sandstone distributionand potential - texas a&m universitytertiary formations,gulfcoast oftexas...

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79-4

Sandstone Distribution and Potential forGeopressured Geothermal Energy Production

in the Vicksburg FormationAlong the Texas Gulf Coast

ByR.G. Loucks

REPRINTED FROM TRANSACTIONS OF THEGULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES

VOL. XXVIII, 1978

Bureau ofEconomicGeologyThe University of Texas at Austin

W. L.Fisher, Director1979

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Sandstone Distribution and Potential for GeopressuredGeothermal Energy Production in the Vicksburg Formation

Along the Texas Gulf CoastR. G. Loucks

AbstractPotential geopressured geothermal reservoirs in the Vicksburg Formation are limited to Hidalgo County along

the Lower Texas Gulf Coast. In Hidalgo County, an area of approximately 385 square miles (designated the Vicks-burg Fairway) contains up to 1,300 feet of geopressured sandstones with fluid temperatures greater than 300°F. In-place effective permeability, however, averages less than 1 millidarcy in the Vicksburg sandstones because of finegrain size and extensive late carbonate cementation. Also, areal extent of indivdiual reservoirs is limited in a dipdirection by growth faults and in a strike direction by the lenticular geometry of the sandstone bodies.Under pres-ent criteria for geothermal fairways, the Vicksburg has minimal potential because of inferred low reservoir deliver-ability, which is constrained by low permeability and limitedreservoir continuity. If future tests indicate that lowerpermeabilities are acceptable, the Vicksburg Fairway should be reconsidered because of the presence of extremelythick sandstone bodies.

IntroductionGeneral statement

Geopressured geothermal prospects of the OligoceneVicksburgFormation (Fig. 1) along the Texas Gulf Coastare minimal because of low permeability and restrictedareal extent of reservoirs. This appraisal of the Vicks-burg Formationresults from aprogram being conductedby the Bureau of Economic Geology to evaluate thepotential for producing geopressured geothermal energyfrom onshore Tertiary formations along the Texas GulfCoast. Two other Tertiary units have been investigated:the FrioFormation (Bebout,Dorfman,and Agagu, 1975;Bebout, Agagu, and Dorfman, 1975; Bebout, Loucks,Bosch, and Dorfman, 1976; and Bebout, Loucks andGregory, 1977), and the Wilcox Group (Bebout, Gaven-da, and Gregory, 1978).

Figure 1—

Tertiary formations, Gulf Coast of Texas

iPublished with permission of the Director, Bureauof Economic Geology,The University of Texas at Austin.2Bureau of Economic Geology, The University of Texas at Austin,Austin,Texas 78712.

An area of approximately 385 square miles (11 x 35miles) was identified as the Vicksburg Fairway in Hi-dalgo County along the Lower Texas Gulf Coast (Fig.2). A geopressuredgeothermal fairway is an area whichmeets a set of easily recognizableminimum requirementsnecessary for the production of geothermal energy, andwhich therfore warrants further investigation. Theserequirements demand a sandstone volume of 3 cubicmiles (for example,300 feet by 50 square miles) and sub-surface fluid temperatures greater than 300°F. Furtherinvestigation of a fairway area involves evaluation ofreservoir permeability and continuity, parameters whichare more difficult to analyze.

An area with deep, thick sandstones in the VicksburgFairway was recognized by Bebout (1976) in a prelimi-nary study of geopressured geothermal corridors inTexas(Fig. 3). Bebout also indicated that the Vicksburg For-mationinan areaof the Upper Texas Gulf Coast (Fig. 3)may have potential, but no prospective sandstones wereidentified during subsequent studies reported herein.

Method of study

Stratigraphic and structural cross sections along theLower Texas Gulf Coast were constructed on a griddowndip of the Vicksburg fault zone, where deep elec-trical-log data are available (Fig. 2). These cross sectionsare regional, and interpretive where data are sparse orabsent. Cross sections were not prepared for the Middleand Upper Texas Gulf Coast because few deep wells areavailable downdip of the Vicksburg fault zone (Fig. 4),and the deep data which are available indicate that nosandstones of interest occur in the zone.

