鄂尔多斯盆地南部延长组致密砂岩储层特征及其成因

Characters and causes of tight sandstones of Yanchang Formation in southern Ordos Basin,China

鄂尔多斯盆地南部延长组含有非常丰富的油气资源,油藏具有低孔、低渗、低压的特征,为典型的致密砂岩油藏。通过大量的铸体薄片、物性测试、扫描电镜分析等,对三叠系延长组致密砂岩储层特征及形成机理进行深入讨论。认为延长组致密砂岩具有非常复杂的孔隙结构,大量微孔的存在使致密砂岩储层具有非常高的束缚水饱和度;高排驱压力使油气充注时阻力过大,油气仅充注在致密砂岩背景中的优质储层。延长组砂岩储层致密的最主要因素是强压实,强胶结,溶蚀作用相对较弱。绿泥石环边的出现可以减缓上覆地层的压实作用,而相对较少的塑性颗粒可以降低因压实作用而导致的塑性颗粒变形。后期溶蚀作用的强弱与溶蚀前储层的储集物性和胶结物成分有很大的关系,砂岩储层在溶蚀前孔隙较发育,则后期的溶蚀作用愈彻底;浊沸石胶结越发育,后期的溶蚀愈强烈。延长组优质储层的发育往往与弱压实、强溶蚀和裂缝发育相伴生。

There are plenty of petroleum resources in the tight sandstone reservoirs of the Upper Triassic Yanchang Formation. The reservoirs are characterized by low porosity, permeability and pressure, and are the typical oil pools of tight sandstones in China. The authors analyze the characters and formation mechanism of the tight sandstone reservoirs by thin section, petrophysics and SEM. The result shows that the tight sandstones have complex pore construction and high irreducible water saturation caused by many micropores. The fine reservoirs are rich in petroleum because the tight sandstones have high capillary pressures. The main causes of forming tight sandstones are the strong compaction and cementation. Solution of organic acid is late because of the low temperature. So, the solution is weak. Chlorite rims can retard the compaction. Similarly, little plastic particles can also decrease their shape change caused by compaction. The later solution is determined by the former petrophysies and cementation. If pores are well-developed, the solution will develop as well. Similarly, if the laumonite cementation is strong, the later solution will more strong. The weak compaction, strong solution and fractures are the keys for the developing of high quality reservoirs.