geology and geomorfology uinta basin

AAPG Bulletin, v. 85, no. 9 (September 2001), pp. 1661–1678 1661

Structure and geomorphologyof the Duchesne graben,Uinta basin, Utah, and itsenhancement of ahydrocarbon reservoirAlicia Groeger and Ronald Bruhn

ABSTRACT

The Duchesne fault zone (DFZ) in the Uinta basin of northeasternUtah is a system of normal faults and joint zones that forms promi-nent, east-trending lineaments tens of kilometers long. The Du-chesne graben, an asymmetrical fault-bounded trough in the west-ern part of the DFZ, is the focus of this article. A master faultbounds the southern margin of the structural half graben, dipssteeply northward, and accommodates about 200 m of slip. Thisfault either terminates downward by 1400 m depth or flattens intoa north-dipping, low-angle detachment fault at about 1000 mdepth. The half graben is developed along the crest of an open, low-amplitude flexure in the southern limb of the Uinta basin syncline.This faulted flexure trapped hydrocarbons that migrated southwardfrom depth in the Uinta basin. Intense fracturing within the halfgraben also created migration pathways for hydrocarbons and gen-erated shallow reservoirs in the Green River Formation.

INTRODUCTION

The Uinta basin is located between the southern flanks of the UintaMountains and the northern edge of the Colorado Plateau provincein the central Rocky Mountains of the United States (Figure 1).This basin is an important onshore petroleum province that isknown for the prolific lacustrine source rocks in the Eocene GreenRiver Formation (Figure 2). Widely distributed oil and gas fieldsand tar sand deposits formed as hydrocarbons generated in thedeeper parts of the basin migrated updip into stratigraphic andstructural traps in the limbs of the Uinta basin syncline (Fouch etal., 1992). Swarms of gilsonite dikes were also injected by hydraulicfracturing in the southeastern part of the basin.

Copyright �2001. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received April 8, 1999; revised manuscript received September 26, 2000; final acceptanceNovember 9, 2000.

AUTHORS

Alicia Groeger � Department of Geologyand Geophysics, University of Utah, Salt LakeCity, Utah, 84112; [email protected]

Alicia Groeger has a B.S. degree in geology(University of North Carolina Chapel Hill,1995) and spent one year of undergraduatestudy at the Geosciences Institute of GottingenUniversity in Germany. She then received herM.S. degree in geology (University of Utah,1997) with a concentration in structure andtectonics. Since then Alicia has been living inPeru, working in exploration geology and insustainable development, cultivatingresponsible relationships between largemining companies, local peasant communities,and the Peruvian national park system.

Ronald Bruhn � Department of Geologyand Geophysics, University of Utah, Salt LakeCity, Utah, 84112; [email protected]

Since receiving his Ph.D. in geology (ColumbiaUniversity, 1976), Ron Bruhn has worked as aprofessor of structure and tectonics at theUniversity of Utah, where he has receivednumerous teaching and research awards.Bruhn’s research interests include theapplication of structural geology to problemsin petroleum geology, seismic hazards, andmining. He is an internationally recognizedexpert on the origin, hydrologic properties,and modeling of fracture systems in faultzones.

ACKNOWLEDGEMENTS

This work was supported by a Department ofEnergy grant awarded to R. L. Bruhn at theUniversity of Utah as a subcontract from TerraTek Inc. We thank S. R. Bereskin for initiatingthe project and providing both technical ex-pertise and encouragement. Richard Jarrardprovided invaluable advice on the interpreta-tion of well logs. We also thank Robert Balou,John Bartley, and Dave Chapman for theirthoughtful comments and suggestions.

1662 Duchesne Graben, Uinta Basin (Utah)

Figure 1. (a) Physiographic provinces located in the Colorado Plateau and Rocky Mountain region. (b) Outline of the Uinta basinshowing the surrounding uplifts, the Duchesne fault zone (DFZ), and gilsonite dikes (G). The Duchesne graben is bounded by arectangle.

Figure 2. Depositional history and stratigraphic nomenclaturefor the Uinta basin (modified after Bruhn et al., 1986). Cross-hatched regions represent periods of erosion or nondeposition.

The Duchesne fault zone (DFZ) is a series of nor-mal faults and joint zones (Figure 1) that extends formore than 60 km across the southern flank of the Uintabasin (Ray et al., 1956). Although the DFZ has beenrecognized for many years, little published informationexists concerning the age and structure of the faultzone or its hydrological properties. This lack of infor-mation is surprising because several oil fields are lo-

cated in the fault zone (Clem, 1985), which is ideallylocated to trap hydrocarbons migrating up the south-ern limb of the Uinta basin syncline.

The structure and geomorphology of the Du-chesne graben are the focus of this article, which seeksto explain the feature’s effect on production from thelocal hydrocarbon reservoir. The Duchesne graben islocated in the western part of the DFZ, where oil hasbeen produced since 1951. This area is characterizedby excellent bedrock exposures and pronounced geo-morphic expressions of faults and joint zones (Figure3). Well data also exist to constrain the subsurfacestructure. Our goal was to determine the structure ofthe graben, to infer possible effects of faulting on hy-drocarbon migration and trapping, and to evaluate evi-dence for active faulting.

