Role of Sequence Stratigraphy in Reservoir Characterization and Solving Waterflood Production Problems, Grayburg Formation (Permian-Guadalupian), Eunice Monument South Unit, Lea County, New Mexico

Abstract

The Grayburg Formation was deposited upon a carbonate ramp as shallowing-upward fifth-order parasequences of interbedded carbonates (dominant) and siliciclastics (subordinate). Sandstones were deposited as offshore shallow-marine blankets, and form some but not all parasequence bases. Seaward, carbonates were deposited as ooid grainstones/mud-poor packstones in a high-energy shoal complex. Landward, carbonates are mud-rich packstones/wackestones that were deposited in a low-energy back-shoal environment. Reservoir-quality porosity (grainstones/mud-poor packstones) is preserved and enhanced by (1) depositional controls, such as bed thickness, grain richness, and grain size; (2) dissolution during subaerial exposure; (3) dolomitization (early diagenesis); and (4) dissolution during deep burial (late diagenesis). Thinner bedded, mud-rich packstones/wackestones contain lower reservoir porosity updip, and eventually form a stratigraphic trap. Using reservoir characteristics and outcrop studies, and applying sequence stratigraphic interpretation have enhanced reservoir management during initial waterflood and reservoir fill. Early water breakthrough occurred in EMSU well 119. Here, a high-porosity high-permeability bed within one parasequence was connected with EMSU injection well 108. Once reservoir connectivity was understood the flow unit in well 119 was squeezed off. The injection well immediately built up pressure and redirected the waterflood. Later, an oil bank arrived at EMSU well 109, another offset producer.