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ORO/4891· .\ -_ ..... ---_... - ..
cr 01 H[ RMAL RESOURCES, WILCOX GROUP, IlXAS GULF COAST
By D, G, Bebout V. J, Gavenda A. R. Gregory
January 1978
Work Performed Under Contract No. EY-76-S-05-.:.g~1
\ . Bureau of Economic Geology ", The University of Texas at Austin Austin, Texas \ l
( ~ ; ,,\ \ .... \ i , '
U. S. DEPARTMENT OF ENERGY Geothermal Energy ,
NOTICE
rhlS r'T'HI .W~~ I'Tl'I':lIl'd as an account of W(}I~: sponsored b) the United Slates Government. Neither the United Stat~s nor the United States Department of Fnergv, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty. express or imphed, or assumes any legal liabilit}, or responsibility for the accuracy, compkteness or usefulness of any information, apparatus, product or l'ruccss disclosed, or represents that its use wouldnot infringe privately owned fights.
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GEOTHERMAL RESOURCES, WILlOX GROUP, TEXAS GULF COAST
8y
D. G. Bebout, V.J. Gavenda, and A. R. Gregory
Assiste.d by
S. C'. Claypool
J. H. Han
J. H. Sea
Prepared for The U. S. Department of Energy, Di vi s ion of Geothermal Energy In partial fulfillment of CONTRACT NO. AT-E(40-l)-4891 (EY-76-S-05-489l) January 1978
Bureau of Economic Geology The University of Texas at Austin
w. L. Fisher, Director
,B.URElAU OF. ECONOMIC GEOLOGl TABLE OF CONTENTS
Introduction...... 1
Wilcox Regional Setting ...... 6
Wilcox Stratigraphic Sections ...... 12 Lower Wilcox Sandstone Distribution ...... 39
Upper Wilcox Sandstone Distribution ...... 43
Formati on Pressures ...... , 47
Formation Temperatures ...... ; ...... 58 Ha rri s County Area...... 59
De Wi tt County Area ...... 59
Area Including Portions of Live Oak, McMullen, Duval, Webb, and Zapata Counties ...... 60 Geotherma 1 Fa i rways...... 66 Acknowl egments...... 76 References ...... 77
Tables ...... 81
i i INTRODUCTION
Areas \'iith the pot.ential for (:onlaining geopressured geothermal fluids in cconomic qUillllities (rwoHl0rrnal fairways) occur in the l·Jilcox Group where the qul fl'J;,rri-dippillt] <;andstone/stlale \vedge thickens abruptly across a complex grmvth-fdult system.
Thi s n~(J i ulI;ll S Lurly () f UI(~ ~)Jtlds tone di 5 tri buti on in the ~~i 1cox
Group is part of IIluch bro,](lcl' illvL'sti~Jation aimed at assessing the po-
tcntiill for' the ploducLioll of ~JI'oLlien"i1l cnerSIY froln the geopressured zone of the onshore Tertiary along the Texas Gulf Coast (Dorfman and
Deller, 1975, 1976). The objective of the study is to identify areas where the Wilcox Group contains significant thicknesses of sandstone o with subsurface fluid temperatures higher than 300 F. These favorable areas are termcd geothermal fairways and are areas in which additional, more detailed work is recommended in the search for prospective geopres- sured geothermal test-well sites. Reports summarizing similar studies of regional assessment of the Frio Formation and a prospective test-well site have been published by the Bureau of Economic Geology (Bebout,
Dorfman, and Agagu, 1975; Bebout, Agagu, and Dorfman, 1975; Bebout,
Loucks, Bosch, and Dorfman, 1976; Bebout, Loucks, and Gregory, 1977). The geothermal potential of the Vicksburg Formation is summarized by
Loucks (1978). Funding for the entire geopressured geothermal assess-
.., ' ment proqrarn has been provided by the Division of Geothermal Energy, U. S. Deparbnent of Energy. ,r The Wilcox and Midway Groups, Lower Eocene, comprise the oldest I thick sandstone/shale unit of the Gulf Coast Tertiary (fig. 1). The ," .~ i Wilcox sandstones and shales outcrop in a 10 to 20 mile-wide band which t I ,l:
1 occurs subparallel to and 100 to ;0 miles inland from the present-day coastline (fig. 2). From the outcrop, the Wilcox dips into the subsur face as one of at least eight sandstone/shale wedges (Hardin, 1961; fig. 3). The sedimerlts of the updip portion of the wedges were deposited primarily by fluvial processes. Uowndip, the sediments transported across the fluvial plain were deposited in huge deltaic complexes and some sediments were reworked by marine processes into barrier bars and strandplains. An understanding of the environmental setting was devel oped almost 40 years ago as noted by Deussen and Owen (1939) and has been expanded and refined since then·.by Culbertson (1940), Echols and Malkin (1948), and Hargis (1962) and emphasized most recently by Fisher and McGowen (1967). Growth faults developed at the ancient shorelines of several of the larger wedges where thick sections of sand and mud were deposited on the unconsolidated offshore mud of the previous wedge (fig. 3). Subsidence along these faults resulted in the isolation of thick sandstone and shale units and prevented escape of pore fluids laterally during subse quent compaction resulting from loading; vertical escape of pore fluids was prevented by the low vertical permeability of the shales. The lack of circulation in these growth-faulted sections is responsible for the increase in pressure gradient from the normal '.464 psi per foot to be tween .7 and 1.0 psi per foot and the increase in the temperature gra dient from the normal 1.00F per 100 feet to between 1.5 and 2.00F per 100 feet. This downdip section with high pressure gradient and temper ature exceeding 3000F comprises the Wilcox geothermal corridor (fig. 2).
2 SYSTEM SERIES GROUP/FORMATION ~~-C=====lf====:~===' ====----__-_-1 Recent Undi f ferentlaled Quaternary f------I- Plel'Slocene " Houst_o_n-----I ~~cel1!:.~_ .__ G~a=d _____",," Fleminq Miocene Anahuac
- ?-?---h-:-:~-:~.:-~:-:~-:~-:.~:.:.:.:.:.:.:.:.:.:.:.:.:.:.: .:-:.:-:.:.:.:.:.:.:.:.:-:-:.:.:-:.:.:-:.:-:. Tertiary OligoceJle ?~~~!~o.:.:· :':';';';';';';' :':':' ;':': I--___._-+.~ ... ~.. ~.V..... 'c~~urg .•...•.••.•..•...... Jackson Claiborne Eocene ~~~?L' "-"'-"'-"'-"'-"'-"- Midway
Figure 1. Tertiary formations, Gulf Coast of Texas. The Wilcox Group, the subject of this geothermal report, is shown by the diagonal pattern; the geothermal potential of the Frio and Vicksburg Formations, shown by the dot pattern, has been re viewed in other Bureau of Economic Geology reports (Bebout et.al., 1975a, 1975b, 1976--,in press; Loucks, 1978).
, ,
3 I ------______J I
CJ 80 120 I I I Mile'
Figure 2. Wilcox geothermal corridor characterized by high pressure gradients and temperatures exceeding 300°F.
