A Review of Porosity Semivariogram Parameters from Giant (and Not so Giant) Carbonate Reservoirs
W. Scott Meddaugh
Chevron Energy Technology Company, Houston, TX
The stochastic algorithms that are used to populate reservoir models typically use the semivariogram as the measure of spatial continuity. For reservoirs with abundant, good quality well log data the semivariogram parameters can be defined and their uncertainty established. For fields with few wells the semivariogram parameters are usually taken from an analog or inferred from the likely geometry of depositional elements. In data limited cases, it is difficult to assess appropriateness of the analog derived parameters or assign appropriate ranges to the parameters as part of an uncertainty assessment.
A survey of porosity semivariogram parameters from carbonate reservoirs in the Permian Basin, western Canada, Kazakhstan, and the Middle East with abundant porosity well logs shows the following: (1) the semivariogram range parameter is typically 1000-2000 m; (2) most reservoirs do not show significant anisotropy; (3) the variation of semivariogram parameters between reservoirs is similar to the variation by stratigraphic interval within reservoirs (about a factor of three); (4) the minimum reported range is 150 m and the maximum is 6500 m; (5) there no correlation of the range parameter with depositional age and only a slight correlation with depositional setting; (6) there is no difference in the range parameter for limestone dominant or dolomite dominant reservoirs, (7) the vertical range parameter varies from less than 1 m to 15 m and shows little correlation with age, setting, or mineralogy. The vertical range parameter obtained from core data is generally lower than that obtained from well log data.