Cretaceous and Tertiary petroleum systems in the Catatumbo basin (Colombia)

Transcription

1 Cretaceous and Tertiary petroleum systems in the Catatumbo basin (Colombia) J. NAVARRO COMET & A. ALAMINOS MARTINEZ CEPSA E.P., S.A., c/ Ribera del Loira, 50, 28042, Madrid, Spain ABSTRACT It is known that around 90% of the world s hydrocarbons have been generated by a relatively small number of ultra-rich source intervals deposited during specific periods of geological time. Many mechanisms have been proposed to explain this particular stratigraphic source rock distribution (oceanic anoxia, biologic evolution, global climate changes, etc), but its origin is still in debate. It is clear that basins where one or more of these world class source rocks are present (e.g.: northwest South America, central North Africa) makes them extraordinarily prolific. However, in most cases all the hydrocarbons found in those areas are systematically considered to be uniquely sourced by these ultra-rich intervals, neglecting the importance of other apparently less relevant organic-rich intervals. The Catatumbo basin is the southern extension into Colombia of the prolific petroleum Maracaibo basin of Venezuela. Oil production in the Catatumbo basin comes from Cretaceous (20%) and Tertiary (80%) reservoirs. The region is characterized by the widespread presence of La Luna formation, an ultra-rich petroleum source rock of Late Cretaceous age. Since the early exploration days, many oil seeps were found associated with outcrops of La Luna formation, clearly indicating its enormous generation potential. The use of geochemical analytical techniques indicated that many of the hydrocarbons found in the Maracaibo basin are genetically related to this ultra-rich source rock sequence. However, there are strong geological and geochemical evidences suggesting that other organicrich intervals of Early Tertiary age, not so obvious, but with excellent source rock properties and thermally mature, are responsible for most of the commercial oil found in the Catatumbo basin in Colombia. INTRODUCTION The Catatumbo basin is the name given in Colombia to the southern edge of the Maracaibo basin of northwestern Venezuela (Figure 1). The Maracaibo-Catatumbo basin is bounded by the Perija Range on the west and the Merida Andes to the southeast. The convergence to the south of these two elements marks the southern end of the basin. 73º 72º 71º Cuiza fault Santa Marta Massif 11º 10º 9º 8º Perija Range Tibu Baja Guajira Rio Zulia COLOMBIA Oca fault La Paz Boscan Urdaneta Maracaibo Basin Merida Andes Gulf of Venezuela Maracaibo Centro VENEZUELA Catatumbo Basin Lagunillas Falcón Basin Bachaquero PANAMA Figure 1. Location of the Catatumbo basin, which is the southern portion into Colombia of the prolific petroleum Maracaibo basin of Venezuela. Blue rectangle indicates area cover in Figure 3. The Maracaibo-Catatumbo basin is considered one of the most prolific hydrocarbon basins in the world, with estimated recoverable oil reserves to be more than 50 billion barrels and gas reserves greater than 50 trillion cubic feet (Talukdar and Marcano, 1994). Oil production principally comes from sandstone reservoirs, located on anticlines with multiple pay horizons, from Cretaceous to Tertiary age. Originally, the organic-rich, bituminous, black calcareous shales of La Luna formation of Late Cretaceous age were considered the most important, if not the only, oil source rock in this region (Blaser & White, 1984). Although La Luna formation is considered the main source rock in the Maracaibo basin, geochemical analysis suggests that source rocks of Early Tertiary age may be responsible for most of the commercial oil found in the Catatumbo basin (Figure 2). Misoa Trujillo Mene Grande ECUADOR Oil & gas fields CENOZOIC MESOZOIC BASEMENT VENEZUELA COLOMBIA BRASIL

2 0 ft AGE QUATERNARY TERTIARY CRETACEOUS PLIOCENE MIOCENE OLIGOCENE EOCENE PALEOCENE SENONIAN CONIACIAN TURONIAN - CENOMAN. ALBIAN APTIAN FORMATION NECESIDAD GUAYABO LEON CARBONERA MIRADOR LOS CUERVOS BARCO CATATUMBO URIBANTE MITO - JUAN COLON LA LUNA COGOLLO AGUARDIENTE MERCEDES TIBU PRE-CRETACEOUS BASAMENT LITHOLOGY PETROLEUM SYSTEMS Figure 2. Generalized stratigraphic column of the Catatumbo basin. Two petroleum systems, Cretaceous and Tertiary, can be differentiated, each characterized by different oil qualities and reservoir properties. Major production in the Catatumbo basin has been obtained from Barco (220 MMBO) and Mirador reservoirs (140 MMBO). Some 100 MMBO are estimated to have been produced from all the Cretaceous reservoirs. The above situation is also common in other basins, where the presence of ultra-rich source rocks makes all the oil found to be immediately ascribed as generated by these intervals, whereas the importance of other sources is not considered. There are many examples worldwide, such as central North Africa, where the presence of two ultra-rich source rocks intervals: Early Silurian and Late Devonian (Frasnian) ages, makes these to be thought as the origin of all the Paleozoic-sourced hydrocarbons of the region. These two source rocks are impressive, regionally extensive, with TOC values that may reach up to 15% and thickness up to 500 