Llanos Basin

The Llanos Basin () or Eastern Llanos Basin () is a major sedimentary basin of in northeastern Colombia. The onshore foreland on Mesozoic rift basin covers the departments of Arauca, Casanare and Meta and parts of eastern Boyacá and Cundinamarca, western Guainía, northern Guaviare and southeasternmost Norte de Santander. The northern boundary is formed by the border with Venezuela, where the basin grades into the Barinas-Apure Basin.
Description.
The northeastern part of Colombia is characterized by its wavy plains, called "Llanos Orientales", as part of the bigger Llanos that extend into Venezuela. The landscape is similar to a savanna, it is poor in trees, and located between the Eastern Ranges of the Colombian Andes in the west, the Vaupés Arch in the south and the Guiana Shield in the east.
Geologically, the Llanos Basin underlies this typical landscape of the Llanos. An area where transport occurs mostly by small boats along the many rivers and the "buses of the Llanos", the Douglas DC-3 planes. The basin covers an area of and contains a stratigraphic column from the Paleozoic to recent. Several of the formations in the basins are source rocks (Gachetá, Los Cuervos, Carbonera C8), reservoir rocks (Mirador, Barco, Guadalupe and the uneven numbered members of Carbonera). Seals are formed by the shaly intervals (even numbered) of the Carbonera Formation, Los Cuervos, and León.
The basin is the main petroleum producing basin of Colombia, with four of the nations biggest oil fields located in the Llanos Basin. Major fields are Rubiales, Colombia's biggest and most recent giant discovery sealed by a complex of hydrodynamic processes, and Caño Limón, at the border with Venezuela.
Major concerns in the basin for the production of petroleum are biodegradation, hydrocarbon migration, fault seal capacity and water flow.
Hydrography.
The Llanos Basin is crossed by numerous rivers, all belonging to the Orinoco watershed. From north to south:
Flora and fauna.
Fauna.
Among other species, Lynch's swamp frog ("Pseudopaludicola llanera") is endemic to the Llanos, with the species epithet referring to the plains. Also the whip scorpion "Mastigoproctus colombianus" is reported from the Llanos Basin.
Geodynamic situation.
The country of Colombia spreads out over six tectonic plates, clockwise from north:
The Llanos Basin is situated entirely on the South American Plate, bordering the North Andean Block or North Andean microplate in the west. The basin is one of three Colombian basins on the South American Plate, to the south the Caguán-Putumayo Basin and to the southeast the Vaupés-Amazonas Basin. The northern boundary of the Llanos Basin is formed by the Colombia-Venezuela border where the basin grades into the Barinas-Apure Basin on the Venezuelan side. The Catatumbo Basin, representing the Colombian portion of the larger Maracaibo Basin borders the Llanos Basin in the northwest and the western boundary is formed by the foothills ("Piedemonte") of the Eastern Cordillera Basin, the covering the Eastern Ranges of the Colombian Andes.
Tectonics.
The basin is bound to the west by the Eastern Frontal Fault System, a long fault system connecting the North Andes and South American Plates and thus the Eastern Cordillera Basin and the Llanos Basin. The fault system has an average strike of 042.1±19, but this orientation varies greatly along its course. The 1827, 1834, 1917, 1967, 1995, and 2008 earthquakes were all caused by fault movement as part of the system.
Basin history.
The tectonic history of the Llanos Basin, a foreland basin formed on top of Mesozoic rift basins, Paleozoic metasediments and Precambrian basement underlain by continental crust, goes back to the Early Jurassic.
The Andean orogeny, represented by the tectonic uplift of the Colombian Eastern Ranges and its northern extension, the Serranía del Perijá, caused tilting and uplift in the Llanos Basin. During the Andean orogenic phase, the paleotemperatures in the basin dropped considerably; in the Baja Guajira area from in the Early Miocene to in the Late Miocene. In the Late Miocene to Pliocene, the major faults to the southwest of the Cocinetas Basin, the Oca and Bucaramanga-Santa Marta Faults were tectonically active.
Basement.
The stratigraphy of the Llanos Basin ranges, depending on the definition from either Jurassic or Paleozoic to recent. The basement is formed by the westernmost extensions of the Guiana Shield. Remnants of these Precambrian formations are found as inselbergs in the far east of Colombia (Cerros de Mavecure), in the Serranía de la Macarena to the southwest of the basin and in the tepuis of the Serranía de Chiribiquete to the southeast.
The Proterozoic crystalline rocks are overlain by metamorphosed sedimentary and igneous rocks ranging in age from Cambrian to Devonian. Younger and contemporaneous Paleozoic deposits are only found in the subsurface and in regional correlative units as the Floresta and Cuche Formations of the Altiplano Cundiboyacense to the direct northwest and the Río Cachirí Group of the Cesar-Ranchería Basin farther northwest of the Llanos Basin.
The units found in the Llanos Basin pertain to the Farallones Group and comprise the Valle del Guatiquía Red Beds, Pipiral Shale and the Gutiérrez Sandstone.
Petroleum geology.
The Llanos Basin is the most prolific hydrocarbon basin of Colombia, hosting well-known petroleum deposits as Caño Limón, Rubiales and other fields. Nine of the twenty most producing oil fields of Colombia are situated in the Llanos Basin.
Fields.
Based on data released in March 2018, Colombia is the 21st oil producer in the world. Daily production dropped in 2017 to . In 2016, twenty oilfields produced 66% of all oil of Colombia, listed below in bold. The total proven reserves of Colombia were in 2016.
Major oil fields in the Llanos Basin are:
Mining.
Mining activities in the Llanos Basin are restricted to certain areas, resulting in less conflicts, more common with indigenous peoples in the Amazonian part of Colombia.
In San José del Guaviare platinum is mined.
Paleontology.
Compared to many fossiliferous formations in Colombia, the Llanos Basin has been lean in fossil content. Most of the basin stratigraphy is only known from wells.
Paleozoic outcrops surrounding and perforating the planar geography have provided fossils dating back to the Cambrian; the Duda and Ariarí Formations.
Several fossiliferous formations of contemporaneous depositional environments have provided many unique fossils indicative of paleoclimatic conditions; turtle fossils were described from Los Cuervos in the Cesar-Ranchería Basin, and the Mirador Formation in the Catatumbo Basin direct northwest of the Llanos Basin has provided many fossil flora.
Sources.
Bibliography.
Hydrodynamics.
 