Changes

==Geological Setting==

==Geological Setting==

The LBC is a flat area of about 200,000 km² (77,220 mi²) between the Eastern Cordillera (westward), the [[Macarena Range]] (southward), and the border of [[Venezuela]], which corresponds for a large part to large rivers ([[Orinoco river|Orinoco]], [[Meta river|Meta]], etc.). The LBC is currently the foreland of the Eastern Cordillera, a branch of the [[Andes]]. The [[cross section]] in [[:file:M114CH08FG02.jpg|Figure 2]] shows this west–northwest dipping [[monocline]]. Southward, the Macarena Range exposes [[Cretaceous]] sequence and separates the LBC of the [[Putumayo basin]], which is the southern foreland of the Colombian Andes.

The LBC is a flat area of about 200,000 km² (77,220 mi²) between the [[Eastern Cordillera]] (westward), the [[Macarena Range]] (southward), and the border of [[Venezuela]], which corresponds for a large part to large rivers ([[Orinoco river|Orinoco]], [[Meta river|Meta]], etc.). The LBC is currently the foreland of the Eastern Cordillera, a branch of the [[Andes]]. The [[cross section]] in [[:file:M114CH08FG02.jpg|Figure 2]] shows this west–northwest dipping [[monocline]]. Southward, the Macarena Range exposes [[Cretaceous]] sequence and separates the LBC of the [[Putumayo basin]], which is the southern foreland of the Colombian Andes.

