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Chapter 17

A New Paradigm for Gas Exploration inAnomalously Pressured “Tight Gas Sands” in

the Rocky Mountain Laramide Basins

Ronald C. SurdamInstitute for Energy Research, University of Wyoming

Laramie, Wyoming, U.S.A.

ABSTRACT

A significant portion of the Cretaceous shales in the Rocky MountainLaramide Basins (RMLB) are overpressured on a basinwide scale. Thechange of pressure regime from normally pressured to overpressured coin-cides with marked changes in the geochemical and geophysical properties of the Cretaceous rock/fluid system. Sandstone bodies within the overpres-sured shale section are subdivided stratigraphically and diagenetically intorelatively small, isolated, gas-saturated, anomalously pressured compart-ments. The driving mechanism of the pressure compartmentalization is the

generation and storage of liquid hydrocarbons that subsequently react togas, converting the fluid-flow system to a multiphase regime in which capil-larity controls permeability.

A new exploration paradigm and an exploitation strategy have been creat-ed that significantly reduce exploration risk in the RMLB. Two elements cru-cial to the development of prospects in the deep, gas-saturated portions of the RMLB are (1) the determination and, if possible, three-dimensional eval-uation of the pressure boundary between normal and anomalous pressureregimes and (2) the detection and delineation of porosity/permeability“sweet spots” (i.e., areas of enhanced storage capacity and deliverability) inpotential reservoir targets below this boundary. Certainly there are othercritical aspects, but completion of these two tasks is essential to the success-ful exploration for the unconventional gas resources present in anomalouslypressured rock/fluid systems in the RMLB.

INTRODUCTION

Only recently have techniques been developed toexamine in detail the boundary between normallypressured (pressure gradient = 0.40–0.50 psi/ft) and

anomalously pressured (pressure gradient ≠0.40–0.50 psi/ft) rock/fluid systems (Surdam et al., 1995,1997). Numerous gas discoveries made in the West-ern Canada Basin in the last decade demonstrate thatthe delineation of this pressure boundary is an

Surdam, R.C., 1997, A new paradigm for gas explo-ration in anomalously pressured “tight gas sands”in the Rocky Mountain Laramide Basins, in R.C.

Surdam, ed., Seals, traps, and the petroleum sys-tem: AAPG Memoir 67, p. 283–298.



284 Surdam

important aspect in searching for unconventional gas

accumulations in basins characterized by anom-alously pressured sections (Surdam et al., 1995). Forexample, Leach (1994), using an enormous databasein the Louisiana Gulf Coast, demonstrated that themajority of oil and gas discoveries are within 2000 ft(610 m) of the top of the anomalous pressure (i.e., thepressure boundary). Surdam et al. (1995) have shownthat 80% of the gas production from Cretaceous rocksin the Rocky Mountain Laramide Basins (RMLB)(Figure 1) is from an interval extending from thepressure boundary down to 2000 ft (610 m) belowthis boundary (Figure 2). Surdam et al. (1995) suggestthat establishing the shape of the top of the anom-alously pressured rock can be critical in the detection

of commercial gas accumulations (Figure 3). Positiverelief on the pressure boundary surface commonlycan be correlated to commercial gas accumulations(Figure 4).

In addition, unlike conventional gas accumulations(e.g., with gas cap, oil leg, and oil/water contact),accumulations associated with anomalously pres-sured regimes (e.g., depletion drive) like those in theRMLB (Figure 1) are not necessarily constrained byeither structural closure or stratigraphic pinch-out.Consequently, when conventional exploration tech-nology is used in the search for these unconventional

gas accumulations, results can be disappointing andcostly. Thus, delineating zones of enhanced storagecapacity (i.e., porosity) and deliverability (i.e., perme-ability), or “sweet spots,” is also an important part of the new exploration paradigm.

In order to demonstrate the applicability of a newexploration paradigm, this chapter will focus specifi-cally on the Cretaceous sandstones of the Mesaverde

Group in the Washakie Basin, but will be generallyapplicable to all basins of the RMLB, which have verysimilar fluid-flow characteristics. It is not the goal of this chapter to provide a detailed overview of the pres-sure regimes in the RMLB, or even in the WashakieBasin. Previous research on the Mesaverde Group has been published in annual and topical technical reportsavailable through the Gas Research Institute (GRI)(Surdam et al., 1993, 1995; Iverson, 1995; Jaworoski etal., 1995).

The aim of this paper is to (1) summarize previouswork on the delineation the pressure boundary in theWashakie Basin, (2) discuss a methodology for detec-tion and delineation of hydrocarbon production sweet

spots beneath the basinwide pressure boundaries inLaramide basins, and, most importantly, (3) discussthe new exploration paradigm (Figure 3) for basin-center, or deep-basin gas, in the RMLB.

GEOLOGICAL SETTING

The Washakie Basin is the easternmost subbasin of the Greater Green River Basin (GGRB). The subbasinis a symmetrical structural and topographic basin bounded on th e east by th e Sier ra Madre, on th enorth by the Wamsutter arch, on the west by the RockSprings uplift, and on the south by Cherokee Ridge

(Figure 5). It is 42 mi (68 km) north-south and 54 mi(87 km) east-west; its area is roughly 2200 mi 2 (3550km2). The surface elevation in the basin ranges from6100 to 8700 ft and averages 7000 ft (2130 m). The dipof the Cretaceous strata into the basin is approxi-mately 8° along the eastern flank and 15° along thewestern flank (Love, 1970). Precambrian rocks lie atdepths o f >32,000 ft (9750 m) at the center of theWashakie Basin. Approximately 32,000 ft (9750 m) of Cambrian through Tertiary sedimentary rocks arepresent in the deeper part of the Washakie Basin(Hale, 1961). The deepest part of the Upper Cretaceoussection, in the basin’s center, is at ~14,000 ft (4270 m)depth (Roehler, 1969).

