鄂尔多斯盆地奥陶系生物礁及其天然气勘探前景

Ordovician Reefs and the Prospect of Gas,Ordos Basin

鄂尔多斯盆地西南缘中上奥陶统不仅存在着珊瑚礁、层孔虫礁和藻礁,而且发育规模较大的藻粘结岩、障积岩体,礁体的底部和礁前普遍发育颗粒滩相沉积。滩相沉积物将相对独立的各个生物礁互相连通,形成规模较大的生物礁复合体。该盆地内部的早奥陶世生物礁以层状藻礁为主,发育于吕梁隆起西侧和伊盟隆起南坡以及中央古隆起北端,表现为2种生态特征,一种为藻类垂直生长形成藻叠层石礁,另一种为藻类粘结灰泥形成球壳状藻灰泥球。生物礁的原生生物孔隙和粒间孔隙受成岩作用影响,已基本消失殆尽,但成礁岩石白云岩化比较普遍,且晶间孔和晶间溶孔发育,储集物性较好,已钻井产能较高,属于优质储层,也是鄂尔多斯盆地下古生界天然气勘探新领域。

Not only coral reefs, coral stromatoporoid reefs and algal reefs, but also relatively large scale algae bonding rocks and bafflestones are distributed in the Middle-Late Ordovician Series in the southwestern margin of the Ordos Basin, and at the same time grain bank facies sediments are widely developed at the bottom and the front of the reefs. The shoal facies deposit connects the relatively independent reefs, forming relatively large scale biohermal complexes. The intrabasinal reefs are mainly layered algal reefs, distributed in the west side of the Luliang uplift, the south slope of the Yimeng uplift, and the north side of the centeral palaeohigh. There exist two kinds of ecological characteristics, one is algae growing vertically and forming algal stromatolite, the other is algae binding plaster and forming algae lime mud sphere. Influenced by diagenesis, the origin biological pores and intergranular pores of the reefs nearly disappeared completely. But the dolomitization is common, intracrystalline pores and intracrystalline dissolved pores are well developed, the physical property is good, and the drilled wells have good deliverability. Dolomitized reefs are high-quality reservoirs and are a new target for the Early Palaeozoic gas exploration.