Impact of biostratigraphy on oil exploration

C. JARAMILLO & M. RUEDAInstituto Colombiano del Petróleo, Piedecuesta, Colombia

ABSTRACTMicropaleontology is the main biostratigraphic

tool used in Colombia. More than 40.000 palynologicaland foraminiferal samples have been analyzed by theindustry in the last 70 years. However, there is not yet avalidated published zonation that can be used by theindustry. Lacking a formal zonation costs millions ofdollars derived from unreliable correlations, misfits inage determinations, and a general mistrust on the tool.The need for a reliable zonation has become moreevident recently because exploration has moved towardmore complex or/and unexplored areas. In such areas,a good biostratigraphy can be a very important tool forfield-mapping, well-dril l ing, testing structuralinterpretations, and correlating reservoirs.

The Colombian Petroleum Institute established afive-years research program to produce a reliablepalynological zonation for Colombia. A preliminarypalynological zonation has already been produced. Thiszonation has 34 zones ranging from the Late Aptian tothe Middle Miocene. The zonation has beensuccessfully tested in several areas. We reinterpreted arecently well drilled in the Llanos foothills. The well wasinterpreted reaching the Guadalupe formation. Ourmodel indicated that the well only reached the Cuervosformation. A sidetrack and a new exploratory well in thearea were subsequently drilled validating the proposedmodel. Also, we have used the biostratigraphic modelin the Arauca field. The Mirador formation is not presentin the area and there is a hiatus from the UpperPaleocene to the lower Oligocene. These two examplesare positive tests that the zonation is reliable and can beapplied to oil exploration.

RESUMENLa principal herramienta bioestratigráfica que ha

utilizado la industria del petróleo en Colombia ha sido lamicropalentología, como lo demuestra el calculo de unacantidad superior a 40.000 muestras analizadas parapalinología y/o foraminíferos durante los últimos 70años. Sin embargo, a la fecha no existe una zonaciónpalinológica válida publicada, lo cual representa un altocosto económico, como resultado de la incongruenciaen las determinaciones de edad, la incertidumbre en lascorrelaciones y la consiguiente desconfianza en laherramienta. La necesidad de una zonación confiableha sido cada vez más evidente, en la medida que laexploración se ha concentrado en áreas más complejasy/o inexploradas, donde una buena bioestratigrafía esfundamental durante la cartografía geológica, el controlde la perforación, la comprobación de interpretacionesestratigráficas, estructurales y la correlación dereservorios.

El Instituto Colombiano del Petróleo estableció unprograma de investigación a cinco años, con elpropósito de generar una zonación palinológica para las

distintas cuencas en Colombia. Aquí se presenta unazonación preliminar, representada por 34 zonas, quecomprende el intervalo Aptiano Tardío a Mioceno Medioy ha sido utilizada con éxito en varias áreas. Porejemplo, la reinterpretación de un pozo en elpiedemonte llanero, para el cual se había interpretado laprofundidad final dentro de la formación Guadalupe,nuestro modelo indicaba que el pozo terminó en laformación Los Cuervos, comprobándose estainterpretación en el “sidetrack” y un pozo exploratorioadicional. Nuestro modelo bioestratigráfico también seaplicó en la interpretación del campo Arauca,permitiendo establecer la ausencia de la formaciónMirador y el reconocimiento de un hiato que comprendeparte del Paleoceno Superior al Oligoceno Inferior. Estepar de ejemplos nos demuestran que la zonaciónpropuesta es confiable y puede aplicarse con éxito en laexploración de hidrocarburos.

INTRODUCTIONMicrofossils, mainly foraminifera, pollen,

spores and dinoflagellates are the mainbiostratigraphic tools used in Colombia for oilexploraton. Over 40.000 samples haven beenanalyzed by the industry for palynological andforaminiferal content in the last 70 years. In spiteof that, there is not yet a validated publishedzonation that can be used by the industry. Thelack of a formal zonation that can be used byconsultants and oil explorers can costs millions ofdollars derived from unreliable correlations, misfitsin age determinations, data that is not comparableamong biostratigraphic consultants, and a generalmistrust on the tool. The need for a reliablezonation has become more evident in the past fewyears because exploration has moved toward morestructurally complex or/and unexplored areaswhere seismic information could not be good oreasy to interpret. In such areas, a reliablebiostratigraphy can be a very important tool forfield-mapping, geological control during well-drilling, testing stratigraphic and structuralinterpretations, and correlating reservoirs.