STRUCTURAL GEOLOGY OF THEDUCHESNE GRABEN

Surface Geology

The Duchesne graben is a faulted trough approxi-mately 16 km long and up to 500 m wide (Figure 4).Topographic relief in the eastern part of the grabenis 15–30 m (Figure 5), but farther west the graben is

Groegerand

Bruhn1663

Figure 3. Aerial photograph of the Duchesne graben. The prominent lineament bounds the south side of the graben as indicated by arrows. Drill pads of the Duchesne oil andgas field are located at the eastern end of the graben. The town of Duchesne is visible at the top center of the photograph.


Figure 4. (a) Structural map of the Duchesne graben in Sec. 13-24, T4S, R5W and Sec. 14-23, T4S, R4W; cross section along lineAA� is shown in Figure 6. Continued.

dissected by erosion in Indian Creek Canyon, wherethe topographic relief is up to 100 m. The longest faultsand most persistent photolineaments are located along

the southern boundary of the graben, where faulting ismore continuous than along the northern flank (Figure4). This asymmetry is illustrated in the cross section

Groeger and Bruhn 1665

Figure 4. Continued. (b) Map of lineaments and major stream channels in and near the Duchesne graben. Lineaments are dashedwhere covered. Numbers 1–4 indicate the locations of topographic profiles in Figure 5. Letters indicate the stream channels profiledin Figure 12.

along Indian Canyon, where the well-developed north-dipping master fault crops out on the southern side ofthe graben (Figure 6a).

Faults in the Duchesne graben cut shale, sand-stone, and lacustrine limestone of the Eocene UintaFormation in the southern limb of the Uinta basin


Figure 5. Topographic profiles 1–4 across the Duchesne gra-ben. The profiles are located on Figure 4b. The vertical exag-geration is 6�. NL marks prominent lineaments in the northernpart of the graben; SL and SL� mark prominent lineaments inthe southern part; E marks ephemeral stream channels.

syncline (Figures 2, 4a). Individual beds range from 10cm to 15 m thick. The thickest beds are sandstone, butwhite limestone beds provide the best marker horizonsfor correlating strata across faults. Bedding dips 4–11�NNE in this limb of the syncline except in the vi-cinity of the Duchesne graben, where the beds areflexed into a low-amplitude anticline that is breachedby normal faults (Figure 6b). Bedding dip increasesfrom 4–6�NNE to 8–11�NNE in a belt approximately2.5 km wide immediately north of the graben. Stratawithin the graben are faulted into narrow blocks andslivers with bedding tilted at various angles, up to amaximum of 30�S. Bedding dips 11–16�SSW in a beltapproximately 300 m wide immediately south of thegraben but resumes the regional dip of 4–11�NNE far-ther south.

Normal faults bound small horst blocks along theflanks and within the central topographic trough (Fig-ure 4a). Faults are well displayed in the first drainageeast of Cottonwood Canyon, where normal faults dis-rupt strata in a belt approximately 100 m wide in thesouthern wall of the graben (Figure 7). Displacement

ranges from several centimeters to several meters onfaults in this array.

Strata in the fault-bounded blocks are intenselyjointed. The prominent joint set strikes east-west, par-allel with the normal faults, but subsidiary sets strikenortheast-northwest, and most joints in these sets ter-minate against longer east-west–striking joints (Figure8). Spacing between joints is less than 1 cm in somemicritic limestone beds, is wider in thicker bedded orcoarser grained limestone, and is up to 20 cm in fluvialsandstone beds that are several meters thick. Calciteveins heal some of the east-west joints, and blebs ofsolidified hydrocarbon or gilsonite occur within theveins at one outcrop near the northern boundary of thegraben. Sandstone beds also contain conjugate defor-mation bands that are incipient normal faults in whichshearing has collapsed the pore space and caused ca-taclasis of rock grains.

Subsurface Geology

Logging data were compiled and analyzed from 31wells in the Duchesne oil field. Gamma-ray and elec-trical logs were used to correlate selected horizons inthe subsurface. Well drilling history and production re-ports were used in conjunction with the petrophysicallogs to identify major mechanical boundaries in thestratigraphy and to identify anomalous intervals wherewells may have penetrated faults. Twenty-seven wellsare located north of the graben, one well is locatedwithin the graben, and three wells are located south ofthe graben (Figure 9).

Nine marker beds were selected for subsurface cor-relation based on their gamma-ray patterns (Figure10). These markers include shale, sandstone, and lime-stone beds that are easily identified in gamma-ray logsand are regionally distributed throughout the subsur-face. Each marker was identified using the originalgamma-ray logs because depths to formation or markerbed tops listed in well completion reports were foundto be incorrect in several cases. The marker beds spana depth range of approximately 2 km and can be iden-tified in the Uinta, Green River, and Wasatch forma-tions (Figure 10).

