鄂尔多斯盆地西南部晚古生代早—中期物源分析

The Analysis of Sediment Provenance in Early-Middle Period of Late Paleozoic in the Southwest of Ordos Basin

鄂尔多斯盆地西南部物源主要为太古界的中—深变质、强烈岩浆活动和混合岩化的复杂变质岩系及下元古界的一套变质火山岩—沉积岩系,在构造上具有稳定与活动的双重性。在不同物源影响区内,轻、重矿物组分特征也明显不同。镇原—庆阳地区的石英主要是棕色变质成因岩石,长石以碱性长石为主,母岩区为祁连—北秦岭造山带的太华群、秦岭群及宽坪群的变质石英砂岩、石英片岩、石英岩、花岗岩或花岗片麻岩等。综合分析,区内有3个不同的物源区,岩屑组成及重矿物组合在南、北物源分布区中明显不同,而且从盒8期青铜峡—固原断裂以西地区开始隆升,西部物源逐渐补给,并伴有火山活动。在REE配分模式上,南或西南物源的与海原地区的差异较大,与陇西古陆的一致,而且此物源的影响区距源区近,砂体厚度大,物性较好,山西组长石溶孔特别发育,并存在次生溶蚀型孔隙发育带,为最有利的勘探目标区。

The source rocks of the southwestern Ordos Basin were the mesometamorphic, magmatic active and migmatizational complex metamorphic rocks of Archaean and the metavolcanite-sediment rocks of the Lower Proterozoic. The tectonic character had steady-active dualism. In different source distributing regions, there were quite differences between the characters of light minerals and those of the heavy ones. The quartz was brown metamorphosed rocks, and the feldspar was mainly alkalescent feldspar, the source rock of which was the quartzsandstone, quartz schist, quartz rocks and granite or granitic gneiss of the Taihua Group, the Qinling Group and the Kuanping Group. After being analysed synthetically, the study area had three source regions. The constituent of clastic debris and heavy minerals had clearly different characters in the southern and northern source distributing regions. In the period of member 8 of Xiashihezi Formation, the western region of Qing- tongxia-Guyuan rupture began to upheave, and the western sediment provenance began to supply the old sediment gradually, where volcano movements once existed. In the southern or southwestern source near the source regions, the sandstone was thick and the reservoir properties were better, which were quite different from the Haiyuan source. What＇s more, the sandstone was consistent with that of Longxi ancient land. The feldspar-dis solved-pores especially developed in Shanxi Formation and existed in the secondary dissolved pores belts, which was the best exploring target plot.