渤海地温场特点的初步研究

渤海位于华北断块区的东部,是华北古生代地台之上发育起来的中、新代断陷盆地,是陆上埕宁隆起和辽河、黄骅及济阳三大拗陷的自然延伸地带。渤海海域的地热特点,一直为人们所关注。在研究渤海的形成和演化时,有人对渤中凹陷的地热状况作了推论;有人根据少量的地热资料认为渤海及其邻近地区是一个高热流异常区。我们根据渤海海域的地温数据,和近年对华北平原地温场的研究结果进行对比,着重对本区地壳浅部的地热状况进行分析和讨论,提出一些初步的看法。

A PRELIMINARY RESEARCH ON THE GEOTHERMAL CHARACTERISTICS IN THE BOHAI SEA

In this paper the temperature distribution pattern in shallow parts of the Earth’ crust for the area studied on the basis of 30 measurements of bottom-hole temperature in prospecting wells, the temperature data of oil-bearing layer in 35 oil producing wells and two reliable heat flow, values are obtained. These results are compared with the distribution characteristics of geothermal pattern and heat flow data in North China Plian and show good agreement between them. The brief conclusions are as follows.1. The geotemperature pattern of Cenozoic sedimentary cover is thoroughly eon-trolled by the structure form of the basement rock in the region studied. The configuration of isotherm and isocline of geothermal gradient reflects the regional tectonic trend(NE, NNE, nearly EW)and structural framework of basement rock. Usually, high temperature appears in the basement uplift, and low temperature-depression. Temperature(T)at 2000m depth is about 83-110℃, and geothermal gradient(G)of Cenozoic sedimentary cover reaches 3.5-5.0℃/100m in uplift zone, In depression area, temperature and gradient and relatively low, T_2000m = 73-83℃, G = 3.0-3.5℃/ 100 m. This phenomenon can be regarded as the result of redistribution of heat flow in the upper-most part of the crust and has been proved by finite element model calculation. 2. Heat flow value of 1.4 HFU at site No. 1 is regarded as upper limit for the deep depression region, whereas heat flow value of 1.6 HFU at site No. 2 is eonsidered to be a reasonable value for the low uplift region. The statistical data indicated, that the conductivity of Cenozoic sedimentary cover in North China Plain is 3.7-4.2 TCU with the average value of 4.0 TCU, and the mean geothermal gradient of Cenozoic sedimentary cover in the Bohai Sea is 3.8℃/100m. Therefore, a average heat flow value of 1.5 HFU for the whole region is obtained.3. The relatively high regional heat flow(1.5 HFU)is closely related to the history of geological evolution in this region. Recent studies suggest that during late Cretaceous and early Tertiary this region had suffered a fifting development stage, resul-tingin extension at surface and high temperature at deep. However, the high temperature state could not continue since the end of early Tertiary owing to the cessation of rifting development. All these led to the appearence of present thermal state in this region, i.e. neither similar to those of Pre-Cambrian old stable Craton nor to the Cenozoic tectonically active unites nor the recent continental rifts.