The Depth-Area-Thickness (DAT) Method for Calculating Gross Rock Volume: A Better Way to Model Hydrocarbon Contact Uncertainty

James, Bill; Grundy, Allen T.; and Sykes, Mark A.^1
1[email protected]

Gross Rock Volume (GRV), the volume of rock between a top and base reservoir surface and above a known or postulated hydrocarbon-water contact in a geologic trap, is often the most influential parameter in determining the magnitude of resource volumes contained, or potentially contained, in that trap. In any petroleum volumetric analysis it is therefore essential to calculate both the best estimate and the range of uncertainty for GRV accurately and appropriately.

However, geologic traps exhibit highly variable and often complex geometries. They range in shape from simple anticlines, resembling an upturned bowl, to all manner of intricately structured features with variable flank dip, overturned limbs and multiple culminations. There is thus a need for a single and reliable equation that can calculate GRV for this diverse suite of trap configurations.

Direct input of a GRV range, for example calculated in a mapping tool as a result of combining explicit choices of closure area, reservoir thickness and hydrocarbon contact depth assumptions, should be avoided. This is because the GRV of a trap, and its range of uncertainty, is a product of the interaction between these three largely independent variables.

Several methods have been developed over the years to estimate GRV on a screening basis. These frequently involve conflation of the trap’s area of closure, the hydrocarbon column height, estimated reservoir thickness and a wedge correction or geometry factor. Whilst these methods are stable, and quick to use, for all but the most simple traps they provide only a rough estimate of GRV.

The depth-area-thickness (DAT) method of calculating GRV provides greater accuracy and flexibility in calculating GRV and its range of uncertainty. This is because it allows hydrocarbon contact elevation ranges, and closure area and reservoir thickness uncertainties to be modeled independently.

The DAT method achieves this by defining a mathematical abstraction of the trap geometry in area-depth space into which an contact elevation or range can be convolved and modeled. This allows for a rapid GRV calculation.

The DAT methods requires that the geologist generate an table of depth-area-thickness values for a selection of contours along the flanks of the trap. This table of values can be calculated by hand, or in a geologic mapping application by using either standard functions or bespoke workflows or macros.

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013