The crystalline basement of the Llanos is covered by [[Paleozoic]] series that consist of siliciclastic [[Silurian|Siluro]]-[[Devonian]] [[deposit]]s; a strong unconformity separates these, locally highly deformed, series from the [[Mesozoic]] and [[Tertiary]] deposits. The Mesozoic [[tectonic]] context was mainly extensive in Colombia, but the [[depocenter]] was located westward of the LBC (Villamil, 1999<ref name=Villamil1999>Villamil, T., 1999, Campanian-Miocene tectonostratigraphy, depocenter evolution and basin development of Colombia and western Venezuela: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 153, no. 1–4, p. 239–275, [https://www.sciencedirect.com/science/article/pii/S0031018299000759 DOI: 10.1016/S0031-0182(99)00075-9].</ref>). The Lower [[Cretaceous]] series, known in the Eastern Cordillera (Campos Alvarez et al., 2007<ref name=Camposalvarezetal2007>Campos Alvarez, N. O., and B. P. Roser, 2007, Geochemistry of black shales from the Lower Cretaceous Paja Formation, Eastern Cordillera, Colombia: Source weathering, provenance, and tectonic setting: Journal of South American Earth Sciences, v. 23, p. 271–289.</ref>), are restricted to the western side of the [[Guaicaramo fault]] and are so absent in the Llanos [[foreland]]. In the west of Colombia the compression that leads to the [[Andes]] formation began during the [[Cretaceous]] times; the first [[accretion]] started during the [[Aptian]], whereas subsidence continued eastward with the deposition of the [[Une formation|Une]], [[Gacheta formation|Gacheta]], and [[Guadalupe formation]]s in the Llanos (Sarmiento, 2001<ref name=Sarmiento2001>Sarmiento, L. F., 2001, Mesozoic rifting and Cenozoic basin inversion history of the Eastern Cordillera, Colombian Andes. Inferences from tectonic models: Ph.D. Thesis Vrije Universiteit, Amsterdam, The Netherlands, 297 p.</ref>). Subsequent accretion took place westward at the end of the [[Maastrichtian]], resulting in the beginning of [[uplift]] of the Central Cordillera and the first [[inversion]] in the now Magdalena Valley (Casero et al., 1997; Sarmiento, 2001). In the Llanos, [[subsidence]] continued in a very poorly deformed basin during the [[Paleocene]], with the [[deposition]] of the [[Barco formation|Barco]], [[Los Cuervos formation|Los Cuervos]], and [[Mirador formation]]s (Cooper et al., 1995<ref name=Cooperetal1995>Cooper, M. A., F. T. Addison, R. Alvarez, A. B. Hayward, S. Howe, A. J. Pulham, et al., 1995, [http://archives.datapages.com/data/specpubs/memoir62/35cooper/0659.htm Basin development and tectonic history of the Llanos Basin, Colombia]], in Petroleum Basins of South America: [http://store.aapg.org/detail.aspx?id=476 AAPG Memoir 62], p. 659–665.</ref>; Bayona et al., 2008<ref name=Bayonaetal2008>Bayona, G., M. Cortés, C. Jaramillo, G. Ojeda, J. J. Aristizabal, and A. Reyes-Harker, 2008, An integrated analysis of an orogen–sedimentary basin pair: Latest Cretaceous–Cenozoic evolution of the linked Eastern Cordillera orogen and the Llanos foreland basin of Colombia: GSA Bulletin, v. 120, no. 9/10, p. 1171–1197.</ref>; Reyes-Harker et al., 2015<ref name=Reyesharkeretal2015>Reyes-Harker, A., C. F. Ruiz-Valdivieso, A. Mora, J. C. Ramirez-Arias, G. Rodriguez, F. de la Parra, et al., 2015, Cenozoic paleogeography of the Andean Foreland and retroact hinterland of Colombia: [http://archives.datapages.com/data/bulletns/2015/08aug/BLTN11110/BLTN11110.html AAPG Bulletin], v. 99, no. 8, p. 1407–1454, DOI: 10.1306/06181411110.</ref>). A compressive context predominates in the Eastern Cordillera since the [[Eocene]]. After the inversions of the Mesozoic extensional [[graben]]s, a thrust regime developed, and the [[Llanos Orientales]] became a classical, but rather external, [[foreland]] during the [[deposition]] of the [[Carbonera formation|Carbonera]] and [[Leon formation]]s (Moretti et al., 2009<ref name=Morettietal2009a>Moretti, I., J. C. Mondragon, J. C. Garzon, G. Bosio, and J. M. Daniel, 2009a, Structural style and decollement levels in the Llanos Orientales Basin (Colombia). X Congress Bolivariano, Conference Proceedings.</ref>; Mora et al., 2015<ref name=Moraetal2015>Mora, A., W. Casallas, R. Ketcham, D. Gomez, M. Parra, J. Manson, et al., 2015, Kinematic restoration of contractional basement structures using thermokinematic models: A key tool for petroleum system modeling: [http://archives.datapages.com/data/bulletns/2015/08aug/BLTN11108/BLTN11108.html AAPG Bulletin]], v. 99, no. 8, p. 1575–1598, DOI: 10.1306/04281411108.</ref>). The Eastern Cordillera is a double verging mountain belt: the western flank in the Magdalena Valley was initially the more active one but the active [[compression|compressive]] front has now shifted to the Llanos where the [[Miocene]] [[foreland]] is currently affected by the compression that propagates eastward. The structure that corresponds to the former Early [[Cretaceous]] basin edge is the [[Guaicaramo fault]] (Bayona et al., 2008<ref name=Bayonaetal2008 />), but some [[thrust]]s and blind thrusts exist eastward of it (called compressive front in the figures).

