BURIAL HISTORY AND THERMAL EVOLUTION OF ...

World Academy of Science, Engineering and Technology 71 2010

BURIAL HISTORY AND THERMAL EVOLUTION OF THE CRETACEOUS NAPO FORMATION IN THE ORIENTE BASIN OF ECUADOR J. Estupiñán1, A. Permanyer2, R. Marfil3 , L. Barbero1

(1) Facultad de Ciencias del Mar y Ambientales: Dpto. de Ciencias de la Tierra, UCA, 11510 Cádiz, Spain. E-mail, [email protected], [email protected] (2) Dpt. de Geoquímica, Petrología i Prospecció Geológica. Universidad de Barcelona, 08028 Barcelona, Spain E -mail, [email protected] (3) Facultad de Ciencias Geológicas, Dpto. de Petrología y Geoquímica. UCM, 28040 Madrid, Spain. E-mail, [email protected]

Abstract Numerical 1D basin modelling was performed for Cononaco-4 well in order to assess the thermal maturity of the Cretaceous Napo (Oriente Basin, Ecuador) formation and its evolution through time. Mean random equivalent vitrinite reflectance results, using vitrinite-like particles for Napo Shale and burial temperatures [1] pyrolysis Tmax values and fluorescence colours indicate that the Napo Shale is immature to early mature in the East and South parts of the basin and clearly more mature in the South Western part of the basin. Thermal gradient used in the models were constrained by Rock - Eval data and vitrinite analyses from de interbedded shale in U and T interval.The paleotemperature evolution is related to the deepest burial. Peak temperature was reached in the middle Miocene where the oil has been generated. Thermal history modelling indicates that the present-day geothermal gradients and heat flows vary at differents locations. The model indicates that Cononaco- 4 well is beginning to generate the oil when the basin has been buried about 6427 feet (1959 m) since 28,4 M.y. The principal episode in the generation of hydrocarbon occurred over 9368 feet (2855 mt) since 7 M.y with the temperature near to 112ºC.

Keywords. Thermal history, vitrinite reflectance, Oriente basin, Ecuador.

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INTRODUCTION

The Oriente Basin of Ecuador located in South America, is a prolific sub-Andean petroleum province that lies at East of the Andean Cordillera. The Oriente Basin has 100 oil fields with reserves of about 30 MMbbl of oil in situ [2]. The eight petroleum source rocks were selected from interbeded shale in U and T Napo Formation from differents wells in the oriente basin. The present work proposes the integration of the geochemical analysis included Rock-Eval pyrolysis, organic petrography and kinetic 1D modelling. The integration of these data provides a discussion of petroleum generation and migration history.

DATA USED IN THE STUDY

The Cononaco-4 well (Figure 1) located in Sacha – Shushufindi corridor “Central Play” in the Oriente Basin, was selected for thermal modelling using BasinMod 1D (Platte River Association, Inc.). The input data (stratigraphic, geology) were collected [3], Baldock [4, 5]. Tops and bases from the well based in the correlation of the wells used the drilling logs. The initial porosity and matrix density were determined from study of petrology thin sections and well logs (neutron-density). Matrix thermal conductivity and heat capacity were adopted from the default values in the Basin Mod 1D.

THERMAL MATURITY

Observed thermal maturity for this study include Eq Ro, and Rock- Eval Tmax [6]. The Eq Ro data by the C29 (20S/20R) steranes isomerisation believe to be equivalent to unsuppressed vitrinite reflectance, and calibrated in units of “mean randon vitrinite reflectance” [1]. The Tmax (ºC) from Rock Eval is the temperature at the maximum S2 pick (pyrolysable hydrocarbons, mg HC/g rock) as a function of the thermal maturity [7, 8], The organic matter of the shale interbedded in the Napo Cretaceous of the Oriente Basin is mixed of type II and III (Figure 2). In this study, some Tmax was affected in its true values because of (1) presence of natural bitumen in the sample and (2) recycled organic matter. Abnormal Tmax data from the maturity

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trend in a well profile should be ignored, the Basin Mod 1D was used for filtering Rock Eval data and burial temperatures were used [1].

DATA FOR THE MODELLING IN THE BASINMOD 1D

The data used for modelling were: 1. - Stratigraphic thickness based on well completation reports. 2. - Percentages of four lithologies (sandstones, shale, limestone, siltstone). 3. - Absolute ages. The geological time scale is referenced [9]. 4. - Measured thermal maturity including Eq Ro, burial heating Tpeak, Rock – Eval Tmax 5. - Thermal parameters including formation temperatures (from drill wells tested) using the Horner plot correction method, and BHT (Bottom Hole Temperature). Default thermal conductivity and heat capacity. The Geothermal gradient used 23ºC, heat flow range (20-45 Mw/m2) determined [10] for the fore-arc basin [2] 6. - Erosional Thickness base on the chronostratigraphic scale from the depositional events in the Oriente Basin. 7. – Porosity evolution from well logs (density-Neutron) and petrography analysis.

