Impact of biostratigraphy on oil exploration

Transcripción

1 Impact of biostratigraphy on oil exploration Instituto Colombiano del Petróleo, Piedecuesta, Colombia ABSTRACT Micropaleontology is the main biostratigraphic tool used in Colombia. More than palynological and foraminiferal samples have been analyzed by the industry in the last 70 years. However, there is not yet a validated published zonation that can be used by the industry. Lacking a formal zonation costs millions of dollars derived from unreliable correlations, misfits in age determinations, and a general mistrust on the tool. The need for a reliable zonation has become more evident recently because exploration has moved toward more complex or/and unexplored areas. In such areas, a good biostratigraphy can be a very important tool for field-mapping, well-drilling, testing structural interpretations, and correlating reservoirs. The Colombian Petroleum Institute established a five-years research program to produce a reliable palynological zonation for Colombia. A preliminary palynological zonation has already been produced. This zonation has 34 zones ranging from the Late Aptian to the Middle Miocene. The zonation has been successfully tested in several areas. We reinterpreted a recently well drilled in the Llanos foothills. The well was interpreted reaching the Guadalupe formation. Our model indicated that the well only reached the Cuervos formation. A sidetrack and a new exploratory well in the area were subsequently drilled validating the proposed model. Also, we have used the biostratigraphic model in the Arauca field. The Mirador formation is not present in the area and there is a hiatus from the Upper Paleocene to the lower Oligocene. These two examples are positive tests that the zonation is reliable and can be applied to oil exploration. RESUMEN La principal herramienta bioestratigráfica que ha utilizado la industria del petróleo en Colombia ha sido la micropalentología, como lo demuestra el calculo de una cantidad superior a muestras analizadas para palinología y/o foraminíferos durante los últimos 70 años. Sin embargo, a la fecha no existe una zonación palinológica válida publicada, lo cual representa un alto costo económico, como resultado de la incongruencia en las determinaciones de edad, la incertidumbre en las correlaciones y la consiguiente desconfianza en la herramienta. La necesidad de una zonación confiable ha sido cada vez más evidente, en la medida que la exploración se ha concentrado en áreas más complejas y/o inexploradas, donde una buena bioestratigrafía es fundamental durante la cartografía geológica, el control de la perforación, la comprobación de interpretaciones estratigráficas, estructurales y la correlación de reservorios. El Instituto Colombiano del Petróleo estableció un programa de investigación a cinco años, con el propósito de generar una zonación palinológica para las distintas cuencas en Colombia. Aquí se presenta una zonación preliminar, representada por 34 zonas, que comprende el intervalo Aptiano Tardío a Mioceno Medio y ha sido utilizada con éxito en varias áreas. Por ejemplo, la reinterpretación de un pozo en el piedemonte llanero, para el cual se había interpretado la profundidad final dentro de la formación Guadalupe, nuestro modelo indicaba que el pozo terminó en la formación Los Cuervos, comprobándose esta interpretación en el sidetrack y un pozo exploratorio adicional. Nuestro modelo bioestratigráfico también se aplicó en la interpretación del campo Arauca, permitiendo establecer la ausencia de la formación Mirador y el reconocimiento de un hiato que comprende parte del Paleoceno Superior al Oligoceno Inferior. Este par de ejemplos nos demuestran que la zonación propuesta es confiable y puede aplicarse con éxito en la exploración de hidrocarburos. INTRODUCTION Microfossils, mainly foraminifera, pollen, spores and dinoflagellates are the main biostratigraphic tools used in Colombia for oil exploraton. Over samples haven been analyzed by the industry for palynological and foraminiferal content in the last 70 years. In spite of that, there is not yet a validated published zonation that can be used by the industry. The lack of a formal zonation that can be used by consultants and oil explorers can costs millions of dollars derived from unreliable correlations, misfits in age determinations, data that is not comparable among biostratigraphic consultants, and a general mistrust on the tool. The need for a reliable zonation has become more evident in the past few years because exploration has moved toward more structurally complex or/and unexplored areas where seismic information could not be good or easy to interpret. In such areas, a reliable biostratigraphy can be a very important tool for field-mapping, geological control during welldrilling, testing stratigraphic and structural interpretations, and correlating reservoirs. We present here a preliminary palynological zonation for Colombia that has been produced by the Colombian Petroleum Institute (ICP) as the first result of a five-year research program aimed to produce and publish a reliable palynological zonation for Colombia. METHODS Over fifty sections with palynological information from outcrops, well cores, and well

