The gray diagenetic facies of the Permian LyonsSandstone, which is associated with all knownpetroleum accumulations in the formation, formedlate in the history of the Denver basin as an alter-ation product of the formation’s red facies. The redfacies that makes up most of the sandstone containsiron oxide coatings, quartz overgrowths and calcitecements. The gray facies, which occurs locally in thedeep basin, is distinguished by pore-filling dolomiteand anhydrite cements and by a lack of iron oxideand calcite. The dolomite and anhydrite cementsoverlie bitumen that was deposited by migrating oil,and hence formed after oil was first generated in thebasin, late in the Cretaceous or early in the Tertiary.The isotopic composition of oxygen in the dolomiteranges to such light values that the cement musthave formed deep in the basin in the presence ofmeteoric water.
The gray facies likely formed in a regime ofgroundwater flow resulting from Laramide uplift ofthe Front Range during the Tertiary. In our model,
saline groundwater flowed eastward through thePennsylvanian Fountain Formation and thenupwelled along the basin axis, where it dischargedinto the Lyons Sandstone. The saline water mixedwith more dilute groundwater in the Lyons, driving areaction that dissolved calcite and, by a common-ioneffect, precipitated dolomite and anhydrite. Thefacies’ gray color resulted from reduction of ferricoxide in the presence of migrating oil or the Foun-tain brine. Underlying source beds by this time hadbegun to generate petroleum, which migrated bybuoyancy into the Lyons. The association of the grayfacies with petroleum accumulations can beexplained if the Fountain brines discharged acrossaquitards along the same fractures that transmittedoil. As petroleum accumulated in the Lyons, thenewly formed cements prevented continued migra-tion, as is observed in shallower strata, by sealing oilinto the reservoirs from which it is produced today.
INTRODUCTION
Geologists have long puzzled over the nature ofdiagenetic alteration in the Permian Lyons Sandstoneof the Denver basin because the occurrence of dia-genetic cements in the formation mirrors the distri-bution of petroleum reservoirs. Most of the Lyonsconsists of a red diagenetic facies. The red Lyons isfamiliar to many geologists because it crops out atthe basin’s western margin on the Flatirons, a seriesof hogbacks along the Front Range of the RockyMountains, and because of its use as a distinctivebuilding stone in the area. The facies, which is bar-ren of petroleum, is characterized by hematite stain,clay minerals, quartz overgrowths, and calcitecements. A gray diagenetic facies occurs locally deepin the basin. The gray Lyons, distinguished by anhy-drite and dolomite cements that overlie bitumenstains and by a lack of calcite and hematite, hosts allknown oil fields in the formation.
1Manuscript received, August 5, 1992; revised manuscript received,September 13, 1993; final acceptance, September 24, 1993.
2Department of Geology, University of Illinois, Urbana, Illinois 61801.Present address: Department of Geology, Temple University, Philadelphia,Pennsylvania 19122.
3Department of Geology, University of Illinois, Urbana, Illinois 61801.We thank Roger Burtner of the Chevron Oil Field Research Company for
supplying Lyons core samples, Keith Hackley and Jack Liu of the IllinoisState Geological Survey for helping make isotopic measurements, andRichard Hay, Randy Cygan, Wendy Harrison, and Steve Johansen for adviceand helpful discussions. We appreciate assistance in the laboratory fromTom Anderson, Steve Altaner, Jay Matthews, and Eric Daniels, and help incomputer programming from the Hydrogeology staff. Jessie Knox and JoanApperson drafted the figures. This study was supported by National ScienceFoundation grants EAR 85-52649 and EAR 8601178, and the generosity ofAmoco Production Research, Arco Oil and Gas, British Petroleum Research,Chevron Oil Field Research, Conoco, Du Pont, Exxon Production Research,Illinois State Geological Survey, Japan National Oil Company, LawrenceLivermore National Laboratory, Marathon Oil Company, Mobil Research andDevelopment, Sandia National Laboratory, Texaco, and Union Oil ofCalifornia.
Groundwater Flow, Late Cementation, and PetroleumAccumulation in the Permian Lyons Sandstone,Denver Basin1
Ming-Kuo Lee2 and Craig M. Bethke3
AAPG Bulletin, V. 78, No. 2 (February 1994), P. 221-241.
origin of the gray facies nor the reason for its associa-tion with petroleum accumulations. The anhydrite anddolomite cements of the gray facies might have formedsoon after the Lyons was deposited, as suggested bythe high intergranular volume preserved in the sand-stone. Anhydrite occupies up to 25% and dolomitecements occupies up to 15% of the rock’s pre-cementpore volume. For this reason, Dimelow (1972) suggest-ed that these cements might have precipitated from for-mation water deposited with the sediment.
Dolomite and anhydrite might also have precipi-tated in an environment open to flow of salinegroundwater. For example, they could have formedwhere evaporated seawater infiltrated carbonateplatforms in sabkha environments (Butler, 1969; Pat-terson and Kinsman, 1977; Achauer, 1982); somegeologists working in the Denver basin have sug-gested such an origin to us. Levandowski et al.(1973), on the basis of the cement distribution in thesandstone, argued that the cements precipitatedwhen underlying strata compacted during burial,driving saline groundwater upwards through theLyons.
Anhydrite and dolomite cements of the grayLyons Sandstone might also have formed in an envi-ronment open to regional groundwater flow, longafter the formation was buried. Such an origin is notunusual: dolomite cements can form where freshand saline groundwaters mix (Badiozamani, 1973;Land, 1973; Harrison, 1991), where migratinggroundwater reacts with host sediments (Banner etal., 1988; Gregg and Shelton, 1989; Bethke and Mar-shak, 1990), and where carbon dioxide escapes frommigrating brine (Leach et al., 1991).
In this study, we use hydrologic and geochemicalmodels to develop an explanation of the origin ofgray Lyons facies that is consistent with petrographicobservations as well as the stable isotopic composi-tions of the Lyons cements. The results of our model-ing suggest a strong relationship between diageneticalteration of the sandstone, the basin’s paleohydrolo-gy, and the accumulation of petroleum in the Lyons.
GEOLOGIC SETTING
The present-day Denver basin is an elongate,asymmetric structure whose axis runs north-south inColorado and Wyoming, just to the east of the FrontRange uplift. The Laramide and Front Range upliftsbound the basin to the west. Basin strata extendeastward across parts of eastern Colorado, southeast-ern Wyoming, southwestern Nebraska, and north-western Kansas. The deepest part of the basin liesnear Denver, Colorado, where more than 4 km ofsediments are preserved. Because of the basin’shydrocarbon potential, its structure, stratigraphy, andtectonic history have attracted considerable study
The Denver basin was a marine shelf that subsid-ed slowly through most of the Paleozoic and Meso-zoic. Figure 1 shows the stratigraphic column nearthe Front Range. Pennsylvanian sediments consistmainly of carbonate and shale, although sandstonespredominate in the west along the uplift of theancestral Rocky Mountains. During the Permian, abroad sea intermittently covered a surface of lowrelief where sediments, including the Lyons Sand-stone, were deposited in environments ranging fromfluvial to normal marine to hypersaline. During theMesozoic, Triassic and Jurassic sediments consistingmainly of sandy shale buried the Paleozoic section.The basin downwarped rapidly in the Cretaceous,when an interior seaway covered much of westernNorth America. During this period, more than 3 kmof shale and shaly sandstone were deposited at the
218 Lyons Sandstone, Denver Basin
Figure 1—Stratigraphic column for the Denver basinnear the Front Range (after Weimer, 1973).
basin’s depocenter.With the onset of the Laramide orogeny near the
end of the Cretaceous, the area west of the present-day basin began to be uplifted. Orogeny continuedinto the Tertiary and reached its peak during theEocene. The basin tilted eastward as the Front Rangebegan to emerge. Uplift resumed during the middleOligocene (Trimble, 1980; Morse, 1981). The mostrecent uplift, about 5–6 million years ago, causedabout 450–600 m of Tertiary sediments to be erodedfrom the basin (Trimble, 1980). With this final adjust-ment, the basin assumed its present configuration.
