The Origin of Jonah Field, Northern Green River Basin, Wyoming

C H A P T E R 8

The Origin of Jonah Field, Northern Green River Basin, Wyoming

ROBERT M. CLUFFThe Discovery Group Inc., Denver, Colorado, U.S.A.

SUZANNE G. CLUFFThe Discovery Group Inc., Denver, Colorado, U.S.A.

ABSTRACT

Prolific production at Jonah field and many other fields in the Green River basin is dependent on the presenceof overpressure. In the Gulf Coast and other areas, plots of shale resistivity and shale sonic transit time versus depthhave been used to identify overpressured zones. The same technique has been proposed to map overpressurecompartments and their boundaries in the Rocky Mountain region using well logs or, alternatively, intervalvelocities determined from seismic data. At Jonah field, the top of the overpressure (determined by continuousgas flaring during drilling) correlates within a few hundred feet to a drop in shale resistivity and increase in shaletransit time. However, studies of nearby wildcat wells and detailed cross sections through both overpressuredand normally pressured wells show that the log anomalies extend significantly beyond the overpressured area.Velocity and resistivity changes in the area around Jonah tend to follow a stratigraphic boundary near the baseof the Tertiary Fort Union Formation instead of tracking the top of the overpressured volume.

Early studies of Jonah field considered the hydrocarbons in the field to be derived from vertical migrationof gas from regional overpressure conditions 2000–3000 ft (610–915 m) deeper. The upward migration waspresumed to be controlled by the presence of extensive microfractures that form a leakage chimney betweenlarge sealing faults. This study suggests that the log anomaly both within and surrounding Jonah provides analternative interpretation. Until the middle Tertiary, overpressure conditions extended up to the base of theFort Union and resulted in undercompaction of Cretaceous shales, as reflected by resistivity and velocityanomalies. Late Tertiary relative uplift initiated slow leakage of the overpressure conditions wherever thesystem was not tightly sealed. As a result, the top of the overpressure has been dropping with time over mostof the northern Green River basin. The sonic and resistivity anomalies are irreversible, and the logs reveal thesignature of the former overpressured and undercompacted conditions.

Based on this new model, Jonah field represents a high remnant of the former regional top of overpressureinstead of a leakage chimney from a deeper overpressured generation cell. If this model is correct, explorationmethods should focus on the seal conditions that prevent leakage instead of fracture models that promoteleakage.

INTRODUCTION

Basin-centered Gas Accumulations andthe Sweet-spot Concept

Basin-centered gas accumulations have been a majorfocus of exploration efforts in the Rocky Mountain region

since the mid-1980s. The concept of a basin-centeredaccumulation dates back to the work in the Elmwortharea of Alberta, where Masters (1979, 1984) describedwhat was termed a ‘‘deep basin gas field’’ in the westernCanada sedimentary basin. Subsequent work in otherRocky Mountain basins by the U.S. Geological Survey and

Jonah Field: Case Study of a Giant Tight-Gas Fluvial Reservoir, J. W. Robinson and K. W. Shanley, eds.: AAPG Studies in Geology 52 and RockyMountain Association of Geologists 2004 Guidebook

127

others (McPeek, 1981; Spencer, 1983; Law, 1984; Law andDickinson, 1985) described continuous gas accumula-tions in the central parts of most basins, several of whichare not very deeply buried today. Thus, the concept wasextended to include ‘‘basin-centered gas’’ as opposed tojust deep basin gas. Basin-centered gas accumulations areconsidered by many to be a worldwide phenomenoncommon to sedimentary basins throughout the Phaner-ozic (Law and Spencer, 1998).

Basin-centered gas has never been tightly defined inthe literature. Most papers cite the following commoncharacteristics for a basin-centered gas accumulation (e.g.,Law, 2002): (1) a large, regionally pervasive gas-saturatedsection; (2) the entire interval is abnormally pressured,either overpressured or underpressured; (3) fields com-monly lack downdip gas-water contacts; (4) gas is pro-duced with minimal associated water; (5) low permeabilityof both the enclosing strata and the reservoir rocks; and(6) no obvious updip seals or trapping conditions definethe gas accumulation. Because of the lack of obvious trapsor downdip free water that are characteristic of conven-tional structural and stratigraphic hydrocarbon accumu-lations, basin-centered gas has been grouped with otherunconventional or continuous hydrocarbon accumula-tions, including shale gas, coalbed methane, and gas hy-drates for resource-assessment purposes. Despite theirseemingly continuous nature over large volumes of rock,commercial production is, with few exceptions, restrictedto a small fraction of the overall accumulation.

The observation that economic gas production tendsto be highly localized in basin-centered accumulationsled to the development of the ‘‘sweet-spot’’ concept forlow-permeability reservoirs. A sweet spot is an area ofenhanced porosity and permeability that is thought toprovide higher deliverability and, therefore, better gasrecovery than the surrounding areas (Surdam, 1997; Law,2002). The origin of the term is obscure. The first pub-lished use we know of in this context was by Sauvageau(1984) and subsequently in the geological literature byBillingsley and Reinert (1994) for the area of prolificproduction from the Almond Formation (Cretaceous,Mesaverde Group) near Wamsutter in the eastern GreenRiver basin.

Sweet spots are thought to be controlled by (1) depo-sitional trends of enhanced porosity and permeability; (2)enhanced fracturing and, consequently, higher bulkpermeability; or (3) overpressuring and improved reser-voir properties in a pressure compartment (Surdam, 1997).Examples of the first two types of sweet spot have beenextensively discussed in the literature on basin-centeredgas. The third type, an anomaly in an overpressuredcompartment, is poorly understood. Because several of

the larger basin-centered gas fields in the Rocky Moun-tain region are significantly overpressured, it has beenproposed that overpressuring is somehow causally relatedto the production of sweet spots, and they might be con-centrated near the upper boundary of the overpressuredintervals (Surdam et al., 1994, 1997; Jiao and Surdam, 1997;Surdam, 1997; Law, 2002).

