Abstract We have estimated the Curie point depth (CPD) in the northwestern corner of South America and the southwestern Caribbean Sea from spectral analysis of magnetic anomalies extracted from the World Digital Magnetic Anomaly Map. To do this, we performed three different spectral methods and chose the model that best fits the geologic and geophysical characteristics of the study area. Then, we calculated the geothermal gradient from these CPD values to assess the likelihood of the hydrocarbon “Golden zone” being present in some of Colombia’s sedimentary basins. Similarly, we tried to establish empirical relationships between CPD, geothermal gradient, and heat flux. Our results show that the CPD lies between 12.6 km (7.8 mi) and 74 km (45.9 mi). The shallowest depths (<25 km [<15.5 mi]) are in the offshore Venezuela and Colombia basins of the Caribbean Sea, the onshore eastern Llanos and Caguan–Putumayo Basins, and southwestern Venezuela. The greatest depths (>50 km [>31 mi]) occur in parts of the western and central Cordilleras, Santander massif, and middle Magdalena, Catatumbo, Barinas-Apure, and Vaupes-Amazonas Basins. Based on the results, we found a relationship between an unexpected zone of deep CPD values (40–47 km [24.8–29.2 mi]) in the Colombia Basin and the presumable presence of an abnormal thick Caribbean Plateau with a continental inheritance. On the other hand, the contrasting deep and shallow CPD values in the Caribbean support the interpretation of flat subduction of the Caribbean plate beneath South America with a flexural topographic bulge toward the Sinu−San Jacinto and lower Magdalena Basins. Partial erosion of this bulge could have resulted in shallowing of the CPD with a consequent increase in geothermal gradient and heat flux. Also, we found a CPD shallowing beneath Caguan–Putumayo and eastern Llanos Basins. Finally, based on the calculated geothermal gradient values in Colombia, we consider that the Golden zone of hydrocarbon occurrence most likely exists in the Choco–Uraba, eastern Cordillera, Guajira−Los Cayos, eastern Llanos, and lower Magdalena Basins, while the Golden zone would be absent only in the Vaupes–Amazonas Basin.

Abstract Sedimentation has continued in the Maracaibo Basin since the formation of its earliest oil-bearing strata. The present lake is reputed to be one of the most productive waters in the world. Perhaps no more favorable place can be found for examining the conditions presumed to precede the accumulation of organic matter in sedimentary deposits. The lake appears to have been fresh in the recent past. Sea water gained access to the basin when the rising sea level drowned its approaches to a present depth of 60 feet. Since then marine sediments have greatly reduced the sill depth. Escaping fresh water limits the entrance of sea water, which occurs only during three months of the dry season. The lake water contains about one part in thirty of sea water, uniformly distributed by an active wind-driven circulation. In the deeper basin, with depths of 100 feet, the salt content is substantially higher. The oxygen content of this "deep water" is reduced and over a large area of bottom is anaerobic. The fauna of the lake on superficial observation is limited in variety and numbers. Marine forms are restricted to species adapted to life at very low salinities. The significant bulk of organic matter is produced by fresh-water species of planktonic algae. These at times produce prominent surface blooms. The potential fertility of the lake water is revealed by its phosphorous content. The total phosphorous is about 1.4 microgram atoms per liter in the mixed layers above the "deep water." This is more than commonly found in surface waters of the sea, but is less than that of the deep water of the Pacific and Indian Oceans. The phosphorus, and presumably other nutrients, are derived from land drainage and are accumulated in the lake by evaporation and by the sinking of organisms into the deep water. One third of the phosphorus is present in inorganic form available for plant nutrition. Consequently, growth is not limited by its availability in the photic zone. In the "deep water" the phosphorus content is manifoldly increased by the sinking of organisms. The oxygen available is insufficient to oxidize this accumulation, and conditions unusually favorable for the entrapment of organic matter in sediments result. Sedimentation is active in the lake, particularly in the southwest quarter, where the Catatumbo River is building an extensive delta. Its water discolors the lake for a great distance along the shore. The lake bottom consists of soft blue mud which, in the anaerobic region, is black for several inches below the surface and smells strongly of H 2 S. The mud is sufficiently unconsolidated to permit a test pile to sink 30 feet under its own weight. It is concluded that Lake Maracaibo is an unusually favorable place to study the processes which lead to the entrapment and the subsequent transformation of organic matter in sediments. "Petroleum in its natural occurrence is closely associated with marine or semi-marine nearshore sediments. The relation is practically universal and has persisted since Cambrian time, very probably without material alteration. The study of the depositional zones of present-day seas and oceans therefore appears essential to an understanding of oil occurrence, particularly in its initial stages." Eugene Stebinger (1950).

Abstract Geologic Problem Solving with Microfossils: A Volume in Honor of Garry D. Jones SEPM Special Publication No. 93, Copyright © 2009 SEPM (Society for Sedimentary Geology), ISBN 978-1-56576-137-7, p. 29–40. The two major oil fields in Colombia discovered in the last fifty years are the Caño Limón and Cusiana fields. Caño Limón is located in the eastern region of the unfolded Llanos of Colombia, and Cusiana is located in the leading thrust sheet of the Llanos Foothills. Paleogene strata in both areas were part of a large foreland basin active since the latest Cretaceous. In both cases the main reservoir is a quartz arenite unit, informally called the Mirador formation, that has always been assumed to extend as a continuous Eocene sandstone layer from the Llanos Foothills into the Llanos Basin. However, recent palynological data suggested that this unit is diachronous across the Llanos and Llanos Foothills. Here, we dated 44 sections in the Llanos Basin and Llanos Foothills using a new zonation that is proposed for the region. Biostratigraphic results constrain the age of the Mirador Formation in the Llanos Foothills as early to middle Eocene with no evidence of a biostratigraphic gap with underlying early Eocene strata. In most of the Llanos Basin, including Caño Limón, the quartz arenite unit has an Oligocene age and rests unconformably upon Upper Cretaceous or Paleocene strata. Additionally, there are areas in the Llanos Basin where mudstone, not sandstone, is the dominant facies overlying the unconformity, suggesting that the basal sandstone in the Llanos Basin is not a laterally continuous body of rock. The absence of lower to middle Eocene quartz arenite beds in most of the Llanos Basin can be explained either by bypass or accumulation and subsequent erosion. These results imply a new paleogeography for the time of accumulation of Eocene and Oligocene reservoir units, a different model for basin evolution, and a different fluid-migration history to explain how the Caño Limón and Cusiana oil fields were filled.

Abstract Hydrocarbon potential in the Guajira Basin is associated with two different processes for hydrocarbon generation: biogenic and thermogenic. Biogenic hydrocarbons are generated from immature source rocks exposed to bacterial activity. Thermogenic hydrocarbons are produced from source rocks that have attained a sufficient depth of burial and thermal maturity level. Source rocks in the basin may include Cretaceous, Eocene, Oligocene, and Miocene organic rich strata. The Cretaceous rocks can be equivalent to the La Luna Formation of the Maracaibo area with a kerogen type II. Source rock of Tertiary units can be type III, which give the basin its predominantly gas-prone character. Recent geochemical data indicate a late Cretaceous source rock in the deep offshore area in the western part of the basin in preserved stratigraphic sections associated with the South Caribbean deformed belt.