ft, representing vast amounts of organic matter incorporated into the sediments. Their origin and widespread distribution is not clear and is still in debate: oceanic anoxia, biologic evolution, global climate changes, etc. However, besides these two ultra-rich sources there are other Paleozoic intervals (e.g.: Ordovician and Carboniferous shales) locally present in certain areas, finely disseminated along the section, with less impressive source potential, but able to S º API S > 40 º API generate important quantities of hydrocarbons (Malla et al, 1998). This paper aims to describe and characterize the petroleum systems in the Colombian portion of the Maracaibo-Catatumbo basin and to discuss other source rock candidates apart of the ultra-rich La Luna formation. EXPLORATION HISTORY Since the early exploration days, numerous oil seeps, some of them accompanied by gas are known in the region (Miller, 1917; Notestein et al., 1944). In the boundary between Colombia and Venezuela many place-names were given because of the presence of petroleum at surface, such as Petrólea in Colombia and La Alquitrana in Venezuela (Figure 3). The first oil activities in the region started on 1905, when the general Virgilio Barco obtained a concession from the Colombian government for oil exploitation and refining. A mule trail and a small still for oil refining were built. Oil was mined at Petrolea, a surface asymmetric anticline where The principal oil producing horizon is a petroliferous black shale and limestone of Cretaceous age which wherever found in this region, appears to be more or less petroliferous (Miller, 1917). This petroliferous black shale and limestone was later known in the literature as La Luna formation. This formation appears commonly impregnated with oil, filling fractures, vugs and fossils, suggesting the oil was indigenous, i.e., sourced by the organic matter in La Luna formation (Hedberg, 1931). Although this formation was ascribed as the main source horizon for petroleum, other petroliferous formations were recognized higher in the Tertiary section (Miller, 1917; Hedberg, 1931, 1968). Exploration in the area began in 1914 with the drilling of the first well in Venezuelan territory by the Colon Development Company (Shell subsidiary). The well, located in Rio de Oro, a surface anticline straddling Venezuela and Colombia, produced small amounts of oil from the Barco formation of Paleocene age (Figure 3). An offset well in the Colombian side of this anticline also flowed small amounts of oil, but rapidly dropped and the well was abandoned. The Barco concession was cancelled by the Colombian government in 1926 and a new concession was awarded to the Colombian Petroleum Company (Colpet) in 1931 (Notestein et al, 1944). Exploration activity consisted of surface geological surveys, outlining the regional

3 Rio de Oro Puerto Barco Orú Tibu Orú Yuca VENEZUELA La Palma Los Manueles Socuavo Tarra West Tarra Concordia Puerto Barco, Orú and Yuca (1958). In most of the fields, oil was tested at two or more stratigraphic horizons, from Lower Cretaceous up to Paleocene. The most significant discovery was Tibú, with estimated recoverable reserves greater than 250 MMBO, mostly coming from the Barco formation of Paleocene age. The other discoveries were quite small, with field reserves from 1 to 10 MMBO. On 1962, Richmond Petroleum Company (now ChevronTexaco) discovered the Rio Zulia oil field with 39º API oil at the Mirador sandstones of Eocene age, revealing a high-quality reservoir, laterally continuous, with a strong water drive and with ultimate reserves estimated in 140 MMBO. 8º 30 N Sardinata Petrolea Carbonera During the 80s, two minor gas discoveries were made in the basin: Cerrito-1 (Amoco, 1980) and Cerro Gordo-1 (Texaco, 1987), where subeconomic amounts of dry gas were produced from the organic-rich shales of La Luna formation, acting as both source and reservoir. 8º 00 N Rio Zulia West Cerro Gordo Rio Zulia COLOMBIA Cerrito Ureña Cúcuta km La Fria The most recent discovery in the basin is the Zulia West field (Cepsa, 2002) with the drilling of RZW-2 well, located 4 km northwest of the Rio Zulia field (Figure 3). The well tested oil from the Barco formation, flowing 1,071 BOPD 39.6º API and 2.6 MMSCFGPD on choke 48/64. Some oil staining in cuttings and oil-bearing fluid inclusions were found in the Mirador reservoir, but flowed fresh water on testing. The well was completed in the Barco formation and was produced during three months, recovering a total of 24,441 barrels of oil, 96.8 MMSCF and 6,776 barrels of fresh water. However, pressure and flow decline suggesting an isolated and small size reservoir and the well was plugged and abandoned. Caño Limón 72º 30 W La Alquitrana PETROLEUM SYSTEMS Figure 3. Map showing the location of oil (green) and gas (red) fields in the Catatumbo basin of Colombia. For area location see Figure 1. stratigraphy and detailed mapping of surface anticlines. In 1933 an exploration well was spudded on the Petrolea anticline (Figure 3). It blew out and caught fire while drilling the Cogollo limestones at less than 200 ft depth, just below La Luna formation. Oil gravity was 47º API with a high gas-oil ratio. Initial oil reserves in the Cretaceous