Whole-Rock Elemental Data as an Aid in Log Interpretation: Low-Resistivity Reservoir Case Study

Abstract

Over the years, low resistivity pay has become recognized as a worldwide phenomenon, unfortunately only a few of these reservoirs are successfully identified and evaluated using standard logging data. This is especially true in the Cretaceous clastic reservoirs of the Orinoco Oil Belt. The primary goals of this investigation were to understand the causes of low resistivity in the pay zones, the nature of the gamma ray response, and to investigate the potential of whole-rock elemental data for characterizing these reservoirs.

It is well known that improvements in missed pay identification must include integration of geological, petrophysical, and reservoir engineering data. The first step was to characterize the composition of the Creataceous formation in well-constrained core samples from an older vertical well, X-23. Thin section and X-ray diffraction (XRD) analyses showed the samples to contain significant amounts of detrital clay, most of which is kaolinite. Some authigenic kaolinite may also be present, but is minor. The source of low resistivity in the X-23 reservoir section can be logically interpreted to be formation water present in microporosity associated with the bundant clay minerals.

For the second step, a test was performed on cuttings samples from the Cretaceous reservoir in the recently-drilled X-272 horizontal well. No core was available from this well. Whole-rock elemental data was obtained for 50 elements from 29 washed cuttings samples, using combined WD-XRF (Wavelength-Dispersive X-ray Fluorescence spectroscopy) and ICP-MS (Inductively Coupled Plasma - Mass Spectrometry). Three of these samples were also analyzed for mineralogy by XRD. Results of the combined analyses indicated the samples contained little clay (about 2%), virtually no plagioclase, significant K-feldspar (5-11%), small amounts of carbonate (2-8%), and moderate amounts of heavy minerals. The heavy minerals, as inferred from elemental data (TiO2, Zr, Nb, Th, U and rare earth element (REE)), are likely to be ilmenite, rutile, titanite, zircon, and apatite; monazite, xenotime, garnet, micas, and Th-oxides may be present as well. The low Al2O3 and XRD clay values preclude these elements being primarily associated with clays.

The third step was to assess the gamma and resistivity responses of the logs. In the X-272 horizontal well, the gamma and resistivity responses could not be adequately explained by the composition inferred by the combined mineralogical and elemental analyses. It is therefore suspected that there was significant detrital clay present in the original oil-saturated cuttings samples, but almost all was removed during cleaning of cuttings at wellsite, leaving the sand fraction as a residual sample.

Despite the difficulties in obtaining representative samples from washed cuttings, the elemental data from the sand fraction of the X-272 samples alone shows potential for the definition and chemostratigraphic correlation of distinctive stratigraphic units for the purpose of wellbore positioning. In addition, the utility of the elemental data has provided motivation to develop an improved method of sample cleaning for oil-sands cuttings at wellsite.