Expression of the Colorado Mineral Belt in Upper Cretaceous Niobrara Formation Source Rock Maturity Data from the Denver Basin

Abstract

New source rock maturity data along the Colorado Mineral Belt trend in the Denver Basin reveal that source rocks in the deepest portion of the basin range from the onset of oil generation to wet gas maturity across a distance of less than 30 miles along present day structure. Additionally, sampled rock core and cuttings along a northeast-southwest transect reveal that the Niobrara Formation is within the oil maturity window all the way to the Nebraska-Colorado border. The correlation of these analyses to an identified thermal anomaly demonstrate that maturity along these trends is affected by a historical increase in heat flow that can still be seen in the present-day bottom-hole temperatures. The identified maturity anomaly has significant implications for Niobrara prospectivity within the basin. Crossplotting, mapping, and numerical modeling show the onset of hydrocarbon maturity in the Niobrara is represented by 432 °C Tmax and that hydrocarbon expulsion occurs between 438 °C and 443 °C Tmax. In the Niobrara Formation of the Denver Basin there is a strong correlation between oil and gas shows, elevated bottom-hole temperatures (and thermal gradients), and geochemical maturity parameters. Through mapping of maturity and free hydrocarbon anomalies, more than 80% of the present day production can be predicted with source rock mapping.

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

“…The lowest gradient is for the Schock Errington well to the east (Figure 1). But there is still a variation of nearly 20°C/km in the five wells within the Wattenberg Field, an observation also noted by Thul and Sonnenberg (2018). The variability in those five wells is primarily due to the ca 35°C variations in Tb peak rather than the 30–200 m differences in maximum projected burial depths.…”

“…Prior workers have attributed elevated palaeo heat flows in the Wattenberg Field to heat pulses along the Colorado Lineament that extends eastward into the Denver Basin from the CMB (Higley et al, 2003; Inks et al, 2019; Thul & Sonnenberg, 2018). Magmatism and mineralisation at the north‐west end of the CMB trend is evidenced by ore deposits within 160 km or less of the study area.…”

“…Niobrara thermal maturities estimated from pyrolysis experiments (Landon et al, 2001; Rice, 1984b; Thul & Sonnenberg, 2018) indicate anomalously mature organic matter (vitrinite reflectance ≥1.2; estimated peak burial temperatures ≥150°C) in the greater Wattenberg area. These high temperatures are greater than observed bottom hole temperatures, and have been attributed to either high heat flux associated with the Colorado Mineral Belt (CMB), whose SW–NE trend extends under the study area (Higley et al, 2003; Inks et al, 2019; Thul & Sonnenberg, 2018), and/or thermal insulation by Late Cretaceous and Palaeocene coals subsequently lost to erosion (Coskey & Cumella, 2015; Crysdale & Barker, 1990; Rice, 1984a).…”