摘要： 早期形成的碳酸盐结核在埋藏期间会经历多种碳酸盐矿物相沉淀的复杂胶结作用,岩石学研究是探究结核成因的关键。通过野外剖面观察、岩石学观察和阴极发光技术,分析了鄂尔多斯盆地渭北地区上三叠统延长组长7油层组泥页岩中各种形状的方解石和白云石结核中自生碳酸盐矿物的特征。这些结核为成岩早期的产物,构成结核的自生碳酸盐矿物特征显著: (1)球粒方解石结核中,方解石呈纤维状或刃片状,球粒间充填晶粒方解石或因压实呈贴面结合,纤维状方解石发桔红色和暗红色2种光,刃片状方解石发暗红色光;(2)粉晶方解石结核中,方解石呈他形粒状,含有机质包裹体或纤维状晶形残余,晶间含沥青和纤维状方解石残余,主要发暗红色光;(3)白云石结核有泥晶和粉晶2种晶体类型,粉晶白云石结核含较多泥质,泥质条带或有机质条带处常见纤柱状白云石;(4)沿裂缝充填的方解石和白云石常呈纤维状或纤柱状结构,发暗红色光或不发光。研究区长7油层组碳酸盐结核中的方解石和白云石具有不同的成因类型和复杂的胶结作用:球粒方解石和泥晶白云石代表了结核开始形成时的胶结作用,可以准确地反映结核的成因;粉晶方解石、粉晶白云石反映了交代成因;裂缝中纤维状、纤柱状方解石和白云石集合体则为结核经历了较强压实作用之后充填裂缝而成。

关键词: 鄂尔多斯盆地, 长7油层组, 碳酸盐结核, 自生碳酸盐

Abstract: The early carbonate concretions would undergo the complex cementation of various carbonate mineral phases during the long-term burial process. The detailed petrology studies are the key to explore the genetic mechanism of the carbonate concretations. In this paper,the characteristics of authigenic carbonate minerals constituting the calcite and dolomite concretions that are rich in the shale of Chang 7 oil-bearing interval of Yanchang Formation in Weibei area of southern Ordos Basin,is studied through detailed observation in the field,together with the petrology studies and cathodoluminescence technology. The authigenic carbonate minerals constituting the concretions that may form in the early diagenesis,have marked features as follows: (1)the calcite spheres are composed of fibrous or flaky calcites. The crystalline calcites are filled between the calcite spheres,or the spheres are closely contacted because of compaction. Under the cathodoluminescence condition,the fibrous calcites show red orange light and dark red light,and the flaky calcites show dark light;(2)the calcites developed in fine-grained crystalline concretions show irregular shapes,containing many organism inclusions or residual fibrous textures. It shows that the natural bitumen and residual fiberous calcites appear between the calcite crystals,which presents dark red light under the cathodeluminescence;(3)the dolomite concretions are characterized as micrite and fine-grained crystalline dolomites. Authigenic fine-grained crystalline dolomite concretions usually contain large amounts of argillaceous bands. The fiber columnar dolomites are common in the argillaceous bands or organic matter enriched bands;(4)Fibrous and fiber columnar calcites and dolomites that present dark red color or colorless under the cathodeluminescence,usually grow along the cracks. The calcite and dolomite minerals developed in the carbonate concretions of Chang 7 oil-bearing interval,have different genetic types and complex cementation history. Spheroidal calcites and micritic dolomites produce at the beginning of the cementation,indicating the early genesis of concretions. Fine-grained crystalline calcites and dolomites indicate metasomatism. Crack-filled fibrous,radial and columnar shaped calcite and dolomite aggregates indicate strong compaction after the formation of concretions.

Key words: Ordos Basin, Chang 7 oil-bearing interval, carbonate concretion, authigenic carbonate

中图分类号: 

• P588.24+5