Published reservoir quality data by Ritch and Kozik(1971) and Swanson, Oetking, Osoba, and Hagens (1976)were integrated with data from this study to completethe preliminary analysis of the VicksburgFairway. Finaldetailed analysis of the fairway will come from a site-selection study now in progress at the Bureau ofEconomic Geology.

240 TRANSACTIONS— GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES Volume XXVIII, 1978

Figure 2 — Location of Vicksburg Fairway and lines of sections shown in Figures 9 to 29

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Figure 3 — Geothermal corridors of potentialfairways

Regional geology

Growth faultsGrowth faults, common structural features in the

Gulf Coast area (Bruce, 1973), were formed by contem-poraneous subsidence during sediment loading (Fig. 5).Two major zones of growth faults affected the VicksburgFormation: the Vicksburg and the Frio fault zones (Fig.4).

More than 7,000 feet of Vicksburg sediment accumu-lated on the downthrown side of growth faults alongthe Vicksburg fault zone. Gulfward, on the downthrownside of the Frio fault zone, massive Frio sandstones arejuxtaposed with older Vicksburg shales by movementalong growth faults. Therefore,a basic understanding ofthe depositional and structural history of basal Friosandstones is necessary in order to delineate the distri-bution of the Vicksburg sandstones, which are similar inappearance on electrical logs to the sandstones of theFrio Formation.

Index ForaminiferaIndex Foraminifera are the basis for distinguishing

formations in the otherwise similar sandstone-shalesequences of the Tertiary Gulf Coast basin. Contactbetween the Vicksburg and Frio Formations is tradition-ally picked at the last occurrence of Textularia warreni(Fig.6). This study,however,indicates that in the Lower

Texas Gulf Coast area the contact between the Vicksburgand the basal part of the Frio Formation occurs at thebase of a thick progradational sequence of shale andsandstone which contains T. warreni (Fig. 7). Textulariawarreni, therefore, is a basal Frio index foraminifer anddoes not occur in the Vicksburg Formation.

Updip of the Frio fault zone, the basal Frio Formationis predominantly fluvial in origin and contains onlyT. warreni in marine sandstones and shales which arerestricted to the lower part of the section (Fig. 7). Onthe other hand, gulfward of the Frio fault zone, theentire,predominantly marine basal Frio strata contain afull suite of Frio indexForaminifera.

Vicksburg andbasal Frio sandstone distributionGeneral statement

Downdip of the Vicksburg fault zone, the VicksburgFormation changes from a very prominent sandstonesection along the Lower Texas Gulf Coast to a stand-stone-poor section along the Middle and Upper TexasGulf Coast (Figs. 8 to 29)- The major source of sand inthe Vicksburg was the ancient Rio Grande as indicatedby the abundance of sandstone only in the HidalgoCounty area.In contrast, a sequence of basal Frio sand-stones occurs along the entire Texas Gulf Coast (Figs. 8to 29).


Figure 4 — Location of Vicksburg and Frio fault zones

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Figure 5 — Growth-fault development interpreted from aseismic sectionand shown sequentially bydiagrams

Vicksburg sandstone distributionVicksburgFairway Area.

— In the Vicksburg Fairwayarea along the Lower Texas Gulf Coast, the Vicksburgsection expands from a few hundred feet thick updipof the Vicksburg fault zones to more than 4,000 feet onthe downthrown side of the fault (Figs. 8, 11 to 16). Thelower part of the Vicksburg Formation contains morestandstone than does the upper part of the formation.Massive sandstones in the lower part of the Vicksburgoccur as far downdip as the Frio fault zone. These sand-stones probably extend beyond the Frio fault zone buthave not yet been penetrated. Also, the lower part ofthe Vicksburg Formation expands in thickness on thedownthrown side of several growth faults gulfward ofthe Vicksburg fault zone, isolating sandstone bodies inseparatefault blocks.

