--> --> Facies architecture and sequence stratigraphy of the Codell Sandstone, Denver Basin, CO

AAPG Rocky Mountain Section Meeting

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Facies architecture and sequence stratigraphy of the Codell Sandstone, Denver Basin, CO

Abstract

Based on detailed descriptions of outcrops and cores, the Codell contains eight facies: bioturbated sandy mudstone, bioturbated muddy sandstone, bioturbated sandstone, planar parallel laminated sandstone, hummocky crosslaminated sandstone, wave ripple cross-laminated sandstone, cross stratified sandstone, and laminated mudstone. Facies can be grouped into two facies associations: bioturbated and laminated facies. The bioturbated facies are characterized by a high bioturbation index (BI) and high diversity of trace fossils, indicating low energy, normal marine conditions. The laminated facies are characterized by a lower BI and similar diversity, indicating higher energy (storms), normal marine conditions. The Codell displays an upward coarsening succession of facies interpreted as progradational shoreline deposits. However, the transition from bioturbated facies to laminated facies is “punctuated” by erosional surfaces that compartmentalize the Codell into discrete reservoir flow units. Photomosaic mapping of outcrops reveals that the erosional surfaces and overlying laminated facies are laterally continuous for up to three miles. The laminated facies also have an anomalously high thorium content that creates a “hot” gamma ray marker that can be correlated for several miles between cored wells. The surfaces are nearly horizontal, show no clinoform geometry, and are interpreted as parasequence-scale, low stand surfaces of erosion, formed during times of base level lowering in areas of low accommodation. Storms reworked the underlying sediment into laminated facies. Laminated facies grade upward into bioturbated facies, indicating that base level gradually rose and predominantly low energy conditions returned. Regionally correlatable, sub-horizontal flow units of laminated facies provided important pathways for oil and gas migration in the Denver Basin. They also contain the best reservoir quality sandstone for horizontal well placement.