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Geochemically distinct oil families in the Middle Magdalena Valley, Colombia

The Middle Magdalena Valley (MMV) is an intermontane basin and petroleum province in northwestern Colombia located between the Central and Eastern Cordilleras of the Andes Mountains. During the Cretaceous, restricted marine conditions within a broad foreland basin that included the MMV 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. The purpose of this study performed by graduate student Will Thompson-Butler is to use chemometrics (multivariate statistics) of source-related biomarker and isotopic ratios for oil samples from the MMV to establish distinct oil families and infer their source rock depositional environment, lithology, and organic matter type.

Geochemical data for 96 oil and source rock extract samples were provided by GeoMark Research, Inc. using laboratory methods described in Peters et al. (2007). Source rock extracts and highly mature or biodegraded oils were excluded at this stage to yield a training set of 67 samples. 23 source-related parameters were used to construct a hierarchical cluster analysis (HCA) dendrogram (Figure 1), which defines five oil tribes within the study area.

Figure 1: Hierarchical cluster analysis dendrogram for the 67-oil sample set from the MMV. Built with 23 source-related parameters in the Pirouette software package.

The tribes may originate from different source rocks or organofacies of the same source rock (e.g., La Luna Formation) and they show systematic distributions by map location (Figure 2) and reservoir age (Figure 3). The source-related biomarker and isotopic ratios can be used to infer differences in organic matter type and source rock depositional environment between the oil tribes illustrating regional differences in petroleum systems across the MMV.

Figure 2: Locations of oil samples and tribes in the MMV study area. Note the systematic distribution from south to north of tribes one through five.

Further work will be done to identify the specific source rocks for the oil tribes defined in this study. Alternating least squares to concentration (ALS-C) analysis of the dataset can help identify end members from mixtures amongst the tribes given in the HCA dendrogram. We then plan to use a training set to build a chemometric decision tree for oil-source rock correlation based on the 23 source-related parameters for various source rock extracts from the Tablazo member of the Calcareous Basal Group and different members of the La Luna and Umir formations.

Figure 3: Illustration modified from ANH Report (2007) showing the principle stratigraphic units of the MMV and their petroleum system elements as well as the reservoir intervals for the MMV oil samples organized by tribe.

Hierarchical cluster analysis dendrogram for the 67-oil sample
Locations of oil samples and tribes in the MMV study area
Illustration showing principle stratigraphic units of the MMV and their petroleum system elements