苏里格气田南区下奥陶统马家沟组气藏复杂岩溶储层的精细评价

鄂尔多斯盆地苏里格气田南区下奥陶统马家沟组气藏马五5亚段储层侵蚀沟槽及溶蚀潜坑发育,非均质性强,沿用传统的储层评价方法对储层刻画的精度不够,井位部署难度大。为此,以沉积微相研究为基础,应用残厚法、印模法、地球物理法、层拉平技术对古地貌形态进行量化描述,并结合研究区主要成岩作用差异性特征、岩溶作用特征研究成果,评价储层品质,刻画储层平面展布,落实有效储层发育区。研究结果表明:(1)该区处于岩溶高地—岩溶斜坡过渡带,发育古残丘、古坡地、古沟槽等三级地貌单元,其中古残丘为最有利的地貌单元;(2)纵向上马五5亚段白云岩厚度介于10~20 m,但裂缝—孔洞型储层仅发育2~8 m;(3)横向上该区自西向东岩溶作用减弱,充填程度增强,储层物性逐渐变差;(4)马五5亚段储层具有较强的非均质性,平面上具有明显的分带特征,有效储层总体以孤立状分布为主,可划分为多个独立的地质单元。该研究成果有效地指导了该区下古生界碳酸盐岩气藏的效益开发,天然气产能规模达到预期目标,钻遇日产气量逾100×10~4 m~3的气井14口。

Fine evaluation on complex karst gas reservoirs of Lower Ordovician Majiagou Fm in the south zone of the Sulige Gas Field,Ordos Basin

Erosion grooves and dissolution pits exist in Ma55 of the Low Ordovician Majiagou Fm gas reservoir in the south zone of the Sulige Gas Field, Ordos Basin, thus revealing a strong heterogeneity. Traditional reservoir evaluation methods are not accurate in the prediction of Ma55, making well deployment quite difficult. In this paper, the paleo-geomorphology was described quantitatively by using the residual thickness method, impression method, geophysical method and layer flattening technique based on the sedimentary microfacies studies. Then, combined with the study on the diagenesis difference and the characteristics of karstification, the reservoir quality was evaluated, the plane distribution of reservoirs was described, and the development areas of effective reservoirs were confirmed. The following research results were obtained. First, the study area is located in the transition zone between karst high and karst slope. In this area, three-order geomorphic units are developed, including paleo-monadnock, paleo-slope and paleo-groove, among which paleo-monadnock is the most favorable geomorphic unit. Second, the thickness of Ma55 dolomite is vertically 10–20 m, but the thickness of fractured-vuggy reservoirs is only 2–8 m. Third, karstification weakens, filling degree increases and physical properties of reservoirs gradually deteriorate laterally from west to east. Fourth, the Ma55 reservoir is characterized by a strong heterogeneity and obvious plane zoning. The effective reservoirs are generally distributed in an isolated shape and can be divided into several independent geological units. The research results play an effective role in guiding the beneficial development of Lower Paleozoic carbonate gas reservoirs in this study area. As a result, the natural gas productivity reaches the expected target and 14 gas wells realize the daily gas production rate of more than one million cubic meters.