特殊碳同位素组成白云岩的发现及其意义

据现有文献报导,除前寒武系和第三系外,地史时期中白云岩δ13C值主要分布在-5‰～+5‰的范围。但是,作者在研究准噶尔盆地上二叠统芦草沟组白云岩碳同位素时获得了一批高度富集13C的资料,其中白云岩δ13C分布于7.3‰～ 17.9‰,平均11.7‰(7);白云石δ13C介于7.9‰～16，平均11.6‰(5)。根据区域地质、岩石学和矿物学资料，结合现代沉积剖面、细菌作用模拟实验和古代沉积剖面碳酸盐矿物碳同位素特征，分析了白云岩分布特征。作者认为白云岩中如此高的富集除了全球二叠纪生物埋藏速率较快和研究区古盐度较高外，更主要的因素应与厌氧条件下发酵带内细菌作用引起有机物质碳同位素分馏，形成贫的和富的有关。这一过程进行得愈充分，沉积介质中富集的的浓度越高，形成的白云岩碳同位素组成越重。同时，只有这一作用持续存在才能形成大量富的与黑色页岩互层的白云岩。这一现象从一个侧面说明在芦草沟组沉积时有一个持续时间较长的细菌活动时期，从而，生成了数量可观的以甲烷为主的生物气。由于生物气形成时代较老，易于扩散逸失，它们难于单独成藏保存至今，但作为天然气复合成藏来源之一，则应予关注。

Finding of the Dolostones with Special Carbon Isotopic Composition and Its Significance

According to publications, thevalue of the dolostones mainly ranges from - 5‰ to + 5‰ in a geologicalhistory except the Precambrian and Tertiary. However, we obtained some data of the dolostones which are moreenriched inthan + 5‰ when we researched the dolostones in the Upper Permian Lucaogou Formation. Thevalue of the dolostones varies in the range from + 7.3‰ to + 17.9‰ with an average of + 11.7‰(7) andthat of the dolomites from + 7.9‰ to + 16.1‰, averag + 11.6‰(5). Based on the data of regional geology,petrology, mineralogy of the Upper Lucaogou Formation, the carbon isotopic characteristics offrom modernsedimentary sections in oceans and lakes and the simulation of bacterial process and carbonate minerals in ancientsedimentary sections we analyzed the distribution characteristics of the dolostones and considered that except higherburied rate of the global Permian living things and higher salinity of the researched area the anomally-en-riched dolosdones are mainly due to the formation of- depletedandenrichedcaused by thecarbon isotopic fractionation of organic matter during a bacterial process in a fermentation zone under an anaerobiccondition. The more fully the process carried out, the more enriched inthein the sedimentary mediumwas and the heavier the carbon isotope of the dolostones. Simutaneously only when this process continued could the-enriched dolostones interbeded with black shales be formed. This phenomenon indirectly indicates thatthere was a long continuous period of the bacterial process when the Upper Permian Lucaogou Formation sediment-ed so that considerable amounts of biogenic gas dorminated by mechane could be generated. Because the gas wasformed earlier and was easy to diffuse, it was difficult for the gas to form gas pools and to be preserved till now,but as one of possible sources of natural gas attention should be paid to it.