APRIL 15, 2015 LUNCHEON MEETING
BETH ANN VANDEN BERG,
PhD CANDIDATE OKLAHOMA STATE U.
Using carbonate mudrock pore architecture to provide insight into porosity and permeability trends in unconventional carbonate reservoirs: examples from the Mid-Continent Mississippian Limestone
The Mid-Continent Mississippian Limestone is an unconventional carbonate reservoir with a complex depositional and diagenetic history. Oil and gas have been produced from vertical wells for over 50 years, but recent horizontal activity in low porosity, low permeability zones have illustrated how crucial it is to understand the petrophysical characteristics to better target producing intervals. Because of the wide variability and complexity of pore systems in carbonate reservoirs, simple porosity and permeability transforms developed for siliciclastic reservoirs often provide erroneous results for carbonates. Recent research has only started to identify the complexity of the pore architecture observed in carbonate mudrocks, and the applicability of conventional carbonate pore relationships to describing carbonate mudrock systems.
The current study shows examples of how fundamental relationships between pore shape, porosity, permeability, and acoustic response differ in carbonate mudrocks with microto pico-porosity (<62µm diam.) compared to conventional carbonates with primarily macropore (256-4mm diam.) systems. Quantitative data show positive correlations exist between porosity and permeability, but negative to no correlation between pore shape and associated porosity and permeability. In addition, there is a significant shift in the acoustic response relative to values calculated from empirically-derived equations for porosity in carbonate mudrocks. Deviations from quantitative data trends are explained through qualitative observations of the pore types and differing internal pore geometries. Visual observations of the pore morphology show how post-depositional cementation can increase the complexity of the internal pore network by sub-dividing pores. When correlated to facies, the internal pore geometry helps explain deviations to general relationships between basic quantitative pore architecture measurements, porosity, and permeability. Although there is an added level of complexity in the pore architecture of carbonate mudrocks, there are fundamental relationships that exist between the pore architecture, pore shape, porosity, permeability, acoustic response, facies and sequence stratigraphic framework with variable levels of predictability that, when used as an integrated data set, can be used to enhance the predictability of key petrophysical properties within these types of reservoir systems.