The location and depth of marker bed interceptswere tabulated to produce a set of digital data forgraphical and numerical analysis. A plane was then fitto the well intercepts of each marker bed in the 16northernmost wells using linear, least squares regres-sion (Davis, 1986). These wells were selected becausethey are all located between 0.5 and 3 km north of the


Figure 6. (a) Structural cross section AA� along the eastern side of Indian Canyon (see Figure 4 for location). The light gray layeris a prominent limestone marker bed that can be traced along most of the cross section. Dark gray layers represent a distinct groupof beds that can be correlated across the graben. Minor faults are dashed where projected at depth. (b) Restored structural crosssection AA� across the Duchesne graben. Only faulting was reversed to create this restoration, so the remaining discrepancy acrossthe master fault is attributed to reverse drag and bedding rotation. Notice the low-amplitude anticline that remains after restorationof faulting.

Duchesne graben in an area where there was no evi-dence of faulting on aerial photos and published maps(Ray et al., 1956) (Figure 9). The marker beds dipped9� in a direction N12�E, similar to the bedding orien-tation at the surface. The standard deviation betweenthe best-fit planes and the well intercepts averaged 4.5m for all nine marker beds. Assuming that the residualerror between actual and predicted well interceptdepth was normally distributed, 95% of the well inter-cepts were located within �9 m (2 standard devia-tions) of the best-fit plane. This residual error included

natural variations in bedding thickness, bedding dip,and measurement error. The error analysis implied thata vertical tectonic offset of �9 m or less was probablynot discernible by correlating marker beds betweenwells.

A structural cross section of the Duchesne grabenwas created by projecting the intercepts of marker bedsin each well onto a vertical plane that strikes N12�E,parallel with the regional direction of bedding dip (Fig-ure 10). Several structural features are noteworthy.(1) Bedding dips more steeply north (9�) than regional

1668Duchesne

Graben,UintaBasin

(Utah)

Figure 7. View of the escarpment on the southern side of the Duchesne graben (see encircled person for scale). The photograph is taken looking eastward from a stream cutlocated several hundred meters east of Cottonwood Creek. North is on the left side of the photograph and south is on the right side. White layers are limestone beds that areintercalated with cross-bedded sandstone of the Uinta Formation. The escarpment marks the southern edge of the topographic trough formed by the Duchesne graben. Theescarpment is underlain by intensely faulted and jointed rock. Notice several subsidiary normal faults located south of the escarpment. These faults offset and rotate beddingtoward the south, opposite the regional northward direction of dip.


Figure 8. Line drawing ofjoint traces on horizontal bed-ding surface in the Duchesnegraben. The bed is fine-grained,lacustrine limestone of theUinta Formation cropping outin a horst block within thegraben. Notice the north-northwest–trending andnortheast-trending joints thattruncate mostly against themore prominent east-west–trending set, suggesting that thelatter set of joints is the oldest.Similar jointing may create frac-tured reservoirs in tight sandsand shales of the subjacentGreen River Formation.

Figure 9. Location mapshowing the oil and gas wellsused for correlation of markerbeds in the subsurface of theDuchesne graben.

bedding (4–6�) in a belt approximately 2.5 km wideimmediately north of the graben. This belt of increasednorthward dip is observed in all of the marker beds,

from the shallowest marker B to the deepest CastlePeak marker horizon (Figure 10). (2) Approximately200 m of strata are omitted by faulting of marker B


Figure 10. Cross section showing elevation of marker bedsencountered in the oil and gas wells (Figure 9). The marker bedintercepts in each well are projected onto a vertical plane thatstrikes perpendicular to the trend of the Duchesne graben(plane strikes N2�W). Notice that marker B is faulted down ap-proximately 200 m in the graben relative to its elevation oneither side and that markers D and F are omitted by this fault.Also notice that marker H and deeper markers do not appearto be faulted. See text for discussion.

within the graben (Figure 11), but deeper markers donot appear faulted, and if they are, the vertical offsetis presumably less than �9 m. This implies that thenorth-dipping master fault intercepts the well in thegraben between markers B and F (Figures 10, 11). Dis-placement on the master fault may either die out be-low marker F or be transferred onto a low-angle faultnear the top of the Green River Formation. (3) Markerbed intercepts in the wells south of the graben are con-sistently lower than the expected elevations deter-mined by projecting bedding updip from the referencearea several kilometers north of the graben (Figure 10).This discrepancy is best explained if beds are foldedinto a broad, open anticline whose crest is breached byfaulting in the Duchesne graben. This structural model(Figure 10) is similar to the cross section farther west

in Indian Canyon (Figure 6a), although the net slip onthe master fault at Indian Canyon is significantly lessthan 200 m. Rotation of bedding due to reverse dragis commonly associated with normal fault deformation.Barnett et al. (1987), however, documented that bedscut by faults with less than 1 km of displacement arerotated less than 5�, so that kind of responsive flexurecannot sufficiently account for the about 20� rotationof the Duchesne graben bedding.

