The Shale Sedimentary Characteristics About Upper Triassic Ma'antang Formation In The Southern Part Of Longmen Shan Foreland Basin

The front area of Longmen Shan is full of oil and gas,which shows great potential of oil and gas exploration.So we Will carry out the research of oil and gas,shale gas exploration in Longmen Shan anterior margin,Due to the breach that Ma'an tang provided for the research of Marine shale gas,Through the previous field data collection,the field measured section,drilling core observation,Geochemical analysts have studied the sedimentay characteristics of shale in Ma'antang formation,the formation mechanism of shale and the characteristics of shale gas accumulation condition,which have mainly made the following understanding.(1)There are mainly two types of sedimentary facies,reef facies,tidal flat and lagoon facies.Ma'an Tang Formation developed in reef-shore,shallow lake and delta facies.Black shale mainly exists in upper saddle pond group,and on biological reef distribution,saddle pond shale thickness is thin,The effective thickness is accumulated up to about 50 m.from south to north,has the tendency of thickening.(2The organic carbon content of Maantang Formation is almost no less than 1%,which is greater than 2.5 in ya 'an area.TOC value of Maantang have gradually reduced from ya 'an region of southwest to north east,which suggests that thrust belt area far away from the Longmen mountains have gradually decreased.Various types of organic matter can coexist.It includes type3,? type 2 and ? type 1.Most of them is type3,a few of them is type 1 and type 2.The Ro of Ma'an Tang Formation is greater than 2.0 in ya 'an area,which has gradually decreased from ya 'an area of southwest to the north east direction.It suggests that Ro values decrease gradually away from the longmenshan thrust areas.The porosity of Ma'an Tang is 0.5% ~ 0.5%,which is smaller.The permeability of mud shale in Ma'an Tang is 0.001~0.01×10-3?m2,which is low.The buried depth of the Ma'an Tang Formation shale are mainly distributed in 3100~5900m in southern section of front Longmen mountains.Near the front Longmen mountain region,the buried depth were within 4000 m on average.The effective thickness of rich organic matter shale add up to 50 m around in Ma'an Tang Formation.It is thin in the south,and thick in the north with a tendency to thickening.In Dujiangyan-dayi region that in front of the Longmen mountains,the cumulative effective thickness of rich organic shale is about 100 m and the average depth is less than 4000 m,where the average organic carbon content is 2.11%,the maturity of organic matter is at least 1.25%,the total air content is at least 1.0m 3/t,and the porosity is about 1.1%.Its surface is given priority to with low plains and mountains,and the good preservation condition is a favorable area for Ma'an Tang Formation to explore shale gas.)(3)There are some aspects about Shale formation mechanism of longmen Shan foreland basin,firstly,Carnian Crisis makes the ancient climate from dry to wet and biological mass extinction event happened,raise the productivity of the organic matter.the Carnian Crisis leaded to the input of terrigenous clastic,which caused the productivity of carbonate limited.Because the late detrital material clay and organic matter were brought in,making delta and lacustrine source rock develop,Giant monsoon reached the maximum in the early late Triassic.Giant monsoon drive n currents in the process of the black shale formed in this period provides a strong inc entive for the migration of the organic matter.The structure is relatively stable,continuous subsidence of foreland area is the key of shale gas accumulation in foreland basin,which provides the accommodation spaceas for deposition,buried depth and preservation of shale of rich organic matter.It also provides the thermodynamic conditions for their continuous heating and hydrocarbon generation.The advantageous material foundation of foreland basin and high thermal maturity makes the shale gas of foreland basin basin enrichment.Thirdly,the deflection subsidence caused by the construction load of the orogenic belt causes the foreland basin to have room for space;Fourth,it is located on the edge of the Yangtze continental plate with certain compressive strength,which makes the black shale of the foreland basin to be preserved.