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North Cuba Basin
This province, located in the northern part of the island and the adjacent area offshore (Fig. 1), is over 1,000-kms long and 80- to 100-kms wide. Sedimentary thicknesses greater than 10 kms are present.
The North Cuba Basin is a geologically complex area and includes several disparate geologic entities, including the Yucatan carbonate platform, the Florida carbonate platform, the southeastern Gulf of Mexico, and the fold and thrust belt of Cuba (fig. 1). The tectonic evolution of this area includes counterclockwise movement of the Yucatan Platform, the opening of the Gulf of Mexico oceanic basin, and the formation of the Cuban archipelago. The tectonic history had a direct bearing on the petroleum systems in the North Cuba Basin.
Figure 1. Physiographic features of the northwestern Cuba and the southeastern Gulf of Mexico area. Several Deep Sea Drilling Project (DSDP) wells are shown in red symbols (from Cubapetroleo, 2002).
The onshore part of the North Cuba Basin has a long history of petroleum exploration and production. The first field, Motembo, was discovered onshore in northwestern Cuba in 1881, and Motembo remains the only condensate field in Cuba. More than 20 oil fields have been discovered in Cuba since then, mostly in the North Cuba Basin (Oil and Gas Journal, 1993) (fig. 2; table 1). Although most of the onshore oil fields are small, shallow, and contain heavy oil (Petzet, 2000), the fact that oil exists there is strong evidence that one or more petroleum systems either are or were active in the subsurface of the northwestern part of Cuba. Based on oil production of onshore Cuba and the knowledge gained from several recent geologic and geochemical studies, the offshore is interpreted to have potential for undiscovered oil and gas resources, and was the focus of the present study.
Figure 2. Locations of onshore oil fields of Cuba; most oil fields are in northwestern Cuba in the North Cuba Basin (from Cubapetroleo, 2002).
Table 1. Oil fields of the onshore North Cuba Basin; North Cuba Fold and Thrust Belt Assessment Unit. [AVG, average; - -, no data.]
Geological Evolution of The Northern Caribbean Area
From the earliest studies the geology of Cuba was recognized as a series of north-verging thrust-fault-bounded tectonostratigraphic units (TSU), and the geologic definition of many TSUs was the focus of many previous investigations (figs. 3,4).
Figure 3. An interpretation of tectonostratigraphic units (TSU) of northern Cuba. TSUs are north-verging thrust-fault-bounded rock units that formed mainly as a result of the collision between Cuba and the passive margin of the Bahama Platform during the Paleogene (from Echevarria-Rodriguez and others, 1991)
Figure 4. Map and diagrammatic cross section showing general positions of tectonostratigraphic units (TSU) of central Cuba. The TSUs have been studied and named across Cuba (from Hempton and Barros, 1993).
Eventually, tectonic studies in Cuba and in the northern Caribbean placed these TSUs in a framework of modern tectonic theory (Pindell and Kennan, 2001, 2003; Pindell and others, 2005). Detailed work demonstrated that the TSUs were the product of the collision between shelf, slope, and basinal sediments of the Mesozoic passive margin of the Yucatan and Bahama platforms and the arc-forearc rocks of the leading edge of the Pacific-derived Caribbean Plate as the Yucatan Basin opened in the Paleogene (Pindell and others, 2005). The stratigraphy of Cuba is complex, and many stratigraphic studies reflect the stacked thrust sheets produced during plate collision (fig. 5). However, the general stratigraphy of many TSUs has been interpreted and restored, documenting general stratigraphic relations (fig. 6).
Figure 5. Stratigraphic column showing thrust repetitions in the Jurassic through Tertiary section in the Cuban fold and thrust belt in northern Cuba (modified from Cubapetroleo, 2002).
Figure 6. Lithostratigraphic column for northwestern Cuba and offshore (fold and thrust belt) showing reconstructed stratigraphy of the proto-Caribbean oceanic basin. Placetas and Rosario tectonostratigraphic units) form major thrust sheets (from Sanchez-Arango and others, 2003).
Major events in the geologic history of northwestern Cuba include: (1) rifting between North America, South America, and Africa in Late Triassic-Early Jurassic time; (2) the tectonic evolution and passive-margin sedimentary history of the southeast Gulf of Mexico; (3) the development of the proto-Caribbean ocean basin and its passive margin; (4) movement of the Caribbean plate since the Early Cretaceous; and (5) Paleogene development of the Yucatan Basin and resultant collision and suturing of allochthonous Cuba terranes with the passive margin of the Bahama Platform. These events will be described briefly as each relates to the development of petroleum systems in the northwestern part of Cuba. The tectonic evolution of the Caribbean, especially the origin of the Caribbean plate, is somewhat controversial and is not the primary subject of this report.