section of Petrolea are estimated in 38 MMBO. Subsequent drilling by Colpet confirmed the presence of other oil fields in the Catatumbo basin: Carbonera (1938), Tibú (1940), Sardinata (1941), The review of published literature (Talukdar and Marcano, 1994; Tocco et al, 1995; Yurewicz et al, 1998; Wavrek and Butler, 2000), the access to a great amount of information from the Ecopetrol database and new geochemical data coming from exploration well RZW-2 (Talukdar, 2003) allowed to confirm the presence of two petroleum systems in the Catatumbo basin: Tertiary and Cretaceous. Hydrocarbon production in the Catatumbo basin has come from Cretaceous (20%) and Lower Tertiary (80%) reservoirs, both clearly differentiated in terms of oil properties (Figure 2). Oil characteristics change vertically, from volatile, lighter oil (>40º API) with high gas-oil ratio (GOR) in the Cretaceous section, and black, heavier oil

4 (20-40º API) with lower GOR in the Lower Tertiary section (Figure 2). Thus, based on API gravity, a threshold can be established in the basin to differentiate Cretaceous (>40º API) from Tertiary (<40º API) reservoired oils. More refined geochemical data on oils from the Catatumbo basin also suggest two families (Figure 4). The first family is composed of oils produced from the Cretaceous formations Cogollo, La Luna and Uribante Group in the fields Petrolea, Tibu, Sardinata, Cerro Gordo and Rio de Oro. The second family is made of oils recovered from Barco and Mirador formations in Carbonera, Tibú, Rio Zulia and Rio Zulia West fields. The geochemical data on source rocks is too limited for any definite oil-to-source rock correlation. However, the interpretation of the available geochemical data and some geologic considerations strongly suggest the presence of two important source rock intervals in the basin. d 13 C Aromatics Tertiary -26 Cretaceous -27 d 13 C Saturates Figure 4. Diagram of stable carbon isotopic composition saturates versus aromatics for the Catatumbo oils. Isotopic data clearly show two oil families: Cretaceous and Tertiary. The oil tested in RZW-2 well plots (black point) within the Tertiary oil family area. CRETACEOUS SOURCE ROCK The Cretaceous La Luna formation has long been referred as the only source rock in the Catatumbo-Maracaibo basin (Blaser and White, 1984). Age and facies equivalents formations of La Luna are also considered the main petroleum source rock in Trinidad (Naparima), eastern Venezuela (Querecual), Colombia (Villeta), Ecuador (Napo) and northern Peru (Chonta). Its high organic content, important thickness and regional presence make it one of the most prolific source rocks in the world. The La Luna formation is a 200-ft thick sequence of rhythmically bedded black shales, laminated limestone and calcareous shales with thin beds of black chert and phosphorite, deposited in a shelf-to-slope marine environment. La Luna formation contains excellent oil-prone type II kerogen source rocks that are rich in hydrogen. The organic content (TOC) generally ranges from 1.5 to 10%. Deposition of these sediments resulted from several possible factors, including warm oceanic deep waters with low dissolved-oxygen contents, sluggish circulation in restricted basins and elevated marine productivity. The deposition of the La Luna organic-rich section has also been explained as controlled by plate tectonic readjustments which may have led to intense and volumetrically important volcanism and a dramatic sea level rise (Villamil, 2001). Volcanism activity may supply a large amount of nutrients (silica) that favors planctonic (siliceous) and algal blooms, creating an increase in organic matter productivity. Volcanism may also inject significant volumes of CO 2 into the atmosphere, leading to warm conditions, ecological crisis and mass mortalities. All this favored voluminous organic fallout from the water column to seabed. A continuous changing environment and high sedimentation rates may have considerably contributed to incorporation and preservation of organic matter in the sediments of La Luna formation. Vitrinite reflectance (%Ro) analysis indicates that La Luna source is thermally mature to overmature in large portions over the Catatumbo basin, even close to surface, such as in the Petrolea anticline, where La Luna is found outcropping and showing %Ro >1.3 (Figure 5). Due to its high maturity values, La Luna is able to generate light oils (>40º API) with high GOR, condensates and even dry gas (e.g: Cerrito). In the Catatumbo basin most of La Luna generated hydrocarbons appear to remain trapped in Cretaceous reservoirs, generally characterized by low porosity (< 6%) and low permeability, unless naturally fractured. Apparently, in the Catatumbo basin, little La Luna oil has vertically migrated into Tertiary reservoirs. Vertical migration of fluids into Tertiary reservoirs is very difficult because the presence of the thick (~3,000 ft) super-sealing section provided by the Colon and Mito Juan massive shales. The only viable mechanism for upward migration is along open vertical faults combined with cross-fault migration, but this is difficult because faults tend to be closed in plastic rocks and faults will juxtapose shale against shale.

