鄂尔多斯盆地三叠系长8储集层致密与成藏耦合关系——来自地球化学和流体包裹体的证据

Compaction and hydrocarbon accumulation of Triassic Yanchang Formation Chang 8 Member,Ordos Basin,NW China: Evidence from geochemistry and fluid inclusions

采用物理压碎、酸液处理与分步抽提相结合的方法,对储集层样品中不同赋存状态的烃类进行分离;再利用气相色谱-质谱联用仪进行组分分析,并结合包裹体均一温度、荧光光谱分析,研究了各类烃之间的关系及充注期次。结果表明,鄂尔多斯盆地三叠系延长组长8含油砂岩中存在4种不同赋存状态的烃类,即游离烃、封闭烃、碳酸盐胶结物包裹体烃以及长石与石英包裹体烃。其中,游离烃的含量在抽提物中占绝对优势,平均为93.4%。包裹体烃荧光光谱λ_(max)、QF_(535)和Q——(650/500)等参数对比分析表明,长石、石英、碳酸盐胶结物包裹体烃成熟度依次增大,其中长石和石英中包裹体烃的荧光光谱各项参数数值相近,而与碳酸盐包裹体烃差异较大。C_(29)ββ/(αα+ββ)与C_(29)20S/(20S+20R)值和甲基菲比值对比分析表明,长石与石英包裹体烃、碳酸盐胶结物包裹体烃、封闭烃、游离烃的热演化程度呈现依次增高的趋势,且游离烃与封闭烃的热演化程度相近。成岩序列、热演化程度分析等综合研究认为长8油藏是一个连续成藏过程,该过程包括3期主要充注,其中晚期充注代表了主成藏期,储集层具有先致密后成藏的特征。

Crushing, acid treatment and step wise separation and oil extraction were employed to obtain the different occurrence state hydrocarbons. All these fractions have been analyzed by Gas Chromatography-Mass Spectrometer （GC-MS）. The fractions relationship and related oil charging process can be mirrored through the analysis of fractions weight content and geochemical characteristics, in combination with the research of inclusions homogenization temperature and fluorescence spectrum parameters. Experimental results reveal that there are four state hydrocarbons, i.e. free hydrocarbon, sealed hydrocarbon, hydrocarbon in carbonate cement, and hydrocarbon within inclusions caught by quartz grains and feldspar grains in the oil-rich sandstones of Chang8 Member, Triassic Yanchang Formation in the Ordos Basin. Among them, the overwhelming fraction is the free hydrocarbon, averaged 93.4%. Fluorescence spectrum parameters of λmax, QF535 and Q650/500 show that the crude oil maturity of the inclusions imprisoned in feldspar, quartz, and carbonate cement increase in turn, and the parameter values of the inclusions in feldspar and quartz are similar and much different from those of carbonate cement. The homogenization temperature values of the three type inclusions bear the similar feature. Through analysis of C29ββ/（αα＋ββ） and C2920S/（20S＋20R）, together with methylphenanthrene ratio, it is revealed that thermal-evolutionary degree of the hydrocarbon within inclusions, hydrocarbon in carbonate cement, sealed hydrocarbon, free hydrocarbon reflects an upward trend, and the data of the last two type are similar. Integrated study of diagenetic sequence and thermal evolutionary degree suggest that the Chang 8 sandstones had been compacted before reservoir formation and the reservoirs have experienced three phase of charge events in which the third one played the most important role for reservoir formation.