关于CCSD主孔4820～4930m井段气体异常的初步解释

Gas abnormalities in the 4820-4930 m interval of the CCSD main Hole and their preliminary interpretation

CCSD主孔取心钻进的第153至166回次(对应井深为4820～4930m)施工中,流体地球化学现场实验室监测到非常显著的气体异常。按气体浓度的变化,又可分为两个不同的异常区段,分别位于4820～4860m和4869～4930m井段。两个区段均表现为显著的CO2和H2异常,第一区段的最大值分别为3.6%和12.1%,第二区段的最大值分别为24.8%和18.7%。第二区段除了CO2和H2异常值更高以外,在其后期(井深4906m)还出现了一处比较短暂的He气异常,最高值达到17×10-6,比基础值高出200%。通过对钻井过程的分析及泥浆的离线实验研究认为:微生物对泥浆的分解作用是本次异常的重要原因之一,堵漏剂助长了气体异常的升高和延续;由泥浆分解产生的CO2和H2与来自地下的流体并存是本次异常的显著特点。

In operations of Nos. 153 to 166 runs of core drilling in the 4820 to 4930 m interval of the CCSD main hole, very obvious gas abnormities were monitored from drilling mud by the fluid geochemistry laboratory. According to the concentrations of gas components, the abnormities can be divided into two corresponding intervals: 4820 to 4860 m and 4869 to 4930 m. Both intervals showed strong CO2 and H2, with maximum concentrations of 3.6% for CO2 and 12.1% for H2 in the first interval and 24.8% and 18.7% in the second interval respectively. At 4906 m depth, in addition to abnormally high CO2 and H2 concentrations, a short, obvious He increase was also observed with a maximum concentration of 17×10-6, which is about 3 times the normal value. Analysis of the drilling process and off-line experiments on drill mud and additives showed that the decomposition of drill mud additives by microbes could be one of the major causes of the abnormities and that the loss of circulation additives promoted the increase and continuation of CO2 and H2 abnormalities. The existence of both subsurface gases and gases resulting from mud decomposition by microbes contributes to the gas abnormities in this study.