New Mexico Geological Society Annual Spring Meeting — Abstracts

Fren (Seven Rivers) pool is areally and volumetrically the largest oil field producing from the upper Seven Rivers Formation (Leonardian, Permian) on the northwest shelf of the Delaware basin. The field has yielded 5.4 mmbo from 1943 through 1996 when it was abolished and integrated into the Grayburg Jackson pool. Cores were collected from two wells in the heart of the field, representative of the entire producing interval, to gain a better understanding of the pay. Much of the cores (>90%) are anhydrite, but shale and carbonate laminations are also present. Thin (cm's) laminated dolomite stringers with porosity exceeding 20% comprise the pay zones. Field structure is an east -plunging anticline formed by drape of Leonardian units over the Wolfcampian Abo reef of the Artesia-Vacuum trend.

Cores were examined under ultraviolet light and 62 samples were selected for petrographic analysis based on ultraviolet fluorescence due to oil saturation. Petrographically, the carbonates have undergone extensive dolomitization and replacement by anhydrite. Dolomites range from very finely to finely crystalline. Anhydrite occurs as displacive crystal masses and nodules grown syndepositionally within the algal mudstones and as poikolotopic, sub-hemal to euhedral, coarsely crystalline later replacements and pore fills. The petrographic data indicates that dolomitization was probably syndepositional (evaporative pumping) continuing into shallow burial (possibly schizohaline). Due to the high salinity of the pore fluids, anhydrite and/or gypsum, instead of calcite, plugged some of the surviving primary porosity (intergranular) and secondary porosity. Secondary porosity within the dolomite stringers ranges from fracture, moldic to intercrystalline.

Most of the primary depositional fabrics within the carbonate stringers were obliterated by dolomitization. Evaporative sabkha conditions dominated this area which was located 32 km back of the Capitan/Goat Seep reef. Preserved carbonate fabrics indicate environments ranging from higher-energy back-reeflbeach to sabkha deposits including bioclastic and oolitic packstones, pisolitic packstones to algally-Iaminated fenestral mudstones. Subtle differences in the paleo-topography probably played a role in the deposition of the high-energy beach/shoal units during periods of higher sea-level stands since the field is coincident with the deep-seated Abo reef structural axis. However, depositional environment does not appear to playa large role in secondary porosity preservation. The highest porosity intervals usually correspond to sections where dolomite is the coarsest crystalline and anhydrite replacements and compaction are minimaL Secondary porosity resulting from calcite dissolution after dolomitization may have been preserved from complete destruction by anhydrite/gypsum precipitation due to early oil migration into the carbonate stringers, preventing further porosity occlusion. Alternately, porosity may have been resurrected during initial phases of oil migration by less saline pore fluids migrating through the more permeable carbonates.