塔里木盆地西北缘寒武系肖尔布拉克组烃源岩特征

Characteristics of source rocks in the Cambrian Xiaoerbulake Formation in the northwestern Tarim Basin

近年来,塔里木盆地西北缘地区寒武系油气勘探取得了一系列发现和突破,研究寒武系烃源岩特征对指导下一步油气勘探具有非常重要的意义。前人研究普遍认为,塔里木盆地寒武系海相烃源岩主要集中在下寒武统玉尔吐斯组。为研究塔西北地区寒武系肖尔布拉克组新层系烃源岩特征,对4口钻井(BY1井、KTJ1井、KPN1井和XSC1井)和3条野外露头剖面样品进行了岩石学和地球化学分析。研究结果表明:塔西北地区肖尔布拉克组烃源岩主要为泥质灰岩和泥岩,测试结果显示寒武系肖尔布拉克组下段总有机碳含量(TOC)平均值分别为1.55%,2.39%,0.45%,0.89%(钻井样品)和1.10%(露头样品),表明该组为好-较好烃源岩,有机质母质类型为腐泥型,沥青镜质体反射率(Rb)分布在2.23%~2.57%,有机质已经处于过成熟热演化阶段,为一套天然气气源岩。肖尔布拉克组烃源岩厚度约20~90 m,中-下段烃源岩优于上段。根据钻井及地震剖面发育特征,肖尔布拉克组烃源岩主要分布在以BY1井和KTJ1井为中心的台内洼地,该区为烃源岩最有利发育区。

In recent years,a series of discoveries and breakthroughs in hydrocarbon exploration have been achieved in the Cambrian strata in the northwestern Tarim Basin(hereafter referred to as the study area).Therefore,investigating the characteristics of the Cambrian source rocks holds great significance for guiding subsequent hydrocarbon exploration efforts.Previous studies have generally suggested that the Cambrian marine source rocks in the Tarim Basin primarily occur in the Lower Cambrian Yuertusi Formation.To investigate the characteristics of source rocks in a new sequence in the Cambrian Xiaoerbulake Formation,we conduct petrologic and geochemical analyses of samples collected from four wells(i.e.,wells BY1,KTJ1,KPN1,and XSC1)and three outcrops.The analytical results indicate that the source rocks in the formation in the study area consist predominantly of argillaceous limestones and mudstones.The test results of samples show that the lower member of the Cambrian Xiaoerbulake Formation exhibits average total organic carbon(TOC)contents of 1.55%,2.39%,0.45%,0.89%(for the four wells),and 1.10%(for the three outcrops),respectively,suggesting fair-to-good source rocks.The formation’s source rocks contain sapropelic parent materials,with bitumen reflectance values ranging between 2.23%and 2.57%,suggesting overmature organic matter and natural gas-prone source rocks.These source rocks vary in thickness from approximately 20 to 90 m,exhibiting higher quality in the middle to lower members than in the upper member.Drilling results and seismic profiles reveal that these source rocks principally occur in the intra-platform depression centered on wells BY1 and KTJ1,establishing this depression as the most favorable area for the occurrence of source rocks.