鄂尔多斯盆地上古生界碎屑岩储层中的鞍形白云石胶结物及其对储层的影响

Saddle dolomite cements in Upper Paleozoic clastic reservoir rocks from Ordos Basin and their effect on reservoir properties

鄂尔多斯盆地上古生界碎屑岩中存在鞍形白云石胶结物,它们具有强烈波状消光等鞍形白云石的典型结构特征,主要分布在石炭系本溪组、二叠系太原组和山西组下部等具有海相色彩的地层中,其矿物学、岩石学和地球化学研究有助于加深人们对鞍形白云石形成机制及其与储层发育关系的研究。从薄片分析,化学分析,碳、氧、锶同位素分析,阴极发光分析和X射线衍射分析结果中获得了鞍形白云石的如下信息:(1)鞍形白云石中FeCO3的摩尔分数变化在13%～23%之间,均属铁白云石。(2)鞍形白云石沉淀开始于碎屑石英次生加大之前,沉淀温度80～120℃。(3)鞍形白云石沉淀流体的δ18O值大致变化在+3‰～-6‰之间,沉淀流体盐度高于二叠纪海水。(4)鞍形白云石的碳同位素组成表明与海水有关的无机碳是其主要碳源。(5)鞍形白云石极其富集的放射性成因锶(87Sr/86Sr变化在0.707116～0.725223之间),这与上古生界鞍形白云石本身产于碎屑岩地层有关。(6)低含量的鞍形白云石胶结物对碎屑岩储层物性的影响是正面的,属于保持性成岩作用;但过度的鞍形白云石胶结作用对储层的影响则是负面的,属于破坏性成岩作用。这两种成岩作用鞍形白云石胶结物质量分数的界线可能在5%左右。

The dolomite cements in the Upper Paleozoic sandstones from Ordos Basin display common saddle features including the wavy extinction. The saddle dolomite cements are abundant in the marine environment of the Carboniferous Benxi Formation, Permian Taiyuan Formation and the lower part of the Permian Shanxi Formation. The studies of the mineralogy, petrology and geochemical characteristics of these dolomites may be helpful to identify the origin of the saddle dolomite, as well as the relationship between saddle dolomite and reservoir quality. The analyses of petrography, chemical composition, stable（O, C） and radiogenic（Sr） isotopes, cathodoluminescence image and XRD data suggest that. 1）The high Fe content （mole fraction of FeCO3 is 13%-23%） suggests that these dolomites can be defined as ankerites. 2） The quartz overgrowths in the sandstones occur after the saddle dolomite precipitation and the formation temperature of the saddle dolomite ranges from 80℃ to 120℃. 3） The δ^8O composition of the precipitating fluid varies about -6%‰＋3‰ （SMOW）, which indicates that the fluids responsible for precipitating of the saddle dolomite are slightly more saline than the value of the Permian seawater. 4） The carbon isotopic analyses of these dolomites reveal that the inorganic carbon in seawater is the dominant source for forming the saddle dolomites. 5） The radiogenic ^87Sr/^86Sr ratio of the saddle dolomite varies between 0. 707116 0.725228, which suggests that the Upper Paleozoic saddle dolomite itself had oceured in the clastic rocks. 6） If the saddle dolomite is not the abundant diagenetic constituent（about 〈5 % bulk）, it may have contributed to preservation of porosity and the formation of the saddle dolomite is a retention process. Otherwise, the process of the saddle dolomite development is a destructive process, thus its effect on reservoir quality may be negative.