万安盆地生物礁及碳酸盐台地的发育演化及控制因素

The Evolution and Main Controlling Factors of Reef and Carbonate Platform in Wan'an Basin

生物礁及碳酸盐台地是南海南部重要的油气储层之一,但目前对万安盆地生物礁的识别及碳酸盐台地沉积相带的划分尚不够深入.基于钻井和地震数据对该区的生物礁及碳酸盐台地进行了精细刻画,万安盆地中生物礁及碳酸盐台地的发育可分为4个演化阶段:早中新世时期,碳酸盐台地初始发育,台地规模小、数量少,零星分布于盆地中部;中中新世台地发育进入繁盛阶段,主要分布于北部隆起、中部隆起及隆起周缘的斜坡之上,横向上呈东、西带状展布,此时期的生物礁以台地边缘礁和块状礁为主;至晚中新世时期,碳酸盐台地开始衰退,而生物礁类型全、数量多,包括台地边缘礁、块状礁、塔礁、点礁等;上新世以来,生物礁及碳酸盐台地全面被淹没,盆地内部不再有生物礁及碳酸盐台地的发育.构造作用和相对海平面变化控制了碳酸盐台地的发育演化过程,古近纪基底断裂产生的地形控制了生物礁及碳酸盐台地初始发育位置及后期发育的空间分布,晚中新世以来的快速沉降和相对海平面变化控制了台地的衰退及淹没过程.

The reef and carbonate platform is one of the most important reservoirs in the south of South China Sea,and the reefs and the sedimentary facies of carbonate platform in the Wan＇an basin should be further studied.The high-resolution seismic profiles associated with well data in this paper facilitate the understanding of the reefs and carbonate platforms in the study area.The evolution of reefs and carbonate platform can be divided into four stages.In Early Miocene,a few isolated carbonate platforms that were small in size firstly initiated in center of this basin.They flourished in Middle Miocene and mainly developed around the Northern Uplift,Central Uplift.Transversely,two bands there,namely western band and eastern band,were the distinct characteristic of its spatial distributions in this time.Platform-edge reef and massive reef were the main style of reef and there were also a few point reefs developing on the edge of carbonate platform.In Late Miocene,the carbonate platforms began to backstep to elevated topographic positions in response to a relative sea level rise.While,the number and style of reef increased in this stage,such as platform-edge reef,massive reef,atoll reef and point reef.Tectonics and eustasy are the two main controlling factors on platform development.Tectonics mainly controlled both the initial topography for reef growth and the distribution of platforms,and eustasy controlled carbonate sequence development.Since the Late Miocene,the rapid tectonic subsidence and the rise of relative sea-level should have led to the drowning of carbonate platform.