China’s second major gas hydrate expedition, GMGS2, took place from the Fugro M/V REM Etive between June and September 2013, in the eastern part of the Pearl River Mouth basin, in the South China Sea. The investigated area lies northeast of the Shenhu site, where the first Chinese gas hydrate expedition (GMGS1) was completed in 2007. The GMGS2 expedition was contracted by the Chinese Geological Survey’s Guangzhou Marine Geological Survey (GMGS) and conducted by Fugro, Schlumberger, and Geotek.

The primary objective of the expedition was to accurately quantify gas hydrate in sediment cores and to determine the nature and distribution of gas hydrate within the sedimentary sequence in the basin. To achieve these goals, GMGS2 employed an initial Logging While Drilling (Schlumberger LWD) phase (Leg 1), followed by a coring and sampling phase (Legs 2 and 3), which included some wireline logging.

Sixteen possible drilling sites were identified prior to the expedition, at sites with water depths ranging from 667 m to 1747 m. During the course of the expedition, 13 of the 16 possible sites were investigated. Sites GMGS2-02, -03, -04, -05, -08, -08s, -09, -11, -15 and -16 were first investigated on GMGS2 Leg 1 using LWD techniques, whereas sites GMGS2-01, -07 and -12 were first investigated on GMGS2 Leg 3
using wireline logging. Figures 3 and 4 illustrate the main LWD curves for sites GMGS2-08 and -05.

Of the 13 sites investigated during the GMGS2 expedition, five were selected for further analysis by coring. Sites GMGS2-05, -08, -09 and -16 were cored during GMGS2 Leg 2, while sites GMGS2-07 and -16 were
sampled during GMGS2 Leg 3. Fugro coring tools used for GMGS2 included rotary and non-rotary tools for sampling different lithologies, as well as pressure coring tools designed to recover gas hydrate samples at in situ pressures. A team of GMGS and Geotek scientists and technicians performed a comprehensive suite of analyses of all core material recovered on board the vessel.

Geotek provided a range of geophysical and geochemical core processing and core analysis equipment in containerized mobile laboratories for both pressure cores and non-pressure cores. The equipment suite included the Pressure Core Analysis and Transfer System (PCATS) for analyzing pressure cores up to 3.5 m long and PCATS Triaxial equipment for performing geomechanical tests on samples recovered at full, in situ hydrostatic pressures. Whole core analysis equipment for nonpressure cores included fast automated thermal infrared core logging as well as standard geophysical core logs. An example of a whole core X-ray CT image is shown in Figure 5. Cores were split on board the ship, before additional imaging and XRF measurements were made for detailed sedimentological analysis. A geochemistry laboratory provided a full suite of pore water and gas analyses suitable for determining the nature and distribution of gas hydrate.

Using combined data from logging and core sampling, it was confirmed that nine of the 13 sites investigated contain gas hydrate in one form or another. Gas hydrate-bearing sites include GMGS2-01, -04, -05,
-07, -08, -09, -11, -12 and -16. Note that all five of the cored sites contain gas hydrate. Gas hydrate-bearing lithologies identified from the coring include the following morphologies: a) massive forms of visible gas hydrate (sites GMGS2-07, -08, and -09; Figure 6); b) disseminated gas hydrate in deeply buried fine-grained sediments (sites GMGS2-05, -09 and -16); c) dense, thin veins of gas hydrate in shallow, fine-grained sediments (sites GMGS2-08 and -16); and d) disseminated gas hydrate in coarse-grained sediments (site GMGS2-16). Photos in Figure 7 show the variety of gas hydrate morphologies found at GMGS2. Massive gas hydrate at site GMGS2-09 was associated with a surface carbonate platform, which had been investigated during previous expeditions in the region. The massive gas hydrate features discovered at sites GMGS2-07 and -08 are thought to be features initially formed at the surface (as per site GMGS2-09), in association with carbonate platforms, and subsequently buried. The layers of disseminated gas hydrate lying just above the base of the methane hydrate stability zone in fine-grained sediment sequences are similar to those discovered at Shenhu during GMGS1 in 2007, with gas hydrate saturations of 25-55% of pore volume in formations that show only a 1-2 ohm-m anomaly in the LWD resistivity.

However, the near-seafloor zones containing thin veins of hydrate (up to 33% of pore volume), found at two sites, have not been seen at other locations. At sites GMGS2-07 and GMGS2-16, free gas was observed coming directly from boreholes. Extensive coring and porewater analysis at site GMGS2-16 revealed evidence of a deep, coarse-grained aquifer around 200m below the seafloor which may be supplying both free gas and fluids to the system.

After more than 3 months of expedition time it was concluded that the drilled region is a very active area of methane flux, and that gas hydrate is common in the first 200 meters below the seafloor. This area is of the richest
and most complex marine gas hydrate environments studied to date.