Depositional and diagenetic facies of the Leonardian (Permian) Abo and Drinkard carbonates, West Dollarhide Field, southeastern New Mexico
Johnson, Imelda Gorman
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The Abo and Drinkard units in the West Dollarhide Field, located in Lea County, New Mexico, were deposited on a shallow-water carbonate platform along the westem flank of the Central Basin Platform during lower Leonardian time. Core was available for four wells, complemented by modem log packages for ~60 additional wells. The depositional facies indicate an environmental fluctuation between marginal- to highlyrestricted marine conditions. These facies have been altered by partial to complete dolomitization of some, but not all, of the subtidal- and intertidal-facies. The lower Leonardian Abo High Frequency-Sequence (HFS) was deposited in a low-energy restricted shallow-marine platform environment, in an inner-ramp setting. The basal Abo consists of subtidal bioturbated muddy wackestones to packstones, and silty intertidal laminated mudstones that form weakly cyclical stacked subtidal cycles. Three distinct reservoir intervals are described, but owing to their stacked vertical relationship, they were mapped as one reservoir zone. This zone is laterally compartmentalized into three distinct "pods." The Abo Highstand System Tract (HST) consists of a tidal flat complex that was pervasively dolomitized. The dolomite is invariably fme-crystalhne and the only porosity within this HST is associated with karst features that developed during a relative fall in sea level, which exposed the top of the Abo HST. The end of Abo deposition was marked by the development of a paleosol. This interval of subaerial exposure allowed development of a small karst system. The exposure surface corresponds to a relative fall in sea level that produced a regional 3rdorder sequence boundary. The upper lower Leonardian Drinkard HFS was deposited during a longer-term relative rise in sea-level in a less restricted inner-ramp depositional setting. The basal Drinkard Transgressive System Tract (TST) is characterized by silty tidal flat-capped subtidal facies, whereas the early-HST consists of stacked subtidal cycles and the late- HST is dominated by cryptmicrobial-laminated intertidal caps that overlie shallow subtidal facies. The only laterally continuous reservoir interval developed in this systems tract consists of an ooid grainstone shoal located toward the base of the early HST. This shoal attained a maximum thickness of 6 feet, and its development was restricted to the southem portion of the West Dollarhide Field. This portion of the early HST, as with the basal TST, was not pervasively dolomitized. The best reservoir facies developed within the late HST, in which the change from aggradational to a more progradational style of deposition produced broader facies belts. Four of these reservoir intervals consist of dolomitized shallow-subtidal peloidal packstones. These facies are capped by peritidal cryptmicrobial laminate intertidal and supratidal facies. It is this close association with an evaporitic peritidal environment that is proposed as the source of dolomitizing fluids in the late HST. Sabkha dolomitization led to the development of thin (5 to 15 feet), but laterally continuous, reservoir intervals. Dolomitization is considered to have been syn- to early post-depositional, and fits with either the sabkha or the reflux model for dolomitization of restricted platform carbonates. The majority of Abo facies were completely dolomitized to aphano- to fmecrystalline. In the Drinkard, the nature of dolomitization is facies-specific. Drinkard intertidal facies were replaced by aphanocrystalline dolomite, whereas the grain-rich subtidal facies were replaced by a medium- to coarse-grained dolomite. In general, the mud-rich subtidal facies were not, or were only partially, dolomitized. Two important factors may explain differences betsveen dolomitization in the Abo and Drinkard units: (1) the change in depositional setting, and (2) the nature of high-frequency sea level fluctuations inferred for the Drinkard. Models of depositional, diagenetic and petrophysical characteristics of the Abo and Drinkard units provide a better understanding of the stratigraphic framework on this part of the Central Basin Platform during lower Leonardian time. In addition, the degree of connectivity, or compartmentalization, of potential reservoir intervals can be clearly shown.
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