The idea that relief on the top of the overpressuredsurface may indicate the location of sweet spots has ledto several proposed techniques to identify and map thissurface. These include log-based techniques to identifythe top of the overpressure from sonic-log, resistivity-log, or density-log signatures (Jiao and Surdam, 1997;Surdam et al., 1997) and seismic techniques based oninversion of two-dimensional (2-D) or three-dimensional(3-D) velocity data to find reversals in the normal trendof increasing velocity with depth (Surdam et al., 2001).This study was part of a regional project to map the topof the overpressure surface across the northern GreenRiver basin using sonic and resistivity logs. The goal ofour work was to determine if the overpressure surfacehad a significant velocity contrast that might be detect-able with seismic data and to investigate if known loganomalies that closely coincide with the known top ofoverpressure in the Jonah–Pinedale area might showsome appreciable relief outside the established fields.

JONAH FIELD: AN OVERPRESSURED SWEET SPOTIN THE GREEN RIVER BASIN?

The discovery and early development history of Jonahfield was summarized by Robinson (2000). Jonah fieldwas discovered in 1986–1987 with the drilling of twowells by Home Petroleum Corporation, the Jonah Fed-eral 1-4 and 32-34, both of which encountered continuousgas shows and flared gas from about 8500 ft (2600 m) tototal depth. The initial completion results of these wellswere disappointing, and the field was considered non-commercial. McMurry Oil Company ‘‘rediscovered’’ Jonahfield with the completion of the Jonah Federal 1-5 in 1993,which encountered continuous gas shows from 8429 ft(2570 m) to total depth of the well. Subsequent devel-opment drilling in 1994–1995 outlined a large, poorlydefined area of gas-saturated Lance Formation (UpperCretaceous), where the top of continuous gas showsbegan at or a short distance below the top of the Lance.Sandstones in the overlying Fort Union Formation (Paleo-cene) had low resistivity, appeared wet on logs, yieldedonly minor or no gas shows while drilling, and weredrilled with normal mud weights. An intervening sectionof shale and shaly sandstones, tentatively correlated

Robert M. Cluff and Suzanne G. Cluff128

AAPG Studies in Geology 52 and Rocky Mountain Association of Geologists 2004 Guidebook

with the ‘‘Unnamed unit’’ of Tertiary age on the Pinedaleanticline (Law and Johnson, 1989), yielded erratic showsand appeared to be either normally pressured orperhaps transitional into the overpressured sectionbelow (Figure 1). The initial interpretation of Jonah field,based on the degree of overpressuring in these threewells, continuous gas shows over thousands of feet, lackof significant water production, and inability to detectany obvious structural or stratigraphic trapping compo-nents, was that Jonah must be a basin-centered gasaccumulation (Battin and Clark, 1994).

The best evidence for the top of the overpressuredsection was the onset of continuous mud-log hydrocarbonshows in both sandstones and shales, which generated aconstant gas flare through a gas buster and the need toincrease mud weight to control the well. McMurry, SnyderOil, and other operators in the area called this point the‘‘top of flare,’’ and it was widely assumed to be coincidentwith the top of significant overpressure. Drilling mudweights above the top of flare vary between operatorsbut are generally less than 9.5 lb/gal (1140 kg/m3), whichis equivalent to a pore-pressure gradient of 0.494 psi/ft(11.2 MPa/km). Below the top of flare, the operatorscommonly mud up to 9.7–10 lb/gal (1160–1200 kg/m3)and, in many wells, reach weights greater than 11 lb/gal(1320 kg/m3) at total depth, equivalent to pore-pressuregradients of 0.572 psi/ft (12.9 MPa/km) (Figure 1).Within the field area, there are relatively few publiclyavailable pressure buildup tests. Warner (1998) citedformation pressures in several tests of the early wellsequivalent to pore-pressure gradients of 0.54–0.58 psi/ft(12.2–13.1 MPa/km), whereas the shut-in drillstem testpressures for several dry holes in the area surroundingJonah indicate that the Lance is normally pressuredoutside the field (Table 1). Montgomery and Robinson(1997) reported that pore-pressure gradients in the fieldcould reach as high as 0.70 psi/ft (15.8 MPa/km) basedon the prefracture-stimulation breakdown tests.

In 1994, the McMurry Jonah Federal 4-8 and the SnyderOil Jonah Federal 3-15 were drilled immediately southof the Home Petroleum wells and the Jonah Federal 1-5well (Figure 2). The Jonah Federal 4-8 did not have anysignificant shows down to its total depth of 11,000 ft(3350 m) and had significantly lower porosity and resis-tivity than the nearby productive wells. The Jonah Fed-eral 3-15 also did not encounter strong gas shows until11,240 ft (3430 m), more than 2700 ft (820 m) deeperthan the top of gas shows in the wells just 1.5 mi (2.4 km)north.

