3-D Geopressure Analysis

Abstract

Abstract In clastic sedimentary basins, geopressure is controlled by depositional and diagenetic history as well as structural geometry. The shapes and sizes of pressure compartments depend on the 3-D structure and distribution of lithologies and faults. We developed 3-D geopressure analysis algorithms and methodology which enable interpreters to estimate, analyze, and visualize geopressure in 3-D. Potential benefits include: (1) more accurate pre-drill pore pressure prediction; (2) 3-D definition of pressure compartments; (3) enhanced interpretation of fault seal or leak from pore pressure distribution; (4) enhanced interpretation of fluid migration pathways. Careful velocity analysis after pre-processing of the 3-D pre-stack seismic data produces 3-D RMS velocities at every in-line and cross-line position through a volume. Further lateral smoothing removes velocity errors caused by near surface and acquisition noises. An interval velocity volume is generated from the smoothed RMS velocity volume. Calibration using offset wells is used to transform 3D seismic velocity into 3D pore pressure gradients. In addition, we have also developed an algorithm to generate minimum horizontal stress. Using difference of the minimum horizontal stress and pore pressure, we can calculate an effective stress volume. This is extremely important for casing design and evaluating seal integrity of the prospect. We have generated and analyzed several 3-D geopressure cases in deep water Gulf of Mexico and South China Sea. Relationship between fluid pressure, faults and geometry can be visualized and interpreted inseveral ways: (1) a pressure volume may be combined with amplitude slices - this is best for understanding and visualizing the influence of subtle structures; (2) a pressure volume may be combined with interpreted fault surfaces to visualize the effects of major faults; (3) Interpreted fault slices may be combined with seismic geopressure lines. Pressure volumes are best viewed as part transparent, part translucent, and part opaque.