准噶尔盆地玛湖凹陷西斜坡百口泉组含油储集岩分子与碳同位素地球化学特征及其意义

摘要

在准噶尔盆地玛湖凹陷斜坡区下三叠统百口泉组地层中发现大面积连片岩性油气藏.对西斜坡百口泉组一段和二段8个含油储集岩样分别提取自由态组分、束缚态组分和油气包裹体组分,并进行GC、GC-MS和GC-IRMS分析.结果表明,3类油气组分三环萜烷主峰含量远高于C30藿烷,反映充注到百口泉组(T1b)储层的原油均具有很高的成熟度,相当于Ro＞1.20％.生物标志物指标β-胡萝卜烷/(β-胡萝卜烷+nC20)、伽马蜡烷/(C30藿烷+伽马蜡烷)、三环萜烷/(三环萜烷+藿烷)、Ts/(Tm+Ts)、C29Ts/(C29藿烷+C29Ts)、C30重排藿烷/(C30重排藿烷+C30藿烷)和甾烷C21/(C21+∑C29)比值均显示从自由态组分、束缚态组分至油气包裹体组分成熟度依次降低的特征,表明油气组分在油藏充注过程中不断升高.与此相反,PrC17和PhC18比值以及正构烷烃单体碳同位素组成显示从自由态组分、束缚态组分至油气包裹体组分成熟度依次升高的特征,表明油气组分从成藏早期至晚期成熟度降低.由于充注的原油为不同成熟度油气组分混合产物,生物标志物在原油中属于微量及痕量组分,而Pr、Ph和正构烷烃属于常量组分,PrC17和PhC18比值以及正构烷烃单体δ13C值更能代表主体原油的成熟度.因此,对于玛湖凹陷西斜坡油气藏,主体原油成熟度从成藏早期至晚期是降低的.根据8个油砂样3类油气组分的成熟度可以推断油气充注期介于晚侏罗世-白垩纪之间.盖层密封性是控制这类油气藏成藏的重要因素.在晚侏罗世之前,下伏二叠系盖层一直阻止了油气沿断层运移.至晚侏罗世-白垩纪,下伏盖层的密封性遭受破坏,油气才从下伏二叠系沿断层运聚至百口泉组一段和二段储层(T1b1-2).由于上覆百三段(T1b3)和克拉玛依组(T2k)盖层自始至终保持密封状态,百口泉组一段和二段储层(T1b1-2)成为玛湖凹陷油气的最终归宿.%Massive continuous oil reservoirs have been found within the Lower Triassic Baikouquan Formation in the slope areas of the Mahu Sag of the Junggar Basin.Free oils,adsorbed oils and inclusion oils were obtained from eight oil-containing reservoir rocks within the first and second sections of this formation,and analyzed using GC,GC-MS and GC-IRMS techniques.The amount of tricyclic terpane with the highest peak in m/z 191 mass chromatogram is substantially higher than that of C30 hopane for the three types of oil components from all the eight reservoir rocks,demonstrating that oil components charging these reservoirs have high maturities (equivalent to Ro ＞1.20％).For the eight oil-containing sandstones,the maturity shows a decreasing trend from free oils,adsorbed oils to inclusion oils,demonstrating that the maturity of charging oil increased during oil charging process for the reservoirs,based on biomarker parameters,such as the ratios of β-carotane/(β-carotane+nC20),gammacerane/(C30 hopane + gammacerane),tricyclic terpanes/(tricyclic tcrpanes + C30 hopane),Ts/(Tm+Ts),C29Ts/(C29 hopane + C29Ts),C30 diahopane/(C30 diahopane + C30 hopane) and C21/(C21+-∑C29) steranes.However,the trend is opposite on the basis of both ratios of PrC17 and PhC18 and δ13C values of individual n-alkanes.Both the initial and final charging oils are mixtures with different maturities.Biomarker molecules are minor and trace components while Pr,Ph and n-alkanes are major components of oils.Therefore,the maturity for the majority of oil components decreased during oil charging process for these reservoirs.Oil charging interval was within the Late Jurassic and Cretaceous periods as estimated based on the maturities of the three types of oil components from the eight reservoir rocks.Cap rocks played a critical role in the filling process of these oil reservoirs.Prior to Late Jurassic,cap rocks within the Permian strata below the reservoirs were effective and prevented oil migration from the Permian strata to the reservoirs within the Baikouquan Formation.Within the Late Jurassic and Cretaceous periods,cap rocks within the Permian strata were ineffective,leading to oil migration from the Permian strata to the reservoirs in the Baikouquan Formation.Cap rocks within the third section of the Baikouquan Formation and Middle Triassic Kelamayi Formation always maintained effectiveness throughout the whole geological history.As a result,oils which were generated and accumulated within the Permian strata were finally accumulated in the reservoirs within the first and second sections of the Baikouquan Formation.