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Integrated Petrophysical and High-Resolution Sequence Stratigraphic Characterization of an Unconventional Carbonate Mudrock Reservoir

Abstract

The Mid-Continent Mississippian age limestone is a valuable unconventional carbonate reservoir in Oklahoma and Kansas. Although over 14,000 vertical wells have been producing oil and gas from Mississippian age reservoirs for over 50 years, recent horizontal activity has illustrated how crucial it is to understand the petrophysical and depositional characteristics associated with producing intervals. Petrophysical analysis has been integrated with high resolution sequence stratigraphic analyses of core from North-Central Oklahoma to better understand the distribution of reservoir facies in this unconventional carbonate reservoir. Horizontal porosity in the data set, ranges from 0.5–7%, although porosity values may be as high as 20% locally. Correlative permeability ranges from 0.001md to just over 1.0md. SEM analysis shows the pores are mostly oblong to oval, intercrystalline to vuggy, meso- (4mm-62.5 μm) to nanopore (1μm-1nm) size, while pore throat measurements are consistently in the nanopore range. Acoustic response data show the inverse relationship with porosity in unconventional carbonate mudrocks is consistent with previous work using Mesozoic to Cenozoic age conventional carbonates. However, the carbonate mudrock data from the Mississippian show a significant shift in the median value that appears to be consistent with analysis from Neogene carbonate mud samples. Detailed facies analysis from three cores in North-Central Oklahoma suggests deposition occurred on a regionally pervasive, distally steepened carbonate ramp. The facies stack into shoaling upward packages of weakly calcareous mudstones to wackestones at the base, overlain by progressively higher energy skeletal packstone to grainstone facies. A sequence stratigraphic hierarchy of shoaling upward packages is observed in core and wireline logs at the third, fourth, and fifth order scales. Tying the correlation between the wireline log signature and facies stacking patterns into the sequence stratigraphic framework provides a means for increasing the predictability of reservoir quality units in the subsurface. Augmenting this data with the acoustic response, and characterization of the macro- to nanoscale pore architecture, provides an example of how integrated studies can enhance predictability of key reservoir facies and producing intervals within unconventional carbonate reservoirs.