A Compositional Petroleum Mass Balance in a Mature Zone for Exploration- Reconcavo Basin, Brazil

L. F. C. Coutinho¹, H. L. de B. Penteado¹, François Lorant², and J. L. Rudkiewicz^2
1Petrobras Research Center, Cidade Universitaria, Quadra 7, Ilha do Fundão, 21949-900 Rio de Janeiro, RJ, Brazil
²Institut Français du Pétrole, 1-4 avenue de Bois Preau, 92852 Rueil-Malmaison, France

INTRODUCTION

The Recôncavo Basin, located in northeastern Brazil, is an aborted continental half-graben formed during the fragmentation of the Gondwana supercontinent in the Lower Cretaceous, subdivided into three main compartments: Southern, Central and Northeasthern. The evolution of this basin encompasses sediments from three main sequences:
a) the pre-rift sequence, composed mainly of siliciclastic sediments of continental origin that contain the main reservoirs of the Sergi and Água Grande formations;
b) the rift sequence of continental origin, showing a change from a lacustrine- dominated sedimentation at its base (including the main source rocks from the Candeias Fm. – the Gomo and Tauá members) to a fluvial-dominated deposition at the top;
c) the post-rift sequence, characterized by isolated patches of preserved sediments of Aptian, Tertiary and Recent ages. The main post-rift erosional episodes are represented by two major unconformities with Aptian and Oligocene ages which are coalescing along most of the basin.

The Recôncavo Basin is presently in a highly mature stage of exploration, with the largest fields discovered in the 1950’s, which has made possible the integration of the geological and geochemical data into important regional studies. Nevertheless, there is not a complete understanding of the evolution of the petroleum systems up to now, especially the timing of petroleum generation, the migration pathways and the mass balance of accumulated/generated petroleum, including their compositional aspects.

The main goal of this study was to perform a mass balance of the amounts of generated, expelled and accumulated petroleum in the Recôncavo Basin. To achieve this goal, modeling scenarios, which evolved from 1D to 3D, were planned in order to assess the efficiency of the petroleum system (total charges, losses and accumulations) within the Recôncavo Basin by validating the thermal and pressure regimes and calibrating the volumes of discovered petroleum accumulations.

This study benefits from the integration of compositional primary and secondary cracking schemes, a new approach to TOC restoration based on Rock-Eval and thermogravimetry experiments and a regional analysis of thermal and petrophysical properties of rocks from the basement and the sedimentary section of Recôncavo Basin (Coutinho, 2008). These information were used as an input to the petroleum system models.

THE MODELING APPROACH

The simulations have used several software that performed petroleum system modeling according to the type of data and the required physical descriptions (temperature, crustal thinning, maturity, migration):
a) 1D: SIMOD (Petrobras) for thermomechanical analysis and GENEX (Beicip-Franlab) for maturation modeling;
b) 2D migration model: Temis2D (Beicip-Franlab);
c) 3D migration model: Temis3D (Beicip-Franlab) with compositional simplified migration (drainage areas) and non-compositional Darcy flow modules.

First, 1D simulations provided the thermal history reconstruction that served as an input to 2D and 3D models. The conductive heat flow was calculated following a generalized two-layer model. The relative absence of post-rift section was consistent with high crustal extension factors (Delta) relatively to the sub-crustal ones (Beta). Sensitivity tests were performed with the GENEX software to calibrate the thermal history (vitrinite reflectances and extrapolated temperature data) using the ratio between Oligocene and Aptian eroded thickness and the radioactive depth decay parameter (D) as free variables. The integrated multi-1D scenario was considered as boundary conditions for the 2D and 3D models at the base of sediments.

Thereafter, 2D models were used to perform sensitivity analyses within a geological section in the Southern Compartment of the basin. The tested parameters were permeability and capillary pressure for shales and faults. The best solution, consistent with the petroleum system saturations at reservoir and source rocks, was applied to the 3D model.

