From the time they settle on the seafloor, marine sediments are the locus of active processes involving the fluids present in the pore space. Most of these processes can have distinct signatures detectable by standard exploration techniques. The dissolution/re-precipitation of Opal-A (biogenic silica) into Opal-CT (porcellanite) is at the origin of time transgressive Bottom Simulating Reflectors (BSR) in seismic surveys of marine sediments across the world. The dissociation of gas hydrates at the base of the gas-hydrates stability zone has been associated with reverse polarity BSR. The presence of hydrocarbons in reservoirs can be identified by high negative-amplitude events. The changes occurring in a reservoir because of production can be identified by 4D, or time-lapse, seismics. The longer term migrations that feed shallow reservoirs in active oil field are too slow for such characterization, but can be identified by their influence on the temperature distribution.

This dissertation presents techniques to characterize and quantify these processes with a common approach: 1) identify the specificity of each process through the comparison with accepted models describing ënormalí, passive attributes, and, if necessary, 2) the numerical modeling of the ëanomalousí or, more appropriately, ëdistinctí signature in order to quantify how it affects the sediments. Under this general methodology, the techniques used are specific to each characterization.

In Chapter 1, we describe the elastic properties of marine calcareous sediments across a siliceous diagenetic front. During Ocean Drilling Program Leg 150, a transect was drilled across the on the Eastern U.S. continental slope off New Jersey. At three sites, a diagenetic boundary was recovered, across which Opal-A (biogenic silica, mostly diatoms) is replaced by Opal-CT. By re-precipitating dissolved silica into porcellanite lepisheres, this transformation can generate a sharp increase in the density and velocity of the sediments that creates a positive high amplitude reflector in seismic surveys. Because temperature and depositional history are the primary parameters controlling the reaction, this diagenetic boundary is sub-parallel to the seafloor, producing a time transgressive positive BSR (see Preliminary Figure a). The deployment of a dipole shear sonic tool allowed to measure the elastic moduli of the calcareous marine sediments. The comparison with the classic consolidation models of Gassmann and Wood shows that porosity is the principal factor controlling the properties of the sediments, and that the Gassmann model can be used to describe their mechanical behavior. In particular we show that this model is still valid to describe the change in properties across the diagenetic front, indicating that despite an anomalously strong seismic signature, silica diagenesis can be characterized mostly by a change in porosity preserving the normal elastic behavior of the sediments.

By comparison, Chapter 2 shows how a similar seismic signature can be associated with a profound changes in the mechanical behavior of marine sediments. In this chapter, we present the results of the first deployment of a dipole shear sonic tool in gas-hydrate bearing sediments. Gas hydrates are ice-like particles forming in the sediment pore space under specific conditions of pressure, temperature and free gas influx. In areas with steady seafloor topography, the limit of the hydrate stability zone is merely an isotherm sub-parallel to the seafloor. In most locations, free gas liberated by hydrate decomposition remains trapped under the impermeable hydrated sediments. The contrast between the properties of hydrated sediments above and partially free-gas saturated sediments below creates a negative amplitude BSR that has been used to identify hydrate reserves worldwide (Preliminary Figure b). The most striking difference between the two BSRs in Figures a) and b) is the change in the character of the seismic waveform across the two reflectors. The stiffening of the sediments because of porcellanite precipitation generates lower seismic amplitudes below the Opal-A/Opal-CT BSR. The free gas produces higher seismic amplitudes below the Gas-Hydrate BSR, while the seismic amplitudes are particularly low above this reflector, which has been called the "Blanking effect" of hydrates. The first recording of a shear sonic velocity log in hydrated sediments during ODP Leg 164 allowed us to characterize the in situ elastic properties of such sediments. Comparison with the Gassmann model shows that the presence of hydrates deeply transforms the properties of the sediments. Investigating alternate models to explain these observations, we use the cementation theory to establish the mode of deposition of hydrates in the pore space and the amounts responsible for the observed anomalies. We can however use the Gassmann model to estimate the amount of free gas below the BSR.

