Seismic formation pressure logs are derived from seismic data in this study. The main objective is to delineate the distribution of overpressured zones in the subsurface, and seismic data from Offshore Louisiana are used to illustrate the proposed approach.
The seismic data processing consists of velocity modeling, wavelet processing, and seismic inversion. From the acoustic impedances produced by seismic inversion, seismic velocity and density "logs" are derived at every CDP location using a relationship between sonic velocities and acoustic impedances (AI).
Seismic formation pressure logs are computed against depth, making use of the above seismic velocity and density logs. Formation pressure logs are calculated assuming that compressional velocity, mean density, and depth are proportional to formation pressure. These logs are constrained at every depth point by estimated matrix and fluid compressional velocities (Vmax and Vmin). Vmax and Vmin are derived using porosity and sonic well log information.
Results include profiles of seismic velocity logs, seismic density logs, and seismic formation pressure logs for two intersecting seismic lines from Offshore Louisiana. One well is used to constrain the data processing. The seismic formation pressure sections delineate a large region of overpressured shales in the subsurface.
An important parameter in hydrocarbon exploration and exploitation is formation (or pore) pressure. In particular, detection of abnormally high formation pressures, or "overpressured" zones, can provide valuable information for exploration and exploitation purposes. High formation pressures are common in the Gulf of Mexico.
Overpressured sediments are generally caused by a sequence of events wherein water becomes. trapped by faults or nonpermeable barriers in sediments at depth where, otherwise, the water would have been forced out by normal increasing overburden pressure (Bilgeri and Ademeno, 982). Abnormal fluid pressure is also caused by release of water into the pore system during clay diagenesis (smectiteillite transformation), and other mechanisms as listed in Bruce, 1984.
High formation pressures cause major changes in subsurface rock parameters. In overpressured shales (Which contain pressured water), seismic velocity is lower, density is lower, and porosity is higher than if the pressured water had been able to escape (Bilgeri and Ademeno, 1982).
Detection of overpressured sediments can also contribute to the overall analysis of hydrocarbon potential of a basin. Abnormal pressures exert partial control on the type and quantity of hydrocarbons accumulated because pressure potential determines the direction of fluid flow, and overpressuring partly controls the geometry of growth faults and related folds in basins where shale structures are the dominant type formed (Bruce, 1984).
In the area of hydrocarbon exploitation, knowledge of formation pressure distribution can aid in conducting safe and efficient drilling operations.
In this study, several recent advances in velocity modeling, seismic wavelet deconvolution, and inversion are incorporated to more accurately derive velocity and density "logs" from seismic data in both time and depth domains. (In his paper, seismically derived "logs" refers to a derived section where each output trace, or "log", has been generated from one CDP in the original seismic section).
