GULF COAST ULTIMATE RECOVERY GROWTH-2
Eugene M. Kim, William L. FisherUltimate recovery growth modeling by plays in Texas Railroad Commission Dist. 4 (RRC-4) has revealed significant growth for Wilcox natural gas plays.
Bureau of Economic Geology
As a whole, major natural gas fields of RRC-4 are experiencing and hold future potential for ultimate recovery growth. However, important ultimate recovery growth trends are masked by such total, aggregated analysis.
When disaggregated by plays, significant growth and future potential were revealed for Wilcox natural gas plays. Wilcox natural gas plays were associated with relatively recently discovered fields, deeper reservoir depths, structural complexities due to fault compartmentalization, high initial reservoir pressures, and reservoirs designated as tight gas/low-permeability.
RRC-4 Wilcox gas playsThe play concept is a basic tool for organizing a vast amount of data available from natural gas reservoirs of RRC-4.
The BEG's Atlas of Major Texas Gas Reservoirs1 was used as the primary guide to play delineation. The two major Wilcox natural gas plays in RRC-4 are the Lower Wilcox Lobo Trend (WX-2) [114,094 bytes] and the Wilcox deltaic sandstone in the Rio Grande embayment (WX-4) [107,666 bytes] (Fig. 1 [117,079 bytes]). Major characteristics of these two plays are shown in the tables on p. 80.
Play WX-2 and WX-4 comprise sediments of the Wilcox Group (Paleocene to Lower Eocene), a major natural gas productive formation and the first major progradational episode in the Tertiary System of the Texas Gulf Coast basin. Most Wilcox reservoirs are small and natural gas prone. Intensive exploration and development began in the late 1930s, and since then the focus has been toward even deeper reservoirs.
The extensive Wilcox growth-fault zone of syndepositional normal faults, with associated dip reversals and rollover anticlines, that developed along the unstable Wilcox shelf margin, is the main structural feature responsible for the formation of major natural gas trapping mechanisms.
Reservoirs of play WX-2 are the major low permeability natural gas producers of the Texas Gulf Coast basin and are formally designated as tight gas sandstones in Webb and Zapata counties. Porosity and permeability ranges of 12-25% and 0.0003-0.5 md., respectively, are common for producing sandstones.2 Almost all Lobo sandstones must be stimulated by fracturing techniques.
The complex configuration of faults and unconformities that compartmentalize Lobo reservoirs as well as the characteristics of geopressured, tight gas reservoirs, have imparted high degrees of reservoir heterogeneity in play WX-2. These reservoir heterogeneities require state of the art reservoir characterization techniques as well as advanced recovery techniques in order to fully recover play WX-2's natural gas ultimate recovery growth potential.
Play WX-4 is a large natural gas play in RRC Districts 1, 2, and 4. Within RRC-4, play WX-4 primarily lies in Duval, Zapata, and Webb counties. Play WX-4 is characterized by growth-faulted, deltaic sandstone reservoirs. Deep Wilcox natural gas reservoirs in play WX-4 are highly faulted because of deposition along an unstable shelf margin. Closely spaced growth faults having thousands of feet of cumulative displacement and associated stratigraphic thickness and facies changes characterize the deep Wilcox formation. Traps formed primarily in faulted rollover anticlines on the downthrown sides of growth faults.3
Production in play WX-4 has shifted from the shallowest Upper Wilcox depositional sequence to the deepest ones in the last 20 years. Isolated areas in the Middle Wilcox in Webb County and the Upper Wilcox in Duval, Jim Hogg, and Starr counties have been formally recommended by the RRC for designation as tight gas sandstones.
Historical trends from 1977 to 1993 of annual natural gas ultimate recovery, production, proved reserves, and number of fields are shown for plays WX-2 and WX-4 in Fig. 2 [67,964 bytes]. Natural gas ultimate recovery for plays WX-2 and WX-4 display prominent increasing trends. Particularly noteworthy is that natural gas ultimate recovery for plays WX-2 and WX-4 displays prominent increasing trends during the 17-year time frame.
Growth, future potentialUltimate recovery growth analysis based on a factor of time and drilling activity was conducted for RRC-4 as a whole and disaggregated by its major plays.
The data set used is derived from Energy Information Administration's Oil and Gas Integrated Field File (OGIFF),4 which is the most comprehensive and reliable historical data on natural gas proved reserves, production, and ultimate recovery by field relative to time currently available. Additional production and completion data were derived from the numerous field and reservoir files from the RRC and Lasser Inc.'s Texas Production Database.5
When taken as a whole, 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. 3 [72,625 bytes]). The cumulative growth factor for the total 520 major fields of RRC-4 was calculated at 10.14 (Fig. 4 [92,828 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. This means that the initial ultimate recovery of a newly discovered field in RRC-4 is expected to increase by a factor of 10.14 during a forecast period of 70 years. 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
While ultimate recovery growth analysis based on a factor of time for the total 520 major fields of RRC District 4 revealed some general trends of ultimate recovery growth, more vital meaning can be derived by play specific analysis. A play specific quantification of ultimate recovery growth can be used to determine the plays that hold the most promise of future ultimate recovery growth potential. Out of the selected plays in which ultimate recovery growth analysis based as a factor of time was performed, plays WX-2 and WX-4 experienced the greatest amount of ultimate recovery growth.