Subsequent reprocessing of a regional northeast–southeast 2-D seismic line revealed a subtle fault sep-arating the productive wells from the two dry holes to the

south. This fault zone became known within Snyder Oilas the ‘‘Line of Death’’ and is now known to form thesouth boundary of the field (Robinson, 2000). In 1996,Amoco acquired a 51-mi2 (13,200-ha) 3-D seismic surveyover Jonah field. Although many interesting details ofthe structural configuration of the field emerged fromthese data (Warner, 1998; Vega and Hanson, 2001), themost significant features imaged were the south bound-ary fault and a previously unrecognized western bound-ing fault (Figure 2). Amoco was drilling a well, the Corona7-24, just west of this fault when the preliminary pro-cessing results were delivered. The Corona 7-24 provedto be a dry hole with no significant gas shows down to atotal depth of 10,214 ft (3113 m), a mirror image of theconditions on the south side of the southern boundaryfault. A test in the Corona 7-24 recorded a downholepressure gradient of only 0.447 psi/ft (10.0 MPa/km)(Table 1).

Since the recognition of these two major boundingfaults, all development drilling operations have beenconfined to the areas north and east of the faults or,more properly, fault systems, where the top of the pres-sure surface is approximately 2500 ft (760 m) higherthan the surrounding region (Bowker and Robinson,1997; Montgomery and Robinson, 1997; Warner, 1997a,b, 1998). The operators at Jonah quickly developed asimple model for the distribution of overpressure shownin Figure 3, with a more or less flat-topped overpressuresurface near the base of the Fort Union Formation.Warner (1998) concluded that the top of the pressuresurface is structurally controlled and rises abruptly be-tween the bounding fault blocks (Figure 3). Our detailedmapping of the top of continuous mud-log shows con-firms his interpretation (Figure 4).

To date, attempts to extend production south or westof the intersection of these major fault systems havebeen unsuccessful. The wildcat tests near Jonah fieldhave not encountered overpressure conditions untilthey penetrated the basal Lance Formation or upperMesaverde at depths of 11,000 ft (3350 m) or greater(Table 1). Development drilling has proven that Jonah isa fault-bounded pressure compartment, and it isprobably the best example in the Rocky Mountain re-gion of a field where overpressure conditions pop upabove what has been interpreted as a regional basin-centered gas accumulation. This observation led to therevised interpretation that Jonah may not be a basin-centered gas field but is instead a gas plume or leakagechimney rooted in a deeper, regional basin-centered gasaccumulation. Warner (1997a, p. 1236), for example, statedthat ‘‘minor high-angle faults may be responsible for up-ward migration of overpressured gas into the thermally

The Origin of Jonah Field, Northern Green River Basin, Wyoming 129

Jonah Field: Case Study of a Giant Tight-Gas Fluvial Reservoir

immature upper Lance section where it is trappedbeneath a shale top seal.’’ Law (2002) also concludedthat Jonah is a gas chimney rooted in the regionallypervasive but deeper gas accumulation previouslydescribed by Law (1984).

OVERPRESSURE DETECTION FROM SONICAND RESISTIVITY LOGS

Wire-line logs have been used to detect overpressure,particularly in the Gulf Coast and other Tertiary basins,since at least the mid-1960s (Hottman and Johnson, 1965;MacGregor, 1965; Foster and Whalen, 1966; Ham, 1966).These techniques are based on the response of resistivity,sonic, density, and neutron logs to the porosity of therocks and their ability to quantify changes in porositywith increasing burial depth and compaction. The loss ofporosity in a sedimentary section with increasing burialdepth is commonly approximated by an exponential func-tion of the form proposed by Athy (1930):

f ¼ f0e�bZ ð1Þ

whereF is the porosity at depth Z,F0 is the initial porosityof unconsolidated sediment at the time of deposition (the‘‘mud-line porosity’’), and b is a compaction coefficientrelated to lithology. By taking the logarithm of each sideof equation 1,

lnf ¼ lnf0 � bZ ð2Þ

a semilog plot of porosity versus depth (depth is linear) isexpected to yield an approximately straight line. In thesimple case of a continuously subsiding section with noerosional removal of sediments, the slope of the trend isthe compaction coefficient, b, and the intercept at depth =0 is the log of the mud-line porosity (Figure 5A, B). Ifthere has been uplift and erosion, the porosity at thepresent-day surface will be some value less than the po-rosity of unconsolidated sediment (Figure 5C). Thethickness between the present surface and an extrapo-lated point where the trend reaches the mud-line po-rosity,F0, provides an estimate of the amount of removedsection (Magara, 1976).

Several studies have demonstrated a change in theporosity-versus-depth relationship between the first sev-

eral hundred meters of burial, where mechanical com-paction of the sediments is the dominant process, and thedeep-burial regime, where chemical compaction is dom-inant (Burrus, 1998). Although an exponential functioncan approximate the overall porosity loss with depth, nosingle set of coefficients fits the entire regime fromsurface to deep burial. Consequently, in some basins, itmay be necessary to fit one equation to the section above3000 ft (910 m) with a large value for b and a differentequation with a low value for b in the deeper rocks. Huntet al. (1998), on the basis of shale-cuttings data, evensuggested that the loss of porosity with depth might bemodeled best with a set of piecewise linear instead of ex-ponential equations. Regardless of the subtleties of fittingone or more equations to the overall porosity-versus-depth trend, in the case of the northern Green River ba-sin, several thousand feet of shallow overburden havebeen erosionally removed by late Tertiary exhumation,and any deviations in the shallow regime have been trun-cated. This study determined that a single exponentialequation can be fit to the entire lower Tertiary sectionfrom the base of the surface casing to the top of the LanceFormation. Any error in the model equation at shallow-burial depths will result in increased error in the estimateof removed overburden, but this estimate was not a pri-mary goal of the study.

