岩石热导率校正对大地热流计算值的影响——以渤海湾盆地冀中坳陷为例

Influence of rock thermal conductivity correction on the calculated value of terrestrial heat flow———A case study of Jizhong Depression,Bohai Bay Basin

岩石热导率是计算大地热流的基础热物性参数,热导率的选取和校正是正确评估热流的基础。当前在冀中坳陷乃至渤海湾盆地的大地热流计算中,多以受季节性气温变化和地下水活动等因素影响较小的古近系和新近系砂泥质岩层作为大地热流的计算层位,岩石热导率也多参考区域内已有定值,而忽略了热导率对水饱和度、温度和压力的依赖性。笔者等从岩石热导率的温压校正和饱水校正角度出发,计算并比较了不同校正方法对热流值的影响。主要认识如下:(1)岩石热导率作为1个受多因素综合影响的热物性参数,其大小与岩性和矿物组分密切相关,同时通常与孔隙率呈负相关,而与密度呈正相关;(2)热导率高温实验结果显示,冀中坳陷砂岩样品热导率总体上随温度的升高而降低,在30~130℃的温度区间热导率降低了6.8%~11.3%,热导率下降率具有随热导率的降低而减小的趋势;由于砂岩和泥岩具有相对较浅的埋深和较低的原位温压条件,温压对热导率的正负效应可以在一定程度上相互抵消,对于热导率小于2 W/(m·K)的样品,温压对其的影响较小。(3)饱水校正后的冀中坳陷JZ03井和JZ04井古近系—新近系砂泥岩热导率的平均偏差分别高达43.0%、47.5%;(4)JZ03井和JZ04井古近系—新近系岩层热导率在未经任何校正、经含水率校正及经孔隙率校正情况下的大地热流值分别为43.05 mW/m^(2)、45.39 mW/m^(2)、61.64 mW/m^(2)和52.81 mW/m^(2)、59.81 mW/m^(2)、78.14 mW/m^(2);最终选取经几何平均模型饱水校正后的热导率计算得到的61.64 mW/m^(2)和78.14 mW/m^(2)两个热流值分别作为冀中坳陷牛北斜坡和高阳低凸起中部的大地热流值。

Objectives:Rock thermal conductivity is the basic thermophysical parameter for calculating terrestrial heat flow,the selection and calibration of this key parameter is the basis for correct thermal assessment.Currently,in the calculation of terrestrial heat flow in the Jizhong Depression and even in the Bohai Bay Basin,the Paleogene and Neogene sandy—mud strata,which are less affected by seasonal temperature changes and groundwater activities,are mostly used as the terrestrial heat flow computing formation,and the rock thermal conductivity is also mostly referred to the existing fixed values in the region,but ignore the dependence of thermal conductivity on water saturation,temperature and pressure.This paper calculated and compared the effects of different correction methods on heat flow values from the viewpoint of water saturation correction and temperature—pressure correction of rock thermal conductivity.As a thermal physical property parameter affected by multiple factors,rock thermal conductivity is closely related to lithology or mineral composition,and generally negatively correlated with porosity,and positively correlated with density.The thermal conductivity of sandstone samples in Jizhong Depression at high temperature showed that overall the thermal conductivity decreases with increasing temperature,and the thermal conductivity decreased by 6.8%~11.3%in the temperature range of 30~130℃,the thermal conductivity decrease rate decreases with decreasing thermal conductivity;since sandstones and mudstones have relatively shallow burial depths and low temperature and pressure in situ conditions,meanwhile,the positive and negative effects of temperature and pressure on thermal conductivity can be offset each other to some extent,therefore,especially for samples with thermal conductivity less than 2 W/(m·K),the effect of temperature and pressure on the samples can be ignored.The thermal conductivity average deviation of Tertiary sandstone and mudstone in well JZ03 and well JZ04 in Jizhong Depression after water saturation correction is up to 43.0%and 47.5%respectively.The terrestrial heat flow values calculated without any correction,corrected for water content and porosity for the thermal conductivity of the Tertiary stratum in wells JZ03 and JZ04 are 43.05 mW/m^(2),45.39 mW/m^(2),61.64 mW/m^(2) and 52.81 mW/m^(2),59.81 mW/m^(2),78.14 mW/m^(2),respectively;Finally,the values of 61.64 mW/m^(2) and 78.14 mW/m^(2) calculated from the thermal conductivity corrected by the geometric mean model water saturation corrected are selected as the terrestrial heat flow values of Niubei slope and the central part of the Gaoyang low bulge in Jizhong Depression,respectively.