AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
The Mississippian Madison Formation: Controls on the Development of a Regionally Extensive Evaporite Paleokarst Complex
(1) University of Texas, Austin, TX.
As carbonate exploitation shifts focus from matrix porosity/permeability systems to breccia and vuggy porosity systems, improved understanding of paleokarst systems will be integral in guiding secondary and tertiary recovery projects. The distribution of heterogeneities within evaporite paleokarst is uncertain and outcrop analog data will better guide subsurface models.
The Madison of the Big Horn Basin consists of four 3rd-order composite sequences, capped by a major 2nd-order unconformity, onlapped by the siliclastic-dominated Amsden Formation. CS 3 and 4 contain evaporite-dominated TSTs that later are occupied by regionally extensive 5-8 m thick evaporite paleokarst. Previous work suggest solution
-enhanced fractures were conduits during dissolution and infill but no controlling mechanism for formation of the stratiform paleokarst and associated spacing/orientation of fracture/
vertical
collapse pipe systems has been established. We seek to determine controls on fracture spacing/density associated with this karst event.
Within the Big Horn Recreation Area, 7 sections were measured from the middle of CS 3 to the top of CS 4. Extensive photopan mapping (~25 km) of karst infill and distribution of 125 solution
-enhanced features were characterized. Integration of differential GPS and terrestrial lidar (~13 km with average spot spacing 5 cm) was used to model paleotopography of the Pennsylvanian unconformity. Distribution of mapped
solution
features reveal fracture spacing ranging 17m-120m on limbs of a fold trending 130°. Spacing increases to 143m-850m at a distance of 1km from the fold axis. Increased fracture intensity relative to this structural orientation is consistent with Ancestral Rockies uplift and implies dissolution occurred during Early Pennsylvanian.
The base CS 4 paleokarst complex contains relict anhydrite and is equivalent to evaporites in the subsurface. A seven-fold facies scheme reveals evolution of the karst system by a combination of vertical
and lateral migration pathways. Paleo-talus cones within the caves indicate maximum cave room heights of 10-15 m. Lack of welding following the evacuation of evaporites indicating contemporaneous infill. Relict pillars are absent, arguing for removal of stratiform evaporites. The combination of relict evaporites and cave-fill facies signifies the evaporite removal zone is a low-permeability zone within reservoirs, whereas the extensively fractured CS 4 roof system is highly prospective.