The present study is concerned with the Upper Cre-taceous rocks in the basin, which include, from oldest toyoungest, the Mesaverde Group (especially the AlmondFormation), the Lewis Shale, the Fox Hills Sandstone,and the Lance Formation. The Lewis Shale, which repre-sents a major marine transgression, is ≤2700 ft (820 m)thick, and is composed mainly of shale. The shale of thelowermost Lewis is black, carbonaceous, and biotur- bated; in places, shell debris is abundant. The Lewis seaopened eastward into the main part of the North Ameri-can Interior Seaway (Winn et al., 1985). The maximumextent of the Lewis transgression was to areas of the

Figure 1. Index map showing the locations of mostof the Rocky Mountain Laramide Basins (RMLB).



Rock Springs uplift in the west and the Wind Riveruplift in the north. The Almond Formation is theuppermost sandstone of the Mesaverde Group (Miller,1977). It was deposited in marginal-marine andmarine environments during the final transgression of the Cretaceous seaway into southwestern Wyoming.The Almond ranges in thickness from 300 to 800 ft(90–240 m) (Roehler, 1990), and is composed of sand-stone, siltstone, shale, and coal beds.

Depositional patterns in the Upper CretaceousMesaverde Group and the Lewis Shale show a contin-

uous trend toward strandline irregularity. The upperpart of the Lewis Shale and the Fox Hills Sandstonerepresent the withdrawal of the Cretaceous seawayfrom the area (Law et al., 1986). The overlying LanceFormation was deposited primarily in an alluvial-plain environment. In response to continued Laramidedeformation and the emergence of adjacent forelanduplifts, an internal drainage system developed on theLance Formation. A similar trend in the eventualdevelopment of internal drainage during the Late Cre-taceous has been noted in the Uinta Basin of Utah(Fouch et al., 1983).

PRESSURE BOUNDARY

The trend in acoustic velocity on sonic logs is a con-ventional method used to detect anomalous pressurein shales (Powley, 1982, personal communication) andto delineate the pressure boundary. Typically, amarked decrease in velocity (i.e., increase in sonic-transit time) is interpreted as indicating lower effectiverock stress and, hence, overpressuring (Figure 4). Pow-ley (1982) cites many examples of pressure compart-ments delineated on the basis of analyzing trends in

transit time from sonic logs. Almost all of the soniclogs from the Washakie Basin have a marked decreasein velocity in the deeper part of the well logs similar tothat illustrated in Figure 4. Thus, the pressure bound-ary can be neatly mapped regionally using thismarked decrease in acoustic velocity.

In order to study the basinwide configuration of overpressuring in the Washakie Basin, three west-eastcross sections across the basin were constructed from41 sonic logs filtered to include only fine-grained rocks(defined as those intervals with gamma-ray log valuesgreater than 65° API). Figure 5 shows the locations of

A New Paradigm for Gas Exploration in Anomalously Pressured “Tight Gas Sands” 285

0 100 200 300 400 5005000

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Figure 2. Plots of cumulative production vs. distance to the top of the anomalously pressured zone for majorgas reservoirs (>5 bcf) in the Washakie, Powder River, Bighorn, and Wind River basins. Plots indicate that>80% of the gas production from Cretaceous rocks in these basins is from an interval extending from the pres-

sure compartment boundary down to 2000 ft (610 m) below the boundary.



286 Surdam

these cross sections, which are denoted T13N, T15N,and T17N for the townships they occupy. The threecross sections are of almost equal length and run fromR102W to R91W.

Figure 6A–C provides a better understanding of thepressure regime in the Upper Cretaceous shales in theWashakie Basin. These figures show a panel of soniclogs with the velocities corrected for compaction usingthe method outlined by Surdam et al. (1995; Figure 7).

Surdam et al. (1995) outline in detail the detection anddelineation of the regional pressure boundary in this basin. The following is a brief synopsis of the resultsreported:

• In all areas studied away from the WashakieBasin margin, the onset of anomalous pressures(the overpressure top) is marked by a transitionalzone of slightly overpressured rocks, ~500–1500 ft(~152–457 m) thick. It appears that overpressur-ing within the Cretaceous shales developedregionally as one basinwide compartment (Law,1984). In the Washakie Basin, the present-daydepth of the overpressure top ranges from 8000 to

10,000 ft (2440–3050 m).• Cross section T13N (Figure 6A), in the southernpart of the Washakie Basin, shows that the UpperCretaceous Almond Formation is buried to>14,000 ft (4270 m) depth [equivalent elevation,–7000 ft (–2130 m)]. The top of overpressuringcuts across structural and stratigraphic bound-aries and is not horizontal, but has a wavy form.Toward the west, the top of overpressuring occursat about 8000 ft (~2440 m) depth [equivalent ele-vation, –1000 ft (–310 m)], and toward the east at9000 ft (~2745 m) depth [equivalent elevation,

UNCONVENTIONAL CONVENTIONAL

"Pressure Seal" n o

r m a l

p r e s s u r e

a n o m

a l o u s

p r e s s u r e

"Sweet Spots"

30005000

2000

4000

6000

8000

10000

12000

14000

D e p t h , m

Velocity, m/s

Velocity from Sonic Logs

Figure 3. Schematicdiagram illustrating thetwo elements crucial tohydrocarbon explorationin gas-saturated, anom-alously pressured rock:(1) the pressure boundaryand (2) sweet spots. Gas

accumulations below thepressure boundary areindependent of structuralclosure or stratigraphicpinch-out.