For purposes of hydrologic analysis, strata in ourstudy area can be divided into five hydrostratigraphicunits (Belitz and Bredehoeft, 1988): (1) Pennsylvani-an-Permian, (2) Permian-Triassic, (3) Triassic-Jurassic,(4) basal Cretaceous sandstones, and (5) Cretaceousshales. Mississippian carbonates form a sixth unitthat, because of extensive secondary porosity, consti-tutes an important aquifer system in the northernGreat Plains (Downey, 1982; Back et al., 1983). Thislowermost unit, however, is absent in our study area.
The Pennsylvanian-Permian unit consists primari-ly of interbedded carbonates and shales, but sand-stones predominate toward the basin’s western mar-gin. In the western basin, sandstones of the FountainFormation lie at the base of the unit. These sand-stones grade eastward to dolomite and shale. Sec-ondary porosity and permeability are less welldeveloped in carbonates of this unit than they are inMississippian limestones, which constitute aquifersin the southern basin.
Interbedded red beds and evaporites dominatethe strata of the Permian-Triassic unit. In eastern Col-orado, the Permian Lyons Sandstone lies betweenanhydritic siltstones. The Lyons, a regional aquifer,serves as a supply of potable water where it is shal-lowly buried and as a petroleum resource in thedeep basin. The overlying Triassic-Jurassic unit con-tains shale interbedded with local sandstone layers.The unit acts as a regional aquitard.
The basal Cretaceous sandstones, which includethe Dakota Sandstone, make up the most importantaquifer system within the basin, providing the mainwater supply for consumption and irrigation in thecentral Great Plains area. The Cretaceous shale unitconfines the aquifer system. This unit, the thickest inthe basin, is predominantly shale, but contains minoramounts of limestone, sandstone, and chalk.
DEPOSITION AND DIAGENESIS OF THE LYONSSANDSTONE
The Denver basin in the Permian contained twomajor subbasins where evaporites accumulated: theAlliance basin to the north and the Sterling basin tothe south (Figure 2). The Lyons Sandstone formed as
a nearshore deposit along a band between theslightly emergent ancestral Rocky Mountains to thewest and the evaporite basins to the east. Fluvial,marine, and eolian processes transported sand to thecoast (e.g., Blood, 1970; Walker and Harms, 1976;Adams and Patton, 1979), where the Lyons wasdeposited eastward about 150–200 km from the pre-sent-day position of the Front Range. Figure 3 showsthe relationship of the Lyons Sandstone to aquitardsand other aquifers within the basin. Anhydrite-richsiltstones bounded the Lyons above and below. ThePennsylvanian Fountain Sandstone underlies theLyons to the west, but pinches out into carbonateand shale facies as it grades eastward.
Lyons sediments have been extensively alteredsince deposition in a pattern suggesting that the pro-cess of diagenesis was closely related to oil accumu-lation (Levandowski et al., 1973). Figure 4 shows thatthe zone of greatest cementation lies close to allknown Lyons oil fields, including Pierce, Black Hol-low, and New Windsor fields. Two diagenetic faciesof the Lyons (one red and the other creamy gray) can
Lee and Bethke 219
Figure 2—Isopach and lithofacies map of Permian sedi-ments in the Denver basin (after Martin, 1965). Line AA'shows the cross section considered in this study, whichruns from Big Thompson Canyon to the Keota oil field.
be distinguished by mineralogy and color. The redLyons is nonpetroliferous. Hematite and clay rim thegrains of quartz sand and much of the pore space isfilled by quartz overgrowths and calcite cement (Fig-ure 5A). Textural relations in the facies suggest a gen-eral paragenetic sequence of hematite and clay over-lain by secondary quartz and then calcite. Detailedpetrographic study of samples from outcrops alongthe Front Range (Hubert, 1960), however, shows thatthe Lyons contains at least two generations of calcitecements that precipitated at different times. Some cal-cite formed even earlier than the hematite, which iscommonly cited as the first phase to form in the dia-genetic history of the sandstone.
The gray Lyons facies, which is associated with oilfields, is distinguished (Levandowski et al., 1973) byanhydrite and dolomite cements that overlie organicmatter (Figure 5B). The organic matter, which isbitumen deposited by migrating petroleum, coversquartz grains and their overgrowths. For this reason,
the anhydrite and dolomite cements must haveformed after oil first migrated into the Lyons. Togeth-er, the anhydrite and dolomite occupy as much as
220 Lyons Sandstone, Denver Basin
Figure 3—Cross section AA' (see Figure 2 for location) through the Denver basin from Big Thompson Canyon to theKeota oil field (after Levandowski et al., 1973). The section shows the relationship of the Lyons Sandstone to otherhydrostratigraphic units within the Denver basin.
Figure 4—Relationship of areas of the most intensiveanhydrite cementation (ruled pattern) to Lyons oilfields (solid pattern). Open circles show locations ofwells where samples were taken for isotopic analysis byLevandowski et al. (1973) and in this study.
40% of precement porosity, or about 8% of the for-mation’s total volume.
Calcite cements are ubiquitous in the red Lyonsfacies, but have yet to be observed in the gray facies.Calcite cementation almost certainly preceded deposi-tion of anhydrite and dolomite because some calcitein the red facies underlies the early hematite cement(i.e., Hubert, 1960), which in turn underlies the quartzovergrowths, whereas the anhydrite and dolomiteoverlie the quartz overgrowths and later bitumen. Ifthe gray facies is an alteration product of the redfacies, the alteration process must have consumed thecalcite. Alteration, in contrast, seems to have con-sumed little quartz. The photomicrograph of the detri-tal quartz grains (Figure 5C), made by scanning elec-tron microscope after carbonate and sulfate in thesamples had been chemically digested, shows shal-low solution pits indicating that quartz surfacesbeneath the late cements were only slightly etched.
ISOTOPIC COMPOSITIONS OF LATE CEMENTS
Oxygen in Carbonate Cements
Levandowski et al. (1973) analyzed the δ18O andδ13C compositions of carbonate cements; their resultsare shown in Table 1. The analyses did not distin-guish among carbonate minerals, but the dominantcarbonate mineral from gray facies samples isdolomite. The cements range in δ18O from +8.8 to+21.2‰ relative to SMOW. These values are signifi-cantly lower than would be expected in marine car-bonates, which normally fall in the range +30 to+32‰ (Figure 6).
The low oxygen ratios implicate meteoric water inthe emplacement of the late dolomites, especiallythose samples strongly depleted in 18O, and suggestthat the cements formed at depth in the basin. Usingthe equation of Northrop and Clayton (1966), wecan estimate at any temperature the fluid composi-
tion that would be required to explain dolomite of agiven composition. At 25°C, groundwaters with δ18Ovalues in the range –26 to –13‰ would be required.Such fluids are much lighter than any seawater,especially seawater concentrated by evaporation,and are difficult to reconcile with the likely composi-tions of meteoric water in the latitude of about 40°N.
