Research On The Tectogenetic Movement And The Fluid Migration In Dabashan Thrust And Fold Belt And Its Foreland

Dabashan Thrust and Fold Belt and its foreland,which located in the transitional zone between Qinling Orogen and Sichuan Basin,are important tectonic belts adjacent to the southern margin of Chinese Central Orogen.Previous exploration has shown that the thrust and fold belt is a place rich in oil and gas potentially.The discovery of Puguang Gas Field indicate that foreland of Dabashan Thrust and Fold Belt,which has been taken as strategically exploration area,is a wonderful perspective region in exploration on oil and gas.On the basis of detailed structural analysis in field,the stages of post-Mesozoic tectonic movements have been identified and the tectonic orders have been built.By the large number of data of striae measured in the field,the paleotectonic stress fields of five tectonic movements have been obtained.The first stage of tectonic movement shown compression in north-south direction roughly and the attitude ofσ₁,σ₂ andσ₃ are 3°∠2°,93°∠0°and 184°∠88°respectively.The second shown compression in NE-SW direction and the attitude ofσ₁,σ₂ andσ₃ are 40°∠2°,310°∠2°and 175°∠88°.The third stage shown NW-SE directional compression and the attitude ofσ₁,σ₂ andσ₃ are 143°∠1°,53°∠2°and 267°∠88°.The attitude ofσ₁,σ₂ andσ₃ of the fourth tectonic movement are 106°∠16°,231°∠64°and 10°∠20°,which indicate a nearly east-west compression.The fifth stage of tectonic movement behaved as NE-SW extension and the attitude ofσ₁,σ₂ andσ₃ are 238°∠90°,127°∠0°and 37°∠0°.The present tectonic framework of the research region mainly formed during the second stage of tectonic movement under the NE-SW compression.Two phases of fluid inclusions,which possess homogenization temperatures of 140℃～210℃and 230℃～270℃,have been identified by the data of fluid inclusions entrapped inside the veins filled in the faults.On the plots of homogenization temperature vs salinity,the plotting points distribute on two clusters, lower and higher salinity respectively,which also indicates two stages of fluid flow.The Indosinian NE-SW compression plays an important role in the forming and distributing of oil and gas in this region.The later structures developed by eas t-west compression overriding the structures formed by NE-SW compression,and the superimposed fold is the most advantageous area for the accumulating of oil and gas.