Figure 4. Plot of velocity vs. depth for theMcPherson Springs 14-2 well, Sec. 14, T13N, R94W,Washakie Basin.



–2000 ft (–610 m)]. In the eastern portion, somelenticular compartments are shown.

• Cross section T15N (Figure 6B), in the central partof the basin, shows the Almond Formation is

buried down to 15,000 ft (4570 m) depth [equiva-lent elevation, –8000 ft (–2440 m)]. The top of over-pressuring is almost horizontal and occurs at~10,000 ft (3050 m) depth [equivalent elevation,–3000 ft (–910 m)].

• Cross section T17N (Figure 6C), in the northernpart of the basin near the Wamsutter arch, showsthat the Almond Formation is buried more shal-lowly there than in other areas of the basin, at<13,000 ft (3960 m) present-day depth [equivalentelevation, –6000 ft (–1830 m)]. The lower part of the basin appears to narrow more than along the

other cross sections. The boundary between nor-mal and anomalous pressure appears horizontal,and occurs at ~11,000 ft (3350 m) depth [equiva-lent elevation, –2000 ft (–610 m)]. Some lenticular

compartments are shown.• Viewed south to north, the top of overpressuringin the Washakie Basin appears to be bowl-shaped,deeper in the center and shallower at the northand south margins.

In summary, the top of overpressuring in theWashakie Basin is an uneven surface. Above the over-pressured zones, there is a transitional zone that isthicker in some areas than in others. Through the useof the sonic-log panels, the top of the overpressuredzone can be clearly determined. However, the bottom


28N

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Green River

Rock Springs

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G R E

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T17N

Figure 5. Location map of the sonic log profiles collected in the Washakie Basin. The cross sections shown areT13N, T15N, and T17N.



288 Surdam

1.8E4 5.4E4 9.0E4 1.3E5 1.6E5 2.0E5 2.3E5 2.7E5 3.1E5 3.4E5-8000

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Washakie Basin, Wyoming

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Distance, ft

9500

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A

B

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Figure 6. Griddedvelocity profiles, afterdecompaction, derivedfrom a panel of soniclogs through theLower Tertiary andUpper Cretaceousstratigraphic section in

the southern WashakieBasin, Wyoming. (A)Decompacted velocityprofile for crosssection T13N, R102W-91W. (B) Decompactedvelocity profile forcross section T15N,R102W-91W. (C)Decompacted velocityprofile for crosssection T17N, R102W-91W. For localities ofcross sections, see

Figures 1–5.



of the overpressured zone has not been identifiedregionally, as only a few wells have been drilled throughthe basinwide overpressured shale compartment in theWashakie Basin. In the paper, the discussion is directedat the exploration for gas beneath the pressure bound-ary, in the gas-saturated portion of the column.

SWEET SPOTS

Sweet Spot Definition

As shown in Figure 3, the key to gas production below the regional pressure boundary is the detectionand delineation of so-called “sweet spots.” In thispaper, sweet spots are defined roughly as those reser-voir rocks that are characterized by porosity and per-meability values greater than the average values fortight sands at a specific depth interval. In other words,sweet spot sandstones represent enhanced hydrocarbon storage and deliverability within a target reservoir

interval. Figure 8 is an example of sweet spot sand-stones in the Almond Formation of the WashakieBasin. As Iverson (1995) points out, these storage/deliverability sweet spots translate directly intoenhanced hydrocarbon production.

To illustrate the importance of sweet spots to gas pro-duction in anomalously pressured rocks, consider thegiant Hoadley gas field in the Western Canada Basin(Figure 9). As Chiang (1984) pointed out, “[t]he discoveryof the Hoadley gas field not only adds a great amount of gas reserve in Canada [~10% at 6–7 tcf] but strongly indi-cates the overlooked potential of both tighter sand reser-

voir rock and sweet spots in it. Prior to discovery, theHoadley glauconitic sand bar had been penetrated byhundreds of wells, none recognizing that this sand barcomplex contains a giant reserve of recoverable gas.” In brief, the keys to unlocking the hydrocarbon reserves atHoadley (6–7 tcf of gas and 350–400 million bbl of con-densate) were recognizing that (1) a regional pressure boundary existed and (2) that sand ridges and conglom-eratic beaches represented sweet spots below the pres-sure boundary (Figure 10). Once explorationistsrecognized these keys, the focus of their efforts shiftedfrom conventional hydrocarbon accumulations in struc-tural and stratigraphic traps in the normally pressured,water-saturated portion of the column to porosity/per-

meability sweet spots below the pressure boundary inanomalously pressured, gas-saturated rock. The spectac-ular results of this exploration shift are summarized inFigure 10. The history of the giant Hoadley gas field dis-covery is a beautiful example of the importance of sweetspot delineation and detection when searching for gas beneath the regional pressure boundary.

Sweet Spot Importance

The importance of porosity/permeability sweet spotsto production in basins characterized by regionally


0

5

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Tight Gas SandSweet Spot

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11680-12934

Mesaverde

PZone:

Figure 8. Plot of porosity vs. depth for Almondsandstones from 2300 measurements in theWashakie Basin. Shaded area shows a sweet spot inthese tight gas sandstones.

