大牛地气田石炭—二叠系海陆过渡沉积体系与近源成藏模式

鄂尔多斯盆地上古生界下部由海向陆演化过程中由下向上形成了3套沉积体系:障壁海岸-潮坪沉积体系、三角洲沉积体系和冲积平原-河流沉积体系,形成相带宽阔、横向分布稳定的陆源碎屑含煤层序,这些沉积体系提供了丰富的烃源岩和储集岩,并且呈大面积同层位共生、或近距离叠置的产状,使得在盆地内部缺乏大规模运移通道的条件下天然气能够聚集成藏。大牛地气田位于鄂尔多斯盆地伊陕斜坡北部,上石炭—下二叠统发育的多层叠合大型岩性圈闭构成气田的基本框架,在具有异常压力而又分布广泛的上二叠统上石盒子组厚层泥质岩的封盖下,表现出近源箱形成藏特征,这是鄂尔多斯盆地内部上古生界的主要成藏方式。

Permo-Carboniferous paralic depositional systems in the Daniudi gas field and its near-source box-type gas accumulation-forming model

In the Ordos basin,three depositional systems formed during the evolution of the lower Upper Paleozoic strata from marine to continental facies; they are in ascending order the barrier coast-tidal flat,delta and alluvial plain-braided river depositional systems,in which there occurs a terrigenous clastic coal-bearing sequence that has wide facies zones and a persistent lateral distribution. These depositional systems provided abundant source rocks and reservoir rocks and occur extensively in the same horizon or overlap each other over short distances; as a result,gas can be accumulated even though the conditions of large-scale migration passageways are lacking in the interior of the basin. The Daniudi gas field is located in the north of the Yishaan slope of the Ordos basin,and its basic framework is a large multi-layered lithologic trap developed in the Upper Carboniferous-Lower Permian. Sealed and covered by the argillite of the widespread Upper Permian Upper Shihezi Formation with unusually high pressures,this trap exhibits the features of near-source box-type accumulation. This is the main Upper Paleozoic accumulation-forming manner in the interior of the Ordos basin.