Petrology, Depositional Environments, and Petroleum Potential of the Codell Sandstone and Juana Lopez Members of the Carlile Shale, Southeast
Authors:   , Galperin Pavel 
Published: February 2023

The deposition of Middle and Late Turonian (Upper Cretaceous) strata in southeastern Colorado occurred in an epeiric sea that occupied the western interior of the American continent. During that time, large portions of all continents were flooded by shallow marine seas. Parts of the upper Carlile Shale and the lower part of the Niobrara Formation were deposited in the final stage of the Greenhorn marine cycle and during the beginning of the Niobrara marine cycle. The members of the Carlile Shale include (in ascending order): 1) Fairport Shale, 2) Blue Hill Shale, 3) Codell Sandstone, 4) Upper Carlile Shale, 5) Juana Lopez Shale, and 6) Sage Breaks equivalent shale and its southern equivalents in the Carlile Shale. The first three members were deposited during the Greenhorn regressive cycle, and the other three during the Niobrara transgression. These two cycles are separated by a disconformity on the top of the Codell Sandstone. Petrologic study of a Codell well-core (Forest Oil, T19S, R69W, Section 4, Colorado), employing petrographic, scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and well-log data were used to characterize the unit in the subsurface. Subsequently, these criteria permitted correlation of subsurface data to the published outcrop descriptions, and provided evidence necessary for the paleoenviromental interpretation and reservoir characterization of the Codell Sandstone. The Codell Sandstone in the southern Denver and northern Raton basins can be divided into three units: very silty, thin, fine-grained sandstone beds separated by shale partings; medium-grained, bioturbated, caleareous sandstone; and very fine-grained, non-calcareous sandstone that contains significant amounts of clay and silt. The vertical succession of facies of the Codell displays progressively shallowing environments of deposition. In ascending order these are: 1) a transitional zone from the shore to the shoreface (Lower Codell), 2) a barrier-island complex (Middle Codell) and 3) lagoons (Upper Codell). As regression proceeded, lagoons prograded seaward on top of the shoreface sands. In the north and northeastern parts of the study area, sub-aerial exposure led to erosion of the barrier islands. Eroded sediments were carried by longshore currents and redeposited to the south (e.g., Pueblo area). The beginning of the Niobrara transgression is recorded in the deposition of the Juana Lopez Member of the Carlile Shale, a transgressive condensed section. At the initial phase of sea level rise, water depths were very shallow, probably even shallower than those of Codell time. The energy of waves and currents was very high; this resulted in the development of the scour surface on the top of the Codell. With increasing water depths during Niobrara time, energy was decreasing. The deposition of the Sage Breaks equivalent shale member and the Fort Hays Limestone Member records this event. Analysis of reservoir geometry combined with petrophysical data reveal that the barrier-island complex of the Middle Codell is a possible target for oil exploration. Source intervals are located within the Carlile Shale and they are rich enough to generate hydrocarbons for the possible reservoirs. However, diagenetic changes, such as compaction and carbonate cementation, significantly reduced porosity, and authigenic clay precipitation affected permeability. This information suggests that significant commercial discoveries within the Codell Sandstone interval in southeastern Colorado are unlikely to occur.