A New Technique for Velocity Modelling for Migration and Pore Pressure

Abstract

Earth models solely based on tomography may be non-unique, especially in presence of anisotropy or subsalt where incidence angles are small. The latter is a major problem for subsalt pore pressure prediction. In the method proposed here we constrain the tomography using geology in conjunction with thermal history modelling and rock physics principles. This is termed as Rock-physics guided (RPG) velocity modeling for migration and pore pressure. A novel feature of this technology is that it uses predicted pore pressure as a guide to improve the quality of earth model in an iterative manner. Thus, we produce a velocity model that not only flattens the CIP gathers but also limits the velocity field to its physically and geologically plausible range without well control. This yields both a better image and reliable pore pressure in complex geologic area such as including carbonates or salt. Since the entire workflow is done in depth domain, the structural image and the attendant pore pressure will share the same earth model in 3D so that no extra velocity analysis needs to be performed after imaging, the pore pressure model can be directly used by the drilling community. We illustrate the process by recent examples from Asia as well as from the Gulf of Mexico. Introduction Traditionally, imaging velocities need to be conditioned prior to pore pressure analysis. This leads to multiple earth models with considerable uncertainty on pore pressure and furthermore, de-focusing of the image and mis-positioning of the structure when these models are used in a migration algorithm such as reverse-time migration (RTM). In the current approach, we use rock physics based pore pressure model in conjunction with velocity modeling techniques, e.g., reflection tomography, to alleviate the problem. First, rock-physics template (RPT) is created to describe the functional relationship between seismic velocities and pore pressure that is consistent with local geology. This model is thermal history dependent and it allows for both mechanical and chemical compaction – that is, a situation where effective stress is not directly related to porosity as is the case with mechanical compaction. This model is then introduced in the tomography workflow and an anisotropic velocity model is created by iterations. Our studies indicate that chemical compaction is as important as mechanical compaction in understanding the variation of seismic velocity and effective stress in depth. Neglect of chemical compaction is found to underestimate pore pressure at greater depths by as much as 3 ppg. We illustrate the workflow with two applications: one on a test data set from the Stampede Area of the Gulf of Mexico (Green Canyon; REV II multi-client dual-coil data; subsalt prospect) and the other from a deepwater project in Asia (also from our multi-client library). Method Burial history of rocks is a combination of both mechanical and chemical compaction. Mechanical compaction reduces porosity with increase of effective stress (defined as the difference between overburden and pore pressure). Chemical compaction, also referred to as burial metamorphism or diagenesis, is a result of mineral composition changes in the rock framework due to burial history dependent thermal alteration of rocks. These process releases "bound" water and lowers the effective stress acting on the rock matrix. Examples of chemical compaction are smectite to illite, kaolinite to chlorite and illite to muscovite in clastics as well as diagenetic alteration of carbonates and kerogene conversion leading to hydrocarbon generation. Proper description of chemical compaction is important for understanding the changes in seismic velocity due to changes in effective stress and hence, pore pressure. Chemical compaction processes are modeled here by using principles of chemical compaction and thermal history (Dutta, 1986; Dutta et al 2014).