Estimate of gas hydrate saturations in the Krishna-Godavari basin, eastern continental margin of India, results of expedition NGHP-02

Abstract

A total of 25 drill sites were established in the Krishna-Godavari (KG) and Mahanadi Basins along the eastern margin of India during the India National Gas Hydrate Program Expedition 02 (NGHP-02) with the goal to identify and characterize gas hydrate occurrences in sand-rich reservoir systems. The objective of this study is to characterize the gas hydrate bearing zones linked with two seismic inferred horizons (R1 and R2) and estimate gas hydrate saturations at two sites in Area B of the KG Basin: Sites NGHP-02-22 and NGHP-02-23. The upper seismic-imaged reservoir section (reflector R1) contains both pore-filling and fracture-filling gas hydrate, while the lower seismic-imaged reservoir section (reflector R2) has only pore-filling gas hydrate. Since electrical resistivity seems to be the most pronounced physical response to the presence of gas hydrate, we have analyzed the resistivity log data using Archie's empirical equation from two sites based on an isotropic reservoir model. In order to assess the gas hydrate saturation, we have also performed isotropic velocity modelling using the three-phase Biot equation (TPBE) and density log derived sediment porosities. At Site NGHP-02-22, the average gas hydrate saturation estimated using the resistivity log data and P-wave velocity modelling are 29% and 17%, respectively in the depth interval of 207–290 mbsf whereas at Site NGHP-02-23, the average gas hydrate saturation obtained from resistivity log data and P-wave velocity modelling are 25% and 60%, respectively in the depth interval of 271–288 mbsf. Estimated results are further validated with the pressure core derived gas hydrate saturations to ensure their reliability at the respective drill sites. It was found that P-wave velocity log-inferred gas hydrates show a slight mismatch with the pressure core derived hydrate saturations while, the electrical resistivity log based result depicts a major mismatch with the pressure core derived hydrate saturations. The reason for this discrepancy is likely due to isotropic resistivity analysis of the reservoir section having anisotropic properties.