鄂尔多斯盆地中部马五_7亚段颗粒滩沉积特征及空间分布

Sedimentary Characteristics and Distribution of Grain Shoals of Ma5_7 Submember in the Central Ordos Basin

鄂尔多斯盆地中部马家沟组地层蕴含丰富的油气资源,以马五7亚段为主要储集层位的盐下碳酸盐岩油气勘探近期取得了重要突破。基于钻井、测录井、岩芯及薄片等资料,对盆地中部马五7亚段颗粒滩沉积特征、纵横向展布规律进行综合分析,进而探讨了研究区沉积古地理格局及其颗粒滩发育控制因素。结果表明:研究区马五7亚段主要由晶粒白云岩及各类型颗粒白云岩构成,纵向上发育潟湖—颗粒滩—灰泥丘—台坪、潟湖—颗粒滩—滩间海—颗粒滩、潟湖—颗粒滩—灰泥丘三种沉积相序,颗粒滩位于向上变浅旋回的中上部,且常与微生物丘构成丘滩复合体建造,具有良好的储集意义。横向上,马五7亚段发育两套较大规模的颗粒滩,层位稳定且连续性较好,整体表现为中西部厚而东部较薄的特征,平面上则呈南北向发育并沿东部凹陷环带状分布。研究区沉积规律表明马五7亚段相对缺乏潮坪相沉积特征,推测其沉积环境更符合局限—蒸发台地。沉积期海平面的频繁变化控制了滩体发育规模及其垂向叠置样式,区内西高东底的古构造格局及其内部的微地貌起伏则决定了马五7颗粒滩平面分布的差异性。

Plentiful oil and gas resources are contained in the Majiagou Formation of Ordovician system in the northeastern Ordos Basin,particully the recent oil and gas exploration has made important breakthroughs in the subsalt of carbonate rocks of Ma57 sub-member as the main reservoir. Based on wells drilling data,logging,drilling cores,thin section and other related information,the Ma57 sub-member of Ordovician system in the central Ordos Basin has been comprehensively analyzed for sedimentary characteristics of the grain banks and their vertical and lateral distribution patterns,in oder to search for the paleogeography for sedimentary patterns as well as the controlling factors on the development of grain banks in the study area. The grain banks of Ma57 sub-member are mainly consisted of fine crystal grain dolostone and other kinds of grain dolostone. Three types of upward sedimentary sequences were developed including lagoon-grain bank-mounds-platform inner flat,lagoon-grain bank-interact bank sea-grain bank and lagoon-grain bank-mounds. The grain bank formed at the upper part with upward shallowing cycle and became mound-shoal complex with microbes for the most time,showing important reservoir significance. In the lateral direction,the Ma57 sub-member had developed two larger grain bank layers with stable location and quite good continuity,being the east thick and midwest thin for the overall performance characteristics,and for the plane performance characteristics on north-south direction and circularly distribution along eastern depression. Further analysis considering the sedimentary regular patterns indicated the Ma57 sub-member was relatively lacked with tidal flat face,thus the suspected restricted-evaporate platform was much more reasonable. The frequently changed sea levels in depositional stage controlled the development scale of grain bank and its placed order,as well as the ancient tectonic framework with lower west and higher east and its inner micro-topography fluctuation might influence the plane distribution of grain bank of Ma57 sub-member.