4- -, i A A' i COASTAL PLA:N CO~HI"'ENTAL Si-";::_=- ---- S_:~"'::
c..., i TEXAS COAST~L
P' Figure 3. Depositional style of the Tertiary along the Texas Gulf Coast (Bruce, 1973). WILCOX REGIONAL SETTING The Wilcox Group is less than 2,000 feet thick updip, near the outcrop, and more than 8,000 feet thick downdip, at depths greater than 10,000 feet. The Wilcox Group comprises a wedge of sandstone and shale which outcrops several hundred feet above sea level at its updip limit; down- dip more than 100 miles, the Wilcox is deeper than 10,000 feet below sea level (figs. 4 and 6). Regional dip averages 100 feet per mile. Where the Wilcox depocenter prograded gulfward of the underlying Lower Creta ceous Stuart City shelf margin (Edwards and Sligo) a 20 mile-wide band of growth faults formed (fig. 5) suggesting that the rigid carbonate shelf controlled the location of growth-fault formation. The Wilcox sandstone and shale section thickens abruptly downdip of the Stuart City shelf margin (fig. 4, Detween wells 7 and 8). The area of steeper contours on the structure and thickness maps (figs. 6 and 7) in the central part of the trend coincides with the area where growth faulting is best developed. The base aT the Wilcox Group has been recognized as transitional with the underlying marine Midway Group and the upper Midway is believed by many to be a marine facies of the lowermost Wilcox fluvial and del taic facies (Culbertson, 1940; Echols and Malkin, 1948; Johnston, 1977; Townsend, 1954). The top of the Wilcox Group is picked by us and by many others (Culbertson, 1940; Echols and Malkin, 1948; Fisher, 1969; and Murray, 1955) as the top of the Carrizo Sandstone because of the similarity in composition and depositional style of the Carrizo with the 6 underlying upper Wilcox. Johllston (1977) considHs that the Carrizo genetically belongs to the upper Wilcox but classif~~s it with the Claiborne Group because the original definition of the Carrilo by Plummer (1932) placed the formation in the Claiborne. Others have considered the Carrizo Formation as part of the overlying Claiborn~ Group because of the presence of an unconformity at the base of the Carrizo (Townsend, 1954) and of reworked Wilcox sandstone within the Carrizo (Todd and Folk, 1957). However, these criteria are not con- sidered valid because unconformities and evidence of reworking are common within fluvial sections (Fisher, Brown, Scott, and McGowen, 1969) . 7 H :,-." ~~:: -~- .::-::....~ .:.;:;:. ~, • ~~':'T._ ... T --- -- r >-- ,- -' ...... , -J ---. ~ "':,.: ~ ~ -- I~------~ \ -- "\ ',,~. -, >- --, -' ~- ,. .;-...; >- !",- ~ ) '~ -'''.- r ------"'- co ',"':" --, "-;, '-''-\ ,-- -", -, - --, -.~: " .... '>\ '--,. Figure 4, Regiona' dip section (Bebout, Luttrell, and Seo, 1975; sec tion HH'). The Wilcox Group thickens abruptly just Jowndip of the Lower Cretaceous Stuart City shelf ~a~gin by mears of a comDlex s1ste~ of growth fault~, The location of t~is secti~n is shown 0r rjaure S. )/ ( ~ . _ .1 " .. 1 J", .' ...... ··f"v ~,. ~ ,::-' r~" t'.· ..... ~ ,.. \ ;, /.Jl._ ·<·~.:>,;··.:~,:+.<~~,--t ·1~., l-_ ... t S,~ I \ ~: : 1 1, J\ I : f ,\ .) 1 " EXPLANATION L~~~J WI1CO' Outcrop ~ Stuort C,ty shelf morQ'" ~~ Foulls ~... Salt domes , -, .... ' ... ~ .. Figure 5. Faults 1n the Wilcox Group. Most of these faults are growth faults which were active during deposition of the Wilcox. Courtesy of Geomap Company, Houston, Texas. The location of the regional dip section (fig. 4) 15 also shown. 19 20, " .! '" i .. ----.- ... ----.-~... - )7 .' , .. .' ,- ----""; --- ;~ __ :~_..-.--- J"_,,,c' ~ ______/------6(;(":' __ --2- N I I I ;' . l ' .. ; .. " ..... -" ...... tt· ' ~~.. ', ':",' _.~' , 1 I' ,. 3,1 l', " 2, "\ 1 .' ,I. . ' -t-:' ~; I ' [d WilCOx. Outcrop Sea Level Dalum Coni our Interval· 1,000 It ,. ]~~ 'F'" ,'" ~ 'II ,,,,.,,q lfJ' . \ Figure 6. Structure on top of the Wi 1cox Group. 10 I~~~",' , '.,,\:," " '" c..,,~,. \~ .~ -.. ~ ....,. -).~. ~ "" ~ I N I :1 , . 'I I '5:, , .. (/ ;':i -~\Z "i'>/~/ .-..:/ ,. ~ Wilcox OutCrop 'I \ , I Contour Interval ~ I.(XX) ft. I II \'1 ;\ " 1 \ I \-;, ' ...... ,\ Figure 7. Total thickness of the Wilcox Group. 11 WILCOX STRATIGRAPHIC SECTIONS The upper and lower Wilcox are two major progradational cycles separated by a transgression. Control for this study was based on wells chosen so as to provide stratigraphic dip sections spaced 15 to 20 miles apart along the entire Texas Gulf Coast (fig. 8). The resulting 21 sections consist of 10 to 15 wells each and extend from near the Wilcox outcrop to the most down dip limit of correlation of the formation. Strike sections were con- structed to insure consistency between dip sections. Data from these sections are supplemented by denser control used by Fisher and McGowen (1967). Datum for the sections is the top of the Wilcox Group. Growth faults present at the downdip end of the sections have been omitted from the sections because they tend to obscure correlation of sandstones from well to well. The Wilcox Group is divided into the upper and lower parts based on the recognition of two major progradational cycles (fig. 9). Each cycle consists dominantly of shale at the base and grades to dominantly sand stone at the top. The upper and lower Wilcox are most easily separated in the middle part of the dip sections and are more difficult to recog- nize updip, where the section is dominantly sandstone, and downdip, where the section is dominantly shale. The lower Wilcox in many places appears to be made up of several smaller cycles such as in Dip Section 4, Wells 5 to 9 (fig. 13). The lower Wilcox of this report corresponds essentially with the lower Wilcox of Fisher and McGowen (1967). The upper Wilcox consists of the Carrizo Formation, upper Wilcox, and middle 12 Wilcox of Fisher and McGowen. The Wilcox sections (figs. 10-32) show high sandstone content throughout except for the downdip end where growth