鄂尔多斯盆地中部奥陶系马五_5亚段白云岩成因

Genesis of the dolomite in Ma 5 subinterval, Ordovician in central Ordos basin.

鄂尔多斯盆地中部的长庆气田奥陶系马家沟组主要产气层均为白云岩。该地区马家沟组马五５亚段在区域上普遍为厚层泥晶灰岩，白云岩夹于灰岩中，呈透镜状，分布具有穿层及分带性，从南向北分布层位逐渐升高。通过对其白云岩的岩石学特征、微量元素、碳同位素、氧同位素等研究，认为白云岩的形成与混合水白云化作用有关。白云岩储集层以微晶白云岩、粉晶白云岩为主，主要储集类型为裂缝孔隙型，具有较好的储集条件。预测盆地北部乌审旗—什汗木利地区，马五５亚段沉积期水体浅，利于大气淡水混入，有可能形成多个白云岩透镜体，是下步勘探值得重视的地区。图１表１参２（梁大新摘）

Main gas producing strata in Changqing gas field, middle part of Ordos basin are dolomite in lithology. In this district Ma 5 subinterval, Majiagou Group is a thick micrite section in which dolomite is inter bedded in micrite in lenticular form, distributed throughout the bed and in a belt form, and is raised higher and higher from South to North. By studies on lithological characters, trace elements, C isotopes, and O isotopes it is considered that, its genesis is related to the dolomitisation of mixed water. Reservoir dolomite is mainly composed of dolomicrite and dolosilts, the main reservoir type is of fracture porosity with rather superior reservoir properties. It is predicted that Uxin Qi Shihanmuli district located in the northern part of this basin where water depth in the aquifer was relatively shallow in the period of deposition of Ma 5 5, and was favorable to the mixing of meteorite water to form multiple dolomite lens; is a district with priority in further exploration.