致密砂岩储集层微观结构特征及成因分析——以鄂尔多斯盆地陇东地区长6段和长8段为例

Micro pore and throat characteristics and origin of tight sandstone reservoirs: A case study of the Triassic Chang 6 and Chang 8 members in Longdong area, Ordos Basin, NW China

通过铸体薄片、扫描电镜、X射线衍射、恒速压汞等方法,对比了鄂尔多斯盆地陇东地区上三叠统长6段、长8段致密储集层微观结构的差异,建立了相应的孔隙演化模式,探讨了主要成岩作用对不同储集层致密化的控制。研究结果表明,喉道是控制长6段、长8段储集层物性的主要因素;储集层渗透率越低,喉道半径越小、分布越集中、占有效储集空间比例越大。明确长6段和长8段储集层差异:(1)随渗透率增大,长8段较长6段储集层中相对大喉道对渗透率的影响作用明显增大;(2)长6段储集层中纳米级喉道与孔喉半径比的相关性更强。沉积作用决定储集层原生孔隙结构,成岩作用是储集层致密的主控因素。受长6段、长8段储集层岩石组构和绿泥石含量差异的影响,压实作用使埋深和地温更大的长81亚段储集层减孔量(15%)小于长63亚段储集层(17%);而硅质、钙质及黏土矿物胶结充填孔隙、堵塞孔喉是造成长6段、长8段储集层渗透率差异较大的关键。

The microstructure differences of the Triassic Chang6and Chang8members tight reservoirs in the Longdong area of Ordos Basin were compared by means of cast thin sections,scanning electron microscope,X-ray diffraction,and constant rate mercury injection.Their pore evolution models were established,and the effects of main diagenesis on densification were examined.The throat is the main factor controlling the physical properties of the Chang6and Chang8members reservoirs:The lower the permeability,the smaller and the more concentrated the throat radius and the larger the proportion of the throats in the effective storage space.There are several obvious differences between Chang6and Chang8members:(1)with the increase of permeability,the contribution of the relative large throats to the permeability in the Chang8member reservoir is more than that in the Chang6member reservoir;(2)the control effect on pore-throat ratio of the nano-throats in the Chang6member reservoir is more significant.The sedimentary action determines the primary pore structure of the Chang6and Chang8members sand bodies,and the diagenesis is the main factor controlling the densification of the reservoirs.Because of the difference in rock fabrics and the chlorite content of Chang6and Chang8,the strong compaction resulted in less porosity reduction(17%)of the Chang81reservoir with larger buried depth and larger ground temperature than the Chang63reservoir(19%).The siliceous,calcareous and clay minerals cement filling the pores and blocking the pore throat,which is the key factor causing the big differences between the reservoir permeability of Chang6and Chang8members.