The crystalline basement of the Llanos is covered by [[Paleozoic]] series that consist of siliciclastic [[Silurian|Siluro]]-[[Devonian]] [[deposit]]s; a strong unconformity separates these, locally highly deformed, series from the [[Mesozoic]] and [[Tertiary]] deposits. The Mesozoic [[tectonic]] context was mainly extensive in Colombia, but the [[depocenter]] was located westward of the LBC (Villamil, 1999<ref name=Villamil1999>Villamil, T., 1999, Campanian-Miocene tectonostratigraphy, depocenter evolution and basin development of Colombia and western Venezuela: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 153, no. 1–4, p. 239–275, [https://www.sciencedirect.com/science/article/pii/S0031018299000759 DOI: 10.1016/S0031-0182(99)00075-9].</ref>). The Lower [[Cretaceous]] series, known in the [[Eastern Cordillera]] (Campos Alvarez et al., 2007<ref name=Camposalvarezetal2007>Campos Alvarez, N. O., and B. P. Roser, 2007, Geochemistry of black shales from the Lower Cretaceous Paja Formation, Eastern Cordillera, Colombia: Source weathering, provenance, and tectonic setting: Journal of South American Earth Sciences, v. 23, p. 271–289.</ref>), are restricted to the western side of the [[Guaicaramo fault]] and are so absent in the Llanos [[foreland]]. In the west of Colombia the compression that leads to the [[Andes]] formation began during the [[Cretaceous]] times; the first [[accretion]] started during the [[Aptian]], whereas subsidence continued eastward with the deposition of the [[Une formation|Une]], [[Gacheta formation|Gacheta]], and [[Guadalupe formation]]s in the Llanos (Sarmiento, 2001<ref name=Sarmiento2001>Sarmiento, L. F., 2001, Mesozoic rifting and Cenozoic basin inversion history of the Eastern Cordillera, Colombian Andes. Inferences from tectonic models: Ph.D. Thesis Vrije Universiteit, Amsterdam, The Netherlands, 297 p.</ref>). Subsequent accretion took place westward at the end of the [[Maastrichtian]], resulting in the beginning of [[uplift]] of the Central Cordillera and the first [[inversion]] in the now Magdalena Valley (Casero et al., 1997; Sarmiento, 2001). In the Llanos, [[subsidence]] continued in a very poorly deformed basin during the [[Paleocene]], with the [[deposition]] of the [[Barco formation|Barco]], [[Los Cuervos formation|Los Cuervos]], and [[Mirador formation]]s (Cooper et al., 1995<ref name=Cooperetal1995>Cooper, M. A., F. T. Addison, R. Alvarez, A. B. Hayward, S. Howe, A. J. Pulham, et al., 1995, [http://archives.datapages.com/data/specpubs/memoir62/35cooper/0659.htm Basin development and tectonic history of the Llanos Basin, Colombia]], in Petroleum Basins of South America: [http://store.aapg.org/detail.aspx?id=476 AAPG Memoir 62], p. 659–665.</ref>; Bayona et al., 2008<ref name=Bayonaetal2008>Bayona, G., M. Cortés, C. Jaramillo, G. Ojeda, J. J. Aristizabal, and A. Reyes-Harker, 2008, An integrated analysis of an orogen–sedimentary basin pair: Latest Cretaceous–Cenozoic evolution of the linked Eastern Cordillera orogen and the Llanos foreland basin of Colombia: GSA Bulletin, v. 120, no. 9/10, p. 1171–1197.</ref>; Reyes-Harker et al., 2015<ref name=Reyesharkeretal2015>Reyes-Harker, A., C. F. Ruiz-Valdivieso, A. Mora, J. C. Ramirez-Arias, G. Rodriguez, F. de la Parra, et al., 2015, Cenozoic paleogeography of the Andean Foreland and retroact hinterland of Colombia: [http://archives.datapages.com/data/bulletns/2015/08aug/BLTN11110/BLTN11110.html AAPG Bulletin], v. 99, no. 8, p. 1407–1454, DOI: 10.1306/06181411110.</ref>). A compressive context predominates in the Eastern Cordillera since the [[Eocene]]. After the inversions of the Mesozoic extensional [[graben]]s, a thrust regime developed, and the [[Llanos Orientales]] became a classical, but rather external, [[foreland]] during the [[deposition]] of the [[Carbonera formation|Carbonera]] and [[Leon formation]]s (Moretti et al., 2009<ref name=Morettietal2009a>Moretti, I., J. C. Mondragon, J. C. Garzon, G. Bosio, and J. M. Daniel, 2009a, Structural style and decollement levels in the Llanos Orientales Basin (Colombia). X Congress Bolivariano, Conference Proceedings.</ref>; Mora et al., 2015<ref name=Moraetal2015>Mora, A., W. Casallas, R. Ketcham, D. Gomez, M. Parra, J. Manson, et al., 2015, Kinematic restoration of contractional basement structures using thermokinematic models: A key tool for petroleum system modeling: [http://archives.datapages.com/data/bulletns/2015/08aug/BLTN11108/BLTN11108.html AAPG Bulletin]], v. 99, no. 8, p. 1575–1598, DOI: 10.1306/04281411108.</ref>). The Eastern Cordillera is a double verging mountain belt: the western flank in the Magdalena Valley was initially the more active one but the active [[compression|compressive]] front has now shifted to the Llanos where the [[Miocene]] [[foreland]] is currently affected by the compression that propagates eastward. The structure that corresponds to the former Early [[Cretaceous]] basin edge is the [[Guaicaramo fault]] (Bayona et al., 2008<ref name=Bayonaetal2008 />), but some [[thrust]]s and blind thrusts exist eastward of it (called compressive front in the figures).

