GSTT (1991) an Evaluation of Post Middle Miocene Geological Sequences Offshore Trinidad

70

AN EVALUATION OF POST-MIDDLE MIOCENEGEOLOGICAL SEQUENCES, OFFSHORE TRINIDAD

Neil PayneTrinidad and Tobago Oil Company Limited

Pointe-a-Pierre, Trinidad

ABSTRACT

The Post-Middle Miocene sediments in the offshore areas around Trinidad can be divided into two (2) main depositionalsequences, which can be distinguished using seismic response differences, stratigraphic relationships, relative age datingand by identUication of depositional regime. They consist of an older progradational series [Sequence (A)] and a younger,retrogradational or aggradational series [Sequence (B)]. Sequence (A) sediments filled the already existing Southern Basinand the syndeposltlonal Northern Basin with sandy clastic facies up to Mid-Pliocene time. Toward the end of the Pliocene,subsidence and uplift occurred concomittantly along a series of northwest-trending (Type II) faults and northeast-trending(Type I) faults respectively. This resulted in the formation of sub-basins in which Sequence (B) sediments were deposited,and the simultaneous uplift of the Southern Basin. Oil migrated into this high along basement-related Type Ifaults and splaysof these, becoming trapped in the associated structures. Additional transcurrent movement occurred along Type II faults,displacing, tc various degrees. the compressive structures formed by the Type I faults.

INTRODUCTION

In this paper the term "Post-Middle Miocene" refersto thatgeological period which is younger that the Globorotaliamenardii planktonic zone of Middle Miocene age. Thesediments of this period have accounted for over eightypercent (80%) of Trinidad's oil production and areconsidered to be highly explored and in a mature state ofdevelopment. However, there remain some unansweredquestions about the stratigraphic and structural evolutionof the various sub-basins of this period and, as a result,their hydrocarbon potential cannot be considered fullyexplored.

This paper focuses onthetwo oil-producing sub-basins ofthe post-Middle Miocene period, the Northern andSouthern Basins. The north coast Tobago Basin was notincluded as rt is considered to be in a different petroleumprovince and has been recently discussed in a paperpublished by Robertson and Burke (1989).

The analyses were done with the aim of (i) establishinginter-basinal, relative time-stratigraphic correlations andidentifying common tectonostratigraphic relationships,(ii) examinin!;J the timing and style of structuring and (iii)making conclusions on the mode of oil accumulation andrecommending methods for further exploration anddevelopment.

The area of study lies between the Northern Range to theNorth and the Amacuro Delta to the South. To the westand east, tho limits were set at the Venezuelan territorialboundary and the Trinidad and Tobago deep wateracreages respectively (Fig. 1).

DATABASE AND METHODOLOGY

The data used comprises a small part of an extensivenetwork of reflection seismic lines and exploration wellsthroughout the study area (Fig. 1). The line numbers andwell names have been omitted and replaced by simplenumerical designations, but their correct locations areindicated.

The method adopted was an integrated one, in whichvarious data types were analysed to arrive at one commoninterpretation. However, the reflection seismic toolprovided the basis for all interpretations as it allowed forunique subsurface imaging of regional stratigraphic andstructural relationships, a main objective of the study.

Seismic Data

The seismic data examined included all available surveysin their respective areas. Most ofthe deeper, regional datawere older lines shot mainlyfor reconnaissanca purposes.These are stili some ofthe most valuable data for regionalwork and have generally improved with reprocessing.

Well Data

The well data used consisted mainly of wireline logs andof palynological, lithological and biostratigraphic reportsand summaries.

The palynological zonation system was developed byE.Gonzales, and is familiar to most of the oil companiesoperating in Trinidad. This zonation shows someconsistency with seismic correlation and was applied ona broad basis for correlating formations and seismicevents in time across major fault boundaries. Due to the

Transactions ofthe 2nd Geological Conference oftheGSn; Ed. ](A Gillezeau; Published byGSn; 1991

2nd Geological Conference of the Geological Society of Trinidad and Tobago - 1991

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71

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Figure 1: Location Map, -.-.-= Limit of Study; = Location of seismic Hnes and sections;

----- = Sub-basin limits

limited information available to this writer on the value ofthis classification in defining absolute time markers, thezones will be used In this paper as a relativechronostratigraphic tool only.

Lithological and biostratigraphic summaries of mainlybenthonic foraminiferal data were used to identifyformations and the paleoenvironments.

In the Post-Middle Miocene, offshore Trinidad, two (2)such sequences have been identified. These can bedistingUished mainly by their reflection seismic characterand seismic/stratigraphic relationships. TI1eycan also bebroadly identified using palynological (p()lIen) and, to alesser degree, lithological and paleontological(foraminifera) data.

STRATIGRAPHY

Depositional Sequences

The term "Deposltlonal sequence" is used in this paper tosignify a genetically related group of sediments which isgovernedby a particular setofexistingtectonic conditions.

