2.2.1 PETROLEUM SYSTEMS AND STRATIGRAPHY
The Upper Magdalena Valley (UMV) and Putumayo basins are two important oil- producing basins in southwestern Colombia (Figure 2.1). The southern UMV, the Neiva Sub-basin, is an intermontane basin located between the Central Cordillera and Garzón Massif, the Putumayo is a foreland basin located to the east of the Garzón Massif. Mora et al. (2010) proposed that during the Mesozoic and Paleogene the UMV and Putumayo basins were part of the same extensive mega-basin, and they were later separated during the Andean orogeny when the Eastern Cordillera and Garzón Massif were uplifted.
The Putumayo Andean foreland basin in Colombia is the northern extension of the Oriente and Maranon basins of Ecuador and Peru (Figure 2.1; Higley, 2001). The Putumayo basin is relatively unexplored compared to the more mature Llanos basin to the north and the basins to the south, and offers prospective exploration opportunities. It covers about 50,000 km2, between 0° - 3° N. Lat. and 74° - 77° W. Long. (Figure 2.1). The Putumayo basin has proved reserves of more than 365 million barrels of oil equivalent to date in 19 crude oil fields (Higley, 2001). In 1963 Texaco discovered the major Orito oil field with reserves on the order of 250 MMBO (Barrero et al., 2007). The hydrocarbons are sourced within three petroleum systems (Figure 2.3): (1) a nearshore marine clastic sequence of Lower Cretaceous Caballos Formation; (2) a proximal sequence of Upper
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.Figure 2.2 a) Geologic location map of the northern and southern study areas, b) Geologic location map of the southern Putumayo/San Gabriel study area.
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Cretaceous Villeta Formation; and (3) a distal marine carbonate sequence of the upper Villeta and Eocene Pepino (Gonçalves et al., 2002; Wolaver et al., 2015). The principal reservoir rocks are shallow-marine sandstones of the Caballos and Villeta Formations. Interbedded organic-rich source rocks and shales serve as source rocks and seals. The tectonic deformation in the Putumayo foreland basin has been variously described as being produced by dextral wrench faulting, rift basin inversion, “thin-skinned” thrust faults, and “thick-skinned” basement faults, e.g., Portilla (1991), Jimenez (1997) and Witte et.al (2013).
Between 1962 and 1982 several oil fields were discovered north of Neiva UMV in the Cretaceous Monserrate Formation In 1984 oil was discovered along the Dina-San Jacinto fault in the Aptian-Albian Caballos Fm (Sarmiento and Rangel, 2004). By 1994 542.1 MMbbl of recoverable oil and 91.9 BCF of gas had been discovered in more than 30 accumulations and 287.5 MMbbl of oil had been produced. A total of 191 exploration wells and more than 300 appraisal and development wells have been drilled in the UMV. Buitrago (1994) defined two petroleum systems in the southern Neiva sub-basin related to source rocks in the Villeta Group (Figure 2.3).
Vitrinite reflectance results for the Putumayo Cretaceous source rocks indicate thermal immaturity (Aguilera et al., 2010). Petroleum generation likely occurred to the west under the present Cordillera or within the UMV (Gonçalves et al., 2002; Wolaver et al., 2015). In the Neiva Sub-basin of the UMV Tmax pyrolysis data indicate intervals of early mature source rock in the lower part of the Villeta Group (Sarmiento and Rangel, 2004). Oil generation modelling indicate that the lower part of the Villeta Group reached the top of the main oil window at 3505 m during the beginning of the Miocene, and the bottom of the
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late oil window at 4572 m at the end of Miocene (Sarmiento and Rangel, 2004). Basement for the Putumayo and Neiva UMV basins ranges from Grenville-age crystalline rocks to Triassic and Jurassic synrift siliciclastic and volcanic deposits of the Saldaña or Motema Formation (Figure 2.3, Mojica and Bayer, 1987; Ramos, 2010). Basement in the Putumayo and Neiva Sub-basins is overlain by two major megasequences separated by a regional unconformity.
The Aptian to Paleocene sedimentary rocks constitute a major marine megasequence (Figure 2.3). The source rocks and the main reservoir rocks are part of this megasequence. During Aptian to Maastrichtian times the sedimentation of the Caballos and Villeta Formations took place during regional thermal subsidence. The Villeta Formation which is the primary source rock in the basin with TOC 1.0-1.7%, has type II and III Kerogen and API 25-30 on the west side of the basin. Villeta sandstones have proven reservoir potential with average porosities of 12-18% and an average permeability 67 MD. From the Maastrichtian to Early Paleocene, the final accretion of the Western Cordillera caused the inversion of ancient normal faults, uplift of the Central Cordillera and a marked change of sedimentation from marine (Villeta Fm.) to continental (Rumiyaco Fm.) conditions (Montenegro and Baragan, 2011).