faults are abundant and the proportion of shale increases significantly. Most sections show a marked decrease in the amount of sandstpne within the downdip-most we 11 s i ndi Cd ti rl9 a very low probabil ity of the occurrence of s i gnifi cant quantities of sandstone basinward of present control. The sandstone distribution in the Wilcox is considerably different from that of the Frio (Bebout, Dorfman, and Agagu, 1975; Bebout, Agagu, and Dorfman, 1975; and Bebout, Loucks, Bosch, ahd Dorfman, 1976). Frio stratigraphic dip sections are readily divided areally from updip to downdip into three parts: the updip part which consists of thin, scattered sand stones in a dominantly shale section; the main-sandstone depocenter which consists of thick sandstones and thin shales; and the downdip part which is dominantly shale. The top of geopressure, marked by the black arrow on the strati graphic sections (figs. 10-32), occurs well beneath the base of the Wilcox along the updip two-thirds of the dip sections; within the zone of growth faulting, along the downdip third of the sections, the top of' geopressure occurs mostly within the upper Wilcox. Subsurface fluid temperature of 300oF, also indicated on the cross sections, occurs 1,000 to 1,500 feet beneath the top of geopressure. 13 % 00' " '\.)10"; " ' : ' " ' 31 00' '( I, r N I II '0 '" ~ __ :t9~1p ~ !!f1 Il,h,;".,.,. Figure 8. Well-log control and location of cross sections (figs. 10 to 32). Supplemental well data from Fisher and McGowen (1967) and Fisher (1969) are not shown. CUi f ~jo I Muellpr 8S-2IE-1 - ---~------Bose of Wilcox " OCXJ Figure 9. Electrical log snowlng division of the Wilcox Group into the lower and upper parts, each of which represents I Mljor pro gradational cycle. is ..,C;~:. ":'f" ,;,:;~::.,. ", "'')' ;:1'i;" >1:(',( '~~i;~;:" "'V,~ .. ~ I . -. ., ,. , ~ , r" j o~ 1 , < ; '. I., " 1 i ; . ( .,. I; " .co:" 1., ~ t ;,f ' I,I-: ,'. 29~ I"".'. " ,'". _ '"~\''-".- . "tii, .00 ''. ,", ,;"". "." .,,', _ ,I ~' ...: '" '060 '. ..•.;c, , , ) I ~ ) , ", 3J~0 .. C '" It:+ "" li'" M,. ., " , ~ I," ,,' , I'.\_', """'''0' {'<(' ) Ii..,i if \ "': ., I,) , , I ~ I . .1 .'" 'i Lf , ~ ~ "m,.., ...,n '\.." "r ' "r:.' "", , I". ,I , I J. l' ;: "-t~" ,-., ,r. I , ~, '-! =--: ' 1 1; " , , , lC I ' , , , , , i' , , ~LOWEO I,.' j l( ~ \ w,,,ox ~ --~~.-~;;;( f, . ~\l) )~ i. ~ " r -,.. W'''-. 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' ." _.L ,,' 'r'~ t,l, ~, 1.,. t -. \ , " .... ~'<)~§ 1'::J ~ / / o / M / Q) I ~ / :::J / en , , I , ...... "',\, .. c~'""':~.:~ ::;~~;:::':::;J:;:-'~.~:-:--_ . -r- {I ~-- I I ,I I ,~l,,:·) ','" .....,','~ ,,, ~., ,V~~ >"~'I ),':".,\"'::" "'::,( '~'-:~~ -i)/ I J~I ;,:;-: j r / I / / I I ~) 0'-' ,.e~., 0) .~'W·"'" ~,,<; ~: '~ ; ~, ~ I i / I~ "-, ....,,' .," ,,,,\J~. '""'. '"'~ I I I I r. """- .~~_ ~j:::--VY-'" :'~'JA""vJ ~"'\~ 'f j / / / / / .C.,.: { - ... ('.~, ~: ~I a,.1 36 .•,0' >; .: ... '. , « .' ' c 0 ...... , .\.' ;:' . .: r I. u I (lJ ,.-; Vl ClJ ...:.: . J '.,l..' .f' " " 'J.'-' •. :"" \ , f .... · I, ...... ;·, ...... ,~ Vl E E E U ~ S ~ ''- ~;;~.~/ .L: 0.. 1:;\"" r~~' .~",~>~",_At" ..' ...-..;'(""- ro .:;" ~ 0'> ,il ...... , ,v •.•... ,._,." ... :',',1 f' '~~>"~~'~ __ '\".'~,!ill ..."..( ...... J'... \ ... J..,J, .....':J-.- n,.1.,...... ) .. ~ .. L,o ..... I./-J.... ; ~ '~,~, ."," ... _' ,(=r:1 .f'J - .....'J.. •• }.-. ,;, • ·r··~··J 01, ...... 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""/~. <~ r('i'i'I!jj~'l" '~'wt .r'" C- .1 ,~, :1" F:'::.J;:~: 'c ,',~ ~ !~ 38 LOWER WILCOX SANDSTONE DISTRIBUI rON Net-sandstone and sandstone-percent maps outline a high-sandstone trend which is broad and lobate along the Upper and Middle Texas Gulf Coast and narrow and straight along the Lower Texas Gulf Coast. Sandstone-distribution maps of the lower Wilcox (figs. 33 and 34) are based on the control provided by the wells on the stratigraphic dip sections prepared during this study and by maps previously prepared by the Bureau of Economic Geology as part of an extensive study of the lower Wilcox (Fisher and McGowen, 1967). These maps show a high-sand stone trend, subparallel to the present-day Gulf Coast, which is broad along the Upper and Middle Texas Gulf Coast (up to 80 miles wide) and narrow along the Lower Texas Gulf Coast (approximately 20 miles wide). Sandstone thicknesses range from a maximum of greater than 2,000 feet to the north to slightly more than 400 feet to the south. Along the northern two-thirds of the trend, both the net-sandstone and sandstone-percent maps (figs. 33 and 34) show a very lobate pattern on the gulfward side of the broad sandstone trend (from De Witt County on the south to Sabine County on the north); this lobate pattern is very similar to that illustrated by Fisher and McGowen (1967) who interpreted it as representing deposition by a high-constructive delta system (the Rockdale Delta System). The southernmost delta lobe, the Guadalupe Delta of Fisher and McGowen, was subsequently cut by a large erosional feature named the Yoakum Channel by Hoyt (1959). Fisher and McGowen believed the Yoakum Channel may have been cut by turbidity currents which resulted from the movement of deltaic sediments down depositional 39 slope. In the axis of the channel the entire lower Wilcox high-sand stone section has been removed and replaced by the dominantly shale section of the lower part of the upper Wilcox (figs. 22 and 23). South from ne \4i tt Coun ty. the net-sands tone and sands tone-percent maps show a marked contrast from the broad, lobate trend to the north to the narrmv. straight trend to the south. This strike-dominanted trend was considered by Fisher and McGowen (1967) as having been deposited as strandplain and barrier-bar depositional systems and was named the San Marcos Strandplain and Cotulla Barrier Bar System~. The emphasis of this study is on the downdip-most sandstone units where reservoirs with potential for production of 300°F geothermal water occur; here, the configuration of the sandstone lobes on the net-sand stone map conforms very closely with that published .by Fisher and McGowen (1967). Updip, on the other hand, the well control used in this study is sparser than that used by Fisher and McGowen; thus, the net sandstone maps are somewhat different. 