STRATIGRAPHIC, SPATIAL, AND STRUCTURAL STYLE OF NATURAL CARBON DIOXIDE GAS ACCUMULATIONS IN THE GREATER PERMIAN BASIN

The Permian Basin of west Texas and southeastern New Mexico is one of most important and prolific petroleum-producing basins in the United States. Commercial production of oil and gas began in the early 20^th century, and in the last few decades, carbon dioxide-enhanced oil recovery (CO₂-EOR) has become increasingly important. In some regions of the basin, natural CO₂ and small quantities of other non-flammable gases comprise most of the gas in the reservoirs.  However, limited physical and chemical data exists on CO₂ reservoirs and CO₂ systems in general in the Permian Basin. These high-CO₂ gas fields are the current focus of a United States Geological Survey research initiative on CO₂-EOR, natural CO₂ resources, and analogues for anthropogenic CO₂ storage. Overall, natural gas reservoirs in the basin containing CO₂ are most abundant west of the Central Basin Platform, in a northwest to southeast trending fairway extending between the Delaware to Val Verde Basins. The CO₂ fields occur mainly within block-faulted structural or anticlinal closures that occur along regional lineaments. In southern portions of this trend, reservoirs traps are associated with structural elements of the Marathon-Ouachita Thrust Belt. In both areas, geologic structures are important constraints on reservoirs.  Limited data on the CO₂ fraction within these fields suggest a south and westward trend of increasing value; however, these trends are general and contain anomalies. Overall, the maximum amount of CO₂ within individual reservoirs ranges from less than 5 %, to over 95 %—internally, reservoirs can exhibit variations in CO₂ content.  Reservoirs with high quantities of CO₂ occur in multiple units that range from Ordovician to Permian in age and mostly in carbonate rocks. In some areas the CO₂ fields are stratigraphically stacked; yet in other areas, gas fields overlying CO₂ reservoirs lack appreciable CO₂. Increased understanding of natural CO₂ systems in the Permian Basin may lead to predictive modeling for exploration of this natural resource, potentially increasing the supply of natural CO₂ available for EOR operations.