Marine and Petroleum Geology 26 (2009) 590–605 Contents lists available at ScienceDirect Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo Modeling an atypical petroleum system: A case study of hydrocarbon generation, migration and accumulation related to igneous intrusions in the Neuquen Basin, Argentina F. Rodriguez Monreal a, *, H.J. Villar b, R. Baudino c, D. Delpino a, S. Zencich a a b c YPF S.A. Exploración Argentina Onshore, Esmeralda 255, C1035ABE, Buenos Aires, Argentina GeoLab Sur S. A. Italia 1616, 1602 Florida, Buenos Aires, Argentina YPF S.A. Exploración, Esmeralda 255, C1035ABE Buenos Aires, Argentina a r t i c l e i n f o a b s t r a c t Article history: Received 7 September 2007 Received in revised form 28 February 2008 Accepted 2 January 2009 Available online 15 January 2009 In the Altiplanicie del Payún area (Neuquen Basin, Argentina), immature source rock sections intruded by up to 600 m thick Tertiary laccoliths show full spectrum maturity aureoles over hundreds of meters from the contacts. Commercial oil accumulations (20–33 API) and oil shows are located along the entire column, both in sandstone/carbonate and fractured igneous reservoirs. A challenging numerical model that included the emplacement of the intrusive bodies, with extreme temperature ranges and unusually short calculation time steps, has been done with the aim to better understand hydrocarbon generation and migration processes related to these thermal anomalies. A 2D petroleum system model achieved satisfactory results when accounting for thermal maturation, oil and gas generation, composition, migration, and known accumulations. This atypical petroleum system is characterized by thermal anomalies lasting thousands of years that are the result of the progressive cooling of the igneous intrusions. Host source rocks were exposed to a wide range of temperatures that varied in time and space generating different maturity products. High generation pressures, source rock fracturation, and convective water flows facilitated migration and mixing of hydrocarbons and the charge of the igneous bodies as they were fractured during thermal contraction. Oil and source rock analyses were done in order to calibrate and support modeling results. Geochemical correlations suggest an in situ generation related to the igneous intrusions. In particular, diamondoids’ measurements show mixing processes of high-mature (cracked) with low-mature hydrocarbons. Oils generated and cracked close to the laccoliths and oils generated from more distant source rock sections, less affected by the thermal anomalies, were probably mixed as they migrated towards the igneous bodies and shallower reservoirs. The integration of a wide set of geochemical analyses and recent modeling techniques based upon time and temperature calculation performance enhancements, led to novel insights on this atypical petroleum system. Ó 2009 Elsevier Ltd. All rights reserved. Keywords: Neuquen Basin Magmatic intrusions Thermal anomaly Atypical petroleum system Basin modeling Petroleum geochemistry Diamondoids Overpressure 1. Introduction Petroleum systems with hydrocarbon generation processes other than burial maturation have been classified as atypical by Magoon and Dow (1994). Several hydrocarbon accumulations worldwide are considered to be generated by the thermal effect of igneous intrusions on source rocks (Zhenyan et al., 1999; Gonzaga et al., 2000; Araújo et al., 2000; Othman et al., 2001; Parnell, 2004). * Corresponding author. Tel.: þ34 917538803; fax: þ34 913480777. E-mail addresses: frodriguezmo@repsol.com, frmonreal@yahoo.es (F. Rodriguez Monreal). 0264-8172/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2009.01.005 Within the Neuquen Basin the relationship between igneous rocks and hydrocarbon generation was suggested in the first decades of the last century by Groeber (1929). Thermal maturation effects of igneous bodies on source rocks have been well described showing different ranges of maturity varying with distances and intrusives thicknesses (Dow, 1977; Rullkötter et al., 1988; Zalán et al., 1990). Simoneit et al. (1978, 1981), Clayton and Bostick (1986), Raymond and Murchison (1992), and Maowen et al. (1998) have described geochemical anomalies in hydrocarbon fluid inclusions and source rock extracts related to the high thermal stresses and maturation processes induced by the magmatic intrusions. Dutkiewicz et al. (2004) pointed to mixing of different maturity range products from several migration pulses F. Rodriguez Monreal et al. / Marine and Petroleum Geology 26 (2009) 590–605 and probable reverse migrations processes towards the intrusive bodies in fluid inclusion studies. Polyanskii et al. (2002) modeled the development of convection cells related with sill intrusions showing fluid flow velocities of up to 94 m/yr. However, generation and migration processes related to igneous intrusions are not yet well understood. The main reasons are probably their low natural occurrence and the difficulty to differentiate hydrocarbons related to igneous intrusions from those related to burial generation in areas where both processes coexist. Advances in numerical simulations have allowed for the performing of thermal models which 591 account for source rocks maturation related to igneous intrusions (Galushkin, 1997; Baudino et al., 2004, 2005; Rodrı́guez et al., 2005, 2006). Hydrocarbon expulsion and migration are still difficult to model due to an insufficient understanding of these natural processes in such non-conventional scenarios and also because of host rocks’ petrophysical changes induced by the igneous intrusions. An integrated study combining petroleum geochemistry and 2D thermal modeling has been performed in the Altiplanicie del Payún area, Neuquen Basin (Fig. 1), where immature source rocks were Fig. 1. Location of the ADP study area, main geological features and petroleum fields of the Neuquen Basin. 