Geochemical Composition of Acidolysised Light Hydrocarbons from Palaeozoic Sedimentary Rock,Tarim Basin

Key words: 
• Tarim basin /
• Palaeozic /
• light hydrocarbons from rock

Abstract: Tarim Basin, with an area of 560,000 square kilometers and a sedimentary sequence of 14,000 meters in thickness,although nearly more than ten medim-sized oil&gas fields discovered but without a giant one in ten years exploration,is still a perspective area for potential petroleum exploration due to its attractive petroleum geological settings. Because of its abundance in gas-condensate,a number of studies focused on the light hydrocarbons in oil&gas have been presented.For better understanding the genetic relationship of light hydrocarbons among rock-oil-gas, this paper deals with the composition characteristics of the light hydrocarbons acidolysised from the Palaeozoic sedimentary rock and their comparison with those of oil and gas. The experimented core samples,with their depth ranged from 4 000m-5 877m and geological time dated to Cambrian,Ordovician and Carboniferous repectively,are located on Lunnan,Tazhong etc.six oil-gas fields or structures. The core samples firstly were smashed under the lower temperature and of which in 60-100 mesh sized were collected to be used in extracting light hydrocarbons.After the acidolysis device,along with 40g sample and 50ml distilled water in it, was evacuated to vacuum,it was heated to 70-80℃ by water bathing,and then dripping phosphoric acid(85%) to react with carbonate minerals to release the light hydrocarbons reserved and CO2 produced,the later was neutralized using KOH(saturated) finally in gas-washing flask and the accumulated light hydrocarbons was analyzed by HP5880 GC equipped with HP-PONA column and data processing system. The data calculated from the acidolysised light hydrocarbons(Table 1)display a complete different distribution compared with the marine oil-gas nearby.(1) Heptane values ranging from 12.0%-24.6% and the average is 19.0%.Isoheptane values ranging from 1.7 to 2.91 and the average is 2.15.Both are much lower than those of the marine oil-gas nearby,for example,both averages of Lunnan structure are 35.8% and 2.9 respectively.(2)Lower content of light aromatic componds,with the average of 0.95% in C_6-7calculating unite,compared with that of 4.96% in oil and gas-condensate.(3)Different distribution pattern in alkane and cycloalkane,namly ∑nC_6-7＜∑iC_6-7and ∑CPs＞∑CHs,contrary to the distribution patterns in oil and gas-condensate.(4)Lighterδ¹³C₁‰ values,averaged -54.8‰,as well as lower (C₁+ C₂)% values( ＜50%),compared with natural gas.Meanwhile one similarity on Mongo K Index is recognized,namly in Tazhong and Yaha oil - gas and rock all possess higher Mongo K values compared with other oil&gas bearing structuresBased on the distributive characteristics of acidolysised light HCs,conclusions obtained as follows:(1)The lower maturity acidolysised light HCs obtained in the experiment is the earlier HCs which was sourced mainly from marine bacteria and algae and preserved in the crystal lattice of carbonate mineral.the reservior rematuration is not effective even under the depth of 4 500m-5 900 during such a long geological period mainly because of the increasingly higher pressure and the lower earth thermal gradient as well as the lower geothermal flux in recent and late geological in Tarim Basin.(2) The similar higher Mongo K values coexisted in rock-oil-gas indicat that there is some relationship between the Ordovician rock and the oil-gas in Tazhong and Yaha oil field.(3)Because of the maturity deviation reflected by light HCs between the rock and oil&gas deposites nearby,it demonstrats that the higher matured marine oil&gas is mainly sourced from the deeper Plaeozoic sources perhaps in the sedimentary center of the Basin by undergoing a long distance of migration,to which the Palaeozoic rock near the reservior contribute little at least in low molecule HCs and also no exchang of light HCs between them perhaps due to the relative closing property of the crystal lattice.