Porosity Preservation and Early Freshwater Diagenesis of Marine Carbonate Sands: ABSTRACT

Abstract

Observations from Holocene and Pleistocene limestones of south Florida and the Bahamas provide the basis for a general outline of freshwater alteration of marine, primarily aragonitic, sands. Porosity and mineralogic data suggest that metastable carbonate recrystallization takes place before significant porosity loss. The outline proposes the following main points: (1) porosity is only slightly modified during mineralogic stabilization and early cementation; (2) secondary porosity development during early cementation preserves overall porosity; (3) early cements formed during metastablephase recrystallization are almost entirely autochthonous on a reservoir scale. The time of stabilization may be as short as 104 years but may be slowed for long periods (e.g., b salt water intrusion, dry vadose conditions, stagnant water). Major porosity reduction occurs after stabilization and takes longer periods of time > 106 years (e.g., extended period of subaerial exposure, burial diagenesis). Our observations of the effects of early freshwater diagenesis underscore the importance of later diagenetic events in porosity reduction of limestones. This outline suggests that reservoir limestones developed through early freshwater diagenesis of aragonitic sands should be characterized by high porosity, most of which is secondary. Conversely, high-porosity reservoirs with considerable amounts of primary pore space have either escaped pervasive freshwater diagenesis or are developed in sediments that were originally calcite. End_of_Article - Last_Page 460------------