用裂变径迹研究地质热历史的方法及其在临清坳陷的应用

THE FISSION TRACK ANALYSIS METHOD FOR EVALUATION OF GEOTHERMAL HISTORIES AND ITS APPLICATION IN THE LINQING SEDIMENTARY BASIN

通过碎屑磷灰石中^(238)U裂变径迹的分析来研究沉积盆地地质热历史的实验方法,是基于裂变径迹在地质时间内受温度作用而发生退火的行为。对于几十百万年的时间,磷灰石中裂变径迹的退火温度约在70～125℃范围,这样的范围符合石油形成所需的古温度环境。分析的样品共20个,取自临清盆地某石油探井不同深度的岩心。用外探测器法测量了各样品的磷灰石裂变径迹表观年龄T,同时也测量了古径迹的水平投影长度分布。径迹的观测是用1500倍光学显微镜进行的。方法的精确度主要取决于古径迹数的多少和热中于通量的误差。测得的表观年龄T和古径迹相对平均投影长度P随井深的变化表明,在2100～4000m,T和P随井深增加而显著减小,这一井段就是磷灰石古径迹的退火带。在1600～2100米井段,T和P保持不变趋势,由此得出该井地层的原始年龄约为85百万年。所有样品的古径迹投影长度分布都呈单峰形状,故此地区没有发生过剧烈的古地温波动变化,地质环境比较稳定。对于多数样品,自发和诱发裂变径迹密度(ρ_8和ρ_i)之间的相关系数rsi大于0.8,这也表明了该井大部井段地层相对稳定。实验结果表明,测量古径迹投影长度是揭示古地温状况的一种有效而简单的方法。

In this paper an experimental technique for investigating geothermal histories of sedimentary basins by analysis of uranium fission tracks in detrital apatites is presented. This method is based on the fact that^(238)U fission tracks in minerals can be annealed by heating over a geological period. In the case of apatite the annealing zone occurs between about 70° and 125℃ for a time span in the order of 10 million years (Myr) which is remarkably conforms to the condition favorable for generation of petroleum. The twenty samples analysed were collected from the cores at various depths of exploratory boreholes in the Linqing sedimentary basin. The apatite fission track age T of these samples have been determined by the external detector method,and the distributions of the projected lengths of fossil tracks in these samples have been measured as well. The fission tracks were observed with an optical microscope at a magnification of 1500X. The precision of this method depends mainly on the counts of the fossil fission tracks and the error of the thermal neutron fluence. The obtained relations between the age T and (mean projected length Pof the fossil track relative to the depth of burial) show that T and P are de-creased with increase of depth of burial in a range of 2100—4000 meter, and thus this zone is the apatite fossil fission track annealing zone. The fact that T and Premain fairly constant in 1600—2100 meter range shows that the primary age of the formations penetrated by the borehole is about 85 Myr. The fact fhat all fossil track projected length distributions have a similar form with a peak shows that no strong palaeotemperature fluctuation occured in the region in which the borehole is located, thus this region was in a fairly stable geological state. For large part of the samples r_(si)>0.8 (r_(si) is a correlation coefficient between spontaneous and induced fission track densities), which also shows that the region was in a stable geological state. The experimental results show that measuring fossil track projected lengths is more powerful to reveal palaeotemperature and is very simple,