Feature Article Relationship of Gulf Coast Basement Tectonics to Present and Future Oil and Gas Fields by J .C . Pratsch, Petroleum Geologist, Houston

Introduction

There is a continuous debate among Gulf Coast Region explorationists to what ex- tent basement structure affects visible struc- tures in Phanerozoic (Paleozoic to Quater- nary) sediments. General opinion favors the interpretation that regional Gulf Coast structural features are formed by salt diapirism, glide and shear tectonics related to the Mesozoic opening of the Gulf of Mexico basin. Discussions have been based largely on subjective concepts, less on data or comparisons with other areas.

If a positive relationship exists between basement structure and shallower structure, oil and gas exploration should be using deep-structure data such as seismic, grav- ity, and magnetics more widely than at present. Basement features can be defined by detailed gravity, magnetic, and seismic data, yet databases are rare, and no large- scale, detailed maps exist. This study uses the Tectonic Map of North America (Muehlberger, 1992) (Figure 1) and the Depth to Basement Map of the Gulf of Mexico Region (Rosenthal and Buffler, 1990) (Figure 2). The first source concen- trates on the onshore area: the second in-

cludes depth data from the offshore. Fig- ures 3 to 6 show the location of oillgas pro- ductive trends in the area in Mesozoic and Paleogene reservoirs. Oil and gas produc- tion in Neogene reservoirs is related more to depositional processes (reservoir loca- tion) and to major vertical and lateral hy- drocarbon migration.

When basement structures are defined by potential tools (gravity, magnetics), these data and their interpretations become a major parameter in strategic exploration decisions (Pratsch, 1996). An overall geo- logical concept thus must combine these data with the petroleum-geological history of the region and compare with other pro- ductive basins both geologically similar and dissimilar. The analysis presented here de- fines practical approaches in future oil and gas exploration in the Gulf Coast Region.

Likely Location of Future Petroleum Prospects

Basement structure in the Gulf Coast re- gion created regional depocenters where Mesozoic and Paleocene source beds were deposited and matured thermally. Regional structural highs became focal points for

N O R T H A M E R I C A N

lateral oil and gas migration from these depocenters. The best future petroleum prospects in the onshore and offshore Gulf Coast region will be found on regional highs that lie near regional structural lows that are hydrocarbon-generating depocenters. Major oil and gas fields will also be found in regional lows and on their flanks in the offshore, just as they occur inside basins in the onshore. However. the final reserve distribution will always favor near-by regional structural highs.

In a worldwide study covering more than 100 producing basins across the world, it was found that 25% of a basin's area car- ries 75% or more of the basin's oil and gas reserves (Lawrence and Pratsch, 1980). Why, from a petroleum-geological view- point, should the Gulf Coast region behave differently from any other producing basin in the world? Basement structure can and will be used to identify prospective areas in the Gulf Coast Region. This knowledge should be obtained prior to investing in more intensive, detailed, and expensive exploration data.

Results of Examining Basement Struc- ture Maps

On Figures 1 and 2, major basement fea- tures in the onshore are clearly visible. Regional basement structural trends are northeast in the western part of the region; they change to northwest in the eastern part. North-south and west-east trends dominate the central portion in east Texas and cen- tral Louisiana. Transform faults trend north- west in the west in Louisiana andTexas and northeast in Alabama and northern Florida. In an alternate view. basement structures trend generally northeast-southwest, paral- lel to the Paleozoic (or even older) conti- nental margin of the North American cra- ton, and all northwest-southeast trends are secondary cross/transform elements.

On a regional scale, a triangular stress dis- tribution system exists. It is caused by a primary north-to-south major compression field. Northeast-trending faults contain a left-lateral shear component, northwest- trending faults will contain a right-lateral shear component. Transform faults trend

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ntrnhwc~t in nnchore eastern Texas. such ;IS lhc Nrwenc tri~nsfer raults rn;~ppeil hy I-lnh. er :il I I WJh!. The untlcrlying rcgionnl we*.; ~ ~ W I I I I\ derived f r o n ~ long-l:~cting tectonic h>rcc\ c;luiecl by the rising ;\hxth Arncric:~n cr:lllm and the suhsidinr G l f G w t px\\ i \c III~~III hi~sin.

