Abstract

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-18α-neohopane/trisnor-17α-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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