Using equation 2 to predict porosity is only valid for asingle and constant lithology. Different lithologies com-pact at different rates and have different characteristicvalues for the constant b. Because of this, basin compac-tion studies commonly focus on fine-grained rocks andassume all shales in the section compact at similar rates.If there are interbedded shales of distinctly different lithol-ogies, for example, organic-rich shales or hard siliceousshales, they may deviate from the ‘‘normal’’ shale com-paction trend. Shale points of similar lithology can beselected using various criteria for shaliness and lithology,such as a gamma-ray (GR) log, spontaneous-potential (SP)log, and neutron-density porosity separation. Several typesof geophysical logs are linearly related to porosity andcan be substituted into equation 2 as proxies for porosity,including sonic transit time, bulk density, and neutron-porosity logs. Resistivity (or its inverse, conductivity) alsovaries as a function of porosity, although the relationshipis nonlinear, and other effects, such as surface conduc-tivity of the clays, salinity changes of the interstitial wa-ters, and hydrocarbon saturation, all contribute to shale

Figure 1. Type log from the Home Petroleum Jonah Federal 1-4, the first well drilled at Jonah field, showing the major stratigraphicunits, key markers, mud-log gas shows (Tgas), and mud weight while drilling (MUDW). Tfu0 = base of Tertiary Fort Union Marker;GR = gamma-ray log; ILD = deep induction log; DT = sonic transit time.

mThe Origin of Jonah Field, Northern Green River Basin, Wyoming 131


Table 1

Formation Pressure Data in the Jonah Field Area.

Well Name

Location(Township,

Range, Section) Test Interval (ft) Formation

BottomholePressure

(psi)

PressureGradient(psi/ft)

EstimatedPermeability

(Md) Source

Wells inside JonahJonah Federal 1-4 T28N, R108W, 4 10,204–10,434 Lance 5600 0.543 13 Warner, 1998Jonah Federal 1-5 T28N, R108W, 5 9695–9924 Lance 5700 0.567 14 Warner, 1998Jonah Federal 1-6 T28N, R108W, 6 6507–9731 Lance 5450 0.581 7.4 Warner, 1998Jonah Federal 1-7 T28N, R108W, 7 9556–9580 Lance 5450 0.570 29 Warner, 1998Jonah Federal 2-5 T28N, R108W, 5 core 9581–9591 Lance 8.2 Warner, 1998Stud HorseButte 11-27

T29N, R108W, 27 core 10,411–10,429 Lance 14.3 Warner, 1998

Tests of wells outside JonahCorona Unit 7-24 T29N, 109W, 24 core 9560–9675 Lance 14 Warner, 1998Corona Unit 7-24 T29N, 109W, 24 9560–9675 Lance 4300 0.447 13 Warner, 1998Sublette Flat 1 T27N, R107W, 7 6925–7112 Fort Union 2207 0.319 IHS EnergySublette Flat 1 T27N, R107W, 7 10,236–10,267 Cretaceous 1985 0.194 IHS EnergySublette Flat 1 T27N, R107W, 7 10,369–10,414 Rock Springs 2606 0.251 IHS EnergyOld Road Unit 1 T27N, R108W, 27 8344–8406 Mesaverde 2890 0.346 IHS EnergyLombard CanyonUnit A-1

T27N, R108W, 33 7944–7981 Ericson 1783 0.224 IHS Energy

Prairie Hen Unit 1-A T27N, R110W, 30 6202–6264 Mesaverde 2339 0.377 IHS EnergyWardell Federal 1 T28N, R108W, 9 11,011–11,100 ? 5642 0.512 IHS EnergyGolden Gate Unit 1 T28N, R109W, 27 11,598–12,104 Blair 5465 0.471 IHS EnergyFerry Island Unit 1 T28N, R109W, 29 8581–8614 Lance 3384 0.394 IHS EnergyPinedale Unit 7 T30N, R108W, 15 8040–8062 Fort Union 3955 0.492 IHS EnergyNew Fork Unit 2 T30N, R108W, 2 9463–9516 Fort Union 3325 0.351 IHS EnergyWagon Wheel 1 T30N, R108W, 5 7340–7400 Fort Union 2297 0.313 IHS EnergyWagon Wheel 1 T30N, R108W, 5 10,978–11,070 Lance 6748 0.615 IHS Energy

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conductivity. Both sonic logs and resistivity logs haveproven useful for compaction studies and overpressureestimation. Density and neutron logs are considered lessreliable because of their sensitivity to borehole condi-tions and washouts.

For compaction to proceed normally, fluids must beable to freely escape as the section compacts, which im-plies normal or hydrostatic pore pressures. If a compact-ing section is unable to continuously expel fluids, usuallybecause of low permeability, rapid deposition, and a lackof continuous carrier beds to conduct the fluids away,part of the overburden will be borne by the interstitialfluids, and overpressure will start to develop. As a result,

the rate of compaction is reduced and does not follow anexponential loss of porosity with depth. This deviationfrom a normal compaction trend caused by vertical load-ing is known as ‘‘disequilibrium compaction.’’ Both qual-itative overpressure detection and quantitative estimatesof the magnitude of overpressure are based on deviationsof the observed compaction-versus-depth trend from anideal compaction curve fit through the shallow, normallypressured section.

Disequilibrium compaction is thought to be the mostcommon cause of overpressuring in young, rapidly sub-siding sedimentary basins, such as the Gulf Coast basin(Osborne and Swarbrick, 1997; Swarbrick and Osborne,

Figure 2. Structure map on the base of the Fort Union Formation (Tfu0 horizon) at Jonah field. Key wells mentioned in the text arehighlighted.



