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Diagenesis and petrophysics of the Upper Cretaceous, Pictured Cliffs Formation of the San Juan Basin, North West New Mexico and South West Colorado
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Diagenesis is the major factor affecting reservoir producibility of the Upper Cretaceous Pictured Cliffs Formation across the San Juan Basin. Four diagenetic periods have been delineated, early, burial, maximum burial, and recent. Basinal variations in pore water chemistry, and thermal history results in different authigenic assemblages to the north and south of the basin. Based on authigenic clay type and abundance, three clay zones exist within the basin. Zone I-the smectite/kaolinite zone; Zone 2-the kaolinite/iHite-smectite zone; and Zone 3-the illite-smectite zone. These zones correlate to changes in cumulative hydrocarbon production, feldspar content, porosity types and porosity and permeability. These lateral variations correlate to three petrophysical rock types in the Pictured Cliffs Formation. Capillary pressure determines the producibility of a formation. In the Pictured Cliffs the synthetic capillary pressure characteristics of the three petrophysical rock types are a result of the diagenetic assemblages specific to each zone. Petrophysical rock type 1 appears to be the best reservoir, but in fact is water saturated, forming a relative permeability barrier to gas migration updip to the south. Petrophysical rock type 2 coincides with the zone of maximum hydrocarbon production. This is the best reservoir in the Pictured Cliffs sandstones. Petrophysical rock type 3 forms a "diagenetic seal" to the north of the basin. Production "hot spots" may exists in zone 3, in sandstones with higher percentages of feldspars and rock fragments relative to quartz, since they have a greater potential for preserving secondary porosity generated by carbonate dissolution. Log analysis revealed that the Waxman-Smits shaly sand calculation yielded the highest hydrocarbon saturations. A lab derived cementation exponent (m = 1.6) used in the Archie equation in sands with < 9% authigenic clay gave similar saturations to those of the Waxman-Smit method. The Dual Water and Fertl (1975) methods gave similar, but less optimistic hydrocarbon saturations. Clay volume (Vcl) determined from the gamma ray and from x-ray diffraction (xrd), yielded very similar water saturations. However if core porosity and Vcl from xrd were used in the equations, water saturations were much lower.
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Includes bibliographical references: p. 105-110.
Issued also on microfiche from Lange Micrographics.
Goberdhan, Helene C (1996). Diagenesis and petrophysics of the Upper Cretaceous, Pictured Cliffs Formation of the San Juan Basin, North West New Mexico and South West Colorado. Master's thesis, Texas A&M University. Available electronically from
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