鄂尔多斯盆地苏里格气田中奥陶统马家沟组五段凝块石类型与古盐度初步分析

Thrombolite types and seawater palaeosalinity of the 5th Member of Middle Ordovician Majiagou Formation in Sulige Gas Field,Ordos Basin

微生物碳酸盐岩是近年来地学界研究的一个热点,其时空分布非常广泛,并可形成重要的油气储集层。在鄂尔多斯盆地北部苏里格气田的中奥陶统马家沟组马五1+2段和马五4段,笔者分别发现了非钙化微生物成因的凝块石微生物岩。依据显微结构,将凝块石划分为蠕虫状凝块石、网状凝块石和房室孔洞状凝块石3种类型。通过与基里巴斯现代超盐湖微生物碳酸盐形成环境的对比,推测在华北地台云坪广布、坪中有湖的大背景下,凝块石是在相对海平面上升、盐度大约在60‰~149‰之间的条件下形成的,而含石膏结核碳酸盐岩是在相对海平面下降、盐度达到149‰以上的条件下形成的。进一步研究发现,蠕虫状凝块石泥粉晶白云岩的白云石晶间孔和溶蚀孔非常发育,网状凝块石和房室孔洞状凝块石的溶蚀残余孔较发育,推测凝块石微生物岩具有较好的物性,是未来油气勘探的重要目标岩性之一。

Studied on well cores of the 5th Member of Ordovician Majiagou Formation in the Ordos Basin,it reveals that the microbialites in the 2nd layer of Ma45,the 2nd layer of Ma25,the 4th layer of Ma15 can be divided into three distinct types:wormlike thrombolite,bacterial netlike thormbolite and chamber-patches thrombolite.By comparing with formation conditions of the modern microbial carbonate in the hypersaline lakes in the Republic of Kiribati,it is inferred that(1)the studied thrombolites formed in hypersaline lakes with salinity of 60‰~149‰as the relative sea level rose,and(2)the gypsum-bearing carbonates formed in the hypersaline lakes with salinity of above 149‰.The wormlike thrombolites have high contents of intercrystaline pores and dissolution pores,and the netlike and chamber-patches thrombolites have high contents of relict pores,which makes thrombolites as an important exploration target.