Genesis and Emplacement of Oil in the San Andres Formation, Northern Shelf of the Midland Basin, Texas

San Andres oil constitutes more than 80 percent of the total production from the Northern Shelf of the Midland Basin, Texas. The San Andres and Clear Fork carbonate rocks of the Northern Shelf contain sufficient amounts of lipid-rich organic material to rank them as potential petroleum source beds. Organic maturation of these rocks, as revealed from vitrinite reflectance and kerogen color, however, is not sufficient to have initiated catagenesis. Therefore, oil within Northern Shelf reservoirs was derived mostly from other sources.San Andres oils have a common source, as evidenced by their remarkably uniform composition, which is revealed in liquid and gas chromatography, and Bartlesville Energy Technology Center (BETC) distillate analyses. Wolfcampian basinal clastics and dark argillaceous limestones of the northern Midland Basin are the most likely source rocks for this oil. Vertical expulsion of basinal oil through fractures into overlying shelf and shelf-margin carbonates has occurred along the Lower Permian Abo Reef trend. This reef hingeline is a narrow belt of shelf-margin buttresses ranging in age from Strawn to Clear Fork.The trapping mechanism in the Northern Shelf is a combination of structural and facies control. Good reservoir conditions exist in San Andres strata that are draped and subsequently fractured over the subjacent shelf-margin buttress. Such fracturing should have enhanced conditions for development of secondary porosity during periods of subaerial exposure. Above the Abo Reef trend, a thick porous zone exists in the lower San Andres and upper Clear Fork Formations; shelfward, this porous zone grades into discrete porous layers resulting from cyclic sedimentation in shallow inner-shelf and sabkha environments. These Upper Permian carbonates tend to lose porosity in a northward (updip) direction, where conditions were more evaporitic. This updip change from porous to nonporous facies provides the porosity pinch-out in the vast Levelland - Slaughter - Cato trend of Texas and eastern New Mexico.Late Cretaceous uplift in New Mexico exposed Permian strata, initiating a west-to-east flow of relatively fresh ground water. Passage of this meteoric water through San Andres and Clear Fork reservoirs caused downdip degradation and flushing of the oil. As a result of this ground-water movement, oil/water contacts tilt downdip 0.3° to 0.5°, and oil production is slightly offset downdip from local structural highs. The San Andres and Clear Fork oil was degraded by anaerobic sulfate reducing bacteria, which resulted in an enrichment of sulfur and light aromatics and a slight depletion of saturated hydrocarbons. This biodegradation progressively increases updip, as evidenced by higher sulfur contents and lower API gravity.