Pre-Conference Field Trip
Unconventional Reservoirs and Stratigraphy of the Southern Denver Basin: Graneros, Greenhorn, Carlile and Niobrara Formations
Sponsored by the PTTC
||Saturday, 23 August – Sunday, 24 August
||8:00 AM–4:00 PM
||Jeffrey A. May (Geologic Consultant, Littleton, Colorado) and Tofer Lewis (Enerplus, Denver, Colorado)
||Guidebook, transportation, lunch, refreshments and one overnight accommodation in Pueblo, Colorado
||Departs and returns to the Colorado Convention Center
West of Pueblo, Colorado, the Arkansas River has cut across the broad Rock Canyon Anticline, revealing approximately 4,000 feet of Middle to Upper Cretaceous strata. Excellent exposures, as well as abundant macro- and micro-fossils plus bentonites, make this a classic area for defining the physical, bio- and chrono-stratigraphy of the Cenomanian to Campanian stages (99.6 to 77.6 Ma). Both small-scale depositional cycles as well as high-energy event beds are superimposed on large-scale, transgressive-regressive successions. Of special note are exceptional mudrock and tight sandstone outcrops of the Graneros, Greenhorn, Carlile and Niobrara formations, currently of great interest for oil and gas exploration and the focus of this field trip. G.K. Gilbert first described much of this section in the late 1800s. He observed rhythmic alternations of argillaceous shale, calcareous shale and limestone in the Greenhorn and Niobrara sections, postulating that intermittent transport of terrestrial clay into the ocean diluted biogenic carbonate sedimentation. Gilbert believed these patterns likely reflected periodic variations in the Earth’s orbit, now known as Milankovitch cycles.
We will examine outcrops illustrating the detailed structural, stratigraphic and sedimentological variability within the contact between the Greenhorn and Niobrara Cycles. This contact has long been recognized as a significant regional unconformity throughout the Cretaceous Western Interior. Exposures in the southern Denver Basin preserve distinct facies and surfaces within the Codell Sandstone and Juana Lopez members of the upper Carlile Shale and the basal Fort Hays Limestone Member of the Niobrara that otherwise are not observed through the central and northern portions of the basin.
During our trip we will view another significant feature: the Global Boundary Stratotype Section and Point (GSSP) for the Cenomanian- Turonian boundary. This horizon occurs within the Bridge Creek Limestone Member of the Greenhorn Formation. The Cenomanian- Turonian GSSP is defined here by widespread bentonites with radiometric ages of 93 to 93.5 Ma and a positive excursion in C-13 isotopes corresponding to a global oceanic anoxic event (OAE II).
This field excursion ultimately provides an opportunity to examine geologically significant and well-exposed reservoir intervals, source-rocks and stratigraphy of the Graneros, Greenhorn, Carlile and Niobrara formations. We will compare and contrast depositional and erosional events in argillaceous, siliceous and calcareous successions. Finally we will relate parameters important when evaluating unconventional plays — such as lithology, organic-carbon content, chronostratigraphic framework and mechanical stratigraphy — to the underlying controls of sea-level change, orbital and climatic cycles and oceanic oxygenation and circulation.