GEOMORPHOLOGY

The geomorphology was evaluated for evidence of ac-tive faulting and to investigate the origins of the pro-nounced photolineaments observed on aerial photo-graphs (Ray et al., 1956) (Figure 3) and satelliteimagery. The geomorphology was studied by field-checking photolineaments, analyzing stream drainagepatterns, constructing stream channel profiles, and cal-culating stream gradients. Stream profiles and gradientswere determined from 1:24,000 scale topographicmaps with 12 m contours.

Streams head in the highlands on the southernlimb of the Uinta basin syncline and flow northeast-ward across the graben to the Duchesne River in thecenter of the basin (Figure 4b). The streams are allephemeral, with the exception of Indian Creek, whichis a perennial stream. Stream channels are marked bya crudely rectangular pattern with longer north-north-east–trending reaches linked by shorter east-trendingjogs. This pattern is caused by a combination of differ-ent factors: streamflow down the regional structuralgradient defined by bedding surfaces; preferential ero-sion along east-striking joints; and possibly, eastwardtilting related to uplift in the Colorado Plateau andWasatch Mountains.

The eastern part of the Duchesne graben is a nar-row topographic trough that is bounded by fault-linescarps and eroded joint zones (Figure 5). Movement ofsediment within the graben is largely confined to thevicinity of ephemeral stream crossings. Colluvium andtalus have accumulated along the flanks of the grabenbetween the stream channels, and wind-borne andabandoned fluvial deposits form mounds in the centerof the trough. Farther west in Indian Canyon, the gra-ben is strongly dissected because of downcutting byperennial streamflow and erosion in ephemeral tribu-taries that extend both eastward and westward alongthe graben. The tributary canyons are each approxi-mately 4 km long (Figure 4b). The southern walls of


Figure 11. Gamma-ray logsof two wells: the PetroglyphOperating Co. 7-16D Ute Tribalwell located just north of thegraben, and the Coors EnergyCo. 10-16D Ute Tribal well lo-cated inside the graben. Markerbeds are denoted by both lower-case and uppercase letters, andthe latter are shown on thecross section in Figure 10. Theinferred position of the north-dipping master normal fault is410–450 m below the drill padof the 10-16D Ute Tribal well.Projection of the fault intersec-tion from the wellbore to thepoint where it crops out alongthe southern wall of the grabenindicates a dip angle of about75�, consistent with surfacemeasurements.

these canyons lie close to the southern structuralboundary of the graben, but erosion by mass wastingand fluvial processes has extended north of the grabeninto unfaulted bed rock.

Stream gradients generally decrease at a stream’spoint of entry into the graben but remain nearly con-stant where streams exit across the northern boundaryof the graben (Figure 12). Cottonwood Creek is an ex-ception to this rule. Cottonwood Creek flows into thegraben at a left-stepping fault jog in the southern flankand exits the north flank where groundwater haspooled and saturated the soil. The stream gradient in-creases just north of this latter location. The jog alongthe southern side of the graben coincides with the pro-jected locations of several normal faults mapped in

nearby outcrops (Figure 4). Several knickpoints are de-veloped along a 500 m–long stretch of the channel ex-tending upstream across the southern flank of the gra-ben. A deep gully extends upstream from the middleof the graben to the last of these knickpoints, wherewater seeps from a spring. These geomorphic featurescould be indicative of active faulting, but there are noscarps in Quaternary deposits.

The origins of the photolineaments mapped byRay et al. (1956) were determined during field map-ping and analysis of drainage patterns. Many linea-ments are defined by fault-line scarps or by depressionseroded along closely spaced faults and joints. Otherlineaments are cliff faces formed by mass wastingalong east-west–trending joint surfaces. The longest


Figure 12. Topographic pro-files (scale on left side) andchannel gradients (scale onright side) of several streamsthat cross the Duchesne graben.Stream profiles a–h are locatedon Figure 4b. Vertical exaggera-tion is 3�. NL and NL� are in-tersection points of lineamentsthat occur in the northern partof the graben. SL and SL� areintersection points of linea-ments that occur in the south-ern part of the graben. The vmarks the confluence ofephemeral stream channels.

lineaments are located along the southern flank of thegraben where the master fault zone crops out (Figure4a). Lineaments along the northern flank of the grabenare shorter and more dispersed, reflecting subsidiaryantithetic faults and joints related to hanging-wall flex-ure above the north-dipping master fault (Figure 4a).Lineaments in the central trough are also formed byerosion of faults and joints, but these lineaments areless conspicuous than those located along the grabenflanks because the bed rock is partly buried by alluvialand windblown deposits.

DISCUSSION

Structure of the Duchesne Graben

Viable hypotheses for the origin of the Duchesne gra-ben must incorporate several key structural features:

• The zone of master normal faulting is located alongthe southern boundary of the graben and dips 75�N(Figure 4a). South-dipping faults located along thenorthern boundary are antithetic to this master fault.

