内蒙古东北部林西组碳酸盐岩结核的成因及油气地质意义

Origin and petroleum geological significance of carbonate rock concretes in Linxi Formation,northeast of Inner Mongolia,China

内蒙古西乌珠穆沁旗哈日根台苏木地区上二叠统林西组深水外陆棚灰黑色泥页岩沉积中发育大量的碳酸盐岩结核。这些结核呈透镜状、结核状及丘状,在产状上与现代海底发育的及古代的冷泉碳酸盐岩极为相似。通过对碳酸盐岩结核的微观组构、物质成分和碳同位素特征的研究,发现碳酸盐岩结核中发育较多的气孔构造、生物化石及草莓状黄铁矿;碳、氧同位素结果显示碳酸盐岩结核的碳同位素存在明显的负偏,表明其中有有机来源的碳。这些特征与现代海底由于天然气渗漏形成的冷泉碳酸盐岩特征一致,因而推测研究区发育的碳酸盐岩结核为古代海底天然气渗漏形成的冷泉碳酸盐岩。这表明在林西组沉积时期,海底存在天然气水合物,可能与海底早期沉积的灰黑色泥页岩中甲烷的释放有关。烃源岩潜力分析也表明,研究区大量发育的灰黑色泥页岩具有一定的生烃潜力,林西组具有较好的油气勘探前景;但是由于地层时代较老,成岩作用及构造运动的影响,现今的勘探难度较大。

There develop a number of carbonate concretes in the black mud shale sediments on the deep outer continental shelf of Upper Permian Linxi Formation in the sum area of Harigentai village,Xi Ujimqin Qi,Inner Mongolia.These carbonate concretes show lentoid,concretionary and mound structures in the field,similar to the cold spring carbonate rocks formed on the ancient and modern ocean floor.The micro fabric,matter composition and carbon isotopic characteristics are researched in detail.Microscopic observations show that there are many vesicular structures,organisms and pyrite to aggregate in the carbonate concretes.The stable carbon-oxygen isotopic characteristics show that in the carbon isotope exist negative values,indicating the origin of the organic carbon.These features are similar to that of the cold spring carbonate rocks formed on the modern ocean floor because of natural gas seepage.Therefore,the paper infers that the carbonate concretes in study area are the cold spring carbonate rocks formed on the ancient ocean floor because of natural gas seepage.This suggests that during the deposition of Linxi Formation,there were gas hydrates on the ocean floor,which might be relative to the methane release from the deep grey-black mud shale deposited earlier.A potential analysis about hydrocarbon source rocks also show that the deep grey-black mud shale wide developed in study area have a certain hydrocarbon-generating potential,and the Linxi Formation has a relatively good potential for petroleum exploration.But because of the relatively old strata,and the influences of diagenesis and tectogenesis,the petroleum exploration is rather difficult in present.