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Exploration risk reduction

An AVO analysis in the offshore Middle Miocene, Central Gulf of Mexico

Geophysical Development Corporation, Houston, Texas, U.S.

Corresponding author: sbarml@wt.net

The first 20% of the full text of this article appears below.

In exploration, seismic attributes of various types are typically used in concert with traditional prospect analysis to reduce risk. For example, a bright trough/peak seismic amplitude on stacked zero-phase data is commonly associated with petroleum-filled sands in Plio-Pleistocene sediments of the Gulf of Mexico. For deeper and older rocks, the AI (acoustic impedance) difference between sands containing petroleum and surrounding shaly rocks becomes slight and does not produce a diagnostic seismic amplitude on stacked data (Class II AVO environment). In this situation, AVO analysis on CMP gathers may highlight sandy intervals and distinguish which pore fluid is present.

AVO response depends on both the AI and Poisson's ratio contrast between the sand and surrounding rocks. Sands generally have a lower Poisson's ratio than shales, and gas sands have a lower Poisson's ratio than brine sands in the same geologic environment. When this is true, some discrimination for both lithology and fluid type can be accomplished. Keep in mind that with AVO analysis one is looking at the properties of the interfaces between layers and can only determine the target layer properties by assuming the properties of the surrounding sediments.

Predicting lithology and fluid type is not as simple as it might seem. Factors such as net/gross, porosity, pressure, properties of the surrounding rocks, and thickness of the sand bodies all influence the final seismic response. To minimize the influence of these factors, a few analog wells should be used to constrain some of these horizon characteristics.

	Predrill analysis

 
The key horizons for this prospect were arched in a four-way structural closure of Middle Miocene- age at approximately 3.0 s (Figure 1). The predicted depth range for the target horizons was greater than 15 000 ft below mud line. Maximum temperature expected at TD was about 300°F, and pore pressure was . . . [Full Text of this Article]