The two sequences represent (i) an oIdElr, high-energyprogradational to aggradational series [Sequence (A)]and (iI) a younger, lower-energy, retrogradatlonaltoaggradational series [Sequence (6)]. Sequence (A)sediments are generally of UMI to rnlddle PII age, whileSequence (6) sediments are from upper 1;)11 to Recent inage (Figs. 2,3).


The Northern Basin

Thestratigraphic sequence forthe Northern Basinoffshoreis best observed in Gulf of Paria wells (Fig. 2), where it ismore or less the same as that displayed on the Geologicalmap of Trinidad (Kugler 1959).

The older series, Sequence (A), estimated by correlationto be UMI to PI in age in the north, consists of a massiveconglomerate, the Cunapo Formation, which aggradesalong the southern flank of the Northern Range andinterfingers southwards with the Manzanilla Formation, aprograding deltaic sequence. The members of theManzanilla formation, from oldest to youngest, are: theSan Jose Calcareous Silt, the Montserrat GlauconiticSandstone and the Telemaque Sandstone. The formertwo are interpreted to be shallow-marine shelf sands,while the latter is a brackish-water deltaic sand with somelignite beds. In the extreme south of the Basin, theManzanilla Sands are fine-grained and mature, and thetop of the sequence is of PII age, indicating a southwardsdirected proqradatlon (Figs. 2, 4B).

In the proximal eastern offshore area, the ManzanillaFormation is assumed to exist, based on extrapolation ofKugler's map and sections. However, the well N-1, whichwas drilled further offshore and to the north, encounteredmainly shales of undifferentiated Miocene and Plioceneage (Fig. 2), indicating that in this area the high-energy,sandy facies may have been replaced by a lower-energy,sand-starved facies.

Themaximum drilled thicknessofthe ManzanillaFormationand equivalents is about 6,500', but this should increasealong the basin axis where the sequence has not beenfully penetrated.

On seismic, the ManzanillajCunapo group appears as aseries of discontinuous or chaotic events, sometimeswithbedding attitudes indicative of progradation (Figs. 4, 6).

Sequence (13) in the Gulf of Paria consists mainly of theshaly Sprin~lvale and Talparo Formations.

The Talparo is a dominantly marginal marine to brackishclay formation with few, but widespread sand bodies.These sands are excellent seismic and well log markers,the most prominent being the Durham Sands at the baseofthe Talparo and the Sum Sum Sands which occur atthebeginning of PIli and Pleistocene times respectively.

The Sprin~lvale Formation represents the onset ofSequence (B) conditions and contains abundant shellfragments and some lignite beds. It isan intermediate unitand in SOmE! areas, such as along the flanks of the CentralRange, is overlain unconformably by the Talparo group(Barr and Saunders 1968).The thickness of the formationvaries widely, indicative of changing conditions during itsdeposition.

72

Sequence (B) can easily be recognised and distinguishedfrom Sequence (A) on seismic, appearing as a series ofcontinuous, high frequency reflectors. The Springvale isoften seen as a zone of poor reflectors between theManzanilla and Talparo Formations (Fig. 4).

The Southern Basin

The Southern Basin sediments have been divided into aneastern and awestern facies, based largely on lithologicaland environmental variations, but both belong geneticallyto the same deltaic regime. Sequence (A) and Sequence(B) sediments can be distinguished from each other,based on seismic character differences, but Sequence(B) remains sandy, and changes in lithology andenvironment are not easily Identified. However, carefulexamination of electric log curves reveals a subtle changefrom progradational (coarsing upwards) to aggradationalor retrogradational (fining upwards) trends in Sequence(B) (Fig. 2).

In the Erin Syncline and west Columbus Channel areas(Fig. 1) Sequence (A) is represented by the Cruse andForest Formations (Fig. 2). These formations include thepro-deltaic Lower Cruse clays which provide much of thediapiric material associated with the uplifted areas of theSouthern Basin. The Lower and Middle Cruse sands aremainly turbidites associated with these deep-water clays,whereas in the case of the Upper Cruse and ForestFormations, delta front bars and channels form the mainsand bodies (Fig. 2). The entire sequence can be as muchas 10,000 feet thick.

Sequence (B) consists of the upper Morne L'Enfer andErinFormations. Ingeneral,the Morne L'Enfer is interpretedto be a marginal-marine to lower delta-plain deposit inwhich a large variety of sand deposits occurs.

The Erin Formation, which unconformably overlies theMorne L'Enfer in the Erin Syncline (Fig. 9 and Barr andSaunders 1968) is a sand and clay sequence whichcontains much evidence (e.g. abundant plant remains) ofbeing a terrestrial deposit. Wells drilled in the ColumbusChannel have documented Talparo type environments.