Lee and Bethke 221
Figure 5—(A) Photomicrograph showing (undercrossed nicols) the red facies of the Lyons Sandstone.Hematite (arrows) coats the original surfaces of quartzgrains (Q) beneath overgrowths of secondary quartz(O). Calcite cement (C) fills some pore spaces. Scale bar= 100 µm. Sample 13398 from Horsetooth Reservoir. (B)Photomicrograph showing (under crossed nicols) thegray facies of the Lyons Sandstone. Dolomite (D) andanhydrite cements (white arrows) fill pore spaces thatare coated with organic matter (black arrows). Organicmatter coats quartz grains (Q) and overgrowths. Scalebar = 100 µm. Sample 13390-6 from the Calco 1 Ferchwell. (C) View under a scanning electron microscope athigh magnification showing the pre-cement surface of adetrital quartz grain. The quartz is lightly pitted. Sample14435-21 from the Troy 1 Jones well.
High-altitude runoff of the Rocky Mountains in theTertiary and present-day have δ18O values as low as–14‰ (Taylor, 1974; Lander and Anderson, 1989).Since this part of the North America craton was notin far northern latitudes during the past 300 m.y.(Irving, 1979), it is unlikely that δ18O values of mete-oric waters in this area could range as low as –26‰.The 18O contents of the cements appear incompati-ble with an early diagenetic origin, and therefore,preclude (barring the possibility of later isotopicreequilibration) the possibility that the cementsformed in a sabkha environment.
Formation temperatures of about 100°C would beexpected in the Lyons Sandstone at present burialdepths of about 3 km (Table 1), assuming a normalgeothermal gradient of about 30°C/km. Groundwaterhaving δ18O values between –13 and about 0‰would be required to precipitate dolomite at this tem-perature. Because Tertiary rainfall in this area couldhave δ18O values down to –14‰, infiltration of mete-oric water deep into the basin could explain even thelightest 18O compositions determined. The scatter inthe δ18O data (Figure 6) could arise from mixing invarying proportions of meteoric water with a basinbrine. If a brine had a δ18O value of about 0‰, forexample, mixing it with meteoric water having a δ18Ovalue of about –13‰ would produce fluids spanningthe required range in isotopic composition.
Carbon in Carbonate Cements
The small CO2 contents of many groundwaterslead many investigators to assume that the δl3C com-position of a carbonate cement simply reflects theδl3C of its precursor mineral (Mattes and Mountjoy,1980; Meyers and Lohmann, 1985). This assumptionmay not be true, however, if the sediment is open toflow of a groundwater charged with CO2. Especiallyin sandstone, where carbonate minerals occupy asmall fraction of the rock volume, carbonate dis-solved in the fluid can represent a second importantreservoir of carbon.
In the red facies, the δl3C of calcite cementsranges widely from –26 to –2‰ (Figure 6). Thesesamples likely represent calcite from different originsand generations, as already discussed. The depletedδl3C values (< –20‰) might result from oxidation oforganic matter, whereas the heavier l3C compositionssuggest that these calcite samples could have formedfrom seawater or from isotopically heavy fluids inthe deep basin.
On the other hand, dolomite cements of the grayfacies span a smaller range and are enriched in l3Ccompared to many of the calcite samples. It is diffi-cult to argue from these data that the dolomitereflects the carbon isotopic compositions of a calciteprecursor. The composition is also inconsistent withoxidation of organic matter. The dolomite can be
222 Lyons Sandstone, Denver Basin
Table 1. Isotopic Composition (‰) of Carbonate and Anhydrite Cements in the Lyons Sandstone
Sample Number* Well Depth (ft)** δ18OSMOW** δl3CPDB** δ34SCDT†
*Numbered by Chevron Oil Field Research.**Levandowski et al. (1973).†This study. ‡Outcrop samples.††Sample contains insufficient anhydrite for analysis.
interpreted to reflect the composition of a deep CO2-charged groundwater that has δl3C values similar tothat of seawater, which ranges from –4 to +4‰. The“normal” carbon-isotopic composition of thedolomite cements likely reflects in large part thecomposition of marine carbonate that dissolved intoa brine deep within the basin, rather than carbonderived from early calcite cements or from oxidationof hydrocarbons in the Lyons Sandstone. The carboncomposition of dolomite is compatible with a fluid-mixing origin suggested by the oxygen composition;in a fluid mixture, the dissolved carbon derived froma CO2-charged brine overwhelms that from meteoricwaters; hence, the carbon isotopic composition ofdolomite that forms from such a mixture will largelyreflect the δl3C values of species in the brine.
Sulfur in Anhydrite Cement
We analyzed the sulfur isotopic composition ofanhydrite cements from nine Lyons core samplesfrom various wells, using the method of Westgateand Anderson (1982) for extraction and analysis. Thecements have sulfur isotopic compositions (Table 1)ranging from +9.6 to +12.5‰ relative to the CDTstandard. These values span almost exactly the rangein isotopic composition of sulfate derived from Per-mian evaporites worldwide, which is +9.6 to +13.0‰(Claypool et al., 1980). For reference, the composi-tion of sulfate from modern seawater is about +20‰.
The sulfur isotopic composition of the anhydrite
cements suggests that the anhydrite could haveformed from seawater during the Permian or fromsulfate that was derived from evaporite rocks in thebasin and imported into the Lyons Sandstone. Theanhydrite is unlikely to have formed from sulfurderived from H2S, organics, or from sulfide mineralsthat oxidized to form sulfate. Sulfur isotopes arestrongly fractionated between sulfide and sulfate,mostly as a result of the bacterial reduction of sulfatein the near surface; consequently basin sulfide reser-voirs are typically depleted in 34S (Thode et al, 1951).The 34S values of reduced sulfide dissolved in naturalwaters normally range from –32 to 0‰ on the CDTscale (Hartmann and Nielsen, 1969). Sulfide thatreoxidizes does not fractionate significantly, so anysulfate derived from the oxidation of sulfide wouldbe depleted in 34S. The anhydrite cements are tooenriched in 34S to be consistent with such an origin.
The anhydrite is more likely a late diageneticphase, because the cement precipitated after oil firstmigrated into the Lyons. Oil usually forms late in abasin’s history because source beds must be deeplyburied long enough to become thermally mature.We show in the next section that most Paleozoic oilin the Denver basin did not form until the Late Cre-taceous or early Tertiary, long after the sandstonewas deposited. In addition, even seawater evaporat-ed to the point of anhydrite saturation would containonly about 14 g/kg of dissolved sulfate to accountfor 0.7% of the sandstone’s compacted volume,much less than is present in the formation. For thesereasons, anhydrite cements in the Lyons likelyformed long after burial in an environment open togroundwater flow, rather than precipitated from sea-water or as pore fluid deposited with the sediment.
PETROLEUM GENERATION AND MIGRATION
Reservoirs in Cretaceous strata provide most ofthe oil and gas produced from the Denver basin. Ofthis oil, most samples can be traced by geochemicalcorrelation to source beds, including the Carlisle,Graneros, and Mowry shales and the GreenhornLimestone, which also lie in the Cretaceous section(Clayton and Swetland, 1980). Some oil in the Creta-ceous section has migrated as far as 150 km from thedeep basin toward the basin’s eastern flank. Oil pro-duced from the Lyons, on the other hand, is a Paleo-zoic variety that is chemically distinct from oils com-mon in younger rocks (Clayton and Swetland, 1980).Unlike overlying strata, the Lyons Sandstone is notknown to have served as a carrier bed for long-dis-tance migration.
