为了分析鄂尔多斯盆地西北部奥陶系已发现气藏的气源，探讨该区奥陶系天然气藏的分布规律，综合利用地球化学实验分析与石油地质综合研究，对研究区天然气组分进行了分析。研究表明，研究区奥陶系天然气干燥系数在0．958～0．986。气源分析认为，奥陶系天然气主要为来自石炭系－二叠系煤系烃源岩生成的煤成气，但不排除有奥陶系海相气源的少量混入。奥陶系烃源岩分析结果亦支持这一结论。研究区奥陶系泥岩和碳酸盐岩的有机质丰度普遍不高。其中，泥岩有机碳含量以乌拉力克组为最高，平均为0．54％；其次是克里摩里组泥岩，平均0．52％；拉什仲组泥岩最低，平均为0．43％；灰岩有机碳含量3个组的平均值分别为0．35％，0．31％和0．25％。综合分析表明，鄂尔多斯盆地西北部奥陶系天然气藏的形成和分布主要受构造、储层、盖层、优质烃源岩及天然气运移通道5大因素控制，天然气主要富集在以下5要素的耦合处：斜坡高部位、溶蚀缝洞储层发育带、乌拉力克组－拉什仲组盖层分布区、优质烃源岩有利区及上古生界天然气“倒灌”运移通道发育区。%In order to determine the source of natural gas and to reveal the factors controlling the accumulation and dis-tribution of natural gas in the Ordovician reservoirs of the study area, we studied the composition of the natural gas through geochemical analysis and comprehensive geologic studies.The Ordovician gases are revealed to be entirely dry gas and their dryness indices range from 0.958 to 0.986.Geochemical analysis indicates that the Ordovician gases are mostly derived from the Carboniferous-Permian coal measure source rocks, whereas the marine Ordovician source is assumed to be a minor contribution.This is also supported by the Ordovician source rock analysis,which has revealed that the organic matter content of the mudstones and the carbonates is commonly not high.The average TOC of the mud-stones in the Wulalike,Kelimoli,Lashizhong formations are only 0.54%,0.52%and 0.43%,respectively,and that of the carbonates in the above three formations are 0.35%,0.31% and 0.25%,respectively.Comprehensive studies suggest that the gas accumulation and distribution is principally controlled by structure, reservoirs, cap rocks, quality source rocks and migration pathways.Gases mainly enrich in places where the following 5 factors couple,namely struc-tural highs of the slope,highly developed reservoirs with dissolution fractures and vugs,cap rocks of the Wulalike and Lashizhong formations,quality source rocks,and migration pathways for downward charging of gas from the Upper Pale-ozoic source rocks.
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