40 .\ '-1 "-.. .. ' , i 'Of J .-",",\ ~ i ". J " N i i II ,,' ," J' 2 • .\ ,. • Jl . EX PLANA T ION \:::::::::1 1200 - 1600 It. o 1600-Zoooft . .. >200011. ~ W.lcox Outcrop ,, Contour Interval = 400 ft ,with supplementary 100 ft contour rn downdlp areo -~ ...... I(II._I.~I Figure 33. Net sandstone - lower Wilcox. 41 I'·· N , I ,', I' .1 t • I, "j",',' 'f , J .. _---f " [XPLANATION [3 40- SO percenl 'IV] D :1.)-60 p~rcen! o /,/,~""" [-~.-J NIICIJ'( OT)IU(.. p Contour Inte 5per~:~::-IOpercent Wlnl rl I '0 conlour ,n dOWnd,~J~:'~~mefilary ",."",,9 10 '!l,,"",,9 '1" '9 '~¥' .p ~ lqo ItdO~.'~:~·· Figure 34 Sandstone percentage - lower Wi' cox. 42 UPPEf~ WILCOX SMDSTON[ DISH fl3UTIOi'J The hi9h-sandstone trr>nd, iJ'; defined by the net-sandstone and sands tOIlC percent maps. is broJd to the south along the Lower Texas Gulf Coast, is slightly r~c1rrower to Ihe north along the r~iddle Texas Gulf Coast, and broader aCluin along tile Upper Texas Gulf Coast. The sdIHJstonf'-disLribuLiot1 maps of the upper \~ilcox (figs. 35 ane! 36) shovls t.he trend to be br'OtHl (5-10 miles wide) both to the south alo:](\ the Lower Texas Gul f (OdSt allel to the north along the Upper Texas Gulf Coast and to be narrow (40 miles wide) in the central part along the Middle Texas Gulf Coast. The net-sandstone and sand percentage values are larger along the Lower and Upper Texas Gulf Coast and smaller along the Middle. The dip-oriented lob~te pattern typical of the lower Wilcox is not present on tre upper Wilcox net-sandstone or sandstone-percent maps (figs. 35 and 36). In contrast, the upper Wi1cox sandstone distribution maps show major 3andstone bodies oriented at a slightly oblique angle to depositional strike. These upper Wilcox sandstones were interpreted by Fisher (1969) to have been deposited as strandplains of a high-destruc tive delta system. The net-sandstone map (fig. 35) shows the tendency for the sandstone bodies to be orie~ted at a low angle to strike but dces not show the very digitate downdip edge illustrated by Fisher (1969, fig. 66). The differences betVieen these two maps are attributed to differences in well control and to differences in criteria used in picking upper Wilcox. Only the deeper wells that penetrated both the upper and lower Wilcox were used in this study whereas Fisher used many 43 of the shallo'd I'lells as \'Iell. In ,!,dition, the upper Wilcox of this study includes both the upper and ~iddle Wilcox of Fisher, thus result ing in the inclusion of units from a much thicker section than considered by Fisher (1969). Thinner mapping units commonly show better definition of the original depositional patterns. In tile Upper vJilcox the Yoakum Channel appears only on the sand stone-percent map (fig. 36) because the thick channel fill included in the upper Wilcox is dominantly shale, thus lowering significantly the sandstone percentage. The net-sandstone map (fig. 35), in contrast, does not show the channel because the normal upper Wilcox sandstone section was deposited across the already filled Yoakum Channel. 44 '.' N ,I." , ,/ ". 10 f l ";' I j i: \·l '" ) I' ,/ \', ,l. ,\ " ,: i/.,: I. ").',: / I " ,: "...1 EXPLANATION o 600-800tt. o 800-IOOOtl. "~I m >IOOOtt ~ Wilcox Outcrop Contour Interval' 200ft ,. '? 10 Mole. " " Ifi lqo 1(, ...... , ••• " .. ">H,::,:",9. 'fI' 'If " Figure 35. Net sandstone - upper Wilcox. 45 / /", N ,\ I " EXPLANATION F~] 50·60pfccent l J 60· 70 percent D > 70perr:ent C"""~ WdcoxOutcroD ' " '. ' \ '. \ Contour Inter vol ;:: 10 percent \ ' ~, : 10 ~ If' ",,1 •• l?, fl " t.", ~ 100 11:.10 .... ·." ,!!"",,,,9 " Figure 36. Sandstone percentage - up per Wil cox. 46 FORMATION PRESSURES The top of (jp(lpressurr. deterillined from resistivity log':;, :,"ili,: logs, and drillinq mud \-Jcights, occurs at depths of 8~000 to lO,IYlC}f'''(:;t. in areas \vhere the Wilcox is dominantly shale and at depths of 11,000 to 13,000 feet where the Wilcox is dominantly sandstone. Knowl edqe of the firs t occurrence of geopressures and the pressu:'" depth profile in a geologic section is valuable for ~mproving drilling and completion techniques and for l11akir.g reserve estimates and perfor- mance predi cti O:lS. I nfortllati on obta i ned about geopressures before drilling permits the use of minimum safe mud weights; helps prevent lost circulation, blowouts, gas cutting, wall sticking, and heaving shales; and assists in the selection of proper casing, casing seats, and well- head equipment. Hence the well can be drilled faiter at reduced costs. In exploration, the identification of geopressure tops in regional control wells permits contour mapping that helps define favorable sand- stone ratio environments and establishes probable depths at which sub- sequent wells will encounter geopressured formations. The !lIap of depth to top of geopressure in the Wil cox Group (fi g. 37) reflects the configuration of the net-sandstone maps. The top of geopressure is relatively shallow. less than 9,000 feet below sea level, downdip of the main-sandstone depocenter; in contrast, top of geopres- sure occurs deeper than 13,000 feet in areas of major deltaic lobes, such as northern Harris County where the contours form salients in the downdip direction and conform to the lobate sandstone patterns. Fluid pressure in shales is determined by using data from both resistivity and sonic logs. A reliable tool for dete(ting and evaluat- 47 ing geopressured formations is tl" short normal curve or amplified short norma 1 curve of the e 1ec tri ca 1 10']. Thi s res i s ti vity method (Hottmann and Johnson, 