Several theories explaining the source of theLyons oils have been proposed: the oil formed from(1) adjacent Permian and Pennsylvanian shales(Levandowski et al., 1973), (2) the Permian Satanka
Lee and Bethke 223
Figure 6—Isotopic compositions of calcite (opensquares) and dolomite cements (solid squares) in theLyons Sandstone. Dolomite falls near the normal rangeof δ13C for marine carbonate, but is depleted in 18O.
Formation (Berman, 1978), or (3) the Permian Phos-phoria Formation near the Idaho-Wyoming border(Dimelow, 1972; Momper, 1978). The third theoryseems unlikely because the oils differ in carbon iso-topic composition from that of Phosphoria oils in theBig Horn basin (Clayton et al., 1987). Middle Penn-sylvanian black shales, however, are rich in organicmaterial and are considered to have excellent sourcepotential over much of the northern Denver basin(Clayton and Ryder, 1984; Clayton, 1989); the Lyonsoils could well have been derived from these beds.In a Cretaceous reservoir, Clayton (1989) found oilfrom a Middle Pennsylvanian source, demonstratingthat oil has migrated vertically upward through thePaleozoic and into the Mesozoic section. There is noreason to believe that oil could not have migratedvertically from a Pennsylvanian source to Permianreservoirs.
Dolomite and anhydrite cements in the grayLyons Sandstone, as already discussed, formed afteroil first migrated into the formation. We can place alimit on the age of the cements by calculating whenoil generation began in Pennsylvanian rocks.According to kinetic theory, time and temperaturecontrol the generation and preservation of oil in sed-imentary basins. For our calculations, we use thesimple model of Lopatin (Lopatin, 1971; Waples,1980). In the model, source-bed maturity isdescribed by a time-temperature index (TTI). TheTTI of a sediment is given as the sum of maturitiesdeveloped in each temperature interval
Index n varies from the lowest to the highest temper-ature interval encountered by the sediment. Theinterval from 100–110°C was chosen as the baseinterval and given the index value n = 0. For each10°C increase or decrease in temperature, the indexvalue increases or decreases by 1. ∆tn is the length oftime (in million years) that the sediment spends with-in each temperature interval. The factor 2n assumesthe doubling of the maturation rate with each 10°Crise in temperature. According to the method, sourcebeds are immature until they attain TTI values ofabout 15. Oil forms when TTI falls between 15 and160, which corresponds to vitrinite reflectance mea-surements (Ro, in percent reflectance in oil) from 0.65to 1.3. When the TTI exceeds 160, oil begins to breakdown to form natural gas.
For a given burial history, models of thermal mat-uration depend on the choice of thermal conductivi-ty and basal heat flow into the basin. We assumethat sediment thermal conductivity (K, in cal/cm·s°C) increases during compaction according to thecorrelation with porosity φ:
K= (5.35 – 4.4φ) × 10–3, (2)
which is taken from the data of Sclater and Christie(1980) for North Sea shales. We calibrate heat flow,assuming that it remained constant through time,against the maturity in Cretaceous source beds deter-mined by Clayton and Swetland (1980) by vitrinitereflectance measurement. Heat flow values, rangingfrom 1.2 to 1.8 HFU (1 HFU = 10–6 cal/cm2·s), repro-duce individual measurements made near our studyarea. Much of this range, however, likely reflects theuncertainty in estimating the depth of past burial andthe timing of uplift and exhumation for samplestaken from outcrop in the western basin. We findthat a heat flow of about 1.5 HFU, near the averagefor continental crust, gives the best fit to the patternof maturity basinwide in Cretaceous strata.
Basin strata were rather shallowly buried duringmost of the Paleozoic and Mesozoic, until thick shalesequences were deposited in the Cretaceous. Forthis reason, even the deepest strata did not becomethermally mature until late in the basin’s history. Fig-ure 7 shows how the thermal maturity of Pennsylva-nian source beds from the deepest basin evolvedthrough geologic time, as calculated for differingheat flows. According to the calculation, the deepestsources began to generate oil between 78 and 50Ma, during the Late Cretaceous or the early Tertiary.
224 Lyons Sandstone, Denver Basin
Figure 7—Evolution of thermal maturity in Pennsylvani-an source beds beneath the Lyons Sandstone, calculatedassuming differing heat fluxes of 1.2, 1.5, 1.8 HFU (50,63, 76 mW/m2, respectively) from basement. The oilwindow is the area between the two dashed lines. Thetime-temperature index (TTI) was calculated usingLopatin’s method (Waples, 1980). As the buried sedi-ment passes from one temperature interval to the next,the index value n in equation 1 increases suddenly by 1,which causes the small kinks on the TTI curves.
(1)
Assuming our best-fit value for heat flow of 1.5 HFU,the first oil probably formed about 68 Ma, near theend of the Cretaceous.
Most Paleozoic oil in the basin, according to theseresults, migrated from source to reservoir in the Ter-tiary, after infilling of the basin was complete. Thistiming provides a further argument that the cementsof the gray Lyons Sandstone formed late in thebasin’s history. The inferred timing also suggests thatthe cements did not precipitate from fluids remobi-lized by compacting sediments, as some have sug-gested, because sediment compaction was completeor nearly so by the time oil generation began.
BASIN PALEOHYDROLOGY
We used BASIN2, a numerical model of groundwa-ter flow in sedimentary basins, to simulate fluidmigration through the Denver basin in the geologicpast. The model calculates the groundwater flow thatarises from sediment compaction and topographicrelief, the transfer of heat by conduction and advect-ing groundwaters, the maturity of petroleum sourcebeds through time, the evolution of porosity and per-meability, and cementation by migrating groundwa-ter. The mathematical basis of the calculation tech-nique is described in detail elsewhere (Bethke, 1985;Bethke et al., 1988, 1993; Corbet and Bethke, 1992).
We show the results of two reconstructions of thebasin’s paleohydrology. The first simulation depictshow the basin’s groundwater flow regime evolved asthe basin subsided and infilled with sediments; the sec-ond portrays flow resulting from Eocene uplift of theFront Range along the basin’s western margin. Model
results provide quantitative estimates of various param-eters needed to evaluate diagenetic models, includingpast rates and directions of fluid migration as well aspressure and thermal gradients along flow paths.
Table 2 shows the hydrostratigraphy assumed inthe simulations. The basin surface in our calculationsis held at a constant temperature of 10°C and atmo-spheric pressure. The sides of the basin remain athydrostatic pressure and are open to groundwaterflow. The bottom of the cross section is the contactwith crystalline Precambrian basement, which wetake as a barrier to fluid flow and the source of aconstant heat flux of 1.5 HFU.
Each stratigraphic unit in the calculations is com-posed of varying fractions of four rock types: sand-stone, carbonate, shale, and evaporite-rich siltstones.We calculate the evolution of porosity and perme-ability as each rock type is buried, using correlationsshown in Table 3; thermal conductivity is set accord-
Lee and Bethke 225
Table 2. Hydrostratigraphy Assumed in the Hydrologic Models
Table 3. Correlations used in the Hydrologic Models toCalculate Porosity and Permeability
Porosity* Permeability**φ0 b (km–1) φ1 A B kx/kz
Sandstone 0.40 0.50 0.05 15 –3 2.5
Carbonate 0.40 0.55 0.05 6 –4 2.5
Shale 0.55 0.85 0.05 8 –7 10
Evaporite 0.55 0.85 0.05 8 –7 10
*φ = φoexp(-bZ) + φ1, expressed as a fraction; Z is burial depth (km). **log kx (µm2) = Aφ + B; kX ≤ 1 µm2; 1 µm2 ≈ 1 darcy.
