Petrophysical Characterization of Unconventional Carbonate Mudrocks of the Southern Mid-Continent: Mississippi Lime and the STACK Plays (Sooner Trend Anadarko Canadian Kingfisher Counties)

Abstract

Petrophysical characterization and quantification of original hydrocarbons in place and producibility in unconventional reservoirs remains a challenging question when evaluating reservoir potential. Porosity in the Mississippian-aged carbonate mudrock reservoirs of the southern midcontinent is less than 10%, with the majority of pores in the micro- to nano-pore class size and an average permeability of 0.1 millidarcies. The goal of this study is to evaluate the pore-scale controls on reservoir properties and develop proxies that characterize carbonate mudrock pore systems by combining facies and pore architecture (size and shape) analysis with nuclear magnetic resonance (NMR).

Pore architecture data from optical micrographs, ion-milled scanning electron microscope (SEM) mosaics and micro-CT images span over six orders of magnitude and are correlated to 100% brine-saturated NMR plug measurements of transverse relaxation times (T₂), porosity, and permeability.

Representative samples selected from cores in the study area include calcareous siltstones and grain-rich packstones to grainstones. The dominant pore types observed in these rocks are mineral- and grain-related interparticle, intraparticle, shrinkage and matrix pores, including filled and open microfractures.

T₂ plots display four geometrically distinct modal distributions which are comparable to pore shape and size distribution obtained from image analysis. These NMR results indicate a grouping of fabrics which correlate with porosity-permeability relationships and dominant pore types in four petrophysically-significant facies. An arbitrary clay-bound water cutoff of T₂ ≤ 3 milliseconds (3 x 10^-3 seconds) is excluded from the porosity and T₂ geometric mean in the process of estimating permeability. This modification significantly increases the correlation between NMR-derived and lab-measured permeability in clay-rich facies (from R = 0.24 to R = 0.53).

Petrographic assessment indicates that the primary cause of pore-scale heterogeneity and varying petrophysical response is related to post-depositional diagenesis such as silicification, cementation, dissolution, and mineralization along pores and pore throats which produce complicated pore systems and affects matrix permeability. These insights reveal that incorporating information on mineralogy, diagenesis, and pore types into formation evaluation workflows in carbonate mudrock reservoirs is critical to understanding storage and producibility.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018