南海礼乐盆地新生代构造热演化特征及其影响因素

Cenozoic tectono-thermal evolution features and influence factors of the Liyue basin,South China Sea

为深入认识新生代礼乐盆地的热体制特征,利用耦合岩石圈变形、热演化和沉积过程的热力学数值模型,重建了8条骨干剖面的构造热演化史,并对主要构造单元的热体制进行了分析.结果表明:张裂阶段,热流总体上随时间增加,张裂结束时,海底热流一般介于70~80mW·m^-2,基底浅埋区热流高于邻近凹陷区内热流;裂后阶段,非礁体发育区热流逐渐降低,现今海底热流一般介于65~70mW·m^-2,局部区域热流因岩体侵位而有所增高,礁体发育区受到礁体与周围海水热交换的影响,海底热流降低或为负值,而基底热流可以达到70mW·m^-2左右.进一步分析表明,礼乐盆地新生代热体制主要是在古近纪岩石圈强烈减薄基础上,叠加了晚期岩浆侵位、基底起伏、沉积过程以及海底地形等局部因素影响的结果,礁体发育区热体制还受到礁体与周围海水热交换的影响;盆地凹陷中心区生油门限深度一般介于2000~2500mbsf,门限温度介于90~110℃;礁体发育区生油门限深度明显大于邻近的北1凹陷沉积中心区.

To further understand the Cenozoic thermal regime of the Liyue Basin,based on crustal stretching factor analyses,we used a numerical model of coupled geothermal processes,lithosphere thinning and depositional processes to reconstruct the tectono-thermal evolutional histories of eight backbone profiles in the Liyue basin,and analyzed the thermal evolution features in the main structural units and their influence factors.The results show that,Liyue basin was greatly extended and thinned during the synrift phase,and the crustal stretching factors are generally larger than 1.8 in most regions of its sags;During the synrift phase,heat flow increased generally with time,and at the end of the synrift phase,seafloor heat flow were as high as about 70~80 mW·m^-2,and heat flow in basement high regions was generally higher than heat flow in sag regions.During the postrift phase,due to deposition and thermal relaxation,heat flow decreased gradually to 65~70 mW·m^-2 at present in the regions where reef bodies were not developed continuously,while in the SW and NW regions of the basin,heat flow becomes higher due to recent magmatic intrusion.Seafloor heat flow and thermal gradient in reef body developing areas are quite low,and even negative due to heat exchange between the upper part of reef bodies and the surrounding low temperature seawater.Further analyses suggest that,the thermal regime of the Liyue basin results mainly from intense lithospheric thinning,and other local influence factors such as recent magmatic intrusion,basement topography,sedimentation and seafloor topography.In addition,the thermal regime in the reef body developing area has been influenced greatly by the heat exchange between the upper part of reef bodies and the surrounding low temperature seawater.The oil and gas threshold depth and temperature are 2000~2500 mbsf and 90~110℃in the central regions of the sags,respectively,which are slightly larger than the threshold depth and temperature in the surrounding basement rise area.In the reef body developing area,due to the heat exchange between overlying reef bodies and surrounding seawater,the thermal maturity of the underlying source rocks is much lower,and the oil and gas threshold depth are obviously larger than the threshold depth in the central regions of the sags.