High-Resolution Chemostratigraphic Analysis of Wolfcamp D Shale-Unit in Upton and Midland Counties - Is it possible to identify stratigraphically significant surfaces using δ13Ccarb?

Abstract

The Late Paleozoic Ice Age represents a dynamic period in Earth system history recording a shift from icehouse to greenhouse conditions. Concomitant with this change was a series high-frequency, high-amplitude sea level fluctuations leading to the deposition of “Kansas type” cyclothems in the Mid-Continent, and a similar rhythmic expression of interbedded shales and carbonates in the Midland Basin. Geochemistry has the potential to highlight certain environmental conditions that may not be readily apparent at the core or outcrop scale. Stable isotope geochemistry is a particularly powerful tool when examining mud-rich successions, because changes in organic matter partitioning may be recorded first in δ13CDIC of sea-water and consequently in δ13Ccarb of marine rocks. Provided significant post- depositional alteration has not occurred, δ13Ccarb may also be used to document distinct chronostratigraphic time horizons within the subsurface. Here we use principle component analysis of a high- resolution XRF dataset in tandem with δ13Ccarb measurements to document periods of organic matter enrichment as well as post-depositional organic matter destruction. We identify several thin bed intervals (<6 in.) marked by sharp negative δ13Ccarb excursions and sedimentary features consistent with low sedimentation. These intervals are interpreted to have undergone significant bacterially mediated diagenesis and may mark periods of limited or non- deposition. We hypothesize these intervals also have the potential to be used as chemostratigraphic tie-points within the basin. Results of this study indicate that while post-depositional diagenetic processes in these Late Pennsylvanian mudstones have altered the primary δ13Ccarb signal, δ13Ccarb may still be used to capture potential correlations intra-basin. The workflow presented herein may aide correlation in mud-rich environments of deposition.

AAPG Datapages/Search and Discovery Article #90343 ©2019 AAPG Southwest Section Annual Convention, Dallas, Texas, April 6-9, 2019