鄂尔多斯盆地奥陶系风化壳古油藏的确定及其意义

岩心观察与岩石薄片分析表明,鄂尔多斯盆地中央隆起带奥陶系风化壳储层沥青含量丰富,其光性特征显示为石油裂解的残余物,是古油藏存在的直接证据,沥青热演化程度高,如旬探1井的沥青反射率为2.35%～3.15%,换算成镜质华反射率为1.85%～2.35%.根据沥青充填特征、面积系数,并结合电测资料确定的古油层的厚度一般为10～22 m.TOC、有机地球化学和岩相分析表明,西缘坳陷平凉组自东而西逐渐增厚的泥页岩含有机质丰富,以咸水低等生物的有机质为特征,是中央隆起带古油藏的可靠烃源岩.成藏史分析表明,J₂-K₁期的埋深加大和古地温升高,导致古油藏石油裂解转化为古气藏;K₂-E时期,燕山运动致使鄂尔多斯盆地构造反转,陕北斜坡形成,古油藏被破坏,一部分油气向浅部运移散失,一部分向陕北斜坡高部位运移,可能是盆地中部大气田的主要气源之一.

Determination and its significance of ancient oil pools in Ordovician weathering crust, Ordos basin

Core observation and thin-section analysis reveal that the Ordovician weathering crust reservoir within the central uplift belt of Ordos basin is abundant in bitumen,which shows optical features of oil pyrolysis remnant——the direct evidence of the existence of ancient oil pools.The thermal maturity of bitumen is high,resulting in high bitumen reflectivities.For example,the reflectivity of bitumen from Xuntan-1 well ranges from 2.35% to 3.15%(1.85%～2.35% of vitrinite reflectance).Based on the bitumen filling and area coefficient,and well logging interpretation,the thickness of ancient oil polls is estimated to be 10～22 m.Analyses of TOC,organic geochemistry,and petrographic facies suggest that the mudstone in Pingliang Formation gradually thickens from east to west in the western margin depression of the basin,contains rich organic matters of saline organisms of lower form,and can serve as reliable source rocks for the ancient oil pools in the central uplift belt.Pool-forming history indicates that the burial depth and palaeogeotherm of Ordos basin were increased during the Middle Jurassic-Early Cretaceous time,resulting in the pyrolysis of ancient oil pools into ancient gas pools,and that the structural reversal of Ordos basin was caused by Yanshan Movement during the Later Cretaceous-Eogene time,resulting in the formation of the Shanbei slope and the destruction of the ancient oil pools.Part of oil and gas migrated and dissipated towards the shallower layers of Ordos basin,and part headed towards upper part of Shanbei slope,becoming one of the important sources for major gas fields in the central part of the basin.