1998). Other mechanisms proposed as common causesof overpressuring are summarized in Table 2. Numericalmodels have shown that most of these mechanisms otherthan disequilibrium compaction are unable to accountfor the very high overpressures observed in many basins(Swarbrick et al., 2002), including the Greater Green Riverbasin. One notable exception may be fluid expansionresulting from hydrocarbon generation (Spencer, 1987)or thermal cracking of oil to gas in a closed system (Barker,1990). Gas generation from type I or type II kerogen isassociated with a volume expansion of as much as 200%and is capable of generating significant overpressure atleast locally within rich source rocks (Burrus, 1998; Meis-sner, 2000). Law and Dickinson (1985) proposed thatbasin-centered gas accumulations in the western UnitedStates formed by hydrocarbon generation induced over-pressure, wherein gas was forced into low-permeabilitysandstone reservoirs, displacing all movable water andcreating regionally extensive overpressure cells. As a noteof caution, although the volume expansion associatedwith kerogen conversion to gas may be large, the gas

generated expands into a very much larger volume ofreservoir and nonsource rocks and will suffer a substan-tial pressure drop as it expands.

Sonic-log and Resistivity-log Anomaliesin the Jonah Field Area

Figure 6A and B are examples of typical wells in theJonah area illustrating the sonic-log and resistivity-loganomalies. These logs have been filtered using the GR logto display only points with high shale volume, but noother lithologic criteria were used to discriminate be-tween types of shale. A linear trend of resistivity and sonictransit time is observed from the top of the logged intervalto a point near the top of the Lance Formation. At thispoint, both curves drift off the trend to lower resistivitiesand higher transit times (lower velocities), and a new,subparallel trend is established within several hundredfeet. The top of the overpressure, as determined by con-tinuous mud-log shows and shown by the total gas curvein the far right track, occurs within a few hundred feet

Figure 3. Cross section illustrating the relief on the top of the overpressured section (shaded gray) at Jonah field (modified afterWarner, 1998). The top of the overpressure is generally encountered in the shaly Unnamed unit between the massive Fort Unionsandstones and the top of the Lance. SOCO = Snyder Oil Company; MOC = McMurry Oil Company; T/OP = top of overpressure.



of the transition to the lower resistivity and velocitytrend. Through the transitional zone, the sandstones mayyield strong gas shows, but the gas drops back to back-ground levels in the interbedded shales. We interpretthe sandstones between the normally pressured FortUnion and the top of the sonic anomaly as slightly over-pressured or normally pressured gas sandstones. All ofthe Jonah field logs we have examined have similarprofiles, where the resistivity and the sonic logs breakback from the shallow-burial trend at nearly identicaldepths. This is important because most wells were notlogged with a sonic tool, and therefore, resistivity be-comes the key measurement for mapping the top of theanomaly.

The sonic trend above the top of the Lance can beprojected upward to a theoretical mud-line interval tran-sit time of 189 �s/ft (620 �s/m) (Figure 5C). The differ-ence between the projected sediment thickness to themud line and the actual thickness to the present-day sur-face of the Earth provides an estimate of the amountof overburden removed since maximum burial. For theexamples shown in Figure 6, more than 5000 ft (1500 m)of section has apparently been removed. One problemwith this calculation is the extremely low slope of thetrend line. Even small changes in the projected trendto honor values in a particular shale zone can changethe removed overburden estimate by hundreds of feet.Furthermore, as discussed previously, any deviation of

Figure 4. The top of the overpressure determined from continuous mud-log shows at Jonah field.



the actual trend from the assumed exponential trend atshallow-burial depths results in additional error in theremoved overburden estimates. Dickinson (1989) esti-mated 1100–1500 m (3600–4900 ft) of erosion at theWagon Wheel well on the Pinedale anticline. Coskey(2004; Figure 4), using vitrinite reflectance data to esti-mate the amount of removed overburden in the Jonah–Pinedale area, concluded that from 1400 to more than6000 ft (430 to >1830 m) of section has been eroded,with a best fit estimate of 3600 ft (1100 m). The timingand precise rates of erosion are unknown, but all theoverburden was likely stripped during the post-Mioceneregional exhumation of the Rocky Mountain region(Dickinson, 1989).

Because of the coincidence of the log anomaly withthe top of the overpressure at Jonah field, it seemsreasonable to expect the top of the overpressure to trackthe log anomalies throughout the northern Green Riverbasin. Figures 7 and 8 are cross sections through Jonahfield and adjacent wildcat wells, showing the resistivity-log, sonic-log, and mud-log gas curves. The sonic-log andresistivity-log anomalies do not track the top of the over-pressure beyond the limits of the field; instead, they re-main at approximately the same stratigraphic positiondespite the fact that the top of the overpressure deepensby 2000–3000 ft (610–910 m) to the west and south.Mud weights, gas shows, and a few well tests outside theJonah compartment, but within the anomalous sonic and

Figure 5. Sonic transit time (DT) vs. depth trends for a compacting section of a uniform shale lithology. (A) DT plotted on alinear scale versus depth displays an exponential curvature; (B) DT on a semilog scale versus depth is a linear trend; (C) trendfor the same well if several thousand feet of section was eroded. Projecting the observed DT at the surface (141 Ms/ft; 463 Ms/m)upward to the original mud-line DT (189 Ms/ft; 620 Ms/m) yields an estimate of the removed overburden to be 5000 ft(1500 m).



resistivity interval, indicate that this section is normallypressured to slightly underpressured (Table 1). The under-compaction signature recorded by the logs and the top ofpresent-day overpressure are obviously decoupled in thearea surrounding Jonah field.

DISCUSSION

Sonic and Resistivity Anomalies as PaleopressureIndicators in the Rocky Mountain Region

In many Tertiary basins, including the Gulf Coast re-gion of the United States, burial depths have steadily

increased with continuous subsidence. The rate of sub-sidence and rate of sediment input to these basins havevaried over time, but the creation of accommodationspace has kept pace. In these basins, which form theoverwhelming majority of the case histories for over-pressure detection from logs, a close correspondencebetween the top of the overpressure conditions and theinitial deviation from the local compaction trend onresistivity and sonic logs has long been recognized.

