Identification Of Marine Gas Hydrate Morphology

As an effective and clean energy,nature gas hydrate has two different morphologies: poreand fracture-filling.Identification of gas hydrate morphology is significant for resource evaluation,seafloor stability,and climate change.Through rock physics modeling,we made thorough analysis of physical properties of gas hydrate-bearing sediments?GHBS?.Based on the modeling results,we proposed three different methods to identify gas hydrate morphologies,which are successfully verified by the field measurements from the South China Sea.The theoretical modeling results shown that the P-wave velocity of fracture-filling GHBS is lower than that of pore-filling GHBS,but its resistivity is much higher.We found a cross plot between the P-wave velocity and the resistivity is significantly different between two morphologies,which can be used to identify the hydrate morphology.The borehole data from Sites 05,08,and 16 are extracted to test this method.The cross plots of the field data fit well with the theoretical cross plots.Gas hydrate saturations estimated by fitting the cross plot also agree well with that estimated from the pore-water chlorinity measurements.Because the formation mechanisms of the pore-and fracture-filling gas hydrates are different,their density variation with hydrate content exhibit different trends.Based on this difference,we found their ?V_p^0.4 versus hydrate concentration exhibit opposite trends.Therefore,the hydrate morphology can be easily identified by comparing the measured ?V_p^0.4 to the ?V_p^0.4 of hydrate-free sediments.We again use the well logging data at the three sites to verify the proposed method.The data points within the hydrate intervals that are consistent with the theoretical results account for 78%,92%,74%,65%,and 83% of the total,respectively.Apart from the above two methods,which are based on the well logging data,we proposed that the amplitude variation with angles?AVA?of the bottom simulating reflectors?BSR?of the gas hydrate-bearing sediments can be used to distinguish the two different morphologies.Based on the isotropic and anisotropic formulas for P-wave reflection coefficient,we obtained the AVA patterns for the BSRs of the pore-and fracture-filling GHBS,which belong to class III and IV,respectively.The field AVA patterns of the pre-stack seismic data at Sites 08 and 16 agree with the theoretical patterns,which verifies the correctness and reliability of the proposed method.