Gas composition and isotopic geochemistry of cuttings, core, and gas hydrate from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well

Abstract

Molecular and isotopic composition of gases from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well demonstrate that the in situ gases can be divided into three zones composed of mixtures of microbial and thermogenic gases. Sediments penetrated by the well are thermally immature; thus the sediments are probably not a source of thermogenic gas. Thermogenic gas likely migrated from depths below 5000 m. Higher concentrations of gas within and beneath the gas hydrate zone suggest that gas hydrate is a partial barrier to gas migration. Gas hydrate accumulations occur wholly within zone 3, below the base of permafrost. The gas in gas hydrate resembles, in part, the thermogenic gas in surrounding sediments and gas desorbed from lignite. Gas hydrate composition implies that the primary gas hydrate form is Structure I. However, Structure II stabilizing gases are more concentrated and isotopically partitioned in gas hydrate relative to the sediment hosting the gas hydrate, implying that Structure II gas hydrate may be present in small quantities.

Deep basin gas: New insights from kinetic modelling and isotopic fractionation in deep-formed gas precursors

“…Although mostly Cretaceous sediments were identified as major oil source rocks, their potential to form gas is still very high. Gas hydrates such as those found in the Richards Island area were proposed to be fed by overmature Cretaceous sediments at depths greater than 6000 m (Lorenson et al, 1999). However, the terrestrial character of most Tertiary sediments deposited within the Mackenzie Delta/Beaufort Sea also underlines the importance of organic rich shale or coal layers for the generation of gas (Dixon et al, 1992).…”

Gas hydrate systems at Hydrate Ridge offshore Oregon inferred from molecular and isotopic properties of hydrate-bound and void gases

“…5a). Because C 3ϩ hydrocarbon gases are not included into sI gas hydrate (Sloan, 1998), the molecular properties may suggest that some shallow gas hydrates represent crystal intergrowths of dominant sI gas hydrate (composed of C 1 , C 2 , CO 2 and in some cases H 2 S) with small volume of structure II (sII) gas hydrate that includes C 3ϩ gases (Lorenson et al, 1999;Yousuf et al, 2003). One gas hydrate from 7.3 mbsf at Site 1248B contains more C 3 than C 2 (Table 3, Fig.…”

Microbial community distribution in sediments from the Mount Elbert Gas Hydrate Stratigraphic Test Well, Alaska North Slope

“…From isotope geochemistry of the gases in the strata at Mallik (Lorenson et al, 1999) and at the Mount Elbert Gas Hydrate Stratigraphic Test Well (Mount Elbert well) (Lorenson et al, 2011) it appears that microbes do not produce much, if any, of the methane that is present as hydrate. However, we do not have much information about the types of cells present, the abiotic factors that control the distribution of the cells, or whether and how their metabolic activity may alter the gases and fluids in the subsurface.…”