特低渗砂岩储集层孔隙结构差异与低电阻率油层成因——以鄂尔多斯盆地中部烟雾峁区块为例

Pore structure differences of the extra-low permeability sandstone reservoirs and the causes of low resistivity oil layers: A case study of Block Yanwumao in the middle of Ordos Basin, NW China

以鄂尔多斯盆地中部烟雾峁区块三叠系延长组长6油层组为例,通过岩心观察、实验分析、地质研究、测井解释、试采验证等,探讨孔隙结构差异对储集层及油层岩石电学特征的影响,论证并揭示了孔隙结构差异与低电阻率油层的成因关系。区内长6油层具有正常电阻率油层和低电阻率油层两种表现形式,发育孔隙型单孔介质和微裂缝-孔隙型双孔介质两种结构类型;微裂缝的发育极大地改变了储集层的微观孔隙结构,孔隙结构差异则对特低渗砂岩储集层及其油层的岩石电学特征产生了重要影响;正常电阻率油层具有明显的孔隙型单孔介质特征,主要集中于长61亚油层组和长622、长623小层,低电阻率油层具有明显的微裂缝-孔隙型双孔介质特征,主要集中于长621小层和长63亚油层组。钻井液沿微裂缝对油层形成了超深侵入,导致电阻率大幅降低及油层的低电阻率现象,低电阻率油层比正常电阻率油层具有更好的储产能力。

The influence of pore structure difference on rock electrical characteristics of reservoir and oil reservoir was analyzed taking Triassic Chang6reservoir in Block Yanwumao in the middle of Ordos Basin as an example.The relationship between the pore structure difference and the low resistivity oil layer was revealed and demonstrated through core observation,lab experiments,geological research,well log interpretation and trial production etc.The results show that there were two kinds of oil layers in Chang6Member,normal oil layer and low resistivity oil layer in the region,corresponding to two types of pore structures,pore type mono-medium and micro-fracture?pore type double-medium;the development of micro-fracture changed greatly the micro-pore structure of the reservoir,and the pore structure difference had an important influence on the rock electrical characteristics of the extra-low permeability sandstone reservoir and oil reservoir;the normal oil layers had obvious characteristics of pore-type mono-medium,and were concentrated in Chang61,Chang622and Chang623;the low resistivity oil layers had obvious characteristics of micro-fracture?pore type double-medium,which were mainly distributed in Chang621and Chang63.The mud filtrate penetrated deep into the oil layers along the micro-cracks,leading to sharp reduction of resistivity,and thus low resistivity of the oil layer;the low resistivity oil layers had better storage capacity and higher productivity than the normal oil layers.