The [[lithology|lithological]] column of the area is presented [[:file:M114CH08FG02.jpg|Figure 2]] with a [[cross section]] that shows the [[pinch out|pinchout]] of the various series toward the east and the sand–shale alternating [[sequence]]s.

The [[lithology|lithological]] column of the area is presented [[:file:M114CH08FG02.jpg|Figure 2]] with a [[cross section]] that shows the [[pinch out|pinchout]] of the various series toward the east and the sand–shale alternating [[sequence]]s.

A regional fault system oriented mainly northeast–southwest is defined based on the seismic interpretations, and previously published works show that the faults have been reactivated as normal and strike slip faults during the whole Tertiary (Moretti et al., 2009a). The inversion structures are only visible westward near the Eastern Cordillera front, and the Leon shale appears to be an efficient disharmonic level. These faults, despite their rather limited offsets, play an important role as lateral barrier in the fluid flow history and have to be taken into account in any migration and accumulation modeling. The majority of the traps in the LBC are structural; they are bordered and closed eastward by these small offset east dipping normal faults, which are considered as seal based on the exploration results. Stratigraphic traps also exist, mainly eastward.

A regional [[fault]] system oriented mainly northeast–southwest is defined based on the [[seismic]] interpretations, and previously published works show that the faults have been reactivated as normal and [[strike slip]] faults during the whole [[Tertiary]] (Moretti et al., 2009<ref name=Morettietal2009a />). The [[inversion]] [[structure]]s are only visible westward near the [[Eastern Cordillera]] front, and the [[Leon shale]] appears to be an efficient disharmonic level. These faults, despite their rather limited offsets, play an important role as lateral barrier in the [[fluid flow]] history and have to be taken into account in any migration and accumulation modeling. The majority of the [[trap]]s in the LBC are structural; they are bordered and closed eastward by these small offset east dipping normal faults, which are considered as [[seal]] based on the exploration results. [[Stratigraphy|Stratigraphic]] traps also exist, mainly eastward.

==Petroleum System==

==Petroleum System==

The LBC is a prolific hydrocarbon province that produces about 70% of Colombian oil and focuses high expectations in exploration, especially for heavy oil. Water in this petroleum system plays an important role: as already noted, about 80% of the total produced fluids, even in the deepest northern area, are not HC but fresh water.

The LBC is a prolific [[hydrocarbon]] province that produces about 70% of Colombian [[oil]] and focuses high expectations in [[exploration]], especially for heavy oil. Water in this [[petroleum system]] plays an important role: as already noted, about 80% of the total produced fluids, even in the deepest northern area, are not hydrocarbon but fresh water.

