高黏土粉砂岩夹层的压汞−核磁联合评价与甜点识别——以古龙凹陷青一段PⅡ井为例OA

[Objective]Continental interlayer-type shale oil in China has become a critical strategic resource for increasing reserves and production.High-clay siltstone interlayers in the First Member of Qingshankou Formation(Qing-1 Member)of Gulong Depression,Songliao Basin,are important carriers of shale-oil micro-migration and local accumulation.Nevertheless,controlled jointly by clay minerals,fine throats,and heterogeneity in pore-throat connectivity,high-clay siltstone interlayers within shale-oil sequences commonly show distinct oil-bearing characteristics but poor fluid mobility.Existing studies lack quantitative coupling relationships between pore-throat parameters derived from high-pressure mercury intrusion porosimetry(MIP)and fluid-mobility indicators obtained from nuclear magnetic resonance(NMR)for such high-clay interlayers,and sweet spot classification still largely relies on empirical judgment.This study targets high-clay siltstone interlayers of the Qing-1 Member in the Gulong Depression,Songliao Basin.Under the constraints of the same well and identical lithofacies,this study aims to clarify the intrinsic relationship between pore-throat structure and fluid mobility,establish a quantitative sweet spot identification criterion,and mitigate misjudgment risks caused by merely using oil-bearing indicators for reservoir evaluation.[Methods]Ten groups of paired core samples from Well PⅡ were collected,and the mineral difference index(MDI)≤15%was used to ensure mineralogical comparability between MIP and NMR subsamples.Integrated laboratory tests,including high-pressure mercury intrusion porosimetry,two-dimensional NMR under original oil-bearing conditions,and centrifugal NMR after oil washing and simulated formation-water saturation,were conducted to obtain key parameters such as median pore-throat radius r50,sorting coefficient σ,mercury withdrawal efficiency,transverse relaxation time T2 cutoff value,movable-fluid saturation SMF,and saturated-state geometric mean of T2.A composite movable-oil index(CMOI=So×SMF/100)was constructed to jointly characterize the contributions of both oil-bearing characteristics and fluid mobility.Based on micro-scale computed tomography(CT)scanning and scanning electron microscopy(SEM)observations,microscopic mechanisms of throat narrowing and pore-throat connectivity impairment induced by clay film coating and bridging infill were revealed to explain the statistical patterns observed in the macroscopic experiments.[Results]For high-clay siltstone interlayers,pore-throat parameters from MIP showed significant correlations with NMR mobility indicators.With increasing clay content,r50 decreased,while SMF,CMOI,and the saturated-state geometric mean of T2 decreased simultaneously.Affected by clay-bound water,the T2 cutoff value was not suitable for cross-sample comparison.Instead,the saturated-state geometric mean of T2 was more suitable for characterizing differences in sample mobility.Based on the analysis of well-interval samples,a empirical sweet spot identification combination was proposed:r50>0.4 μm,SMF>40%and CMOI≥20%.Four evaluation zones(Type A-D)were further defined on the SMF-r50 cross-plot.The samples occurred only in types A,B,and D,with no Type-C samples observed.Type-B samples were characterized by large pore-throat sizes but limited mobility,reflecting the reservoir characteristics of"large pore-throat size does not guarantee high mobility".[Conclusion]Constrained by the sample size and sampling scope,the proposed identification criteria are only applicable to the same-lithofacies interval of the Qing-1 Member within Well PⅡ.Further sample expansion and verification are required for threshold calibration.The joint discrimination scheme combining CMOI and SMF-r50 can effectively distinguish oil-bearing reservoirs with limited mobility,and it can be applied to relative ranking and sweet spot classification of analogous siltstone interlayers within this well interval.