碳酸盐岩-膏盐岩高频沉积旋回组合生-储特征——以鄂尔多斯盆地奥陶系马家沟组中-下组合为例

Source-reservoir characteristics of high-frequency cyclic carbonate-evaporite assemblages:A case study of the lower and middle assemblages in the Ordovician Majiagou Formation,Ordos Basin

针对碳酸盐岩-膏盐岩沉积组合内是否发育烃源岩,能否提供充足烃源,源、储是否匹配等制约鄂尔多斯盆地奥陶系马家沟组中-下组合勘探领域评价的问题,通过陕473和桃112两口马家沟组全取心井的岩心观察和高频沉积旋回分析,取得了3项认识。①碳酸盐岩-膏盐岩沉积组合包括由未云化灰岩段、白云岩段、白云岩-膏盐岩薄互层段和膏盐岩段构成的2类8种气候逐渐变干旱的沉积旋回;白云岩段和白云岩-膏盐岩薄互层段共发育10类111个高频沉积旋回,具有旋回频繁、单层厚度薄的特点。②基于高频沉积旋回分析,识别出马家沟组中-下组合发育两类烃源岩:一类为优质烃源岩,即毫米级高TOC(有机碳含量)(>2.50%)黑色泥质白云岩;另一类为中等烃源岩,即低TOC(1.21%)灰黑色白云岩。优质烃源岩发育具有相控性和层控性特征,分布于膏盐湖周缘的白云岩-膏盐岩薄互层段中,计算天然气资源量为8504.94×10^(8) m^(3)。③识别出马家沟组中-下组合两类相控白云岩储层,分别为古隆起及周缘的颗粒滩白云岩储层和膏盐湖周缘的微生物白云岩储层,源-储匹配良好。上述认识为鄂尔多斯盆地马家沟组中-下组合的持续勘探提供重要信息,指明了方向。

The lower to middle carbonate-evaporite assemblages of the Ordovician Majiagou Formation in the Ordos Basin are potential exploration targets but questions have been raised concerning whether there are source rocks developed,how well they are developed and if they match well with reservoirs.Detailed observation of cores and analyses of high-frequency sedimentary cycles from wells Shan 473 and Tao 112 encountering the Majiagou Formation show that:①Carbonate-evaporite assemblages consist two kinds/eight types of sedimentary cycles(recording gradually dry-up climate)of limestone(without dolomitization),dolostone,thin dolomite-evaporite interbeds,and evaporites,of which the dolostone and thin dolomite-evaporite interbeds are further recognized to contain 10 types/111 high-frequency sedimentary cycles characterized with frequent cycles and thin single layers.②Based on analysis of these high-frequency sedimentary cycles,two types of source rocks are identified:the high-quality source rocks consisting of black argillaceous dolostone in millimeter-scale lamina with high TOC(>2.50%)and the medium-quality source rocks consisting of grey-black dolostone with low TOC(1.21%).The high-quality source rocks are facies and horizon conditioned and mostly distributed in the interbedded thin dolomite-evaporite sections around the evaporitic lake and with gas resource amounting up to 850.494 BCM.③Two types of facies-controlled dolostone reservoirs are recognized:the dolo-grainstone reservoir on the paleo-uplift and its periphery,and the microbial dolostone reservoir around the evaporite lake,both are favorably located with source rocks.The above understandings may be used as guidance to the future exploration in the lower and middle assemblages of Majiagou Formation,Ordos Basin.