• Normal faulting breached a low-amplitude flexure(Figure 6). Near the center of the graben, the am-plitude of the flexure is about 100 m and the wave-length is about 1.5 km.

• Correlation of marker beds between oil wells indi-cates that this low-amplitude fold persists downwardinto the Wasatch Formation at a depth of 2 km (Fig-ure 10).

• The east-central part of the graben shows 200 m ofdisplacement on the north-dipping master fault.This displacement either dies out at depth or is trans-ferred into nearly horizontal displacement where themaster fault flattens into a gently dipping detach-ment fault that is subparallel with bedding(Figure 10).


Figure 12. Continued.

• Displacement on the master fault dies out laterallyaway from the center of the Duchesne graben (Fig-ure 4).

We propose two different structural models for theDuchesne graben: a detachment fault model and a pla-nar master fault model. A detachment fault was mod-eled using the geometrical construction proposed byGroshong (1989). Groshong’s model assumes that asteep-dipping, planar master fault either intersects orbends abruptly into a horizontal to low-angle detach-ment fault at depth. The depth of the detachment is afunction of the dip angle of the master fault, the widthof the belt of flexure in the hanging wall, and the dipangle of bedding in the hanging-wall flexure. In theDuchesne graben, the master fault dips 75�N, thehanging-wall deformation zone is 500 m wide, and

hanging-wall strata dip approximately 30�S in the cen-tral part of the Duchesne graben. Based on these inputparameters to Groshong’s (1989) model, the depth tothe detachment is 930 m (Figure 13).

Displacement on the detachment is fixed by equat-ing the area of the hanging-wall basin with that of arectangle defined by the depth to, and the displace-ment along, the detachment fault. The area of the tri-angular basin is 5 � 104 m2, corresponding to a slip ofapproximately 55 m on the detachment fault. Anti-thetic faults are not explicitly included in Groshong’s(1989) model, but the south-dipping antithetic normalfaults in the Duchesne graben presumably accommo-date rotation and extension within the hanging wall ofthe master fault. The estimated depth of the detach-ment fault is near the transition from a thick-beddedsandstone and limestone section in the Uinta and upper


Figure 13. Two structural models for the Duchesne graben.(a) North-dipping master fault that flattens into a low-angle de-tachment fault. (b) The planar fault model in which displace-ment dies out downward along the north-dipping master fault.In both models, faulting breaches an open, low-amplitude an-ticline. Dashed lines indicate zones of intense jointing and an-tithetic faulting.

Figure 14. Drilling anomalies associated with borehole en-largement (open circles) and high drilling torque (solid circles)for wells in the Duchesne oil field (Figure 9). The locations areprojected from each wellbore onto a vertical plane that strikesperpendicular to the graben (plane strikes N2�W). The drillinganomalies are defined as borehole diameter and torque thatare at least three times greater than the average value in a 100m vertical section of the wellbore. Notice that there are manyanomalies in the lower Uinta and uppermost Green River for-mations, suggesting that these rocks are more brittle andstronger than those in the subjacent part of the Green RiverFormation. The downward transition from more brittle to lessbrittle rock may limit the depth of normal faulting (Figure 13b)or localize the position of the inferred detachment fault(Figure 13a).

Green River formations to an underlying section ofthin-bedded shale and limestone (Figure 14). This lith-ologic transition is an important mechanical boundarybased on borehole caliper and drilling torque measure-ments (Figure 14). Rocks in the base of the Uinta For-mation and uppermost Green River Formation are ap-parently stronger (higher torque during drilling), morebrittle, and perhaps more fractured (greater boreholediameter) than the underlying shale and limestone.This mechanical boundary is a likely position to trans-fer normal fault displacement onto a detachment faultin underlying shale beds. The detachment need notcontinue for any great distance downdip to the north,however. The estimated 55 m of displacement on thedetachment fault may die out northward within theshale beds if shearing becomes distributed by flowwithin the shales.

The alternative, planar fault model requires thatdisplacement on the master fault decrease with depth.We used a fault displacement gradient magnitude pro-posed by Nicol et al. (1995) to estimate the maximumdepth of normal faulting for this structural model.Fault displacement dies out at a depth of about 1365m, given 200 m of net slip at 400 m depth in the grabenwell and a fault-displacement gradient of 0.2 belowthat depth on the 75�-dipping master fault. South-dipping, antithetic normal faults are secondary struc-tural factors in this model as in the listric master faultmodel. The mechanical transition in the upper part ofthe Green River Formation (Figure 14) is also impor-tant in this model. Normal fault displacement may beabsorbed below this depth by flow and distributedfaulting in the Green River shale and limestone unit.


Origin of the Duchesne Graben

The origin of the Duchesne graben must be consideredin the context of both regional and local structure. TheDFZ is a series of en echelon grabens and fractures ex-tending more than 60 km across the southern limb ofthe Uinta basin syncline (Ray et al., 1956). The lengthof the fault zone suggests that a deep-seated fault orfracture zone may be developed in the underlying base-ment, but there is no evidence for a basement-involvedstructure in regional magnetic or gravity patterns, atleast to our knowledge. The relatively small fault dis-placements of tens to hundreds of meters suggest thatthe DFZ is not associated with basement-involvedfaulting of significant magnitude.

