1. seismic technology
Seismic exploration is the most commonly used method for gas hydrate exploration at present. The velocity of gas hydrate deposits is relatively high, but the stratum under gas hydrate deposits is generally a hydrocarbon gas (free gas) accumulation area, and the sound velocity is low, so the strong acoustic impedance at the bottom boundary of hydrate will produce strong reflection, showing a unique reflection interface on the seismic reflection profile. In addition, because the boundary of the gas hydrate stability zone is roughly distributed at the same seabed depth, and the reflection at the bottom of the hydrate stability zone is also roughly parallel to the seabed, this technology is named BSR (Figure 14- 10). With the wide application of multi-channel reflection seismic technology and the improvement of seismic data processing technology, the characteristics of BSR in seismic profile, such as high amplitude, negative polarity, parallel to the seabed and intersecting with submarine sedimentary structures, have been easily identified. It has been proved that hydrocarbon gas above BSR exists in the form of solid natural gas hydrate, and hydrocarbon gas below BSR exists in the form of free gas. BSR is the earliest, most reliable and intuitive geophysical sign to confirm the existence of natural gas hydrate. At present, most of the confirmed submarine gas hydrates are found by identifying BSR on the reflection seismic profile.
2. Logging technology
The functions of logging technology mainly include: ① determining the depth distribution of natural gas hydrate and sediments containing natural gas hydrate; ② Estimation of porosity and methane saturation; ③ Using borehole information to correct seismic and other geophysical data. Logging data is also an effective means to study the sedimentary environment and evolution of the main strata of natural gas hydrate near the well point.
On the conventional logging curve, gas hydrate deposition mainly shows the following abnormal phenomena, as shown in figure 14- 1 1: ① high resistivity; ② The sound wave time difference is small; ③ The amplitude of spontaneous potential is not large; ④ High neutron logging value; ⑤ High gamma value; ⑥ Large aperture; ⑦ There is obvious gas emission during drilling, and the measured gas value is high.
Fig. 14- 10 BSR seismic profile in Black Ridge area.
(According to Collett et al., 2009)
Fig. 14- 1 1 logging response characteristics of natural gas hydrate layer
3. Geochemical technology
Geochemical technology is an effective means to identify the occurrence state of submarine gas hydrate. The fluctuation of temperature and pressure can easily decompose natural gas hydrate, so there are often geochemical anomalies of natural gas in shallow seabed sediments. These anomalies can indicate the possible location of natural gas hydrate, and then the source of natural gas can be judged by its hydrocarbon composition ratio (such as C 1/C2) and carbon isotope composition. At the same time, the application of offshore methane field detection technology can delineate the high methane concentration area and determine the prospective distribution of natural gas hydrate.
Under the current technical conditions, the main signs of geochemical exploration of natural gas hydrate include: the decrease of chlorine or salinity in pore water, the low redox potential of water, the low sulfate content and the change of oxygen isotope. When analyzing geochemical data, we should treat them differently and comprehensively according to the specific actual situation.
4. Marking mineral technology
Typomorphic minerals that can indicate the existence of natural gas hydrate are usually carbonates, sulfates and sulfides with specific composition and morphology. They are a series of typomorphic minerals formed by the interaction of ore-forming fluids with seawater, pore water and sediments in the process of sedimentation, diagenesis and supergene.
When the fluid under the seabed enters the vicinity of the seabed in the form of overflow or seepage, it will produce a series of physical, chemical and biological effects. When the fluid containing saturated gas moves from deep sea to shallow sea bottom, it quickly cools to form natural gas hydrate, accompanied by authigenic carbonate and chemical energy autotrophic biota dependent on this fluid. Because of its low temperature, these fluids are called "cold spring" fluids, which are different from the high temperature fluids in the deep crust and are one of the most effective indicator minerals for finding natural gas hydrates.