Reports: AC847341-AC8: Controls on Marginal Marine and Nonmarine Stratigraphic Architecture: New Constraints from the Cretaceous Straight Cliffs Formation, Utah

Cari L. Johnson , University of Utah

This research seeks to define sedimentary facies architecture and reservoir potential in the Cretaceous John Henry Member (JHM) of the Straight Cliffs Formation, southern Utah. We are conducting a field-based analysis of shallow marine and nonmarine clastic deposits, with an emphasis on developing well-controlled facies models to improve prediction in analogous subsurface reservoirs. The study area around the Kaiparowits Plateau features high-quality, three-dimensional exposures of JHM strata. It is therefore possible to physically track facies transitions in all directions, as well as the major surfaces bounding stratigraphic packages. Our results bear directly on fundamental principles of sequence stratigraphy and their applications to energy exploration. Fieldwork to date has focused on a cross section across the southern Kaiparowits Plateau, where we focus on the marine-nonmarine transition from east (paleoshoreline) to west. Key results to date are summarized below:

1. Marginal marine strata of the JHM near Rogers Canyon and in Left Hand Collet Canyon (southeast Kaiparowits Plateau) were deposited within a moderately high accommodation and high sediment supply setting that facilitated preservation of both transgressive and regressive marginal marine deposits. Complete transgressive-regressive cycles (T-R cycles), comprising barrier island lagoonal transgressive deposits interfingered with regressive shoreface facies, are distinguished based on their internal facies architecture and bounding surfaces. The preservation of transgressive facies under moderately high accommodation and sediment supply conditions greatly affects stratigraphic architecture of T-R cycles. Acknowledging variation in T-R cycles, and recognizing transgressive successions that correlate to flooding surfaces basinward, are both critical to achieving an accurate sequence stratigraphic interpretation of  high-frequency cycles.

2. Marginal marine strata are correlated to paralic and coastal plain strata in the Kelly Grade area (south-central Kaiparowits Plateau). New outcrop and subcrop data from this area illustrate regional nonmarine-marine stratigraphic correlations and address sequence stratigraphic models based on such correlations. Three facies associations identified in fluvial and paralic sections correlate to downdip marine and shoreline equivalents as follows: Facies association 1 (FA-1, the lowermost interval) consists of tidally influenced, laterally restricted fluvial channel belts, coastal mire, and shoreface sandstone. FA-1 correlates to a lower marine package that shows net progradation and consists of vertically thick, laterally extensive regressive shoreface sandstones intercalated with transgressive lagoonal deposits. Facies association 2 (FA-2, the middle interval) consists of laterally restricted, highly sinuous fluvial channel belts, lagoonal and estuarine coastal plain mires, bay-head deltas, isolated distributary channels and tidal channels. FA-2 correlates downdip with a middle marine package shows net transgression and consists of vertically thin, laterally restricted regressive shoreface deposits intercalated with thick transgressive lagoonal deposits and barrier island sandstone. Facies association 3 (FA-3) consists of laterally extensive, low sinuosity fluvial channel belts and vertically amalgamated fluvial channel belt complexes, and floodplain overbank. The marine equivalent of FA-3 shows net progradation and consists of vertically thick, laterally extensive regressive shoreface sandstones intercalated with transgressive lagoonal deposits. Preserved within each marine package are multiple transgressive-regressive cycles, but the fluvial architecture does not appear to respond to this scale of cyclicity. The observed evolution of fluvial systems and the inferred relationship to relative sea level change is distinct from previous interpretations of these strata.

3. Work on the most proximal alluvial-fluvial facies at Rock House Cove and at Ty Hatch Mesa (southwest Kaiparowits Plateau) defines broad depositional trends that relate to possible incised valley formation and fining-upward fill near the base of the JHM, versus a coarsening-upward trend at its top. Seven depositional units identified in proximal fluvial sections correlate basinward to three previously identified depositional units (facies associations (FA) of Gallin, 2010) in paralic sections and seven equivalent shoreface sandstones near the Kelly Grade.  The lower John Henry Member in the southwestern Kaiparowits Plateau consists of depositonal units 0 through 3 (DU-0 through DU-3). DU-0 consists of tidally-influenced, laterally restricted channel belts. DU-2 consists of laterally extensive single story laterally accreting channel belts. DU-0 through DU-2 correlate downdip to the tidally-influenced, laterally restricted fluvial channel belts and coastal mires of FA-1 and supply sediment for the thick, landward extending and net prograding shoreface sandstones A and B. DU-3 consists of thick floodplain muds and isolated channel belts that correspond to bay head deltas and lagoonal deposits of FA-2 and correlate to the Christiansen and lower Rees coal zones. The marine equivalents of DU-3 shows net transgression and consist of thin, laterally restricted C, D, and E shoreface sandstones. The upper John Henry Member in this area consists of DU-4 through DU-6 and grades from laterally restricted channel belts with low vertical amalgamation, to highly vertically amalgamating downstream accreting river channels. These depositional units are consistent from proximal to distal alluvial facies in the John Henry Member and correlate to the net prograding thick, landward extensive, net progradational F and G shoreface sandstones. Changes in fluvial architecture in proximal fluvial strata appear to respond to net progradation and transgression of the coeval shoreline, ~ 60 km to the east. These correlations refine previous cross section models of John Henry Member deposition.

4. Reservoir quality of these alluvial-fluvial depositional units in the southwestern Kaiparowits Plateau was evaluated based on grain size, average porosity, depositional unit net to gross, channel belt size, internal connectivity (heterogeneity) and channel belt connectivity. Highly amalgamated sandstone bodies of DU-1 in the lower John Henry Member and DU-6 in the upper John Henry Member present the greatest reservoir potential, whereas the isolated channels of DU-3 present the lowest reservoir potential. Two trends in reservoir analog quality (upward decreasing and upward increasing) correspond to defining trends in fluvial architecture (upward fining and upward coarsening) and demonstrate that models of alluvial architecture deposition may be used to predict reservoir quality. Outcrop interpretations, aided by Gigapan mosaics and a terrestrial Lidar database, are now integrated into preliminary synthetic seismic sections, to model imaging quality of analogous reservoir systems in the subsurface.

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