In the East Coast, progradation of Sequence (A)sediments towards the northeast is clearly visible onseismic data located on the undeformedAmacuro Platform(Fig. 4). The depocenter for all this sediment was the largeColumbus Basin areawhere the Lower Cruse, Gros Morneand Mayaro Formations of UMI to PIIage were deposited(Fig.3).ln the proximal Columbus Basinarea,this sequenceis 11,000feet thick (Bane & Changpong 1979)I whereas inthe more distal areas thicknesses of over 20,000 feet areestimated from seismic data. The top of the sequence inthis area has also been interpreted to be possibly asyoung as Pili, attesting to the time-transgressive extent ofthe progradation.

The LowerCruse, assumed to havebeen the first deposited


73

-1000'

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-2000'

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-4000'P

P II P 1I1-seee:

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-7000' -.-8000'

.:.:.

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-13000' p )

-14000' Nl0

':'~'.-;.:.-ISOOo- -:.:

)....:.

-\6000' ....~~....."....:.

-17000' S4S2 00

Figure 2: A comparison of selected well logs from offshore areas around Trinidad.Northern Basin Formations/Members: Ml=Manzani11a; TM=Telemaque; SP=Springvale; T=Talparo;cO=Cunapo; MO=Montserrat Glauconitic Sand; DS=Durham Sand; SS=Sum Sum Sand.Southern Basin Formations/Members: LC=Lower Cruse; MC=Middle' Cruse; UC=Upper Cruse;F=Forest; ML=Morne L'Enfer; L7=Lot 7 Silt; LG=Lower GrosMorne; UG=Upper Gros Morne;P=Palmiste and Post-Palmiste. Palynolo~~ Zones: PL=Pleistocene; PI-PI I I=Pliocene;UMI-UMI I=Upper Miocene; P=Pliocene (undl ferentiated); M=Miocene (~ndifferentiated);MM~iddle Miocene or older; K=Cretaceous.

f.~~~J Progradational series (Sequence A)

c=:J Retrogradational or aggradational series (Sequence B)


NOA:THERN BASIN SOUTHERN BAS I N

74

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PRE-MIDDLE

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POST PALIrtISTE

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ERIN

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PALYNO AGE .ZONES

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...,....z,.dznen

.Fi9u~e 3~: Generalized stratigraphic chart of Trinidad offshore areas (modified after Kugler, 1959).Note strongly Prograding Sediments of Sequence (A).

pro-delta clays here, have been displaced to a largedegree by diapiric activity into the cores of uplifted ridgesand rollover anticlines.

Sequence (B) formations in the east coast are not welldocumented ill the literature. Although the section remainsquite sandy, e.g. the Palmiste Formation (Fig. 2), theseismic characer resembles that of the Northern Basinequivalents (Fig. 4, 8), indicative of a similar change indepositional n~gime.This is supported by paleontologicaldata, e.g. in the Palmiste clay, which displays distinctmarine features.

UNCONFORMITY ANALYSIS ANDTIMING OF UPLIFTS

Northern Bnin

In the eastern Northern Basin (Fig. 5) Upper Miocene,Pliocene [Sequence (A)] and possibly older sedimentsonlapped the Northern RangejTobago metamorphicbasement. Within the Pliocene, uplift began along thebasin axis and southern limits, reSUlting in a narrowing ofthe Upper Pliocene basin. Offshore, uplift and erosionceased towards the end of the Pliocene, and a relativelyundeformed Pleistocene basin was formed and filled withSequence (6) sediments (Fig. 5A). Onshore, however,compressive stresses persisted longer and only a thinPleistocene (Upper Talparo) cover was deposited (Fig.

5B, Kugler 1959).

In the Gulf of Paria (Fig. 6), simultaneous tectonic eventstook place. In the vicinity otthe EIPilarfaulttrend, collapseand rotation occurred during and at the end of Sequence(A) with the formation of a conglomerate-filled half-graben(Fig. 6B). As in the east coast area, uplift ceased at the endof the Pliocene and Upper Talparo clays (Pleistocene)were deposited unconformably on the Cunapoconglomerates. Just south and east of EI Pilar (Fig. 6A)subsidence occurred slightly earlier with Lower Talparoclays being deposited on the unconformitysurface, whichis also not very strongly erosional in this area. In well N-2(Fig. 2), the uppermost Cunapo Formation displays aretrogradational log character. Indicating the onset ofSequence (B) conditions even before the main erosional

.phase.

Collapse and rotation within the northern Gulf of Parlaoccurred along majornorthwest/southeast-alignedfaults,resulting in a wide rifted basin with two (2) prominent horstfeatures, the Gulf and Domoll Highs (Fig. 13). Along thebasin axis, this activity continued into the Pleistocene,resulting in the accumulation of thick Sequence (A) andSequence (B) sediments along a major structurallineament, the Central Range (or Warm Springs) Fault.South of this Fault zone lies the original Central Range, anow buried high on WhiChS~qu~nc~ (A) ~edim~nt~ onlapand overwhich Soquence (B)sedimentshavetransgressed(Figs. 9,10).


