伊拉克米桑油田群中白垩统Mishrif组岩溶特征及作用模式

Karst characteristics and karstification model in the Mid-Cretaceous Mishrif Formation of Missan oilfields,Iraq

通过对伊拉克米桑油田群多口井的岩心观察,结合铸体薄片、扫描电镜、成像测井、地球化学资料及区域沉积构造背景分析,发现中白垩统Mishrif组沉积时期局部的构造运动和海平面升降导致了2次沉积间断,从而在碳酸盐岩沉积内部存在2个准同生期风化壳岩溶不整合面。不整合面上下岩性及沉积特征差异明显:不整合面之上为泥质灰岩,岩性致密,为非储层;之下为生屑(厚壳蛤)灰岩,发育大量的溶缝和溶洞,对应储层物性较好。生屑灰岩中部分溶缝和溶洞被早期大陆成岩暴露环境中的渗流粉砂所充填,不整合面之下见溶蚀作用形成的示底构造;阴极发光和碳氧同位素同样表明不整合面之下曾经历过大气淡水的影响。本文研究成果可为研究区层序划分和储层成因机制的研究提供重要参考。

Combining core observation of wells with thin sections,scanning electron microscopy,imaging logging,geochemical testing and regional sedimentary tectonic setting analysis,two hiatus caused by regional tectonic movement and sea level eustacy are identified in the Mid-Cretaceous Mishrif Formation of Missan oilfields,Iraq,in which exists two corresponding penecontemporaneous weathering karstification unconformities. Obvious changes of lithology and sedimentary characteristics exist along the 2 unconformities： argillaceous limestone develops underlying the unconformity surfaces,and it is dense rock and non-reservoir; bioclastic（ rudist） limestone with a great number of dissolution caves and fractures develops overlying the unconformity surfaces,which is good reservoir. Partial dissolution caves and fractures of bioclastic limestone are filled by vadose silt produced by the vadose zone in the exposed environment during early continental diagenesis. In addition,a geopetal microstructure due to dissolution develops below the unconformity surface. The cathodoluminescence and carbonate and oxygen isotopes also show that the strata below the unconformity surface have experienced the influence of atmospheric fresh water. The research result can provide reference to sequence division and reservoir genesis for the study area.