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Stratigraphic cyclicity and reservoir heterogeneity within upper San Andres and Grayburg strata (upper Permian-Guadalupian), Maljamar field, Se New Mexico
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Cores, logs, and 3D seismic data from Maljamar field (Lea County, southeast New Mexico) were examined in this study and used to construct a detailed sequence stratigraphic framework. Upper San Andres strata are divided into three high-frequency sequences (HFSs). The "maximum flooding intervals" of these HFSs consist of stacked and amalgamated meter-scale cycles which contain mostly subtidal carbonate facies (reservoir facies). In contrast, late highstand to early transgressive systems tracts ("Minimum accommodation intervals") contain cycles that are dominated by non-reservoir peritidal carbonate facies. "Maximum flooding intervals" of individual HFSs therefore comprise the primary flow units. Diagenetic overprinting of primary depositional fabrics, however, has complicated this picture and resulted in extremely complex permeability distributions. Two scales of meteoric diagenesis affected upper San Andres strata at Maljamar field: 1)Short-term (cycle-scale) subaerial exposure events resulted in local porosity inversion (interparticle to moldic) and variable degrees of cementation/dissolution of carbonate sand-shoal deposits.) Long-term subaerial exposure events related to HFS boundaries resulted in locally extensive dissolution and collapse of upper San Andres strata. Cavernous porosity was later plugged with massive anhydrite, resulting in the degradation of reservoir quality. In the overlying Grayburg Formation, cycles consist of mixed sandstone and shallowwater carbonate facies. In the study area, contacts between massive, quartzose sandstones and underlying shallow marine carbonate facies are typically gradational, suggesting that sandstones along the middle Grayburg platform were deposited during high-frequency base-level falls. Massive sandstones are interpreted as eolian sand sheet (erg margin) deposits. Petrographic evidence for an eolian origin of Grayburg sandstones includes the virtual lack of textural variation between sand grains, early evaporate cements, and rare occurrence of low-angle ripple laminae, interpreted as wind-ripple laminae. The interpretation of Grayburg sandstones as dominantly eolian sediments that were deposited during high-frequency base-level falls and lowstands provides an explanation for)the poorly developed cycle stacking patterns that are typical of mixed sandstone-carbonate strata of the Grayburg Formation-nation. Siliciclastic facies largely accumulated above marine base level, effectively filling "negative platform accommodation," and resulting in amalgamation-nation of sandstone cycles during times of reduced long-term accommodation. Because sandstone facies are interpreted as the most "regressive" facies within the Grayburg Formation, vertical trends in the dominance of nonmarine sandstone versus shallow-marine carbonate facies (visible in gamma-ray logs) can be used to constrain detailed sequence stratigraphic and lithologic correlations.
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Modica, Christopher James (1997). Stratigraphic cyclicity and reservoir heterogeneity within upper San Andres and Grayburg strata (upper Permian-Guadalupian), Maljamar field, Se New Mexico. Master's thesis, Texas A&M University. Available electronically from
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