Core lithologies and their constraints on gas-hydrate occurrence in the East Sea, offshore Korea: Results from the site UBGH1-9

• Published in: Marine and petroleum geology 2011-11, Vol.28 (10), p.1943-1952
• Main Authors: Bahk, Jang-Jun, Um, In-Kwon, Holland, Melanie
• Format: Article
• Language: English
• Subjects: Foundry sands; Gas hydrate; Grain size; Lithology; Marine; Mud; Offshore; Offshore drilling rigs; Offshore engineering; Sand; Saturation; Sediments; Ulleung Basin
• ISSN: 0264-8172; 1873-4073
• DOI: 10.1016/j.marpetgeo.2010.12.003

Drilling at the site UBGH1-9, offshore Korea in 2007, revealed varied gas-hydrate saturation with depth and a wide variety of core litholgies, demonstrating how the variations in the lithology are linked with those in gas-hydrate saturation and morphology. Discrete excursions to low chlorinity values from in situ background chlorinity level occur between 63 and 151 mbsf. In this occurrence zone, gas-hydrate saturations estimated from the low chlorinity anomalies range up to 63.5% of pore volume with an average of 9.9% and do not show a clear depth-dependent trend. Sedimentary facies analysis based on grain-size distribution and sedimentary structures revealed nine sediment facies which mainly represent hemipelagic muds and fine- to medium-grained turbidites. According to the sedimentary facies distribution, the core sediments are divided into three facies associations (FA): FA I (0–98 mbsf) consisting mainly of alternating thin- to medium-bedded hemipelagic mud and turbidite sand or mud beds, FA II (98–126 mbsf) dominated by medium- to very thick-bedded turbidite sand or sandy debris flow beds, and FA III (126–178 mbsf) characterized by thick hemipelagic mud without intervening discrete turbidite sand layers. Thermal anomalies from IR scan, mousse-like and soupy structures on split-core surfaces, non-destructive measurements of pressure cores, and comparison of gas-hydrate saturations with sand contents of corresponding pore-water squeeze cakes, collectively suggest that the gas hydrate at the site UBGH1-9 generally occurs in two different types: “pore-filling” type preferentially associated with thin- to medium-turbidite sand beds in the FA I and “fracture-filling” type which occurs as hydrate veins or nodules in hemipelagic mud of the FA III. Gas-hydrate saturation in the FA II is generally anomalously low despite the dominance of turbidite sand or sandy debris flow beds, suggesting insufficient methane supply. ► Varied gas-hydrate saturation are linked with lithology changes. ► One type of hydrate resides in discrete turbidte sand layers within mud. ► The other type is associated with hydrate veins in thick mud. ► Lack of hydrate in sand-rich interval suggests bypassing of methane supply.