592 F. Rodriguez Monreal et al. / Marine and Petroleum Geology 26 (2009) 590–605 locally affected by thermal anomalies generated by laccolith intrusions. Regardless of the overall source rock immaturity, numerous commercial oil (20–33 API) accumulations occur in this particular area. The main reservoirs are fractured and overpressured laccoliths intruded in the Vaca Muerta black shales (Fig. 2). Abundant oil shows have also been reported in both shallow and deep sandstone/carbonate reservoirs. Trap formation is also related to Tertiary laccoliths emplacement. Significant quantities of black oil expelled from analogous regionally immature source rock sections in contact with Tertiary igneous intrusions have been observed in Cara Cura outcrops, 30 km west of the study area (Zencich, unpublished result). Additional commercial oil and gas fields within the Neuquen Basin produce from sills and laccoliths (Delpino and Bermúdez, 2008), but the regional thermal maturity of the source rocks makes it difficult to differentiate burial from igneous related generation processes in these deeper areas. The purpose of this article is to discuss the patterns of this atypical petroleum system emphasizing the role of igneous intrusions as a trigger for local source rock maturation and hydrocarbon generation and migration. 2. Geological and tectonic setting The sub-Andean Neuquen Basin is located in the mid-western part of Argentina (38 S, 69 W). Covering an area of approximately 1.000.000 km2, it is one of the largest gas and petroleum productive provinces in the country. It has been a region of cyclical marinecontinental sedimentary filling since the Early Triassic rifting period. The basin operated as stable back-arc depocenter from Lower Jurassic to Lower Cretaceous. When Andean deformation moved from west to east in the Upper Cretaceous, the basin evolved into a foreland depocenter (Vergani et al., 1995). A fold and thrust belt was developed on the western margin of the basin by the Mesozoic and Tertiary Andean deformation that became more intense during the Neogene. At present-day, the triangular shape of the Neuquen Basin is defined to the west by the north–south elongated Andean fold-and-thrust belt, to the south by the ENE trending Huincul High and to the north-east by the Platform area. The complex evolution of the basin is reflected in four main petroleum systems, with different source rocks and a variety of reservoirs. The Altiplanicie del Payún exploration area (ADP in the following text) is located in the north-centre of the Platform area (Fig. 1). A detail of the study area stratigraphic record is shown in Fig. 2. 3. Petroleum systems, source rocks and thermal maturity The general features of the petroleum systems occurring in the Neuquen Basin have been extensively documented by Uliana and Legarreta (1993), Villar et al. (1993), Urien and Zambrano (1994), Villar and Talukdar (1994), Cruz et al. (1996, 1998, 1999, 2002), Legarreta et al. (1999), Gulisano et al. (2001), Veiga et al. (2001), Villar et al. (2005). Recent publications by Legarreta et al. (2004, 2005) have Fig. 2. Schematic stratigraphic column, source rock intervals and main reservoirs of the ADP study area. F. Rodriguez Monreal et al. / Marine and Petroleum Geology 26 (2009) 590–605 593 Fig. 3. Isopach maps and maturity trends (Ro%) of the Vaca Muerta and Agrio source rocks (modified from Legarreta et al., 2004). Notice that both source rocks are regionally immature in the study area. Black dots represent studied oils produced from ADP fields. White dots represent studied oils from deeper mature areas. discussed and quantified the generation–accumulation efficiency of the four charge systems of the basin, known as Puesto Kauffman (Early Jurassic), Los Molles (Middle Jurassic), Vaca Muerta (Late Jurassic) and Agrio (Early Cretaceous), and have focused on estimating the remaining exploratory potential. The stratigraphy, organic facies and hydrocarbon-generating capacities of these four source horizons have been thoroughly discussed by Uliana et al. (1999) and Legarreta et al. (2000). The Tithonian Vaca Muerta Fm. (Fig. 3a) is considered the main source rock of the basin. These bituminous shales, deposited under anoxic conditions of shelf and slope marine settings, have high organic matter contents (TOC 3–8% reaching values up to 10–12%) and an amorphous marine Type I–II kerogen. The Vaca Muerta source rock matured during the Upper Cretaceous and Miocene. The Early to Late Hauterivian Agrio Fm. (Fig. 3b) was deposited as a transgressive organic-rich marly shale. Its kerogen is amorphous marine Type II to II–III and the TOC content ranges from 2 to 3% with peak values up to 5%. The thermal evolution of the Agrio source rock occurred between the Eocene and the Late Miocene. Organic-rich, oil-prone intervals of both Vaca Muerta Fm. (w125 m thick) and Agrio Fms. (w250 m thick) are typically recognized in the ADP area, displaying TOC contents frequently in the range of 2–6% and averaging hydrogen indices (HIs) around 550 mg HC/g TOC. However both units are largely immature to marginally mature in the ADP area, where vitrinite reflectance (Ro) span values mostly from 0.4 to 0.6% (Fig. 3). 