We present here a prel iminarypalynological zonation for Colombia that has beenproduced by the Colombian Petroleum Institute(ICP) as the first result of a five-year researchprogram aimed to produce and publish a reliablepalynological zonation for Colombia.

METHODSOver fifty sections with palynological

information from outcrops, well cores, and wellC. JARAMILLO & M. RUEDA

ditch cuttings were used for this study. The datacomes from outcrop, core and ditch-cuttings datafrom internal reports in Ecopetrol biostratigraphydatabases.

The palynological data were then analyzedusing the technique of graphic correlation(Edwards, 1984; Edwards, 1989; Shaw, 1964).This technique does not assume a-priori that anytaxon is an index guide or represents a time line.Instead, the technique relies on looking at thewhole assemblage to find the true stratigraphicrange for each taxon (Edwards, 1989), which isdone by comparing taxon ranges in differentsections and finding lines of correlation.

All morphological data for the species usedin the zonation are built in a morphologicaldatabase developed by ICP (Jaramillo, 2004)

THE ZONATIONThe preliminary palynological zonation

produced by ICP zonation has 34 zones rangingfrom the Late Aptian to the Middle Miocene. Zoneshave an average of 3 million year, although zonesin the Tertiary have more resolution that those inthe Cretaceous. This difference is due to betterquality and cuantity of the data coming fromTertiary sediments in Colombian basins. Thiszonation can be applied to Colombian sedimentsand is more reliable for Colombia than previouszonations published for northern South America(Germeraad et al., 1968 and Muller et al., 1987).The zonation is shown in Figures 1 and 2.

TESTING THE ZONATIONThe validity of a zonation can only be tested

by their level of prediction. Here, we present twocases were the zonation has been applied andhave shown to be useful to the oil explorers.

The Gibraltar case.The zonation was used for the interpretation

of Gibraltar 1, a well recently drilled in the northernLlanos foothills,. During the original drilling, thestratigraphic position of the final depth of the wellwas difficult to establish. Incompatiblebiostratigraphic interpretations were produced byfour different biostratigraphic consultants, rangingfrom Paleocene to upper Cretaceous for thedeepest 4000 feet of the well. The finalinterpretation was that the Upper Cretaceous,Guadalupe Formation had been reached and thatthe strata were dipping almost horizontally (Villamilet al., 2004).

The ICP zonation was used to re-interpretthe well. Four palynological zones for the CuervosFormation were identified and it was interpretedthat the last 4289 feet drilled were in the lowerCuervos Formation, that the well was moving

stratigraphically toward older rocks, and that nofaults or repetitions were evident (Jaramillo et al,2004). The biostratigraphic interpetration wasconfirmed by several other tools (Villamil et al.,2004).

A sidetrack was drilled successfully reachingthe Barco Formation after drilling 1500 feet as thegeological model proposed by Ecopetrol hadpredicted. A subsequent well, Gibraltar 2, wasdrilled further confirming the model (Villamil et al.,2004).

The Arauca case.The Paleogene stratigraphy of the Arauca

field has always been controversial (ESSO, 1980,1981, 1982, 1983; Navas, 1985; Gilbert, 1992;ICP, 2000). Some authors believe that Miradorformation is present in the area, while others haveargued that the Mirador is missing. The zonationwas used to help solving this problem. Thepalynological analyses shown that there is not arock record in the Arauca field during the time ofdeposition of Mirador in the Llanos foothills (Figure3). The sandstones previously interpreted asMirador, were deposited during the age ofpalynological zone Pa 7 (F. perforatus), LatePaleocene. During this time, the lower to middleCuervos formation was being accumulated in theLlanos foothills. In the Arauca field there is anunconformity that encompasses the palynologicalzones Pa 8 to Ca 3 (latest Paleocene to earliestOligocene, Figure 3). During this time the upperCuervos, Mirador and lower to middle C8 werebeing accumulated in the Llanos foothills. Thisinterpretation is similar to a recent 3D seismicinterpretation of the field (Gilbert, 1992), andseems to be related to the evolution of the Araucahigh and the Apure basin in Venezuela where theEocene is also missing from the Guafita and LaVictoria fields (Monroy y van Erve, 1988).

