| Computer simulations of both depositional processes and hydrocarbon migration were used to quantify the effects of lithologic heterogeneity on hydrocarbon migration in sedimentary rocks. The depositional simulations were carried out with a program called SEDSIM that generates a three-dimensional spatial distribution of clastic sediments produced in response to topographic gradients, fluid and sediment discharge of inflowing streams, tectonic subsidence, and sealevel changes.; The resulting statistical distributions of grain sizes are then transformed to their corresponding petrophysical flow properties, including porosity, permeability, capillary pressure, and relative permeability. A test case involved sandstone deposits of the Woodbine Formation of East Texas in which simulations were compared with seismic sections and well logs. The results suggest that porosity and permeability in the Woodbine sandstones depend on their primary depositional characteristics even after compaction and cementation.; To represent oil migration, I developed a three-dimensional two-phase flow simulator called MIGRAT that couples the movement of oil and water with compaction according to Terzaghi's principle of effective stress. Use of MIGRAT involves two steps: First, carrier and reservoir beds are simulated with SEDSIM, and then the rate of sediment load that affects the carrier and reservoir beds is estimated by backstripping methods. The loading beds cause compaction and pore-water expulsion to occur in carrier and reservoir beds, providing the setting in which oil migration occurs.; As a test case, SEDSIM and MIGRAT were used to simulate hydrocarbon migration in the Quaternary Tulare Formation at South Belridge oil field, California, where variations in sediment grain size, permeability, and oil saturation are strongly interdependent. Beds that form the fluvio-deltaic component of the Tulare Formation were simulated and oil was then allowed to migrate within them. Simulated permeabilities and oil saturations were compared with actual data, revealing that oil migrates first into channel deposits that have the lowest entry pressures, thereby producing the highest oil saturations. However, saturation decreases rapidly as grain sizes decrease, suggesting that a continuum exists between beds that serve as carrier and reservoir, and beds that serve as seals. |
