Keys to Evaluating "Paleokarst" Reservoirs - Guidelines From Quaternary Carbonates and Enhanced Petrography
Jeffrey J. Dravis
Paleokarst is often invoked to explain reservoir development in ancient brecciated carbonates. However, the paleokarst model is valid only when fabrics seen in Quaternary cave systems are present and petrography of the non-brecciate zones supports subaerial exposure. Quaternary caves typically exhibit early fractures, microscopic to cavernous secondary porosity, solution collapse and brecciation, cave cements, and reddish-brown cave fill. Inter-cavern rocks show fabric-selective dissolution, porosity inversion, precompaction cements, and capping laminated soil crusts.
In brecciated Ellenburger (Ordovician, W. Texas) and Upper Elk Point (Devonian, W. Canada) reservoirs, major discrepancies exist between what the paleokarst model predicts and what is actually observed. Reddish calcretes, cavernous porosity and cave cements are absent. Enhanced petrography reveals relict sutured grains, implying burial dolomitization. These same dolomites are brecciated and breccia clasts contain stylolites rotated at different angles to each other and the horizon, implying burial brecciation. Fractures cutting these burial dolomites and stylolites are burial in origin. Preserved secondary porosity is unrelated to brecciated zones and facies controlled. Therefore, brecciation and reservoir quality in both sequences were created by burial dissolution of replacement dolomites and not by subaerial karstification. Brecciation alone does not prove the unconformity-related paleokarst model or the timing of porosity development.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California