With growing interest in natural gas hydrate, it is increasingly important to be able to determine the volume of gas hydrate and included natural gas within a gas hydrate accumulation (as discussed in Collett and Ladd, Chap. 19, this volume). The primary objective of this study was to develop quantitative geochemical log evaluation techniques, which will permit the calculation of gas hydrate saturations in gas hydrate-bearing sedimentary units with downhole neutron spectroscopy logs. To obtain this objective the study was subdivided into two general sections: (1) neutron spectroscopy log response modeling, and (2) field data characterization. During the response modeling phases of this study, new neutron spectroscopy log evaluation techniques were developed that deal specifically with gas hydrate. These newly developed gas hydrate log-evaluation techniques were tested and applied in the field data characterization portion of the study.
Neutron-induced gamma-ray spectroscopic downhole logging is well established as a means of evaluating the chemical composition of hydrocarbon reservoirs. The geochemical logging tool (GLT; Fig. 1) provides measurements of most of the elements present in rock-forming minerals, which can be used to construct detailed mineralogic models. In addition, GLT-derived data on the carbon and oxygen content of hydrocarbon reservoirs can be used to determine oil and gas saturation information. In this report, we calculated the theoretical carbon and oxygen content of several hypothetical gas hydrate occurrences and constructed a series of carbon/oxygen "fan charts" that can be used to calculate gas hydrate saturations. We also assessed the effect of complex reservoir conditions (including the presence of clay and dispersed organic carbon) on the GLT gas hydrate saturation calculations. In the field data characterization phase of this study, carbon/oxygen elemental data from the GLT was used to determine gas hydrate saturations at all three sites (Sites 994, 995, and 997) drilled on the Blake Ridge during Leg 164 of the Ocean Drilling Program (ODP).