Abstract:

Based on the high-precision petrophysical experiments and the data acquired by new logging techniques, a logging-based characterization was conducted on three key parameters (lithofacies, effective porosity, and movable oil porosity) of shale oil reservoirs in the Fengcheng formation of Mahu sag, Junggar Basin. The following results are obtained. First, according to the core characteristics, FMI images, pore types, and mineral contents, the Fengcheng shale oil reservoirs are divided into felsic, dolomitic/calcareous, and clayey mixed lithofacies. The diamictite index and micro-pore index are constructed to identify lithofacies. Second, a tight reservoir analysis (TRA) experiment is performed to measure porosity of rock samples, confirming that the felsic lithofacies presents the best physical properties. Using TRA experiment results to calibrate the NMR logging data, an effective porosity characterization model with variable T₂ cutoff of different lithofacies is established. Third, through comparison of multi-state T₁-T₂ NMR experiments, and considering the NMR logging responses, the positions of movable oil, bound oil, capillary bound water, asphalt and clay bound water of three lithofacies are revealed, and the shale oil occurrence identification chart is formed. According to the characterization results of lithofacies, effective porosity and movable oil porosity, together with the fluid production profiles of key wells, it is clarified that the interbedding of felsic lithofacies and clayey mixed lithofacies forms a favorable lithofacies combination in the Fengcheng formation. The research provides technical support for the experimental analysis of other continental shale oil reservoirs in China, and contributes a reference for shale oil reservoir evaluation.

Key words: Mahu sag, Fengcheng formation, shale oil reservoir, lithofacies, NMR, effective porosity, movable oil