The upper part of the Vicksburg Formation containsmassive sandstone beds immediately downdip of theVicksburg fault zone, but toward the Frio fault zone(gulfward) these sandstones grade to shale. Sandstonesin the upper part of the Vicksburg Formation are tooshallow to have fluid temperatures greater than 300°Fand are not prospective geothermal reservoirs. Onlysandstones in the lower part of the formation are suffi-ciently hot to qualitfy the area as ageothermal fairway.

A generalized fades tract exhibited by the VicksburgFormation progresses gulfward from thin sandstoneswithin a shale sequence (fluvial) to a massive sandstonesequence composed of numerous standstone units sepa-rated by thick shales (high constructive lobate to elongatedeltas). The tract then grades again into thin sandstoneswithin a shale sequence (distal delta) and finally intoa shale sequence (pro-delta to slope). Immediately updipof the Frio fault zones, a vertical sequence of the Vicks-burg Formation grades from massive deltaic sandstonesto slope shales, indicating a major marine transgression.

South of the Vicksburg Fairway Area. — The entireVicksburg Formation south of the fairway and downdipof the Vicksburg fault zone is predominantly shale witha few thin siltstone lenses up to 4 feet thick (Fig. 9-These strata were deposited in distal delta to prodeltaenvironments and are unfavorable for geothermalreservoirs.

North of the VicksburgFairway Area.— North of the

fairway along the Lower Texas Gulf Coast, the lowerpart of the Vicksburg Formation grades laterally fromsandstone to shale, followed by a similar change in theupper part of the formation (Figs. 17, 19, 21, 23, 25, 27,and 29). Along the Middle and Upper Texas Gulf Coast,most of the Vicksburg sequence is shale, as indicatedby published cross sections (Gregory, 1966, fig. 30) andscattered deep wells. Some sandstones occur in the upperpart of the Vicksburg section, but these sandstones aretoo shallow to have sufficiently high fluid temperaturesfor production of geothermal energy.

Basal Frio sandstone distributionAlong the Lower Texas Gulf Coast, basal Frio strata

expand across the Frio fault zone which is located nearthe underlying Vicksburg shelf edge (Figs. 8 to 29).


Figure 6—

Index Foraminifera

Updip of the Frio fault zone,basal Frio stratagrade gulf-ward from a fluvial system containing scattred sandstonebodies within a shale sequence, to a delta system com-posedof massive sandstones.

The basal Frio delta system thickens across the Friofault zone, juxtaposing downthrown,massive Frio deltaicsandstones against upthrownprodelta slope shale depositsof the VicksburgFormation (Figs. 8 to 28). Swansonandothers (1976) inferred that some massive Frio sandstonesdowndip of the Frio fault zone in Kenedy County wereactually Vicksburg. However,according to StratigraphicDip Sections 6 and 7 (Figs. 19 and 21), these deepmassivesandstones must occur within the FrioFormationbecausethe facies tract updip of the sandstone is predominantlyprodelta to slope shales of Vicksburg age. The sedimentdispersal system for the deep sandstones in KenedyCounty is composed of fluvial massive sandstone faciesin the basal Frio sectionupdip of the Frio fault zone.

Vicksburg sandstone composition, diagenesis,andporosity

Vicksburg fine-grained sandstones deposited in thezone of geothermal reservoirs were chemically unstableand, thus, have undergone intense diagenesis. Sandstonegrains which are composed of quartz, feldspar, and car-bonate and volcanic rock fragments are classified (Folk,1974) as arkoses and volcanic lithic arkoses (Fig. 31).

Figure 7 — Electrical log showing contact between Vicksburgand basal Frio sandstones

Intense diagenesis of the sandstones has destroyed mostprimary porosity through cementation by quartz, calcite,and Fe-rich dolomite or ankerite. Most remainingporosity in the deep Vicksburg Formation is secondaryinorigin and results from the leaching of feldspar grainsand carbonate cements. Similar leaching and late-stagecementation occurred in the Frio Formation in SouthTexas at depths of 8,000 to 11,000 feet (Lindquist, 1977;Loucks,Bebout, and Galloway, 1977).

