DISCUSSION AND REPLY

Pore-throat sizes insandstones, tightsandstones, and shales:DiscussionWayne K. Camp1

INTRODUCTION

Nelson (2009) prepared a nice compilation de-scribing the wide range of pore-throat sizes thatspan a continuum from conventional sandstonereservoirs to unconventional tight-gas siliciclasticreservoirs and shale seals. In this article, Nelson(2009) proposes a new and rather unique defini-tion for unconventional reservoirs (including tight-gas sands) that lacks sufficient supporting evidenceor references to support this definition. Nelson(2009, p. 329) defines a conventional reservoiras, “…one in which evidence that buoyant forcehas formed and maintained the disposition of oiland gas is present,” and thus for unconventionalreservoirs, it follows that Nelson concludes (p. 330)that, “In these systems, evidence for buoyancy as adominant force in the disposition of oil and gas islacking.”

It appears that the sole argument for non-buoyant or buoyancy-subordinate unconventionalreservoir systems is the perceived lack of evidenceof buoyancy, which is generally not good science

Copyright ©2011. The American Association of Petroleum Geologists. All rightsreserved.1Anadarko Petroleum Corporation, 1201 Lake Robbins Drive, The Woodlands, Texas77380; wayne.camp@anadarko.com

Manuscript received February 17, 2010; provisional acceptance March 26, 2010;revised manuscript received April 9, 2010; final acceptance December 14, 2010.DOI:10.1306/12141010019

AAPG Bulletin, v. 95, no. 8 (August 2011), pp. 1443– 1447 144

(absence of evidence is not evidence of absence).It would be much better to present a clear andconcise hypothesis followed by objective obser-vations that support the conclusions. I appreciatethat such a thesis may be beyond the scope of aGeologic Note article, but until such evidence ispresented, it is difficult to have intelligent dis-cussions and design experiments to test the hy-pothesis of nonbuoyant gas systems.

Although it is not clear from the definition ofNelson (2009), the reader is left to conclude thatthe natural laws of buoyancy are not dominantfactors during the evolution of tight-gas sand ac-cumulations from hydrocarbon migration to thepresent-day observed distribution of hydrocarbonsand water. This conclusion is misleading and ig-nores a significant body of recent published re-search (Shanley et al., 2004; Cluff et al., 2005;Fassett and Boyce 2005; Camp, 2008).

Nelson (2009, p. 339) further concludes thatonemay simply use ameasured pore-throat diametercutoff in considering the function of buoyancy inthe disposition of fluids in consolidated siliciclasticreservoirs and thereby classification as conventionalor unconventional petroleum accumulations. Re-viewing figure 2 (p. 332) of Nelson (2009), thepore-throat radius cutoff separating his interpretedconventional (buoyancy-driven) from unconven-tional (nonbuoyancy-driven) reservoirs is approxi-mately 1 to 2 mm.

BUOYANCY

Buoyancy is simply the upward force that keepsthings afloat, such as oil on water in a beaker. Theforce of buoyancy is equal to the weight of thefluid displaced as defined by Archimedes morethan 2000 yr ago. Therefore, buoyancy is only afunction of the difference in density between hy-drocarbons and water and not a function of thesize of the container (or pore-throat diameter), un-less the size of the void space is insufficient to

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contain molecules of gas and water, which is un-likely for tight-gas reservoirs (figure 2 of Nelson,2009).

Unfortunately, Nelson (2009) does not spec-ify what lack of evidence he is relying upon toconclude that buoyancy is not a controlling forcein certain low-permeability gas accumulations, nordoes he provide evidence of, or a model to de-scribe, the development of nonbuoyant petro-leum systems.

An AAPG Hedberg Conference was held inVail, Colorado, in 2005 to specifically address tight-gas sandstones. At that time, considerable contro-versy existed with what has been called “basin-centered gas systems” (Cumella et al., 2008). Theidea that buoyancywas not a factor in basin-centeredand other tight-gas systems is generally attributedto the following observations in many tight-gas–producing regions: (1) lack of significant waterproduction, (2) lack of clear gas-water contacts,and (3) lack of a well-defined conventional trap-ping mechanism.

Based predominantly on bottom-hole pressureobservations from the San Juan Basin, NewMexico,Nelson and Condon (2008) deduce that buoyancycannot exist within underpressured gas accumu-lations in low-permeability sandstones of the Mesa-verde Group if (my emphasis) all movable waterhas been displaced and no bottomwater is present.They envision any water that may be present asforming a very thin film on mineral grain surfaces,filling only the smallest voids, and that the irre-ducible water, therefore, acts only as a part of thesolid rock container holding the gas and impart-ing no buoyant force (Nelson and Condon, 2008).Unfortunately, Nelson and Condon (2008) do notinclude any Mesaverde water saturation or capil-lary pressure measurements in support of theirconclusions.