Late Triassic to Middle Jurassic Rifting
The continents of North America, South America, and Africa composed the supercontinent of Pangea in the late Paleozoic and Triassic time (Salvador, 1991). In the Late Triassic, rifting began between North America and Africa and then between North and South America. Rifting continued through the Early and Middle Jurassic (Callovian), forming stretched or attenuated continental crust between the diverging continents (Marton and Buffler, 1993; 1994). During rifting, the extensional regime resulted in the formation of graben and half-graben structures in many areas of stretched continental crust, and these structures were filled with typical synrift sedimentary facies. The rift-related structures formed in the area underlain by continental crust in the northwestern part of offshore Cuba, and rift structures underlie part of the Bahama Platform (Sheridan and others, 1983; Ladd and Sheridan, 1987).
Facies of the rift-related grabens and half grabens include coarse red clastics, marine clastics, marine mudstones, and evaporites. These strata have been described from exposures on Cuba as the San Cayetano Formation (Haczewski, 1976) and as the Eagle Mills Formation from the subsurface of the northern Gulf Coast (Salvador, 1991). The synrift San Cayetano Formation might contain petroleum source rocks.
As rifting waned in the Middle Jurassic, evaporitic conditions within the extensional province resulted in the deposition of widespread evaporites (halite and anhydrite) known as the Louann Salt in the northern part of the Gulf of Mexico and the Campeche Salt in the southern part. Evaporites also were deposited in the Bahama area (Walles, 1993), but these evaporites are not stratigraphically connected to the evaporites in the Gulf (Iturralde-Vinent, 2003). As rifting continued, the continental crust was stretched to the point where individual crustal blocks were mobilized, and sea-floor spreading commenced in the central part of the Gulf of Mexico as the Yucatan crustal block began to rotate counterclockwise. The rift-related structures formed during this time might have their own source rocks, reservoirs, and traps (Magnier and others, 2004).
Opening of the Gulf of Mexico in the Late Jurassic
In Callovian and possibly into Oxfordian time, the Yucatan crustal block began to rotate counterclockwise from its pre-rift position to its present configuration (fig. 7). The Yucatan block rotated about a hypothetical pole in Florida (Pindell, 1993), and rotated along a western transform margin in Mexico known as the Tamaulipas fault system. In the process, sea-floor spreading formed oceanic crust that floors the central part of the Gulf of Mexico. The spreading also resulted in the separation of the Callovian salt into two accumulations-the northern Louann Salt and the southern Campeche Salt. Thinner salt accumulations might exist to the east in the northwestern Cuba area. In about Valanginian time, the Yucatan block docked in its present position following its counterclockwise rotation, and sea-floor spreading ceased in the central part of the Gulf of Mexico.
Figure 7. Reconstruction of middle Oxfordian paleogeography showing the partially opened Gulf of Mexico as Yucatan rotated counterclockwise, formation of incipient proto-Caribbean oceanic crust as South America drifted away from North America, and deposition of the San Cayetano Formation and related rocks along the passive margin of Yucatan Platform. Dashed black lines are uncertain geologic boundaries; dashed red line is the Bahama Fracture Zone (modified from Pszczolkowski, 1999). E, Escambray terrane; P, Pinos terrane; SO, Sierra de los Organos terrane; SR, Southern Rosario terrane; NR, Northern Rosario terrane; BFZ, Bahama Fracture Zone.
The margin of the Gulf of Mexico from the Oxfordian to the Valanginian was passive (fig. 8). Several depositional units of this time interval are interpreted to be significant petroleum source rocks in the Gulf of Mexico, and they might have extended into the northwestern Cuba area. The basinal facies of the Oxfordian Smackover Formation is known to be a prolific source rock in the northern Gulf of Mexico (Sassen and others, 1987). The basinal facies of the Tithonian (Pimienta Formation) is well known as the source for giant oil accumulations in the southern part of the Gulf of Mexico (Magoon and others, 2001), and the coeval Bossier Formation is a potential source rock for gas in the northern Gulf of Mexico Basin (Wagner and others, 2003). The Tithonian organic-rich source rock facies, like the Oxfordian shales, might have extended into the northwestern Cuba area, which would have been a deep-water environment during the Late Jurassic.
Figure 8. Reconstruction of Tithonian paleogeography showing further counterclockwise rotation of Yucatan and the opening of the Gulf of Mexico and formation of proto-Caribbean oceanic crust. Tithonian source rocks are an important component of the Jurassic-Cretaceous Composite Total Petroleum System in the North Cuba Basin. Dashed black lines are uncertain geologic boundaries; dashed red line is the Bahama Fracture Zone (modified from Pszczolkowski, 1999). BFZ, Bahama Fracture Zone.
