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AAPG Bulletin, V.
Umir Formation: Organic geochemical and stratigraphic assessment as cosource for Middle Magdalena basin oil, Colombia
1Ecopetrol, Instituto Colombiano del Petroleo, P.O. Box 4185, Kilometro 7 autopista a Piedecuesta, Piedecuesta, Colombia; email: [email protected].
2Stanford University, Department of Geological and Environmental Sciences, Stanford, California, 94305-2115; current address: Biomarker Technology, 2501 Blucher Valley, Sebastopol, California, 95472; email: [email protected]
3GEMS Ltda., Calle 51 #26A-06, Of. 304, Bucaramanga, Santander, Colombia; email: [email protected]
4GEMS Ltda., Calle 51 #26A-06, Of. 304, Bucaramanga, Santander, Colombia; email: [email protected]
5Ecopetrol, Instituto Colombiano del Petroleo, P.O. Box 4185; Kilometro 7 autopista a Piedecuesta, Piedecuesta, Colombia; email: [email protected].
Antonio Rangel received an M.Sc. degree in geochemistry from the Central University of Venezuela in 1986. Since 1987 he has been working in the geochemistry section of the Colombian Petroleum Institute. During this period, he has led several geochemical studies applied to petroleum exploration and production in different Colombian basins. His main research interest is the use of biomarkers to assess organic facies variability and paleoenvironmental changes and to refine oil-source correlations.
Mike Moldowan attained a B.S. degree in chemistry from Wayne State University in 1968 and a Ph.D. in chemistry from the University of Michigan in 1972. Following a postdoctoral fellowship in marine natural products with Carl Djerassi at Stanford University, he joined Chevron's Biomarker Group in 1974. The Chevron biomarker team was focused on pioneering research on the application of biological marker technology to petroleum exploration. Moldowan joined the Department of Geological and Environmental Sciences at Stanford University as professor (research) in 1993. In 1986, Moldowan served as chair of the Division of Geochemistry of the American Chemical Society, and he has twice been awarded the Organic Geochemistry Division of the Geochemical Society Best Paper Award, for publications he coauthored in 1978 and 1989.
Christian H. Niño received a degree in geology from the Industrial University of Santander, Colombia, in 1997. His work has focused on geochemistry projects applied to petroleum exploration as outsourcing of the Colombian Petroleum Institute. Since 1999 he has been working as one of the managers of GEMS Ltda., a services company for the oil industry. His main interest is focused on organic geochemistry and petroleum systems.
Pedro Parra is a geologist who graduated from the Industrial University of Santander, Colombia, in 1993. Since then, he has been working for the oil industry, participating in several organic geochemistry projects. Currently he is the manager of GEMS Ltda., a services company for the oil industry. His main interest is focused on organic geochemistry and petroleum systems.
Blanca Giraldo has been employed by the Colombian Petroleum Company/Colombian Petroleum Institute in the organic geochemistry group of the Exploration Division since 1987. She received her M.Sc. degree in geology from the National University, Medellin, Colombia, in 1987. Her research interests include organic petrology, thermal maturity studies for petroleum exploration, and palynofacies studies. Her recent work has focused on the Mesozoic-Cenozoic sequence in the Middle Magdalena and Llanos basins of Colombia.
We are grateful to Ecopetrol and Colciencias for the financial support of this project, as well as for the approval to publish. We extend their appreciation to the laboratory personnel, Paulina Gomez, John Javier Gallegos, Jaime Moldowan, and Geoff Bott, for their analytical support and to Jeremy Dahl, Michael Clark, Colin Baker, Felix Thadeu Texeira Goncalves, and an unidentified referee for their thoughtful reviews and comments.
A detailed geochemical and stratigraphic study conducted on a sedimentary sequence of the Upper Cretaceous Umir Formation, in a section outcropping on the eastern flank of Middle Magdalena basin (MMB), Colombia, identifies this formation as a new hydrocarbon source rock, not previously considered as a source for MMB oil.
The Umir Formation is composed of coal, coaly shale, claystone, siltstone, and sandstone. Based on Rock-Eval data, the coal and coaly shale show good potential for oil generation. Certain biomarkers are characteristic of the depositional environment of the Umir Formation and related oil and contrast it with the La Luna Formation and related oil. For example, biomarkers of the Umir Formation show pristane/phytane ratios greater than 1; predominance of C29 steranes; low oleanane/hopane ratios (<0.08); diasteranes/steranes ratios greater than 1; 22,29,30-(trisnor-18a-neohopane/trisnor-17a-hopane) (Ts/Tm) ratios less than 0.46; and relatively low C35/C34 hopane values (<0.94). Values of the bicadinane index of 0.15-0.25 in Umir samples sharply contrast with those of La Luna samples, which are in the range 0-0.07.
Oleanane/hopane ratios do not show a good correlation with other plant indicators, for example, C29 sterane percentages and bicadinane indices. Low oleanane/hopane may reflect a predominately conifer rather than angiosperm contribution to the depositional system, which is also supported by observations of abundant resinite in thin sections. The diasteranes/steranes ratio displays an inverse correlation with total organic carbon (TOC) content and hydrogen index (HI) from Rock-Eval pyrolysis, suggesting that this parameter strongly depends on oxic depositional conditions. The biomarker results of the Umir Formation show two main sources of organic matter in competition, marine algal vs. higher plant.
A detailed biomarker comparison of Santos-31 oil reservoired in the middle-upper Eocene Esmeralda and La Paz formations, sampled in a field located in the eastern area of the Middle Magdalena basin, shows it to be sourced from the Umir Formation, establishing a new Umir-Esmeralda/La Paz(!) petroleum system in the MMB. This contrasts with a good correlation of Suerte oil in the same geographic area with La Luna source rocks.
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