Diagenesis of Larger Foraminifera (<I>Lepidocyclina</I>, Sp.) and its Effect on Porosity in the Limestone Reservoir Rocks of Bombay Offshore Area

Abstract

The tests of the larger benthonic foraminifera (Iepidocyclina, sp.) in the limestone reservoir rocks of Bombay Offshore area show various progressive stages of diagenetic alteration, starting from the initial stage of the pillars and septae of the test being clarified to aggregates of microfibrous calcite enclosing polygonal areas suggestive of original chambers, to tests completely rendered to aggregates of fibrous calcite proceeding as parallel brush like rays from the axial plane of the test, with the total destruction of its original Skeletal morphology. Evidence is put forward to show that the sheaf like bundles of radiating fibrous spar are of pore filling and syntaxial overgrowth origin than one of aggradational neomorphism. In calcarenitic rocks of such foraminiferal assemblages excessive overgrowth cementation may remove original intergranular porosity, thus reducing considerably the oil and gas reservoir capacity of the rocks.