1965) relies on the observation that shale resistivity Rsh increases with depth as the porosity and water content of shales de- creases under condi t ions of normal compacti on. Geopressured sha 1es depart frolll thrc: nonn.) 1 Lt'end and Iower values of Rsh are recorded be cause of the illueased porosity and water content of the geopressured shales. The amount of divergence of Rsh from the established normal compaction trend is a measure of the pore fluid pressure in the shale and also in adjacent sandstones. Normal procedure for detecting geopres sures involves a semi log plot with Rsh plotted on the logarithmic scale and depth on the linear scale (fig. 38). A cap rock with higher than normal resistivity is frequently observed above the top of geopressure. The transition zone from hydropressured to geopressured conditions may be sharp and definitive (fig. 38) or gradual and somewhat equivocal. Sonic logs are probably more reliable than electrical logs for locating and evaluating geopressures. When the shale transit time 6tsh is plotted on sernilog paper versus depth (fig. 39), a normal compaction trend line is established and the geopressure top is located at the depth where the plotted data for 6tsh depart from the normal trend. Top of geopressure contour lines for the Wilcox (fig. 37) were drawn from data determined primarily by analysis of resistivity logs. Supplementary data from sonic and gamma-ray logs were also used. For example, top of geopressure for the Superior Oil Company, #1 T. J. High tower and Humble Oil and Refining Company, B. E. Quinn #B-l, Liberty 48 County, Texas is marked by the il rows at about 12,200 feet in the geo logic section (fig. 27). A plot of the logarithm of Rsh versus depth for these two wells show departures from the normal shale compaction trend that are identified as top of geopressure (fig. 40). It is impor tant to pick Dilly th-ick high-quality shales to estublish the trend line. For example, iln effort was made to select shales that were at least 10 to 30 feet Lhick ilnd to avoid silty, limey, and washed-out shales. Re sistivity data for shales at depths less than 4,000 or 5,000 feet were disregarded because these shallow formations contain fresh water with high resistivity values that cannot be used to establish the compaction trend. Discontinuities observed in the trend line may be caused by an abrupt change in lithology or differences in the geologic age with consequent drastic changes in shale properties. A major change in bit size may also affect resistivity values. Bentonitic shales and shales located near salt masses should be avoided because these have low resis tivity (high salinity) which may falsely indicate higher than normal pressures. The presence of gas-cut mud and muds containing additives to combat lost circulation may also contribute to spurious resistivity data. In using well logs for detecting geopressures, the quality of the logs should be considered because logging tools can malfunction in deep holes where temperatures and pressures exceed the safe rating speci fications of the logging instruments. Mud weight versus depth recorded on resistivity plots (fig. 40) provides a first approximation of the location of the transition zone. Although mud weights alone are inac- 49 \ curate and can be misleading fOI picking geopressure tops, a mud weight of 12 to 13 lbs/gal (0.623 to 0.675 psi/foot) is used to first approxi mate the depth of the transition zone in the Wilcox when the quality of well-log data is questionable. Depth to geopressure top can then be adjusted after correlations with suitable logs and pressure data from nearby wells. t~ud weights are not recommended for quantitative evalua tion of geopressure. Detailed analytical procedures for quantitatively determining the geopressure profile are available (Hottmann and Johnson, 1965; Pirson, 1970; Fertl, 1976), but these methods were not deemed necessary for this regional study .. At depths less than 8,000 to 10,000 feet in the Wilcox, fluid pres sure gradients are commonly below or slightly above hydropressure level. Below 10,000 feet, however, most of the g~s and oil producing reservoirs are geopressured and the gradient increases with depth. Bottom-hole shut-in pressures (BHSIP) from drill-stem tests in the De Witt County area clearly show that most geopressured formations first occur between depths of 9,000 and 10,000 feet (fig. 41); generally, gradients increase with depth to a maximum of about 0.85 psi per foot. Limited BHSIP data for the area including Live Oak, Duval, Webb, and Zapata Counties indi cate that geopressures in the Wil cox reservoir occur at depths between 7,700 and 12,000 feet and gradients are less than 0.90 psi per foot (fig. 42). Similar data for deep wells in the Harris County Area show that geopressures in the Wilcox lie between 10,000 and 15,300 feet and gradients range up to 0.87 psi per foot (fig. 43). 50 . 1,21' 18 " , J7,, .' " " .' , Hi" 15,, N I i il j'.> ( , ~ , t' , 11 1 i; 'i 'j ,', l n \ ~ ...... ~ :'.~...... ,.:" ...... -!;-- " t'1 ,I " '/ ,- 1 " ~~ Wilcox OUlcrop Contour Interval,:; I,OOOft '" 0 "·"'>H.-,:: .. ,9 Figure 37. Top of geopressure within the Wilcox Group, Texas Gulf Coast. !:II 1 -r T-- .. --r--,-,.---r----- .:.. . . 5 7 0= 0 9 2 '"0. ~ 0 II Cop Rock Top Mud wI. #/901 of 13 Gtloprenure .;=-.-~-- ~ 9j8"~ U..2 -~, \ t-~_ \ !.U -~ \ 1L1 15 .. --~- \ -, \ 7"~ 17 7 ..... --~ -.. \ 18 3 \ Figure 38. Shale resistivity plot for a Gulf Coast well (after Fertl, 1976). 52 ? ,--,-,,,,. T--~-'--~-~~~~~ 1.3 "Is" ,1·· Figure 39. Shale transit time for a Gulf Coast well (after Fertl, 1976). 53 2 ",,--~- 2: iii 1 x 102 x x x X X "xx X '(, x x X X r * 4 Xxl H''i-L~U'., '..... 8R--r 4 _ (XX THE SUPER:OR OIL CO bL~ ,~ \~; ,--,v . } NO I T J HIGHTOWER 'Jx 8 ::.: G~""'J,\ :'.~J 3-! x {')'.",~~-) , , (3N-40E-9) ';( '-- 1 ...... <-, \ \\, r- ' X r w : ~ LL w 6'f- lJ..i LL NORMAL LL "'JCR~~L C::',\~?':C:: ,=,~~ LL .. COMPACTION LL T,~E'~~ o TREND o X Xx (f) 8' ~ s- o c..,-, z Z <[ <[ ~ r (f) MUD WT 'J\."L.': .::" II'" (f) MUD WT ::J ~ -TOP WILCOX ::J #/GAL #/GAL x x o 'Oi-, 0 0 - I' I x r- I \ r- iO 8 \ \ - \ L \ , x I I I \x x I I \ f- tIII x\X r- 127 ,x TOP OF 0.. 12 f- 0..,2- x \ TOP OF \ x W x ~ W I x '\ .... ;::;E~PRESSURE o 144 x x \ GEOPRESSURE i o ! 165 ~ ( \ I X i X 14 :.. 14 _ I':: '" X " v X X I x :~\ I X * \ I +16 ' I I ~ 16 __} __ IE _~_ i67. I I I LL'~ ____ ~X ~_ 03 t I 2 5 10 03 2 "J RSh ' OHM - METERS Rsh, OHM - METERS Figure 40. Shale resistivity versus depth plots show tops of geopressJre for two wells about 10 miles apart. Liberty County, Texas. DE WITT COUNTY AREA () (WILCOX) o o 1- W \ \ W \ " LL LL \0 \ \ ~ o (f) o z 14 16 IBL----L· _---.L~ ______.l ______.1_. __ ~~_'_~______..L"_____ • ___----l.- ____ -----..-J. _____ ~_. __ L...______....l_.._ ____ L __--'-_-'--I _ --...... -...J. ______o 2 4 6 e 10 I? 14 ~ If! BOTTOM HOLE SHUT-IN PRESSURE (PSI )(1000) Figure 41. Bottom-hole shut-in pressure versus depth plot for the De Witt County area. 55 , .- .. -r ..... _- -. T- _. -'-r -----,-- - _.- i - '\ ! \'" i \'" "'-"-. \ "', \ " \ '" \ UVE Ut,:\f<.[)UVAL.WEBB,AND ZAPATA COUI~TY AREA \ \ (WIl_CUX) \, ~ L r) (j) () "'- .T~ 1-- CL uJ 0 14 ! i ! \ I 1 \ I \ 1 18 _. ___ .___ -.1. ______L _. _____ J ____ _ 1 ______L _____ ---l._•.• ____ l ___ . __ .1 __ ...____ .1 __ I ___ -..l __ ._~ __ .. L ______~-----L--...J..... ___...... I.._ ••_~_ • .L.-__ ~.Jj o 2 466 10 12 14 16 18 BOTTOM HOLE SHUT -IN PRESSURE (PSI x 1000) Figure 42. Bottom-hole shut-in pressure versus depth plot for the area including Live Oak, McMullen, Duval, Webb, and Zapata Coun ties. 56 \~~ \ \ \ HAIlRIS COUNTY :'Ht~A \ , (WILCOX) \ (, ~- o \'\ 0 o 'AL ____ J ___. __ __L. _ L . \ l L __ "._~_ ..L ___.. L __ ---L...... -_; _ o f, B 10 12 11 BOTTOM-tKJLf SHUT-IN PR[S'3UR[ {P"-)I X 10(0) Figure 43. Bottom-hole shut-in pressure versus depth plot for the Harris County area. 5J FORMATION TEMPERATURES In the geopressure zone the geothermal gradient reaches as high as 2.7oF per 100 feet, resulting in the occurrence of the 300 0 F isotherm as shallow as at ]0,850 feet in depth. Fonnati on tClllp~ratures have been recorded in thi s report primari ly to serve as a tool to delineate thick sandstone reservoirs with tempera tures greater than 300 0 F. However, subsurface temperature also must be known in order to determine the amount of methane dissolved in the water (Bebout, Loucks, and Gregory, in press). Temperature plays a major role in sandstone, shale, and hydrocarbon diagenesis (Galloway, 1974) which, in turn, control porosity, permeability, and elastic properties of the reservoir rocks. The role of temperature in stimulating diagenetic conversions of clay minerals and dewatering of clay-rich sediments in the Gulf Coast subsurface enivronment is well documented (Burst, 1959, 1969). Alterations in permeability, porosity, and elastic properties caused by changes in temperature have a substantial influence on the bulk volume, pore-fluid volume, and fluid deliverability of reservoirs. Heat effects on elastic properties, in particular, may have an impact on the interpretation of seismic and well-log data that are used for find- ing, producing, and evaluating geothermal reservoirs. The point at which 300 0 F occurs in the Wilcox is indicated where possihle on all wells on the stratigraphic sections (figs. 10-32). From these sections the updip-most occurrence of sandstone in the Wilcox with temperatures higher than 300 0 F is available and has been plotted on the net sandstone maps of lower and upper Wilcox (figs. 44 and 45). Minimum 58 thi ckness of sands tone penetrat" j by deep wells with temperatures hi gher than 3000 F are shown, gulfward of the 300 0 F isotherm. The average geothermal gradient of the Wilcox is not linear with depth but goes through a 3-stage change of slope that differs for shal ~ow, medium, and deep formations. Generally, gradients increase and higher temperatures occur shallower toward the southwest part of the Wilcox trend (table 1). Gradients are moderate (1.4 to 1.90 F per 100 feet) at depths less than about 9,000 feet in shallow formations that generally lie above the Wilcox. Gradients are highest (2.1 .to 2.7oF per 100 feet) from about 9,000 to 14,000 feet and are lowest (1.2 to 1.50 F per 100 feet) in the lower Wilcox from 14,000 to 19,000 feet. Harris County Area This area includes parts of Harris, Liberty, Austin, and Colorado Counties and portions of adjacent counties (fig. 46). The geothermal gradient in the upper Wilcox is about 2.1oF per 100 feet in the depth interval 8.300 to 14,000 feet. The lower Wilcox from 14,000 to 18,000 feet has a gradient of about 1.2oF per 100 feet. Formations above the Wilcox (0 to 10,000 feet) have a geothermal gradient'of about 1.4oF per 100 feet. A subsurface temperature of 300 0 F occurs at 13,050 feet below sea level. De Witt County Area This area includes portions of DeWitt, Karnes. Goliad, Victoria, and Lavaca Counties (fig. 47). The geothermal gradient in the upper 59 Wilcox (7,400 to 14,000 feet) is lbout 2.6oF per 100 feet. A gradient of 1.2oF per 100 feet occurs in the lower Wilcox at depths between 14,000 and about 19,000 feet. Shallow formations above the Wilcox (0 to 8,600 feet) have a geothennal gradient of about 1.SoF per 100 feet. A temperature of JOOoF occurs at a depth of 11,700 feet. Area Including Purtions Qf Live Oak, McMullen, Duval, Webb, and Zapata Counties Geothermal gradients in this southwestern part of the Wilcox trend (fig. 48) range from 1.90 F per 100 feet for formations above the Wilcox (0 to 9,700 feet) to 2.70 F per 100 feet for the upper Wilcox (6,800 to 14,000 feet)' to 1.50 F per 100 feet in the lower Wilcox (14,000 to 16,000 feet). A temperature of 3000 F occurs at a depth of 10,850 feet. Temperature data used to obtain gradients (figs. 46, 47, and 48) were taken from well logs and corrected to approximate thermal equili- brium by the empirical relation developed by Kehle (1971). where TE = equilibrium temperature, of TL = bottom-hole temperat~re from well logs, °F, and 0 = depth, feet. 60 J7,. ,/ ~}, " , \ ,'. ' "., ' ./.',/ 3.1 ,', J', 2. I I 1, II " " ,;J EXPLANA r ION ,oJ , /, 1-"'J 1200 - IfiCO II o 1600 - 20c0 II nra ~ " 2000 II 1 \ . L~__ ~l Wilcox Qulerop \ \. l' ~. Contour Interval" 400 fl ,with supplementary 100 fl contour In downdlp area '0...... lrj,::,;",90 Figure 44. Net sandstone of the lower Wilcox with the 200 and 300 0 F isotherms. 