An extensive logging and coring appraisal program of a deepwater Gulf of Mexico sandstone reservoir resulted in conflicting technical data. Most notable was a significant difference in resistivity responses of the sands despite being above the oil-water contact. It was critical to establish the source of the resistivity reduction in order to accurately determine in-place oil volumes and improve future field development decisions.
Wireline data consisted of conventional triple-combo and the latest generation of wireline logs, along with rotary/whole core and formation tester pressure and fluid samples. Of particular importance was nuclear magnetic resonance (NMR) and spectroscopy technology that provide complementary "examination" of fluid and rock matrix respectively. Spectroscopy data suggested that the wells contained similar clay volume, while the NMR indicated that pore-size variation (T2 distribution) resulted in variation of irreducible water saturation that led to variation in resistivity.
However this conclusion was not supported by initial results of routine and special core analysis. Core laser particle grain-size analysis (LPSA) and petrography modal point count analysis indicated that there was no significant variation in grain size and mineralogy between the two wells. Initial X-ray diffraction (XRD) tests also indicated significant differences to those from log derived clay volumes. Subsequent additional validation tests of the XRD volumes using a second commercial laboratory, internal in-house testing, and back calculated modal point count volumes revealed XRD clay volumes which agreed more closely with those derived from the "geographical area calibrated" spectroscopy log volumes. Additional special core analysis (SCAL) was used to establish rock texture, sorting and irreducible water saturations (Swirr) through the use of capillary pressure and NMR measurements. A review of the extensive amount of technical data secured on this reservoir highlights the frequent occurrence of conflicting technical data and the need to establish the sources of the differences and a closer review of core measurement methods, procedures and associated uncertainties. The premise that core data provides the "calibration" standard to which log data should be calibrated may not always be correct. This in turn impacts the selection of petrophysical parameters and associated computations. The resolution of conflicts in technical data at the early appraisal stage provides a better direction for future well placement and development planning.
In order to evaluate the potential of a Gulf of Mexico deep water prospect, three wells were drilled. During the pre-spud formation evaluation planning meetings, two critical formation evaluation technical goals were identified; 1.) Develop a comprehensive logging/fluid/coring program in order to adequately develop seismic, geological, petrophysical and reservoir models, and 2.) Due to the high daily rig cost, minimize the rig time for securing logging while drilling (LWD) and wireline data.
A comprehensive logging program was developed which consisted of two components; 1.) LWD logs consisting of multi-depth resistivity, gamma ray, neutron, density, sonic, formation pressure and checkshot velocity survey, and 2.) Wireline logs consisting of multi-depth resistivity, gamma ray, neutron, density, dipole sonic, NMR, spectroscopy, resistivity imager, pressure/fluid samples, and rotary cores.
