Guizhou-guangxi Paleozoic Reservoir Fluid Accumulation Mineralization Relationship

Based on field observation, the fluid inclusion technique has been used to study the Paleozoic reservoir fluid and its relationship to hydrocarbon accumulation and ore mineralization in the Nanpanjiang-Youjiang regions, South China.Fluid inclusions in the paleo-oil reservoirs and the gold deposits in the studied area vary greatly both in size and in form. In addition to abundant liquid H₂O and liquid-vapor H₂O inclusions, CH₄ and CH₄-H₂O inclusions are dominated in the paleo-oil reservoirs, while CO₂ and CO₂-H₂O inclusions occur mainly in the gold deposits.In the Permian reef limestone reservoirs, homogenous temperatures of aqueous inclusions range from 90℃to 160℃, the salinity of the fluid is typically below 5 wt NaC1ï¼…. Homogenous temperatures and salinity of aqueous fluids in the sedimentary rock-hosted disseminated gold deposits are generally higher than those in paleo-oil reservoirs, typically varying from 150℃to 250℃and from 3.5 to 8.4 wt NaC1ï¼…, respectively. Pressure of the reservoir fluid varies from 42×10⁵ to 654×10⁵ Pa. Laser Raman microspectrometry analyses on single fluid inclusions indicate that the vapor phases of hydrocarbon inclusions in the paleo-oil reservoirs are dominated by CH₄ and subordinately CO₂, while the vapor components of fluid inclusions in the gold deposits are mainly CO₂, with minor amounts of N₂ and CH₄.There are three stages of solution pore-and fracture-filling calcite in the Permian reef paleo-oil reservoirs. Liquid H₂O and liquid-vapor H₂O inclusions with homogenization temperatures between 77-84℃are dominated in the first stage calcite, although hydrocarbon inclusions are occasionally also seen. Fluid inclusions in the second stage calcite are predominantly liquid C₂H₆ inclusions and subordinately vapor CH₄ inclusions, bitumen inclusions, and aqueous inclusions. Homogenization temperatures of the aqueous inclusions vary from 91℃to 103℃. The third stage calcite is characterized by abundant vapor hydrocarbon inclusions with homogenization temperatures between 117-155℃. It is thus indicated that with increased burial temperature and maturity of organic matter the major components of organic inclusions vary gradually from liquid hydrocarbons to vapor hydrocarbons. Hydrocarbon filling time of the corresponding three stages is estimated to be 235-237.5 Ma, 230-232.5 Ma, and 227.5-185 Ma, respectively.Based on fluid inclusion data, the paleogeothermal gradient during Permian and Triassic is calculated to be 4.30-4.35℃/100 m in the Yangba area, with an average of 4.33℃/100 m. The burial depth of Permian strata in the Well Yang-1 is estimated to be 2564-3187 m, and the denudation thickness of the Permian stratigraphy is estimated to be 2069-4606 m.Figures of temperature, salinity, CH₄ molar percentage and fluid potential contours of the Permian reservoirs in the study area show that the hydrocarbon reservoir fluid migrated in general from south to north and accumulated in the Ceheng and Wangmo areas. Coupled with the cap rock conditions for hydrocarbon conservation, it is suggested that the most favorable prospect area for hydrocarbon reservoirs is in Banjie, Yangba, Huarong and Leyuan.The origin and evolution of hydrocarbon and ore-forming fluids as well as their relationship to petroleum accumulation and ore mineralization were discussed. A model for reservoir formation and ore mineralization was established based on systematic and comprehensive investigation.