4. Igneous activity in the Altiplanicie del Payún area The stratigraphic record in the ADP area comprises several Tertiary laccoliths that reach thicknesses of up to 600 m and area extensions averaging 3.5 km2. Three main igneous bodies intruding the Vaca Muerta Fm. are recognized in the area. Named for discussion purposes as Southern Laccolith, Middle Laccolith, and Northern Laccolith (Figs. 4 and 5), they reach thicknesses of 160 m, 110 m and 600 m respectively. Laccoliths depths are between 1820 and 2460 m below surface. Most of the present-day structures in the area are in response to the deformation caused by the intrusion of these igneous bodies. The laccoliths can be identified on seismic by their domal structure and the concentric faulting that they generate in the overlying sedimentary column. From Ar40/Ar39 dating of core and cutting samples, the laccolithic field of ADP was formed during an acidic alkaline intraplate extensional event between 49 and 46 Ma. The intrusion temperature is estimated in 980  C, taking as reference the emplacement temperature of the dome at Mount St. Helens of similar chemical composition (Friedmann et al., 1981). 5. Oil occurrence and thermal maturity anomalies Intermediate oil (20–33 API) accumulations are located along the entire sedimentary column in the ADP area, both in fractured intrusives and sandstone/carbonate reservoirs (Fig. 5). Some CO2 and methane accumulations have also been reported in the laccoliths and shallower reservoirs. Well tests (TST) show that fluids in igneous reservoirs are overpressured around 4 MPa above the regional hydrostatic gradient. Oil shows and accumulations are more frequent and extend to shallower reservoirs in areas where laccoliths are thicker. Geochemical analyses of oils and source rocks have been performed on samples from six wells within the study area, labeled as A, B, C, D, E and F in Fig. 5. 594 F. Rodriguez Monreal et al. / Marine and Petroleum Geology 26 (2009) 590–605 Where laccoliths intruded the Vaca Muerta source rock, a wide maturity range can be identified through variations of the hydrogen indices (HIs) and vitrinite reflectance (Ro%) data over thicknesses of more than 400 m from the contacts (Fig. 6). Conversely, wells drilled a few kilometers far from igneous intrusions show immature source rock sections. The maturation trends are illustrated by the HI logs of Wells A–F (Fig. 6). In Well A, where the Vaca Muerta Fm. is intruded by the 160 m thick Southern Laccolith, HI values of the Agrio source rock decrease towards the intrusive body while the Vaca Muerta shales are overmature and show very low HI values. In Well B, approximately 5 km north of Well A, both Agrio and Vaca Muerta Fms. are immature to low-mature and preserve original HI values, except for a limited transformation of the upper Vaca Muerta source rock due to the presence of a minor sill. Well C, intruded by the Middle Laccolith (110 m thick), shows again an overmature pattern for the Vaca Muerta Fm. (HI values close to zero) but low conversion of the Agrio shales (high HI values; Ro: 0.68%). In Well D, located tens of meters from the Middle Laccolith flank, the Agrio shales are immature (Ro: 0.4%), while the Vaca Muerta source rock is moderately transformed, according to partial depression of the HI values, probably due to lateral thermal influence of the igneous body. Well E, where no igneous rock was drilled, presents again an immature pattern for both source sections similar to those of Well B. Finally, Well F, that encountered the 600 m thick Northern Laccolith, shows strong overmaturity for both Agrio and Vaca Muerta Fms. 6. Geochemistry of oils and source rocks’ extracts Fig. 4. Laccolith distribution and structural features in the ADP area. Notice the concentric faults related to the emplacement of the igneous bodies. A, A0 and A00 denote the trace of the modeled cross-section in Fig. 5. Several GC, GCMS, and diamondoids’ analyses on oils and source rock extracts have been performed in order to investigate generation and migration processes related to the presence of igneous intrusions. Particularly, source rock extracts from Wells A and D, oils produced from the Middle and Northern Laccolith reservoirs (Wells C and F) and oils from deeper and shallower reservoirs (Wells C, D and F) have been studied. The results were compared with analyses of oils generated in deeper kitchen positions of the basin (Fig. 3), south of the ADP area. The latter were used as reference for the geochemical signatures of hydrocarbons Fig. 5. S-NW structural cross-section in the ADP area (see Fig. 4 for