库车前陆盆地流体化学、成因与流动

FLUID CHEMISTRY, ORIGIN AND FLOW IN KUQA FORELAND BASIN

库车前陆盆地寒武系 -第三系流体分析显示 :寒武系 -奥陶系和侏罗系油田水与白垩系 -第三系有所差异 ,前者总矿化度相对较低 ,为 90g/L～ 1 1 0g/L ;相对富Ca2 + 和HCO-3 ;后者总矿化度介于 73g/L～ 3 1 0g/L,主要为 1 3 0g/L～ 2 90g/L,相对富K+ +Na+ 和Cl-;而大宛齐第三系油田水则相对富Ca。影响水化学演化的因素包括 :膏盐类、沸石类矿物溶解以及白云化作用和富镁绿泥石形成等。白垩系油田水δ18O、δD关系显示为淡水蒸发成因 ,且在轮台断隆带油田水87Sr/86Sr向西降低 ,指示了富87Sr的流体来自东部的碎屑泥岩 ,这与本区煤成油运移方向是一致的。而 2个寒武系 -奥陶系油田水87Sr/86Sr高达0 .71 71 6,并富Li、B、Sr元素和δ18O ,被认为来自结晶基岩 ,与氮气、氦气及部分甲烷的来源一致。

Oilfield water and natural gas chemistry and isotopic composition are systematically measured in Cambrian to Tertiary reservoir rocks from the Kuqa foreland basin, Northwest China. It can be found that there exists some difference in water chemistry between the Cambrian\|Ordovician reservoir rocks and the Cretaceous\|Tertiary ones. Oilfield waters from the Cretaceous were derived from freshwater evaporation as evidenced by δ\{\}\+\{18\}O\|δD. Relative enrichment of Ca\+\{2+\}, Sr\+\{2+\}, Na\++ and Cl\+- may be considered to be the result of dissolution of salt, anhydrite, laumontite; and of depletion in Mg due to dolomitization of calcite and Mg\|chlorite formation. \{\{\}\+\{87\}Sr\}/\+\{86\}Sr ratios in 10 oilfield water samples range from \{0.71034\} to \{0.71716\}, which are more radioactive than seawater. It is explained as the result of extensive water\|rock interaction and brine migration. High \{\{\}\+\{87\}Sr\}/\+\{86\}Sr ratios in the oilfield waters from Tertiary and Cretaceous decrease westward, suggesting the oilfield waters were derived from mudstones in the east, which is consistent with oil migration direction. And the waters from Cambrian\|Ordovician carbonate rocks with \{\{\}\+\{87\}Sr\}/\+\{86\}Sr up to \{0.71716\} and high Li, B, Sr concentrations and relatively heavy δ\{\}\+\{18\}O are considered to be from the crystalline basement. Gas isotope geochemistry shows that the δ\{\}\+\{13\}C of methane in some samples is high, ranging from -17.3‰ to -17.9‰ PDB with δ\{\}\+\{13\}C\-\{CH\-4\}>δ\{\}\+\{13\}C\-\{C\-2H\-6\}>δ\{\}\+\{13\}C\-\{C\-3H\-8\}>δ\{\}\+\{13\}C\-\{C\-4H\-\{10\}\}, indicative of inorganic origin of methane. δ\{\}\+\{15\}N of N\-2 range mainly from 3.35‰ to 16‰ relative to air, and \{\}\+3He/\+4He from \{3.39\}×10\+\{-8\} to 6.12×10\+\{-7\}. It can be inferred that radioactive decay and N\-2 release from NH\++\-4 of the crystalline basement are most likely to be the source of N\-2 and He in natural gases.\;