Carbonate reservoir evaluations for either Exploration or Production studies are markedly different than those for clastic ones because of several important factors:
Carbonate reservoir evaluations for either Exploration or Production studies are markedly different than those for clastic ones because of several important factors:
First, for drilling depths equivalent to at least 100o C, numbers of potentially important reservoir-modifying processes in carbonates far exceed those typically associated with adjacent clastic reservoirs. Some carbonate-specific processes like early hydrocarbon charging can help preserve excellent carbonate reservoir properties to great burial depths – far off the burial trends expected from simple compaction.
Second, carbonate reservoirs affected by processes of karsting and/or fracturing have variations in poroperm architecture that span a much larger spatial scale ranging from log-defined matrix properties near the wellbore to un-loggable megafabric properties developed far beyond the depth of investigation of typical logging tools. This huge spatial range of property development requires high levels of data integration to define - especially seismic and dynamic data where scales of investigation are appropriately large.
Third, hydrocarbon column heights may extend far beyond apparent structural closure in carbonate traps because of favorable depositional architectures and subsequent development of top and lateral seals by chemical compaction of flanking beds. Excellent carbonate reservoir rocks can pass laterally into effective carbonate seals in trap geometries that are generally not translatable to clastic systems.
Finally, commonly-used approaches like defining net-to-gross ratios and porosity cutoffs often do not work to properly characterize carbonate reservoirs unless the evaluation has a proper petrophysical context. For example in an all-carbonate stratigraphic interval, high-porosity carbonate intervals may actually be non-pay due to dominance of microporosity with associated low hydrocarbon saturations, whereas low-porosity carbonate intervals may be productive pay due to processes such as natural bed-bound fracturing that increases hydrocarbon flow to the wellbore. Related petrophysical factors like wettability and relative permeability also often describe the fundamental quality of a carbonate reservoir in dissimilar ways than a clastic reservoir – with large implications for hydrocarbon recovery efficiencies under various types of IOR/EOR development concepts.