location), denoting source rock distributions, laccolith emplacements and oil and gas occurrences. Labels O1–O7 identify the location of the analyzed oils from the study area. Fig. 6. Hydrogen index (HI) trends in key wells of the study area. Note the high HI values of immature intervals contrasting with the progressive decrease towards the laccoliths’ contacts. Measured vitrinite reflectance (Ro%) values are given as reference. Labels R1–R6 identify the position of rock layers used for detailed extract analysis (gas chromatography and biomarkers). Fig. 7. GC and biomarker signatures of oils and source rocks’ extracts from the ADP study area compared with oils from southern deeper kitchens of the Neuquen Basin (see Figs. 3, 5 and 6 for sampling location). Abbreviations. TAS: Triaromatic Steroids; RP3, RP5, RP6: Ring preference at C3, C5, C6. (Mango, 1994). 596 F. Rodriguez Monreal et al. / Marine and Petroleum Geology 26 (2009) 590–605 generated through normal burial processes, that is, excluding the thermal effect of intrusive bodies. The studied samples of the ADP area show a strong geochemical identity, which allows assigning a genetic relationship between oil accumulations and local source rocks. The general molecular patterns of all oils and rock extracts are consistent with aquatic organic matter sources deposited in a marine anoxic depositional environment with very minor terrestrial contribution and variable carbonate influence. Several cross-plots of selected biomarker parameters, such as C30 S þ R Sterane Index vs. C26/C25 Tricyclic Terpanes ratio and C23/C24 Tricyclic Terpanes vs. Sterane/Hopane ratios (Fig. 7a and b), or distributions such as C26-C27-C28 TAS (Triaromatic Steroids; Fig. 7c), indicate variations of organic facies of the ADP samples with respect to oils from deeper kitchen positions southwards of the study area and strongly suggest an in situ generation instead of an alternative long distance migration. Moreover, the good discrimination from GC data of the Mango’s ring preference (Mango, 1994) at C7 (Fig. 7d) reinforces the identity of the ADP oils and supports the statement for their local origin. Regarding thermal maturity, biomarker parameters such as bbS/ (bbS þ aaR) sterane isomerization or aromatic hydrocarbon ratios such as MPI 1 (methylphenanthrene index) show immature signatures for rock extracts not affected by the thermal aureole of igneous intrusions (rock extracts R3, R4 and R5 in Fig. 8) which correspond to a 0.4–0.6% VRE (vitrinite reflectance equivalent) range derived from Rock Eval, Ro and TAI data. Conversely, ADP oil samples show typical oil-window maturities (0.7–0.9% VRE) in the range of thermally affected source rock samples (R1, R2 and R6 with VRE values of 0.6–1.0%). Fig. 8. Cross-plot of bbS/(bbS þ aaR) (C29; m/z 217) steranes vs. MPI 1 showing maturity comparison of ADP oils and source rock samples. Regionally immature source rocks show VRE (vitrinite reflectance equivalent) values lower than 0.6% while source rock samples affected by igneous intrusions show higher maturity levels in the range 0.6–1.0% VRE, similar to that of the ADP oils (see Figs. 5 and 6 for sampling location). Evidence derived from diamondoids’ analyses on Altiplanicie del Payún oils help us to understand the generation processes associated to the thermal effect of the intrusive bodies. In order to define the general maturity and hydrocarbon cracking trends of the basin, additional analyses were performed on several oils from southern kitchen areas. According to the concepts put forward by Dahl et al. (1999), oils with high concentrations of both diamondoids and biomarkers (in particular 3- and 4methyldiamantane and C29aaR Sterane) can be considered as mixtures of cracked, high-mature oils/condensates, with normal maturity ‘‘black’’ oils. While C29aaR Sterane concentrations of the ADP oils correspond to mid-maturity stages within the basin trend, several samples show moderately augmented diamondoid concentrations and a shift from the general maturity-cracking trend (Fig. 9a). These oils are interpreted to be the result of the mixture of hydrocarbons generated close to the laccoliths and hydrocarbons generated from more distant source rock sections less affected by the thermal effect of the igneous bodies (Fig. 9b). This appears to be the case of oil samples O2 and O7, respectively pooled in the Middle and Northern Laccoliths igneous reservoirs and of oil samples O5 and O6, probably generated from source rock sections above the Northern Laccolith and mixed during migration towards shallower reservoirs (Fig. 5). On the other hand, oil samples O3 and O4, not showing mixing signatures, are probably generated from source rock sections slightly affected by the Middle Laccolith and laterally migrated to adjacent reservoirs. 7. 