The La Luna formation: chemostratigraphy and organic facies in the Middle Magdalena Basin

A detailed geochemical study and a sequence stratigraphic interpretation have been conducted on a sedimentary sequence of the Upper Cretaceous La Luna Formation, in a section outcropping in the eastern ̄ank of the Middle Magdalena Basin (MMB), Colombia. The goals were to evaluate geochemical variability related to lithofacies and organic facies changes, characterize depositional environment and investigate the possible relationship between geochemical data and sequence stratigraphic cycles. The La Luna Formation is composed of organic-rich sediments of monotonous appearance, with good to excellent potential for oil generation. Most of the bulk, petrographic and biomarker parameters display a relatively narrow range of variation. However, the geochemical variations are sucient to di€erentiate organic facies types B, BC and C in the Salada Member, B and D in the Pujamana Member and B in the Galembo Member. Certain biomarker ratios are consistent within the La Luna Formation and are characteristic of its depositional environment, for example, average ratios of diasterane/sterane are lower than 1, Ts/Tm averages are less than 0.33, the C35/C34 hopane ratio is more than 0.92, and oleanane/C30 hopane ratios range from 0.02 to 0.19. Regarding depositional condition indicators, the C35/C34 hopane ratio shows a good positive correlation with HI. This suggests that in carbonate environment changes in this parameter are more strongly related to redox condition than to changes in carbonate content. Regarding the possible relationship between organic matter characteristics and sea level changes, in regressive carbonate shelves during shallow stages, HI tends to increase and TOC tends to decrease, while in regressive siliciclastic shelves, both TOC and HI decrease continuously. Some biomarker ratios (oleanane/C30 hopane, C20/C23 tricyclic, Ts/Tm) increase during base level falls. Regarding d C/C isotope composition, the aromatic fraction and whole bitumen display an isotopic shift associated to the main deepening event in the section. # 2000 Elsevier Science Ltd. All rights reserved.