Secondary Porosity in Miocene Sandstones of Louisiana Gulf Coast and Its Significance in Reservoir Properties: ABSTRACT

Abstract

Petrographic examination of Louisiana Gulf Coast Miocene sandstones recovered from depths ranging from 9,600 to 20,100 ft indicates that considerable porosity enhancement has occurred. Dissolution of authigenic calcite and dolomite is the primary mode of porosity enhancement. Relatively minor but locally significant secondary porosity is formed by dissolution of framework feldspars, rock fragments, and bioclasts or their replacements. Siderite dissolution is negligible. The degree of porosity enhancement is determined largely by the original textural and compositional characteristics of a sediment. Relatively coarse-grained, well-sorted, matrix-free sands are subject to early diagenetic pervasive carbonate cementation and replacement. Sandstones later affected by extensive dissolution of pervasive carbonate cement and replacements contain excellent pore networks and exhibit maximum deliverability. Poorly sorted, fine-grained, matrix-rich (e.g., bioturbated) sands undergo little effective porosity enhancement. Consequently, dirty sands retain inferior reservoir characteristics. Compaction and cementation generally diminish reservoir quality with increasing burial depth and aside from simple porosity loss, reservoir productivity and recovery efficiency are affected adversely by secondary pore geometry modification accompanying deep burial. Pore interconnection decreases and pore-throat geometry declines from tabular to lamellar with increasing depth. Pore to pore-throat ratios increase with depth, particularly in sandstones containing abundant secondary moldic and over-sized pores. Premature production decline in ultra-deep reservoirs can, in places, be attributed to reservoir-stress sensitivity associated with lamellar pore throats, very large pore to pore-throat ratios, and high overburden pressure. End_of_Article - Last_Page 266------------