Finding highly permeable sandstone reservoirs in thedeep Vicksburg is unlikely because Vicksburg sand-stones are fine grained, chemically immature, and highly

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altered. TheFrio Formation in the South Texas HidalgoFairway (Bebout, 1976) exhibits similar properties andwas judged to have no potential for the production ofgeothermal energy because of extremely low per-meability.

Vicksburg Fairway

Pressure, temperature, and salinityIn the Vicksburg Fairway the top of the geopressured

zone (pressure gradient greater than 0.7 psi per foot)ranges from a depth of 7,000 to 11,000 feet (Fig. 32);average depth is 8,700 feet (Swanson and others, 1976).The top of the geopressured zone is indicated on strati-graphic dip sections (Figs. 9, 11, 13, 15, 17, 19, 21, 23,25, and 27). In the McAllen Ranch Field (Fig. 32) thetop of geopressure ranges from 7,000 to 8,400 feet deep(Ritch and Kozik, 1971), and the transition zone fromhydropressure into geopressure is 500 to 700 feet thick.In this field the pore-pressure gradient in the geopres-sured zone is from 0.86 to 0.92 psiper foot.

In the McAllen Ranch Field the 300°F isotherm islocated approximately 2,700 feet below the top of geo-pressure at an average depth of 11,400 feet (Figs. 9, 11,13, 15, 17, 19, 21, 23, 25, and 27) as indicated by datafrom Swanson and others (1971; Fig. 32). Fluid tempera-tures in the McAllen Ranch Field range from 240°F at9,000 feet to 340°F at 14,000 feet (Ritch tnd Kozik,1971); therefore the thermal gradient in this field is2.O°F per 100 feet. This thermal gradient agrees withthe 2.1°F per 100 feet plotted for the geopressured zonein the VicksburgFairway (Fig. 33). Inaddition, Swansonand others (1976) report ageothermal gradient of 1.8°Fper 100 feet in the hydopressured zone in the McAllenRanch Field; this is very closs to the I.9°F per 100 feetfor the Vicksburg Fairway (Fig. 33).

Available salinity data from two Vicksburg fields,Jefferies andMcAllen Ranch (Fig. 32), indicate that theaverage salinity range in the geopressured VicksburgFairway is from 14,000 to 40,000 ppm (Ritch and Kozik,1971; Swansonandothers, 1976).

Porosity and permeabilityLow permeability is the factor precluding production

of geopressured geothermal energy from the VicksburgFairway. Detailed investigations by Ritch and Kozik,(1971) and by Swanson and others (1976) concluded thatporosity andpermeability in the geopressuredVicksburgFormationare low.

Effective in-place permeability based on flow testsfrom six geopressured Vicksburg gas fields (Swansonand others, 1976) averages less than 1millidarcy (Table1). Swanson and others also note that permeability is afunction of depth, decreasing by approximately oneorder of magnitude for every 2,000 feet of depth (Fig. 34)between 6,000 to 14,000 feet.By superimposing the depthrange for the 300 °F isotherm onto a depth-versus-effective permeability plot (Swanson and others, 1976),it can be readily observed that there are essentially no

effective permeabilities greater than 1.0 millidarcy(Fig. 34).

Detailed porosity and permeability data for the geo-pressured Vicksburg Formation of the McAllen RanchField were summarized by Ritch and Kozik (1971) asfollows:

Based on the available data of 535 quality coresamples, the majority (60.7%) had air permeabilitiesless than 1md. with 78.3% and 87.8% having airpermeabilities less than 2 and 5 md. respectively.Only 6.2% had air permeabilities greater than 10md. Porosities for these 535 samples ranged fromabout 16 to 25%. The average porosity for all 535samples was about 19%.