Opening of the Proto-Caribbean Ocean Basin
As South America continued to drift away from North America, sea-floor spreading was initiated south of the Yucatan block and the Bahama Platform in about Oxfordian time, forming what has been called the proto-Caribbean ocean basin (fig. 8; Pindell, 1993). As sea-floor spreading continued, the drift of South America from North America led to the development of a passive margin along the south edge of the North American plate. The passive-margin strata associated with the proto-Caribbean plate are now known from the many TSU exposures on Cuba, and these strata are important for the interpretation of petroleum source rocks, reservoirs rocks, and seal rocks in the subsurface of northwestern Cuba.
Passive-margin conditions existed from about Oxfordian through the Late Cretaceous, during which time several potential petroleum source rocks were deposited along the passive margin (fig. 9), including mudstones of the Cenomanian-Turonian, which are known source rocks in the U.S. Gulf Coast.
Figure 9. Reconstruction of Hauterivian-Barremian paleogeography showing complete opening of the Gulf of Mexico, further opening of the proto-Caribbean oceanic basin, and development of the passive margins of the Bahama Platform and Yucatan Platform with sediments now named as terranes. Dashed black lines are uncertain geologic boundaries; dashed red line is the Bahama Fracture Zone (modified from Pszczolkowski, 1999). SO, Sierra de los Organos terrane; SR, Southern Rosario terrane; LE, La Esperanza terrane; NR, Northern Rosario terrane; P, Placetas terrane; C, Camajuani terrane; R, Remedios terrane.
Oil potential of the northern province
Twenty-one medium and small-sized oilfields have been discovered in the Northern Province, of which Varadero and Boca de Jaruco are the most important in terms of volumes of recoverable reserves.
Hydrocarbon traps are associated with asymmetric folded structures, with dimensions in the south of between 3.0-5.0 and 1.5-2.0 kms. Further north, the dimensions of fold-structures increase to 30-40 sq. kms or more. These structures are located offshore, mostly near the limit of the overthrust belt. According to seismic data (CDP), it is possible to identify relatively-flat (25-30°), incoherent, non-extensional and irregular seismic horizons, which correspond to thrust surfaces between different or the same TSU. In the upper parts of the sections, flat-lying, continuous, uniform, extensive reflections can be correlated. Their depths are limited by a prominent reflection event, related to the seismic impedance contrast associated with the base of post-orogenic Middle Eocene-Quaternary sediments of the Northern Oil Province.
The oil and gas accumulations so far discovered are associated with four allochthonous TSUs (Zaza, Placetas, Camajuani and Remedios), which are located partially offshore in western Cuba, and onshore in central and eastern Cuba. The overthrust belt is bounded to the north by an area of differing structural and sedimentological configuration corresponding to the Florida-Bahamas carbonate platform.
In seismic sections across the northern platform area, extensive flat-lying reflections are revealed, those with the highest amplitudes being associated with the Upper Cretaceous unconformity; high acoustic impedance is observed in all lines.
Jurassic-Cretaceous Composite Total Petroleum System
A large body of geochemical data strongly indicates that several petroleum source rock units are present in the North Cuba Basin (Moretti and others, 2003b). The source rocks may be thermally mature at depth in the fold and thrust belt and in the deeper parts of the foreland basin, but the shallower, stratigraphic intervals of potential Cretaceous source rocks to the west of the fold and thrust belt are not thermally mature.
Petroleum from the thrust belt and from the foreland basin might have migrated updip into traps in the thrust belt and in the foreland basin (Lopez-Rivera and others, 2003a, b), and possibly also migrated laterally to the margins of the Yucatan and Bahama carbonate platforms. Oil shows in core from DSDP Site 535 and from wells along the southwest margin of the Bahama Platform indicate that migration of petroleum occurred within this composite total petroleum system.
Although oil is the hydrocarbon found in the onshore fields in Cuba, oil and nonassociated gas accumulations might be present in the deeper parts of the thrust belt and in the foreland basin.
Table 2. North Cuba Basin assessment results [MMBO, million barrels of oil; BCFG, billion cubic feet of gas; MMBNGL, million barrels of natural gas liquids. Results shown are fully risked estimates. For gas fields, all liquids are included under the NGL (natural gas liquids) category. F95 denotes a 95-percent chance of at least the amount tabulated. Other fractiles are defined similarly. Fractiles are additive under the assumption of perfect positive correlation. Gray shade indicates not applicable.]
Data source: Oil and gas exploration in Cuba. G. Echevarria-Rodriguez, G. Hernandez-Perez, J. O. Lopez-Quintero, J. G. Lopez-Rivera, R. Rodriguez-Hernandez, J. R. Sanchez-Arango, R. Socorro-Trujillo, R. Tenreyro-Perez and J. L. Yparraguirre-Pena. Journal of Petroleum Geology, vol. 14 (3), July 1991, pp. 259-274
Executive Summary—Assessment of Undiscovered Oil and Gas Resources of the Jurassic-Cretaceous Composite Total Petroleum System in the North Cuba Basin, Cuba, 2004. U.S. Geological Survey, Reston, Virginia: 2008
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