The tn;~itm!y o f o i l :~nd €:IS reserves in the Gulf Cmrt drcpw;mr ;oc;~ wi l l he ihund It\c;rted <,n h;~sement slruulural h ighr LPr;11\cl1. I(,')hl. To illustrate this point. the 1n:lin oi l and IS producing trendi and ar- exs m the C;ulS('o;ist Region i n M e s w ~ ~ i c :!nil P:~leo$w reservoirs lI:ails. IL!O1) are s l i ~ w n in I:tgl~r?c 3 to 6. GulfC'n:~\t N e w genc ir.;er\orr d iwihut ion h l l owcd Ter- tixy w ~ m e n t ; ~ t i n n Inlore than h i~wment stnlcti~rc I'rlxlucing drep-watrrure;~ trend dirccticms .Ire Inlt \.<.I h l l y realized due to [lie rel;~tivel! small nua~ber~~f~liscc~veries. Yet. initial CI:II:I ;q)pe:!r to suppurt thih in- terpretxttv I f'rabch. lW15l.

The Cnncept

Repion;~l rtnrcti~r;~lly pncitivc l'c;~tures in the tm4iore s m he shown tu have :~l'iecred &lcrcmuc ;m'rmi:q scdimentation.This st~crnent i\ <,?en cleh:~tctl t>r cven rejected I;v 111~. c,ll;htrr ;ire;t.Twnsoppc~niny p i n t s are:

S;m .Vi~rcos arch. nonhcast Alaharna. and Strut11 Florid;, nre:ls. T h i s i s expected tohold Inte l i l r h:~senient movement In llw o I l N ~ t m Guli. Thereh~rc. the chances Car findin: I a r ~ e repional lli~li:. om1 loa,s exprewt l :,I ~op-h;~scrncnl level ;~nd in blesuzoic and P~leopene sediments :Ire r e d and re;~lislic. Ii so. c~l'l'shore Gulf Coast hasetnenr SIIUC-

lure wi l l inilicatc tiil/g;~s :cncr;~tin: struc- tural l o w ("depocenters") ilnd ~ill:'i~\ ~IC-

cumu1;tting l ~ i y l ~ s ("trap centers").

2. Arecently presented TGS regiomd seis- mic line at the GCAGS Ssn Antunio Cun- Serrnce. IWh. :icn>ss the Texas o f f h ~ r c sliell region, showcd :In interprol:~lion of rcnii~rhi~hly wonp stn~ctural expression at the h;aeofthe Jur;~ssic "1.11uxnn Sorn~;~lion" s;11t. The overlyin: LVes<>z<,ic-P;~lco~enc sedimentaq sectinn. ~Ieposited hehre lhe Miocene. uw shown to he v i approxi- mately con\l;mt il~iclinc%c. an indication o f re i l~~ced sediment supply i n (?) deep w;~- ~ers. C\S one r e d t . "h;wtnent structure" W:IP also expreshed at I'deogcne levels

(prior to thick hliccenc;~nd ywngerhctls). R:lse-s;~lt .;ln~crure m:!) include sonic to- pogr;lpIiy deveh~ped t l ~ ~ r i n ~ p~:-s;dt (?Tri- ;Irw. Early Jur:~sricl t~mc: In~wever. the m<,st lihcly conclusi<m ir that ;dl pre-h,li- triene slnlclllre i s 121;llerl h, i1:l~erllelll de- rormalion I l l e l h : ~ r occurred :I$ 1312 as liocene time IWi l iox t'~mn;!li<~n depusition time), I f 511. (hen oi l :~nd ?:IS l ~ ~ k e d lrom t l w e prim:~ry field.;. :mcl hydrnc;~rhons migratud intoyounp?r. il iocene toQu:iter- n;q-;qe reserv~lirs~

Area hy Area Compur iwn

I n the florid:^ 11n5ht,rc. llir Crel:icem~s Sonnil:~n(I I'orm:~lion r b i l production i s present i n n punion 01' the rc~.ic:nd hase-

Houston Geologlcol Soc ie ty

ment structural low trending parallel to the coast in the South Florida Basin. expressed by gravity and sediment-thickness and fa- c i e ~ maps. The explanation seems to lie in the restricted occurrence of thermally ma- ture Sunniland formation source beds in the deepest portion of a structural low area: a regional basement structural feature (a structural low) controls the occurrence of oil and gas accumulations!

