The Process Of Resource Exploration And Estimate For Submarine Gas Hydrate

Natural gas-hydrate is an unconventional ice-like potential clean energy, rich in methane. One unit volume of natural gas hydrate can produce 164 units volume of methane gas. Natural gas hydrates are widely distributed in the slope area of the main land and islands, the active and passive uplifted area of continental margin, polar continental shelf, oceans, as well as deepwater in inland lakes and permafrost. Gas hydrates have gradually been attached importance to by scientists and governments all over the world, for its giant reserves, wide distribution, high energy density, and non-pollution properties. Gas hydrates were considered to be the alternative energy for petroleum and coal. So the exploration and resource evaluation for gas hydrate have become one of the hottest topics in geosciences.The exploration of submarine gas hydrate can be traced back to the late 1970's. Drilling was carried out on Gulf of Mexico by DSDP leg 66 and leg 67, and received gas hydrate core 91.24 m from seafloor, which approved the existence of submarine gas hydrate deposit for the first time. So far, more than 220 sea areas in the world have found the existence of gas hydrate directly or indirectly. But exploration and resource evaluation for submarine gas hydrate are still imperfect and nonsystematic.Although the explorations in foreign countries on submarine gas are much earlier than those in China, and their researches are much further, there is still no systemic report about the exploration process in those countries, nor formal criterion in China. In order to afford a systematic exploration criterion during submarine gas hydrate exploration, this paper puts forward the "Phasing method" and resource evaluation methods through summarizing the exploration experiences of conventional mineral resources, which include solid mineral resource, coal, petroleum, seabed nodules and uranium ore, and through integrating the submarine gas hydrate deposits exploration in foreign countries. So this paper will have a positive significance in submarine gas hydrate survey, and will provide some foundation and reference for the government to set up a standard in this field.Through systemic research, this paper divides gas hydrate exploration into four stages, which are respectively independent but also relevant: prospective survey stage, reconnaissance stage, comprehensive survey stage and exploration stage.Prospective survey stage is a primary stage which focuses on previous large-scale survey of temperature and pressure condition, as well as documents collecting of this region. The main target is to determine the distribution of gas hydrate and gas source rocks condition preliminarily, and to delineate the prospective areas for reconnaissance stage. The main objects of evaluation in this stage include natural gas existing status, gas hydrate existing parameters and prospective areas. The main purpose of this stage is to understand the present probability of gas hydrate in this area.Reconnaissance stage is the 2nd stage. In this stage, offshore seism is the most important project, and the main purpose of which is researching the geometric shape and spatial variation about gas hydrate deposit. The main evaluation contents in this stage include the genesis of gas hydrate, the method of exploring technology of hydrate and the favorable areas of hydrate. The main purpose of this stage is to understand the genesis of gas hydrate and to find the evidence which can indicate the presence of hydrate.Comprehensive survey stage is the extensive period of Reconnaissance stage. By using high resolution 3-d seismic, deep sea drilling, PCS and so on, we can find out the favorable horizons for the exploration stage. In this period the major targets are to confirm the occurrence of bonanza, change of thickness, shape, size and the relationship between hydrate and surrounding rock, and to classify the favorable horizons. The main objects of evaluation in this stage include geophysical and geochemical data as well as gas hydrate district. The main purpose of this stage is to understand the accumulation regularity on the basis of obtaining information from geophysical and geochemical data reasonably, and to figure out a much more credible reserve data.Exploration stage is the highest stage which researches the hydrate body elaborately by integrating information from well and seism. The main target in the stage is to define the occurrence, effective thickness, the shapes and scale of the hydrate body and the results can serve for the following development periods. The main objects of evaluation in this stage include the mining technical conditions and hydrate body, with the purpose of mastering the spatial distribution beneath seabed. Proven reserves and controlled reserves should be counted out, and related date and information are provided for the following development periods.The main methods of hydrate resource calculation include volume method and Monte-carol probability statistical method. Monte-carol probability statistical method is suitable for early stage of gas hydrate survey. In this paper we follow volume method basically. The emphasis is to confirm the parameters in different survey stages, such as area, effective thickness, porosity of sediment, saturation of hydrate and so on. As the exploration degree increases, the parameters of gas hydrate occurrence become much more accurate, and the results are much more creditable. In the beginning we use conjecture method, then go to indirect method and direct method at last. In conjecture method, the parameters are got from analog, and the results can provide an inferred reserve or potential reserve. In indirect method the information which was extracted from geophysical and geochemical data inversed the necessary parameters like area, thickness, porosity and saturation, and the results can provide a divinable reserve. In direct method the occurrence of gas hydrate beneath seafloor is presented objectively by deep water drilling (PCS) and measure in suit, and the results can provide a controlled reserve or proved reserve.There are still many problems in exploration stage division and resource evaluation, such as projects arrangement, projects density and so on, what's more, resource evaluation for submarine gas hydrate is still imperfect. All these problems will be improved in the following practical works.