Numerous studies of the petroleum system have been published; they allow us having a good knowledge of the foreland geometry and temperature evolution through time. The precise analysis of hydrocarbon charge in the LCB is out of the scope of this study but since water and hydrocarbons are migrating together in the foreland, the main features of this system in terms of fluid circulation have to be summarized. More details could be found in Villegas et al. (1994), Moretti et al. (2009b), Mora et al. (2010), Person et al. (2012), and Vayssaire et al. (2013).

Numerous studies of the [[petroleum system]] have been published; they allow us having a good knowledge of the foreland geometry and temperature evolution through time. The precise analysis of [[hydrocarbon]] charge in the LCB is out of the scope of this study but since water and hydrocarbons are migrating together in the [[foreland]], the main features of this system in terms of [[fluid]] circulation have to be summarized. More details could be found in Villegas et al. (1994<ref name=Villegasetal1994 />), Moretti et al. (2009<ref name=Morettietal2009b>Moretti, I., C. Mora, W. Zamora, M. Valendia, G. Rodriguez, and M. Mayorga, 2009b, Petroleum system variations in the Llanos Basin (Colombia). X Congress Bolivariano, Conference Proceedings.</ref>), Mora et al. (2010<ref name=Moraetal2010>Mora, A., B. K. Horton, A. Mesa, J. Rubiano, R. A. Ketcham, M. Parra, et al., 2010, Migration of Cenozoic deformation in the Eastern Cordillera of Colombia interpreted from fission track results and structural relationships: Implications for petroleum systems: [http://archives.datapages.com/data/bulletns/2010/10oct/BLTN09111/BLTN09111.HTM AAPG Bulletin], v. 91, no. 10, p. 1543–1580.</ref>), Person et al. (2012<ref name=Personetal2012>Person, M., D. Butler, C. W. Gable, T. Villamil, D. Wavrek, and D. Schelling, 2012, Hydrodynamic stagnation zones: A new play concept for the Llanos Basin, Colombia: [http://archives.datapages.com/data/bulletns/2012/01jan/BLTN11019/BLTN11019.HTM AAPG Bulletin], v. 96, no. 1, p. 23–41, DOI: 10.1306/08101111019.</ref>), and Vayssaire et al. (2013<ref name=Vayssaireetal2013>Vayssaire, A., H. Abdallah, W. Hermoza, and E. Figari Negri, 2013, Regional study and petroleum system modeling of the Eastern Llanos, Search and Discovery Article 10564, accessed May 31, 2017, http://www.searchanddiscovery.com/pdfz/documents/2014/10564vayssaire/ndx_vayssaire.pdf.html.</ref>).

The source rocks, not only Cretaceous but also Tertiary in age, are only mature westward near the Eastern Cordillera thrust front. Source rock maturation started at the Miocene (Moretti et al., 2009b; Vayssaire et al., 2013). However, the true deepening of the basin is recent and mainly due to the molasses (Guayabo Formation) deposits. During the past 10 m.y. the increase in burial and so maturation and expulsion of hydrocarbons resulted in a currently very active petroleum system. Hydrocarbons are migrating eastward in the different carrier beds and are trapped in structural and stratigraphic plays. Except in the narrow foothills ring where the structures are thrust anticlines, the structural traps consist mainly in the crest of faulted blocks bordered by east dipping normal faults. The stratigraphic prospects are either the channel of the Carbonera Formation or the pinchout of the sandy series eastward.

The [[source rock]]s, not only [[Cretaceous]] but also [[Tertiary]] in age, are only mature westward near the [[Eastern Cordillera]] [[thrust]] front. Source rock maturation started at the [[Miocene]] (Moretti et al., 2009<ref name=Morettietal2009b />; Vayssaire et al., 2013<ref name=Vayssaireetal2013 />). However, the true deepening of the [[basin]] is recent and mainly due to the [[molasse]]s ([[Guayabo Formation]]) [[deposit]]s. During the past 10 m.y. the increase in burial and so maturation and expulsion of [[hydrocarbon]]s resulted in a currently very active [[petroleum system]]. Hydrocarbons are [[migration|migrating]] eastward in the different carrier beds and are trapped in [[structure|structural]] and [[stratigraphy|stratigraphic]] [[play]]s. Except in the narrow foothills ring where the structures are [[thrust]] [[anticline]]s, the structural [[trap]]s consist mainly in the crest of faulted blocks bordered by east dipping normal faults. The stratigraphic prospects are either the channel of the [[Carbonera Formation]] or the [[pinch out]] of the sandy series eastward.

