Research On Estimation Of Submarine Gas Hydrate Saturation Using Marine CSEM Method

The utilization study of new energy is imminent because of the depletion oftraditional hydrocarbon resources. As a new energy, submarine gas hydrate has greatpotential for energy supply and its products are clean. Besides, submarine gas hydrateis also closely related to environment and methane as the main ingredient of submarinegas hydrate has a very important influence on global climate. Decomposition ofsubmarine gas hydrate is an incentive to seabed geological disasters, and it may causethe seabed geological damage. In order to effectively use submarine gas hydrate andavoid damaging the environment and ecology, it is necessary to study the physicalproperties, detection methods and content estimation methods of submarine gashydrate.Traditional methods commonly used to explore submarine gas hydrate are mainlyseismic and well logging methods. In seismic exploration, bottom simulating reflector(BSR), blank zone and bright spots sometimes can be seen as signs of gas hydratereservoir. Although BSR in the seismic profile is believed to signify the presence of gashydrate, studies have shown that BSR doesn’t always mark the presence of gas hydrateand the seismic method can’t detect the upper boundary of the submarine gas hydratereservoir and the content of gas hydrate. It requires certain conditions and high-cost toexplore submarine gas hydrate with well logging method. As an important and newlyarisen means of marine exploration, a lot of research for the theory and application ofmarine controlled source electromagnetic (CSEM) method have been done. Studiesfound that submarine gas hydrate can be detected by the resistivity differences betweenthe submarine gas hydrate and surrounding rock. Many countries and regions havecarried out submarine gas hydrate exploration using marine CSEM method. MarineCSEM method can be used to determine the distribution and saturation of submarinegas hydrate together with seismic, well logging, geothermal heat flow and coreanalysis methods comprehensively. Firstly，this article briefly describes the working mode of marine CSEM methodin exploring for submarine gas hydrate. Then, the one-dimensional electromagneticfield expressions in frequency domain and study the algorithm of two-dimensionaladaptive finite element method based on posteriori error estimation are derived. Theone-dimensional model and two-dimensional model with gas hydrate reservoir arebuilt according to the actual geological and geophysical data of South China Sea,Shenhu area and Gulf of Mexico. After that, the electromagnetic response ofone-dimensional model and two-dimensional model are calculated based on forwardmodeling theories. By comparing the amplitude and phase responses generated byone-dimensional and two-dimensional gas hydrate model, the effectiveness of marineCSEM method to detect submarine gas hydrate is proved.This paper also discusses the exploration parameters such as frequency, offset andtransmitter height in submarine gas hydrate detection using marine CSEM method onthe basis of forward modeling. It is found that geological conditions and characteristicsof target geological bodies must be taken into account when determining theexploration parameters. Appropriate frequency and offset can be determined withfrequency-offset-normalized amplitude plot and transmitter height can be determinedin accordance with the seabed topography, instrument control precision and otherfactors. To compute the resistivity values used in gas hydrate saturation calculation,this paper also introduces the principle of one-dimensional Occam inversion andtwo-dimensional equivalent of apparent resistivity calculation and calculates resistivityof the one-dimensional and two-dimensional gas hydrate model.To calculate submarine gas hydrate saturation, this article discusses the use ofArchie law and determines its parameters on basis of previous works and summarizescommon parameters values of South China Sea, Shenhu area and the Gulf of Mexico.With resistivity from Occam inversion, the equivalent apparent resistivity calculationand Archie parameters from relevant areas, the saturation of one-dimensional andtwo-dimensional submarine gas hydrate model can be obtained. Finally, the estimatedsaturation values and values from actual data are compared by each other. The study ofthis paper will give some supports in the actual detection and content estimation of submarine gas hydrate.