61 (' , , , '.:21 , ' , . I, 10 l't "I J .1' .. ,.) .1 . ,,: i: ,':" /' '/.', ....I EXPLANATION CTI 600- 800 fl, D 800-1000fl, >IOOOft \ o -\ \ \ \ \ ~ Wilcox Ou1crop "I '\ \ \ \ Conlour In'erval' 20011 10 0 A~ l5l .9 ~o Ap 6fJ 8,'1 100 1(·10 .... ·." Figure 45. Net sandstone of the upper Wilcox with the 200 and 3000 F isotherms. 62 I' HAfllilS COUNTY ARE A (WILCOX) " ~. I - .-l 1"/' ,;/ If i\ I x x x x ,- *" ,,' "'"- n" VI 0 ;j .:0'" 0 0 x )(R() ~ w a r}lo 0 0 €)') 'D I () R I I Cl () Gh',lDIDIT o WILCOX ," jOF /IOOFT Cl ABOVE WILCOX o o (J () (' 0 00 0 GRADlliV r 0 Cf.l 12aF /IOOFT /.- :;DO I J6() 140 I I 300 340 BOT TOM - HOLE TU.,4PfRATURf (-F) Figure 46. Temperature-versus-depth plot and geothermal gradients for the Harris County area. 63 Dr 'Iv,; I COUfHY I\Rt.1I (WILCOX) \ ',:.'. i' ,:".:. T r" ~- ('1'-" Jr,' ~'l' l '_':)Vl~""T \, i ',f GOLIAD 10 , r) o 1 " Cl 0 )(n) 0 0 00 () 0 0 (1 0 ~ c 0 0 R \ C\ 00 o Wi! (OX 0 CI 0 n • ABOv( WILCOX o o o (.H4{)/Uv; o l"oF / Ie \)[1 o o o 0 Will Ul\; flHl (('111,'1 lin 1(1 f()UIlIfIHIlIM TfMf'IHtl.TlIIH o "., .. I Figure 47. Temperature-versus-depth plot and geothermal gradients for the De Witt County area. i LIVE OAK, DUVAL, WEBB, ZAPATA COUNTIES AREA II II M( ~,~ .I', I l • ~ _c.~ rp'nr~rGI' 'I' dolo tJ'Jljn\~ '1 , 1"'11 "J ,1, c;ll'.ll ! f ,~" J ,I '}C'f 'l'I'r, x x [~J"\ x x ZAPATA co I . I .... [ ZJI.~/~.:' 81- '-, .' I ': "'- o If a w o o "l o o ,0 o WILCOf 8 0 00 x ABOVE WILCOX 000 o () o 14 o ~ o 0 o 0 o GRA[JIEIVT /5°F//OOFT o '\ '\ '\ \ I I I I I I .L 1 1 I 100 140, 18n no <60 K>O .,4() '8<) 41e Figure 48. Temperature-versus-depth plot and geothermal gradients for the area including Live Oak, McMullen, Duval, Webb, and Zapata Counties. 6!.i . GEOTHERMAL FAIRWAYS Eight geotherillal fairways have been identified as having thick sandstone units with subsurface temperatures higher than 300°F. Geotherillal fairways are areas in which thick sandstone sections have subsurface temperatures in excess of 300°F. These fairways are readily identified by super"imposing the corrected 300°F isotherm on the net-sandstone maps (fig~). 44 and 45). Fairvlays occur downdip of this isotherm. Eight Wilcox geothermal fairways have been recognized in this way (fig. 49); two of the fairways occur in the upper Wilcox (Live Oak and Webb) and the remaining six, in the lower Wilcox (Liberty, Harris, Colorado, De Witt, and Zapata. The fairways range in areal extent from more than 1,000 to less than 50 square miles and contain total net sandstone varying from 3,600 to less than 300 feet (table 2). The top of geopressure occurs at depths between 9,000 to 13,300 feet and the 300°F temperature, at depths of 10,200 to 13,800 feet. In all fairways, except Zapata, the individual sandstone beds range in thickness from 10 to 60 feet; in the Zapata Fairway individual beds appear to be as thick as 150 feet. Typical electrical logs from wells in these fairways are shown on Figures 50 to 57. 66 19 :10. ,', , ,- 18, ,i' .'. ". 21 ; 'i /~ ;, ,J. I ,I \ '. \' 'I ,I. e " " .., .' , '" " "- '; " .)') , \ , . ," . ," ) .' ;, " . '. If~ ,J " ,,\ , " ,II I EXPLANATION Upper Wilcox FOlrways /1 Lo ...... er Wilcox FOlrways \ \ ~' ZAPATA \ \ WilCOX Ou1crop \\ \ \ ],0, ,-. r • • " ~ 'Ii""".8 'QO " "j •• Figure 49. Wilcox geothermal fairways. Well logs representing each of these fairways are shown on Figures 50 to 57. Dots show locations of representative well logs. 67 LIBERTY COUNTY SUPERIOR #1 H,qhlower 3N-40E-9 L.llie 18 Well @ S ( Top of --<-o:;·-tI Lower \WCox GR ~ t (2. ( I 3 13.000 1 1 I f l I Figure 50. Representative well log from the Liberty Fairway. , , , 68 HARRIS COUNTY TEXACO # I MI'rqale 14,(XX) I N 3~ [ 8 Line 16 Well® \ Top of t I Lower Wilcox 12,000 15,(XX) 16,(XX) 17[.XYJ Figure 51. Representative well log from the Harris Fairway. COLURADO COUNTY HUMf1L r- #1 Karctwiu )S )I[ H. l_mel4 Well (I?) Top of Lower Wilcox 12,000 <.".~-~ 13,000 Figure 52. Representative well log from the Colorado Fairway. 70 DE WI n COUNTY L Cl~Jl STAR # I Hiller 85 -22.E-6 LmelO Wel([) I ) i ~ Top of ~.:~ Lower Wilcox I [ II,CX)] :n - ~ ? r' ~. ! l j \ 30(0 1 ) ;: E: 12,000 13,000 ~'"i ~ Bose Of Wilcox 14,000 Figure 53. Representative well log from the DeWitt Fairway. 11 LIVE OAK cour~ lY )TANDAf~D Of' TEXAS 0# 1 Burns 15S-15E-5 LineS Well@) lOP 12,000 13,000 14,000 Figure 54. Representative well log from the Live Oak Fairway. 72 DUVl\1 COUNTY Top of Lower Wilcox 12,000 10,CXX) 13,000 II,eX!) 14,OCO Figure 55. Representative well log from the Duval Fairway. n WEBH COUNTY HtlMmJ,etol 11 I Perez 2(:') IOE-8 Line 2 Well@ 12,000 Top of -- Lower Wilcox Figure 56. Representative well log from the Webb Fairwav. 74 ZAPATA COUNTY LONE STAR PRODUCING CO #1 Zamora 27S-SE -3 10,000 11,000 Figure 57. Representative well log from the Zapata Fairway. 75 ACKNOWLlDGt~uns We should like to express our appreciation to W. L. Fisher and J. H. McGowen who provided to us all of the materials used for their exten sive study of the Wilcox Group and to W. R. Kaiser and J. E. Johnston who aided in organizing this project during its initial stages and revievJed the conclusions and made Illany helpful suggestions during the final stages. Geomap Company, Houston, Texas, gave us permission to publish the location of faults in the Wilcox from their Gulf Coast mapping service. 