志丹-安塞地区延长组下组合方解石与浊沸石胶结物含量变化对储层致密化差异的控制

Content variation of calcite and laumontite cements and its control on the difference of lower part of Yanchang Formation tight reservoir in Zhidan-Ansai,China

为探讨志丹-安塞地区三叠系延长组下组合(长8、长9和长10)致密砂岩储层致密化过程和主要原因,运用铸体薄片观察、扫描电镜、包裹体测温等手段,通过全区下组合464个储层样品的统计分析发现,方解石与浊沸石纵向含量分布具有明显的此消彼长关系。在研究区西部,自长8、长9至长10段,方解石胶结物的含量逐渐减少,而浊沸石胶结物的含量逐渐增加;在研究区东部,从长91段向上部的长8段和下部的长10段,同样表现为方解石胶结物减少而浊沸石胶结物相对增加。分析表明,方解石与浊沸石胶结物含量变化与烃源岩的分布层段和有机质成熟过程中流体的释放密切相关。在研究区的西部,下组合的油气主要来源于长73烃源岩;在研究区的东部,下组合的油气主要来源于长91烃源岩。烃源岩成熟之前,储层中先发育浊沸石胶结物,形成储层的早期致密化;烃源岩成熟之后,随着有机酸和CO2的大量排出,发生浊沸石胶结物的不断溶蚀、油气充注和晚期方解石胶结物的不断充填,形成储层的晚期致密化。靠近长73和长91烃源岩的层段以先致密后成藏再致密为主,远离烃源岩的层段以先致密后成藏为主。

The densification process and its main reasons of the lower part of Yanchang Formation tight reservoir in Zhidan-Ansai are studied on the basis of thin section examination,scanning electron microscopy,fluid inclusions and other analytical methods.Analyses of reservoir samples in the study area show that the longitudinal distribution of calcite and laumontite displays wane and wax relation.In the west part of the study area,the content of calcite cement from Chang 8 to Chang 10 decreases gradually and the content of laumontite cement increases gradually.Similar situation exists in the east part of the study area.The research indicates that the content variation of cement is closely related to the distribution of source rock layers and also to the fluid formed in the organic matter maturation process.In the west part of study area,oil and gas of the lower part of Yanchang Formation are mainly originated from Chang 73 source rocks,while in the east part of the study area,oil and gas are mainly from Chang 91 source rocks.Before the maturation of organics,laumontite cementation dominates in the reservoir and early stage densification occurs.After the maturation of organics,extensive discharge of organic acid and CO2 leads to the dissolution of laumontite cements and to the infilling of oil and gas,associated with calcite cementation and late stage densification of the reservoir.Source rocks near Chang 73 and Chang 91 experienced early stage densification followed by late stage hydrocarbon formation and densification.The rocks far away from the source rock experienced initial densification followed by late stage hydrocarbon accumulation.