In foreland basins, including Laramide basins of theRocky Mountain region, the geological history is morecomplex. These basins have typically seen a principalsubsidence phase, during which most of the basin-filling sediment was deposited, followed by one or more

Table 2

Common Mechanisms for Abnormal Pressure Development in Sedimentary Basins(after Osborne and Swarbrick, 1997).

Overpressure Mechanisms Cause Expected Magnitude (psi)

Stress-related compactionDisequilibrium compaction vertical loading (rapid burial) thousands, limited by rock failureTectonic compression lateral loading thousands, limited by rock failure

Fluid-volume increaseAquathermal expansion heating and expansion of water with

increasing burial depth and temperature�100

Mineral transformations gypsum ! anhydrite dehydration 7–100smectite ! illite transformation unclearsmectite dehydration �100

Hydrocarbon generation kerogen ! gas transformation 70–6000 in source rock itselfkerogen ! oil transformation 70–6000 in source rock itselfoil ! gas (in-situ cracking) hundreds in reservoir rock

Fluid movementOsmotic pressure salinity contrasts across semipermeable

shale membraneshundreds, but theoretically asmuch as 3000

Hydraulic or potentiometric head elevated recharge areas tens to hundreds, dependingon elevation difference

Hydrocarbon buoyancy hydrocarbon-water density contrasts 1–1000, depending on columnheight and density differenceof phases

Underpressure MechanismsDifferential dischargeDifferential gas flowOsmosisRock dilatancy uplift and removal of overburdenPore-fluid cooling cooling and contraction of pore

fluids with relative uplift



periods of relative uplift, during which substantial amountsof sediment were eroded. If an interval of strata in oneof these basins becomes overpressured at any time in its

burial history because of disequilibrium compaction andthen is subsequently uplifted, the present-day pressuremay not accurately reflect the net state of compaction.

Figure 6. Typical sonic transit time-resistivity anomalies in the Jonah area. (A) Jonah field well, McMurry Oil Stud Horse Butte 15-28(SW SE Sec. 28, T29N, R108W), with less than 300 ft (90 m) between the log anomaly and the top of overpressure; (B) a well outsidethe overpressured area, HS Resources Holmes Federal 5-1 (SW NW Sec. 1, T27N, R109W), with more than 3800 ft (1160 m)between log anomaly and the top of overpressure. GR = gamma-ray log; Rsh = filtered deep resistivity in shales; DTsh = filtered sonictransit time in shales; Tgas = mud-log total gas curve.



Figure 7. Southwest–northeast cross section across Jonah field showing position of resistivity anomaly relative to the top of overpressure. A large-scale version ofthis cross section is included on the CD-ROM with this publication as Appendix E. JF = Jonah Federal. SHB = Stud Horse Butte.

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Figure 8. Northwest–southeast cross section across Jonah field showing position of resistivity anomaly relative to top of overpressure. A large-scale version of thiscross section is included on the CD-ROM with this publication as Appendix E. JF = Jonah Federal. SHB = Stud Horse Butte.

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Total pore pressure decreases with uplift and erosionbecause the trapped fluids cool and the pore systemdilates as the net overburden stress is reduced. Leakageof the overpressured fluids can occur if faults and frac-tures are activated and form permeable pathways forfluids to migrate away from the overpressured interval. Ina leaky section, the top of the overpressure will dropstratigraphically as the interval is uplifted.

Sonic and resistivity logs in the Rocky Mountain regionprobably respond to undercompaction of the sedimentsin an overpressured section just as they do in youngerbasins. Where overpressure results from disequilibriumcompaction, the sonic-log and resistivity-log anomalieswill mark the top of the overpressure during the com-paction phase. The undercompacted rock fabric is anirreversible change that does not diminish as the sectionis uplifted. The only way to compact the rocks to thedegree expected from a normal compaction gradient andthereby erase the undercompaction signature would beto rebury them to their former maximum depth in anopen state, where pore fluids can freely escape. There-fore, in a section that has undergone relative uplift andfluid leakage during the uplift phase, the top of thepresent-day overpressure will lie somewhere below thelog anomalies that mark the top of the overpressureduring the burial and compaction phase. This decouplingof present overpressure with the position of the log anom-aly is exactly what we observed in the area surroundingJonah field.

Rathbun (1968) described conductivity anomalies in thenorthern Green River basin that he attributed to changesin pore pressure, and these were used as mappable pa-rameters. Evers and Ezeanyim (1983) developed a set ofcalibration charts to predict pore pressure from conduc-tivity and sonic transit time anomalies and presented acomparison of the predicted and actual pressures for theWagon Wheel 1 well on the Pinedale anticline. Theyconcluded that the method was accurate in this area towithin ±0.03 psi/ft (0.679 MPa/km) of depth, or 300 psi(2.069 MPa) at 10,000 ft (3000 m). Subsequently, Prensky(1986), in a study of well-log responses in the northernGreen River basin, found a poor correlation betweenthe top of the conductivity anomaly in the Pinedale areaand the top of the overpressure estimated from mud-weight data. Noting the close proximity of the conduc-tivity anomaly in wells on the Pinedale anticline with theCretaceous–Tertiary boundary and with a major changein GR intensity at approximately the same depth, Prensky(1986) concluded that the conductivity anomaly was astratigraphic phenomenon related to a change in bulkmineralogy near the base of the Fort Union. He furtherconcluded that shale conductivity and shale transit time

plots would not be useful for pore-pressure prediction inthe Green River basin because of complex changes inlithology through time, the lower salinity of pore waterscompared to the Gulf Coast basin where the techniqueswere developed, and differences in the mechanism pres-umed to be responsible for overpressuring (hydrocarbongeneration versus disequilibrium compaction). Prensky(1986) did not offer any explanation for the observationmade by Rathbun (1968), which this study confirms, thatthe conductivity anomaly seems to cross stratigraphyfrom east to west, until it is present near the base of theHilliard Shale on the west flank of the basin and in the LaBarge platform area.

