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Yucatan Platform
The onshore and offshore Yucatan Block covers approximately 450,000 km2 of Mexico, Guatemala, and Belize. The block is a Paleozoic cratonic element whose edges have been extensively modified since it was isolated as a discrete microplate between spreading centers during the Jurassic separation of North and South America. Since the Late Jurassic, the Yucatan Block has been mantled by a variable thickness of carbonates and evaporites comprising the core of the Yucatan platform. Nomenclature for this rather uniform depositional sequence (Figure 1) varies from country to country; e.g. the Hillbank, Yalbac, and Barton Creek Formations in Belize; the Coban and Campur Formations in Guatemala; and the Cretacico Medio, Cretacico Superior, Icaiche, Chichen Itza, and Carrillo Puerto Formations in Mexico.
The platform carbonates continue to the west beyond the limits of the Yucatan Block into the Reforma Trend of Mexico and the Sierra de Chiapas of Mexico and Guatemala, where they are named the San Ricardo, Sierra Madre, and Ixcoy Formations. The southern margin of the Yucatan Block is truncated by Tertiary through Holocene left-lateral displacement of the Chortıs Block of Guatemala and Honduras along the Cuilco-Chixoy-Polochic and Motagua-Cabanas Fault Systems with pieces of the original Yucatan Block possibly dispersed along the Nicaragua Rise as far east as Jamaica. Its eastern edge, or Yucatan Borderland (Marton and Buffler, 1994), was dismembered by Paleogene strike-slip faults during the relative northward motion of Cuba, with displaced fragments of the original Yucatan Block incorporated into western Cuba (Iturralde-Vinent, 1994).
Figure 1. Generalized stratigraphic column for the Yucata´n Block.
Deformational events that have influenced the petroleum and mineral resource potential of the Yucatan Block include:
1) Late Triassic to Middle Jurassic rifting (Marton and Buffler, 1994);
2) Late Cretaceous suturing along the southern margin of Yucatan (Beccaluva et al., 1995);
3) Cretaceous-Tertiary (K/T) asteroid or cometary impact (Sharpton et al, 1996); and
4) Cretaceous to Paleogene(?) westward tilting manifested by the shallow basement (<1 km) in eastern Yucata´n (Marton and Buffler, 1994) and wells indicating deep basement (> 6 km) to the west (Lopez-Ramos, 1973). Depositional episodes related to these tectonic events include:
1) Early to Middle Jurassic red bed and eolian deposition;
2) Late Jurassic to Early Cretaceous marine transgression;
3) Late Jurassic through Holocene carbonate and evaporite (mainly gypsum-anhydrite) accumulation; and
4) Mass wasting and brecciation at the K/T boundary as a result of the Chicxulub impact event.
Tertiary sedimentation marked the return of carbonate platform deposition with the local exception of the Macuspana Basin. Despite thick sedimentary section and hydrocarbon production in Guatemala, most of the Yucatan Block has no regional seismic coverage. Exploration wells are sparse (less than one well per 20,000 km2 irregularly distributed, and mostly drilled without seismic control (Figure 2). Prospecting for metallic minerals has not been feasible beneath the generally featureless surface carbonates.
Figure 2. Geological elements of the Yucata´n Block. CC = Chicxulub Crater, CS = Campeche Sound, IPB = Intraplatform Basin, LLA = La Libertad Arch, MB = Macuspana Basin, MM = Maya Mountains, RT = Reforma Trend, SC = Sierra de Chiapas, 1 = Xan field, 2 = Eagle-1 well, 3 = Yucata´n-1 well.
HYDROCARBONS
The presence of one or more hydrocarbon systems in the Yucatan Block is known from the occurrence of oil in Guatemala and Belize.
Rift Play
Drilling in Mexico and Belize and outcrops in the Maya Mountains indicate that the crystalline crust is generally granitic with pre-Pennsylvanian metasedimentary and metavolcanic components (LopezRamos, 1973; Steiner and Walker, 1996). These authors also mention low-grade Pennsylvanian and Permian metasedimentary rocks encountered by drilling and in outcrop. This basement complex corresponds to the hinterland of the Ouachita belt of Arkansas, Oklahoma, and Texas, and may be the ‘‘Llanoria’’ of Flawn et al. (1961). The continental basement of Yucatan is stretched, since much of the block is covered by sedimentary overburden as much as six km in thickness; an impossibility on unstretched continental crust at isostatic equilibrium. Linear gravity anomalies within the Yucatan Block suggest that this crustal stretching produced a series of horsts and grabens in this continental block between the Gulf of Mexico and the Proto-Caribbean Sea spreading ridges in the Jurassic (Marton and Buffler, 1994). Few wells drilled in the Yucatan Block have reached Jurassic rocks or basement. However, a 36-m-thick section of Jurassic dolomite was described by Lopez-Ramos (1973) at the depth of 3140 m in the Yucatan-1 well (Figure 2).
The hydrocarbon source potential of this sequence is confirmed by the presence of light oil in the Eagle-1 well of Belize whose biomarkers suggest derivation from Late Jurassic or Early Cretaceous marl ( J. Zumberge, personal communication, 2000). Exploration objectives of this play would be the synrift and early post-rift sandstones on the flanks and crests of horst blocks, and carbonates deposited during transgression and platform building (Figure 3).
Figure 3. Diagrammatic southwest-to-northeast-oriented section across a gravity low that may represent an intraplatform basin and underlying rift illustrating the possible distribution of source rocks, carbonate buildups, and anhydrite seals, as typified by the Xan oilfield of Guatemala.
