Origin and Analysis of the Nano/Micropore Network in the Upper Cretaceous Ozan and Annona Chalks in the Caddo/Pine Island Field in Northwest Louisiana

Abstract

The Campanian Annona Chalk is a shallow (1400 to 1700 ft, however it may have been buried deeper) producing reservoir of light oil (API gravity 42 to 44). The reservoir is a fractured, slightly argillaceous lime chalk, as is the Ozan chalk below. Both units were deposited on the Upper Cretaceous drowned shelf in a moderate-depth, aerobic setting as evidence by numerous burrows and a high ratio of benthic to planktonic foraminifera. Clay-size material (up to 20%) composed of smectite and microquartz is slightly higher in the Ozan than the Annona. Major allochems are benthic and planktonic foraminifera and fragments of echinoids, ostracods, and bivalves in a finer matrix of coccoliths and coccoliths elements. The coccoliths and associated elements range in size from less than 400 nm to a few microns. The pore network resulting from this fine-grain size and burial cementation produces a reservoir composed of nano- to micropores. Pore throats are in the nanometer range. Porosity averages between 23 to 27%. The origin of this extremely fine pore network is depositional. The original pores between the coccoliths and fragments are interparticle. Minor intraparticle pores are associated with the foraminifera and some voids in the coccoliths. The coccosphere bodies easily breakdown to coccolith plates and individual elements. With the degradation of the polysaccharides (organic matter that hold the coccolith together) the coccoliths fragments separate. This results in finer material and associated finer pores. Later cementation will reduce these early formed pores. The minor amount of clay affects pore size by dividing the interparticle pores into multiple smaller pores. The clay also appears to promote pressure solution and enhanced cementation. The result of all these processes is to produce a porous reservoir with modest permeability. Natural and induced fractures form the collection network for the oil, but the nano- to micropores are the storage component of the reservoir.

AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014