Table 1. Representative in-place effective permeabilityin geopressured gas fields along the Vicksburg FaultZone inHidalgoCounty

These porosity and permeability values from theMcAllen Ranch Field are surface readings to air at oneatmosphere. Ritch and Kozik point out from isostaticcompaction studies, using brine-filled core samples, thatporosity readings taken at the surface should be reducedby 1.5 porosity percent. Also, air permeability readingsshould be reduced by one order of magnitude to allowfor the effects of brines and differential pressure; forexample 1.35 millidarcys air permeability reduces to 0.24millidarcys in-place effective permeability.

Sandstone thickness and lateral continuity

Sandstone reservoirs in the Vicksburg Fairway occurin units up to 1,100 feet thick separated by shalesequences up to 1,600 feet thick. Within the sandstoneunits, individual sandstone beds range in thickness froma few feet to 120 feet. Sandstones are separated by shalebeds ranging in thickness from a few feet to 30 feet(Figs. 11, 13,and 15).

Lateral continuity of potential reservoirs is controlledby areal distribution of the sandstones which, in turn,depends on depositional environment. Ritch and Kozik(1971) show that Vicksburg sandstones in the Vicksburg

Field Name Permeability (md)

South Kelsey 0.27McMoran 0.69McAllen Ranch 0.60Arrowhead 0.65McCook 0.21

Jeffress 0.20


Fairway weredeposited by a large delta system (Fig. 35).Deltaic deposition(Figs. 11, 13, 15, and 17) occurred inseveral progradational episodes, indicated by the numer-ous individual coarsening-upward sequences. Sand-stone isopachous maps of the Jeffress Field areademonstrate that deltas in the lower part of the Vicks-burgFormation were probably high-constructive systemswith a strong dip orientation.Lateral continuity of in-dividual sandstone beds in these deltaic sequences isexpected to be poor and continuity probably will notextend more than a few miles in a strike direction.Lateral continuity of sandstones in a dip direction isinterruptedby numerous growth faults in the lower partof the Vicksburg Formation.Poor lateral continuity ofsandstone reservoirs is also further documented bySwanson and others (1976), who report that many geo-pressured fields inSouth Texas consistof one single well.They note that the complexity of faulting within theVicksburg increases with depth. Faults are primarilygrowth faults spaced approximately 4 miles apart (Figs.11, 12, and 15).

RecommendationsAccording to present criteria established for identify-

ing geopressure geothermal fairways, the VicksburgFormation is not a promising source of geothermalenergy. Nevertheless, because of the presence of highfluid temperatures and thick sandstone bodies in theVicksburg Fairway, it is recommended that takeover ofa previously drilled well or a currently drilling well("well-of-opportunity") that is uneconomical for hydro-carbon production be considered. An experimental testwould provide data about deliverability of fluids fromlow-permeability reservoirs and about dissolved methanecontained in deep geothermal fluids.

Acknowledgements

Many people contributed to this investigation andreport. Their time and interest are gratefully acknowl-edged: S. C. Claypool, V. J. Gavenda, J. H. Han, P. E.Luttrell,K. L. Milliken, and M. G. Mosely assisted inthe investigation; D. G. Bebout and W. E. Gallowayoffered useful ideas and criticism; George Forming(Tenneco Oil Company) contributed information in theearly stages of the project that insured its proper direc-tion and emphasis.

Funding for the entire geopressured geothermal assess-ment program has been provided by the Division ofGeothermal Energy,U. S. Departmentof Energy.