75

Southern Basin

In the Southern Basin (Fig. 7), uplift along the SouthernRange /Galeota Ridge occurred at similar times to theNorthern Basin, beginning at the end of Sequence (A) (PIItime) and continuing Into the Pleistocene. Here also, localunconformity/pinchouts can be seen at the base ofreflectors of Pleistocene and PIli age respectively.

In the Columbus Basin (Fig. 8), unconformities were alsoformed due to rollovers and slumping across northest/southwest-trending faults. Growth across fault throws islargest in the Pleistocene (proximal areas) and IntraPleistocene (distal areas), indicating that the uplifts alongthe Southern Range during Sequence (B) time hadnarrowed the depocenter of the Columbus Basin. Theclearly discernible difference between the prograding orchaotic reflectors of the Sequence (A) (Gros Morne/Mayaro) and the parallel, high frequency reflectors ofSequence (B) (Palmiste and younger) reflects the changein depositional conditions and is strongly reminiscent ofsediments in the Northern Basin, where related changesoccurred at the same time.

Central Range

The relationship between the Central Range and theNorthern and Southern Basins is a complex one andinvolves the juxtaposition of both older and youngerHighs.

Figure 9 is a seismic section across the Southern Gulf ofParia. It shows that three (3) high areas exist along theNorthern/Southern Basin Boundary, each with a differentstructural expression.

The northernmost feature, the Central Range Fault, is ayoung massive flower structure which runs along the axisof the Northern Basin. .

The Brighton/North Marine High is an older, now buried,feature, on which Manzanilla and Springvale sedimentsonlap both from the north and south (Fig. 10).

The Soidado High is the present-day Northern Basin/SouthernBasinboundarywhereUpperTalparo [Sequence(B)] sediments onlap Cruse, Forest and Lower MorneL'Enfer sediments [Sequence (A)], which themselvesappear to be locally thrusted over Manzanilla [NorthernBasin Sequence (A)] sediments in a complexcompressional event.

Figure 9 also shows that the Soidado High, the ErinSyncline and the Southern Range are young features(Post-Sequence (A) time) and that the Erin Formation, alargely terrestrial deposit, represents the fill of this newlyformed basin.

---,

Figure 10 (A-C)shows some interpreted seismic sectionsalong the present-day Northern Basiin/Southern Basinboundary in the North Marine/ S()ldado area. Thisboundary is now actually fault controliled (the Los BajosFault) and the original Central Range, now masked by theSoldado uplift, is recognised by the onlap of Manzanilla[Sequence (A)] sediments on the Brighton/NM5 High.Cruse and Forest [Southern Basin Sequence (A)]sediments onlap or thin against an older feature which isno longer in place (Fig. 10 A,B). Assuming that theManzanilla sediments of the North Soidado Graben musthaveextended some distance to the South, it Isquite likelythat they are either (i) an onlap facles of the SouthernBasin, or (ii) the rift system of the Northern Basin whichextendedfurther souththanthe present-dayCentralRange.

In the east coast area, the onlapping of sediments withboth Sequence (A) and Sequence (8)) characteristics isseen in the vicinity of the Darien Rid!~e (Fig. 11). Thesesediments, which represent Southern Basin onlap of theCentral Range, should be of the Gros Morne/Mayaro/Palmiste suite, but this information was not available tothis writer.

STRUCTURAL ANALYSIS

For some time now, it has been postulated that obliquecollision has occurred in the Tertiary period between theCaribbean and South American plates, and that strike sliptectonics have played a major role in the structuring ofTrinidad during the late Pliocene to Pleistocene (Wilson1968, Persad 1986, Tyson 1988) (Pindell and Barrett,1988; Robertson and Burke, 1989). This study providesstrong evidence to support this.

Structural Lineaments

Figure 12 is a map of structural trends throughout thestudy area. With the exception of the Northern Range,Central Range and Erin Syncline onshore (Kugler 1959),the data have been extracted from sub-surface mapsdrawn at or near the end of the Pliocenli![(top of Sequence(A)]. Two main fault types exist: (1) a series of long,anastomosing east-west or northeast-southwest trendinglineaments (TypeI)and (2)a subordinat groupof numerousnorthwest/southeast-trending faults (Type II).

Type I faults are compressional or transpressional,exhibiting either flower structures on setsmic (Fig.9) and/or possessing diapiric cores, as in the cases of theSouthern Range anticline (Fig. 9) and Darien Ridge (Fig.11). Based on extrapolation, it is reasonable to assumethat these faults must exist onshore, and that correlativefeatures, e.g.the Balata/Penal-BarrackporeHigh (Kugler,1959), are also Pliocene Uplifts of the same type.

