Abstract

This study integrates interpreted high resolution facies from whole core CT (X-ray computed tomography) scans, standard well logs, and standard whole core plug analyses. The analyses were made (mostly) on strata penetrated by the La Luna-1 well in the Cretaceous, calcareous-siliceous mudstones of the Salada Member of the La Luna Formation, Middle Magdalena Basin, Colombia. The focus of this report is how high-frequency CT data, after classification into four facies using a simple two-parameter grid of bulk density versus average atomic number, can be used to assign initial facies probabilities to each (digitized) sample used in a probabilistic clustering procedure (PCP) that uses standard well logs as clustering variables. This procedure permits the high-definition CT facies to be directly related to the well log data, and the results of the PCP provide a mathematical model (generated within the clustering software) that permits the prediction of (upscaled) CT facies in non-cored wells.

This procedure works because the CT facies provide relative rock type data (silica rich vs calcareous rich) and relative reservoir quality data (organic rich and/or high porosity vs organic poor and/or low porosity), so that when these interpreted properties are associated with the samples used in the PCP, the results provide a depth profile that depicts the probability that a given sample belongs to each of the CT high-definition facies. Because the resulting probabilistic facies profile is inherently upscaled due to use of low-resolution well logs as the clustering variables, the results can also be compared with the results of standard core plug analyses (e.g., porosity, total organic carbon, XRD mineralogy, etc.). Thus, this study is an example of how data at three different scales can be integrated – in this case: CT data, core plug data, and well log data.

The most noticeable sedimentology-stratigraphy pattern revealed by this study is a cyclic pattern of two end-member rock types: calcite-rich, low-porosity, low TOC mudstones, and calcite-poor (silica- and clay-rich), high-porosity, high TOC mudstones. The frequency of the cycles averages about two meters. Preliminary indications are that individual beds associated with these cycles are laterally continuous and might be correlated over distances of ten kilometers or more.

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