Before going to the 3D maturation and migration results, one must consider that the choice of closed- or open-system primary cracking kinetics has a great impact on the timing and extent of petroleum formation and expulsion, and consequently on the numerical results for the petroleum mass balance. Currently, it is widely accepted that the characteristics of the natural environment are best reproduced with the results of closed-system pyrolysis because of: a) a better reproduction of oil compositions (e.g. Ruble et al., 2001); b) a good calibration of source-rock maturation (e.g. Lewan & Ruble,, 2002) and c) the recognition of NSO compounds as major precursors of petroleum (Behar et al., 2007 and 2008). Therefore, it was decided to convert the original open-system kinetic parameters of the Candeias source rocks to new ones that could represent their equivalent transformation characteristics in closed-system experiments. The elements for that conversion were provided by a series of comparative pyrolysis performed by Lorant (2006) using Brazilian source-rock samples. Lorant (2006) proposed a special rule to derive the closed-system transformation ratios based on Rock-Eval-derived kinetics. A simple shift of the activation energies (Ea) by -3Kcal/mole was sufficient to reproduce the maturation scenario (transformation ratios) observed in closed pyrolysis experiments using the same pre-exponential factor (A).

The 3D maturation modeling included both open- and closed-system converted kinetics. The results with the latter kinetics fitted better the transformation ratio obtained from natural series data of Recôncavo Basin than those from open-system kinetics. This thermal maturity validation was a prerequisite for a more appropriate reconstruction of the original organic carbon content (TOC) distribution using a specific TOC versus Transformation Ratio (TR) law obtained by means of thermo-gravimetry experiments (Coutinho, 2008). The closed-system maturity scenario was consistent with an earlier timing for petroleum generation and a greater amount of generated petroleum.

3D compositional expulsion models coupled with a simplified migration scheme (drain analysis) was then performed. Pyrolysates from source rocks of the Candeias Fm. presented compositions characterized by a predominance of NSO compounds, as opposed to C₁₅+ saturates-rich oils in the basin. Some authors consider that oils enriched in the saturated fractions probably result from the interaction of processes of primary and secondary cracking in the source rocks favored by the retention of heavy compounds (Aromatics C₁₅+ and NSO) before petroleum expulsion (Penteado, 1999; Behar et al., 2001; Behar & Lorant, 2006; Behar et al., 2008). Retention coefficients of NSO and aromatic C₁₅+ fractions inside the source rocks were tested and calibrated with the compositions (C₁₅+ compounds) and volumes in the 12 largest oil fields with reservoirs of the Sergi Fm. that encompass 415 Mm³, or 36 % of total accumulated volume (VOIP) in the Recôncavo Basin. Concomitantly, secondary cracking parameters of the highly labile NSO compounds were adjusted to values similar to those of their kerogen precursor.

The best solutions obtained for the parameters of heavy compounds retention were: a) Southern Compartment - 96% for the NSO’s and 71% for the C₁₅+ aromatics and b) Central and Northeastern compartments – 96% for the NSO’s and 81% for the C₁₅+ aromatics.

The compositional calibration was a prerequisite for the calculations of the maximum expulsion efficiency of oil in the Recôncavo Basin, which is here understood as the maximum percentage of the oil potential based on the hydrogen index that can be converted to mobile fractions. The maximum expulsion efficiency was estimated based on a mass balance with the stoichiometric coefficients of primary and secondary cracking.

A direct and linear relationship between the maximum expulsion efficiency (MEE) and the Hydrogen Indices (HI) was obtained based on the immature studied kerogens, following the equation: MEE = 0,0878HI - 22,286.

For the conclusion of the petroleum mass balance proposed, the oil masses were computed using a non-compositional 3D migration following the Darcy law.

The objective in this part of the study was the reproduction of the retained oil masses along the secondary migration pathways and the calibration of accumulated petroleum amounts in the 21 largest fields in the basin. Primary cracking kinetic parameters converted for closed-system and adjusted values of Hydrogen Index proportional to the maximum expulsion efficiencies were applied.