Figure 7. Plot of velocity vs. depth corrected for com-

paction for the McPherson Springs 14-2 well, Sec.14, T13N, R94W, Washakie Basin. Cross-hatchingshows area where rocks are anomalously pressuredand gas saturated.



290 Surdam

significant anomalous pressures is undeniable. This is because sweet spots typically provide a large continu-ous horizontal fluid conduit into an otherwise inacces-sible large volume of low-permeability rock (e.g., tightsandstone). Above the pressure boundary, the rockstend to be more porous and permeable due to less bur-ial and, consequently, less cementation. Also, rocksabove the boundary tend to be in a single-phase fluid-flow system; here, low-permeability rocks tend to beflow barriers and not seals, except very locally and inclose proximity to conventional hydrocarbon accumu-lations. In contrast, rocks below the pressure bound-ary are in a multiphase fluid-flow system, androck-fluid characteristics regionally are dominated by

capillarity. As a consequence, low-permeability rocks below the pressure boundary, including tight sands, become capi llary seals. Simply stated, these tightsands give up their fluids only when their displace-ment or threshold pressures are exceeded. Thus, theimportance of sweet spots becomes apparent; sweetspots allow the fluid flow in the reservoir volume,including both the sweet spot and adjacent tightsands, to be manipulated. For example, pressure dif-ferentials between a sweet spot and the adjacent tightsands can be created such that displacement pressurescan be overcome, resulting in the drainage of much

larger volumes than is possible from the sweet spotalone. An example of this production mechanism isshown in Figure 11. As a consequence, fields like theStandard Draw-Echo Springs not only have producedmore gas than could be stored in the sweet sand (i.e.,an Almond sandstone), but also show little or no

drawdown after 14 years of production (Figure 12;Iverson and Surdam, 1995). Iverson (1995) discusses indetail the significance of reservoir sweet spots to pro-duction in the Washakie Basin.

Sweet Spot Characteristics

Sweet spots are characterized by important attri- butes in addition to enhanced porosity and permeabil-ity. One of these is the tendency of a particulardepositional facies to have the maximum sweet spotpotential in a formation. Typically, in every basin andtargeted reservoir interval, one or two specific deposi-tional facies will comprise the porosity/permeability

sweet spot. In the Hoadley field, the best reservoirfacies are the eolian beach ridges and conglomeratic beaches; in the Mesaverde Group in the WashakieBasin, the group of marine bar sands collectivelyknown as the Upper Almond bar sands comprise thesweet spot (Figure 13). Thus, one of the first steps inevaluating targets for gas production beneath the pres-sure boundary is the determination of which deposi-tional facies has highest potential for enhanced storagecapacity and deliverability.

For those basins with production histories, thesize of the sweet spot targets can be determined by

Gas Well

Sand Ridge

0 10 mi

M A R I N E

B A R R

I E R B A

R

L A G O O N

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PRESSURED

OVER

PRESSURED

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Ar c h

F o o

t h i l l s

P e a c e R i v e r

S w

e e t g

r a s s

A r c h

Canadian Shield

Edmonton

S a s

k a t c

h e

w a

n

A l b

e r

t a

B r i t i s h

C o

l u m

b i a

Elmworth

Hoadley

Calgary

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o u n t a

i n s

D i s

t u r

b e d B e l t

0 100

Miles

200 Figure 10. Map showing facies of the Hoadley barri-er bar complex. Well distribution is related to thepressure boundaries. Modified from Chiang (1984).

Figure 9. Location map showing the largest gasfields in the Alberta Basin of Canada. Modifiedfrom Chiang (1984).



constructing a potentiometric surface map of the basin(Figure 14). Our potentiometric surface constructionsutilize pressures from drill-stem tests (DSTs). Allavailable nonzero data are used. Poor tests and dataare indicative of low-permeability rocks and tests tooshort to accurately measure pressure. Good tests anddata are indicative of high-permeability rocks. Ourpotentiometric surface maps delineate which rocks arepermeable and which are not, and give good approxi-mations of the size of permeable sweet spots withinthe basin of interest (Figure 14).

The degree to which the sweet spots are in fluidcontinuity is of particular importance in evaluatinggas column heights and designing well stimulation

strategies. Fluid continuity commonly can be evalu-ated by considering the initial production pressures of all available reservoirs from a variety of fields vs.depth (Figure 15). Typically, reservoirs above the pres-sure boundary follow a hydrostatic gradient withdepth. Below the pressure boundary, reservoir sand-stones follow either a single, but different, gradientwith depth, or they exhibit significant scatter.

When the sandstones follow a single gradient, it isa pressure-depth gradient parallel to the regionallithostatic gradient but shifted to much lower pres-sures (Figure 15). In such a case, each reservoir below

the pressure boundary is affected not only by theweight of the water column above the boundary butalso by the weight of the rock and fluid column fromthe pressure boundary down to the depth of the reser-voir. Under these conditions, there is no fluid continuity between individual reservoirs; they are compartmental-ized and isolated (Figure 15).

In contrast, if the reservoir sandstones show signifi-cant scatter from a single gradient below the pressure boundary, there is a much greater potential for fluidcontinuity (connectivity) between individual reser-voirs or fields. The tendency of connected reservoirs below the pressure boundary to follow a gas gradientwith depth provides some evidence of this. In other

Laramide basins, we have noted that at and immedi-ately below the pressure boundary, individual reser-voirs follow a gas gradient before changing to asandstone pressure gradient with greater depth (Fig-ure 15). This suggests that, in some basins, there ismore reservoir connectivity than at the top of theanomalously pressured rock.

