Low-porosity highly fractured carbonate rocks in the deep Ordovician Formation in the Langgu depression of China remain active exploration targets. However, the seismic characterization of fractured reservoirs has been a challenging problem for decades. By integrating logs with seismic data acquired from the Huabei Oilfield in the Langgu depression, we improved a rock-physics-based method of estimating fractured zones using post-stack seismic data combined with stress field analysis. Using an FMI image log, porosity, density as well as sonic logs, fractured zones and heterogeneous rock types are classified. Two reservoir types are identified as having different elastic properties: the high-porosity zone (ф>6.5%) and the fractured zone (ф<6.5%). Further, by using a pore structure parameter γ from a rock physics model, it has been found that acoustic impedance correlates well with both porosity and this pore structure parameter, for the oil reservoir of the ultra-deep carbonate buried-hill of the Ordovician Age. Fractured zones in the studied reservoirs have a signature of acoustic impedance lower than 16500 (g/cc*m/s) and a pore structure parameter higher than 14. Quantitative geological interpretation further indicates that fractured zones are mostly located along the faults whereas high-porosity zones are distributed along the unconformity surface formed by dominating diagenetic processes.
- Sun, Yuefeng Mollie B. & Richard A. Williford Professor