The Appropriate Use of Seismic Data for Estimating Petroleum Resources
John Rhodes, MXD, LLC
Seismic data is generally the key data source for resource evaluation in all categories except proved reserves. The use of seismic data for classification of proved reserves in particular is the subject of frequent disagreement because: 1) seismic quality, 3-D in particular, has advanced significantly and rapidly 2) seismic data is generally precise but not accurate and 3) reserves classification is historically an engineering discipline, and engineers are typically not well versed in seismic methodology. Seismic data is generally accurate enough for estimation of the areal geometry attributes such as dip and faulting for proved reserves. With comprehensive interpretation seismic data may be accurate enough for estimating gross thickness. Seismic data is generally not accurate enough for the quantification of porosity or net-to-gross in the proved reserves category but often useful for the other resource categories. Seismic data is not accurate enough for the quantification of Sw within a hydrocarbon accumulation.
Fair quality data is more difficult to evaluate than good or poor data because judgment becomes more significant. Probabilistic estimations yield a specific numerical value for designating the resource category, but judgment is still significant in the estimation and less transparent. The consistency of resource estimation to which we aspire is aided by rules, standards and guidelines. Neither the PRMS nor the SEC reserves definitions specifically address rules for the application of seismic data to resource estimation except in the case of proved reserves and 3-D seismic data with a flat or bright spot. An attribute should be used for quantification of resources only when: The attribute is well defined (the data is good), the attribute is clearly correlated to well data (synthetics, analogs), and the basis of the attribute is known and can be modeled. Seismic data is commonly used with a bias, either too punitively or too optimistically. More definitive data sources such as petrophysics and well test data should be used to clip the high-side and low-side estimates from seismic data. No data, including seismic data should be used to classify resources unless that data clearly contributes to understanding the reservoir. Otherwise, generalized approximations and assignments are better because they are defined, repeatable and no less accurate. In particular, the reservoir parameters net-to-gross, porosity, and Sw are better defined by other data sources unless the seismic data is of good quality and considerable effort has gone into the interpretation and modeling of the attribute(s) used for the estimation. Individual judgment is essential in the resource estimation process and frequently results in a wide variation of results. The consistency and clarity sought by the application of standards are often at the expense of accuracy, but must be in place if resource estimations are going to be a relevant.
AAPG Search and Discovery Article #90098©2009 AAPG Education Department, Houston, Texas 9-11 September 2009