张裂型盆地地热参数的垂向变化与琼东南盆地热流分布特征

海洋热流数据是开展海洋地球动力学研究和油气资源评价的基础数据.为深入认识琼东南盆地的地热特征,本文首先利用耦合沉积作用与岩石圈张裂过程的数值模型分析了张裂型盆地主要地热参数的垂向变化特征;并通过钻孔资料的详细分析,获得了琼东南盆地44口钻孔的热流数据;结合海底地热探针获取的热流数据,对琼东南盆地地热特征及其主要影响因素进行了简要分析.结果表明:沉积作用的热披覆效应对表层热流有较明显的抑制作用,由于沉积物生热效应与披覆效应的共同作用,同一钻孔处海底表层热流与钻孔深度3000~4000m处热流或与海底间的平均热流差异很小,可以一起用于分析琼东南盆地的热流分布特征;莺歌海组、乐东组热导率随深度变化小于黄流组及其下地层热导率的变化,钻孔沉积层平均热导率约为1.7 W·(m·K)-1,钻孔地层生热率一般低于2.5μW·m-3,平均生热率为1.34μW·m-3,平均地温梯度主要介于30~45℃/km,热流介于50~99mW·m-2,陆架区热流主要集中于60~70mW·m-2,深水区钻孔具有较高的地温梯度和热流值;从北部陆架与上陆坡区往中央坳陷带,热流值从50~70mW·m-2,增高为65~85mW·m-2,并且往东有升高趋势,在盆地东部宝岛凹陷、长昌凹陷与西沙海槽北部斜坡带构成一条热流值高于85mW·m-2的高热流带.进一步分析认为,琼东南盆地现今热流分布特征是深部热异常、强烈减薄岩石圈的裂后冷却作用、晚期岩浆热事件、地壳与沉积层的生热贡献以及沉积作用的热披覆效应等多种主要因素综合作用的结果.

Vertical variations of geothermal parameters in rifted basins and heat flow distribution features of the Qiongdongnan Basin

Marine heat flow data are essential for study of marine geodynamics and reliable evaluation of hydrocarbon resources. Detailed temperature and well log data of commercial drills are invaluable for acquisition of surface heat flow data, thus to understanding the geothermal features of the Qiongdongnan Basin. Since heat flow values from various methods of calculating heat flow might be different, the present vertical variations of some main geothermal parameters were calculated to facilitate explanation of such differences.#br#A numerical model coupling the heat transfer, sedimentation and lithospheric stretching was employed to predict the present vertical variations of some main geothermal parameters in a rifted basin, then well log data of sonic, density, neutron porosity, resistivity, and gamma ray were used to calculated average thermal conductivity following the method of Goutorbe et al.(2008), and GR (total gamma-ray logs) was used to calculate average heat generation rates with the relationship of Bucker & Rybach (1996). A total of 44 heat flow data were determined with the calculated thermal conductivity and average geothermal gradient.#br#The results show that the thermal blanketing of the sedimentation could greatly reduce the heat flow value in the strata, and due to the opposite thermal contributions of sedimentary heat generation and thermal blanketing, the difference in the average heat flow between the seafloor and the deep drill hole temperature measured in a range of 3000~4000 m, and the seafloor heat flow is very small. Thus both of the heat flows could be put together for analyzing the heat flow distribution of the Qiongdongnan Basin. The thermal conductivity variation with depth of the Yinggehai and Ledong groups is much smaller than that of those older groups, with an average of about 1.7 W·(m·K)^-1. The average Cenozoic heat generation is generally lower than 2.5 μW·m^-3, with an average of 1.34 μW·m^-3. The average geothermal gradient ranges from 30 to 45 ℃/km, and the heat flow ranges from 50 to 99 mW·m^-2. The sedimentary contribution to surface heat flow is larger than 20 mW·m^-2 in the western basin and about 10 mW·m^-2 in the eastern basin, respectively. The heat flow in the shelf is mostly in the range of 60~70 mW·m^-2, and the geothermal gradient and heat flow are generally much higher in the deep-water area. In conjunction with the seafloor heat flow data, heat flow in the shelf and upper slope area are generally lower than 70 mW·m^-2, while in the deep-water area, heat flow values are generally in the range of 70~85 mW·m^-2. There is a high heat flow zone with values higher than 85 mW·m^-2 trending in northeast in the eastern part of the study area.#br#In the study area, the difference between the average drill heat flow and the seafloor heat flow is very small, thus both can be adopted for analyzing the heat flow distribution of the Qiongdongnan Basin. The geothermal gradient and heat flow are generally much higher in the deep-water area than the northern shelf, and there is a high heat flow zone trending in northeast in the eastern part of the study area. Further analysis suggests that the present heat flow distribution features resulted from multiple factors such as thermal anomalies in the upper mantle, the cooling of the highly thinned lithosphere, recent extensive magmatism, the crustal and sedimentary heat generation, and the sedimentary thermal blanketing.