南海礼乐盆地礁体水热循环及其对地温场的影响

南海礼乐盆地自渐新世以来持续发育碳酸盐岩礁体,礁体区地温场可能受到礁体与周围低温海水间水热循环的扰动.为了解礁体与周围海水间的热交换过程及其对礁体区地温场的影响,以便从位于礁体区的钻井测温数据中提取深部热信息,本文以过礁体区S-1井的地震剖面为基础,在流热耦合条件下对礁体内的水热活动进行数值模拟.结果表明,礁体与海水间存在水热交换,该水热活动对礁体区温度场有明显的扰动,使得礁体上部温度和地温梯度明显降低,进入礁体下伏地层后,地温梯度逐渐趋于正常;水热循环对礁体区地温场的影响程度与礁体的厚度和渗透率密切相关,礁体厚度越大、渗透率越高,礁体及其下伏地层温度越低;计算剖面中,2100 m厚礁体之下可能存在约400 m厚、渗透率约为3×10^-12 m^2的高渗碎屑岩层,高渗层上覆礁体平均渗透率估计介于1×10^-13~5×10^-12 m^2之间.分析表明,在受水热活动影响的礁体区,礁体下伏地层中的热流可近似代表礁体区的深部热背景,S-1钻井深部热流介于65~75 mW·m^-2之间.

Hydrothermal circulation and its effect on geothermal field in the reef developing area of the Liyue Basin,South China Sea

In the South China Sea,reefs have developed in the Liyue Basin from Oligocene.Geothermal fields in these reefs may have been disturbed by the hydrothermal circulation between reef and surrounding seawater.In order to illuminate the heat exchange process between reef and seawater and its effect on the geothermal field,so as to obtain the deep thermal information of Liyue Basin from observed temperature of drilling,the hydrothermal activity within the reef under the fluid-thermal coupling condition is numerically simulated in a profile of Liyue Bank.The results show that there is a hydrothermal circulation between reef and seawater.The hydrothermal activity significantly disturbs the temperature field within reef,making the temperature and geothermal gradient of the upper part decrease obviously.Geothermal gradient gradually tends to be normal under the reef.Variation of thickness and permeability of the reef have a great influence on the temperature within the reef and the underlain formation.The higher the reef thickness,the higher the permeability,and the lower the temperature.In the profile,there may be a locally hyperpermeable clasolite about 400 m thick underlain the 2100 m thick reef.Permeability of the clasolite is about 3×10^-12 m^2 and the average permeability of the reef is in the range of 1×10^-13~5×10^-12 m^2.Further analysis showed that,in the reef developing area affected by hydrothermal activities,the heat flow under reef could reflect the deep thermal state.Heat flow in the deep part of drilling S-1 is about 65~75 mW·m^-2.