南海北康盆地热流分布特征及其构造控制因素探讨

Heat Flow Characteristics and Controlling Factors of the Baikang Basin in South China Sea

热流是天然气水合物储量估算和油气资源评价的基础数据。南海实测热流数据虽然多,但分布不均。为深入认识北康盆地的热流特征,本文首先利用北康盆地获得的实测热流数据和前人研究成果,绘制了北康盆地热流分布图;根据热流和地温梯度变化特征,重点讨论了北康盆地热流异常区的成因机制,并根据区域地质构造背景,对北康盆地热流分布主要的控制因素进行了简要分析。数据统计结果表明:北康盆地热流变化范围为43.0~115.0mW/m^2,平均为76.8±21.7 mW/m^2;地温梯度变化范围为49.0~133.1℃/km,平均地温梯度为82.2±22.4℃/km。热流整体偏高,具有西高东低,南部高、东南部热流最低的热流特征,且在盆地西北部具有明显的热流异常。分析认为:(1)热流异常区中B区是由于海底高导层埋深影响,A区和C区是由海底流体渗漏区的地下水循环导致热流异常,推测A区为补给区,C区为排泄区;(2)北康盆地热流分布明显受构造作用的控制。西北部及南部受廷贾断裂控制,而东南部为古俯冲带及古洋陆过渡带,西高东低的热流分布特征还与盆地经历的3次快速沉降作用有关,但北康盆地拉张的构造背景引起整体热流值偏高。这对北康盆地油气和天然气水合物的研究均有重要意义。

Heat flow is the basis data for gas hydrate reserves estimation and the oil and gas resource evaluation. The existing heat flow data measured in the South China Sea are huge but the data distribution is uneven. In order to understand the geothermal feature of the Beikang basin, this study mapped the distribution of hot flow in the Beikang basin using the measured heat flow data and previous research results. According to the characteristics of heat flow and geothermal gradient,the formation mechanism of heat flow anomaly zone in the northwest of the basin was discussed. The regional geology structure was used to analyze the characteristics of heat flow and geothermal gradient of Beikang basin and their main influence factors. The main controlling factors of heat flow distribution were briefly analyzed with the regional geological tectonic background. Statistics results show that： Heat flow varies from 430 to 1150 mW/m2 with an average value of 768±217 mW/m2, meanwhile the geothermal gradient ranges from 490 to 1331 ℃/km with the mean of 822±224 ℃/km. Heat flow is generally high, characterized by higher in the south and west than in the east, with the lowest in the southeast. And there is distinct heat flow anomalous zone in the northwestern basin. Further analysis shows： （1） Heat flow anomaly in district B is mainly influenced by the burial depth and/or thickness of high conductive layers in the crust, and districts A and C are caused by the hydrothermal circulation of seafloor fluid seep system. It can be inferred that district A should be recharge area and district C be the discharge area; （2） Heat flow distribution of the basin is obviously controlled by tectonic, with northwest and south controlled by Tinjar fault and southeast by continent ocean boundary and ancient subduction zone. The distribution feature high in west and low in east was likely related to three rapid sedimentation events,but the regional high heat flow is due to the stretching of the basin. It is of great significance to the research of hydrocarbon and gas hydrate.