Structurally controlled hydrothermal dolomite reservoir facies: An overview: Discussion

Abstract

One of the issues of the AAPG Bulletin (v. 90, no. 11) was concerned with “hydrothermal dolomite reservoirs,” specifically “hydrothermal dolomite fields” (Davies and Smith, 2006). The term “hydrothermal dolomite” is confusing and meaningless (Machel and Lonnee, 2002). To study the origin of dolomite and its hydrothermal depositional setting, I examined modern settings, including the Mediterranean Sea (Friedman, 1991) (Figure 1). The shallow-water facies in this modern setting consists of dolomite and gypsum. The rocks range from almost pure gypsum to pure tough dolostone so thoroughly cemented that sampling required hammer and chisel. X-ray diffraction analysis indicates the presence of minor anhydrite. Sporadic aragonitic gastropods occur, but fossils are otherwise absent. Under the microscope, it is a peloidal micrite (dolomicrite). The δ13CPDB of the dolostone is −39.3 to −31.2‰. These strongly negative δ13C values suggest that carbon of the dolomite came from methane. Methane forms when all sulfate has been reduced. Bacterial reaction was responsible for dolomite formation through the methane pathway. This syngenetic dolomite is like most dolomites of hydrothermal origin, but my students and I avoid this confusing bandwagon term. Figure 1 Approximate temperatures at which Holocene intertidal carbonate sediments formed, calculated from isotopic signatures. Three classes of dolostone have been recognized, which have been termed syngenetic, diagenetic, and epigenetic (Friedman and Sanders, 1967). The purpose of this classification has been for the orderly discussion of the subject and to facilitate descriptive study and genetic interpretation. The term “syngenetic” has been adapted for characterizing biological and chemical origin in its …