Patterns of Porosity and Cement in Ooid Reservoirs in Dogger (Middle Jurassic) of France: REPLY

Abstract

In the Chailly oil field (Dogger, Middle Jurassic) of the Paris basin, France, an ooid facies is the producing reservoir, but not all wells are productive. The initial porosities of modern oolitic sands range from approximately 35 to 45%. In the ooid facies of the Chailly wells of this oil field the only cement is a fringe of radial crystals coating the ooids; most initial porosity, an estimated 30%, has been preserved. In the St. Brice-Villeneuve wells initial pore space has been obliterated in two ways: by pressure solution which created a tightly compacted oolitic limestone lacking porosity, and by cementation with calcite crystals (generated elsewhere by pressure solution) of noncompacted ooids. In the Marolles wells of this oil field porosity is present above a paleo ater table in the former vadose zone, but original pores have been eliminated below the paleowater table in the former phreatic zone. Thus the patterns responsible for the presence or absence of porosity are related to pressure solution with its attendant reduction of porosity and generation of cement which occludes pores elsewhere in noncompacted ooids, and the position of a paleowater table in the original depositional environment.