四川广元上寺剖面上二叠统大隆组有机质分布特征与富集因素

Distribution Characteristics and Enrichment Factors of Organic Matter in Upper Permian Dalong Formation of Shangsi Section,Guangyuan,Sichuan Basin

四川盆地上二叠统大隆组富有机质硅质泥岩是页岩气勘探的一套重要目的层系.利用扫描电子显微镜和元素地球化学等方法探究大隆组有机质类型、分布特征及富集因素,对该地区非常规油气勘探具有指导意义.根据扫描电镜图片,上寺剖面大隆组有机质按形态可分为形态有机质、弥散有机质和沥青.三者成因不同,形态有机质是选择性保存的结果,以离散状分布在所有岩性样品中;弥散有机质是大隆组有机碳总量(total organic carbon,TOC)的主体,主要分布在硅质泥岩样品中,硅质灰岩样品中极少,体现了粘土矿物的吸附作用;沥青是可溶有机质运移进入孔、缝系统,经高热演化后形成的块状有机质,主要分布于硅质灰岩和灰岩样品中.该剖面地球化学数据显示缺氧沉积环境有利于有机质保存,但海洋表层生产力才是控制TOC含量变化的主要因素.研究表明,晚二叠世海平面上升导致上寺剖面大隆组表层生产力增加及底层海水缺氧,该组富有机质黑色硅质泥岩段高TOC是沉积环境与矿物吸附共同作用的结果,同时也反映了原生有机质保存情况.更好的理解泥质烃源岩中有机质赋存类型有助于烃源岩评价及非常规油气勘探开发工作.

The black siliceous mudstones of the Upper Permian Dalong Formation in Sichuan basin are important target zones for shale gas exploration.To facilitate the unconventional hydrocarbon resources exploration in this area,scanning electron microscope（SEM）and elemental geochemistry were used to investigate organic matter types,distribution characteristics and enrichment factors in Dalong Formation.According to SEM pictures,organic matter in the Dalong Formation can be divided into morphological organic matter,amorphous organic matter and bitumen.Morphological organic matter is the result of selected preservation,discretely distributed in all lithology samples;disperse organic matter,which can be seen in siliceous mudstones but rarely in siliceous limestone samples,is the principle part of TOC（total organic carbon）of the Dalong Formation,reflecting the adsorption of clay minerals;bitumen formed from dissolved organic matter migration and subsequent high thermal evolution,is distributed in siliceous limestone and limestone samples,rarely in siliceous mudstone samples.Geochemical data indicate that anoxic environment is beneficial to the preservation of organic mat-ter,but the productivity is the main factor to control the change of content of total organic carbon（TOC）.Comprehensive correlation research shows that the sea-level rise of the Late Permian led to the increased productivity and anoxic water column in the adjacent oceanic basin.The high content of TOCin organic-matter-rich black siliceous mudstone is the result of sedimentary environment and mineral absorption,and also reflects preservation state of primary organic matter.Migration bitumen should be excluded for hydrocarbon source rock evaluation.