The Duchesne graben breaches an open, low-amplitude flexure or anticline that continues below thedepth of discernible normal faulting (Figure 13). Theorigin of this flexure is difficult to ascertain using theavailable data. Beds are locally flexed by reverse dragadjacent to normal faults at some localities (Figure 6)and tilted by block rotation between faults. This latterphenomenon is well displayed along the southern flankof the graben (Figure 7). This smaller scale flexing androtation, however, does not sufficiently account for thelarger scale folding in the broad anticline that persistsbelow the depth of discernible faulting (Figures 10,13). This anticline may have originated in the southernlimb of the Uinta basin syncline during compressionaldeformation in the Laramide orogeny. If folding andfaulting were coeval, then the graben presumably ini-tiated where beds were extended above the neutralsurface of the low-amplitude fold (Figure 13). The agesof folding and normal faulting are difficult to deter-mine, however, because late Tertiary strata have beenstripped by erosion from the region surrounding theDuchesne graben (Fouch et al., 1992).

Regional geological relationships can be used,however, to develop a plausible, if speculative, historyfor the Duchesne graben. Normal faulting at the east-ern end of the DFZ is thought to predate both theinjection of northwest-trending gilsonite dikes and theformation of northwest-trending joints (S. R. Bereskinand R. L. Bruhn, 1996, unpublished data). This tem-poral relationship is consistent with the fact that north-west-trending joints in the Duchesne graben are alsoyounger than the east-west–trending normal faults andjoints, although there are no gilsonite dikes (e.g., Figure8). If one assumes that the gilsonite dikes and north-west-trending joints formed during the peak period ofsource rock maturation in the Uinta basin, then the

DFZ may be older than 30–35 Ma (Fouch et al., 1992).This early Oligocene age coincides with the culmina-tion of the Laramide orogeny in the Uinta Mountainsand the maximum burial of hydrocarbon source rocksin the Uinta basin.

We have reason to suspect that normal faultingand jointing continued episodically during the Neo-gene, even if the faulting began earlier. The Neogenewas marked by uplift and fracturing in the ColoradoPlateau just south of the DFZ (Chidsey and Laine,1992). Normal faulting also occurred in the easternUinta Mountains (Hansen, 1984) and locally along thenorthwestern edge of the Uinta basin (Hecker, 1993).An estimated 1–2 km of overburden were strippedfrom the DFZ during regional uplift and erosion in thesouthern part of the Uinta basin while maturation ofhydrocarbon source rocks continued to generate highfluid pressure in the Green River Formation (Fouch etal., 1992). Reduction in confining pressure concomi-tant with generation of high fluid pressure presumablyreduced the mechanical stability of faults and en-hanced jointing. Notably, the Duchesne graben is pres-ently located at the southwestern edge of a broad zoneof high fluid pressure in the Green River Formation(Fouch et al., 1992; McPherson, 1997).

The narrow but distinct topographic troughs andprominent photolineaments within the DFZ have beeninterpreted as evidence for Quaternary faulting(Hecker, 1993). No compelling geological evidenceexists for Quaternary faulting in the Duchesne graben,however. The graben is highly dissected in Indian Can-yon, where the topographic trough is deepened bystream action and widened by erosion of the northernwall (Figure 3). No fault scarps exist in alluvial depos-its, stream knickpoints are located several hundred me-ters upstream from the master fault in the southernwall of the graben, and there is little change in channelgradient where streams exit through the northern wallof the graben (Figure 12). The small spring, water-saturated soil, and changes of channel gradient in LittleCottonwood Creek (Figures 4, 5) could be caused byactive faulting, but ponding of northward-flowinggroundwater against impermeable faults is a plausiblealternative.

Spatial and Dimensional Analysis of Structural Lineaments

Fracture density and length are important parameterscontrolling fluid migration pathways and reservoir per-meability. The orientations, lengths, and midpoint co-ordinates of photolineaments were compiled from


Figure 15. The number of lineaments N(L) of length equal toor greater than a specified length L plotted at logarithmic scale.The lineaments are divided into two different types, thoseformed by joint zones and those associated with faults. Joint-related and fault-related lineaments in the northern half of thegraben are indicated by open and solid triangles, respectively.Open and solid circles indicate joint-related and fault-relatedlineaments, respectively, in the southern half of the graben.

1:40,000 scale aerial photographs of the Duchesne gra-ben. The origins of most photolineaments were deter-mined during geologic mapping, and we were able tocorrelate lineaments across roads, streams, and areascovered by Quaternary deposits. Lineaments wereclassified as either eroded features along faults or aseroded features along joint zones. Lineaments werethen separated by location into two structural domains:one in the northern half of the graben, where beds areflexed from north-dipping to south-dipping in thestructural hinge zone, and the other in the southernhalf of the graben, where the master fault zone cropsout (e.g., Figures 4, 15).

