Extracting seismic uncertainties from tomographic velocity inversion and their use in reservoir risk analysis

Abstract

Structural information in subsurface seismic images is critical for reservoir delineation, reserve estimation, and well planning. However, by its very nature, it is also uncertain. One cause of the image uncertainty is the migration velocity model that directly affects the position of migrated events, both laterally and vertically. (The term "velocity" is meant in the broad sense; i.e., it also includes the anisotropy parameters.) We present a method that accounts for uncertainties in a velocity model estimated by tomography and translates them into the migrated domain. Standard-deviation attributes on target horizon positions or layer thicknesses are extracted. The method includes quality controls for validating the estimated uncertainty attributes before integration with other downstream or interpretative information. The method is demonstrated on a North Sea area covered by data from multiple seismic surveys. We observe that uncertainties increase with model complexity or depth and decrease as the illumination diversity increases. The computed uncertainty maps constitute a valuable source of information for hierarchizing (both qualitatively and quantitatively) different areas in the survey. For the purpose of reservoir risk analysis, we combine our technique with other information (e.g., interpretation uncertainties) to map how uncertainties in the depth of the structural spill point impact the gross rock volume (GRV) estimation of a reservoir.

Tomographic model uncertainties and their effect on imaged structures.

“…For this purpose, target horizons are zero-offset re-migrated in each perturbed model where both V p and ε are simultaneously perturbed. A standard-deviation-like attribute on the depth position of each horizon is thus derived (Messud et al, 2017;Messud et al, 2017b). The geological horizon Base Flett around 2000m depth (Figure 3), chosen for its structural complexity, illustrates the estimate of uncertainty on horizon depth position.…”

“…Indeed, the migration velocity model directly affects lateral and vertical positioning of migrated structures. Messud et al (2017;2017b) have proposed a strategy aiming at assessing structural uncertainties associated with ray-based tomography. After the final tomography step, the method generates a series of equi-probable velocity model perturbations that are both tomographically-consistent and within a confidence level defined by standard deviation.…”

A workflow for seismic imaging with quantified uncertainty

“…Several works discuss how to express the uncertainty related to velocity-model building and measure the impact on the migration techniques [1,2,3,4,5,6]. Typically, uncertainties are characterized through a probabilistic perspective and expressed as an ensemble of possible realizations of a random variable, making the Monte Carlo (MC) method the standard tool to manage and quantify uncertainties [7, 8, 9, 10, 11? ].…”

Appraising structural interpretations using seismic data — Theoretical elements

“…By the reference position of a given horizon or fault, one often means the position obtained from the tomographic velocity model (e.g. Messud et al, 2017).…”