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Challenges associated with amplitude-bearing, multiple-zone prospects

Rose and Associates, LLP, Houston, Texas, U.S.

Corresponding authors: G. Citron, gcitron1@houston.rr.com and P. Rose: prrose@onr.com

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

Explorers are frequently faced with assessing amplitude-bearing, multiple-zone prospects, especially when exploring in Tertiary basins such as the Gulf of Mexico. Some or all of these zones are often incorrectly described as "direct hydrocarbon indicators." In this paper, we deal with best practices of consistency, calibration, and measurement for amplitude anomalies to ensure appropriate valuation of such prospects.

When a zone is identified as "amplitude-bearing," a series of questions about the anomaly's attributes and data quality helps address the appropriate range of uncertainty associated with productive area, average net pay, and recovery factor. Discussion should focus on the anomaly's (1) strength and polarity relative to known lithologic (and various fluid-bearing) sequences and its presumed downdip water leg, (2) conformance relative to downdip structural contours, (3) AVO and fit to depositional model, and (4) additional specific facets of amplitude quality.

Results of drilled amplitude targets can be evaluated in the same vein, producing a matrix populated by data that can serve as reality checks on the assigned geologic chance of success (Pg). Evaluation schemes should not necessarily equate "amplitude anomaly" with "direct hydrocarbon indicator," because the latter implies more certainty of hydrocarbon presence and a much narrower range of outcomes that is often not warranted.

In multiple-zone prospects, each zone must be evaluated separately, followed by a discussion of the geologic phenomena to describe the nature and amount of dependency between zones. The resultant Pg and reserves distribution from the multiple-zone analysis both need to arise from the same conditions of dependence.

Most companies that explore in the Tertiary basins deal with amplitude-bearing targets, a benefit of the increased detection capabilities and resolution of modern seismic data. Prospectors often feel obligated to generate prospects with amplitudes at several levels that can be penetrated by the same well bore. This paper seeks . . . [Full Text of this Article]