Depth Uncertainty Estimation for a Subsalt Development in the Lower Tertiary, Deepwater Gulf of Mexico
Authors: Yang Yi , Chao James , Almoughraby Bassel , Osypov Konstantin , Medina Raquel , Amaru Maisha , Hoelting Cory 
Published: May 2013

Abstract
Deepwater oil and gas development projects are extremely challenging. Specific factors contributing to deepwater subsurface characterization challenges include: limited well count, large variation in gross rock volume (GRV) estimates of low relief structures produced by even minor depth uncertainty, and complex overburdens (e.g., complex allochthonous salt). In response to these challenges, we conducted a multi-organization depth uncertainty analysis over the St. Malo subsalt, Lower Tertiary development in deepwater Gulf of Mexico. The project applied the Quantitative Uncertainty Analysis (QUA) method (Osypov et al., 2008). QUA is a linearized Bayesian velocity model (and subsequent depth) uncertainty analysis process sampling a set of anisotropic tomographic models. Bayesian methods require a prior model, which is a quantitative representation of our current subsurface understanding. Those familiar with Bayesian methods (e.g., from geostatistical reservoir characterization) know the importance and impact of properly specifying this prior model. To specify this prior we integrated information from various subsurface disciplines (geophysics, geology, drilling). The depth uncertainty estimates that were obtained are being used in building the next generation static model for St. Malo.

Subsurface setting and reference imaging
A collaborative, multi-organization depth uncertainty analysis was conducted (involving two E&P companies and a geophysical contractor) over the St. Malo subsalt, Lower Tertiary development in deepwater Gulf of Mexico, see Figure 1. The range of water depths in the St. Malo area is approximately 6,500 - 7,400 ft. The St. Malo structure is a 4-way closure with the top of the shallowest reservoir between 27,000 and 29,000 ft. This structure lies completely beneath a fairly tabular and " well behaved" allochthonous salt body, which has maximum thickness of approximately 11,000 ft.
In terms of subsalt seismic imaging, St. Malo is easily one of the very best imaged structures in operator's deepwater Gulf of Mexico portfolio. It is also arguable that it is one of the best imaged subsalt structures anywhere in the deepwater Gulf of Mexico, which made it an excellent candidate for the first subsalt application of the Quantitative Uncertainty Analysis (QUA) method/workflow. QUA is a linearized Bayesian velocity (and subsequent depth) uncertainty analysis process, which assumes that both the prior distribution and the likelihood function follow multi-dimensional Gaussian distributions, see Osypov et al. (2008) and Osypov et al. (2011) for more details. Lewis and Hoelting (2012) describe a completely different approach to a similar velocity and depth uncertainty problem without salt in the overburden (their method is based on a production anisotropic MVA workflow used by the operator, see Neal et al. (2009)).
The mean of the prior distribution is the " current" transversely isotropic (TI) imaging model, consisting of v0, d, and e (Thomsen, 1986). For this imaging model the symmetry axis is vertical (i.e., the imaging velocity model used as the reference model for the linearized QUA analysis at St. Malo is VTI).