In Alabama. the Jurassic-reservoired oil fields lie on basinward-dipping structural noses that are subsurface extensions of the Appalachian basement structure. Oil mi- grated updip from the oil-generating re- gional structural low into stluctural highs. The structurally prominent Monroe uplift has concentrated major gas accumulations.

The hydrocarbon-generating Mississippi salt basin and the East Texas basin contain locally derived oil and gas fields. The re- cent Jurassic carbonate play on the flanks of the East Texas basin most likely is tied in the subsurface to basement tectonic con- trol on the location of carbonate build-ups.

Other areas with mappable basement struc- ture and mature Mesozoic source beds will soon be found productive within the trend.

In East Texas, Upper Cretaceous oil mi- grated laterally liom the regional basement low in the south into pre-existing traps on the regional basement high (Wescott and Hood. 1993). The Sabine Uplift structural high played a role in concentrating the fz- mous oil pools of the East Texas field.

In Southeast Texas. numerous oil and gas fields occur along long Tertiary syn-sedi- mentary growth fault systems. These growth fault systems are parallel to regional lineaments as expressed by geological maps and magnetic data (Figures I and 2). It is obvious that there are geological connec- tions between basement faults and/or frac- ture zones and the location and develop- ment of these Tertiary syn-sedimentary growth faults.

In the offshore Gulf of Mexico region, deep basement features with dimensions similar to regional features onshore are indicated.

- I Figure 3. ' ~ u l f Coast Region p rodk ing Areas with Upper Jurasslc

R e S e ~ o i r S ( from Fails. 1990)

L Figure4, Gulf Coast Region Producing Areas with Cretaceous Reservoirs

(from Fails. 1990)

Shallow hydrocarbon fields will be found at the end points of lateral and vertical hy- drocarbon migration from deep basement structural features (Pratsch. 1996). Struc- tural lows may be preferred hydrocarbon generating area.;, structural highs may be preferred hydrocarbon accumulation areas.

Conclusions and Practical Applications

As mentioned in the beginning of this ar- ticle. oil and gas fields in any producing basin are concentrated on basement-rooted regional highs. In a worldwide study cov- ering over 100 producing basins across the world, it was found that 25% of a basin's area carries 75% or more of the basin's oil and gas reserves (Lawrence and Pratsch. 1980). A similar concentration of reserves on regional structural highs will also be found in the Gulf Coast offshore region. There is no reason why. from a petroleum- geological viewpoint, this region should behave differently than any other produc- ing basin in the world. Thus, basement structure can and will be used to identify most prospective areas in the Gulf Coast

Figure 5. Gulf Coast Region Producing Areas with Eocene Reservoirs (from Fails. 1990): Horizontal Lines -Yegua Fm, remaining areas Wilcox. Sparta, Cockfield Fms

Figure 6, Gulf Coast Region Producing Areas with Oligocene ReSe~0ir.S (mainly Frio Fm) ( from Fails. 1990)

14 Houston Geological Society June 1997

Hub. S.. Watkins. IS.. Kasand4 R.FiL. J.C. Bryant, S.. Silver. Id&. BrsdsbPW, RE. Xuq F. .ad J. Xi. 1996, R@od Scmcnmaod'REconicsonmS~m~ In: SPlucnvlt Fnmewe of the Northwr Oulf of W c o (J,A.loaes aod R.LFned e&J. GdfcoutAswahtion ofGwbgi- all sOdetiC;I, p.39-Sl.

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lbmtbl. D.B. and RT. B a r . 1990, DepmtoBasement OulFofMuiooRegim: University d T w s I n s t i ~ a for Oeophy8- ics. (It is a shon 45-minute drive bmn dowmown Houston M a g 1.45 South, Bloadway Boulev~andZOmStrrsL The Centerwan toopen tochepublicin May. 1997.).

June 1997 Houston Gedogfcal Society