The absence of the Mirador formation in theArauca field may have strong implications in thestratigraphic, structural and paleogeographicinterpretations currently held for the north of Llanosand Llanos foothills, including the Caño Limonfield. A model explaining the absence of Mirador inthe field still needs to be proposed.

CONCLUSIONSBiostratigraphy can be extremely useful to

hydrocarbon exploration!We have helped to drill wells, improve

structural and stratigraphic models, help seismicinterpretations, refine outcrop maps, and evendecide where new wells are going to be drilled.Biostratigraphy has become a member of theEcopetrol exploration team rather than an outsider

C. JARAMILLO & M. RUEDA

70.00

75.00

80.00

85.00

90.00

95.00

100.00

105.00

110.00

Maa

stric

htia

n

71.30

Cam

pani

an

83.50

Santonian

89.00

Turonian

Cenomanian

98.70

Alb

ian

112.20

114.90Apt

ian

Pa04 - E. protofranciscoiPa03 - B. andreeviPa02 - S. echinatus

Pa01 - DuplotriporitesPa00 - F. margaritae

71.30

Cerodinium

77.40

Dinogymnium

88.50

D. senonicus

91.30

Interval Zone

Triorites

Afropollis95.80

E. castelaine

101.60

E. protensus

104.20

C. polygonalis

106.90

Odontichitina

109.50

Cribroperidinium

112.20

Cyathidites

Subtilisphaera

1817

1615

14

71.30

13

77.40

12

88.50

1191.30

109

8

95.80

7

101.60

6

104.20

5

106.90

4

109.50

3

112.20

21

TIME Palynological Zone

Senegalinium

Trichodinium

Unipontidinium

Coniacian

Figure 1. Cretaceous palynological zonation for Colombia. Time scale after Berggren et al. 1995

TIME

15.00

20.00

25.00

30.00

35.00

40.00

45.00

50.00

55.00

60.00

65.00

11.20

Mid

dle

Mio

cene

16.40

Ear

ly M

ioce

ne

23.80

Olig

ocen

e

33.70

Eoc

ene

55.60

Pal

eoce

ne

11.20

Crassoretitriletes - Ca11

E. maristellae - Ca10.5R. crassianulatus - Ca10.4

Cyclusphaera - Ca10.3R. planopolaris - Ca10.2

Marine E. maristellae - Ca10.1

Marine - Ca9.2P. pachyermatus - Ca9.1

R. minutiporus - Ca8L. tibuensis - Ca7

23.80

Janduforia - Ca6

26.80J. seamrogiformis/M. grandiosus/ M.f. var.

minutus/V. usmensis acme zone - Ca 5

28.80

R. "esponjosus" Zone Ca4

31.20

Nothofagidites Zone Ca3

33.70

E. trianguliformis Zone Ca2

39.20

S. aff. echinatus Zone Ca1

44.70

Mi2 - C. dorogensis

50.10

Mi1- S. spiralis

Pa7 - F. perforatus

Pa6 - B. annae

Pa5c - Mauritiidites

Pa5b - Colombipollis

Pa5a - S. baculatus

Palynological Zone11.20

35

16.40

3433

32

31

23.80

30

26.80

29

28.80

28

31.20

27

33.70

2639.20

25

44.70

24

50.10

2321

20

60.30

19

Figure 2. Tertiary palynological zonation for Colombia. Time scale after Berggren et al. 1996

Pa9 - EsterilPa8 R. magdalenensis 22.1

22.2

17370 17500 1751017300

ARAUCA 1 ARAUCA 2 ARAUCA 3ARAUCA 4

1 km

Pa 7 zone (Late Paleocene)

Ca4 to Ca5 palynologicalzone (early Oligocene)

ARAUCA 2

ARAUCA 4

ARAUCA 1

ARAUCA 3 N1Km

Figure 3. Dating the Arauca wells using the palynological zonation shown in this work. There is not a rock record in the Arauca fieldduring the time of deposition of the upper Cuervos, Mirador and lower and middle C8 in the Llanos foothills. The unconformityencompasses the Pa 8 to Ca 3 (latest Paleocene to earliest Oligocene).

