Intracrystalline Porosity--A Newly Discovered Pore Type in Dolostone Reservoirs and Implications for Dedolomitization and Pseudospiculites: ABSTRACT

Abstract

Intracrystalline porosity consists of hollow dolomite crystals. It forms when anhydrite, which had replaced cores of dolomite crystals, was dissolved. Significant volumes of intracrystalline porosity have been preserved in subsurface Permian dolostones of the Permian basin and in outcrops of Lower Cretaceous dolostones of Texas. Anhydrite can be seen replacing organic-rich cores of dolomite crystals in Permian and Jurassic (Smackover) dolostones. Dolomite crystals, which replace original carbonate, commonly appear because of organic inclusions. After dolomitization, continued flux of the dolomitizing fluid causes precipitation of clear rims on the cloudy crystals. Anhydrite tends to replace the organic-rich cores, leaving the clear epitaxial rims to form pill box structures. When anhydrite was dissolved, many hollow dolomite crystals collapsed and fragments became transported as vadose internal sediment. Obtuse and acute angles of these hollow dolomite rims superficially resemble sponge spicules, and pseudospiculites are layers of vadose internal sediment in which numerous fragments of hollow dolomite rims have been deposited in solution channels. Hollow dolomite crystals and pseudospiculites constitute evidence for disconformity and also represent subtle indications of vanished sulfates. In many instances, calcite cement was precipitated within intracrystalline pores and is misinterpreted as partial dedolomitization. Such calcite cement can be distinguished from dedolomite, because the calcite is not syntaxial with dolomite rims, as it would be in dedolomitization. In some Permian dolostone intervals, intracrystalline porosity is the predominant pore type. Molds formed by dissolution of replacement porphyroblasts and nodules of anhydrite also commonly were formed along with intracrystalline pores and all represent tertiary (third order) voids. End_of_Article - Last_Page 491------------