No. 8, 1980, pp. 1,158-78.
4. Kosters, E.C., Bebout, D.G., Seni, S.J., Garrett, C.M., Brown, L.F., Hamlin, H.S., Dutton, S.P., Ruppel, S., Finley, R.J., and Tyler, N., Atlas of major Texas gas reservoirs, Bureau of Economic Geology, 1989, 161 p.
5. Energy Information Administration, The 1993 oil and gas integrated field file, digital data file, 1993.
6. Marsh, R.G., How much oil are we really finding?, OGJ, Vol. 69, No. 4, 1971, pp. 100-104.

The Authors