Type II faults are extensional to transtensional and appearto be both basement-involved and detached. Evidence ofthe former can be seen in the vicinity of the EI Pilar faultzone (Fig. 680, the Los Bajos Fault and in the Columbus


N

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Figure .. :(A) North-South progradation of Orinoco delta sediments on the

Amacuro Platform into the Columbus Basin. PL=Pleistocene;TP=Top Progradation; PS=Prograding Sequence; TK=Top Cretaceous;MM=Near Top Middle Miocene.

(8) Characteristic Seismic line, Northern Basin, Gulf of Paria.Note similarities in seismic response between Upper and Lowersequences with Figure 4(A). UT=Upper Talparo; SS=Sum Sum Sands;LT=Lower Talparo; DS=Durham Sands; SPV=Springvale; MZ=Manzanill~

o Sequence (B)

o Sequence (A)


77

5Km

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Fi¥ure 5: Structural-Stratigraphicre ationships along the EasternNorthern Basin.

(A) Offshore East Coast showingon1appi ng Mi ocene and Pl i 0cene sediments being stronglyuplifted in the Middle Pliocene with subsequent re 1axation of stress and theformation of a Pleistocenebasin with marked erosionalunconformity at its base.PL=Pleistocene; P=Pliocene(undifferentiated); M=Miocene(undifferentiated); MM=MiddleMiocene; LK=Lower CretaceousUK=Upper Cretaceous

(8) Onshore Caroni Basin. Similar situation with onlapping Springvale and Manzanilla sediments (UMI-PIIage) being uplifted along the axis of the Northern Basin. Collapse has also occurred along the ElPilar trend. Note that there is little Pleistocene present.UT=Upper Talparo; LT=Lower Talparo; SPV=Springvale; MZ=Manzanilla; c==J Sequence (B);

H::}}:) Sequence (A) .


N

5 km

EL PILAR TRENDGULFHIGH

s

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( A)

Figure 6: Structural-Stratigraphic relatio~ships along theWestern Northern Basin.

(A) Unconformable relationship between Cunapo Conglomerate(CUN) and the overlying Talparo formation. Notesouthward di rected s1umpi og of Cunzpo and Manzanill asediments.

(B) Collapse and rotation along a fault within the £1Pilar trend. The age of the sediments is not known,but seismic character correlations hove been used.

SS=Sum Sum Sands (Upper Talparo)j DS=Durham Sands (LowerTalparo); MZ=Manzanillaj MM=Middle Miocene; K=Cretaceous;LK=Lower Cretaceous; (A)=Sequence Aj (B)=Sequence B


N L.--...;5::..;k:;.m~_..J sGALEOTA

RIDGE

2

5-6

2

Figure 1: Structural/Stratigraphic relationships alongthe South Flank of the CaleotaRidge/Southern Range uplift.

DATA COuRTESY AMOCO TR'NIDAD LTD.

\.o

PL=Pleistocenec==J Sequence (B)1;0:/4 Sequence (A)

(A) Eastern Columbus Channel. Initial uplift hereappears to be slightly earlier (End PI) but themain unconformity is still at the base ofSequence (B) (PIT time). Note otherunconformities at base PIII and basePleistocene.

(8) Proximal Columbus Basin area showing goodseismic to palynology tie. Uplift started atthe end of pII time and continued into Recentwith intermittent interruptions, especially atBase Pleistocene.

s(8)

s-z

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2

3

4

5

6

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3

4

5

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Fi~ure 8: Deformation in the Columbus Basin. Collapse along main listric fault (a) appears to be related to a deeperlYlng break in the underlying platform sequence, assumed here to be Cretaceous. Faults (b) and (c) also appear to bevertical.

MM=Middle Miocene to Lower Tertiary Shales; TP=Top Prograding Unit; K=Near Top Cretaceous;1),::1 Sequence (A)

o Sequence (B);


NIOkm

s

<XI.....

<NORTHERN BAS I N SOUTHERN BAS IN

>CENTRAL RANGE

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CENTRAL "ANOEFAULT

NORTH MARINEHIGH

SOLOAOOHIGH

LOS BAJOS FAULT

ERIN SYNCLINE SOUTHERNRANGEOIAPIR

COLUMBUSCHANNEL

DATA COURTESY TRINIDAD AND TOBAGO OIL COMPANY LTD. AND T'UHMAlit LTD.

Figure 9: Seismic montage across the Southern Gulf of Paria showing structural/stratigraphic relationships between theNorthern and Southern Basins.Sequence (A): L7=Lot 7 Silt; TF=Top Forest Fm.; TC=Top Cruse Fm.; TMC=Top Middle Cruse Fm.; MZ=Manzanilla.