2 ditch cuttings were used for this study. The data comes from outcrop, core and ditch-cuttings data from internal reports in Ecopetrol biostratigraphy databases. The palynological data were then analyzed using the technique of graphic correlation (Edwards, 1984; Edwards, 1989; Shaw, 1964). This technique does not assume a-priori that any taxon is an index guide or represents a time line. Instead, the technique relies on looking at the whole assemblage to find the true stratigraphic range for each taxon (Edwards, 1989), which is done by comparing taxon ranges in different sections and finding lines of correlation. All morphological data for the species used in the zonation are built in a morphological database developed by ICP (Jaramillo, 2004) THE ZONATION The preliminary palynological zonation produced by ICP zonation has 34 zones ranging from the Late Aptian to the Middle Miocene. Zones have an average of 3 million year, although zones in the Tertiary have more resolution that those in the Cretaceous. This difference is due to better quality and cuantity of the data coming from Tertiary sediments in Colombian basins. This zonation can be applied to Colombian sediments and is more reliable for Colombia than previous zonations 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 ZONATION The validity of a zonation can only be tested by their level of prediction. Here, we present two cases were the zonation has been applied and have 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 northern Llanos foothills,. During the original drilling, the stratigraphic position of the final depth of the well was difficult to establish. Incompatible biostratigraphic interpretations were produced by four different biostratigraphic consultants, ranging from Paleocene to upper Cretaceous for the deepest 4000 feet of the well. The final interpretation was that the Upper Cretaceous, Guadalupe Formation had been reached and that the strata were dipping almost horizontally (Villamil et al., 2004). The ICP zonation was used to re-interpret the well. Four palynological zones for the Cuervos Formation were identified and it was interpreted that the last 4289 feet drilled were in the lower Cuervos Formation, that the well was moving stratigraphically toward older rocks, and that no faults or repetitions were evident (Jaramillo et al, 2004). The biostratigraphic interpetration was confirmed by several other tools (Villamil et al., 2004). A sidetrack was drilled successfully reaching the Barco Formation after drilling 1500 feet as the geological model proposed by Ecopetrol had predicted. A subsequent well, Gibraltar 2, was drilled 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 Mirador formation is present in the area, while others have argued that the Mirador is missing. The zonation was used to help solving this problem. The palynological analyses shown that there is not a rock record in the Arauca field during the time of deposition of Mirador in the Llanos foothills (Figure 3). The sandstones previously interpreted as Mirador, were deposited during the age of palynological zone Pa 7 (F. perforatus), Late Paleocene. During this time, the lower to middle Cuervos formation was being accumulated in the Llanos foothills. In the Arauca field there is an unconformity that encompasses the palynological zones Pa 8 to Ca 3 (latest Paleocene to earliest Oligocene, Figure 3). During this time the upper Cuervos, Mirador and lower to middle C8 were being accumulated in the Llanos foothills. This interpretation is similar to a recent 3D seismic interpretation of the field (Gilbert, 1992), and seems to be related to the evolution of the Arauca high and the Apure basin in Venezuela where the Eocene is also missing from the Guafita and La Victoria fields (Monroy y van Erve, 1988). The absence of the Mirador formation in the Arauca field may have strong implications in the stratigraphic, structural and paleogeographic interpretations currently held for the north of Llanos and Llanos foothills, including the Caño Limon field. A model explaining the absence of Mirador in the field still needs to be proposed. CONCLUSIONS Biostratigraphy can be extremely useful to hydrocarbon exploration! We have helped to drill wells, improve structural and stratigraphic models, help seismic interpretations, refine outcrop maps, and even decide where new wells are going to be drilled. Biostratigraphy has become a member of the Ecopetrol exploration team rather than an outsider

3 70.00 TIME Maastrichtian Palynological Zone Pa04 - E. protofranciscoi Pa03 - B. andreevi Pa02 - S. echinatus Pa01 - Duplotriporites Pa00 - F. margaritae Senegalinium Campanian Cerodinium Trichodinium Unipontidinium Dinogymnium Santonian Coniacian Turonian D. senonicus Interval Zone Cenomanian Triorites Afropollis E. castelaine E. protensus Albian C. polygonalis Odontichitina Cribroperidinium Aptian Cyathidites Subtilisphaera Figure 1. Cretaceous palynological zonation for Colombia. Time scale after Berggren et al

