Favorable oceanic deposits are those with a high SH in high-quality reservoirs. The challenges facing commercialization of marine GH are likely to be higher than those in the Arctic, given the higher cost deep water operations. Installed infrastructure and access to markets (e. g., in the GOM), or lack of an Arctic GH option (as is the case of India or Japan), may make this an attractive option. The oceanic deposits that serve as models for the evaluation of marine GH prospects are described below.

Offshore Japan— Nankai Trough. This area has probably experienced the largest investment and most advanced field research activity because of the intensive Japanese effort to evaluate the potential and feasibility of gas production from hydrates. Following the drilling of an exploration well in 2000 [190], a multi-well exploration program was conducted in 2004 at 16 locations in three different sites (Kumano Basin, Second Atsumi Knoll, and Offshore Tokai) that had been selected on the basis of the BSR signature [189] at water depths of 720-2,033 m (Fig. 8). A total of 32 wells were drilled, and a comprehensive evaluation was conducted. The experimental program focused heavily on the practicalities and challenges of well construction in hydrate sediments [189]. An offshore production test appears to be the next logical step [48,96, 172, 189].

МЕТІ “Tokai-oki to Kumano-nada”
exploratory test wells (2004)

Tokai-Oki

S>QQ2%D Seismic Survey Area

MITI “Nankal-Trough

2001 2D Seismic Survey Area

Water Depth 722m ~ 2033m Drilling Length 263m ~ 451m (LWD)

Fig. 8 The hydrate deposit areas in the Nankai Trough region offshore Japan, and the drilling sites of the 2004 Drilling Program [96]

GOM— Oligocene Frio Formation, Tigershark Deposit. Smith et al. [179] described this first documented case of high-SH hydrate-bearing sand in the Alaminos Canyon Block 818 of the GOM (Fig. 9). Log data from an exploration well in about 2,750 m of water at the site indicated the presence of an 18.25-m thick sandy hydrate-bearing layer (HBL) (3,210-3,228 m drilling depth) at a relatively high temperature (about 21°C), with a high porosity p (about 0.30), Darcy-range intrinsic permeability k, and with the base of the GH stability zone occurring at or slightly below the base of the hydrate [26,179].

This deposit belongs to the third tier in the resource pyramid of Fig. 2, with ini­tial SH estimates ranging from 0.6 to over 0.8 [26] . Preliminary simulations with synthetic data (describing GH reservoirs under the Tigershark conditions) indicate that such systems can reach gas production rates well in excess of 2.8 x 105 m3/ day= 10 MMSCFD [131,132].

GOM—GC955 and WR313 Deposits. The reservoirs are very recent findings that were first identified during the “Leg II” logging-while-drilling operations conducted in the deepwater GOM in April 2009 [11, 24, 142] by the GOM Gas Hydrates Joint Industry Project (JIP) [74,115,117,118]. The sites for Leg II drilling of the JIP were selected based on an integrated geologic-geophysical approach designed to identify accumulations of gas hydrate at high saturation SH (>50%) in sand reservoirs (Fig. 10).

AC818 Keathley Canyon

Fig. 10 Seafloor map of the Gulf of Mexico depicting the location of JIP Leg II drill sites: AC818, WR313, and GC955 [135]

The GC955 accumulation involves a sand-rich channel-levee complex at the mouth of the Green Canyon embayment. Two industry wells in the block have confirmed the presence of clean, thick sands in close proximity to the most clearly imaged channel axis. Logging results from Leg II showed a thick accumulation of gas hydrates within porous sand lithologies [54]. The drilling depth to the top of the sand unit was 389 mbsf (feet below sea floor). At the H-well location [54], approximately 100 m of GH-bearing media involved a sequence of thin horizontal sand-shale interbeds at approximately the 0.3-0.6-m scale. GH occurs in pore-filling mode in three separate zones with SH in the 50-85% range, with no indication of free gas.

The WR313 sites are within the highly dipping eastern margin of the Terrebonne salt-withdrawal mini-basin with significant sand deposition (see [115,117,118,177]). Two horizons were targeted for drilling in JIP Leg II: Well WR313 “G” and well WR313 “H” targeted the informally named “blue” and “orange” horizons, respec­tively. LWD data obtained at the G-well indicated a sand-rich interval with thinly interbedded sands and shales (0.3-1.5 m). The drilling depth to the top of the unit was 852 mbsf. About 12 m of GH-bearing sands within a gross interval of 21 m were identified, with SH typically ranging from 40% to over 70%. The primary con­trol on gas hydrate occurrence in this unit is availability of suitable reservoir condi­tions [9, 11] . The WR313 “H” horizon occurred approximately 140 m below the blue horizon, and consisted of two massive, clean sands with sharp bases and tops. Drilling depth to the top of the upper unit was 806 mbsf. The upper “orange” sand is 4 m thick, with SH as high as 90% [54]; the lower unit is about 7 m thick, with SH ranging between 35 and 80%.