Relationship of Diagenesis to Sweet Spots

Yin and Surdam (1995) describe in detail the diage-netic history of Almond Formation sandstones. The


Lower AlmondLaminated SandsPD = 400 psi Pf = 5200 psi

(Pp = 4800 psi) φ = 3%K = 0.002 md

Upper Almond Bar SandPD = 20 psi Pf = 5200 psi(Pp = 3000 psi) φ = 12%K = 0.2 md

Lewis ShalePD = 2000 psi

In order for gas to movefrom laminated sandstones

to "bar" sand ∆P > 400psi

Figure 11. Schematic diagram of the production mechanism at work in a sweet spot. The threshold pressure ofthe sands must be exceeded to allow gas to migrate from the tight, laminated sandstones to the adjacentporous, permeable sweet sands.



292 Surdam

Almond sandstones and, for that matter, all MesaverdeGroup sandstones lose porosity with progressive burialat a greater rate than is typical for most sandstones (Fig-ure 8). This loss of porosity is the result of both mechan-ical compaction and cementation. Some Almondsandstones (i.e., tidal channel) lose very little intergranu-lar volume (i.e., ~ minus — cement porosity) with pro-gressive burial and suffer porosity loss primarily from

cementation (Yin and Surdam, 1995). In contrast, manyof the other depositional facies in the Almond sand-stones lose significant intergranular volume during the burial due to compaction and grain deformation. Volu-metrically, the most damaging cements in MesaverdeGroup sandstones are quartz and late dolomite. Anotherimportant factor in porosity damage is the presence anddeformation of ductile lithic framework grains (espe-

cially in shoreface sandstones).To find those Almond sandstones that are charac-

terized by enhanced porosity (i.e., sweet spots), explo-rationists should target sandstones where quartz andlate dolomite cementation have been inhibited andlithic framework grains are sparse. The tidal channelsandstones contain less lithic grains and late dolomitecement than the shoreface sandstones. In addition,abundant early calcite cement in the tidal channelsandstones provided an opportunity for enhancementof porosity through its subsequent dissolution. Thus,the tidal channel sandstones are usually associatedwith permeability sweet spots. However, some quartz-rich sandstones are tightly cemented by quartz over-

growths at great depth, where both porosity andpermeability have been seriously damaged.

The five most important processes that inhibitcementation during diagenesis are as follows:

• Grain-rimming clay rims (inhibition of quartzcementation)

• Dissolution of early carbonate cements (preserva-tion of intergranular volume)

• Overpressuring (compaction retardation)• Liquid hydrocarbon accumulation (cementation

abatement)• Fracturing (porosity/permeability enhancement)

0.1

1

10

number of wells

Champlin #226

decline at -0.0545/yrabandon at 30 MMscf/yrEUR = 55 Bscf at 2068

100

1000

1980 1984 1988 1992

YEAR

G a s P r o d u c t i o n

( B s c f / y e a r )

, ,

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Figure 12. Plot of gas production vs. time showingthat production has declined much less than expect-ed in the Standard Draw area.

Figure 13. Diagrammaticcross section showingrestored Upper Cretaceousrocks across northern Utahand southern Wyoming.Rocks of continental originare shaded; alluvial-plainand marine-shoreline, shelf,slope, and basin sandstoneand siltstone units areshown by dot pattern;marine shale and limestone

are unpatterned. The loca-tion of subaerially formedunconformities is indicatedby the squiggly line.Modified from Roehler(1990).



Yin and Surdam (1995) have shown that i n theMesaverde Group sandstones (including the Almondsandstones) in the Washakie Basin, clay coats are volu-metrically insignificant and dissolution of carbonatecements is only locally important. Moreover, as Surdamet al. (1995) have demonstrated, overpressuring of theMesaverde Group is coincident with maximum burialand the oil-to-gas reaction. As a consequence, overpres-suring of these rocks occurs relatively late in the burialhistory, after the sandstones have suffered extensive

cementation. Thus, fracturing and/or early migration of liquid hydrocarbons appear to be the two processes thatmight play a role in enhancing porosity/permeability inthe Mesaverde Group sandstones in the Washakie Basin.

Relationship of Fracturing andRegional Lineaments to Sweet Spots

Iverson (1995) used oil and gas production data toclassify oil and gas fields within the Washakie Basininto four categories. Cumulative production of >$500million constitutes a largest field; $50–$500 million, asignificant field; $5–$50 million, a small field; and <$5million, a smallest field.

Figure 16 shows the spatial relationship betweenthe regional lineaments and the hydrocarbon accumu-lations. There is an obvious relationship between thedistribution of hydrocarbon accumulations in fieldswith >$5 million cumulative production and the mostprominent regional lineaments. For example, most of the accumulations in these fields are either cut or ter-minated by the lineaments (Figure 16). Conversely,few of the smallest fields (<$5 million) are associatedwith mapped regional lineaments (Figure 16).

A good illustration of this relationship is the hydro-carbon accumulation at the Standard Draw-Echo

Springs field (Figure 17). One lineament forms thenorthernmost limit of production in this field, whereas


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Figure 14. Contoured potentio-metric surface of drill-stem testdata from the MesaverdeFormation in the Washakie Basin.Modified from Heasler andSurdam (1992).

Figure 15. Pressure profile for the major Cretaceous gasreservoir pressures in the Washakie Basin. Above thepressure boundary, the reservoir pressure follows ahydrostatic gradient. Below this boundary, the reser-voir pressure is parallel to the regional lithostatic gradi-ent, but is offset to lower pressures than this gradient.



