Natural gas hydrate, or gas hydrate, is a crystalline solid formed in high pressure, low temperature environment where sufficient gas and water are present, which looks like ice and can be lit on fire, hence known as combustible ice. Gas hydrates are usually found in oceanic sediments at water depths greater than 300 m or in permanent tundra at depths of not less than 130 m below the surface. In China, combustible ice has been discovered in the tundra of the Qilian Mountains (the land-based area) and under the South China Sea and the East China Sea. In 2008, samples of combustible ice were collected in the northwestern Qilian Mountains, leading to a major discovery of gas hydrate in tundra in the medium and low latitudes. Those deposits are located at 133-396 m depths in the form of white icy flakes in rock fracture or fine disseminated substance that is barely visible to the naked eye in rock pore. China announced in May 2017 a breakthrough for mining combustible ice in the Shenhu area of the South China Sea. On March 28, the first test well was drilled. At 14:52 pm of May 10, the samples collected caught fire, indicating the success of extracting gas from maritime sediments at depths of 203-277 m, 1266 m under the sea. By 10:00 am of May 18, an average of 16,000 cubic meters of gas have been extracted per day for 8 consecutive days, surpassing the goal of 10,000 cubic meters of gas per day for a week consecutively.
CNKI International Publishing Center invited Lu Zhenquan, Director of Natural Gas Hydrate Office, Gas and Oil Survey Center, China Geological Survey, to talk about the differences between oceanic and continental gas hydrates in China in terms of gas source, deposition mode, and exploration and mining, in an attempt to give an overview on gas hydrate survey in China.
Lu told us, gas hydrates extracted from the tundra of the Qilian Mountains are quite different from those extracted under the South China Sea in many ways, for example:
1. Different geographical conditions for the formation of gas hydrateIn the tundra of the Qilian Mountains located in the medium latitude and high attitude, frozen soil of certain thickness and an appropriate geothermal gradient are sufficient conditions for the formation of gas hydrates in the hydrate stability zone, while under the South China Sea, the necessary conditions include water at certain depths and an appropriate geothermal gradient.
2. Different types of sediments under which gas hydrates are found
Gas hydrates are extracted from the consolidated sediments in Jurassic coal measure strata and in argillaceous rock fracture in the tundra of the Qilian Mountains, while they are found in the unconsolidated Neogene sediments under the South China Sea.
3. Different features of occurrences
Gas hydrates present as flakes in argillaceous rock fracture and disseminated substances in siltstone pore respectively in the tundra of the Qilian Mountains and under the South China Sea.
4. Different composition and formation
Gas hydrates found in the tundra of the Qilian Mountains are comprised of heavy hydrocarbons like ethane, propane and butane apart from methane generated by oil pyrolysis, while biogenic methane constitutes gas hydrates discovered under the South China Sea.
5. Different structures
Gas hydrates in the tundra of the Qilian Mountains and under the South China Sea form structure II and structure I respectively.
6. Differences in features of distributions
Gas hydrates are unevenly distributed in the tundra of the Qilian Mountains, appearing at intervals of hundreds of meters in the transverse direction and sporadically or discontinuously in the longitudinal direction. By contrast, they are evenly distributed under the South China Sea, extending a few square kilometers or even a few tens of square kilometers in the transverse direction and a few meters or even hundreds of meters in the longitudinal direction.
7. Differences in deposition modes
The deposition of gas hydrates in the tundra of the Qilian Mountains follows the mode of gas source, formation and coupling with frozen soil. In the late stage of middle Jurassic period, hydrocarbon generated from Trias and other source rocks accumulated locally under the actions of compression fractures in the same period or later age and Jurassic argillaceous rock. They may be mixed with some coal gas and biogenic gas. At the time not later than the late pleistocene epoch, those gases were partly converted into gas hydrates after coupling with frozen soil in the hydrate stability zone. Part of gases remains free or in absorbed state above the zone.
Under the South China Sea, gas hydrates are deposited by seepage or diffusion. Organic matters in the unconsolidated Neogene sediments produced biogenic gases under the effects of microorganisms. Those gases together with the pyrolytic gases underneath or in the deep area moved upwards by seepage nearby the fracture zone, while they penetrated sedimentary bed by diffusion and moved upwards in other zones. Once arriving at the hydrate stability zone, they were converted into gas hydrate, which occurred in the strata where oil shale is commonly seen. Oil patch, oil immersion and other signs of oil and gas are usually shown along with gas hydrates.
8. Differences in the strategies of exploration and mining
An integrated method of exploration involving geology, geochemistry, geophysics and drilling (including well logging) is adopted due to a lack of an effective technology system for the exploration of gas hydrates in the tundra of the Qilian Mountains. To the contrary, a system of effective exploration methods such as well logging and seismic survey has been developed for the exploration of gas hydrates under the South China Sea. Exploration is carried out in good order by using technologies in geology, geochemistry, seismic exploration, drilling and well logging. Currently in both sites, reducing pressure is mainly incorporated in the trial mining program, but from the actual effect point of view, the South China Sea has achieved greater success.
JTP's LIU Kang contributed to this report.