Porous Arab-D Dolomite Rhombs: A question of Leached or Arrested Crystal Growth?

F.O. Meyer and D.C. Cantrell

Saudi Aramco, Box 5000, Geological R&D, X-4800, Dhahran 31311, Saudi Arabia Aramco Geological Research and Development, New Technology Division, March 1999

Recent investigation on the development of Arab-D dolostones revealed new data about the development of porous dolomites rhombs. Porous crystal fabrics make up a significant component of partially and completely dolomitized intervals in the Arab-D reservoir. Rhombs with internal voids may account for up to 80 percent of the dolomite crystals present in partially dolomitized intervals and nearly 10 percent of those present in pure dolostone. Intracrystal porosity varies from only a few as much as 90 percent. The observed pore space generally exhibits highly irregular shapes and variable distribution patterns. However, some porous dolomite exhibits crystals fabrics that mimic the micro architecture of precursor grains. Curved crystal faces after thin shelled grains and crystals with internal pores that preserve foraminiferal chambers offer compelling evidence that these porous dolomite fabrics have a constructional origin. Such examples plus the observation that many intracrystalline pore walls are planar rather than pitted surfaces suggests that many porous dolomite rhombs may actually be a case of arrested crystal growth rather than selective leaching of unstable dolomite phases.

If valid, the constructive crystal growth hypothesis holds two significant implications for the Arab-D. Firstly, the porous dolomites can not be regarded as examples of dedolomite. Secondly, the presence of porous dolomite is not unequivocal evidence for an intraformational exposure surface.

Meyer, F.O., and D. L. Cantrell, 2000, Porous Arab-D dolomite rhombs: A question of leached or arrested crystal growth: GeoArabia, v. 5, p. 144