Petrophysical, XRF Elemental, and Geomechanical Comparison of the Greenhorn and Niobrara Formations, Wattenberg Field, Denver Basin

Abstract

The Niobrara and Greenhorn Formations are two carbonate intervals within the Upper Cretaceous strata of the Western Interior Basin. The carbonates are dominantly coccolith/foraminiferal chalks and shaly chalks/marls. The Niobrara is subdivided into four main chalk benches (A, B, C, and Fort Hays). The Greenhorn is subdivided into the Bridge Creek Limestone, Hartland Shale, and Lincoln Limestone Members. The limestone members are foraminiferal chalks and shaly chalks/marls. These two formations are compared to each other on the basis of petrophysical, XRF elemental, and geomechanical data. The chalk beds in the Niobrara (A, B, C, Fort Hays) have relatively low abundances of detrital-related elements (Al, Ti, K, Si) and elevated values of the biogenic indicator element (Ca). The Bridge Creek Member of the Greenhorn has similar low detrital elemental indicators and elevated Ca values (chalk indicator). The Lincoln Limestone Member has higher values of the detrital indicators, lower values for Ca, and thus is more clay rich. From a petrophysical analysis, the Bridge Creek is similar to the chalk beds in the Niobrara. The Bridge Creek has elevated resistivity and similar Neutron-Density porosity readings. The Lincoln Member has lower resistivity and separation in the Neutron-Density curves, indicating an increase in clay content. Poisson's ratio and Young's Modulus data are also compared between the Niobrara and Greenhorn Formations. The Bridge Creek has elevated and similar Young's Modulus values compared to chalk beds in the Niobrara. The brittleness index for the Bridge Creek is also similar to the Niobrara chalk beds. The interpretation of the well-log geomechanical data suggests that the Bridge Creek is more brittle than the Lincoln Member and is similar to Niobrara chalk beds. This comparison suggests that the Bridge Creek is similar to the Niobrara chalk beds and should be targeted as a possible unconventional play.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016