Sensitivity tests on the lithological parameters of faults (permeability) and shaly rocks (capillary pressure) were undertaken, with choice of values lying on those that allowed the best match of pressure and petroleum masses distribution through the sedimentary section.

The best-case scenario allowed us to reproduce 90% of the total volume of oil in place within the 21 greatest fields (871.4 Mm³ out of 970 Mm³).

THE RESULTING PETROLEUM MASS BALANCE

The resulting petroleum mass balance is presented in Fig. 1.

The expelled petroleum mass calculated for the entire basin was 20.0 x 1012 kg, corresponding to 82% of the total generated mobile fraction; from this total, 1.55 x 1012 kg have been accumulated in reservoirs, of which 50% are in the Southern and 43% in the Central Compartment, respectively. A petroleum system efficiency (ratio of accumulated divided by the expelled mass) value of 6.3% was estimated for the Recôncavo Petroleum System. The petroleum masses in the secondary migration pathways have been calculated to be 3.24 x 10¹² kg, with 79.0% in the Southern Compartment and 20.9% in the Central Compartment. The lateral losses and seeps, not computed in the Temis3D program, were calculated through the deficit in the equation of the petroleum mass balance, and constitute 76% of the total expelled fraction.

BIBLIOGRAPHY

BEHAR, F., LORANT, F., PENTEADO, H., 2001, “Thermal stability of NSO compounds in source rocks: implication for chemical composition of the expelled fluid during primary migration”. In: 21th International Meeting on Organic Geochemistry (IMOG), P/THU3/19, pp. 415-416, Nancy, France.

BEHAR, F., LORANT, F., 2006, “Compositional kinetic schema for oil cracking”, In: Extended Abstracts of X ALAGO Congress on Organic Geochemistry. Associação Latino-Americana de Geoquímica Orgânica (ALAGO), pp. 5-8, Salvador, Brasil.

BEHAR, F., LORANT, F., LEWAN, M., 2007, “Role of NSO compounds in primary cracking and on kinetic parameters determined by open – and closed-system pyrolysis”. In: 23th International Meeting on Organic Geochemistry (IMOG), P142-TU, pp. 367-368, Torquay, Devon, England.

BEHAR, F., LORANT, F., LEWAN, M., 2008, “Role of NSO compounds during primary cracking of a type II kerogen and a type III lignite”, Oil Organic Geochemistry, v.39, pp. 1-22.

COUTINHO, L.F.C., 2008, Análise do balanço composicional do petróleo em uma região em fase de exploração matura – Bacia do Recôncavo, Brasil: Tese de doutorado Universidade Federal do Rio de Janeiro, R.J., Brasil.

LEWAN, M. D., RUBLE, T. E., 2002, “Comparison of petroleum generation kinetics by isothermal hydrous and nonisothermal open-system pyrolysis”. Organic Geochemistry, v. 33, pp., 1457-1475.

LORANT, F., 2006, Tentative experimental calibration of compositional kinetic data generated in open system pyrolysis relative to closed system pyrolysis. Internal report (Petrobras).

PENTEADO H.L.B., 1999, Modélisation compositionnelle 2D de la genèse, expulsion et migration du pétrole dans le Compartiment Sud du Bassin de Recôncavo, Brésil: PhD. Thesis, Université Pierre et Marie Curie, Paris, France.

RUBLE, T. E., LEWAN, M. D., PHILIP, R. P., 2001, “New insights on the Green River petroleum system in the Uinta basin from hydrous pyrolysis experiments”, The American Association of Petroleum Geologist Bulletin, v. 85, n. 8 (August), pp. 1333-1371.

Figure 1. Mass balance equation for the petroleum system of the Recôncavo Basin emphasizing the secondary and tertiary migrations. The lateral losses and seeps (*1) were calculated from the deficit of the masses in the equation.

AAPG Search and Discovery Article #90091©2009 AAPG Hedberg Research Conference, May 3-7, 2009 - Napa, California, U.S.A.