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THE PARADOX Basin is a northwest-southeast trending intracratonic basin that formedin southwestern Colorado, southeastern Utah and adjacent parts of Arizona and New Mexicoduring the late Paleozoic Era. During rise of the adjacent Uncompahgre Uplift (Ancestral RockyMountains) the rapidly subsiding basin was filled with over 2000 m of Permo-Pennsylvaniansediments. Stacked depositional sequences accumulated in three roughly parallel facies belts: anortheastern clastic belt (adjacent to uplift), a central salt and black shale belt, and asouthwestern carbonate belt. Over 400 million barrels of oil have been extracted from upperParadox (Desert Creek and Ismay) carbonates in the southern Blanding Subbasin (Greater AnethField) since 1956. The sedimentology and sequence stratigraphy of Paradox Shelf strata on thewalls of the San Juan River gorge and in the subsurface Aneth Buildup are well documented.Less well documented are the stratigraphy and facies architecture of basinward extensions ofupper Paradox sequences in the northern part of the Blanding Subbasin.Detailed analysis of the lower and upper Desert Creek and lower and upper Ismay 4thordersequences from three cores (Long Point, Lewis Road, Cedar Point) demonstrate theexistence of distinctive basinward depositional trends. Compared to sequences exposed on theParadox Shelf (San Juan River outcrops) and the Aneth Buildup, sequences in the more distalnorthern Blanding Subbasin are thinner, are dominated by muddy carbonate facies, displaylimited occurrences of porous phylloid-algal and oolitic carbonates, contain thicker, morecomplete occurrences of black shale, and possess distinctive suites of lowstand facies (quartzsandstone on the shelf, bedded and nodular evaporates in the basin). Vertically, the four 4th-ordersequences display 2nd-order progradation of the Paradox Shelf through Desert Creek and Ismaytime. Carbonate-starved sequences (4th order) and parasequences (5th order) comprised of muddominatedfacies are succeeded upward by thicker, more grain-rich sequences andparasequences. The implications for the petroleum system relative to established oil and gasfields is that conventional reservoir rock facies are rare, except in small, isolated buildups.Meteoric diagenesis associated with 4th-order lowstands of sea level has reduced overallpermeability. Lowstand conditions also promoted limited precipitation of pore-occludingevaporite cement. The maximum-flood Chimney Rock, Gothic and Hovenweep shales arethicker and contain a more complete succession of basinal cycles than updip occurrences of thesepetroleum source rocks. A suite of samples from the Gothic Shale from the Cedar Point coreindicate higher burial maturity (kerogen has mostly been converted to gas) compared to valuesderived from the outcrop belt and more proximal subsurface samples.
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