2D petroleum system modeling A 2D model including the thermal effect of the three main igneous bodies was built using the Temis2DÒ software of IFP/Beicip-Franlab, accounting for thermal maturation, petroleum generation, migration and accumulation. A detail of the section can be seen in Fig. 10. The stratigraphic column used for this model is represented in detail in Fig. 2. The kinetic scheme as well as the original hydrogen indices and original TOC contents for the Agrio and Vaca Muerta source rocks were defined from the analyses of immature samples of Well E. Thermal boundary conditions are a 1330  C constant basal temperature at the mantle–lithosphere interface and a constant surface temperature of 14  C. The model was calibrated using well data including bottom-hole temperatures, porosity, vitrinite reflectance, and transformation ratio. A default, noncompositional, single liquid phase scheme for hydrocarbon migration was initially used. The emplacement of igneous intrusions within host rocks is considered an instantaneous process (Swanson and Holcomb, 1990; Bruce and Huppert, 1990). Accordingly, laccolith intrusions have been modeled as a single instantaneous event occurring at 47 Ma that corresponds to an average of their radiometric ages (49–46 Ma). The defined petrological parameters for the laccoliths are a density of 2650 kg/m3, a thermal conductivity of 3 W/ m/ C, a mass heat capacity of 1150 J/kg/ C and an intrusion temperature of 980  C, characteristic values of acidic compositions (Friedmann et al., 1981). Time steps for calculation were set to a minimum of 0.0000001 Ma with results recorded every 100 years. From the burial reconstruction the estimated emplacement depth is around 1900 m below paleosurface (Fig. 11). After this magmatic event, the study area was stable and only 300–500 m of extrusive volcanic sediments were deposited during Tertiary times. This burial history resulted in a regional immaturity of the source rocks. Fig. 9. a) Plot of biomarkers (C29aaR Sterane) vs. diamondoids’ (3- and 4-methyldiamantane) concentrations (ppm), modified from Dahl et al. (1999), in selected oil samples of the Neuquen Basin. Low maturity oils show comparatively high amounts of C29aaR Sterane and low presence of 3- þ 4-MD. High maturity oils from kitchen positions basinwards show depressed amounts C29aaR Sterane and augmented 3- þ 4-MD. Most of the ADP oils plot in the field interpreted as mixtures of low and high maturity oils. b) Simplified conceptual model for the mixture of oils in the ADP area. Low maturity hydrocarbons, sourced from distant positions in relation to the intrusive, mix in the reservoirs and/or during migration with mature and cracked hydrocarbons sourced relatively closer to the intrusives. Fig. 10. Modeled 2D section at present-day showing the emplacement of the three main igneous bodies in the ADP area. 598 F. Rodriguez Monreal et al. / Marine and Petroleum Geology 26 (2009) 590–605 Fig. 11. Modeled 2D section at 48 Ma, 1 Ma before the emplacement of the three main igneous bodies into the Vaca Muerta Fm. at 1900 m below surface. 7.1. Maturity calibration and hydrocarbons generation Satisfactory calibration (Figs. 12 and 13) of the vitrinite reflectance (Ro) and transformation ratio (TR) data was achieved in the three thermally affected wells (A, C and F). Good calibrations of the raw data have also been obtained in areas without laccolith intrusions, where source rocks remain immature (Wells B and E). Host rocks show a full spectrum of maturity over hundreds of meters around each laccolith supporting the local generation of hydrocarbons. These maturity anomalies are localized in tens to hundreds of meters around each laccolith as a consequence of high heat flows associated with significant temperature increases during laccolith emplacement and cooling. The model shows that the thermal effect over host rocks is higher in the middle areas of the igneous bodies (Figs. 12 and 13), where the laccoliths are thicker, and it is lower towards the flanks. A reduced thickness of the laccoliths and a larger heat dissipation surface result in a faster cooling in these flank areas. The calculated maturity of the host rocks is lower above the laccoliths while it is higher below. As an example, 300 m above Fig. 12. Present-day vitrinite reflectance after local maturation of Vaca Muerta and Agrio source rocks by thermal effect of the igneous bodies and calibration profiles from Well A (Southern Laccolith), Well C (Middle Laccolith), Well E (non-intruded area) and Well F (Northern Laccolith). F. Rodriguez Monreal et al. / Marine and Petroleum Geology 26 (2009) 590–605 and below the Northern Laccolith, Ro values are 1.64% and 2.97% respectively. The coexistence of a downward heat flow related to the igneous bodies with the regional upward heat flow below the laccoliths possibly results in a slower heat dissipation in this area. Taking as a reference the centrally located Well C within the Middle Laccolith that reaches a thickness of 110 m thick, the maximum observable temperature (100 years after intrusion) attained in the first 10 m from the laccolith contact is 475  C (Fig. 14a). However, the temperature decreases rapidly with time and distance from the laccolith being 350  C at 50 m, 255  C at 90 m, 200  C at 130 m, 155  C at 230 m, 120  C at 370 m and keeps decreasing gradually at larger distances. For the Well F position within the 600 m thick Northern Laccolith, temperatures of up to 550  C can be observed in the first 10 m above the laccolith contact, 500  C at 30 m, 440C at 55 m, 360  C at 100 m, 280  C at 200 m, 200  C at 340 m, 160  C at 475 m and 130  C at 600 m (Fig. 14b). These temperature profiles