Controls on the Architecture of Paleokarst Systems and Associated Reservoir Quality
Loucks, Robert G.; Zahm, Chris
Depositional environment controls initial evaporite stratification and bed thickness. Thick (few meters to tens of meters), relatively pure (contains only a few thin nonevaporite units) evaporite beds may be deposited in some setting, while other depositional circumstances may result in thin interbedding of evaporites and carbonates or evaporites and siliciclastics. Thicker, pure evaporites tend to produce thick breccia deposits following karsting, whereas thin, interbedded lithologies produce broken, irregularly bedded intervals after deflation (dissolution) of the evaporite interbeds. If caverns develop during dissolution, type and amount of superficial fill material available will determine composition and amount of cavern fill, as well as cavern stability. Evaporite areas undergoing dissolution with no source of surface sediment to become cavern fill will form large cavities that are relatively empty except for collapse breccias. These cavities may be more prone to collapse creating suprastratal deformation (damage above a collapsed dissolved-evaporite zone). However, where a surface-sediment source is available, caverns may fill rapidly with this sediment. Rapid filling adds support to the cavern inhibiting collapse and resulting in less suprastratal deformation. Dissolution of a thick evaporite sequence is a large-scale diagenetic feature that affects thousands of square kilometers and produces intrastratal deformation within the interval of the former evaporite bed (meters to tens of meters vertically) and may produce suprastratal deformation, expressed by folds, faults (inverse and normal), and intense fracturing for tens of meters above the collapsed evaporite unit. We observe three distinct stratal relationships for the resultant deformation within this system—substratal, synstratal, and suprastratal—with deformation occurring in three stages—predissolution, active dissolution, and postdissolution (signified by burial). Pore networks within intrastratal zones are fractures, interclast/interparticle pores, and moldic pores after evaporites. However, if the collapsed cavity is filled with argillaceous detritus, the former evaporite zone may form a seal. Pore networks within the suprastratal zone are dominated by enhanced fracturing related to collapse and preexisting matrix pores.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013