Grain-supported Facies Research Articles

Carbonate Porosity Related to Depositional Fabric--Zelten Field, Libya: ABSTRACT

Production from the Zelten field, Libya, is from the highly porous shelf carbonates of the Zelten Member (main pay) of the Paleocene and lower Eocene Ruaga Limestone. Fifteen facies are easily recognized, mapped, and predicted. In the Zelten field, primary and secondary porosities, recorded as high as 40%, are related to the original depositional fabric of the sediment and are, therefore, facies controlled. Porosity is best developed in the coralgal wackestone and packstone and Discocyclina-foraminiferal packstone and grainstone, which together form a northwest-southeast trend across the northern part of the field. Porosity is lowest in the miliolid-foraminiferal-wackestone and argillaceous bryozoan/echinoid-wackestone facies, both of which are blanketlike in distribution over the top of the field and form the cap for the reservoir. Porosity is also low in the argillaceous molluscan-wackestone facies south of and equivalent to the coralgal and Discocyclina-foraminiferal facies. It is concluded that early compaction of the soft carbonate sediments determined the amount of porosity preserved in the reservoir today. The grain-supported facies were not compacted and much of the original primary porosity is presumed to have been enlarged later by leaching. However, the mud-supported facies were compacted; the original porosity was lost early and consequently, later leaching was inhibited. End_of_Article - Last_Page 603------------