are similar to those modeled by Carslaw and Jaeger (1959). In the first 200–400 m from the contacts, maximum temperatures reached are too high for the preservation of liquid hydrocarbons. However, before extreme temperatures are attained at a given position of the source rock, hydrocarbons can migrate tens to hundreds of meters away reaching cooler lateral areas and shallower reservoirs, remaining protected from secondary cracking processes. Host rocks maximum temperatures are much lower towards the flanks of the laccoliths, making these areas more suitable for the preservation of liquid hydrocarbons. Therefore, high maturity hydrocarbon generation and cracking can happen close to the intrusive at the same time that lower maturity products are being generated further away from the laccolith contact. Heat transfer by conduction and convection from the cooling laccolith to the host rocks is a progressive process in time and in space. 599 For comparison purposes, a cut off for the duration of the main thermal anomalies has been defined. The time after intrusions occurrence, at which the temperature 10 m above each laccolith contact drops below 100  C, is approximately 31,000 years for the thinner Middle Laccolith intrusion (110 m thick), 48,000 years for the Southern Laccolith (160 m thick), and 62,000 years for the Northern Laccolith (600 m thick). Background geothermal gradient is completely reestablished in the study area after more than 300,000 years when isotherms become horizontal in all the section. These values are specific for this model and can vary widely in other case studies because they are strongly influenced by laccolith thickness, emplacement temperature and depth as well as host rock petrophysical properties. Present-day laccolith temperatures are between 80 and 100  C. The increase in source rock maturity is extremely rapid in this atypical petroleum system when compared with burial generation processes. An example of vitrinite reflectance and transformation ratio evolution in time and space at several depths above the thickest area of the Middle Laccolith is shown in Fig. 15. 7.2. Migration and accumulation High pore pressure is calculated in source rocks’ sections and igneous reservoirs. Its occurrence could be explained through a combined process of rapid hydrocarbon generation and hydrothermal fluid flows during cooling of the laccoliths. The sudden increase of hydrocarbon saturation and pore pressure, unusual for numerical simulators, induced instability. As a consequence, user defined lithology parameters such as permeability, capillarity, and fracturation thresholds were modified in order to facilitate fluids migration and therefore, calculated pore pressures may differ from the real values. Fig. 13. Present-day transformation ratio after local maturation of Vaca Muerta and Agrio source rocks by thermal effect of the igneous bodies and calibration profiles from Well A (Southern Laccolith), Well C (Middle Laccolith), Well E (non-intruded area) and Well F (Northern Laccolith). 600 F. Rodriguez Monreal et al. / Marine and Petroleum Geology 26 (2009) 590–605 Fig. 15. Vitrinite reflectance (a) and transformation ratio (b) vs. time at 10, 200 and 340 m above the contact of the Middle Laccolith thickest section. Fig. 14. Temperature vs. time, 10 m above the contacts of the Middle Laccolith (a) and Northern Laccolith (b) thickest sections. In this model a limited increase of overpressure is observed (up to 5.5 MPa in the first 10 m above the Middle Laccolith top) followed by a drop out to 0 MPa (after approximately 200,000 years in this case) (Fig. 16a). Comparable values are reached above the Northern and Southern Laccoliths (Fig. 16a) and also inside the igneous reservoirs. However, well test measurements show an overpressure of 4 MPa in the Middle Laccolith reservoir indicating that the drop out has not been completed in the real world and suggest that the original maximum value was much higher than the calculated one. Indeed, in other examples of intrusion modeling, overpressures up to 36 MPa were calculated for cells of host rock 100 years after intrusion (Baudino et al., 2005). The relative contribution of hydrocarbon generation and hydrothermal fluid flows to pore pressure build-up and its real extent are difficult to estimate on the basis of our present-day knowledge of expulsion and migration processes in such a system. Accompanying the overpressure increase, the model simulates an increase in the fracturation ratio (Fig. 16b). The calculated ratio is not high enough to overcome the fracturation threshold of the host rock in this case. However, fracturation ratio values up to 2.1 have been calculated for cells located immediately below the intrusion in more stable models that simulate only water flow and use the original lithology values. This host rock fracturation has important implications on fluid flow. Outcrops generally show fracture networks near the mantelic intrusions. In nature, it is probable that fluid overpressure contributes to the intense fracturation of the host rock increasing hydrocarbon migration (Baudino et al., 2005). Migration processes in this atypical petroleum system are still unclear. Field observations in source rocks affected by