From Bureau of Economic Geology, The University of Texas at Austin (
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Bureau of Economic Geology Technical Seminar, Austin, Texas, September 12, 2003

Origin of Ordovician Brecciated and Fractured Reservoirs; Paleocave, Thermobaric, Tectonic, or All the Above?

Robert G. Loucks


Several origins have been suggested for the formation of breccias and fractures that comprise Ordovician reservoirs throughout the United States. Suggested origins include karst-related paleocave collapse, subsurface thermobaric fracturing, and regional tectonic fracturing. Actually, the brecciation and fracturing in the Ordovician section is a combination of all three processes. The initial brecciation and fracturing is commonly associated with cave formation and collapse. The collapse starts at the surface contemporaneous with cavern formation and continues into the subsurface at least down to 9000 ft of burial. Cave formation is evidenced by (1) detrital cave-sediment fill, (2) younger conodonts in the sediment fill, (i.e. Devonian conodonts in Lower Ordovician cave fill) (3) speleothems, and (4) lateral extent of brecciated pods. Thermobaric processes are suggested by the presence of higher temperature baroque dolomite cements. The baroque dolomite cements passively fill the void spaces created by cave or regional fracture processes. Regional fractures cut host rock, lithified breccias, and lithified sediment fill. They commonly have relatively strong directional patterns. Deciphering the origin of complex breccia and fracture systems is difficult if the complete paragenesis of the system is not understood.