Eastward the reservoirs are shallow, and so at temperatures lower than 80°C (176°F), biodegradation occurs, resulting in heavy oil reserves. Westward the first flow of hydrocarbons reached shallow reservoirs and has also been biodegraded, but the current charge of hydrocarbons that refreshes these reservoirs may consist of high API light oils (Dzou et al., 1999; Ramón et al., 2001). An alternative interpretation of these data has been proposed recently: the mixture between low-gravity and high-gravity HC in the western field could also be due to the mélange between a first charge of only early-mature HC and a more recent charge pulse of late-mature HC (Gonzalez-Penagos et al., 2015). In addition to the fact that there are numerous source rocks, this mixture of different oil at different maturity levels makes it difficult to predict the characteristics of the hydrocarbons (Figure 2).

Eastward the reservoirs are shallow, and so at temperatures lower than 80°C (176°F), [[biodegradation]] occurs, resulting in heavy [[oil]] [[reserves]]. Westward the first flow of [[hydrocarbon]]s reached shallow [[reservoir]]s and has also been biodegraded, but the current charge of hydrocarbons that refreshes these reservoirs may consist of high API light oils (Dzou et al., 1999<ref name=Dzouetal1999>Dzou, L. I., A. G. Holba, J. C. Ramón, J. M. Moldowan, and D. Zinniker, 1999, Application of new diterpane biomarkers to source, biodegradation and mixing effects on Central Llanos Basin oils, Colombia: Organic Geochemistry, v. 30, no. 7, p. 515–534, [https://www.sciencedirect.com/science/article/pii/S014663809900039X DOI: 10.1016/S0146-6380(99)00039-X].</ref>; Ramón et al., 2001<ref name=Ramonetal2001>Ramón, J. C., L. I. Dzou, W. B. Hughes, and A. G. Holba, 2001, Evolution of the Cretaceous organic facies in Colombia: Implications for oil composition: Journal of South American Earth Sciences, v. 14, no. 1, p. 31–50, [https://www.sciencedirect.com/science/article/pii/S0895981101000104 DOI: 10.1016/S0895-9811(01)00010-4].</ref>). An alternative interpretation of these data has been proposed recently: the mixture between low-[[gravity]] and high-gravity hydrocarbons in the western field could also be due to the mélange between a first charge of only early-mature hydrocarbon and a more recent charge pulse of late-mature hydrocarbon (Gonzalez-Penagos et al., 2015<ref name=Gonzalezpenagosetal2015 />). In addition to the fact that there are numerous [[source rock]]s, this mixture of different oil at different maturity levels makes it difficult to predict the characteristics of the hydrocarbons ([[:file:M114CH08FG02.jpg|Figure 2]]).

In addition to these west–east variations, the basin shows a north–south difference; the Carbonera formations are thicker in the south, whereas the Guayabo Formation is up to 6 km (3.7 mi) thick in the north and only 3 km (1.9 mi) thick in the south. As a result, maturation started during the middle Miocene in the south and only recently in the north (Moretti et al., 2009b; Vayssaire et al., 2013).

In addition to these west–east variations, the [[basin]] shows a north–south difference; the [[Carbonera formation]]s are thicker in the south, whereas the [[Guayabo Formation]] is up to 6 km (3.7 mi) thick in the north and only 3 km (1.9 mi) thick in the south. As a result, maturation started during the middle [[Miocene]] in the south and only recently in the north (Moretti et al., 2009<ref name=Morettietal2009b />; Vayssaire et al., 2013<ref name=Vayssaireetal2013 />).

==See also==

==See also==