intrusions show that fractures are filled by hydrocarbons and/or hydrothermal minerals (calcite and gypsum). This implies that one important mechanism responsible for migration is fracturation, as also suggested by modeling results. This fracturation can be originated by fluids overpressure (related to both rapid hydrocarbon generation and hydrothermal fluid flows) and also by mechanical deformation due to the intrusive emplacement and cooling itself. However, the relative importance of each phenomenon and a better understanding of the migration processes are needed to correctly adjust simulation parameters and to obtain more reliable results. It is well known that igneous bodies are fractured while cooling due to mineral crystallization and volume loss. For dacitic laccolith compositions the beginning of the crystallization starts when laccolith temperature reaches 725  C (Swanson and Holcomb, 1990). This fracturation process was simulated increasing the porosity and permeability of the laccoliths lithologies when reaching 725  C, approximately 400 years after intrusion. When the laccoliths acquire porosity, the source rock overpressure favors a reverse fluid flow towards these bodies. The full spectrum of F. Rodriguez Monreal et al. / Marine and Petroleum Geology 26 (2009) 590–605 601 these areas while it remains immature in the Middle Laccolith area. Migration could have been influenced as well by faults created during laccolith emplacement and collapse while cooling, but they have not been included in this model. A detail of petroleum migration towards the Northern Laccolith and shallower and deeper reservoirs is shown in Fig. 18. The model shows how hydrocarbons generated from different maturity sections of the Agrio and Vaca Muerta source rocks can get mixed during migration. 7.3. Compositional modeling and calculation of generated hydrocarbon volumes Fig. 16. Overpressure (a) and fracturation (b) vs. time, 10 m above the contact of the Middle Laccolith thickest section. maturity developed vertically and laterally in the source rocks in time and space within few hundred of meters from the contacts would allow for the mixing of different maturity products. Convective water flows are also developed around the cooling laccoliths that probably drive differently mature hydrocarbons towards these igneous bodies. Modeled accumulations in laccoliths and conventional reservoirs have pattern distributions which are similar to both oil accumulations and oil shows observed in the study area (Fig. 17a and b). As observed both in the model and in the study area, hydrocarbons have reached shallow reservoirs especially in the Northern and Southern laccoliths areas, probably due to higher volumes of hydrocarbons generated by the thermal aureole of these thick igneous bodies. The Agrio Fm. is also thermally affected in A 12 class compositional detailed model for the Middle Laccolith was also performed using a Type II default kinetic scheme (Behar et al., 1997) for the Vaca Muerta source rock. The model shows that oil and gas are accumulated in and around the Middle Laccolith in very variable proportions during and after cooling of the laccolith (Fig. 19). A significant amount of gas is expected to be accumulated in this kind of petroleum system due to the high temperatures reached close to the laccoliths that favor secondary cracking processes. However, exploration and development wells in this area have only found low amounts of thermogenic gases in the igneous and shallower reservoirs. Gas retention in the reservoirs was probably limited, due perhaps to low capillary pressure values of the seals and leakage through fractures and faults related to laccolith emplacement. Modeled oil accumulations show API gravities in the range of 20–40 , similar to those observed in the study area oil samples. Final compositions of the products in this petroleum system are very dependent on expulsion efficiencies, migration pathways, generation, and mixing of different maturity products and secondary cracking. Future research needs to be done in compositional modeling of such timely and spatially highly variable processes. The volume of hydrocarbons generated by the Middle Laccolith thermal aureole has been calculated following the Schmoker method (1994). Original 3.5% for TOC and 550 mg/g for HI average values were obtained from immature samples of the ADP area, in locations not affected by the igneous intrusions (Well E; Fig. 6). From pyrolysis data and model results, a thickness of 125 m of the Vaca Muerta source rock has been thermally affected with 0.9 average transformation ratio over an area of 3.5 km2. Average density for ADP oils is 0.92 g/cm3 at 15  C (23 API). For the Vaca Muerta shales an average density of 2.4 g/cm3 was taken from the model at the age of the laccolith emplacement. The total calculated volume of hydrocarbons generated by the Middle Laccolith thermal effect over the Vaca Muerta source rock is 20.5 million cubic meters oil equivalent (approximately 129 million barrels). In this petroleum system the reservoir and trap formation, as well as generation and migration processes, occur at the same time. Reservoirs are emplaced into the source rock, which act also as a seal. Migration distances are very short, from 0 m at the contact to a few hundred meters for more distant transformed areas of the source rock. Furthermore, reservoirs in shallower structural levels of the stratigraphic column are also charged. All these characteristics favor a high generation–accumulation efficiency. 