| Baiyun sag is located in the northern margin of the South China Sea,which is the key area of gas hydrate research in China.The natural gas hydrate in this area is similar to that found in typical regions of the world,but the difference is more significant.Due to the influence of geological structure and sedimentary environment,most of the hydrate samples drilled in Shenhu sea area of Baiyun sag are distributed in fine-grained muddy and silty sediments,and their saturation value is lower than that of typical hydrate distribution areas in the world.According to the existing research,the hydrate gas source characteristics,saturation and distribution,hydrate layer thickness found at different stations in the drilling area are different.The formation mechanism and accumulation rule of hydrate of different genetic types in the study area are not clear.Carrying out hydrocarbon generation numerical simulation can not only effectively predict hydrate saturation and distribution,but also further explore the occurrence and accumulation mode of complex genetic types of natural gas hydrate,and provide some help for the future study of natural gas hydrate in the area without wells.This paper focuses on the study of hydrocarbon generation and gas hydrate saturation and distribution of different types of source rocks,and draws on the previous experience in numerical simulation of hydrate hydrocarbon generation.Combining with seismic data,core and logging data of hydrate drilling and oil-gas drilling,regional geological structure evolution and thermal evolution,source rock distribution and geochemical indicators and other previous basic research data.Established two-dimensional geological model,temperature model and hydrocarbon generation model in the study area.Based on the simulation of hydrate formation mechanism of different source rock distribution,different fluid migration mode,different gas source types and different saturation distribution,the current hydrate accumulation model in the study area is summarized.The simulation results of present formation temperature field show that the present formation temperature distribution range of Shenhu sea area in Baiyun Depression is 0-400 ?,and there are high temperature anomalies near stations W19 and W17,which may be related to the deep fluid activity.The simulation results of organic matter maturity and hydrocarbon generation rate of source rocks show that the strata of Wenchang Formation,Enping formation and Zhuhai Formation are of high maturity and belong to thermal hydrocarbon generation strata.However,the maturity of organic matter above Zhujiang Formation is relatively low,which belongs to biogenic hydrocarbon generation formation.Combining with the simulated temperature profile and hydrocarbon generation rate profile,it is further divided that the depth range of biological hydrocarbon generation formation is within 1500 m below the sea floor,and the depth range of thermal hydrocarbon generation formation is between 2300-6000 mbsf.In addition,the simulation calculation of hydrocarbon generation amount of source rock shows that there are dozens of times differences between biogenic and thermal hydrocarbon generation due to the small abundance of organic matter.The gas production of the source rock also changes in the horizontal distribution,presumably because the southwest side of the profile is close to the settlement center of Baiyun main depression,and its organic matter abundance is greater than the northeast side.The results of fluid migration simulation show that there are many kinds of fluid migration modes in Shenhu sea area,and the Darcy path migration mode along the low permeability stratum is the most widely distributed.There is a flow path migration mode along the high permeability stratum in Zhuhai Formation,while the breakthrough and fault migration modes control the upward migration of deep fluid.Biogenic gas migrates in the shallow layer along the side wall of the canyon by buoyancy and diffusion;thermal gas migrates to LW3 oil and gas field mainly in the deep side due to the influence of inclined stratum and sandy reservoir distribution caused by basement uplift,and a small part migrates vertically to the bottom of hydrate stable zone through faults,anticlines,domes and other structures.The simulation results of hydrate stability zone show that the bottom boundary of hydrate stability zone calculated by other methods is basically the same.The simulated thickness of the stable zone in SH2 is about 225 m,that in sh5 is about 180 m,that in W19 is about 168 m,and that in W17 is about 241 m,which is consistent with the previous research results.The simulation results of hydrate saturation and distribution show that the formation of discontinuous hydrate layer above the stable zone in Shenhu sea area is mainly related to the biogenic hydrocarbon generation with relatively low organic matter abundance,and the saturation value is relatively low.When the thermogenic gas has a certain contribution to hydrate formation,it will form relatively high saturation hydrate enrichment,and the longer hydrocarbon gas release time is of great significance to the formation of high saturation hydrate enrichment.There are also differences in gas source types of hydrate in different Canyon strips.The formation of hydrate in SH2 station is mainly affected by biogenic gas,while the formation of hydrate in W19 and W17 stations is related to mixture gas,and the thermogenic gas plays an important role.The formation mechanism and accumulation model of gas hydrate of complex genetic type in the study area show that the accumulation and accumulation of gas hydrate are affected by structural activity,stratigraphic subsidence,temperature distribution,hydrocarbon generation from source rock,fluid migration,hydrate generation and other factors.The study area is located in the lacustrine sedimentary environment before the new century,and in the semi deep sea sedimentary environment since the new century.Most of the sediments are fine-grained sediments.In this kind of low permeability stratum,biogenic gas migrates in shallow layer by buoyancy and diffusion,while thermal gas migrates laterally in deep layer under the influence of stratum tilt caused by basement uplift and local high permeability stratum distribution of Zhuhai Formation.At the same time,affected by the tectonic activity and the gas overpressure in the deep anticline and dome,the deep thermal gas can move up to the bottom of the stable zone.The diversity of Hydrate Distribution in the gorge enrichment area of Shenhu sea area is related to the gas supply of different genetic types.The southwest side of the gorge is close to the Baiyun main depression,with large biogenic gas abundance,but less structural activity and limited contribution of deep thermal gas.Therefore,the hydrate genetic type in the southwest side is biogenic.However,the northeast side of the canyon is far away from Baiyun main depression,and the abundance of biogenic gas is relatively small.Affected by basement uplift,the fault system is developed,and the deep thermal gas has a significant impact on the formation of hydrate.Therefore,the genetic type of hydrate in the northeast side of the canyon is the mixed gas. |
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