酒西盆地下白垩统赤金堡组烃源岩孢粉相特征及其石油地质意义

Palynofacies characteristics of source rocks of Lower Cretaceous Chijinbao Formation in Jiuxi Basin and their petroleum geological significance

酒西盆地是中国最早开发的陆相含油气盆地之一,下白垩统是盆地主要的烃源岩。通过对旱峡地区下白垩统赤金堡组烃源岩孢粉相进行系统研究,首次识别出木质-丝炭组合(孢粉相A)、无定形组合(孢粉相B)和无定形-木质组合(孢粉相C)3个孢粉相,分别发育在赤金堡组的下部、中部与顶部、上部。孢粉相A以丰富的木质组织和较多的丝炭为特征,孢粉相B以无定形为主要组分,含较多孢粉体和木质组织,孢粉相C以木质和无定形为主。有机质母质来源自下而上表现出从以陆源高等植物输入为主到低等水生生物为主,再到显示混源特征,最终又变为以低等水生生物为主的演化特征。赤金堡组中部及顶部孢粉相B段烃源岩有机质类型以Ⅱ1型为主,具有较好的生油潜力;而下部和上部孢粉相A段和孢粉相C段烃源岩有机质类型以Ⅲ型为主,生油能力差。孢粉相分析揭示赤金堡组沉积期湖盆经历了断陷形成、强烈扩张、充填收缩和再次凹陷4个演化阶段,沉积环境依次为近源冲积扇—滨浅湖相、半深湖相—深湖相、扇三角洲相和局部半深湖相—深湖相。其中,沉积中期强烈凹陷阶段和末期断陷复活阶段的深水缺氧环境中发育了较好的烃源岩。

The Jiuxi Basin is one of the earliest developed continental petroliferous basins in China,dominated by Lower Cretaceous source rocks.Through the systematic research on the palynofacies of source rocks of Lower Cretaceous Chijinbao Formation in Hanxia area,three types of palynofacies were initially identified in this study,i.e.,the woody tissue-fusain assemblage(palynofacies A),amorphous assemblage(palynofacies B)and amorphous-wood tissue assemblage(palynofacies C),developed in the lower,middle and top parts of the Chijinbao Formation,respectively.Palynofacies A is characterized by abundant woody tissue and high content of fusain.Palynofacies B is mainly composed of amorphous material,containing much sporinite and woody tissue.Palynofacies C mainly contains woody tissue and amorphous material.From bottom to top,the source of parent material for organic matter shows the evolution from the dominant terrestrial higher plant input to the dominant lower hydrobiont,and then the mixed source characteristics;finally,it is dominant by the lower hydrobiont again.The organic matter of source rocks of the palynofacies B section in the middle and top parts of Chijinbao Formation is dominated by typeⅡ1,indicating good oil-generating potential;while that of the palynofacies A section and palynofacies C section in the lower and upper parts is dominated by typeⅢ,showing poor oil-generating ability.The palynofacies analysis reveals that the lake basin underwent four evolutionary stages during the depositional stage of Chijinbao Formation:formation of fault depression,strong expansion,filling and contraction,and re-sagging.The depositional environment was shown as near-source alluvial fan facies-shore shallow lake facies,semi-deep lake facies-deep lake facies,fan delta facies and local semi-deep lake facies-deep lake facies successively.Among them,good source rocks developed in the deep water anoxic environment during the strong subsidence stage and the resurrection stage of fault depression respectively in the middle and final sedimentary stage.