华南下扬子区早寒武世幕府山组沉积环境:来自于全岩地球化学的启示

Sedimentary environment of the Lower Cambrian Mufushan Formation in the Lower Yangtze region:Evidence from whole-rock geochemistry

下寒武统为扬子地块最重要的烃源岩层之一。然而下扬子地区在该层位尚未取得工业油气突破,开展下扬子地区早寒武世幕府山组沉积环境研究,对比分析扬子地块这一时期不同地区的油气地质条件具有重要的意义。下扬子盱眙地区实施了官地1井,获得了下寒武统幕府山组全取心样品。该井地化分析结果显示,在幕府山组钙质/炭质泥岩样品中,大多数大离子亲石元素展示富集特征,而高场强元素及过渡元素显示出亏损特征;幕府山组泥岩样品中稀土元素总量为14.81~107.47μg/g;样品的化学风化指数CIA在64.84至78.81之间变化,结合A-CN-K三角图解,表明幕府山组源区受到中等强度的风化作用;微量元素的富集特征表明幕府山组泥岩沉积于潮湿且低盐度环境,氧化还原敏感参数比值指示幕府山组沉积于硫化还原环境。在Th/Sc-Zr/Sc图解中,大多数样品投在玄武岩与长英质火山岩区间,表明并未经历沉积循环。在Al_(2)O_(3)/TiO_(2)与TiO_(2)/Zr图解中,幕府山组泥岩样品指示出中酸性岩浆物源。与此同时,Cr/V与La/Th-Hf图解表明幕府山组泥岩物源主体为中酸性岩石。

The Mufushan Formation of Lower Cambrian is the most significant hydrocarbon source rock for shale gas in the Yangtze Platform.No exploration breakthrough has been achieved so far in the Lower Yangtze area,compared to the Middle and Upper Yangtze areas.Recently,the well of GD-1 has been completed,for which the Early Cambrian Mufushan(MFS)Formation is completelyly cored.Geochemistry of calcareous/carbonaceous mudstone of Early Cambrian Mufushan(MFS)Formation are carefully investigated for paleo-environment,provenance and tectonic settings.The samples of MFS are characterized by enriched large ion lithophile elements and depleted high field strength elements and transition elements.The analysis results show that the total REE concentrations of MFS mudstones vary from 14.81 to 107.47 ug/g.The Chemical Index of Alteration(CIA)ranges from 64.84 to 78.81.And the A-CN-K plot indicate that the source rocks has undergone a moderate weathering.In the Th/Sc versus Zr/Sc plot,most samples are located in the area between basalt and felsic igneous rocks,with negligible sedimentary recycling.Both the Al_(2)O_(3)/TiO_(2)ratios and TiO_(2)/Zr ratios indicate an intermediate-felsic igneous provenance.The Cr/V ratios and La/Th-Hf diagrams also suggest that most of the materials are derived from intermediate-felsic rocks.