The perceived lack of evidence of water in sometight-gas reservoirs may, therefore, lead to the er-roneous conclusion that gas-water buoyant forcescannot exist. The lack of evidence of water in sometight-gas reservoirs may be caused in part by thelack of detailed studies addressing water produc-tion, the source of produced water, and in-situwater saturation.

1444 Discussion

WATER PRODUCTION

Many tight-gas reservoirs produce somewater. Forexample, the stabilized flow rates after recoveryof fracture stimulation fluids average between500 and 900 mcf/day with 1 to 5 bbl of water perday from the east Texas Bossier sands (Newshamand Rushing, 2009). The low-salinity (40,000–60,000 ppm NaCl) produced water is consistentwith laboratory analyses that indicate that 5 to10 mol% of water vapor may be dissolved in gasat reservoir conditions within the deep (12,500–13,500 ft [3810–4115 m]) high-pressure gradient(0.6–0.9 psi/ft [13.57–20.36 kPa/m]) and high-temperature (280–325°F [138–163°C]) reservoirconditions (Newsham and Rushing, 2009). Con-nate water saturations from gas productive Bossiersandstone reservoirs measured from cores obtainedwith low-invasion oil-based drilling mud rangefrom 5 to 50% (Newsham and Rushing, 2001).

Shanley et al. (2004, 2005) and Cluff et al.(2005) have shown that althoughwater productionfrom the major Rocky Mountain tight-gas fieldsin the Green River, Piceance, and Uinta basins isgenerally low, it is too high to be explained aswater derived from gas condensation. Unfortunate-ly, Nelson (2009) and Nelson and Condon (2008)did not include water production data for theunderpressured Mesaverde tight-gas reservoirs.

WATER SATURATION

Warpinski and Lorenz (2008) report data fromthree wells covering a vertical interval of 4300 to8200 ft (1300–2500 m) within an overpressuredgas-producing region at the multiwell experiment(MWX) site at Ruleson field in the Piceance Basin,Colorado. A pressure core using oil-based drillingmud was acquired to provide the most accuratewater saturations possible (Warpinski and Lorenz,2008). These carefully obtained water satura-tion measurements in low-permeability (<0.1 mdin situ) reservoir-quality (>4%porosity)Mesaverdecore samples range from about 20 to 70%. Irre-ducible water saturationmeasured from high-speed

centrifuge capillary pressure tests are quite high,ranging from 30 to 50%.

Recent capillary pressure data also demon-strate that many tight-gas reservoirs are not at ir-reducible water saturation, but instead, the in-situpermeability may be below the critical water sat-uration required to flow water (Cluff et al., 2005).Note that the critical water saturation is muchhigher than irreducible water saturation. There-fore, one cannot simply assume that the lack ofsignificant water production is evidence for irre-ducible water saturation or lack of buoyancy.

BURIAL HISTORY

The inferences by Nelson and Condon (2008) andNelson (2009) regarding the function of buoyancyin forming and controlling the distribution of hy-drocarbons in unconventional tight-gas systems isbased on present-day observations of structure,pressure, porosity, permeability, and water satura-tion. Law (2002) describes the evolution of basin-centered gas systems beginning as normal-pressuredwater-saturated systems that become overpressuredas a result of disequilibrium compaction and laterhydrocarbon generation (and migration); processesthat should be governed by the natural laws ofbuoyancy. Law (2002) also recognizes that somebasin-centered gas accumulations, which he des-ignates as “indirect systems,” may have originatedas buoyancy-dominated conventional oil traps thatlater converted to gas as a result of thermal crackingof oil with increased burial. Underpressured tight-gas sand reservoirs (such as described by Nelsonand Condon, 2008) are thought to be caused bypartial gas loss and reservoir temperature abate-ment during late-stage uplift and erosion (Law,2002). Thus, present-day observations provide onlya snapshot of an evolving petroleum accumulation.

Coskey (2004) demonstrated the importanceof understanding the geology at the time of hy-drocarbon expulsion and migration before makingconclusions regarding the function of buoyancy.For example, at Jonah field in northwestern GreenRiver Basin, Wyoming, the present reservoir qual-

ity of the Lance sandstone reservoirs ranges from2 to 12% porosity, with 0.0005 to 0.6 md perme-ability (DuBois et al., 2004). However, the mod-eled porosity (and related permeability) in theLance reservoirs was likely significantly higher(16–23% porosity) at the time of hydrocarbonexpulsion and migration (Coskey, 2004).