8. Conclusions In Altiplanicie del Payún area, both the Upper Jurassic Vaca Muerta and the Cretaceous Agrio source rocks are largely immature (0.4–0.6% VRE). However, significant thicknesses of source rock have been locally matured by thermal anomalies related to Tertiary intrusions of alkaline laccoliths. Commercial oil accumulations (20– 33 API) and oil shows have been identified along the entire 602 F. Rodriguez Monreal et al. / Marine and Petroleum Geology 26 (2009) 590–605 Fig. 17. Petroleum saturation (solid colours) and migration (yellow arrows) after local maturation of Vaca Muerta and Agrio source rocks by thermal effect of the igneous bodies at (a) 46.9362 Ma (63,800 years after igneous intrusion) and (b) at present-day (47 Ma after the magmatic event). Oil migrates into the laccoliths and also reaches shallower and deeper reservoirs. Fig. 18. Mixing of hydrocarbons from different maturity Agrio and Vaca Muerta source rock sections, convective water flows and petroleum migration towards the laccolith at 46.9996 Ma. Pink and light blue arrows represent modeled petroleum and water flows. Solid colours represent vitrinite reflectance values. F. Rodriguez Monreal et al. / Marine and Petroleum Geology 26 (2009) 590–605 603 Fig. 19. Oil (a) and gas (b) fractions in and around the Middle Laccolith at 45 Ma. stratigraphic column, both in sandstone/carbonate and fractured overpressured igneous reservoirs. The good geochemical correlation between oils and source rock samples from the study area, having a different molecular signature than burial generated oils from deeper areas of the basin, is interpreted as the result of local maturation processes during emplacement and cooling of the igneous rocks. Furthermore, ADP oil samples show typical oil-window maturities (0.7–0.9% VRE) in the range of thermally affected source rock samples (0.6–1.0% VRE). Mixing processes of high and low maturity products, evidenced by diamondoids’ analyses, support oil generation and migration within full maturity range source rock sections tens to few hundred meters from the contacts. A 2D model that included the thermal effect of the three main igneous bodies (110–600 m thick) was satisfactorily achieved which accounts for thermal maturation, generation, and migration processes. Modeling results are in agreement with the geochemical data and the spatial distribution of known hydrocarbon accumulations. Full maturity aureoles over hundreds of meters of source rocks are developed in time and in space by progressive heat transfer from the igneous intrusive bodies. Thermal anomalies are dissipated from thousands to a hundred thousands years after laccoliths emplacements with intrusion temperatures of 980  C. Convective water flows, high pore pressures, as well as source rocks fracturation characterize this extremely fast generation event, favoring petroleum mixing and migration towards shallower and deeper reservoirs and also feeding the igneous bodies as they get fractured while cooling. Compositional modeling shows that oils with API gravities in the range of production oils are generated and accumulated in and around the igneous bodies. Despite gas generation and accumulation are also expected, and indeed observed in the model, low amounts of gas have been found in the study area, probably due to leakage through seals and faults. The composition of the final products is highly dependent on expulsion, migration, secondary cracking and mixing processes which require future research. The integration of oil and source rock geochemical data with numerical modeling was helpful for the better understanding of these non-conventional generation and migration processes. Important generated volumes and highly efficient migration and 604 F. Rodriguez Monreal et al. / Marine and Petroleum Geology 26 (2009) 590–605 accumulation processes that led to the presence of economical oil fields in the study area, make this atypical petroleum system an interesting exploration target. Acknowledgements This work has been supported by YPF S. A. Exploración Onshore, we specially thank Ricardo Calegari, Pedro Lafourcade, Ricardo Ferrante, Jorge Hechem and Tomás Zapata. We also thank Luis Alvarez, Nestor Vitulli, Hernán Maretto and many other colleagues from E&P Neuquén offices for their contributions in this study. We also acknowledge Beicip-Franlab for its technical support and Rodrigo Ferreira for the artwork. We are thankful to Peter Kukla, André Bender and Peter Mc Gregor for their fruitful reviews. We acknowledge YPF S. A. for authorizing the publication of these data. References Araújo, L.M., Trigüis, J.A., Cerqueira, J.R., da Freitas, L.C.S., 2000. The atypical Permian petroleum system of the Paraná Basin, Brazil. In: Mello, M.R., Katz, B.J. (Eds.), Petroleum Systems of South Atlantic Margins, AAPG Memoir 73, pp. 377–402. Baudino, R., Rodrı́guez, F., Zencich, S., Calegari, R., 2004. 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