The 1D model is used where the transfer of the heat flow is considerate vertical conduction, this values for a back arc basin [10] suggested that heat flow is about 20 -50 Mw/m2 with an average 35 Mw/m2, was assumed steady state, than consider a constant heat flow over the time.

THERMAL MATURITY MODEL

The Cononaco-4 well was drilled on the anticline structure in the centre of the Oriente Basin (Figure 1). The well contains a wide Paleozoic to Tertiary sequences. The measured vitrinite reflectance is 0.75% which corresponds to a burial temperature of 113°C in accordance with [1]. The calculated current heat flow for Cononaco-4 well is 47 Mw/m2 (Figure 3). This data supports the high thermal maturity in Cononaco -4 well.

The model indicates that Cononaco- 4 well is beginning to generate the oil when the basin has been buried about 6427 feet (1959 m) since 28,4 M.y. The principal episode in the generation of

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hydrocarbon occurred over 9368 feet (2855 mt) since 7 M.y with the temperature near to 112ºC (Figure 4).

CONCLUSION

This study shows that the Tmax and Eq Ro in the thermal modeling of individual well (Cononaco4) is a good indicator as the thermal maturity. Eq Ro deduced from C29 (20S/20R) steranes shows good correlation with the measured vitrinite reflectance and can be used when the conventional values of Ro present anomalously. The study indicated that the interbeded shale in U and T Napo Formation is rich in organic matter and potential source of hydrocarbon generation. The thermal model for individual well “Cononaco-4”, presents a good correlation about the hydrocarbon generation and burial depth considering that the model could be affected assuming a vertical thermal conduction without considering the horizontal, do no introduce exact values of time and thickness of the erosion and hiatus

REFERENCES

[1] Baker, C.E. and Pawlewicz, M.J, “Calculation of vitrinite reflectance from thermal histories and Peak Temperatures comparison methods”, Geologycal Survey, 1994, 1, pp 13. [2] Baby, P., Rivadeneira, M., and Barragan, M, “Estratigrafía, Estructura y evolución geodinámica de la Cuenca Oriente”. In La Cuenca Oriente: Geología y petróleo, 2004, pp, 14. [3] Jaillard, E, “Síntesis estratigráfica y sedimentológica del Cretácico y Paleógeno de la Cuenca Oriental del Ecuador”. Convenio ORSTOM-PETROPRODUCCION,1, 1997, pp 1-164. [4] Baldock, J.W, “Geología del Ecuador”, Ministerio de Recursos Naturales, 1982, 1, pp. 1-65. [5] Dashowood, M., and Ebbotts, J, “Aspects of the petroleum geology of the Oriente Basin, Ecuador”, Geologycal Society Special Publication, 1990, 50, pp 89-117. [6] Estupiñan, J, “Control diagenético sobre la calidad de los reservorios de las areniscas “U” y “T” de la Fm Napo del Cretácico de la Cuenca Oriente, Ecuador. Modelización Térmica y su relación con la generación de hidrocarburos”, Tesis Doctoral, Universidad Complutense de Madrid, 2006, pp. 178-191.

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[7] Espitalié, J., Marquis, F., and Barsony, L, “Geochemical logging”. In K.J. Voorhess (Ed.), Analytical pirólisis: Techniques and applications, 1984, pp 276-304. [8] Tissot, B.P., Pelet, R., and Ungerer, P.H., Thermal history of sedimentary basin, maturation indices, and kinetics of oil and gas generation, American Association of Petroleum Geologist Bulletin, 1987, 71, pp. 1445-1466. [9] Beroiz, C.P, “Geología de las cuencas Subandinas y su interés petrolífero”.Tesis de Doctoral, Universidad Complutense de Madrid, 1994, pp. 291-296. [10] Allen, P.A. and Allen, J.R., Basin analysis principles and aplications. Oxford: Blackwell Scientistic Publication, 1990, pp. 282-283 and 301.

FIGURES.

Figure 1. Location map of the Cononaco-4 well in the Oriente basin. Figure 2. Plot diagram showing the relationship between Tmax vs Hidrogen Index Figure 3. Plot diagram showing heat flow vs depth and the % Ro value. Figure 4. Burial history curves of Cononaco-4 well.

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Figure 2

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Figure 3

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Figure 4

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