The implications of the study by Coskey (2004)is that present-day reservoir parameters may notrepresent the conditions of the reservoir during thetime of hydrocarbon expulsion and entrapment,indicating that buoyancy may have, in fact, beenthe dominant force forming the distribution ofhydrocarbons in tight-gas accumulations, althoughno clear gas-water contacts can be easily describedat present. These inferences, as shown by Coskey(2004), can easily be tested with simple modelingexperiments. If it is accepted that buoyancy wasa factor in forming tight-gas accumulations, thenthe unconventional reservoir definition of Nelson(2009) becomes moot.

EXPLORATION CONCEPTS

Coleman (2008, p. 246) reviewed 25 yr of tight-gas exploration in a variety of basins across theUnited States fromwhich he recommends, “…drillas high on structure as possible; tight-gas sand-stones were not always as tight as they are now,and fluid buoyancy worked at one time, even if itmay be overcome by pore-throat friction now.”

In describing the same Upper Jurassic BossierFormation tight-gas sandstone reservoirs in eastTexas and north Louisiana that Nelson (2009)shows as occurring below the 1-mm pore-throatunconventional reservoir cutoff, Blanke (2005)concludes that the Bossier reservoir quality is highlydependent on many of the same conditions gov-erning conventional gas accumulations. The higherquality Bossier sandstone reservoirs (as much as0.2 md permeability) are associated with trapspresent at the time of peak hydrocarbon genera-tion and migration where early formed oil accu-mulations, based on pyro-bitumen “cemented”sandstone (Rushing et al., 2004), appear to have

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inhibited extensive authigenic quartz cementa-tion that is present in poor-quality reservoirs lo-cated in downdip parts of the trap.

Perhaps the lingering misconception that mostlow-permeability (e.g., basin-centered) gas accu-mulations are not governed by the natural lawsof buoyancy has impacted exploration efforts insome areas. As Shanley et al. (2005) noted at the2005 Vail Hedberg Conference, there has notbeen a significant tight-gas discovery in the RockyMountains since the discovery at Jonah field in1992. Tight-gas sand explorations based on per-haps more conventional principles in the eastTexas and north Louisiana Gulf Coast basins from1996 to 2006 have resulted in several large (>1 tcf)gas fields including Dew-Mimms Creek, Vernon,and Amoruso fields.

CONCLUSIONS

Nelson (2009) proposed a new definition for un-conventional gas reservoirs that suggests that thenatural laws of buoyancy are not (and were not)the dominant force in forming and maintainingthe observed distributions of hydrocarbons andwater in tight-gas (basin-centered gas) and otherunconventional gas accumulations. However, noevidence is presented to support this concept, butonly a rather weak statement that evidence forbuoyancy is lacking.

Nelson (2009) further proposes that a naturalpore-throat radius threshold of 1 to 2 mm belowwhich buoyancy becomes the subordinate forceexists, and thus the present-day measured pore-throat radius may be used to separate conventionaland unconventional reservoirs. Again, no evidenceis presented to show the absence, or diminishedfunction, of buoyancy in submicron pore-throatradius gas reservoirs.

The misconception that buoyancy is not a fac-tor in basin-centered and other tight-gas systemsis generally attributed to (1) lack of significantwater production, (2) lack of clear gas-water con-tacts, and (3) lack of a well-defined conventionaltrapping mechanism. Several recent studies haveshown that water is much more prevalent in many

1446 Discussion

unconventional tight-gas accumulations than pre-viously thought, and traps can be described by con-ventional structural or stratigraphic trap mech-anisms. Burial history studies and experimentalmodels show that many unconventional tight-gasaccumulations have evolved with time, and thuslimited present-day observations may lead to mis-conceptions regarding the function of buoyancyin forming and maintaining the distribution of wa-ter and gas.

The contention of Nelson (2009) that buoy-ancy is not a dominant factor in the formation anddisposition of hydrocarbons and water in tight-gassandstone (unconventional) accumulations, whichcan be defined by pore-throat threshold below 1 to2 mm, is inconsistent with several recent studies oftight-gas accumulations and requires further clar-ification and supporting evidence. Otherwise, it isrecommended that this definition be abandoned.

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Camp, W. K., 2008, Basin-centered gas or subtle conven-tional traps?, in S. P. Cumella, K. W. Shanley, and W. K.Camp, eds., Understanding, exploring, and developingtight-gas sands: 2005 Vail Hedberg Conference: AAPGHedberg Series 3, p. 49–62.

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