滇东—黔西中二叠统白云岩特征、成因及其主控因素

Characteristics,Origin and Main Controlling Factors of Mid-Permian Dolostones in Eastern Yunnan and Western Guizhou,China

【目的】中国滇东—黔西地区中二叠统广泛发育白云岩。针对这些白云岩的成因,前人开展了一系列研究,并提出了多种白云岩形成机理。然而,关于镁离子的来源、白云化的主控因素等关键问题,至今仍无定论。这些问题严重制约了滇东—黔西地区的油气扩展勘探。【方法】综合利用露头、岩心和钻井等资料,对滇东—黔西地区中二叠统白云岩的分布规律、岩相学特征和地球化学特征进行了详细分析。【结果与结论】(1)研究区白云岩的形成机理为开放热对流白云化模式。白云化流体的镁离子主要来自海水,少部分来自深部热液流体;(2)白云化流体主要通过亮晶生屑石灰岩中的孔隙,以及断层和裂缝系统运移。在温度空间差异的驱动下,白云化流体沿循环流动路径运移,导致中二叠统石灰岩迅速且广泛的白云化;(3)白云岩发育的主控因素为滩相沉积和峨眉地幔柱(活动范围)。

[Objective]Dolostones are widely developed in the mid-Permian rocks in eastern Yunnan and western Guizhou(China).A series of studies have previously been conducted on their genesis,and various formation mecha-nisms have been proposed.However,the source of Mg ions and the main controlling influences of the dolomitization process remain controversial.These problems severely hinder further exploration for oil and gas in the region.[Methods]This study analyzed in detail the regularity of the distribution of dolostone and its petrographic and geochemical characteristics,using outcrop and core observation as well as drilling data.[Results and Conclusions](1)The formation mechanism of dolostone in the study area is the open thermal convection dolomitization.Mg2+in dolomitizing fluids originates mostly from seawater,with a minor component coming from deep hydrothermal fluids.(2)The dolomitizing fluid mainly migrates through pores in bioclastic grainstone and faults and fracture systems.Driven by spatial variations in temperature,the fluid migrates along the circulation flow pathways,resulting in fast and pervasive dolomitization of Middle Permian limestone.(3)The main controlling factors of dolostone development are shoal facies and Emei mantle plume(active area).