Intraplatform Basin Play
Xan field on the central Yucata´n Block in Guatemala (reserves of ̴ 100million barrels of oil) is on the curvilinear La Libertad Arch south of the 150- to 200-km-diameter gravity low (Lopez-Ramos, 1973) outlined in Figure 2. This field produces 168 API gravity oil from vuggy dolomite in a carbonate buildup of Turonian age. The reservoir is sealed by anhydrite and overlies organic-rich, oil-prone Cenomanian carbonate source rocks. The broad negative gravity anomaly may represent a temporally persistent intraplatform basin with a central concentration of source rocks ringed by carbonate buildups or calcarenite banks of the Xan type (Figure 3). The author speculates that this may be a ‘‘steer’s head’’ basin formed by subsidence over a major graben or failed rift. Hydrocarbon migration out of this basin would be radial, but westward tilt of the Yucatan Block would favor eastward hydrocarbon migration across a wide, unexplored swath of Mexico, Guatemala, and Belize.
Eastward oil migration was confirmed in the Eagle1 well of Belize, where 398 API gravity oil flowed from Lower Cretaceous carbonates just above the crystalline basement at a depth of 600 m. Several other wells in northern Belize (Figure 2) also had oil shows (unpublished oil company data). Relatively shallow burial depth and low organic contents of the Mesozoic strata in Belize are insufficient for hydrocarbon generation, indicating that the Eagle-1 oil had its origin in a relatively distant hydrocarbon generation kitchen regionally downdip in Mexico and/or Guatemala.
It is probable that the Xan reservoir is not the only carbonate buildup on the inner platform of the Yucatan Block. This facies tract is characterized by laterally extensive, eustatically controlled alternations of carbonate and anhydrite along migration pathways radiating from the proposed intraplatform basin (Figure 3). This framework is similar to that of the supergiant oil accumulations on the Arabian platform where oil generated in the intraplatform Hanifa Basin is trapped among cyclic carbonates and anhydrites of the Arab Formation beneath the massive Hith Anhydrite (Wilson, 1985).
Lateral Migration of Hydrocarbons from the Gulf of Mexico
The hydrocarbon accumulations of Campeche Sound (>30 billion barrels of oil) and the Macuspana Basin (>10 trillion cubic feet of gas) indicate the presence of a massive hydrocarbon charge in the area bordering the western Yucatan Block. The westward tilt of Yucatan creates a favorable geometry for capturing hydrocarbons that either have bypassed or spilled from traps in Campeche Sound and the Macuspana Basin. These hydrocarbons could be trapped in carbonate buildups in the platform sequence, or at porosity pinch outs among anhydrite layers that thicken and coalesce towards the central platform (Figure 4).
Figure 4. Diagrammatic west-to-east-oriented section showing the possible migration of hydrocarbons from Campeche Sound into the western margin of the Yucatan Block.
Astrobleme Related Hydrocarbon System
The K/T impact at Chicxulub, with a final crater diameter estimated to be 200 to 300 km (Figure 3), is among the largest preserved impact features on earth (Sharpton et al, 1996). A compelling case has been made for the impact origin of K/T dolomitic breccia reservoirs in the giant and supergiant Campeche Sound fields (Grajales-Nishimura et al., 2000). The possible existence of an impact-related hydrocarbon system within the Yucatan Block is discussed below.
Several wells were drilled into and around the impact crater prior to 1970. Some of these wells (Lo´pezRamos, 1973) penetrated igneous rocks and breccias, originally thought to be of volcanic origin, that have since been determined to be melt rock formed by the Chicxulub impact (Sharpton et al., 1996). The wells outside the crater’s rim penetrated the typical interbedded carbonate-anhydrite sequence of the Yucatan Block (Figure 5). Although a hydrocarbon system would not be expected to survive within the crater, conditions around its periphery may have been conducive to hydrocarbon generation and accumulation.
Figure 5. Diagrammatic west-to-east-oriented section across the Chicxulub impact crater with possible locations of metallic ores and hydrocarbon resources. Note the representation of ore bodies in the impact melt and in carbonatehosted hydrothermal systems adjacent to and immediately overlying the central crater. Carbonates beyond the outer crater may be hydrocarbon-bearing with seals comprising faults, interbedded anhydrites, and the overlying impact breccia. Adapted from Sharpton et al, 1994.
Hydrocarbon Generation
The Yucatan-1 well penetrated Paleozoic volcanic basement at a depth of 3200 m. The low geothermal gradient in the overlying carbonates and anhydrite may have precluded hydrocarbon generation from any possible Mesozoic source rocks in that area. Therefore, hydrocarbon presence in the area could depend upon local, impact-induced heating caused by:
1) heat radiating from impact induced melts and increased geothermal gradient caused by the rapid uplift of deep crustal layers;
2) ‘‘kneading’’ of a large rock volume by the passage of very high-amplitude impact-induced seismic waves;
3) friction along rapidly moving kilometer-scale fault/slide blocks during crater collapse; and
4) heat transfer by post-impact hydrothermal systems.
Reservoirs
Dolomite interbedded with anhydrite is well documented in the target section. The dolomite should be extensively fractured over a wide area around the crater, thereby enhancing any matrix porosity.
Traps
Faulted and tilted strata around the periphery of the post-collapse crater could provide structural traps (Figure 5).
Seals
Interbedded anhydrites, although initially fractured, would quickly heal to form internal seals for the interbedded dolomite reservoirs. Top seal rocks would consist of micritic platform carbonates, intraplatform evaporites, and/or impact breccia similar to welded volcanic tuff (Figure 6). Lateral seals would consist of faults and major fractures made impermeable by fault gouge and frictional melt (pseudotachylite) found as meter-scale dikes in well-exposed major impact structures (Peredery and Morrison, 1984), and by precipitation of hydrothermal minerals in open fractures.
Источник: Economic Potential of the Yucatan Block of Mexico, Guatemala, and Belize. Joshua H. Rosenfeld, 2001
Следующий Бассейн: North Cuba