BEKKOUCHE D., Sonatrach Division Exploration, Algiers, Algeria; M.G. BONHOMME, J. PERRIAUX, Universite Joseph Fourier, Grenoble, France; H. ABDALLAH*, A.M. DOKKA and A. GHOMARI, Sonatrach Division Exploration, Algiers, Algeria.

ABSTRACT: Diagenetic Events, Porosity Development and K/Ar Dating of Clay Minerals in Lower Devonian Reservoirs of Ghadames Basin (Northeast Algerian Sahara).

The Lower Devonian sandstones having been deposited in storm effected, wave dominant coastal plain environments, constitute good oil producing reservoirs in Ghadames basin. In these sandstones, ferriferous chlorite precipitated as an early cement, forming well developed rims, which preserved and protected the interclastic porosity from later diagenetic changes. This resulted in anormalous high porosities of 25 to 30% even at a depth of 3500 to 3600 m. SEM examination showed that pore-communication is poorer in sandstones having thick rims and greater in those with thin rims of chlorite. Where there is no chlorite rim cement, the sandstones were affected by compaction, cementation and authigenic quartz overgrowths that considerably reduced the porosity. The quartz overgrowths are postd ted by carbonate and sulfate cements. They completely occluded the pore space and even replace the grains forming floating type cementation. Later dissolution of these cements created secondary porosity thus enhancing the overall quality and quantity of these sandstone reservoirs. Leaching of K-feldspar may also be observed creating isolated pores, however, the consequent formation of authigenic kaolinite as euhedral pseudohexagonal books partially infills the pore space. The latest diagenetic stage noticed was the formation of illite/smectite and illite, filling and bridging the pores, consequently reducing permeability. Potassium/Argon dating of chlorite rims gave their age to be of 283 +/- 8 Ma, wheras that of illite/smectite, around 190 Ma. A sequence diagenetic model is being worked out in relation to hydrocarbon entrapment.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.