Carbonate Porosity Creation by Mesogenetic Dissolution: Time for a Reality Check

Ehrenberg, Stephen N.¹; Walderhaug, Olav ²; Bjørlykke, Knut ³
(1)Oil and Gas Research Center, Sultan Qaboos University, Muscat, Oman. (2) Statoil, Stavanger, Norway. (3) University of Oslo, Oslo, Norway.

We argue that the highly popular model of porosity creation in carbonate reservoirs by dissolution during deep burial is both unsupported by quantitative evidence and theoretically implausible. This paper was stimulated by the recent reappraisal of sandstone diagenesis by Taylor et al. (2010; AAPG Bulletin, v. 94, p. 1093-1132), which concluded that porosity creation by mesogenetic dissolution in sandstone petroleum reservoirs is generally insignificant. If the generation of acidic fluids by kerogen maturation or other burial processes is judged to be of too small magnitude to have significantly affected most sandstones, then it seems problematic that this same phenomenon should be important for carbonates.

A fundamental problem is the timing of porosity generation: How can it be proven that the pore spaces in a deeply buried reservoir formed after burial and were not already present? It is remarkable that many authors seem unaware of the near impossibility of the task of objectively measuring the relative proportions of an existing pore system that are inherited as opposed to created during mesogenesis. Even where there is textural evidence that pores formed by dissolution during deeper burial, such porosity may merely have been locally redistributed as nearby pore spaces became cemented by the dissolution products. The presence of burial cements in vugs is also poor evidence of late porosity creation because, as expressed by the Rubber Duck Axiom of R.G. Loucks, "What is in a hole might not have anything to do with the origin of the hole."

Because of the ubiquitous presence and rapid kinetics of dissolution/precipitation of carbonate minerals, pore waters in sedimentary basins are expected to be always carbonate-saturated, providing little opportunity for vertical transport of "aggressive fluids," even if the initial generation of undersaturation could occur. Burial diagenetic reactions are controlled by the solid phases because very little solids can be held as ions in solution and because the volumes of water available are in most cases distinctly limited. Deep circulation of marine or evaporated waters along fractures has indeed caused dolomitization in rare documented examples, but most cases of burial dolomite can be regarded as representing burial destruction of largely eogenetic pore space.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.