Abstract

Several processes are suggested for the formation of large-scale, intensely brecciated and fractured carbonate bodies that compose many Ordovician reservoirs. Suggested processes include (1) karst-related paleocave collapse, (2) regional faulting and fracturing, and (3) subsurface thermobaric dissolution, brecciation, and fracturing. The most common process of brecciation and fracturing of Ordovician carbonates is well documented to be associated with cave formation and collapse. With this process, collapse starts at the surface contemporaneous with cavern formation and continues into the subsurface to 9,000 ft or more of burial. Regional faults and fractures cut across undisturbed strata, lithified breccias, and lithified sediment fills. They can have relatively strong directional patterns and generally have limited brecciation associated with them. Higher temperature baroque dolomite and other cements are suggested evidence of thermobaric-fluid alteration of Ordovician carbonates. Some localized dissolution and associated brecciation and fracturing might occur with the thermobaric alteration. Both collapsed paleocave and regional fracturing and brecciation are well-established pathways for later thermobaric-fluid transport.

Deciphering the complete origin of complex brecciated and fractured carbonate systems can be difficult and the inclusive paragenesis of the fractured and brecciated carbonate system must be recognized and understood. It is well accepted that karst-associated cave development, regional faulting and fracturing, and thermobaric processes can affect carbonate strata. The major question this paper addresses is what is the capacity of each of these processes to form large-scale, intensely brecciated and fractured carbonate bodies? Commonly, many of the large-scale, intensely brecciated and fractured carbonate bodies in Ordovician strata formed by a combination of the above processes.