鄂尔多斯盆地中东部上古生界致密气成藏条件差异性分析

Analysis on differences of tight gas accumulation conditions of Upper Paleozoic in central and eastern Ordos Basin

以鄂尔多斯盆地中部苏里格气田和东部临兴气田为研究区,从烃源岩条件、储层条件、盖层条件及源储组合等方面分析二者成藏条件的差异性。结果表明:1)烃源岩条件方面,苏里格气田和临兴气田干酪根以腐殖型为主,中部的总有机碳质量分数略高于东部。由于晋西挠褶带的隆升和紫金山岩体的侵入,临兴气田内局部位置有机质达到过成熟,发育少量混合型干酪根;2)储层条件方面,临兴气田上古生界自下而上发育多套储层,而苏里格气田主力储层较为集中,仅为山西组和下石盒子组,储层物性较临兴气田好;3)盖层条件方面,苏里格气田主要由上石盒子组泥岩的剩余压力对下伏储层形成有效封盖,而临兴气田由于盖层受断层破坏,天然气可垂向运移;4)源储组合方面,临兴气田发育源内、近源和远源3种类型成藏组合,而苏里格气田仅发育源内和近源组合。这些成藏条件差异的主要原因是受晋西挠褶带的影响,盆地东部断层和微裂缝较为发育,可沟通下部煤层烃源岩,气源通过断裂-裂缝复合体向上运移,在浅层形成远源成藏组合。

Taking Sulige gas field in central Ordos Basin and Linxing gas field in eastern Ordos Basin as study areas,the differences of accumulation conditions between the two gas fields are analyzed from source rock conditions,reservoir conditions,cap rock conditions and source-reservoir combination.The following four conclusions are drawn.(1)In terms of source rock conditions,the kerogen of Sulige and Linxing gas fields is mainly humic type,and the content of organic matter in central basin is slightly higher than that in eastern basin.Due to the tectonic uplift of flexural fold belt in western Shanxi and the intrusion of Zijinshan pluton,the organic matter in local area of Linxing gas field has reached over maturity,and a small amount of mixed kerogen is developed.(2)In terms of reservoir conditions,several sets of reservoirs are developed from bottom to top in the Upper Paleozoic of Linxing gas field,while the main reservoirs in Sulige gas field are only concentrated in Shanxi Formation and Lower Shihezi Formation.The reservoir property of Sulige gas field is better than Linxing gas field.(3)In terms of cap rock conditions,the residual pressure of mudstone in Upper Shihezi Formation effectively seals the underlying reservoir in Sulige gas field,while the gas in Linxing gas field can migrate vertically because the cap rock is damaged by faults.(4)In terms of source-reservoir combination,Linxing gas field has three types of combination:in-source,near-source,and far-source,while Sulige gas field only has in-source and near-source combination.The differences of these accumulation conditions are mainly due to the influence of flexural fold belt in western Shanxi.Faults and micro-fractures are well developed in the eastern basin,which can communicate with the source rock of the lower coal seam.The gas source migrates upward through the faults and micro-fractures,and forms far-source combination in the shallow layer.