Determining hydrocarbon saturation in Colombian oilfields is particularly challenging for conventional methods since the formation water in most of the tertiary sand targets is extremely fresh, frequently in the 100 ppm to 3,000 ppm NaCl equivalent range. In these conditions, the uncertainty in resistivity and sigma-based saturation interpretation methods is huge. Recent advances in measurement technology and interpretation including the combined use of dielectric dispersion and radial magnetic resonance provide a considerable uncertainty reduction in water saturation and hydrocarbon mobility evaluation. These open-hole measurements have a very shallow depth of investigation (DOI) and provide valuable information from the flushed zone, at one to four inches within the reservoir.

A new, advanced nuclear spectroscopy tool conveyed on wireline makes a direct total carbon measurement at a DOI from four to nine inches within the reservoir. When the inorganic mineral carbon is subtracted from the total carbon measurement, the resultant total organic carbon (TOC) can be used to directly compute formation hydrocarbon volume and saturation. Validation is available from the slightly shallower dielectric-derived water filled porosity. These combined measurements provide a saturation profile independent of resistivity and salinity, and allow key reservoir evaluation parameters including hydrocarbon type and oil mobility to be evaluated more accurately.

In mature fields, slim carbon-oxygen logging has been the main technique historically for estimating oil saturation behind casing, given its salinity independency. In order to achieve a desirable statistical precision and accuracy, carbon-oxygen data with slim tools requires several logging passes over the zone of interest. A typical acquisition strategy consists of a sigma pass acquired at 1500 ft/h and three or more carbon-oxygen passes acquired around 120 ft/h. In addition to borehole fluid sigma and salinity, these measurements provide information on formation porosity, lithology, matrix properties and oil saturation.

The advanced spectroscopy tool was run in the same wells as the slim carbon-oxygen devices. Comparison of measurements from both services indicates that the advanced spectroscopy device is capable of providing lithology, matrix properties and hydrocarbon volume and saturation from TOC with improved accuracy in a single logging run, at a logging speed five time faster that one single carbon oxygen acquisition pass from the slim logging device. The advanced spectroscopy tool also provides improved precision and the measurement of additional elements compared with previous generation tools, allowing for a more accurate computation of mineral components and matrix properties.

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