294 Surdam

a second parallel subsurface lineament cuts through asaddle in the isopach map of the Almond bar sand-stone. This saddle coincides with some of the mostproductive wells in the field. The coincidence of thin-ning of the bar sand and position of the geophysicallineament suggests that basement faults were reacti-vated during the deposition of the bar sand. The factthat some hydrocarbon accumulations are bounded bythe lineaments further supports the hypothesis that asignificant cause-and-effect relationship exists between the regional lineaments and the more signifi-cant hydrocarbon accumulations in the WashakieBasin (e.g., sweet spots).

Fracturing of reservoir sandstones adjacent to

these lineaments is prevalent (Dunn et al., 1995).Moreover, the smaller scale fracturing adjacent to theregionally prominent lineaments enhances perme-ability (Figure 18; Dunn et al., 1995). Most impor-tantly, Jaworowski et al. (1995) have demonstratedthat the regional lineaments and the associatedsmaller scale fractures in adjacent sandstones typi-cally have the same spatial orientation. There is littledoubt that fracturing has improved deliverability insweet spot sandstones, and has probably signifi-cantly improved the connectivity of sweet spot sand-stones and nearby tighter sandstones.

Relationship of Source Rocks to Sweet Spots

If the relationship between source rocks and reser-voir rocks is examined, the significance of fracturing becomes even more evident. García-González et al.(1993a, b; 1997) demonstrated that coal is the majorsource of liquid and gaseous hydrocarbons in theMesaverde Group sandstones in the Washakie Basin by showing that (1) oi l in the coal was generatedduring the alteration of desmocollinite and liptinitemacerals into waxy oil and inertinite solid residue;(2) the waxy oil was initially stored in porous struc-tures and subsequently in vesicles as the coalmatured under increasing temperature; (3) primary

migration of the oil occurred as the generation of asufficient volume of waxy oil microfractured the vit-rinite-semifusinite vesicles, interconnecting vesicles,and pores; and (4) the thermal cracking of waxy oilto gas generated a sufficient volume of gas to frac-ture the vesiculated coal as pore pressure increasedand allowed expulsion and migration of hydrocarbons out of the coal.

Figure 19 summarizes the maturation scenario forcoals in the GGRB and elsewhere. The only way waxyoil could have been expelled from the coal was either by reacting to gas, as outlined above, or by tectonically

Figure 16. Map showing the relationship between major lineaments and oil/gas fields in the Washakie Basin.Color coding represents the dollar value of cumulative production of each hydrocarbon field. Note the rela-tionship between major fields and regional lineaments. Lineaments mark the position of regionally signifi-cant shear zones characterized by recurrent movement.



fracturing the coal during the generation of the oil.Only fracturing would have resulted in the migrationof oil into adjacent sandstones before diagenesis wascomplete and the sandstones became cemented.Regional lineaments characterized by recurrent move-ment would have been ideal locations for the earlymigration of liquid hydrocarbons out of coals intoadjacent sandstones, which would have resulted in

cementation abatement. Thus, early liquid hydrocar- bo n mi grat io n and fr ac tu ring (i .e., es tab lishin gsource/reservoir connection) were key elements in theformation of reservoir sweet spots in the MesaverdeGroup sandstones of the Washakie Basin.

NEW EXPLORATION PARADIGM

On the basis of the present discussion of reservoirsweet spots and previous discussion of pressureregimes, regional lineaments, fractures, and stratigraphy

(Surdam et al., 1993) a new exploration paradigm has been constructed that significantly reduces explo-ration risk for for hydrocarbons in anomalously pres-sured, gas-saturated rocks beneath the pressure boundary in the Rocky Mountain Laramide Basins(RMLB) (Figure 3).

This paradigm, which focuses on the Almond For-mation in the Washakie Basin, can be used as a basis

for the construction of a new and more innovativeexploitation strategy for gas exploration in anom-alously pressured “tight gas sands” in the RMLB. Theapplication of this paradigm depends on the comple-tion of two crucial tasks: (1) determining and evaluat-ing, in three dimensions, the boundary betweennormal and anomalous pressure and (2) detecting anddelineating porosity/permeability sweet spots.

It is imperative to delineate the posi tion of the boundary between overlying, normally pressuredrock and the underlying, anomalously pressured rock, because 80% of the cumulative gas production in each


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Figure 17. Map showingthickness of the Almondbar sandstone in StandardDraw-Echo Springs. Opencircles indicate wells with10 Bcf production. Tworegional lineaments fromFigures 1–16 are shown.

The top lineament formsthe northern terminus. Thebottom lineament inter-sects the bar sand on a sad-dle on the isopach; thislineament correlates withthe better wells in the field.Note that no correlationexists between the bar sandthickness and the mostproductive wells. Modifiedfrom Christiansen, 1995.



296 Surdam

basin of th e RM LB co me s fr om th e st rat igr ap hi c

interval between the pressure boundary and 2000 ft(610 m) below the boundary (Figure 2). The detectionof any three-dimensional relief on the pressure boundary is also very important, particularly ele-vated portions of the surface. For example, Davis(1984) showed that in the Western Canada Basin,major anomalously pressured gas accumulations areassociated with relief on the surface of the pressure boundary . If the pr essu re boundary surf ace cutsacross stratigraphic units, it can serve nicely as a sealor trap updip (Figure 3). Establishing the position of the pressure boundary has also been shown to beimportant in the Louisiana Gulf Coast, which, unlikethe RMLB, has experienced little or no uplift and ero-

sion, but does exhibit anomalous pressures (Leach,1994). Thus, delineating the pressure boundary iscrucial to any exploration strategy in basins charac-terized by an anomalously pressured stratigraphicinterval.

