Well logging evaluation of Triassic Chang 7 Member tight reservoirs, Yanchang Formation, Ordos Basin, NW China

Taking the Triassic Chang 7 Member tight reservoirs of the Yanchang Formation in the Ordos Basin as study object, transverse relaxation simulation was made using nuclear magnetic resonance (NMR) experimental data, CT images and random-walker algorithm to evaluate microscopic pore-throats of the tight reservoirs. The smoothing degree of well logging curves was utilized to evaluate sand structure of the tight reservoirs; and through multi-well analysis and the calibration of well testing data, a grading chart of tight reservoir productivity was established based on sand structure. Reservoir quality was judged and analyzed by pore level: Transverse surface relaxivity ρ2 was obtained by comparing transverse relaxation time T2 of numerical simulation and that of NMR experiment, and the distribution of pore-throat radius was obtained according to the corrected NMR logging curves. The study shows there is an obvious correlation between productivity and sand structure of Chang 7 tight reservoirs. The more homogeneous the distribution of lithology, physical property and oiliness of the massive sand, and the smoother the well logging curves, the more likely the high oil yield will occur. Based on well logging evaluation and grading chart of productivity of tight reservoirs, sweet spots in well area W of Jiyuan Oilfield in Ordos Basin were identified, the comparison of the evaluation results and well testing data shows the accuracy rate is as high as 94.7%. Key words: tight oil, well logging evaluation, microscopic pore-throat distribution, sand structure, sweet spot distribution, Ordos Basin