4 TIME Palynological Zone Middle Miocene Crassoretitriletes - Ca Early Miocene E. maristellae - Ca10.5 R. crassianulatus - Ca10.4 Cyclusphaera - Ca10.3 R. planopolaris - Ca10.2 Marine E. maristellae - Ca10.1 Marine - Ca9.2 P. pachyermatus - Ca9.1 R. minutiporus - Ca8 L. tibuensis - Ca Janduforia - Ca Oligocene J. seamrogiformis/m. grandiosus/ M.f. var. minutus/v. usmensis acme zone - Ca R. "esponjosus" Zone Ca Nothofagidites Zone Ca E. trianguliformis Zone Ca S. aff. echinatus Zone Ca Eocene Mi2 - C. dorogensis Mi1- S. spiralis Paleocene Pa9 - Esteril 22.2 Pa8 R. magdalenensis 22.1 Pa7 - F. perforatus Pa6 - B. annae Pa5c - Mauritiidites Pa5b - Colombipollis Pa5a - S. baculatus Figure 2. Tertiary palynological zonation for Colombia. Time scale after Berggren et al

5 ARAUCA 1 ARAUCA 2 ARAUCA 4 ARAUCA Ca4 to Ca5 palynological zone (early Oligocene) Datum: Oligocene top Ca 3 Zone (earliest Oligocene) Pa 7 zone (Late Paleocene) 1 km oil 1Km ARAUCA 4 ARAUCA 3 N ARAUCA 2 ARAUCA 1 Figure 3. Dating the Arauca wells using the palynological zonation shown in this work. There is not a rock record in the Arauca field during the time of deposition of the upper Cuervos, Mirador and lower and middle C8 in the Llanos foothills. The unconformity encompasses the Pa 8 to Ca 3 (latest Paleocene to earliest Oligocene).

6 as it was before, helping exploration to be more successful. Colombia: case history of an exploration success in a frontier area: Journal of Petroleum Geology, in press. ACKNOWLEDGEMENTS Thanks to ICP for permission to publish this paper. Special thanks to all palynological consultants and academic researchers who have produced palynological data from Colombia over the past fifty years. Many thanks to the entire biostratigraphic unit at ICP. Thanks to Maria Ines and Camilo for their continuous source of ideas. REFERENCES Berggren, W. A. Kent, D. V. Aubry, M. P. and Hardenbol, J., 1995, Geochronology Time Scales and Global Stratigraphic Correlation: SEPM Special Publication 54, SEPM, p 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 Gilbert, O.E., 1992, Segmentation and Distribution of a Problem Reservoir, Arauca Field, Llanos Basin, Colombia: Reservoir Geophysics, Soc. Expl. Geophys., p ICP, 2000, Registro de Descripción Sedimentológica y Estratigráfica 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 of the Llanos Foothills, Colombia: Improving Palynological Resolution for Oil Exploration, Volume in press, TMS Special Publication. Jaramillo, C.A., and Dilcher, D.L., 2001, Middle Paleogene palynology of central Colombia, South America: A study of pollen and spores from tropical latitudes: Palaeontographica B, v. 258, p Jaramillo, C.A. 2004, A morphological electronic database of Cretaceous-Tertiary fossil pollen and spores from Northern South America; AASP Palydisk 24, Electronic Publication, American Association of Stratigraphic Palynologists. Monroy, Z. and van Erve,A., 1988, Revisión palinoestratigráfica del Cretácico y Terciario de Apure (Venezuela Suroccidental): III Simposio Bolivariano. Exploración Petrolera de las Cuencas Sub-Andinas, p Muller, J., Di Giacomo, E., and Van Erve, A., 1987, A palynologic zonation for the Cretaceous, Tertiary and Quaternary of Northern South America: American Association of Stratigraphic Palynologists Contribution Series, v. 19, p Navas, J., 1985, Campo Arauca: II Simposio Bolivariano Exploración Petrolera en la Cuencias Sub-Andinas, v. I, p 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,