鄂尔多斯盆地中东部奥陶系盐下侧向供烃成藏特征及勘探潜力

Hydrocarbon accumulation characteristics and exploration potential of Ordovician pre-salt formations by lateral hydrocarbon supply in the centraleastern Ordos Basin

鄂尔多斯盆地中东部奥陶系马家沟组发育厚达近千米的碳酸盐岩与膏盐岩共生的地层组合,埋深偏大。这套远离风化壳的奥陶系盐下层系能否具备烃源有效供给及规模成藏,是制约勘探的关键问题。基于奥陶系沉积期后构造演化及其与上古生界煤系烃源层配置关系的研究,认为奥陶系盐下在邻近古隆起区存在与上古生界煤系烃源层直接接触且规模性分布的供烃窗口,窗口区在生排烃高峰期处于构造下倾部位,生烃增压等因素产生运移动力,有利于天然气向高部位运聚;膏盐岩封盖层与白云岩储集体横向连续稳定分布构成良好的储盖组合;盆地中东部奥陶系盐下具有规模成藏的潜力,乌审旗—靖边—延安一带为有利勘探区,值得勘探重视。

The stratigraphic combination of carbonate rocks and gypsum rocks is developed in the Ordovician Majiagou Formation in the centraleastern Ordos Basin with nearly 1000 m thick and relatively large burial depth.The key issue restricting exploration is whether there is effective hydrocarbon supply and large-scale hydrocarbon accumulation in the Ordovician pre-salt strata which is far away from the weathering crust.Based on researches on the structural evolution in later sedimentary period of Ordovician and the allocation relation between the Ordovician strata and the coalbearing source rocks in the upper Paleozoic,it is considered that there is a hydrocarbon supply window for the Ordovician pre-salt strata in the areas adjacent to the paleo-uplift,which is in direct contact with the coal-bearing source rocks of the upper Paleozoic and distributed on a large scale.The window zone was in the downdip structural position during the peak of hydrocarbon generation and expulsion.Favorable factors such as hydrocarbon generation and pressurization provide migration force,which is conducive to the migration and accumulation of natural gas towards high positions.A good reservoir-cap assemblage is formed by continuous and stable lateral distribution of gypsum cap rocks and dolomite reservoirs.There is potential for large-scale hydrocarbon accumulation in the Ordovician pre-salt formations in the central-eastern part of the basin.The area of Wushenqi-Jingbian-Yan'an is favorable for exploration,which should be paid more attention.