Early Meteoric Diagenesis and Porosity Preservation: Stratigraphic Traps in Carbonate Grainstones

Abstract

Abstract It is well established that aragonite sediment can be rapidly cemented in near-surface diagenetic environments when exposed to meteoric (fresh) water. Indeed, there are published examples of calcite cementation of aragonite sediments that resulted in rocks hard enough to require a hammer for sampling, and some of these examples formed in time periods shorter than a human lifetime. Complete mineralogic stabilization requires longer time periods, although within meteoric phreatic lenses in humid environments alteration to 100 percent low-magnesium calcite can occur as quickly as 5,000 years. Early lithification, especially lithification and complete mineralogic stabilization in paleophreatic lenses, has been demonstrated to produce zones within carbonate rocks that are more resistant to compaction and porosity destruction during burial than superjacent and subjacent rocks. Thus, early exposure to meteoric phreatic diagenesis is one mechanism for producing stratigraphic traps; that is, hydrocarbon reservoirs in which the trapping mechanism is not controlled by structural deformation. Introduction Ehrenberg et al. (2012) reviewed numerous published examples of porosity in carbonate rocks interpreted to have formed by late-stage burial dissolution and concluded that most of these interpretations were based on equivocal data. Although Ehrenberg et al. are perhaps too quick to dismiss some very well documented examples of this phenomenon, their principal conclusion seems at least consistent with what is known about the burial diagenetic environment. That is, in contrast with near-surface diagenetic environments in which several mechanisms can move a great many fluid pore volumes through a sediment in geologically short periods of time, fluids in burial diagenetic environments move sluggishly and generally do not have an obvious exit pathway. Thus leaching in burial diagenetic environments, although clearly important in some hydrocarbon reservoirs, is probably neither common nor volumetrically significant in the stratigraphic record. It has been well documented that the general fate of porosity in carbonate sediments and rocks is progressive destruction during burial (for example, Schmoker and Halley, 1982). Thus, porous and permeable rocks at depth, i.e. reservoirs, are the anomalies rather than the generalization. If one accepts these two generalities: late-stage leaching is rare and porosity is normally destroyed during burial, then it follows that porosity preservation during burial is at least one important factor in the formation of hydrocarbon reservoirs. This paper reviews one mechanism for preserving porosity during burial that may be quite common.