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Phylloid-algal mounds form heterogeneous hydrocarbon reservoirs in the southeastern portion (Blanding sub-basin) of the Paradox Basin. Well-studied Lower Ismay mounds exposed along walls of the San Juan River gorge in the vicinity of Eight Foot Rapids, the west limb of the Raplee Anticline, and at the classic Honaker Trail locality (southwestern Paradox Basin) have often been cited as outcrop analogs of productive subsurface mounds. Until now, however, there has not been a complete description of the distribution, size, and spacing of outcropping algal mounds at the classic Eight Foot Rapids locality. The Lower Ismay sequence was analyzed in the context of a build-and-fill model of deposition. There are three facies associations within the sequence: 1) a basal lowstand to middle highstand pre-mound facies association, 2) a late highstand to middle falling stage phylloid algal-dominated relief-building facies association, and 3) a late falling stage, post-mound relief-filling facies association. Above the basal maximum flooding surface (Gothic Shale), the facies succession displays a distinct shallowing upward trend through the Lower Ismay sequence. Mound dimensions and facies stacking patterns permit evaluation of two depositional models. The first is a traditional, moderate- to low-energy model of vertical and radial mound accumulation of phylloid algal plates. The second is a high-energy, tidally influenced model of accumulation wherein mounds become hydrodynamically elongate. Outcrop data indicate that algal-dominated buildups are domal in shape with no preferred axis of elongation. These patterns do not support a hydrodynamic accumulation of loose algal plate fragments. The absence of in-situ algal thalli in all but the upper few tens of centimeters of the mounds, however, argues against a purely biological/ecological origin of mounds. A down-stepping ramp model is proposed wherein a muddy algal facies was deposited at the base of the mounds in the low energy of the outer ramp, followed by a grain-rich algal core in the mid-ramp environment. Mounds tops accumulated in an algal bafflestone facies in the inner ramp setting. Restriction of energy due to basinward algal buildup may have also contributed to deposition of algal bafflestone. Mounds accumulated radially at differential rates and were influenced by these variations in energy. This differential deposition of microfacies and subsequent diagenetic alteration have produced heterogeneities in algal reservoir rock, producing algal mound reservoirs that have a high potential for compartmentalization.

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.

The Chimney Rock and Gothic units of the Pennsylvanian Paradox Formation have long been considered source rocks for the rich hydrocarbon fields of southeastern Utah. Fundamental questions about these units include: source and nature of the organic material, source rock character, and position of the source rocks in the existing sequence stratigraphic framework. The Chimney Rock and Gothic, historically referred to as shales, are composed of calcareous mudstone, dolomudstone, and calcareous sandstone. High total organic carbon (TOC) values are more closely linked to sequence stratigraphic position than lithology. In the Gothic, TOC values decrease upwards. Terrestrial maceral content increases upwards in both the Gothic and the Chimney Rock as determined through point-count and qualitative observation. Pyrolysis indicates that greater than anticipated terrestrial influence is present and is consistent for all wells. No distinct difference in geochemical character exists between the two units. Sequence stratigraphic boundaries appear to be as good as, or better, than traditional lithostratigraphic boundaries for determining high TOC occurrence and source rock location. Within repetitive major sea level transgressions the organic matter that fed the basin evolved from a marine-dominated signature to a terrestrial-dominated signature. / text