Another critical aspect of exploration for gas below the regional pressure boundary in the RMLBis the detection and delineation of reservoir sweetspots, as they typically provide significant continu-ous horizontal fluid conduits into otherwise inacces-sible, large volumes of low-permeability rock (e.g.,tight gas sands). The depositional settings of most of the basins of the RMLB were highly variable, so awide variety of factors control the formation and

position of sweet spots in the anomalously pressuredsection; very rarely is only a single factor responsiblefor the development of a sweet spot (Surdam et al.,1995). Because of this, sandstone reservoir systems inthe RMLB are characterized by multiphase fluid-flow behavior and are dominated by capillarity andrelative permeability; these highly variable sandsgive up their fluids only when their displacement(threshold) pressures are exceeded. Thus, the impor-tance of reservoir sweet spots becomes apparent;sweet spots allow the pressure regime and, hence,the fluid-flow system in the total reservoir volume

(including both the sweet spot and the adjacent tightsands) to be manipulated during production.The following methodology is used to translate

this new exploration paradigm into an explorationstrategy:

• Determination of the position of the pressure boundary (i.e., the boundary between normal andanomalous pressure regimes);

• Evaluation of the three-dimensional (3-D)aspects of the pressure boundary surface, withspecial emphasis on areas characterized by posi-tive relief;

• Determination of which depositional facies have

the greatest potential for enhanced storage capac-ity and deliverability below the pressure bound-ary (i.e., which are sweet spots);

• Documentation of the potential determinative ele-ments that control sweet spot development in thetargeted lithofacies (e.g., fractures, early migra-tion of liquid hydrocarbons, chlorite rims, over-pressuring, and dissolution of early carbonatecement);

• Detection and delineation of sweet spots using2-D and 3-D models of electric log response andseismic data.

B U R I A L

i n c r e a s i n g t h e r m a l e x p o s u r e

DRYGAS

Expulsion of"dry gas"

(kerogen ⇒ gas)

Immature Coal

COAL

Expulsion of"wet gas"(oil ⇒ gas)

WETGAS

WithFaulting

Generation of oilwith storageor migration

Figure 19. Schematic diagram of a typical maturationscenario for coal. Vertical sequence of blocks on theleft indicates maturation scenario in the absence offracturing; the block on the right indicates matura-tion scenario after fracturing has occurred.

Figure 18. Partially cemented (quartz and kaolinite)fracture in the Upper Almond bar sand, northernend of the Echo Springs field, Wyoming.



ACKNOWLEDGMENTS

This study was funded through the Gas ResearchInstitute under Contract No. 5091-221-2146. The origi-nal document was reviewed by Kathy Kirkaldie.Graphic assistance was provided by Allory Deiss.

REFERENCES CITEDChiang, K.K., 1984, The giant Hoadley gas field, South

Central Alberta, in J. Masters, ed., Elmworth—casestudy of a deep basin gas field: AAPG Memoir 38,p. 297–313.

Christiansen, G.E., 1995, Factors influencing differen-tial natural gas production from the upper Creta-ceous Upper Almond Formation, Wamsutter archarea, Sweetwater and Carbon counties, Wyoming:Master’s thesis, University of Wyoming, Laramie,Wyoming, 157 p.

Davis, T.B., 1984, Subsurface pressure profiles in gas-saturated basins, in J.A. Masters, ed., Elmworth—

case study of a deep basin gas field: AAPG Memoir38, p. 189–203.Dunn, T.L., W.P. Iverson, B. Aguado, J. Humphreys,

and R.C. Surdam, 1995, Improvements to reservoirevaluation and characterization, Almond Forma-tion, Green River Basin, Wyoming, in An engineer-ing and geologic evaluation of a horizontal gas wellcompletion in the Almond sandstone-Echo Springsfield, Greater Green River Basin, Wyoming: GRITopical Report GRI-95/0066, p. 31–70.

Fouch, T.D., T.F. Lawton, D.J. Nichols, W.B. Cashion,and W.A. Cobban, 1983, Patterns and timing of syn-orogenic sedimentation in Upper Cretaceous rocksof central and northeast Utah, in M. Reynolds and

E. Dolly, eds., Mesozoic paleogeography of thewest-central United States: Rocky Mountain Sectionof SEPM, Rocky Mountain Paleogeography Sympo-sium 2, p. 305–336.

García-González, M., D.B. MacGowan, and R.C. Sur-dam, 1993a, Mechanisms of petroleum generationfrom coal, as evidenced from petrographic andgeochemical studies: examples from AlmondFormation coals in the Greater Green River Basin, inS. Andrew and B. Strook, eds., Wyoming geology–past, present and future: Casper, Wyoming,Wyoming Geological Association Jubilee Anniver-sary Guidebook, p. 311–324.

Garcia-González, M., D.B. MacGowan, and R.C. Sur-

dam, 1993b, Coal as a source rock of petroleum andgas—a comparison between natural and artificialmaturation of the Almond Formation coals, GreaterGreen River Basin in Wyoming, in D. Howell, ed.,The future of energy gases: U.S. Geological SurveyProfessional Paper 1570, p. 405–437.

García-González, M., R.C. Surdam, and M.L. Lee, 1997,Generation and expulsion of petroleum and gas fromAlmond Formation coal, Greater Green River Basin,Wyoming: AAPG Bulletin, v. 81, no. 1, p. 62–81.