Sequence (B): SS=Sum Sum Sands; UT=Upper Talparo or equivalent; LT=Lower Talparo Fm.; SPV=Springvale

MM=Middle Miocene or older

The Soldado High and Southern Range are Pliocene uplifts against which Talparo and equivalents onlap from the north andsouth. The Columbus Channel and the Northern Basin became separated as a direct result of this uplift. Manzanilla andMiddle to Lower Cruse sediments onlap older, buried Middle-Miocene Highs in the North-Marine/Soldado areas which are nowjuxtaposed as a result of movements along the Los Bajos Fault which have stopped only in Upper Talparo (IntraPleistocene). Note massive Flower structure of the Central Range Fault.

.,1


(Al

CENTRALRANGEFAULT

N

·D·ou··•

!lk..

NM-!l HIGH

---

SOLDADO HIGH ERIN SYNCLINE . SOUTHERNRANGE

s

CENTRALRANGEfAULT

BRIGHTON/NM HIGH

NORTHSOLDADO LOS BAJOSGRABEN FAULT

SOLDADO HIGH ERIN SYNCLINE SOUTHERNRANGE

(B)

te)

CENTRALRANGEFAULT

N

BRIGHTON /NM HIGH

ERIN SYNCLINE SOUTHERNRANGE

5

fto:

Figure 10: Interpreted Seismic Sections in the Southern Gulf of Paria. Note stratigraphic and structural relationships[os Bajos Fault Vicinity•.SS=Sum Sum Sands; DS=Durham Sands; SPV=Springvale; MZ=Manzani11a; E=Eriin j \ L7=Lot 7 Silt;Forest; TC=Top Cruse; LC=Top LOfer Cruse; TL=Top Lengua (Middle Miocene); I:}})::! Sequence (A); Sequence (6);Di aparic Shale

in theTF=Top~


83

NCENTRAL

RANGE DARIEN RIDGE.sOUTHERN

BASIN

s

OATA COUfIITESY TAINIOAO ANO TO.AOO IiIlIlt4lSTIn 01' CNCftOY

Figure 11: Seismic line acrosa the Darien Ridge showing~entra\ Range being onlapped by Sequence (AI and Sequence(B) aedimenta. Note Flower Structure with reverse fault atlower right. HM=Middle Miocene and older.

Figure 12: Structural trend map, Trinidad, ahowlng compreulve (Type I I and extenalonal (Type II I hulta"

-- Type I; -- Type II. Note anti ·clockwise rotation of compressive structures.

A"Arima Fault; e-chupna Fault; P"El Pilar lineament; CR=Central Range Fault; LB=Los Bajoa Fault;,B=Brighton Anticline; CH-Culf High; DH-Oomoil High; S=Soldado High; NM=North Marine High; O=Debe"Wellington Anticline; B/e-earrackpore-Balata Anticline; DR-Darien Ridge, CRaCaleotli Ridge; SR-Southerr,Range; DE=Deformation Edge; RQ..Rock Dome Anticline


Basin, where apparent vertical faults cut Into the TopCretaceous "basement" (Fig. 8).These TypeIIFaults arecommonthroughoutTrinidad (Kugler 1959) and may besingle largedisplacements, e.g.LosBajos, or a series ofcloselyspaced en echelon faults. Evidence of the lattercanbe foundInmanysub-surface oilfield maps(Ablewhlteand Hlgglm;, 1968).

Simultaneous Development ofExterlslonal and Compressional Features

The simultaneous development of extensional andcompressional structures Is a characteristic feature ofstrike-slip tectonlcs (Christie-Blick and Biddie, 1985).

Figure 13 Is, a comparison of two (2) regional schematicseismic sections on the east and west coast areas ofTrinidad. The present-day Northern Basin In the Gulf ofParla appears as a typlcl rift basin, with horsts, grabensand deep-seated, high-angie, normal-throw faults. TheSouthern B,asln on theotherhand, Iscompressional, withIndication of' high angle reverse faulting. Evidence ofcompression IntheNorthern BasinattheendofSequence(A) canalsobeseen, especially Inthe eastcoast (Fig. 5).Fault planes, whenprojectedatdepth,appearto coalesceInto a few major fault zones which are vertical andbasement-related.

These compressive and extensional areas are boundedby malor structural lineaments consisting of both Type IandTypeIIjfaults (Fig. 12). Compressionalongtheformerandtranscurrent movement alongthelatterhave resultedIn the formation of basins and uplifts of the same age,adjacenttoeachother. ThisrelationshipwasdocumentedbyTyson(1 '988) whodescribedtheeffectsoftranscurrentreverse fault couples In the southwestern Trinidad area.An analysis. of Fig. 12 shows that other similar couplesexist, e.g.Intheeastern NorthernBasin, andthatadditionalmajorshealring appears to occuralongTypeIIfaults whena certain strain threshold for the TypeI compressions Iscrossed. AlJood exampleofthisappearstobetheeasternonshore Nc)rthern Basin areawheremovement alongtheChupura F~IUlt (Kugler, 1959) hasapparently pushed androtated the eastern portionsof the Central Range southeastwards. The degree of displacement Is not large Insituations likethese, whererotationleadsto a resolutionof the forces within the sub-block and transcurrentmovementceases.ThestressIsthenprobablyiransformedto anotherblock. In areas of lessrotation, e.g., alongtheLos Balos Fault, lateral displacement should be larger.Thisrotational effectprobablyreflects basement-Involvedadjustment of crustal blocks (Christie-Blick and Biddie,1985) toa general southeast-dlreeted stressfield,resultingout of the i:>b1lque collision of the Caribbean and SouthAmerican ptatss.

