Understanding the Scale of Calcite Mass Transfer in a Nanoporous Reservoir, Cretaceous Niobrara Formation, Denver Basin, Colorado

Abstract

In contrast to open-system diagenesis of mesoporous carbonates, nanoporous carbonate reservoirs like the Cretaceous Niobrara Formation of northeastern Colorado largely undergo closed-system, water-buffered diagenesis in the form of calcite recrystallization and precipitation of internally sourced carbonate cements. Oxygen isotopic evidence from several regional Niobrara diagenesis studies suggests that recrystallization/cementation is a continuous process that increases with burial depth and temperature (Pollastro and Scholle, 1986; Humphrey etal, 2016). However, this simple diagenetic model overlooks significant isotopic variability on a smaller vertical scale—published δ18O-depth curves show repeated 1-2‰ shifts over 3-6 vertical inches in nearly every well analyzed. Such variability implies that diagenetic products are not distributed uniformly, even between single beds, which has implications to the length scales of calcite mass transfer. The scale of mass transfer, in turn, may have profound implications for evolution, abundance, and vertical heterogeneity of pores that gross regional observations do not reflect. In order to understand complexity introduced by spatially heterogeneous calcite diagenesis, qualitative and quantitative assessments of the calcite cements, their timing, and their chemistry have been performed on a much smaller vertical scale than previous works employed. Adjacent mm-scale chalk-marl laminations, lamination bundles, and decimeter-scale chalk-marl couplets were sampled from drill cores to cover the range of observable lithologic variability, and were tested using cathodoluminescence petrography, stable oxygen isotopes, and elemental analysis. Early data suggest that within the Niobrara there are multiple generations of calcite cement that have been previously unrecognized, and that calcite diagenesis has created a repartitioning of calcite between marls and chalks at multiple scales. Preliminary correlations also suggest that these non-uniform diagenetic processes may lead to lithologically selective porosity preservation, rather than a uniform destruction of porosity with increasing burial depth.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017