Macintosh computer display of the history of oil and gas exploration in the Denver basin of Colorado, Nebraska and Wyoming

The history of calcite diagenesis and origin of exceptionally negative oxygen isotope values in chalks of the Niobrara Formation, Denver Basin, USA

“…The topographically driven flow system was probably artesian (over pressured) and that in turn drove the meteoric groundwater upwards into the Niobrara chalks along conduits, such as fractures, the deep‐seated wrench faults of Weimer (1996), and the shallower polygonal fault system of Sonnenberg et al (2016), Radiogenic Sr and Nd would have been derived from rock–water interaction with siliciclastic minerals along the flow path. This flow system would have existed until either gas generation in the mid to late Cenozoic (Higley & Cox, 2007; Landon et al, 2001) created over pressure in the Niobrara chalks and underlying mid Cretaceous rocks (Spencer, 1987; Swarbrick & Osborne, 1998; Swarbrick et al, 2002; Weimer, 1996), or tilting and erosion in the last 27 Myr created the current under pressure in the aquifer rocks (Belitz & Bredehoeft, 1988; Umari et al, 2018). This meteoric flux is known to have driven post‐oil generation burial diagenesis in Lyons (Permian) oil wells in the basin axis (Lee & Bethke, 1994).…”

“…The Niobrara Formation in the Denver Basin has a different burial history along the basin's axis and eastward relative to the basin's far western margin that abuts the Colorado Front Range, which was subjected to Laramide structural deformation (Higley & Cox, 2007; MacMillan, 1980; Tainter, 1984). Only the former is of interest to this study and that burial history can be broken down into three major segments (Figure 5).…”

“…Sites farther east would have attained lesser depths due to thinner Pierre Shale. In the second burial segment, Niobrara rocks in the basin's axis experienced 600–800 m of additional burial due to deposition of Palaeocene alluvial deposits (Higley & Cox, 2007), and thereafter remained at a relatively uniform depth to the end of the Eocene. The last burial segment involved deposition of less than a hundred metres of late Cenozoic sediment, followed by erosional uplift over the last 7 million years of 300–1,200 m depending on location within the basin (Higley & Cox, 2007).…”

“…The Bayesian Information Criteria (Schwarz, 1978) indicates that the non‐linear model, which is a parametric transformation of the depth equation given by Schmoker and Halley (1982), is a more robust representation than a linear model. Depths for shallow wells towards the eastern margin of the basin are plotted at sample depth plus 400 m as a minimum correction for sediment eroded in the last 7 Ma (Higley & Cox, 2007); depths for deeper wells in the basin axis are plotted at sample depth plus 800 m to account for even greater eroded material (File S3).…”