Abstract
The Eocene low-permeability sandstone reservoirs in the Dongying Depression, Bohai Bay Basin, China host a significant amount of oil reserves. The development of the reservoirs has been hampered by our inability to understand the complex and heterogeneous pore structures of the reservoirs. In this study, the pore systems, pore sizes, pore connectivity, and movable fluid distribution of the Eocene Shahejie Formation (Es4) sandstone reservoirs were investigated using an integrated analysis of optical and scanning electron microscopy (SEM), mercury injection capillary pressure (MICP), and nuclear magnetic resonance (NMR). The full-range pore structures of the Es4 sandstone reservoirs were evaluated by using NMR experiments. Various NMR T2 spectral parameters suitable for describing the pore structures and movable fluid distribution were extracted through morphological and statistical analysis of NMR T2 spectra. In combination with corresponding MICP data and petrophysical properties, we have demonstrated the reliability and robustness of the T2 spectral parameters for pore structure characterization. Four types of pore structures (I, II, III, and IV) were distinguished from the NMR T2 spectral parameters in association with other petrophysical properties and macroscopic behaviors. We have demonstrated the effectiveness of using the NMR T2 spectral parameters to characterize and classify micropore structures, which may be applied to effectively evaluate and predict low-permeability reservoir quality.
Highlights
The Eocene low-permeability sandstone reservoir is a major petroleum source in the Dongying Depression, Bohai Bay Basin, China [1,2,3]
We investigated the reliability of using nuclear magnetic resonance (NMR) T2 spectral parameters to approximate the pore size distribution by using the sorting coefficient (σ) and skewness (Skp )
Various quantitative T2 spectral parameters derived from NMR experiments were employed to characterize and evaluate the pore structures of the Eocene sandstone reservoirs in the Dongying Depression, Bohai
Summary
The Eocene low-permeability sandstone reservoir is a major petroleum source in the Dongying Depression, Bohai Bay Basin, China [1,2,3]. Previous studies mainly invoked indirect analysis of NMR data, including qualitative characterization of pore size distribution [30,37], simulation of capillary pressure curves [38,39], and estimation of movable fluid distribution in pore space [40,41]. We attempt to shed light on the application of the NMR T2 spectral parameters in pore structure evaluation. The second section presents the geological background, sampling information, and methodology; The third section investigated the petrophysical properties, pore structures, and effective movable fluid in pore space using an integrated method. The final section discusses the effectiveness of the NMR T2 spectral parameters in pore structure evaluation by establishing a reservoir quality classification scheme for the Eocene sandstones
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