异常高压驱动石油运移成藏——鄂尔多斯盆地延长组多相态烃类沸腾包裹体证据

鄂尔多斯盆地上三叠统延长组裂隙充填的方解石脉中含有大量多相态烃类包裹体,包括固体沥青+液态油+气态烃3相包裹体、气/液比差别很大(最小仅为5%,最大超过70%)的液态油+气态烃二相包裹体以及纯液态油包裹体、盐水+液态油+气态烃3相包裹体等。岩相学观察表明,这些不同相态烃类包裹体是被同时捕获形成的共生不混溶包裹体,气/液比不同的烃类包裹体具有相近的均一温度,分布范围较小(74 ℃～117 ℃),综合分析认为它们是在沸腾状态下被同时捕获的。不同相态烃类包裹体显微傅里叶红外光谱和荧光光谱不同,表明其烃类成分的差异。油/源对比研究表明烃类包裹体与长8油层的原油均来源于长7烃源岩。结合盆地区域构造演化分析认为,延长组长7油层组厚层烃源岩层在埋藏热演化和油气形成过程中出现的过剩压力,在早白垩世油气形成高峰阶段达到顶峰,鄂尔多斯盆地在晚白垩世抬升和剥蚀产生的卸载作用导致延长组长7烃源岩层在超高压下破裂,其中积聚的石油因压力突降而沸腾并沿裂隙排出并运移进入特低渗致密砂岩储层成藏。

Anomalous pressure driving oil migration:Evidence from multiphase boiling hydrocarbon fluid inclusions in Yanchang Reservoir, Ordos Basin

Abundant primary boiling oil inclusions were found contained in calcite veins filled in fractures cutting through compacted fine grain sandstone of Chang 7 and Chang 8 section of Upper Triassic Yangchang Formation in Ordos Basin in centre of north China.Petrography,microthermometry and organic geochemistry of oil inclusions were studied and were correlated with crude oil from Chang 8 and source rocks of Chang 7.Two-phase and three-phase oil inclusions with extremely variable gas/liquid ratios were coeval with one-phase pure oil inclusions.Characteristics of oil inclusions indicate that they have been trapped simultaneously in a case of heterogeneous of liquid oil and vapor at boiling state during the period of calcite filling fractures.Measured T_h of oil inclusions ranged from 74 ℃ to 117 ℃ which are correlated with thermal maturated degree indicated by fluoresce of bright yellow color and fluorescent spectra indexes of oil inclusions themselves and that of source rocks of Chang 7.Results of organic geochemistry studies suggest that oil inclusion and crude oil of Chang 8 derived from the same source rocks of Chang 7.It indicates that oil expelled from source rocks of Chang 7 and migrated into compacted sandstone reservoir with very low permeability at boiling state caused by fracturing under over-pressure due to expanding of accumulated oil generated from thick mudstone and oil shale of source rocks of Chang 7,or because of release of loadage of Upper Cretaceous-Paleocene sediments due to intense uplifting of Ordos Basin after Later Cretaceous when large scale oil generation had taken place.