INVESTIGATING THE NATURE OF THE LIMESTONE-DOLOSTONE TRANSITION IN THE EARLY PERMIAN SAN ANDRES FORMATION, GOLDSMITH LANDRETH FIELD, ECTOR COUNTY, TEXAS

The production of oil from the Permian Basin began more than one hundred years ago and the basin is now one of the largest producing basins in the world. The discovery and production of petroleum from Residual Oil Zones or ROZ’s in the Permian Basin has led to the redevelopment or expansion of producing fields that were once presumed not exploitable. One of these fields is the Goldsmith Field on the Central Basin Platform in Ector County, Texas. In the early 2000’s wells in the field were deepened into the ROZ located beneath the main pay interval of the early Permian San Andres Formation in the Goldsmith Field. Within the ROZ the limestone of the San Andres Formation transitions rapidly from dolomitic skeletal wackestone-packstone to fabric destructive dolowackestone-dolopackstone. We used 103 petrographic thin sections from six wells spanning the limestone-dolostone transition through the ROZ in Goldsmith Field to characterize the preserved carbonate fabrics and evaluate the diagenetic history of these rocks. Eight distinct diagenetic phases were identified from the thin sections. Early diagenetic euhedral dolomite crystals that displace micritic matrix and fill matrix pore spaces are observed in the limestone. Formation of these early diagenetic dolomite crystals is followed with complete infilling of intragranular pore space with euhedral-subhedral dolomite crystals during shallow burial diagenesis. Complete destruction of the original limestone fabric is associated with the formation of subhedral-anhedral dolomite and formation of secondary porosity during intermediate to deep burial diagenesis. Precipitation of anhydrite and silica cements in remaining secondary pore spaces represent the latest diagenetic step that affected the dolomites of the San Andres Formation in the Goldsmith Field. The diagenetic trends interpreted here support a reflux dolomitization model for the early Permian dolomite on the Central Basin Platform of the Permian Basin.