藏北羌塘盆地侏罗纪含颗石藻黑色岩系地球化学特征与地质意义

Geochemical characteristics of the Jurassic coccolith-bearing black rock series from the Qiangtang Basin, Northern Tibet, China and their significances

藏北羌塘盆地侏罗系发育一套富含有机质的黑色岩系,化石较为丰富而倍受地质学家关注。本文选择双湖地区毕洛错剖面,从有机地球化学和元素地球化学方面分析富含颗石藻黑色岩系特征。研究表明,研究区黑色岩系有机碳含量为2.84%~3.71%,氯仿沥青"A"含量为0.18%~0.29%。有机质类型为Ⅱ1型,母质主要来源于浮游生物及少量陆生植物,Tmax为429~435℃,显示该套岩系处于未成熟阶段。研究区样品的主量元素Si、Al、Ca等元素较富集,表明该套黑色岩系以陆源碎屑输入为主;V/Cr、U/Th、Ni/Co、V/(Ni+V)特征比值、U和Mo的富集以及Eu和Ce无明显异常,反映了该套黑色岩系主要沉积在弱还原环境,有利于有机质保存。以上研究表明,研究区含颗石藻黑色岩系为较好的烃源岩,保存条件较好,可以作为进一步勘探开发的有利烃源岩层。

A set of black rock series with rich-organic matters and abundant fossils, developed in Jurassic strata of the Qiangtang Basin in Northern Tibet, China, have been paid more attention by various geologists. In this paper, the coccolith-bearing black shales at the Biluo Co section of the Shuanghu area in the Qiangtang Basin have been selected to study their organic and elemental geochemical characteristics. The results show that their total organic carbon(TOC) contents change from 2.84% to 3.71%, and their chloroform bitumen "A" vary from 0.18% to 0.29%. The organic matters belong to the type Ⅱ1, indicating that the organic matter was sourced from major plankton and minor terrestrial plants. Tmax values varying from 429 oC to 435 oC indicating that the rock series are in immature stage. Furthermore, the major elements such as Si, Al, Ca and so on for samples are relatively enriched, showing that the black rock series are dominantly sourced from the terrestrial clastic input. The characteristic ratios of V/Cr, U/Th, Ni/Co, and V/(Ni+V), enrichments of U and Mo, and features of no obvious Eu and Ce anomalies show that the black rock series were mainly deposited in a weakly reduced environment which is beneficial to the preservation of organic matters. This study indicates that the coccolith-bearing black rock series are relatively good hydrocarbon source rocks, which are well preserved, for further prospective exploration of oil and gas.