226 Lyons Sandstone, Denver Basin
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ing to equation 2. The correlations for porosity andpermeability of all rock types, except the evaporiticsiltstones, are taken from previous studies of basinsfrom the cratonic interior (Bethke et al., 1991);because of a lack of data, correlations for the hydro-logic properties of shales were assigned to the evap-oritic siltstones.
Flow Driven by Sediment Compaction
The first paleohydrologic model simulatesgroundwater flow resulting from sediment com-paction as the basin infilled through most of thePaleozoic and Mesozoic. The simulation followscross section BB' (Figure 8), which traverses thenorthern Denver basin. Figure 9 shows the results ofthe calculation for the final time slice in the calcula-tion, at the end of the Cretaceous. In the calculation,fluids migrate from compacting shale and carbonatestrata upward and downward, primarily into the Cre-taceous aquifer complex and, to a lesser extent, intothe Lyons and Fountain sandstones. These aquifersact as drains that carry fluid laterally from deep stratato the basin margins. Because groundwater flowhere is too slow to alter the thermal structure of thebasin, temperatures in the calculation fall along aconductive gradient that corresponds to the assumedthermal conductivity correlation and heat flux frombasement.
Contours in Figure 9 are equipotentials, which rep-resent the drive for fluid migration; groundwatermigrates toward areas of lower hydraulic potential.The equipotentials in this case (where there is notopographic relief and the reference elevation is sealevel) also show the extent to which pressure exceedshydrostatic. The calculation predicts that excess pres-sures developed in the Cretaceous shales were verysmall, about 1 atm (0.1 MPa), near the basin’sdepocenter. Greater pressures result when permeabil-ity is set smaller, but it seems unlikely that sedimentsin this basin were impermeable enough to allow gen-uine overpressures, such as those observed in theGulf of Mexico basin (e.g., Bethke, 1986) to havedeveloped. Overpressure in the Gulf basin resultsfrom burial rates sometimes exceeding 10,000 m/m.y.(Harrison and Summa, 1991), about 100 times morerapid than rates of burial in the Denver basin.
The role that fluids displaced by sediment com-paction could have played in precipitating cementsis limited by the modest flow rates predicted by thecalculation. The estimated flow velocities (true ratherthan Darcy) through the Lyons Sandstone are every-where less than about 2 cm/yr or 20 km/m.y. Thesmall flow rates reflect the slow rates of burial andcompaction in the basin, which infilled over 30 m.y.at a rate less than 100 m/m.y. Such slow rates of sub-sidence, infilling, and fluid expulsion from compact-ing sediments are common to basins of the cratonicinterior (e.g., Bethke et al., 1991).
Lee and Bethke 227
Figure 9—Calculated groundwater flow driven by sediment compaction at the end of Cretaceous. Contours are cal-culated excess pressures (atm) in the basin. Dashed lines show predicted temperature distributions. Arrows indi-cate the flow directions in the Dakota Sandstone, Lyons Sandstone, and Fountain Formation. Vertical componentsof cross section are exaggerated by about 40:1, as shown by the scale bar.
228 Lyons Sandstone, Denver Basin
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Flow Driven by Topography
The second paleohydrologic model simulatesgroundwater flow driven by Eocene uplift of theFront Range in the western basin (Figure 10). On thebasis of the thicknesses of the Laramie Formationand the Dawson sandstone, which represent materi-als eroded and deposited during the Eocene, Trim-ble (1980) estimated that the Front Range was uplift-ed during the Laramide orogeny to between 2500and 6000 m above sea level. As the Front Rangeemerged during the Laramide orogeny, basin fluidsmigrated eastward in response to the hydraulic gra-dient created by the slope on the water table.Groundwater recharged at high elevation along thebasin’s uplifted western margin and dischargedtoward the eastern margin. Groundwater flow con-tinues in this regime today (Belitz and Bredehoeft,1988), but modern flow rates are presumably some-what smaller now than in the past because the FrontRange has eroded over time.
In Figure 10, we assume that the Front Range inthe Eocene was uplifted 2000 m above the elevationof the basin’s eastern margin. The topographic relief
drives groundwater through the Cretaceous aquifersat velocities of tens of meters per year (Figure 11).The velocities predicted, however, reflect two poorlyconstrained assumptions: the past elevation of thewestern basin and the aquifer permeabilities. Pre-sent-day flow rates in these aquifers are not known,but a model calculated by Belitz and Bredehoeft(1988) predicted flow velocities of about 3 m/yr. Bythis standard, the predicted velocities shown hereare perhaps optimistically high. The predicted flowvelocities are somewhat slower (<20 m/yr) in theLyons and Fountain sandstones because theseaquifers are more deeply buried and hence morecompacted and less permeable, and because neitheraquifer is hydraulically continuous across the basin.The Lyons pinches out near the eastern margin ofthe cross section, and the Fountain sandstone gradesinto dolomite and shale near the basin axis.
The greatest hydraulic potentials in the deepbasin develop within the Fountain aquifer (Figure11). Hydraulic potential is greater deep in thisaquifer than along the land surface directly over-head; therefore, a hypothetical well drilled into theaquifer in the Eocene would have been somewhat
Lee and Bethke 229
Figure 11—Calculated Eocene groundwater flow and the hydraulic potential distribution (gray contours; in atm) in theDenver basin. Calculation shows relatively rapid groundwater migration in response to the uplift of the Front Range.Flow in this gravity-driven regime is much more rapid than in the compaction-driven system shown in Figure 9.
overpressured. Potentials in the Cretaceous aquifercomplex are slightly overpressured to the west andunderpressured to the east; the complex is under-pressured over broad areas in the present day (Belitzand Bredehoeft, 1988). The present-day underpres-suring in these shallow aquifers likely reflects ero-sion of the uplifted basin margin and poor hydrauliccontinuity across Front Range faults.
The Lyons Sandstone serves as an aquifer for east-ward flow. Along the basin axis, the aquifer is also azone of dispersive mixing. Fluids migrating alongthe underlying Fountain Formation dischargeupward across stratigraphy where the sandstonegrades into less permeable facies near the basin axis.As shown in Figure 11, the drive for cross-formation-al flow is provided by the hydraulic potentials in theFountain which, along the basin axis, are greaterthan the potentials in overlying strata. In the calcu-lated flow regime, Fountain fluids along the basinaxis discharge upward across the lower Satanka For-mation and into the Lyons, mix with Lyons ground-waters, and then continue to migrate eastward.
Simple Models of Cementation
Groundwater migrating through a sedimentarybasin alters the sediments through which it flowsbecause mineral solubility varies with temperatureand pressure along the flow path. Quartz, for exam-ple, is increasingly soluble with rising temperature.Groundwater saturated with quartz precipitatesquartz cement as it migrates along a path of decreas-ing temperature, whereas it dissolves quartz as itmigrates toward higher temperatures. It is possible,therefore, to calculate the rates at which variousminerals dissolve and cements precipitate through-out a basin from knowledge of mineral solubilityand the groundwater flow pattern; Appendix 1 givesthe mathematical basis for the calculation. We referto this type of calculation as a simple model ofcementation because it accounts only for the effectsof changing temperature and pressure along flowpaths.
