Acoustic and thermal characterization of oil migration, gas hydrates formation and silica diagenesis

This dissertation presents four processes occurring in marine sediments with distinct signatures in standard geophysical datasets. The diagenetic transformation of Opal-A to Opal-CT, the formation of gas hydrates, fluid substitution in hydrocarbon reservoirs, and fluid migrations in a fault are described with a common approach: identify the specificity of each process and reproduce their ‘distinct’ signature to quantify how it affects the sediments.; In Chapter 1 we describe the changes occurring when dissolved Opal-A re-precipitates into Opal-CT in siliceous sediments. This diagenetic transformation produces regional Bottom Simulating Reflectors (BSRs) in seismic surveys. We use geophysical logs to calculate the elastic moduli of sediments through a BSR offshore New Jersey. Comparison with classic consolidation models shows that the mechanical changes are controlled by a decrease in porosity following the “Gassmann” formulation.; A more common BSR is associated with the occurrence of gas hydrates and underlying free gas. In Chapter 2, logging results from the Blake Ridge show that classic models cannot describe elastic properties of hydrated sediments. The cementation theory describing interactions between grains uniformly coated by hydrates reproduces best the observations. According to this model, Gas hydrates occupy about 10% of the pore space above the BSR and coexist with free gas below.; In Chapter 3, we use various elastic models to interpret changes observed between successive 3D seismic surveys of producing reservoirs. The models are applied to the results of reservoir simulation to translate seismic impedance differences into fluid substitution and bypassed hydrocarbon. This time-lapse methodology is applied to two reservoirs in the Gulf of Mexico.; In the last chapter, we present the thermal regime in the Eugene Island 330 oil field, within an active fault. The temperature distribution reconstructed from 600 Bottom Hole Temperatures displays ∼10°C anomalies overlying salt diapirs and the fault. 3D numerical modeling of heat transfers is used to estimate the contributions of the diapirs and of fluid migrations along the fault. Some of the observed anomalies could result from 5,000 years-long fluid circulations.