鄂尔多斯盆地奥陶系马家沟组五段5亚段斑化白云岩成因

Origin of porphyry dolomite in the 5th sub-member of the 5th member of Ordovician Majiagou Formation (Msub-member), Ordos Basin

廖慧鸿 苏中堂 黄文明 马慧 佘伟 罗振锋 刘国庆 胡孙龙

LIAO Huihong SU Zhongtang HUANG Wenming MA Hui SHE Wei LUO Zhenfeng LIU Guoqing HU Sunlong

成都理工大学 沉积地质研究院, 成都 610059 成都理工大学 油气藏地质及开发工程国家重点实验室, 成都 610059 中国石油 川庆钻探工程有限公司 苏里格项目经理部, 成都 610051

Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu, Sichuan 610059, China State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu, Sichuan 610059, China Sulige Project Management Department, Chuanqing Drilling Engineering Co., Ltd., CNPC, Chengdu, Sichuan 610051, China

鄂尔多斯盆地中东部奥陶系马家沟组五段5亚段发现大量不均一斑化白云岩，多口钻井油气显示良好，成为油气勘探关注的热点，但成因机理尚未明确。依据岩石学特征，辅以地球化学分析，研究了斑化白云岩成因。斑化白云岩可分为不均匀斑化白云岩、明显斑化含灰云岩和略微斑化灰质云岩3类，根据云斑的形态特征认为云斑构造受到生物扰动所致，白云岩斑点与基质的阴极发光相同；斑化白云岩总体具有较高的Fe，Mn，Na和较低的Sr含量，斑点与基质的Fe，Mn和Sr的含量无明显差别；斑化白云岩δ13C值与同期海水相同，δ18O值均偏负，三类白云岩无明显差别；87Sr/86Sr比值高于同期海水。岩石学和地球化学特征表明生物扰动使岩石不均一，扰动程度影响斑化程度，斑化白云岩系生物扰动灰岩经高盐度海水渗透回流白云石化作用而成，白云岩再埋藏后经历了重结晶作用改造。

Large number of heterogeneous porphyry dolomites have been found in the 5th sub-member of the 5th member of Ordovician Majiagou Formation (M55 sub-member) in the central and eastern parts of Ordos Basin, and many drilled wells have shown good oil and gas potential, which has become the focus for oil and gas exploration, but the genetic mechanism is still undetermined. Based on petrological characteristics and geochemical analysis, the origin of porphyry dolomite in this area is studied. Results show that porphyry dolomite can be classified into three types: unevenly, obviously and slightly porphyry dolomite. According to the morphology characteristics of cloud spots, the structure of cloud spots is supposed to be caused by biological disturbance, and the cathodoluminescence of spots and matrix is the same. Porphyry dolomite has relatively higher contents of Fe, Mn and Na and lower content of Sr, but there is no significant difference between the contents of Fe, Mn and Sr in spots and matrix. The δ13C values of porphyry dolomites are highly comparable to those of seawater at the same time, and the δ18O values are all negative with no significant difference among the three types of dolomites. The ratio of 87Sr/86Sr is higher than that of seawater in the same period. Petrological and geochemical characteristics show that biodisturbance causes the heterogeneity of rocks, and the degree of disturbance affects porphyry rate. The biodisturbed limestone of porphyry dolomite series is formed by the dolomitization of the osmotic reflux of high salinity seawater. The dolomite underwent recrystallization after re-burial.

国家自然科学青年基金“鄂尔多斯盆地天环地区下奥陶统白云岩成因机制分析” 41302087

https://doi.org/10.11781/sysydz202205835