Basin “Porosity Floor” — A Valid Rule of Thumb? The Case for Early Numerical Integration

Abstract

Exploration geologists commonly use rules of thumb to make decisions early in exploration projects to focus attention to relevant problems. This approach can be helpful if users consider assumptions underlying these rules and apply the rules in relevant situations. Too often, however, simplified rules replace understanding of physical processes and are applied inappropriately. Some rules are outdated but their use persists even for decades. Example of such a rule is the concept of a basin “porosity floor”. The rule takes a common (average) porosity-depth trend for a shallow section of the basin, extrapolates it to depth and sets a depth limit, below which the porosity is assumed to be too low for a potential reservoir. The rule focuses an explorer to a “relevant” stratigraphic section. A convenient simplification, which is wrong and can be costly. Research on siliciclastic sediments demonstrated that porosity varies widely and predictably as a function of its diagenetic history, which in turn is a function of sediment petrographic properties and burial history (e.g., Taylor et al., 2010). It is easy and far more informative to calculate a few lithology/porosity endmembers (e.g., Rajmon, 2012) rather than rely on meaningless average porosity trends. Even in case of lithologies, for which accurate physical porosity models have not been developed yet, it is better to consider a range of analog scenarios. If explorers relied on the “porosity floor” concept in Brazilian Atlantic margin, the large pre-salt accumulations would not have been discovered. Why is this rule still being used 15-20 years after learning better? Part of the answer is in the complexity of the petroleum systems difficult to master by any single geologist and in the lack of dynamic modeling tools that would guide geologists and make them think like basin modelers. Current basin modeling tools are typically detached from interpretation suites used by geologists and/or too complex for general exploration community. Consequently, basin model is typically built later in the exploration workflow and the initial incorrect perceptions about basin history may persist despite later models. It is critical to perform numerical integration early in, and throughout, the exploration workflow to test initial interpretations and guide the explorers to the right questions and decisions. References: Rajmon D. (2012) AAPG ACE #90142, Long Beach, CA. Taylor T.R. et al. (2010) AAPG Bulletin 94(8):1093-1132.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016