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Geochemically Distinct Oil Families in the Middle Magdalena Valley, Colombia Based on Chemometric Analysis of Source-Related Biomarker and Isotopic Ratios

Abstract

The Middle Magdalena Valley (MMV) is an intermontane basin and petroleum province in northwestern Colombia. During the Cretaceous, restricted marine conditions within a broad foreland basin resulted mainly in deposition of organic-rich carbonate and siliciclastic sediments. The Upper Cretaceous La Luna Formation is the primary regional source rock but other source rocks may contribute. This study uses chemometrics (multivariate statistics) of source-related biomarker and isotopic ratios for 96 crude oil and rock extract samples from the MMV to establish distinct oil families and infer their source rock depositional environment, lithology, and organic matter type. Samples and data were provided by GeoMark Research, Ecopetrol, and PSI and laboratory methods are described in Peters et al. (2007). Source rock extracts and highly mature or biodegraded oils were excluded to yield a training set of 67 samples. The parameters selected for the chemometric analysis include one porphyrin ratio, fourteen terpane ratios, five sterane ratios, and three stable carbon isotope ratios. These 23 source-related parameters were used to construct a hierarchical cluster analysis (HCA) dendrogram, which defines five oil tribes within the study area. The tribes may originate from different source rocks or organofacies of the same source rock and they show systematic distributions by map location and reservoir age. Tribes 1 through 3 are from Paleogene reservoir rocks in the south to central portion of the study area. Tribes 4 and 5 on the other hand are from Cretaceous to early Paleogene reservoir rocks in the northern portion of the study area. The source-related biomarker and isotopic ratios are used to infer differences in organic matter type and source rock depositional environment between tribes. All five tribes show stable carbon isotope ratios for saturate and aromatic hydrocarbons indicating marine organic matter based on the Sofer (1984) diagram. Terpane ratios suggest marine shale to marine carbonate source rocks for the oil samples (Peters et al., 2005). Tribes 1 and 2 show mainly a marine shale source rock, with marine shale to marine carbonate for Tribe 3 and marine marl to carbonate for Tribes 4 and 5. DBT/phenantrene and pristane/phytane ratios support these interpretations. Finally, C27 diasterane/regular sterane and Ts/Tm ratios indicate that Tribe 5 is more thermally mature than Tribes 1 through 4.