Although we concur with Prensky’s (1986) observa-tion that the resistivity and sonic anomalies in the Pine-dale area roughly coincide with the Cretaceous–Tertiaryboundary, it is not clear that this is a result of a change inshale mineralogy. Equally likely is that the sandstonesand shales of the Tertiary are poor seals and had higherfluid transmissibility during compaction, consequentlyfollow a normal compaction-versus-depth curve. Thesandstones and shales in the Lance and at least part ofthe underlying Mesaverde, however, apparently had poortransmissibility and are undercompacted relative to theshallower, normally compacted, and normally pressuredTertiary units. Thus, the log anomalies follow the strati-graphic boundary, at least in this immediate area, becauseof a fluid-transmissibility contrast in the major units, whichmay or may not be related to mineralogical differences.

Surdam et al. (1997, their figure 12) proposed anotherexplanation for the sonic-log anomaly, wherein relativelysmall levels of gas saturation in the shales can generatein a significant increase in sonic transit times. They citedlaboratory data showing that as little as 5% gas saturationin the matrix porosity can result in a strong decrease in thecompressional wave velocity (Timur, 1987). In this model,the top of the overpressure coincides with the onset ofgas generation and results in partial gas saturation through-out the shales. The principal problem with this hypothesisis that an increase in gas saturation in the shales shouldresult in an increase in average shale resistivity and not adecrease. Studies of mature source rock shales invariablydocument a major increase in resistivity that coincideswith the top of hydrocarbon generation because porewaters are displaced by nonconductive hydrocarbons(e.g., Meyer and Nederlof, 1984; Schmoker and Hester,1990), which is the opposite of what we observed in theLance section in and around Jonah. Additionally, avail-able vitrinite reflectance data at Jonah indicate that shalesin the Lance are immature and have not reached theonset of gas generation (Bowker and Robinson, 1997;Coskey, 2004).



Jonah Field: Is It Really Just aPaleopressure Remnant?

The presence of a sonic-log and resistivity-log anom-aly throughout the area surrounding Jonah field suggeststhat the Lance section is undercompacted and, at itsmaximum burial depth, was overpressured over a broadarea of the northern Green River basin. Gas shows andmarginal gas production from nearby wildcats furthersuggest that the Lance section contains some gas, but it isnormally pressured or underpressured outside the fieldboundaries. These two observations, in combination,lead us to propose the following model for the origin ofJonah field.

The top of the overpressure was widely establishedas a regional surface during the early to middle Tertiary.The position of this surface coincides with the anomalywe see today on sonic and resistivity logs. The exacttiming and burial history of the northern Green Riverbasin is poorly constrained, in large part because por-tions of the section are now missing and therefore can-not be directly dated. Maximum overburden was prob-ably attained significantly after deposition of Paleocenestrata but before the middle Miocene. Coskey (2004), forexample, suggests that most of the removed section isWasatch, and erosion begins at about 5 Ma. The strat-igraphic position of the top of the regional overpressurewas controlled by the low permeability of the Lancesection, which is composed of a heterolithic assem-blage of low-permeability sandstones and shales thatimpeded fluid flow out of the section during burial. Thesandstones of the lower Fort Union in the Jonah area havegood lateral continuity, are regionally porous and per-meable, and allowed fluids expelled during compactionto escape the system. Consequently, the Fort Union iscurrently normally pressured, exhibits high water satura-tions, and has probably always been normally pressured.

At its time of maximum burial, at depths 3000–5000 ft(910–1500 m) greater than today, the entire Lance sec-tion was receiving gas charge from deeper, more maturesource rocks (Coskey, 2004). The Lance itself is thermallyimmature in the Jonah area (Warner, 1998; Coskey, 2004).Gas migrated vertically through a regional system of jointsand fractures that must have been sufficiently extensiveto contact a high percentage of the reservoir sand bodies.The formation was somewhat overpressured because theLance had lower transmissibility than the overlying strataand, consequently, was in a state of disequilibrium com-paction, resulting in the sonic-log and resistivity-log anom-alies found today both within and around the field.

The lithostatic overburden decreased in direct pro-portion to the rate and amount of erosion during late

Tertiary uplift. The porosity of the sandstones increasedslightly as the pore system dilated with reduced confiningstress and the section cooled with uplift, resulting in con-traction of the trapped gases. Total reservoir pore pressuredecreased, although the pore-pressure gradient may haveactually increased with relative uplift because of the highcompressibility of gas (Figure 9) (Katahara and Corrigan,2002). Another factor to consider is that during uplift,erosion, and cooling, the top of the active hydrocarbongeneration window falls away from the Lance in roughproportion to the amount and rate of erosion. It is un-likely that active hydrocarbon generation was able tocontribute to overpressured conditions in the Lance onceuplift commenced.