The second mainly continental megasequence extended from Eocene to Miocene (Figure 2.3), and records the sedimentary and tectonic burial responsible for petroleum generation and the deformation events that produced petroleum traps (Sarmiento and Rangel, 2004). During the Early Paleocene to Middle Eocene, increasing convergence rates between the Nazca and South America plates inverted normal faults, leading to the
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Figure 2.3 Stratigraphic columns and petroleum systems of Neiva Sub-basin (UMV) and Putumayo basins, (modified after Buitrago, 1994; Montenegro and Baragan, 2011).
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development of thrust structures, and erosion and deposition of the coarse siliciclastics of the Pepino Formation. Uplift of the Central Cordillera during the Middle Eocene to Middle Miocene times induced flexural subsidence and the deposition of the Orito-Belen Group and the Ospina Formation in a foreland basin. Finally, the intense compression of the Andean Orogeny (Late Miocene to Recent) induced major uplift of the Eastern Cordillera and Garzón Massif, separating the Putumayo basin from the UMV which became an intermontane basin (Gonçalves et al., 2002).
2.2.2 LARAMIDE BASEMENT TECTONICS
One of the most controversial and least understood mountain forming styles is that of the Laramide orogeny (40-70 Ma) in the central and southern Rocky Mountains of the United States. The basement block, “germanotype”, or “thick-skinned” tectonic style of the Laramide orogeny is characterized by broad zones of uniform strike and dip separated by narrow zones of steeper dips or high angle faults. The overlying sedimentary rocks may be folded or “draped” over the faulted basement blocks forming monoclines. Bouguer gravity anomalies directly reflect the movement of the basement blocks. The uplifts have Bouguer gravity anomaly highs indicating that dense basement rocks are involved and that the uplifts are “rootless”, i.e., without crustal thickening. A controversial question has been whether the basement uplifts were formed by compressional tectonics or by vertical tectonics. Proponents of vertical tectonics believe that the magnitude of the vertical displacements exceeded the magnitude of horizontal displacements during the Laramide orogeny (e.g., Stearns, 1975). Subsequent seismic studies have revealed significant crustal shortening by brittle thrust faulting (e.g., Smithson et al., 1978; Erslev et al., 2016).
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The common occurrence of overturned fault slivers of Mesozoic or Paleozoic sedimentary rocks beneath the Precambrian rock hanging-wall thrusts suggest a fault- related folding kinematic model for many of the Rocky Mountain basement uplifts. Low- temperature, basement-involved compressive folds are confined largely to the hanging walls of thrust faults and appear to be produced in response to both propagation and slip on non-planar faults. Kinematic models of three Laramide structures, including the Big Thompson anticline, Colorado (Narr and Suppe, 1994) involve thrust faults propagating through the brittle upper crust along non-planar paths (Figure 2.4). The stratified cover sequence in many cases forms a drape fold or monocline over the propagating basement fault. Other examples of basement fault monoclines are found throughout the Laramide US Rocky Mountains (e.g., Smithson et al., 1978; Miller and Mitra, 2011), the Venezuelan Merida Andes (DeToni and Kellogg, 1993), and the Sierras Pampeanas in Argentina (Jordan and Almendinger, 1986; Ramos et al., 2002). At least 16 wells have been drilled through Precambrian rocks for oil and gas prospecting in the sedimentary rocks that are concealed and virtually unexplored beneath the Rocky Mountain front thrusts (Gries, 1983). The wells set up potential footwall plays and helped define the structural geometry of the mountain-front thrusts, including the angle of the thrust, the amount of horizontal displacement, and the presence or absence of fault slivers containing overturned Mesozoic or Paleozoic rocks. Appendix A Table A.1 lists the well depths, dips of the fault angles at TD, and subthrust fault sliver thicknesses, if available. The mean dip angle from the ten wells is 22° with a large standard deviation. As Figure 2.5 shows, the measured dip angle distribution is bimodal, but the most common mode is low angle thrusting (5 to 30 degrees).
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The measured Garzón thrust (Iskana-1) well dip angle (17°, this paper) lies near the average for the low angle basement thrust dip mode.
2.2.3 GARZÓN MASSIF GEOLOGIC BACKGROUND
The Garzón Massif is an uplifted basement block bounded by the UMV to the northwest and the Putumayo basin to the southeast (Fig 2.1). The Garzón Group basement, part of the Garzón-Santa Marta granulite belt, consists of 1.2 – 1.0 Ga old granulites, gneisses, amphibolites and minor ultramafic and calcsilicate rocks (Priem et al., 1989). Based on similar ages and rock types, Kroonenberg (1982) suggested that the Garzón-Santa Marta granulite belt and the Grenville Province at the southeastern edge of Laurentia were related and formed by the long-lived Grenville collision during the assembly of Rodinia. According to Van der Wiel’s (1991) geochronological studies, the major uplift events in the Garzón Massif from 1.18 Ga to the present day are: 1) 1180 Ma: Relative rock uplift of 6 km, 2) 900 Ma: Orogeny, possibly also related to Grenville, resulting in at least 10 km rock uplift, 3) 12 Ma - Present: 6.5 km of rock uplift related to the Panama Arc-North Andes collision.