The cumulative number of lineaments N(L)greater than or equal to a specific length L was plottedon a graph with log-log scale for analysis of dimensionalscaling (Figure 15). Several important features areevident:

• Lineaments developed along faults range in lengthfrom about 100 m to 16 km. The 16 km–long line-ament is developed in the master fault zone alongthe southern boundary of the graben; the longestfault-related lineament along the northern boundaryis only 2 km long.

• Lineaments formed by erosion along joint zonesrange in length from less than 100 m to more than1 km.

• Many more short lineaments exist than long ones,a typical property of natural fracturing (Cladouhosand Marrett, 1996).

• Length-scaling of fault-related lineaments is similarin the northern and southern parts of the graben,but the ratio of short to long joint zones is greatestin the northern part (Figure 15). This differencepresumably reflects the asymmetry of deformation.Long joint zones are proportionally more frequentin the southern flank of the graben where extensionis focused around the master normal fault zone.Shorter joint zones are proportionally more fre-quent along the northern flank of the graben. Thereis less extension in the north flank, which is a hingezone where bedding is tilted southward in the hang-ing wall of the master fault zone (Figure 13).

Petroleum Geology

One million of the 400 million equivalent bbl of oilproduced in the Uinta basin since 1951 were pro-duced from wells in the Duchesne oil field (Figure16). The percentages of total production from eachof three major producing intervals are as follows: up-per Green River Formation, 13%, dominantly oil;lower Green River Formation, 29%, dominantly oiland gas; and Wasatch Formation, 54%, dominantlyoil (data compiled from the Utah State Division ofOil, Gas, and Mining public records office). Severalwells produce from multiple zones simultaneouslyand account for the remaining part of production.

Producing intervals in the Wasatch and lower-most Green River formations are overpressured.Drill-stem tests in the Wasatch Formation indicatedpressure gradients exceeding 12 kPa/m and locallyapproaching 18 kPa/m, significantly greater than ahydrostatic gradient of 9.8 kPa/m (Spencer, 1987;Chidsey and Laine, 1992). Overpressure presumablyoriginates as solid kerogen is converted to fluid hy-drocarbons during oil generation near the center ofthe Uinta basin and perhaps by dehydration of claysduring burial and compaction (Narr and Currie,1982; Spencer, 1987). Narr and Currie (1982) andSpencer (1987) speculated that high fluid pressure inthe Duchesne oil field was caused when oil migratedupdip to flow beneath and become trapped withinfractured, low-permeability shale in the lower GreenRiver Formation.


Figure 16. Cumulative oilproduction from wells in theDuchesne oil field. Well loca-tions and production intervalsare projected onto a verticalplane striking N2�W, perpendic-ular to the trend of the Du-chesne graben. Detachmentand planar fault models (Figure13) are superimposed on thecross section to compare pro-duction with inferred faultstructure. Production data werecollected from the public rec-ords of the Utah State Divisionof Oil, Gas, and Mining.

The low-amplitude anticline in the Duchesne oilfield presumably formed a trap that localized accu-mulations of oil. Notably, most production in the Du-chesne oil field was from wells drilled into the broadnorthern limb of this fold just north of the Duchesnegraben. The greatest production has been from frac-tured intervals in the Green River Formation near thedepth of the possible detachment fault (Figure 16).Fracturing by normal faulting and jointing enhancedproduction, providing both hydrocarbon migrationconduits and fractured reservoirs that extend to theupper part of the Green River Formation. The impor-

tance of fracturing was exemplified in one well locatedimmediately north of the graben (API well number 43-013-20045). One-third of the total oil produced in theDuchesne oil field comes from fractured Green RiverFormation shales and limestones encountered in thiswell (data compiled from the Utah State Division ofOil, Gas, and Mining).

Oil may also be trapped directly beneath the gra-ben in the crest of the low-amplitude fold and justsouth of the fold axis in the south-dipping beds. Thesetraps may be formed either by anticlinal closure (Fig-ure 13) or by a combination of fold closure and


small-displacement faulting, that is, displacements ofless than 9 m. This hypothesis has not been testedadequately because the only well drilled in the grabenwas dry, and only three wells have been drilled in thesouth limb, one dry hole and two moderate producers.Charging of the traps just south of the graben requiresthat oil migrate either through or below the Duchesnegraben fault zone (Figure 16) before rising upward intothe footwall of the master fault. This migration pathmay be blocked by impermeable faults, making thesetraps particularly risky drilling targets.

Structural mapping, together with the analysis oflineament length-scaling (Figure 15), may be used toinfer plausible dimensions of fracture-controlled res-ervoirs in the Duchesne graben. Numerous horstblocks within the graben may provide fractured reser-voirs with shale top seals and lateral fault seals. Frac-turing within low-permeability beds (Figure 8) mayalso generate fractured reservoirs sealed by low-permeability country rock. We assume that the maxi-mum linear dimension of a fractured reservoir isroughly equal to the longest lineament formed bylinked faults and/or joints. The width is estimated byspacing between lineaments and mapped fault andjoint zones (Figure 4). Most fractured reservoirs in thehanging wall of the master fault are presumably lessthan 2 km long and probably only tens of meters wide.Reservoirs surrounding the steeply dipping masterfault could be longer than those in the hanging-wallhinge zone (Figure 13).