We applied this type of calculation to predict theamounts of anhydrite and quartz that could formand be consumed as a result of (1) compaction-driv-en groundwater flow occurring as the basin subsid-ed during the Paleozoic and Mesozoic, and (2) flowdriven by uplift of the western basin during theEocene. Appendix 2 gives the correlations we usedto compute mineral solubility. We did not attempt tocalculate the distribution of dolomite cementation,because the solubility of this mineral varies morestrongly with pH than with temperature or pressure.
The calculation results for the compaction-flowregime (Figure 12) show the cumulative amounts ofanhydrite and quartz formed or dissolved within the
Lyons Sandstone in this hydrologic regime by theend of the Cretaceous. In the hydrologic models,groundwater flowed upward relative to the subsid-ing strata, but downward over much of the basin rel-ative to fixed elevation. For this reason, groundwaterin the deep basin warms with time and, because ofthe retrograde solubility of anhydrite, precipitatesanhydrite cement. The cumulative amount of anhy-drite precipitated in this regime, however, is lessthan about 0.1% of the formation’s volume. Not onlyis this volume too small to account for the origin ofthe sandstone’s gray facies, but the cement is dis-tributed across the sandstone, not just in deep stratawhere the gray facies occurs.
Cementation rates near the Lyons oil fields calcu-lated for groundwater flow in the Eocene (Figure 13)are small (<1%/m.y.). The model predicts that themost extensive cementation or dissolution occursalong the flanks of the basin, where the fluid isdescending or ascending across temperature gradi-ents. This predicted cementation pattern, again, isnot consistent with the observation that the greatestenrichment of anhydrite cements occurs within theLyons oil fields (Figure 4). Therefore, the simple
230 Lyons Sandstone, Denver Basin
Figure 12—Calculated cumulative volumes (X) of anhy-drite and quartz across the Lyons sandstone at the endof Cretaceous. The cumulative volume is the volume ofthe mineral that formed or dissolved from the Permianto the Late Cretaceous per unit volume of the formation,expressed in percent. This simple model of cementationpoorly predicts the volume percentage of anhydritecements in the gray Lyons sandstone.
ideas of cementation by flow through temperatureand pressure gradients cannot explain the origin ofthe gray facies. These simple models of cementation,however, do not account for other potential causesof cementation in the Eocene flow regime, such asthe mixing of groundwaters of varying compositionor common-ion effects in which species producedby dissolution of one mineral might drive the precip-itation of another.
REACTIONS ACCOMPANYING FLUID MIXING
From the results of paleohydrology modeling, weinterpret that, in the Eocene, the Lyons Sandstone atdepth contained a zone of fluid mixing. The locationof the mixing zone corresponds generally to the areain which rocks of the gray facies are found. In thissection, we use quantitative modeling techniques topredict which chemical reactions occurred and howstable isotopes fractionated during the mixing pro-cess. The modeling results show that fluid mixing
could have driven a reaction by which calcite dis-solved and dolomite and anhydrate precipitated, cre-ating the gray Lyons facies as an alteration productof the red facies.
Reaction Modeling
We used the computer program REACT (Bethke,1992) to solve for the overall reaction that wouldaccompany fluid mixing in the deep Lyons Sand-stone. REACT is one of a class of reaction path models(Helgeson, 1968; Helgeson et al., 1970; Wolery,1979; Reed, 1982; Plummer et al., 1983) that tracethe chemical evolution of systems open to mass flux-es. Calculations here are based on version R46 of thethermodynamic database compiled at Lawrence Liv-ermore National Laboratory (Delany and Lundeen,1990; Johnson et al., 1991), and employ the extend-ed Debye-Hückel method (Helgeson, 1969) for cal-culating activity coefficients.
The conceptual basis of our calculation is shownin Figure 14. A packet containing 1 kg of dilute fluidmigrates along the Lyons sandstone, maintainingequilibrium with calcite and quartz in the formation.The packet, which originated as meteoric rechargealong the Front Range, is initially dilute. A secondfluid discharges upward into the Lyons and mixesinto the fluid packet. The second fluid is moresaline, having reacted with evaporite beds in thePennsylvanian and Permian section. As the fluidsmix over the course of the calculation, any mineralsthat become supersaturated precipitate.
We used an analysis of modern Lyons groundwa-ter to set the fluid’s initial composition (McConaghyet al., 1964, sample C5-69-18bbcc). We assumed thatthe mixing occurred at 100°C. Since McConaghy etal.’s (1964) analysis was made at 51°C, we usedREACT to correct it to the temperature of the calcula-
Lee and Bethke 231
Figure 13—Calculated cementation rates (dX/dt) ofanhydrite and quartz across the Lyons Sandstone dur-ing the Eocene. Anhydrite, which has a retrograde solu-bility, precipitates where the fluid is descending anddissolves where the fluid is ascending. Quartz shows anopposite pattern due to its prograde solubility. Thecementation rate is expressed as a percentage of the for-mation’s bulk volume per unit time (%/m.y.). Again,this simple model of cementation poorly predicts thedistribution of anhydrite cement in the gray LyonsSandstone.
Figure 14—Conceptual model for tracing chemical reac-tion during fluid mixing in the Lyons Sandstone.
tion by heating the analyzed fluid in the presence ofcalcite and quartz. We assumed that the compositionof the Fountain groundwater reflects equilibriumwith minerals in the evaporite strata that lie beneaththe Lyons. The Fountain groundwater in our calcula-tions is a 3-molal NaCl solution that has equilibratedwith dolomite, anhydrite, magnesite, and quartz.
The choice of NaCl concentration represents theupper limit of validity of the activity coefficient correla-tions we used. We fixed pH by setting the CO2 fugaci-ty to reflect a partial pressure of 50 atm. The resultingLyons groundwater is predominantly a sodium-bicar-bonate solution, whereas the Fountain brine is mostlya sodium-chloride-bicarbonate solution (Table 4).
In the model, we set the Ca2+ concentration in thebrine by assuming equilibrium with dolomite, andfixed the fluid’s Mg2+/Ca2+ activity ratio to an end-member value of about 20 by specifying saturationwith magnesite. In fact, we do not know whethermagnesite occurs in the evaporite strata, so the actu-al brine from these rocks could have been undersat-urated with respect to this mineral and hence have asmaller activity ratio than predicted. Fortunately, themineralogic results of the fluid mixing reaction varylittle over a broad range of activity ratios. As long asthe Mg2+/Ca2+ activity ratio is greater than about0.06, calcite is undersaturated in the brine, as wouldbe expected in a saline groundwater from evaporitebeds, and we found that mixing the Fountain brineinto the Lyons Sandstone causes dolomite and anhy-drite to precipitate at the expense of calcite.
The choice of CO2 fugacity for the Fountain brinereflects our interpretation of the isotope data, as pre-viously explained. We might have chosen a differentfugacity, but the degree to which calcite is undersat-urated in the brine, the critical variable in the calcu-lation, is independent of pH or CO2 fugacity. Thefluid has equilibrated with dolomite and magnesite,so the saturation state is fixed by the reaction,
CaCO3 ←→ CaMg(CO3)2 – MgCO3
calcite dolomite magnesite
As a result, calcite has a saturation index (log Q/K,where Q is the activity product for the dissolutionreaction, and K is the equilibrium constant) of about–1.3, regardless of pH or CO2 fugacity. Hence, thereis no petrographic basis for determining the pH ofthe Fountain brine.