ReferencesBebout, D. G., 1976, Subsurface techniques for locating

and evaluating geopressured geothermal reservoirsalong the Texas Gulf Coast: Proceedings SecondGeopressured Geothermal Energy Conference, v. 11,

Resource Assessment, Center for Energy Studies,Univ. Texas, Austin, 44 p.,Agagu, O. X., and Dorfman, M. H., 1975, Geo-thermal resources— Frio Formation, Middle TexasGulf Coast: Univ. Texas, Austin,Bur. Econ. GeologyGeol.Circ. 75-8, 43 p.,Dorfman, M. H., and Agagu, O. X., 1975, Geo-thermal resources— Frio Formation, South Texas:Univ. Texas, Austin,Bur. Econ. Geology Geol. Circ.75-1, 36 p., Gavenda, V. J., and Gregory, A. R., 1978, Geo-thermal resources, Wilcox Group, Texas Gulf Coast:Univ. Texas, Austin, Bur.Econ. Geology Geol. Circ.,Loucks, R. G., Bosch, S. C, and Dorfman, M. H.,1976,Geothermal resources— Frio Formation,UpperTexas Gulf Coast; Univ. Texas, Austin, Bur. Econ.Geology Geol. Circ. 76-3, 47 p.,Loucks,R.G.,and Gregory, A.R., 1977,Frio sand-stone reservoirs in the deep subsurface along theTexas Gulf Coast— their potential for theproductionof geopressured geothermal energy: Univ. TexasAustin,Bur. Econ. Geology Rept. Inv. 91-

Bruce, D. H., 1973, Pressured shale and sediment defor-mation: mechanism for development of regionalcontemporaneous faults: Am. Assoc, Petr. GeologistsBull., v. 57, p. 878-886.

Folk, R. L., 1974, Petrology of sedimentary rocks:Austin,Hemphill Publishing Co., 182 p.

Gregory, J. E., 1966, A lower Oligocene delta in the sub-surface of southeastern Texas: Gulf Coast Assoc.Geol. Socs. Trans., v. 16, p. 227-241.

Lindquist, S. J., 1977, Secondary porosity developmentand subsequent reduction, overpressured Frio For-mation sandstone (Oligocene), South Texas: GulfCoast Assoc. Geol. Socs. Trans.,v. 27, p.99-107.

Loucks,R.G.,Bebout, D. G.,and Galloway, W. E., 1977,Relationship of porosity to sandstone consolidationhistory— Gulf Coast lower Tertiary Frio Formation:Gulf Coast Assoc. Geol. Socs. Trans., v. 27, p.109-120.

Ritch,H. J., andKozik,H. G., 1971,Petrophysical studyof overpressured sandstone reservoirs, VicksburgFormation, McAllen Ranch Field, Hidalgo County,Texas: Society Professional Well Log Analysts,Twelfth Annual Logging Symposium, 14 p.

Swanson,R. X., Oetking, P., Osoba, J. S., and Hagens,R. C, 1976, Development of an assessment meth-odology for geopressured zones of the Upper GulfCoast based on a study of abnormally pressured gasfields inSouth Texas: Southwest Research Institute,San Antonio, Texas, ERDA Contract No. E(11-D-2687, 75 p.

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Figure 8—

Stratigraphicdip section showing Vicksburg and basal Frio sandstone distribution in the Vicksburg Fairway

248 TRANSACTIONS— GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES Volume XXVIII,1978

Figure 9 — Stratigraphic dip section1

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Figure 10 —Structure section 1

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Figure 11— Stratigraphic dip section 2

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Figure 13—

Stratigraphic dip section 3

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Figure 15 — Stratigraphic dip section 4

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Figure23— Stratigraphic dip section 8

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Figure 25—


Figure 26—

Structure section9

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Figure 27—


Figure 28—

Structure section 10

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Figure 29—


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Figure 31 — Vicksburg sandstone composition

Figure 30—

Stratigraphic dip section in Wharton County,Upper Tefxas GulfCoast

268 Volume XXVIII, 1978TRANSACTIONS— GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES

Figure 32—

Reservoir data in the Vicksburg and Frio Forma-tions in HidalgoCounty

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Figure 33 — Depth vs. temperature plot for wells from theVicksburg Fairway


Figure 34—

Depth to top of 300°F isothermoverlaid on ef-fective permeability vs. depth plot for a large number of gaswells inSouthTexas

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Figure 35 — Lower Vicksburg depositional environments

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