In the Southern Basin, deformation wasof a moreplastic.nature, and there was'much. shale flowage Into uplifted

04

areas. This diapiric activity, which Is restricted to theSouthern Basin and Central Range, Is a result of undercompacted oreasily mobilized shales Inthese areas (e.g.,Narlva Formation, LowerCruseFormation) reacting tothecompressive stressorflowing Intoextensional faultzones.The geometric alignment of the major shale ridges Isprobablyrelated to thepositionsofmajorbasement shearzones ordetached splaysofthese (Fig. 12).Theoccurrenceof a large earthquake of 6.2 magnitude on the RichterScale In the distal Columbus Basin In 1988 providesevidence of such recentbasement-related movement Inthat area.

SIGNIFICANCE OF RESULTS TOHYDROCARBON EXPLORATION

The results and conclusions of this study could have asignificant Impact on the finding of more hydrocarbonsboth In the Post-and Pre- Middle Miocene sediments ifsomeof the postulations outlined belowaretested.

Thiswriter believes that the best opportunities will ariseout of the applications of new structural-related Ideas,although some unexplored plays of stratigraphicsignificance stili exist.

Offsetting of Structures/Fields along Type II Faults

Wilson (1968) had Indicated that the LosBalosFault candisplace oil-fields In the Trinidad area. The structuralmodelproposed In this papersupportsthis conceptandIndicates thatnumerousTypeIIfaultdisplacementsshouldexist throughout the Island (Fig. 12), particularty In theSouthernBasin, where compressionwasgreatest. Surfacegeological mapping (Kugler, 1959) maynothavedetectedall the fault traces, and evidence of smaller, en echelondisplacements have been mapped In the sub-surface(Ablewhite andHiggins, 1968). Largeroffsets, liketheLosBaJos, may be present, and could explain the suddendisappearance of a productlve trend or an apparent"shale-out". Byattemptlngtolocatetheseoffsets,valuablenew 011 can be found. One possible example Is theapparent right-lateral displacement of anticlines alongatrend north of and parallel to the Los BaJos Fault(DebeWelllngton-Barrackpore Antl-cllne, Rock Dome) (Kugler1959).

Structural Traps

Theclassicwrenchtectonic regime is a difficultplacetoprospectIn,due to the rapidlychanging trap type alongmajor shear zonesand the resulting poor predictability.Trinidad Is,however, not a classicstrike-slip regime. Due.to thecouplingeffectofTypeIandTypeII faUlts, localized.structural sub·provlncesofaparticularstyleexist. Inareas

. ofpurecompression, e.g.theCentral Range, andapparentdetached thrust r~glme exists (Kugler 1959). In otherareas, classi('~! tr~ps associated withrifted margin basins


SOUTHERN RANGE

85

NORTH

~TOBAGOSHELF

CO .. P"Il •• IOIt

EL PILAR? CENTRAL OARIENRANGE FAULT

+

SCHEMATIC SEISMIC SECTION

EAST COAST, TRINIDADII""I'OH

COLUMBUS BASIN

SOUTH

OEFORMATION EOGE M1ACURO PLATFORM

•.. SCHEMATIC SEISMIC SECTIONWEST COAST, TRINIDAD

••. I

Figure 13: Regional seismic sections on the East and West Coasts of Trinidad. Note rifting androtation lextension) in the Northern Basin and compression in the Southern Basin in the Culf of Paria(Section A). In the East Coast, the compressive Southern Basin is rotated northwards, narrowing theNorthern Basin. The Columbus Basin, on the other hand, remains an extensional area.

L::::::) Sequence (A); W:,j,t] Diapiric shale; 0 Sequence (B)

may occur, e.g. western Northern Basin. This couplingeffect ofTypeI andTypeII faults canpresent a variety ofstructural plays(Harding andTumlnas, 1988, 1989) whichhave not yet been fully explored.

Source Rock Maturity, Timingof Migration and Uplift

The Upper Cretaceous organic-rich, oil-prone facies ofthe Naparima Hill and Gautier Formations has beendetermined geochemlcally to be the only source rock InTrinidad (Rodrigues 1989). A knowledge of the thermalmaturityofthls sequence Isakeyfactorinpredicting whenpeakgeneration, (eqUivalent to expluslon of011) occurredandthereforewhen 011 hadbecomeavailable tobetrapped.Regional geochemical studies In eastern Venezuela(Talukdar et ai, 1988) have concluded that 011 migrationIntothesouthernmostupliftsoftheMaturln SUb-basin (thePlrltal and EIHuesothrusts) has occurred from 5 MAto

present. A series of. thermal history models (Lopatinmethod) by Rodrigues (1986) in the Trinidad area alsoconcluded thatprimarymigrationofUpperCretaceous oilIn the Southern Basin began approximately seven (7)millionyears ago and continued Intothe present.

