Petrophysical study of Pennsylvanian Atoka carbonate bank complex Eddy County, New Mexico
On the northwest shelf of Delaware Basin in Eddy County, New Mexico, Pennsylvanian (Atoka) sediments are represented by an algal bank carbonate complex interbedded with marine shales. The reservoir rocks are low porosity carbonates with neutron-density crossplot porosities of less than 5 percent. The primary objectives for this study are: (1) to determine the depositional and diagenetic factors controlling the reservoir quality; and (2) to predict the location of productive zones. The analysis includes petrographic, petrophysical, x-ray and electron microscopy methods. The lithofacies of bank deposits and their surrounding sediments are: (1) upper shale; (2) crinoidal limestone (grainstone/packstone); (3) nodular shaly limestone; (4) bank margin (algal grainstone and boundstone); (5) algal bank (wackestone); (6) marginal breccia; (7) basal bioclastic micritic pile (wackestone); (8) limy shale; and (9) lower shale. The red alga (Archaeolithophy1lum) is the primary fossil in Atoka carbonates, and indicates that the carbonates were predominantly deposited in relatively shallow water environments at depths less than 100 feet/30 meters. A structural map on top of the Atoka carbonates reveals that the bank complex contains several elongated biostromes (banks). The banks developed along the shelf margin, and then prograded seaward during regression. The banks reached a maximum thickness of 70 feet by vertical accretion. The vertical accretion was interrupted at least four times by sea-level fluctuations. The bank development was terminated by transgression, which caused shifting of bank development northwestward into shallow water. During this transgression, the bank complex was overlain by marine shales (upper shale). The Atoka carbonates have undergone a complex diagenetic history that includes marine, subaerial, and burial diagenetic environments. The most important diagenetic events affecting the reservoir quality are dissolution and cementation. Surrounding shales of the bank complex have been subjected to a high degree of illitization. The illitization reaction released Fe^* and Mg^* ions, which were transferred from shales to adjacent carbonates to form saddle dolomite cement. Determination of hydrocarbon productive zones in the Atoka carbonates was based on the Archie water saturation equation (Swa). The intervals that contained net pay with cutoffs of clay volume (Vcl) < 10%, effective porosity (^e) > 3%, and Archie water saturation (Swa) < 30%, are productive.
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