Despite being particularly noticeable features in seismic surveys because of their extent, BSRs have little economic relevance beside possible landslide assessment - at least until technology allows the exploitation of the estimated 2x10¹⁶ m³ of hydrocarbons trapped in hydrated sediments worldwide. In Chapter 3, we present a more pragmatic application of the same elastic models: to estimate the changes in reservoir fluids associated with differences observed between successive 3D seismic surveys of producing reservoirs. As part of an integrated time-lapse seismic interpretation methodology, petrophysical models are used in combination with reservoir simulation to characterize the changes detected through seismic inversion of 3D surveys (Preliminary Figure c) and to identify bypassed hydrocarbon. This chapter is therefore partly devoted to the selection of the most reliable formulation to convert observed impedance changes into producible resources. Because our objective is to develop a methodology applicable to the time lapse analysis of any reservoir, we present various possible formulations and a complete description of fluid properties. we then apply the methodology to two specific cases in the Gulf of Mexico. The different successes of the two reservoir simulations show the difficulties of time-lapse analysis and allow discussion of the additional steps necessary to conclude the interpretation loop before spudding a new well.

Finally, whereas the present-day dynamics of a producing reservoir can be identified by time-lapse seismics, this technique can not be used to study the natural migrations filling shallow reservoirs with mature hydrocarbons in active oil fields. Such migrations can cover several kilometers vertically and span centuries of periodic fluid expulsions in active faults, and they are too slow to produce noticeable changes in reservoir seismic attributes over a few years. However, the ascension of large amount of fluids from deep sources can generate temperature anomalies detectable for thousands of years. In Chapter 4, we present a complete analysis of the thermal regime in the Eugene Island 330 area, offshore Louisiana, located at the center of an active growth fault system. The reconstruction of the present-day temperature field is done by the correction and the interpolation of 600 Bottom Hole Temperatures (Preliminary Figure d). The analysis of the temperature distribution identifies several high temperature anomalies. Because the area is underlain by high relief salt diapirs, some of these anomalies are produced by the highly conductive salt. This is confirmed by the 3D numerical simulation of the regional conductive regime. The anomalies remaining after removal of the conductive component are attributed to fluid migrations within the active fault, and we use the 3D numerical modeling of the complete thermal regime to estimate the mechanism and duration of the migrations.

This last chapter shows how combining the insight of seismic surveys with the knowledge of the temperature distribution can help understand the dynamics of deep underground processes. In every study presented here, the use of thermal and acoustic signature to characterize the processes presented is related to the time scales involved. The migrations within a fault zone can occur over a few thousand years, but are induced by relatively low pressure gradients with pressure changes to low for time lapse seismic identification. They have however a significant thermal signature because of the large volumes of fluid and of the vertical extent of the migrations. In this study seismic data are only used for the characterization of static features of the system. By comparison, the forced pressure depletion in a reservoir under production and the resulting fluid substitutions can generate impedance changes of about 10% over a few years. As a result, these changes can be detected by the comparison of successive 3D seismic surveys, but are not associated with significant thermal signatures and temperature is here a static attribute. The primary importance of temperature on both silica diagenesis and hydrate stability allows to combine both acoustic and thermal characterization of these processes, but the very distinct reaction rates of opal-CT formation and hydrate decomposition require different interpretation of the thermal significance of the two BSRs. Both reactions obey Arrhenius-type laws but the rate of hydrate decomposition at the BSR is of the order of mol/min [Kim et al, 1987], while it is of the order of mol/kyear at opaline BSR [Mizutani, 1966]. Hydrate decomposition is merely an instantaneous process on a sedimentation time scale and the BSR represents simply a phase change boundary defined by pressure and temperature at the present time. The precipitation of Opal CT in passive margin sediments requires millions of year to produce a BSR [Hein et al., 1978] and the location of the reflector is the result of the integrated deposition history of the overlaying sediments [Langseth and Tamaki, 1992]. In both cases, the depth of the BSR can be used to estimate the regional thermal regime, providing or requiring additional constraints on stratigraphic history in the case of silica diagenesis [Langseth and Tamaki, 1992,Townend, 1997, Ruppel et al. 1995, Barstow et al., 1997]. Proceeding with such thermal characterization of the Blake Ridge and of the New Jersey margin could complete the cycle of the present work. It would also mostly underline what is already apparent in the present chapters and what Baudelaire [1857] already said so much better of sounds, scents and colors: into one deep and shadowy unison, sound, heat and pore fluids correspond.
 
 

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