ThisstUdy hasattempted to demonstrate that collapse/upliftof the Northern and Southern Baslns began duringthePlioceneandtherefore, 011 couldhavemigrateddirectlyInto traps which were forming at the time. In areas ofmaximum collapse (theColumbus Basin andtheWesternGulfof Parla) the timingwould have been the same andtheseareascouldholdmuchpotential foroilaccumulationsIf migration pathways,·traps and seals exist.

Migration Pathwa),.

Themigration pathways for theoilappear to be themajorbasement-Involved TypeI faults andsecondary splays of


these (Figs. 9, 13).The splays could appear as thrusts,either southward or northward-directed, depending onthe particulardirection of stress (Fig. 9). In some casesthere may be only one direction, e.g. onshore CentralRange (Kugler 1959), where strong southward-directedcompressionoccurred. Theidentificationofthesefaultsisimportant in locating accumulations, which should beconcentrated in hanging-wall structures (Talukdaret ai,1988). Thesefaults could be masked by occurring alongbed boundaries (whichwouldformnatural slippageplanes)or may not reach the surface at all, stopping as blindthrustsinthe coresofanticlines.Thismayoffera possibleexplanationfor the markeddecreasein trapped oil withinupperSequenclB (B)sediments (Post-Lot 7silt)throughoutthe SouthernBasin. Thesesediments, although folded to.some extent, may not be in contact with the Type I faultsystemand thereforewould not be accessed by the oil.

Stratigraphy

In most areas of the Northern and Southern Basins,Sequence (A). reservoirs are abundant. In the distalColumbus Basin especially, deep-water turbltes shouldabound.Thesoutbem Rangeanticline,whichwasadeeparea during the deposition of lower Sequence (A)sediments, sboud alsohaveaccumulated someturbidites.TowardstheoriginalCentralRange, stratigraphicthinningof Sequence(A)sedimentsshould provide somedegreeoftrapping by updip ·shaleout",a relatively untestedformof play InTrinidad.

CONCLUSIONS

The conclusions of the study are:

1) A contldent picture of the geological evolutionofthe post-Middlet Miocene period in the offshoreTrinidadarea has been achieved using a regional, integrated,seismic-based approach, in which genetically relateddepositional sequences were Identified, correlated andanalysed.

2) Using this Information in combination with astructuralanalysis, a modelfor continuedexplorationandexploitation of the oil-prone Sequence (A), pre-MiddleMiocene, andtoa lesserdegree,Sequence (B)sediments,has beendeveloped.

3) TheonshoreareasofTrinidad(Kugler, 1959) canalso be confidently re-evaluated using the newdata andmodel presentedIn this paper.

j~CKNOWLEDGEMENTS

Theauthorwould liketo thankthefollowingOrganisationsandpersonsfortheirassistance:TRINTOC, especiallyMr.Wayne Bertrand, EXploration and Production DivisionalManager, for supportingthis study and the publicationof

86

the paper; the Ministry of Energy, Trinidad and TobagoPetroleum Company Limited (TRINTOPEC), AmocoTrinidadOilCompanyLimited,TrinmarLimited,TrlntomarLimited and LAGOVEN, SA, all of which have Willinglyapproved the publication of their data; my colleaguesintheTRINTOC Regional TertiaryStudyTeam,InparticularMessrs Frank Falkenheln, Kenneth Abdulah, UewellynTyson, Oement Ramroop and Ken La Borde for theiruseful discussionsandencouragemenUowrltethepaper,and specialthanksto Mr.PauloPaJagi of Braspetro, who,as former Geological Superintendent of TRINTOC'sOffshoreexploration and DeVelopment Division, Initiatedintegratedregional,seismic-based studiesIntheTrinidadoffshoreareas.

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Harding, T.P. and Tuminas,A.C., 1988. Interpretationoffootwall (Iowside) fault traps sealedby reverse faultsand convergent wrench faults. Amer. Assoc. Petrol.Geol. Bull. v. 72, Nr. 6, p. 738-757.

Harding, T.P. and Tuminas, A.C., 1989. Structuralinterpretation of hydrocarbon traps sealed bybasement normal block faults at stable flank offoredeep basins and at rift basins. Amer. Assoc.Petrol.Geol. Bull. v. 73, Nr. 7, p. 812-840.

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GSTT (1991) an Evaluation of Post Middle Miocene Geological Sequences Offshore Trinidad

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