Abstract

Core data from five key wells spanning the Denver Basin were tied to wireline log data and used to interpret the distribution of the Middle Turonian Codell Sandstone Member of the Carlile Shale across the Denver Basin. The character of the Codell’s upper contact is sharp with a localized top-down truncation across the basin, which is consistent with an associated unconformity surface. In contrast, the Codell’s lower contact varies from being gradational in most of the southern Denver Basin to being unconformable in the northern basin.

Log correlations reveal that the Codell is absent within an elongate northeast-trending swath up to 125 miles wide in northeastern Colorado. This elongate gap is herein referred to as the ‘No Codell Zone’ abbreviated as NoCoZo. Hypotheses to explain the absence of the Codell Sandstone in the NoCoZo include a lateral facies change from sandstone to shale, non-deposition of Codell-equivalent sediments across this area, post-depositional erosion, or a combination of these processes. Correlation of wireline logs across the northern and southern limits of the NoCoZo, combined with outcrop and core observations, suggest top-down erosion of the Codell increasing into the NoCoZo. However, the overlying Fort Hays Limestone is laterally continuous and has a relatively consistent thickness across the NoCoZo, suggesting two tenable hypotheses: 1) the NoCoZo represents an area of post-Codell erosion due to short-lived growth of a broad, low relief uplift that was no longer active during Fort Hays deposition; or 2) a stepped sea level fall and forced regression resulting in non-deposition of the Codell over this broad swath.

North of the NoCoZo, the Codell thickens northward to more than 40 ft into adjacent parts of Wyoming and Nebraska. In this northern area, the Codell has two main lithofacies in three laterally correlative zones, in ascending order: a lower bioturbated siltstone to very fine-grained sandstone ranging from 2 to 20 feet thick, a middle 2- to 10-foot thick laminated to bedded siltstone to fine-grained sandstone, and an upper 5- to 20-foot thick bioturbated siltstone to very fine-grained sandstone. Southeast of the NoCoZo the Codell thickens to as much as 80 feet in an east-trending belt from Pueblo, Colorado, into west central Kansas. The southern Codell can be divided into two coarsening upward parasequences, from basal muddy coarse siltstones to very fine-grained sandstones. The siltstones and sandstones in the southern Codell are mostly bioturbated with locally developed bedded facies at the top.