Ca 3 Zone(earliestOligocene)

oil

Datum: Oligocene top

C. JARAMILLO & M. RUEDA

as it was before, helping exploration to be moresuccessful.

ACKNOWLEDGEMENTSThanks to ICP for permission to publish this paper.

Special thanks to all palynological consultants andacademic researchers who have produced palynologicaldata from Colombia over the past fifty years. Manythanks to the entire biostratigraphic unit at ICP. Thanksto Maria Ines and Camilo for their continuous source ofideas.

REFERENCES

Berggren, W. A. Kent, D. V. Aubry, M. P. andHardenbol, J., 1995, Geochronology Time Scales andGlobal Stratigraphic Correlation: SEPM SpecialPublication 54, SEPM, p. 1-386.ESSO, 1980, Registro Gráfico Compuesto Arauca 1.-, 1981, Registro Gráfico Compuesto Arauca 2.-, 1982, Registro Gráfico Compuesto Arauca 3.-,1983, Registro Gráfico Compuesto Arauca 4.Germeraad, J.H., Hopping, C.A., and Muller, J., 1968,Palynology of Tertiary sediments from tropical areas:Review of Palaeobotany and Palynology, v. 6, p. 189-348.Gilbert, O.E., 1992, Segmentation and Distribution of aProblem Reservoir, Arauca Field, Llanos Basin,Colombia: Reservoir Geophysics, Soc. Expl. Geophys.,p. 73-79.ICP, 2000, Registro de Descripción Sedimentológica yEstratigráfica Corazones Arauca 2, 3, 4.: Internal report,ICP, Bucaramanga.Jaramillo, C.A., Muñoz, F., Cogollo, M., and Parra, F.,2004, Quantitative Biostratigraphy for the Paleocene ofthe Llanos Foothills, Colombia: Improving PalynologicalResolution for Oil Exploration, Volume in press, TMSSpecial Publication.Jaramillo, C.A., and Dilcher, D.L., 2001, MiddlePaleogene palynology of central Colombia, SouthAmerica: A study of pollen and spores from tropicallatitudes: Palaeontographica B, v. 258, p. 87-213.Jaramillo, C.A. 2004, A morphological electronicdatabase of Cretaceous-Tertiary fossil pollen and sporesfrom Northern South America; AASP Palydisk 24,Electronic Publication, American Association ofStratigraphic Palynologists.Monroy, Z. and van Erve,A., 1988, Revisiónpalinoestratigráfica del Cretácico y Terciario de Apure(Venezuela Suroccidental): III Simposio Bolivariano.Exploración Petrolera de las Cuencas Sub-Andinas, p.142-167.Muller, J., Di Giacomo, E., and Van Erve, A., 1987, Apalynologic zonation for the Cretaceous, Tertiary andQuaternary of Northern South America: AmericanAssociation of Stratigraphic Palynologists ContributionSeries, v. 19, p. 7-76.Navas, J., 1985, Campo Arauca: II Simposio BolivarianoExploración Petrolera en la Cuencias Sub-Andinas, v. I,p. 1-23.Villamil T., Muñoz, J., Sánchez, J., Aristizabal, J.J.,Velasco, J., Luna, P.E., Mantilla, A., Fajardo, Peña,L:E:, Paz, M.G., Silva, O., Sánchez, E., and Meza, N.,2004, The Gibraltar discovery, Northern Llanos foothills,

Colombia: case history of an exploration success in afrontier area: Journal of Petroleum Geology, in press.

C. JARAMILLO & M. RUEDA