AVO Calibration of GOM Deepwater: Results from 50 Fields
Authors: Ward Ronald W. , Danbom Stephen H. , McKnight Randy S. , Hebert Michael 
Published: October 2008

Fifty fields in the deepwater Gulf of Mexico (GOM) have been studied to determine the relationship between clastic lithologies, saturating fluids, and seismic response with source-receiver offset (AVO). The fields range from East Breaks / Alaminos Canyon to Viosca Knoll protraction areas. The goal is to predict theoretically and demonstrate with observed data, seismic attributes that are most diagnostic of reservoir sands and hydrocarbon saturation in diverse deepwater locations and at various depths.

The findings of this paper will illustrate the general conclusions for GOM based upon over 3000 km of reprocessed long-offset seismic data. Detailed modeling of key wells has been carried out as well as petrophysical analysis of each well. The stacked and seismic AVO gathers are tied to synthetic data based upon the well logs recorded in each well. A general conclusion is that almost every field has Class 3 AVO anomalies associated with hydrocarbon reservoirs. These Class 3 reservoirs are often less productive than thicker pays that exhibit Class 2 AVO anomalies, which have low reflectivity on the near-offsets. Reservoirs with Class 2 AVO response often do not have anomalous amplitudes on conventional stacked data. Detailed analysis of AVO seismic attributes provides a more reliable picture of the reservoir quality, geometry, degree of saturation, and extent.

In addition to the project wells in each field, dry holes drilled nearby on seismic amplitude anomalies are studied to determine which seismic attributes are indicative of brine saturation versus pay in that area. Modeled and observed examples of this are presented illustrating which attributes are most diagnostic. The role of offset data in identifying sands pinching out against salt is illustrated. Examples of gas-oil and oil-brine contacts in reservoirs are analyzed. The use of multiple attributes such as NI × PR (product), AVO crossplot, and gradient of amplitude envelope seem to be the most diagnostic in the general situation. Several analogs from all areas studied will be discussed.

Improved seismic AVO attributes and calibration utilizing existing and yet to be drilled wells will provide improved reduction of risk as the deepwater GOM exploration becomes a more mature play. This study makes a contribution to aid explorationists in achieving this goal.

1.0 Background

The objective of this project was to use both seismic and well data in the AVO / AVA domain to calibrate seismic data to well synthetic models. On a field-by-field basis, the study sought to learn how seismic attributes of reprocessed 2-D seismic data can be used to predict lithology and fluid properties of reservoirs. The seismic data used in the project were completely reprocessed from field tapes to provide a consistent set of data in which amplitude information was preserved. A detailed description of the acquisition and processing flow is provided in a companion paper (Verm et al., 2002). This study comprises 50 fields as shown on the map in Figure 1.1.