76 REFERENCES Bebout, D. G., Agagu, O. K., and Dorfman, M. H., 1975a, Geothermal re- sources--Frio Formation, Middle Texas Gulf Coast: Univ. Texas, Austin, Bur. Econ. Geology Geol. Circ. 75-8,43 p. ----, Dorfman, M. H., and Agagu, O. K., 1975b, Geotherma 1 resources-- Frio Formation, South Texas: Univ. Texas, Austin, Bur. Econ. Geol- ogy Geol. Cire. 75-1, 36 p. ----, Loucks, R. G., Bosch, S. C., and Dorfman, M. H., 1976, Geo- thermal resources--Frio Formation, Upper Texas Gulf Coast: Univ. Texas, Austin, Bur. Econ. Geplogy Geol. Circ. 76-3,47 p. ,- ----, Loucks, R. G., and Gregory, A. R., in press, Frio sandstone reservoirs in the deep subsurface along the Texas Gulf Coast--their potential for the production of geopressured geothermal energy: Univ. Texas, Austin, Bur. Eeon. Geology Rept. Invest. ----, Luttrell, P. E., and Seo, J. H., 1976, Regional Tertiary cross-sections--Texas Gulf Coast: Univ. Texas, Austin, Bur. Econ. Geology Geol. Circ. 76-5, 4 p. Bruce, C. H., 1973, Pressured shale and related sediment deformation: Mechanism for development of regional contemporaneous faults: Am. Assoc. Petroleum Geologists Bull., v. 57, p. 878-886. Burst, John F., 1959, Postdiagenetic clay mineral environmental rela- tionships in the Gulf Coast Eocene, in A. Swineford, ed., Clays and clay minerals: 6th Natl. Clays and Clay Mineral Conf. Proc., Pergamon Press, 411 p. 77 1969, Dii)~cnesis of Gulf' oast clayey sediments and its possible relation to pctl'olcUIlI migration: The Amer. Assoc. Petroleum Geolo gists Bull., v. 53, no. 1, p. 73-93. Dorfman, M. H., and Deller, R. W., eds., 1975, Proc., First Geopressured Geothermal Energy Conference: Univ. Texas, Austin, Center for Energy Studies, 369 p. ----, and Deller, f\. W., eels., 1976, Proc, Second Geopressured Geo- thermal lnergy Conference: Univ. Texas, Austin, Center for Energy Studies, 5 vols. Deussen, Alexander, and Owen, K. D. ,,1939, Correlation of surface and subsurface formations in two typical sections of the Gulf Coast of Texas: Am. Assoc. Petroleum Geologists Bull., v. 23, p. 1603-1634. Echols, D. J., and Malkin, D. S., 1948, Wilcox (Eocene) stratigrapny, a key to production: Amer. Assoc. Petroleum Geologists Bull., v. 32, p. 11-33. Fertl, Walter H., 1976, Abnormal formation pressures: Elsevier Scienti fic Publishing Co., Amsterdam, The Netherlands, 382 p. Fisher, W. L., 1969, Gulf Coast Basin Tertiary Delta Systems, in Fisher, W. L., Brown, L. F., Scott, A. J., and McGowen, J. H., Delta sys tems in the exploration for oil and gas: Univ. Texas, Austin, Bur. Econ. Geol. p. 30-39. ----,and McGowen, J. H., 1967, Depositional systems in the Hilcox Group of Texas and their relationship to the occurrence of oil and gas: Gulf Coast Assoc. Geol. Socs. Trans., v. 17, p. 105-125. 78 Gallmvay, \~. E., 1974, [jeposi I ion and Diagenetic dltr (', tion of sandstonp in northeast Pacific arc- r"elated basin--implications for graywacke genesis: Geol. Soc. Amer. Bull., v. 85, p. 379-390. Hargis, R. N.• 1962, Stratigraphy of the Carrizo-Wilcox of a portion of South Tf:xc1s and its relationship to production: Gulf Coast Assoc. Geol. Socs. Trans., v. 12, p. 9-25. Hottman. C. [ .• and Johnson, f~. K., 1965, Estimation of formation pres SUI"eS from loq-clerivecl shale properties: J. of Petro Technol., v. 17, p. 717-723. Hoyt, W. V., 1959, Erosional channel in the middle Wilcox near Yoakum, Lavaca County, Texas: Gulf Coast Assoc. Geol. Socs. Trans., v. 9, p. 41-50. Johnston, J. E., 1977, Depositional systems in the Wilcox Group and the Carrizo Formation (Eocene) of Central and South Texas and their relationship to the occurrence of lignite: Univ. Texas, Austin, M. A. thesis, 113 p. Kaiser, W. R., 1974, Texas lignite: near-surface and deep-basin re sources: Univ. Texas, Austin, Bur. Econ. Geology Rept. Inv. 79, 70 p. Kehle, R. 0., 1971, Geothermal survey of North America, 1971 annual progress report: Research Committee, Amer. Assoc. Petro Geologists (unpub. rept.), 31 p. Murray, G. E., 1955, Midway Stage, Sabine Stage, and Wilcox Group: Am. Assoc. Petroleum Geologists Bull., v. 39, p. 671-689. 79 Plrson, Sylvain J., 1970, GeoloJic well log analysis: Gulf Publishing Co., Hous ton, Texas, 370 p. Plummer, F. S., 1932, Cenozoic systems in Texas, in Sellards, E. H., Adkins, W. C., and f)lullllller, F. B., The geology of Texas, v. 1, stt'ati~JrllfJhy: Univ. Texas, Austin, Bull. 3232, p. 519-818. Todd, T. \~., ilnd Folk, fL L., 1957, Basal Claiborne of Texas, record of Appalachian tectonisill durin gists Bull., v. 41, p. 2545-2566. Townsend, J. V., Jr., 1954, The generalized geology of the Wilcox Group of northeast Texas: Gulf Coast Assoc. Geol. Socs. Trans., v. 4, p. 69-74. 80 Table 1. Geotherma' Gradients of the Wilcox Group, Texas Gulf Coast Harris Co. De Wi tt Co. Area of Live Oak, Area Area McMull en, Duva 1 , Webb, and Zapata Counties ------'------Geothermal Gradients Above Wilcox 1.4 1.5 1.9 (0-9,000 ft) Upper Wi 1cox 2.1 2.6 2.7 (9,000-14,000 ft1 Lower Wilcox 1.2 1.2 1.5 (below 14,000 ft) Subsurface Temperatures and Depths 250 0 F 10,700 9,800 9,000 300°F 13 ,050 11,700 10,850 3500 F 15,100 14,900 12,700 81 t ,. c: Ul Table 2. Wilcox Geothermal Fairways C1 0 < t'l ~ ~ t'l Z >-3 ! ~'" Name Typical Number Areal ! Sandstone Thickness Depth to H Z v I >-3 Electrical of Wells Extent " H Total Individual Beds 300 F Top of Geopressure ?5 Log (mi2) (ft ) ( ft) (ft ) (ft ) 0 '"'l '""" C1 I ~ Zapata fig. 57 1 48 340 20-150 1 10 ,200 I >-' ~ 00 fi g. 56 48 400 10-20 10 8,700 ""I Webb 1 1 ,800 Ie ""6 i I Duval fi g. 55 2 140 400 10-50, 11,000 , 9,000-10,000 '"0 "-'"e- N Live Oak fig. 54 2 75 240 10-40 11, 300 9,400 ""~ CO :::0 N De Wi tt fig. 53 6 280 700 10-50 10,500- Ie.. 100-10,700 ro a; 19,900 .... ' g "'" Colorado fi g. 52 2 200 850 10-20 12,300 11,400 Harris fi g. 51 11 1,375 3,600 10-60 11,000- 11,100-13,300 13,500 Liberty fig. 50 1 200 460 10-60 12,500- 12,300 l3 ,800 i --- ,