Hale, L.A., 1961, Late Cretaceous (Montanan) stratig-raphy, eastern Washakie Basin, Carbon County,

Wyoming, in Symposium of Late Cretaceous Rocky,Wyoming and adjacent area: Wyoming GeologicalAssociation 16th Annual Field Conference Guide- book, p. 129–137.

Heasler, H.P., and R.C. Surdam, 1992, Pressure com-partments in the Mesaverde Formation of the GreenRiver and Washakie basins, as determined fromdrillstem test data, in C. Mullen, ed., Rediscover the

Rockies: Casper, Wyoming, Wyoming GeologicalAssociation 43rd Annual Field Conference Guide- book, p. 207–220.

Iverson, W.P., 1995, Detection and delineation of porosity “sweet” zones in Mesaverde Group tightgas sands, in Gas reservoir sweet spot detection anddelineation in Rocky Mountain Laramide Basins:Chicago, Gas Research Institute, Topical Report No.GRI-95/0443, Contract No. 5091-221-2146, p. 31–46.

Iverson, W.P., and R. Surdam, 1995, Tight gas sandproduction from the Almond Formation, WashakieBasin, Wyoming, Paper SPE 29559: Proceedings of the SPE Rocky Mountain Regional Meeting, Den-ver, CO, March, 20–22, p. 163–176.

Ja wo rows ki , C. , R.C. Su rda m, G. Chr isti an sen ,M. Grout, H.P. Heasler, W.P. Iverson, R.S. Martin-sen, M. Olson, and E. Verbeek, 1995, Natural frac-tures and lineaments of the east-central GreaterGreen River Basin: Chicago, Gas Research Institute,Annual Report No. GRI-95/0306, Contract No.5091-221-2146, 81 p.

Law, B.E., 1984, Relationships of source-rock, thermalmaturity, and overpressuring to gas generation andoccurrence in low-permeability Upper Cretaceousand Lower Tertiary rocks, Greater Green RiverBasin, Wyoming, Colorado, and Utah, in J. Wood-ward, F.F. Meissner, and J.L. Clayton, eds., Hydro-carbon source rocks of the Greater Rocky Mountain

Region: Rocky Mountain Association of Geologists,p. 469–490.

Law, B.E., R.M. Pollastro, and C.W. Keighin, 1986,Geologic characterization of low-permeability gasreservoirs in selected wells, Greater Green RiverBasin, Wyoming, Colorado, and Utah, in C. Spencerand R. Mast, eds., Geology of tight gas reservoirs:AAPG Studies in Geology, v. 29, p. 253–269.

Leach, W.G., 1994, Distribution of hydrocarbons inabnormal pressure in south Louisiana, U.S.A., inW.H. Fertl, R.E. Chapman, and R.F. Hotz, eds.,Studies in abnormal pressures: developments inpetroleum science, 38: New York, Elsevier,p. 391–428.

Love, J.D., 1970, Cenozoic geology of the GraniteMountains area, Central Wyoming: USGS Profes-sional Paper 495-C, 154 p.

Miller, F.X., 1977, Biostratigraphic correlation of theMesaverde Group in southwestern Wyoming andnorthern Colorado, in Exploration frontiers of thecentral and southern Rockies: Rocky MountainAssociation of Geologists, p. 117–137.

Roehler, H.W., 1969, Stratigraphy and oil-shaledeposits of Eocene rocks in the Washakie Basin,Wyoming: Wyoming Geological Association 21stAnnual Field Conference Guidebook, p. 197–206.




298 Surdam

Roehler, H.W., 1990, Stratigraphy of the MesaverdeGroup in the central and eastern Greater GreenRiver Basin, Wyoming, Colorado, and Utah: U.S.Geological Survey Professional Paper 1508, 52 p.

Surdam, R.C., P. Yin, M. Garcia-González, D.B. Mac-Gowan, H.P. Heasler, Z.S. Jiao, C. Jaworowski, R.S.Martinsen, W.P. Iverson, J. Liu, D. Britton, andG. Christiansen, 1993, Natural gas resource charac-

terization study of the Mesaverde Group in theGreater Green River Basin: Chicago, Gas ResearchInstitute, Annual Report No. GRI-93/0423, ContractNo. 5091-221-2146, 452 p.

Surdam, R.C., Z.S. Jiao, and J. Liu, 1995, Anomalouspressure regime in the Washakie Basin, Wyoming:Chicago, Gas Research Institute, Topical Report No.GRI-95/0390, Contract No. 5091-221-2146, 24 p.

Surdam, R.C., Z.S. Jiao, and H.P. Heasler, 1997, Anom-alously pressured gas accumulations in Cretaceousrocks of the Laramide Basins of Wyoming: A newclass of hydrocarbon accumulation, in R.C. Surdam,ed., Seals, traps, and the petroleum system: AAPGMemoir 67, p. 199–222.

Winn, R.D., Jr., M.G. Bishop, and P.S. Gardner, 1985,Lewis Shale, south-central Wyoming: shelf, delta

front, and turbidite sedimentation: Wyoming Geo-logical Association 36th Annual Field ConferenceGuidebook, p. 113–130.

Yin, P., and R.C. Surdam, 1995, Effects of diagenesis onpetrophysical properties of reservoir rocks, inD. Jones, ed., Resources of southwestern Wyoming:Cheyenne, Wyoming, Wyoming Geological Associa-tion 1995 Field Conference Guidebook, p. 247–254.

tight gas sands (surdan, 1997)

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