Natural Fractures in Limestone vs. Dolomite: Comparing Fracture Characteristics in the Canyon (Sacroc Field, TX) and the Arbuckle (Wellington Field, KS) Formations

Natural fractures are common in carbonates, even in relatively unstructured, flat-lying strata. In equivalent structural settings, however, fractures in dolomite are typically better developed and have more variety than fractures in limestone. Comparisons of fractures in cores from the Arbuckle dolomites in the Wellington field of southern Kansas and from the Canyon limestones in the Sacroc field of West Texas, show that multiple sets of extension fractures with intersecting strikes are common in both settings, but that fractures are typically taller and more irregular in dolomites. Stylolites and clay partings arrest vertical fracture propagation, thus fractures are short in lithologies that contain numerous such heterogeneities, but they are more closely spaced in the dolomites. Fracture characteristics vary significantly by lithology in the Sacroc field, being shorter but more numerous in dense limestones than in high-porosity limestones. Plugs tested for permeability indicate that stylolites and their associated structures enhance local permeability. Dissolution along fractures was an important process in both lithologies, but is better developed in the Arbuckle dolomites where fracture-controlled dissolution produced irregular vertical slots up to five ft tall and nearly a centimeter wide. Dissolution also opened irregular slots along horizontal planes. The slots are narrow and follow stylolites in the limestones whereas they are wider and more irregular in the dolomites where their parentage is less obvious but probably related to stylolites and bed-parallel shear.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California