鄂尔多斯盆地中部延长组砂岩中碳酸盐胶结物成因与分布规律研究

Origin and Spatial Distribution of Carbonate Cements in Yanchang Fm.(Triassic) Sandstones within the Lacustrine Center of Ordos Basin,NW China

通过岩石学、矿物学和地球化学等多学科交叉渗透的方法,来阐明不同期次碳酸盐胶结物的成因机理、分布规律和主控因素及其产生的储层效应。研究表明鄂尔多斯盆地中部延长组砂岩中,碳酸盐胶结物可分为早、中、晚3期,以早期和晚期为主。早期碳酸盐胶结物是在常温常压条件下,直接从过饱和的碱性湖水介质中析出的产物。而中晚期碳酸盐胶结物主要为含(亚)铁方解石,多形成于晚成岩阶段A2期,与碱性成岩流体中发生的水—岩相互作用关系密切。早期碳酸盐胶结物中碳主要来源于大气水中溶解的CO2和湖相原生碳酸盐岩的碳,而中晚期碳酸盐胶结物中,部分同位素组成较轻的碳可能受到有机碳影响;有机酸引起的长石类骨架颗粒溶解为中晚期碳酸盐胶结物形成提供了主要的钙源。不同期次碳酸盐胶结物空间分布具有一定规律性,主要表现在半深湖相中碳酸盐的碳、氧同位素组成相对较重,主要为无机碳,多为早期碳酸盐;而砂质碎屑流、三角洲前缘和三角洲平原相的碳酸盐多为中晚期胶结物,为碱性成岩环境的产物。在其形成过程中存在有机碳的混入,并且随着离湖中心距离的增大,有机碳混入的比例也随之增大。

Through the multiple discipline methods of petrology,mineralogy and geochemistry,we try to elucidate the forming mechanism of carbonate cements at different phases,distribution and main controlling factor as well as reservoir effects in the Triassic Yanchang Fm.sandstones,which were deposited in the lacustrine center of Ordos basin,NW China.Three kinds of carbonate cements in these sandstones can be subdivided into early,middle and late phase according to their textural relation and composition.The early and late carbonate cements are dominant in the study area.The former is the product due to precipitation of ultrasaline lake water under normal temperature and pressure.The middle and late cements composed of ferrous calcites are closely related to water-rock interaction in the alkaline diagenetic fluid flow occurred at the A2 period of late diagenetic stage.The carbon element in the early cements are dominantly sourced from the meteoric water or lacustrine carbonate layer,whereas the carbon incorporated into the middle and late cements may be mixed by organic carbon from decarboxylation.The dissolution of feldspathic grains provided the main calcium supply to the precipitation of middle and late cements.The spatial distribution shows that the early cements were dominant in the lacustrine facies mudstone with relatively heavy carbon isotopes,implying their inorganic origin.The middle and late cements with relatively lighter carbon isotopes in turbidite,sandy debris sand and delta frontier as well as delta plain sandstones would be formed within the alkaline diagenetic environment.The tendency of mixing of organic carbon in the carbonate cements at different phases indicates that the quantity of organic carbon would gradually enhance with increasing distance from the ancient lacustrine center.