鄂尔多斯盆地东南部上古生界层序地层格架及含气砂体沉积演化特征

Sequence stratigraphic framework and sedimentary evolution characteristics of gas-bearing sandbody in the Upper Paleozoic in southeastern Ordos Basin

鄂尔多斯盆地长期存在"南油北气"的固有认识,晚古生代早期砂体不发育与分布规律不清的认识,长期制约了盆地东南部天然气勘探开发的步伐。因此,加强盆地东南部上古生界煤系含气地层地质方面的研究、尤其是层序地层与储集砂体成因研究,对进一步增储上产具有重要理论与实践意义。作者采用Vail经典层序地层理论与方法,对鄂尔多斯盆地东南部上古生界进行层序地层研究:首先对层序界面进行识别,以岩性、旋回和沉积特征变化作为识别依据,并提出二级层序界面上下岩性组合变化巨大、二级界面之上叠置的低位砂厚度巨大的观点,共将本溪组—石盒子组划分为3个二级层序,11个三级层序;其次根据层序内部砂体堆积、迁移样式对体系域进行了划分。通过大量钻井的层序单元追踪对比与岩心精细描述,重点分析层序格架内主要含气层段本溪组、山西组及盒8段储集砂体的空间叠置关系,结合不同时期基准面旋回变化及其形成机制对其进行探讨,认为优势含气层段储集层中本2段具有障壁迁移、毯式连通,山2段具有低位控砂、面线连通,山1段具有侧向迁移、接触式连通,盒8段具有迁摆叠置、多向连通的特点。

The traditional idea of hydrocarbon distribution ＂oil exists in south and gas in north＂ of Ordos Basin prevailed for a long time,which is contradiction with the latest study.The recognition on lackof sandbodies in the Upper Paleozoic and weak geological research had been constrained the exploration and exploitation in the southeastern Ordos Basin.Therefore,strengthening geological research on the Benxi Formation to Xiashihezi Formation（coal-bearing strata of the Upper Paleozoic）,especially the study of sequence stratigraphy and reservoir sandbody genesis has an important theoretical and practical significance.This paper study sequence stratigraphy of the Upper Paleozoic in the southeastern Ordos based on the theory and method of Vail＇s classical sedimentary sequence theory.First of all,identifying the sequence boundary based on lithologic characteristics,cycle,and change of depositional characteristics.This paper pointed out that the upper and lower lithological assemblages of the secondary sequence boundary are greatly changed,and the lowstand sandbodies thickness on the secondary boundary is significant.The Benxi Formation to Shihezi Formation was divided into 3 two-order sequence and 11 three-order sequence.Secondly,system tracts in sequence were also divided according different stacking patterns and different migration style.The spatial superposition relationship of the sandbody in the Benxi Formation,Shanxi Formation and the Member 8 of Xiashihezi Formation in the sequence stratigraphic framework have been analyzed by means of the sequence of large-scale well logs and the detailed description of the core.The formation mechanism of the time-transgressive base level cycle is discussed considering that Ben-2 Zone was ＂barrier migration,blanket connectivity＂,Shan-2 Zone was a ＂lowstand controlling sand,surface-line connectivity＂features,Shan-1 Zone was ＂lateral migration,contact connection ＂,He-8 Zone has a ＂migration-swings stack,multi-directional connectivity＂features on the gas-bearing strata of the Upper Paleozoic in the southeastern Ordos Basin.