陇东地区三叠系长7段页岩油储层孔隙结构特征及成因机制

Pore structure characteristics and genetic mechanism of Triassic Chang 7 shale oil reservoir in Longdong area

通过高压压汞、低温氮气吸附和核磁共振等实验,表征了鄂尔多斯盆地陇东地区三叠系长7段页岩油储层的孔隙结构特征,并探讨了成岩作用对孔隙结构的影响。研究结果表明:(1)陇东地区三叠系长7段页岩油储层主要为长石岩屑砂岩和岩屑长石砂岩,孔隙度平均为7.55%,渗透率平均为0.149 mD,属于典型的特低孔、超低渗储层,储集空间类型以溶蚀孔、残余粒间孔、黏土晶间孔为主,并发育少量微裂缝;(2)储层整体排驱压力较高,为0.67~27.54 MPa,平均为5.54 MPa,最大进汞饱和度较低,为33.22%~84.35%,平均为64.71%,且平均喉道半径小,为0.008~0.163μm,平均为0.277μm,表现出强非均质性特征;(3)研究区典型样品的等温线形态均呈反“S”型,属于典型的Ⅳ型等温线,易出现H3型“滞回环”现象,孔径小于200 nm的孔隙以平行板状为主;(4)研究区长7段页岩油储层的孔径大多小于30μm,且随着样品物性的变好,储层内较大尺寸孔隙(大于200 nm)的占比逐渐增多;(5)研究区长7段页岩油储层经历了较强压实、较弱胶结与较弱溶蚀等3种成岩作用,其中胶结作用和压实作用具有负面影响,溶蚀作用对储层品质和孔隙结构的改善具有建设性作用。

The pore structure characteristics of Chang 7 shale oil reservoir were characterized by high pressure mercury injection(HPMI),low-temperature nitrogen adsorption(LTNA),and nuclear magnetic resonance(NMR),and the effects of dominant diagenesis on pore structure were discussed.The results show that:(1)The shale oil reservoir of Triassic Chang 7 member in Longdong area is mainly feldspathic lithic sandstone and lithic arkose,with an average porosity of 7.55% and an average permeability of 0.149 m D,belonging to typical ultra-low porosity and ultra-low permeability reservoir.The reservoir space types are mainly dissolved pores,residual intergranular pores and clay intercrystalline pores,and a small amount of micro fractures are developed.(2)The reservoir is characterized by high displacement pressure,ranging from 0.67 MPa to 27.54 MPa,with an average of 5.54 MPa low maximum mercury saturation,ranging from 33.22% to 84.35%,with an average of 64.71%,and small average throat radius,ranging from 0.008 μm to 0.163 μm,with an average of 0.277 μm.,showing strong heterogeneity.(3)The isotherms of the typical samples in the study area are all in an inverse S shape,belonging to a typical type Ⅳ isotherm.The type of hysteresis loop is dominated by type H3,indicating that the shape of pores less than200 nm is mainly parallel plate shape.(4)The pore diameter of Chang 7 shale oil reservoir in the study area is mostly less than 30 μm.With the improvement of physical properties of samples,the proportion of larger pores(greater than 200 nm)in the reservoir increases gradually.(5)The shale oil reservoir of Chang 7 member in the study area has experienced three types of diagenesis:strong compaction,weak cementation and weak dissolution.Cementation and compaction have a negative effect,and dissolution has a constructive effect on the improvement of reservoir quality and pore structure.