EARLY DIAGENESIS AND THE PRESERVATION OF POROSITY IN JURASSIC LIMESTONES

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Abstract
The study of Quaternary sediments has shown that carbonates are susceptible to early lithification, or to dissolution, in many different environments. Most of these surface effects, although reducing primary porosity, rarely destroy it completely. Although similar diagenetic phenomena also occur in ancient rocks, the great majority of limestones do not exhibit early diagenetic fabrics; much sparitic cement is possibly related to carbonate liberated by pressure‐solution within the limestone mass. In spite of the fact that carbonate sediments frequently have high initial porosities, most of it is destroyed by sparitic cement. The exceptional circumstances favouring the preservation of effective porosity may be related to the presence of an early diagenetic phase. A number of reservoirs exhibit early diagenetic cements, dissolution and dedolomitisation fabrics. It is concluded that early lithification tends to retard or prevent subsequent pressure‐compaction and thus reduces the volume of carbonate available for porosity destruction. The recognition of early diagenetic phenomena in ancient limestones therefore has important economic implications.