Bio:

Dawn Hayes is a Senior Geological Advisor for Premier Oilfield Group’s sedimentology team, providing clients with detailed integrated digital core descriptions, sedimentologic interpretations, and reports incorporating multi-faceted reservoir characterization data sets. She has experience working siliciclastic, carbonate, mixed, and mudrock-dominated depositional systems of all types, using core data to characterize both conventional and unconventional hydrocarbon reservoirs and CCUS targets in the U.S. and internationally. Prior to joining Premier in 2020, she was part of Oxy’s Rock & Fluid Properties team and several multidisciplinary Anadarko teams, beginning as a geologist supporting APC’s Marcellus asset development in 2013 and then working as a sedimentologist for both the Geoscience Technology and Delaware Basin Subsurface Integration groups until 2019. She earned her Ph.D. (2013) and M.S. (2010) degrees at Utah State University, where her research focused on documenting the stratigraphic, geochemical, and biotic records of the Earth’s oceans before and after the Neoproterozoic “Snowball Earth” global glaciations. In her life before geology, she completed undergraduate work at DePauw University in Indiana (B.A. Biology 1999) and Idaho State University (B.S. Secondary Education 2002) and taught high school biology and chemistry courses in South Carolina and Florida for 5 years.

Abstract: 

The Wallace Ranch#1 and #2 cores capture 90 feet of Middle Pennsylvanian (Desmoinesian) mudrocks, heterolithics, sandstones, and conglomerates in Eastern Kent County, Texas, comprising part of the Strawn Formation. Deposition occurred across a range of open marine and shallow to marginal marine environments in an epicontinental sea that evolved with sea level cycles at several different scales.  Multiple deltaic systems sourced siliciclastic sediments from the East (Ouachita-Marathon orogeny), resulting in delta progradation across the Eastern Shelf during a highstand systems tract composed of multiple higher-frequency transgressive-regressive cycles.

During the subsequent lowstand systems tract these deltaic deposits were locally eroded, with incised valley fill later deposited that now caps partially eroded deltaic progradational parasequences of the underlying Strawn sand units. Reservoir properties in the Wallace Ranch cores vary across stratigraphy – and between the two cores - with depositional components and near-surface diagenetic features overprinted by burial diagenesis. This short core workshop focuses on reservoir characterization of the facies in these two Strawn Sand cores by integrating detailed digital core description, thin section and scanning electron microscope (SEM) petrography, X-ray fluorescence (XRF), and X-ray diffraction (XRD) data sets.

Both Wallace Ranch cores consist of 1) irregularly laminated and bioturbated heterolithic organic matter (OM)-bearing argillaceous siltstone and very fine to fine-grained sandstone, transitioning upward into 2) bioturbated OM-bearing argillaceous very fine to fine-grained sandstone and 3) bioturbated fine to medium-grained sandstone, capped by 4) faintly laminated fine to medium-grained sandstone. These coarsening-upward packages of sediments are interpreted as having been deposited in tidally influenced prodelta environments that transitioned upward into tidally influenced deltas and bars, representing classic progradational parasequences developed during a highstand systems tract. Above these higher-energy-environment deposits (faintly laminated fine to medium-grained sandstones) in the Wallace Ranch #2 core is a fining-upward package of sediment consisting of the same facies types in the coarsening-upward package below, illustrating a transition back into deltaic and then prodeltaic settings. These deposits are capped by transgressive sediments – OM-rich calcareous silty claystones deposited in a deeper open marine setting. In contrast, the Wallace Ranch #1 core captures an erosional surface just above faintly laminated fine to medium-grained sands, above which sit massive sandy cobble to pebble conglomerates and a sandy skeletal grain-dominated packstone bed. These coarser-grained deposits are interpreted as incised valley fills deposited during a lowstand systems tract.

Visible porosity (at both core and petrographic scales) is by far the highest in the faintly laminated fine to medium-grained sandstone facies hypothesized to have formed in tidally influenced sand bars. The majority of this porosity is interparticle, with sparse moldic porosity where lithic or feldspar grains have partially dissolved. In all sandstone facies (including conglomerates), some amount of calcite and Fe-carbonate cements precipitated in marine phreatic and shallow burial environments are present, occluding interparticle porosity. In the faintly laminated fine to medium-grained sandstone samples, however, carbonate cements are much more rare and consist of sparse patchy poikilotopic calcite that does not completely fill the interparticle pore space. Compaction, especially of ductile lithic grains and clay, along with quartz cementation also occluded porosity to some extent during burial.