An increasing natural gas ultimate recovery trend can be correlated with increasing cumulative well completions for the total 520 major fields of RRC-4. For each of the selected individual natural gas plays of RRC-4, natural gas ultimate recovery growth based as a factor of drilling activity was conducted. When yield per effort was calculated in terms of ultimate recovery versus cumulative well completions, plays WX-2 and WX-4 showed significantly higher natural gas ultimate recovery growth trends (Fig. 5 [100,769 bytes]).
Controls on growthWhen analyzed as a factor of time and drilling activity, Wilcox plays have shown to experience greater ultimate recovery growth compared to other plays in RRC-4.
What makes these plays different? Are these plays different in terms of a definable statistical parameter or characteristic? General statistical comparisons of plays WX-4 and WX-2 with other plays of RRC-4 are undertaken to find clues to these answers.
Discoveries for the total 520 major natural gas fields in RRC-4 peaked in 1955 and showed a steady decline. However, discoveries increased significantly in the 1980s, and new discoveries are still occurring. Plays WX-2 and WX-4 are comprised of relatively recently discovered fields. Although play WX-4 is a mature play, most of its recent production and reserves are from relatively recently discovered fields. Plays with less ultimate recovery growth trends than these plays tend to be comprised by older fields.
Field depths for the total 520 major natural gas fields in RRC-4 generally fall within a range of 10,000-14,000 ft. Noticeable trends toward deeper fields were revealed for plays WX-2 and WX-4, especially for play WX-4 where approximately half of its fields were 15,000 ft or deeper.
A generally decreasing field size trend of newer discoveries is displayed for the total 520 major fields of RRC-4. However, this does not validate that most recent discoveries have been only smaller fields. Since the 1970s, Laredo, J.C. Martin, La Perla Ranch, Fandango, Bob West, McMurrey, Rosita NW, and Vaquillas Ranch fields have all been discovered exceeding 300 bcf in terms of field size. It is interesting to note that these mentioned fields are included among plays WX-2 and WX-4.
Plays WX-2 and WX-4 display high structural complexities due to fault compartmentalization. Both Wilcox plays are affected by intense, regional faulting attributable to the Wilcox fault zone. Play WX-2 is largely comprised of reservoirs formally designated as tight gas/low-permeability by the RRC with fracture stimulation treatments being the normal production practice. Locally, play WX-4 consists of reservoirs designated as tight gas reservoirs. These two Wilcox plays also show relatively high initial reservoir pressures.
A probable important control on high structural complexity, tight gas reservoirs, and high initial reservoir pressures is reservoir depth. 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.7
ConclusionsSignificant historical natural gas ultimate recovery growth and future potential in RRC-4 have been revealed for Wilcox plays both as a factor of time and drilling activity. Undoubtedly, ultimate recovery growth from Wilcox plays 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 must be addressed in more detail. Specific technologies applied and amenable on a play basis must be determined. Detection technology, locational diagnostics, horizontal drilling, directional drilling, hydraulic fracturing technology, measurement while drilling, advanced drilling bits, 3D seismic, and amplitude versus offset are just a few technological advances that have led to an increase in exploration and development efficiency sufficient to offset the depletion effects of declining field size, particularly in natural gas ultimate recovery growth of older, large fields. However, neither the impacts of technology by play nor the play-specific amenability of applying advanced technologies have been assessed or quantified.
Although a significant amount of natural gas ultimate recovery growth has occurred, the economic sensitivity of this resource component has not been examined in detail. Play by play variations in the economic sensitivity of natural gas ultimate recovery growth are essential in determining future potential based on future economic projections. Moreover, cost-benefit analysis of the application of various technologies in achieving natural gas ultimate recovery growth should be addressed.
Future research directions concerning technology and economics may be better analyzed at the field or reservoir level of plays experiencing and holding the greatest natural gas ultimate recovery growth potential. The results may be compiled and extrapolated to the play level. Furthermore, such a detailed and specific analysis will allow a closer examination of the geological, engineering, and production controls on natural gas ultimate recovery growth.
AcknowledgmentsThanks to the U.S. Department of Energy for funding this project and to the U.S. Energy Information Administration for the data.
- 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.
- Robinson, B.M., Holditch, S.A., and Lee, W.J., A case study of the Wilcox (Lobo) trend in Webb and Zapata counties, Texas, JPT, December 1986, pp. 1,355-64.
- Debus, R.W., and Debus, M.M., A review of Upper Wilcox (Eocene) production history and drilling activity in Railroad Commission Dist. 4: A 58 year old emerging play, South Texas Geological Society Bull., April 1998, pp. 19-33.
- Energy Information Administration, The 1993 Oil and Gas Integrated Field File, digital data file, 1993.
- Lasser Inc., Texas Production Database, Digital CD-ROM, 1997
- Marsh, R.G., How much oil are we really finding?, OGJ, Vol. 69, No. 4, 1971, pp. 100-104.
- Kim, E.M., Natural gas ultimate recovery growth modeling by plays in the Gulf Coast basin, The University of Texas at Austin, 1998, 288 p.
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