5 %Ro %Ro MDBRT (feet) 6000 MDBRT (feet) Figure 5. Present-day vitrinite values (%Ro) versus depth for rock samples from the Cretaceous section of the Catatumbo basin. Continuous vertical lines correspond to %Ro = 0.7 (green) and 1.1 (red). Note the general high maturity values, even at shallow depths (less than 2000 ft). Vitrinite values close to surface were obtained from La Luna formation in Petrólea. TERTIARY SOURCE ROCK The presence of a Tertiary petroleum system in the Maracaibo-Catatumbo basin has been suggested in several papers (Talukdar and Marcano, 1994; Yurewicz et al, 1998) and was confirmed by geochemical analysis run in oil and rock samples from exploration well RZW-2. Now this petroleum system is envisaged as the most volumetrically important in the Catatumbo basin. The Maracaibo-Catatumbo basin is well known as a prolific oil province, but is poorly known as a coal-rich basin. Coal beds from Carbonera and mainly form Los Cuervos formations (or lateral equivalents) are mined in Venezuela and Colombia (Blaser and White, 1984; Tocco et al, 1995) since the 19 th century, long before oil was found in commercial quantities. Shales rich in carbonaceous or coaly material have been also recognized by drilling in the Catatumbo formation (Talukdar, 2003) Figure 6. Present-day vitrinite values (%Ro) versus depth for rock samples from the Tertiary section of the Catatumbo basin. Continuous vertical lines correspond to %Ro = 0.7(green) and 1.1 (red). In some wells, values greater than 0.7 % are attained at 2000 ft depth. Vitrinite values at surface were obtained from coal outcrops and mines. These Lower Tertiary formations consist of a series of shales, siltstones and sandstones intercalated with carbonaceous shales and coal beds (10 to 30 ft thick). These sediments were deposited in fresh, brackish to near-shore marine environments. Net coal thickness ranges from 5 to 25 ft. These coals are classified as sub-bituminous to high volatile bituminous. Commonly, the coals and shales show mixed organic facies (Type II/III kerogen) with about 70-75% lipid rich vitrinite, 10-25% massive or undifferentiated unstructured lipids, trace to 5% structured lipids (liptodetrinite, sporinite, resinite, cutinite), 5-10% inertinite and solid bitumen (Talukdar, 2003). Coal and carbonaceous shales had been generally discounted as oil source by most authors and are generally considered as an important source of gas. However, there are many examples where coals and associated shales have been evidenced as oil sources (Scott and Fleet, 1994). The link between oils and coal-bearing strata was first made by Hedberg (1968) who wrote the classical paper discussing the origin of the oil in the Maracaibo basin (Hedberg, 1931). Hedberg

6 (1968) mentioned the oils from the Tertiary of the Barco region of Colombia as an example of waxy oils than could have been sourced by terrigenous vegetable matter. Geochemical analysis in the Catatumbo basin shows that the Lower Tertiary organic-rich beds have potential to generate oil. The TOC is very high (70-80%). Rock-Eval analysis show S 2 values ranging between 150 to 250 mg/gr and hydrogen index (HI) commonly above 200 mg/g, indicating a mixture between kerogen type II and III. Regional vitrinite reflectance values (Figure 6) indicate these coal beds and carbonaceous shales are mature enough (%Ro = 0.7 to 0.8%) to locally generate oil. The presence of oil-droplets (exsudatinite) and surface oil seeps intimately associated to these shales and coals is further evidence that they are able to generate liquid petroleum. Tocco et al (1995) clearly established the oil-to-source correlation between oil seeps in the Mérida Andes of Venezuela with Lower Tertiary coals and carbonaceous shales. Geochemical analysis run in several rock and oil samples obtained in the well RZW-2 (Talukdar, 2003) suggest an excellent correlation of the oil produced from the Barco formation with the organic rich shales and coals of Los Cuervos and Catatumbo formations. Saturate and aromatic stable carbon isotopes and pr/ph ratios for the Barco oil and Los Cuervos and Catatumbo rock extracts are quite similar. The pr/nc17 versus ph/nc18 ratios of the rock extracts also compare well with that of the oil. Furthermore, the biomarker data suggest a general similarity of Los Cuervos and Catatumbo rock extracts with the Barco oil and only minor differences in organic facies and thermal maturity are present. The maturity of the RZW-2 oil is also in agreement with the maturity of these Lower Tertiary source rocks in the well and with other maturity data in the basin (Figure 6). In summary, the carbon isotope, GC/MS, biomarkers and maturity data suggests the RZW-2 oil produced from the Barco sands was sourced, near by or even in place, from coals and carbonaceous shales, located immediately above and/or below the Barco reservoir and do not support its correlation with the Cretaceous La Luna source. CONCLUSIONS Geochemical and geological evidences indicate that the Catatumbo basin in Colombia contains organic-rich intervals within the Lower Tertiary section. These horizons contain carbonaceous shales and coal (Type II/III kerogen) which are mature and capable of generating nearby or in place large volumes of oil. These are postulated as the source for most (80%) of the oil commercially produced in the Catatumbo basin. The confirmation of this Tertiary petroleum system should imply a deep revision of the hydrocarbon exploration potential in the region. REFERENCES Blaser, R., and White, C., 1984, Source-rock and carbonization study, Maracaibo Basin, Venezuela. In: Demaison, G. and Murris, R.J., (Eds.) Petroleum geochemistry and basin evolution, AAPG Memoir 35, p: Hedberg, H.D., 1931, Cretaceous limestone as petroleum source rock in northwestern Venezuela, AAPG Bull., V. 15, No. 3, p: Hedberg, H.D., 1968, Significance of high-wax oils with respect to genesis of petroleum, AAPG Bull., V. 52, No. 5, p: Malla, M.S., Khatir, B., and Yahi, N., 1998, Review of the structural evolution and hydrocarbon generation in the Ghadames and Illizi Basins, In: Proceedings of the 15 th World Petroleum Congress, p: Miller, W.Z., 1917, Geological report, Barco Concession. 32 p. Internal Report. Notestein, F.B., Hubman, C.W. and Bowler, J.W., Geology of the Barco Concession, Republic of Colombia, South America. Geol. Soc. America Bulletin, V.55, No. 10. p: Scott, A.C. and Fleet, A.J. (eds.), 1994, Coal and coal bearing strata as oil-prone source rocks?. Geol. Soc. Spec. Publ. No. 7. Talukdar, S.C. and Marcano, F., 1994, Petroleum systems of the Maracaibo Basin, Venezuela. In: The petroleum system- from source to trap, (Eds. L.B., Magoon and W.G., Dow), AAPG Memoir 60, p: Talukdar, S.C., 2003, Geochemical evaluation of fifty six rock samples and one crude oil from the well Rio Zulia West-2, Catatumbo Basin, Colombia. Report prepared for Cepsa. Tocco, R., Escobar, M., Ruggiero, A. and Galárraga, F., 1995, Geochemistry of oil seeps and rock samples of the early Tertiary section from the North Andean flank of the Venezuelan Andes, Org. Geochem., V. 23, No.4, p: Villamil, T., 2001, New Exploration Plays: a Modification of a Traditional Exploration Approach in Colombia. HGS International Dinner Meeting, Wavrek, D.A. and Butler, D., 2000, Petroleum systems charged by Tertiary-age source rocks: New exploration opportunities in northern South America, Memoria, VII Simposio Bolivariano, Exploración Petrolera en las Cuencas Subandinas, Caracas, p: Yurewick, D.A, Advocate, D.M., Lo, H.B. and Hernández, E.A., 1998, Source rocks and oils families, southwest Maracaibo Basin (Catatumbo subbasin), Colombia. AAPG Bulletin, V. 82, No. 7 p: