摘要
分析泥页岩形成的沉积环境对页岩气储层评价和甜点区优选具有重要意义。为深入探讨敦煌盆地五墩凹陷中间沟组下段富有机质泥页岩的形成环境和有机质聚集机制,以DY1井为研究对象,开展了总有机碳(TOC)含量、显微组成、碳同位素、主量元素、微量元素和稀土元素等测试。结果表明,五墩凹陷中间沟组下段岩性为灰黑色炭质泥页岩、粉砂质泥页岩夹薄煤层等,泥页岩具有较高的TOC含量,质量分数介于0.53%~25.25%,均值为8.18%,成熟度R_(ran)介于0.74%~1.21%。Mo含量、P/Ti值和有机质显微组成表明,中间沟组下段沉积水体具有较低的初级生产力,高丰度有机质主要来自于陆源高等植物,反映了研究区浅水三角洲–半深湖环境下泥页岩的古生产力不是有机质聚集的关键控制因素;V/(V+Ni)、Ce_(anom)、Th/U和U_(EF)-Mo_(EF)协变模式等揭示中间沟组下段泥页岩形成于缺氧环境;Sr/Cu、Rb/Sr和气候指数C等指标反映了温暖–半干旱的古气候条件;Sr/Ba、Ba/Ga、Ca/(Fe+Ca)和Al_(2)O_(3)/MgO等特征指示古水体为淡水–微咸水;Zr/Al、Rb/K和MnO含量等指标反映了沉积水体为浅水–半深水。依据DY1井沉积环境参数与有机质聚集的关系,建立了中间沟组下段富有机质泥页岩的沉积模式,自下至上经历2个旋回4种沉积模式,沉积水体由浅水三角洲–半深湖沉积–浅水三角洲–半深湖沉积演变,富有机质泥页岩形成于低初级生产力、高等植物陆源输入为主以及缺氧的半深湖环境,缺氧条件是中间沟组下段泥页岩有机质聚集保存的关键控制因素。该研究为敦煌盆地侏罗系页岩气成藏机理、资源潜力评价和有利区优选提供了理论支持。
Analysis on the sedimentary environment of shale is of great significance for the evaluation of shale gas reservoir and the optimization of sweet spot.In order to deeply explore the depositional environment of organic-rich shale and the mechanism of organic matter accumulation in the Lower member of Zhongjiangou Formation of Wudun Sag in Dunhuang Basin,the TOC content,maceral composition,carbon isotope,major elements,trace elements and rare earth element were tested based on DY1 well.The results indicate that the lithologies of the Lower member of Zhongjiangou Formation in Wudun Sag are gray-black carbonaceous shale and silty shale with thin coal seam interlayer.The shale has a high TOC content,ranging from 0.53%to 25.25%,averaged 8.18%,with maturity R_(ran) ranging from 0.74%to 1.21%.According to the Mo content,P/Ti ratio and the maceral composition of organic matter,the sedimentation water has low primarily productivity and the rich organic matter is mainly contributed by the terrestrial higher plants,which indicates that the paleoproductivity of shales in the environment of shallow water delta and semi-deep lake is not the major con-trolling factor of organic matter accumulation.V/(V+Ni),Ce_(anom),Th/U and the U_(EF)-Mo_(EF) covariation reveals that the shale in the Lower member of Zhongjiangou Formation was developed in an anaerobic environment.The indicators such as Sr/Cu,Rb/Sr and climate index C reflect that the paleoclimate was warm to semi-arid.The characteristics of Sr/Ba,Ba/Ga.Ca/(Fe+Ca)and Al_(2)O_(3)/MgO suggest that the palaeo-water was fresh to brackish.Besides,Zr/Al,Rb/K and MnO indicate that the sedimentation water was in shallow to semi-deep depth.According to the relationship between the para-meters of sedimentary environment and the accumulation of organic matter in DY1 well,sedimentary models of organic-rich shale in the Lower member of Zhongjianggou Formation were established,with two cycles and four sedimentary models experienced form bottom to top,the sedimentation water evolved from shallow delta to semi-deep lake depos-ition,and then to shallow delta to semi-deep lake deposition again.The organic-rich shale was formed in the semi-deep lake environment with low primary productivity and terrestrial higher plant input and anaerobic conditions,and the an-aerobic condition is the key controlling factor for the accumulation of organic matter in the shale in the Lower member of Zhongjiangou Formation.Generally,this study provides theoretical support for Jurassic shale gas accumulation mech-anism,resources potential evaluation and favorable area selection in Dunhuang Basin.
作者
邓伟
杨涛
樊生龙
刘子锐
万旭辉
肖娟宜
曹涛涛
DENG Wei;YANG Tao;FAN Shenglong;LIU Zirui;WAN Xuhui;XIAO Juanyi;CAO Taotao(Geological Survey of Gansu Province,Lanzhou 730000,China;School of Earth Sciences and Spatial Information Engineering,Hunan University of Science and Technology,Xiangtan 411201,China)
出处
《煤田地质与勘探》
CAS
CSCD
北大核心
2022年第12期114-130,共17页
Coal Geology & Exploration
基金
国家自然科学基金项目(41802163)
湖南省自然科学基金项目(2021JJ30240)
甘肃省基础地质勘查项目(甘国土资财发〔2016〕15号)。
关键词
元素地球化学
有机质特征
沉积环境
中间沟组下段
五墩凹陷
geochemistry of element
organic matter characteristics
sedimentary environment
Lower member of Zhongjiangou Formation
Wudun Sag