The reaction model (Figure 15) traces the chemi-cal consequences of progressively mixing the Foun-tain brine into the Lyons aquifer at 100°C. As calcitedissolves into the undersaturated brine, the Ca2+ andHCO3
– added to solution drive precipitation of anhy-drite and dolomite by a common-ion effect. Theoverall reaction predicted by the model is
5 CaCO3 + 2 SO42– + 5⁄2 Mg2+
calcite→ 2 CaSO4 + 5⁄2 CaMg(CO3)2 + 1⁄2 Ca2+
anhydrite dolomite
This reaction reduces porosity in the sandstonebecause the volume of anhydrite and dolomite pro-duced is greater than the volume of calcite dissolved(Figure 15). The predicted reaction explains the ori-gin of anhydrite and dolomite cements, as well as thelack of calcite in the gray facies. It does not explain,however, the slight predominance of anhydrite overdolomite cements observed in the facies; dolomite isa more voluminous reaction product than anhydritein the calculation results (Figure 15). This apparent
232 Lyons Sandstone, Denver Basin
Table 4. Chemical Compositions (mg/kg) of EndmemberGroundwaters used in Mixing Calculations
Figure 15—Predicted overall reaction resulting frommixing of the ascending Fountain brine into the LyonsSandstone. The sandstone is initially filled with the infil-trating meteoric water. The plot shows changes in min-eral volume as brine mixes into the formation by dis-persion. Positive volume changes indicate precipitation,and negative volume changes show dissolution.
discrepancy might be accounted for by slower pre-cipitation rates of dolomite, or a brine with a higherSO4
2–/Mg2+ ratio than assumed in the calculation.Reaction models, such as the one we used, are
quite useful because they trace the effects among allof the reactions that occur in a system in order topredict the overall reaction. Treating reactions indi-vidually without concern for their interactions canbe misleading. For example, the solubility of anhy-drite varies with temperature and fluid salinity. Thesolubility vs. salinity curves are convex upward (seeFigure 6 of Blount and Dickson, 1969), indicatingthat mixing a dilute and a saline fluid should dis-solve, not precipitate, anhydrite. The flaw in this rea-soning, which would seem to negate the analysispresented above, is that the solubility curve or func-tion represents the single reaction,
CaSO4 ←→ Ca2+ + SO4
2–,anhydrite
in isolation, whereas the reaction model correctlyaccounts for the effects of other reactions occurring inthe system. An analysis that considers only anhydritesolubility would incorrectly predict that fluid mixingcould not account for the cementation observed.
Isotope Fractionation
To test whether the reaction model presented inthe previous section can explain the isotopic compo-sitions of cements in the gray facies of the LyonsSandstone, we traced how the stable isotopes 13Cand 18O fractionate during the predicted reaction.The calculation technique employs mass balanceequations similar to those derived by Bowers andTaylor (1985) and Bowers (1989). The technique, asimplemented in REACT, accounts for fractionationamong the solvent, dissolved species, gases, andminerals. Table 5 shows the fractionation factorsassumed in the calculation. Instead of assuming thatminerals maintain isotopic equilibrium with the fluid,as is commonly done in this type of model, the cal-culation segregates minerals from isotopic exchange.Only the increments of minerals precipitated duringa reaction step are in isotopic equilibrium with thefluid, and only the mass of minerals dissolved over astep affects the fluid’s isotopic composition. In thisway, the calculation procedure models the fractiona-tion that results from the reaction process in theabsence of isotope exchange.
We assumed that the initial system contained ameteoric water having an initial δl8O of –13‰(SMOW), which matches estimates for Tertiary rain-fall in the region, as already discussed. The δl8O ofcalcite was set to +11‰, the mean 18O compositionof this cement. The l8O composition of calcite
assumed in the model, however, has very little effecton the results since the amount of oxygen in the sys-tem provided by water overwhelms the smallamount obtained by dissolving calcite. We furtherassumed that quartz was in equilibrium with the ini-tial fluid, but so little of this mineral reacts that itsisotopic effect can be neglected. The δl3C of the ini-tial fluid was –12‰ (PDB), which was set to reflectthe mean isotopic composition of calcite cements(–11‰) found in the red facies of the Lyons Sand-stone. We further assumed that the Fountain brine,which migrates into the Lyons during the reactionprocess (Figure 14), is isotopically heavier. The brinehas δl8O and δ13C values of 0‰, as might be expect-ed in a sedimentary brine.
Figure 16 shows how the isotopic compositions ofdolomite cements (assuming varying CO2 fugacities forthe Fountain brine) evolved as the Fountain brinemixed into the Lyons Sandstone. Results show that therange in δl8O values observed for the dolomitecements can be explained by mixing in differing pro-portions of Lyons groundwater with Fountain brine, aswe previously hypothesized. The δ13C values predict-ed by the model depend on the CO2 fugacity assumedfor the Fountain brine. As previously noted, the valuechosen for CO2 fugacity has little effect on the miner-alogic results of the reaction path, and hence is notconstrained by petrographic observations.
When we assumed small values for the CO2fugacity of the Fountain brine, the calculation pre-dicted dolomite compositions similar to the δ13C ofthe precursor calcite. Assuming CO2 fugacities in therange of about 25 to 100, however, gives dolomitecompositions near the observed range. In this case,the dolomite δ13C more closely reflected the isotopiccomposition of carbon species in the brine than car-bon from the precursor calcite. A reaction model
Lee and Bethke 233
Table 5. Isotope Fractionation Factors Assumed in theCalculations, Expressed as 1000 ln α. Factors DescribeFractionation at 100°C of 18O Relative to Water, and 13CRelative to CO2*
*Fractionation factors calculated from regression curves compiled by J.K.Bohlke from the following sources: Northrop & Clayton (1966), Malinin et al.(1967), Bottinga (1968), O’Neil et al. (1969), Sheppard and Schwarcz (1970),Clayton et al. (1972), Richet et al. (1977), Chiba et al. (1981).
**Assumed.
that calls on migration of a CO2-charged brine intothe Lyons, therefore, explains both the petrographyand isotopic compositions of cements in the Lyonsgray facies.
The fugacities suggested by the above analysis aresomewhat greater than those measured in the GulfCoast, where CO2 pressures are less than 5 bars at100°C (Smith and Ehrenberg, 1989). The fugacitiessuggested for the Denver basin might better be com-pared to CO2 concentrations and temperatures mea-sured in fluid inclusions in late dolomite cementsthroughout the Ozark region (Leach et al., 1991). TheCO2 contents in fluid inclusions there are greater than0.35 molal, indicating that CO2 pressures at about120°C could range up to at least 40 bars.
DISCUSSION
In this study, we integrated quantitative models ofgroundwater flow and chemical reaction with petro-graphic and isotopic observations to explain the ori-gin of the petroliferous gray facies of the LyonsSandstone. Our results show how past groundwaterflow in the Denver basin drove diagenetic reactionsin the Lyons that are intimately related to the accu-mulation of petroleum into present-day reservoirs.
We argue that the gray facies of the Lyons formedas an alteration product of the sandstone’s red faciesin zones where fluids, including groundwater andpetroleum, migrated into the formation from under-lying strata. In our model, Laramide uplift of theFront Range along the basin’s western margin, whichreached its peak in the Eocene, set up a regime ofeastward groundwater flow. Meteoric waterrecharged into the Lyons and moved deep into thebasin. Flow in the underlying Fountain Formationwas much more restricted because sandstones in thisformation pinched out to carbonate and shale bedsalong the basin axis. The slowly flowing Fountaingroundwater became saline by reaction with miner-als in the evaporite beds.