The sharp difference in pressure gradients across thefield boundaries, with similar reservoir rock propertiesand identical burial histories within and outside thefield, can be explained by a difference in the rate of fluidleakage out of the Lance section during uplift. The Jonahcompartment appears to be very tightly sealed and hasretained its gas charge during uplift. Leakage of fluidsout of Jonah was limited by extremely low-permeabilityseals on three sides of the field. The major fault zones onthe south and west flanks of Jonah form lateral sealingsurfaces that are capable of maintaining a pressure dif-ferential greater than 1000 psi (6.9 MPa) over short dis-tances. The top of the gas accumulation at Jonah co-incides with the shaly, low net-to-gross Unnamed unit.This apparently sealing interval separates overpressured,gas-charged Lance sandstones from water-saturated andnormally pressured Fort Union sandstones.

These three major sealing surfaces, two fault systems,and a top-sealing shale, form a structural trap similar ingeometry to the bow and decking of a ship. Because thebow is fortuitously pointed directly updip, fluids wereunable to leak out of the trap as the system was uplifted.The top-sealing shale, however, is truncated by post-Lanceerosion a short distance to the south and west of Jonah(Coskey, 2004). If the truncation edge of the UnnamedTertiary was within the present field area, anywhere northand east of the intersection of the two major boundingfaults, a large part of the accumulation would have likelyleaked upward into the Fort Union, and the field would bemuch smaller or might not even exist.

In the area surrounding Jonah, gas freely escaped up-ward into the overlying Fort Union section as the intervalwas uplifted. The exact mechanism and pathways for leak-age are speculative, but it is likely that gas leaked thougha system of regional joints and fractures. This regional frac-ture system may very well be identical to that inside thefield, but a key difference is that outside the field, thereare no lateral or top seals to prevent gas from migrating



out of the Lance into the Fort Union. The Lance sandstonesoutside Jonah still retain some gas saturation because min-imal free water was available to be imbibed as the sectionwas uplifted, but they have lost a large portion of theircharge and yield much weaker gas shows during drilling.

Jonah field is a tightly sealed and overpressured com-partment that stands high above the surrounding top ofthe pressure surface, much like a mesa above an erodedlandscape. The top of the overpressure surface surround-ing the field, established in the early Tertiary at maximumburial depth, has been dropping since uplift and ero-sion began in the late Miocene or Pliocene. Because Jonahfield owes its origin to the preservation of overpressureconditions during uplift, and maintaining this state ofoverpressure is a direct result of the unique sealing con-ditions at Jonah, exploration for additional overpressuredaccumulations of this kind should focus on sealing andtrapping conditions that will prevent leakage. This is instark contrast to the prevailing exploration philosophyfor basin-centered gas sweet spots, which emphasizesfinding fractures and areas of enhanced reservoir qualityfavorable for gas leaking upward into shallower horizons.

CONCLUSIONS

The Lance Formation consists of highly discontinuoussandstones in a shale matrix. Fluids contained in the Lancewere unable to freely escape the section during rapidburial, and as a consequence, the fine-grained sediments

are undercompacted relative to their former maximumburial depths. Sonic and resistivity logs through the Lanceand Mesaverde retain a strong signature of reduced ve-locity and reduced resistivity directly related to under-compaction during burial. The top of the anomaloussonic velocity-versus-depth and resistivity-versus-depthtrends generally coincides with the top of the shaly Lancesection and is everywhere beneath the more continuousand permeable Fort Union sandstones.

Coincidentally, the top of the anomalous velocity andresistivity profiles corresponds, within a few hundredfeet, to the top of the overpressured gas in Jonah field,leading to the idea that the geophysical signatures mightbe directly related to the presence of overpressure. Re-gional examination of logs, however, demonstrates thatthe anomalies extend far beyond the field limits andpersist at roughly the same stratigraphic position, clearlydemonstrating that the anomaly is decoupled from present-day pressure conditions.

The best explanation for the similarity of velocity-resistivity profiles inside and outside Jonah is that theyrecord the former top-of-pressure conditions attained atmaximum burial, and the signature was frozen into therocks during subsequent exhumation. Jonah field is thusan anomalous remnant of former regional overpressureconditions that were established during maximum burialconditions in the early or middle Tertiary.

The origin of Jonah field is therefore related to rel-ative rates of fluid leakage out of the Lance section dur-ing relative uplift. Jonah field is tightly sealed on two

Figure 9. Model calculations of5000 ft (1500 m) of relative upliftof the Lance show a net reservoirpressure decrease of approximately15%, but the pore-pressure gradi-ent relative to the surface actuallyincreases from 0.45 to 0.58 psi/ft(10–13 MPa/km).



sides by impermeable sealing faults and is sealed on thetop by a thick, low-permeability shaly interval. OutsideJonah field, fluids were able to leak updip through frac-tures and escaped the system. Where the fluids wereunable to escape, a deep, overpressured system wasable to survive uplift to much shallower depths. Thus, amulti-tcf gas accumulation in the Greater Green Riverbasin owes its origin to the preservation of gas chargeand overpressure over millions of years of regionaluplift and erosion.

ACKNOWLEDGMENTS

The authors acknowledge the contributions of nu-merous colleagues and clients we have worked withover the years on the Jonah problem, many of whomcontributed substantially to the development of the ideaspresented in this chapter. These include Alan Byrnes ofthe Kansas Geological Survey, and John Webb of TheDiscovery Group, with whom we first started working onthe petrophysical aspects of Jonah back in 1994; DennisStorhaug and Denise Delozier, both formerly with SnyderOil Company; Kent Bowker and Deane Foss, both for-merly with Chevron U.S.A.; Dean DuBois, EnCana U.S.A.;Jim Folcik, EOG Resources; and Phil Winner, formerlywith HS Resources. We especially thank Keith Shanleyand John Robinson for uncounted hours of discussion onthe dilemma of the origin of the pressure conditionsaround Jonah and for their support in the preparation ofthis chapter.

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The Origin of Jonah Field, Northern Green River Basin, Wyoming

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