Gas hydrate saturation and morphology from analysis of pressure cores acquired in the Bay of Bengal during expedition NGHP-02, offshore India

Abstract India's National Gas Hydrate Program Expedition 2 (NGHP-02) completed an ambitious pressure coring program as part of their effort to evaluate gas hydrate in coarse-grained reservoirs in the Bay of Bengal. The 75 successful pressure cores were used to measure gas hydrate saturation, examine the morphology of the gas hydrate at in situ conditions, and to provide samples for further laboratory work. Pressure core quality was assessed via pressure-temperature records and X-ray computed tomographic reconstructions of cores. Gas hydrate saturations were measured by using the gas from quantitative degassing experiments in methane mass balance calculations. Gas hydrate morphology was determined from X-ray computed tomographic reconstructions, P-wave velocity, and gamma density measured under pressure using PCATS, the Pressure Core Analysis and Transfer System. Gas hydrate saturations reached 65–85% in the pores of sandy reservoir sediments, and were near full saturation in gravelly sediments. Clayey interlayers between reservoir sediments had gas hydrate saturations near 10%, with no evidence of gas hydrate veins in X-ray computed tomographic (CT) reconstructions. Veins of gas hydrate were visible in X-ray images of clayey non-reservoir sediments; at some locations the full complement of gas hydrate was contained in X-ray-resolveable veins, but in others the veins only accounted for part of the total gas hydrate burden in the sediment, implying that some gas hydrate was finely distributed in the pore space or in veins below the resolution of the X-ray CT data. At one location low-density, biosilica-rich silty clays hosted pore-filling gas hydrate at saturations of 45%, with only slightly elevated P-wave velocities. Gas hydrate saturation and morphology from all the pressure cores are presented here and placed into geological context.