利用NMR与SIP研究湘西北下寒武统页岩孔隙特征

Pore characteristics of the Lower Cambrian shale in northwestern Hunan Province studied using NMR and SIP

湘西北地区牛蹄塘组发育了一套富有机质优质页岩,但对其岩石物理特性研究较少。以HY1井与CY1井页岩为研究对象,通过低频核磁共振、频谱激发极化实验,结合矿物分析、扫描电镜观察等手段,对下寒武统页岩的孔隙特征和岩石电学特征进行了研究。结果表明,HY1井页岩T_(2)呈不连续多峰分布,有明显波谷,以微孔与中孔为主,连通性较差;CY1井页岩T_(2)呈连续多峰分布,无明显波谷,孔隙类型多样,大孔广泛发育且连通性较好。核磁共振(NMR)与频谱激发极化实验(SIP)测试发现,下寒武统页岩没有明显的相移谱峰值,电导率实部与NMR孔隙度、渗透率都没有明显的相关性。在HY1井页岩中,电导率虚部表现出较为集中的分布特征,CY1井页岩电导率虚部表现出较为分散的分布特征,表明两口井页岩孔隙结构相似,但孔隙复杂程度不同。在建立SIP与NMR响应参数相关性时,发现常规电导率虚部与T_(2ML)计算的比表面积间没有明显相关性,而电导率虚部与地层因子的乘积与比表面积有很好的相关性,表明研究区页岩具有复杂的孔隙特征和强烈的各向异性,而在下寒武统地层,地层因子在改变页岩电导率的过程中起着至关重要的调节作用。

A group of organic-rich shale has been discovered in the Niutitang Formation in northwestern Hunan Province,but there are few studies on the petrophysical characteristics of the Lower Cambrian shale in the area.Using the shale samples of the HY1 well and CY1 well as the research objects in this study,the pore characteristics and rock electrical characteristics of the Lower Cambrian shale were investigated using low-frequency nuclear magnetic resonance(NMR),spectrum induced polarization(SIP)experiments,mineral analysis,scanning electron microscope observation,etc.The results showed that various pore types are developed in the Lower Cambrian shale,mainly micropores and mesopores and also macropores and micro-fractures.The distribution of T_(2) shale in HY1 well is discontinuous and multi peaked with obvious trough,mainly microporous and mesoporous,and poor connectivity.There is a discontinuity between the peaks,indicating that the shale of HY1 well has mainly micropores and mesopores,and macropores are less developed with poor connectivity.The T_(2) distribution of CY1 well shale shows a bimodal distribution;the left peak distribution is dispersed,and the peaks are continuous.There is no obvious boundary,indicating that the CY1 well shale has various pore types;macropores are well developed and with good pore connectivity.Combined NMR and SIP tests showed no obvious phase shift spectrum peak in the Lower Cambrian shale.The real part of the conductivity and NMR porosity and permeability show relatively discrete distribution characteristics.The imaginary part of the conductivity in the shale samples of the well HY1 showed a more concentrated distribution,whereas the imaginary part of the conductivity of the CY1 well shale sample showed a more dispersed distribution,indicating that the pore structure of HY1 well is similar to that of CY1 well,but the pore complexity is different.The result between SIP and NMR response parameters revealed no significant correlation between the imaginary part of the conductivity and the specific surface area calculated by T_(2ML),and the product of the imaginary part of the conductivity and the formation resistivity factor showed a good correlation with the specific surface area.This suggests that the shale has complex pore characteristics and strong anisotropy;however,in the Lower Cambrian strata,the formation resistivity factor plays a crucial role in regulating the shale conductivity.