Oil in the Lyons Sandstone probably derived fromsource beds in underlying Pennsylvanian strata. Oilgeneration began toward the Late Cretaceous or theearly Tertiary, before the peak of the Laramide oroge-ny. The orogeny likely produced fractures along thebasin axis along which the oil could migrate upwardby buoyancy across aquitards and into the LyonsSandstone. The regime of groundwater flow set upby the orogeny drove brines along the Fountainsandstone. Where Fountain sandstone facies pinchout, the brines migrated upward across confiningaquitards, likely along the same fractures that trans-mitted the oil, and discharged into the Lyons.
Calcite dissolved as the brine mixed into theLyons because the upwelling brine was undersatu-rated with this mineral after having reacted with
dolomite and evaporite minerals. In a common-ioneffect, calcium from the dissolving calcite reactedwith magnesium, sulfate, and CO2 carried in thebrine to form the dolomite and anhydrite cements ofthe gray facies. This reaction accounts for the iso-topic compositions of the cements. The broad rangeof δ18O in dolomite requires mixing of an isotopical-ly heavy fluid, such as the Fountain brine, withmeteoric water. 13C in the dolomite is consistent withderiving the CO2 by dissolving Paleozoic marine car-bonate into the brine, and 34S in the anhydritematches the composition of sulfate from Permianevaporite beds.
Hematite stains of the red facies were also con-sumed in the mixing zone by reduction in the pres-ence of the migrating petroleum, and perhaps by thebrine itself. Kilgore and Elmore (1989) documenteda similar reaction in which hydrocarbons removedhematite cements from red beds of the TriassicChugwater Formation in Montana. By this process offluid mixing, rocks of the red facies of the LyonsSandstone were transformed locally along the basinaxis into the gray facies.
The association in time and space of oil migration
234 Lyons Sandstone, Denver Basin
Figure 16—Calculated isotopic composition of dolomiteresulting from the mixing of ascending brines into theLyons Sandstone, as shown in Figure 15. Initial isotopiccomposition of the Lyons groundwater is δl8O = –13‰and δ13C = –12‰; the brine has δl8O = 0‰ and δ13C =0‰ (×). The precursor calcite cement (×) has δl8O =+11‰ and δ13C = –11‰, reflecting the mineral’s meancomposition. Each curve shows, for a given CO2 fugacityof brine (on an atmosphere scale), how the isotopiccomposition of dolomite evolves over the course of thereaction. Tick marks along the center curve show water-rock ratios (by mass). Calcite and dolomite composi-tions (boxes) are from Levandowski et al. (1973).
and groundwater mixing explains the fact that allknown petroleum reservoirs in the formation arefound within the gray facies, and the observation thatsome dolomite and anhydrite cements overlie bitu-men stains left behind by migrating oil. The modelalso explains why oil in the Lyons apparently did notcontinue to migrate eastward through the sandstone,as occurred higher in the section along the Creta-ceous aquifers. Since the migration and cementationprocesses were so closely linked, the cements proba-bly began to seal the oil into reservoirs as it accumu-lated, preventing farther migration.
APPENDIX 1: SIMPLE MODEL OF CEMENTATIONBY GROUNDWATER FLOW
To test the relationship between regional groundwater flow andcementation in the basin, we calculated the amounts of anhydriteand quartz that would have precipitated from groundwaters migrat-ing along temperature and pressure gradients. The solubility ofquartz, for example, increases with temperature. Quartz cementsform where groundwater flows in a direction of decreasing temper-ature and hence decreasing solubility. Quartz dissolves wheregroundwater migrates toward higher temperatures because thefluid must acquire silica to remain in equilibrium with quartz.Anhydrite has a retrograde solubility, on the other hand, and thuswould precipitate and dissolve in an opposite pattern.
If a mineral’s solubility depends on temperature and pressurealone, and the fluid can be assumed to remain in local equilibrium,the rate at which the mineral precipitates or dissolves can be com-puted from the velocities of groundwater flow and the temperatureand pressure fields. For simplicity, we represent solubility by thedimensionless ratio Vi of the volume of mineral i that can be dis-solved in per unit volume of groundwater. Similarly, we representcement volume by the ratio Xi of the volume of mineral i presentper unit volume of the formation.
Under these conditions, the cementation rate dXi/dt for minerali is given directly by
(3)
where, T and P are temperature and pressure, φ is porosity, and ν'xand ν'z are lateral and vertical groundwater velocities (cm/yr) incurvilinear coordinates. Dx and Dz are coefficients of hydrodynam-ic dispersion (cm2/yr), which account for diffusion of the solute aswell as mechanical mixing that occurs in a groundwater flowingthrough a porous medium.
The time derivatives d/dt in equation 3 are Lagrangian, that is,taken in the reference frame of the sediments as they are buried oruplifted. The first terms on the right side of equation 3 give thecementation rate caused simply by changing temperature and pres-sure within a sediment, as would occur when a formation is buriedor exhumed. Terms on the next line account for the effects of diffu-sion and dispersion of the dissolved mineral within the basin. Thefinal terms, which are the most significant even at modest flowrates, account for the effects of groundwater advection.
APPENDIX 2: SOLUBILITIES OF ANHYDRITE ANDQUARTZ
The equilibrium solubility of anhydrite depends on tempera-ture, pressure, and fluid salinity. To calculate the role that fluidsmigrating across temperature and pressure gradients play in anhy-drite precipitation, we correlated solubility to values measured byBlount and Dickson (1969) for a 2-molal NaCl solution. Blount andDickson’s measurements span the range of about 100 to 250°C and5 to 1350 bars, so the correlation serves to extrapolate the data totemperatures found in shallow strata. The resulting regression,which gives molal solubility as a function of temperature T (°C)and pressure P (bars), is
By this correlation, anhydrite solubility decreases with increasingtemperature but increases with increasing pressure, especially atlower temperatures. The net effect is that anhydrite is less solubleat depth in a basin than at surface conditions.
Quartz solubility increases rapidly with increasing temperature,but depends only weakly on pressure and fluid salinity over thetemperature range considered in this study (Kitahara, 1960; Waltherand Helgeson, 1977). To calculate quartz solubility, we assume thatthe mineral is soluble as SiO2(aq), as is generally the case exceptunder alkaline conditions, and that the species has an activity coef-ficient of about 1. In this case, the correlation
log mqtz = 1.881 – 2.028 × 10–3Tk—1560/Tk (5)
of Rimstidt and Bames (1980) gives the molal solubility of quartz asa function of absolute temperature Tk (in kelvins).
The dimensionless solubility Vi, the volume of a mineral solubleper unit volume of groundwater, can be calculated from a mineral’smolal solubility, mi, as
where MVi is the mineral’s molar volume in cm3/mole, ρf is the fluiddensity in g/cm3, and TDS is the total dissolved solids (in mg/kg) inthe fluid. Because the largest uncertainty in calculating cementationrates is the estimation of groundwater flow rates, it is sufficient forour calculations to carry ρf as 1 and TDS as 0 in equation 6.
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Adams, J., and J. Patton, 1979, Sabkha-dune deposition in the LyonsFormation (Permian) northern Front Range, Colorado: The Moun-tain Geologist, v. 16, p. 47–57.
Back, W., B. B. Hanshaw, L. N. Plummer, P. H. Rahn, C. T. Rightmire,and M. Rubin, 1983, Process and rate of dedolomitization: masstransfer and 14C dating in a regional carbonate aquifer: Geologi-cal Society of America Bulletin, v. 94, p. 1415–1429.
Badiozamani, K., 1973, The dorag dolomitization model—applicationto the Middle Ordovician of Wisconsin: Journal of SedimentaryPetrology, v. 43, p. 965–984.
Lee and Bethke 235
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