The Pictured Cliffs Sandstone (Upper Cretaceous) is a progradational nearshore-marine sand body that was deposited in the San Juan basin (Colorado-New Mexico) during the last regression of the Western Interior Seaway. Previous workers interpreted the unit as deltaic or beach-barrier environments. In order to interpret the depositional environment of the Pictured Cliffs Sandstone, detailed lithofacies analysis of the exposed outcrops along the northern portion of the San Juan basin is carried out. Gamma-ray log and density log along with well core data are used to determine the subsurface lithofacies. The architecture of the reservoirs is understood by correlating the subsurface well data with outcrop data.
From this study, 14 different lithofacies and 4 lithofacies assemblages are identified and interpreted to be part of storm-dominated clastic shelf and shallow marine strand-plain environments. The most common lithofacies in these environments are hummocky-stratified sandstone (Sh), planar-laminated sandstone (Sl), and scoured erosional surface (Ss). Distal delta-plain deposits are also found within the Pictured Cliffs Sandstone, but there is no local evidence of a delta system or channel deposit. The paleocurrent analysis suggests the paleoflow is to the southwest. The provenance analysis suggests the sediment source is from the Laramide uplift, the San Juan Dome to the north.
Lithofacies assemblages separated by bounding surfaces are classified to represent architectural elements. In this study, thirteen architectural elements are identified of which four are tempestite elements, three are amalgamated tempestite elements, and four are turbidite elements. In addition, four orders of bounding surfaces are identified, which represents the internal architecture of these elements. The geometry of the architectural elements are determined using photomosaics from the outcrops along with the hierarchy of bounding surfaces and lithofacies association found between successive bounding surfaces. The geometries are found to be: 1) lenticular/ribbon, 2) wedge/prism, and 3) tabular/sheet deposits.
In outcrops, the Pictured Cliffs Sandstone is up to 95 m thick, extending from an intercalated basal contact with Lewis Shale (offshore marine) to an abrupt contact with the Fruitland Formation (nonmarine coal beds). The lithofacies are organized into sequences at meso-scale, macro-scale (parasequences), and mega-scale. At the meso-scale, four typical small-scale sequences can be recognized: (1) erosional scours overlain by hummocky-stratified sandstone, followed by planar-laminated sandstone, overlaid by ripple-laminated sandstone and/or mudstones (interpreted as a tempestite sequence); (2) heterolithic, wavy bedded or ripple laminated sandstone and mudstone drapes (interpreted as tidal rhythmites); (3) interbedded mudstone and distal turbidite sequences up to 3-m thick (interpreted as prodelta deposits); and (4) massive sandstones interbedded with planar laminated sandstone up to 3-m thick containing intraclasts and load casts (interpreted as flood deposits). At macro-scale, parasequences up to 25 m thick can be recognized. These parasequences thickens- and coarsens-upward, and are interpreted to be prograding shoreline sequences of: 1) storm-dominated prograding shoreface, 2) prograding shoreface influenced by distal delta, 3) prograding strand-plain. There are seven such parasequences observable in the northern part of the San Juan basin, indicating a subtle relationship between tectonic subsidence and eustasy.
In the subsurface, the thickness of the Pictured Cliffs Sandstone ranges from 30 to 40 meters. Subsurface lithofacies are analyzed using well log and well core data approximately 15 ¿¿¿¿¿¿¿¿¿¿¿¿“ 25 km south of the exposed outcrops. Gamma-ray logs and density logs are used to determine the subsurface lithofacies along with the well core (S023) from Well-1. Lithofacies Sh, Sp, Sl, SMk, SMf, SMw, SMl, and Mm are identified from Well-1. These lithofacies are interpreted as prograding strand-plain environment and represents the contact between the Pictured Cliffs Sandstone and Fruitland Formation. The lithofacies identified from well data are matched with the outcrop section to conform the interpretation.
Gamma-ray log curve patterns are defined to determine the facies assemblages for all the three wells. The shapes of the curves from Well-1 and Well-3 are found to be mostly funnel-shaped representing coarsening-upward sequence with some bell-shaped, cylindrical, and irregular patterns. These shapes of the curves along with the subsurface lithofacies are interpreted to be offshore mudstone, overlain by transition zone interbedded sandstone mudstone, overlain by shallow marine, barrier-island, or wave-dominated shoreface sequence of a prograding shoreline sequence. Well-2 shows irregular patterns with an overall coarsening-upward sequence. The patterns of the gamma-ray logs from Wells — 1, 2, and 3 are used for correlation. The correlation of well logs shows basin scale variations in thickness and architecture of the reservoir sandstone units of the Pictured Cliffs Sandstone in the third dimension.