鄂尔多斯盆地上古生界深盆气藏研究

鄂尔多斯盆地存在前缘坳陷和大型斜坡组成的深盆地结构。上古生界发育海陆交互相和陆相砂泥岩生储组合，大型三角洲体系形成了广泛发育的致密砂岩储集层。煤系地层形成了深盆气藏的主要气源岩，范围广、丰度高的煤系有机质除东北部处于低成熟外，盆地中普遍进入成熟─高成熟演化阶段。深盆气的生成─运移─聚集从三叠纪持续到晚白垩世，其后进入保存期。上古生界形成了一个几乎覆盖全区的特大型深盆气藏，地层压力以异常低压为主，气水分布明显呈现南气北水下气上水的特征，气层分布不受构造控制，预测地质储量为１． ４７ × １０１２～ １０． ５ × １０１２ｍ３。

Deep basin gas trap in the Upper Paleozoic of Ordos basin

The giant gas accumulations in Ordos basin found in low-permeability Paleozoic sandstones provides some of the best available information on basic characteristics of deep basin gas traps. The deep basin gas reservoir mainly consists of deltaic sandstone with varying amount of coarser sandstone and conglomerate. The numerous and mature coalbeds and organic-rich shales associated intimately with reservoir rocks throughout the region provide a prerequisite for deep basin gas accumulation. Gas generated in adjacent source rocks, moved vertically into the sand layers and then slowly migrated updip. This process lasted from late Triassic to late Cretaceous. This special form of deep basin gas trap defies conventional concepts of gas entrapment by turning them virtually upside down. Gas is trapped in the deeper part of the basin and is located in the downdip of an aquifer which outcrops mainly in northeast Ordos. The gas/water contact is really a wide belt, which marks the transition from downdip gas to updip water. Original gas accumulation pressures lie below the regional formation water pressure gradient. Gas-productive units in the deep basin are not controlled by local structure. An estimated 1.47×1012 to 10.5 ×1012m3 of gas in place are contained in the 15,000 km2 Upper Paleozoic gas accumulation.