Sedimentary facies and paleoenvironments of a gas-hydrate-bearing sediment core in the eastern Nankai Trough, Japan

Abstract

From June to July 2012, the Ministry of Economy, Trade and Industry completed a pressure core-sampling well (AT1-C) for determining reservoir properties at the first offshore methane hydrate production test site on the north slope of the Daini–Atsumi Knoll off the Atsumi and Shima peninsulas. Coring through the targeted gas-hydrate-bearing reservoir was completed using the hybrid pressure coring system and the extended shoe coring system. The coring program achieved 61% recovery through 60 m of the hole. The recovered core consists of alternating clayey to sandy siltstones and very fine to fine sandstones. The stratigraphic pattern indicates upward thinning of the sand layer. Grain-size distributions indicate that the transition to mud facies is marked by an increase in fine particles and more poorly sorted sediments. Sedimentary structures and bioturbation characteristic of conventional cores are absent in the pressure cores due to the effects of fluidization attributed to the release of water from dissociation of gas hydrates. The core sediments were subdivided into four facies: Facies 1 (hemipelagic setting or slope) structureless silt; Facies 2 (abandonment surface mud drape) silt-dominated alternating beds of sand and silt; Facies 3 (non-amalgamated channel deposit) proportionally interbedded sand and silt; and Facies 4 (semi-amalgamated channel deposit) sand-dominated, with alternating beds of sand and silt. The results aid the interpretation of subsurface data and to quantitatively constrain geological models, thereby reducing uncertainties in the development of reservoirs.