CONCLUSIONS

The Duchesne graben is a structural half graben lo-cated along the crest of an open anticline. The masterfault crops out along the southern flank of the grabenand dips northward at 75�. About 200 m of verticaldisplacement exist on the master fault in the center ofthe graben, and displacement dies out to the east andwest. The master fault may root into a gently north-ward-dipping detachment fault in the Green River For-mation at a depth of approximately 1 km, or alterna-tively, the master fault may be planar and die outdownward into the Green River Formation above adepth of approximately 1.4 km. No evidence exists forQuaternary faulting, although groundwater movementmay be retarded by impermeable faults and locallychanneled through open fractures to form springs. Hy-drocarbon traps in the Duchesne graben are formed byfaults and possibly by the broad, low-amplitude anti-

cline that is breached by faulting in its upper part (Fig-ure 13). Intense faulting and jointing produce conduitsfor hydrocarbons migrating from depth and form frac-tured reservoirs in the Green River Formation.

REFERENCES CITED

Barnett, J. A., J. Mortimer, J. H. Rippon, J. J. Walsh, and J. Watter-son, 1987, Displacement geometry in the volume containing asingle normal fault: AAPG Bulletin, v. 71, p. 925–937.

Bruhn, R. L., M. D. Picard, and J. S. Isby, 1986, Tectonics and sed-imentology of the Uinta arch, western Uinta Mountains, andUinta basin, in J. A. Petersen, ed., Paleotectonics and sedimen-tation in Rocky Mountain region, United States: AAPG Memoir32, p. 333–352.

Chidsey, T. C., and M. D. Laine, 1992, The fractured Green Riverand Wasatch formations of the Uinta basin, Utah: targets forhorizontal drilling, in T. D. Fouch, V. F. Nuccio, and T. C.Chidsey, eds., Hydrocarbon and mineral resources of the Uintabasin, Utah and Colorado: Utah Geological Association Guide-book 20, p. 123–134.

Cladouhos, T. T., and R. Marrett, 1996, Are fault growth and linkagemodels consistent with power-law distributions of faultlengths?: Journal of Structural Geology, v. 18, p. 281–293.

Clem, K., 1985, Oil and gas production summary of the Uinta basin,in M. D. Picard, ed., Geology and energy resources, Uinta basinof Utah: Utah Geological Association Guidebook 12, p. 159–167.

Davis, J. C., 1986, Statistics and data analysis in geology, 2d ed.:New York, John Wiley and Sons, 656 p.

Fouch, T. D., V. F. Nuccio, J. C. Osmond, L. MacMillan, W. B.Cashion, and C. J. Wandrey, 1992, Oil and gas in uppermostCretaceous and Tertiary rock, Uinta basin, Utah, in T. D.Fouch, V. F. Nuccio, and T. C. Chidsey, eds., Hydrocarbon andmineral resources of the Uinta basin, Utah and Colorado: UtahGeological Association Guidebook 20, p. 9–47.

Groshong, R. H., 1989, Half-graben structures: balanced models ofextensional fault-bend models: Geological Society of AmericaBulletin, v. 101, p. 96–105.

Hansen, W. R., 1984, Post-Laramide tectonic history of the easternUinta Mountains, Utah, Colorado and Wyoming: The Moun-tain Geologists, v. 21, p. 5–29.

Hecker, S., 1993, Quaternary tectonics of Utah with emphasis onearthquake-hazard characterization: Utah Geological SurveyBulletin 127, 157 p.

McPherson, B. J., 1997, A three-dimensional model of the geologicand hydrodynamic history of the Uinta basin, Utah: analysis ofoverpressures and oil migration: Ph.D. dissertation, Universityof Utah, Salt Lake City, Utah, 111 p.

Narr, W., and J. B. Currie, 1982, Origin of fracture porosity—example from Altamont field, Utah: AAPG Bulletin, v. 66,p. 1231–1247.

Nicol, A., J. Watterson, J. J. Walsh, and C. Childs, 1995, The shapes,major axis orientations and displacement patterns of fault sur-faces: Journal of Structural Geology, v. 18, p. 235–248.

Ray, R. G., B. H. Kent, and C. H. Dane, 1956, Stratigraphy andphotograph-geology of the southwestern part of Uinta basin,Duchesne and Uintah counties, Utah: U.S. Geological SurveyOil and Gas Inventory Map OM 171, scale 1:63360, 2 plates.

Spencer, C. W., 1987, Hydrocarbon generation as a mechanism foroverpressuring in Rocky Mountain region: AAPG Bulletin,v. 71, p. 368–388.

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