DEPOSTIONAL ENVIRONMENT AND SEQUENCE STRATIGRAPHY OF THE PENNSYLVANIAN MORROWAN AND ATOKAN INTERVALS AT THE FARNWORTH UNIT, OCHILTREE COUNTY TEXAS

The Morrow B sandstone and the overlying upper Morrow shale and Atokan Thirteen Finger Limestone comprise the reservoir and primary caprocks at Farnsworth Unit (FWU). FWU is operated by Chaparral Energy LLC and is undergoing tertiary oil recovery using anthropogenic CO₂. The Southwestern Regional Partnership on Carbon Sequestration (SWP) is monitoring the injection and migration of CO₂ to aid in evaluating storage permanence at the site.

Accurate geologic characterization of the reservoir and caprock are essential for reservoir modeling and simulation of CO₂ injection. Core from three new wells drilled through the reservoir and caprock were described. Based on sedimentary attributes, core was subdivided into lithofacies, then interpreted for depositional environment and sequence stratigraphy.

A sharp erosional contact separates coarse-grained Morrow B reservoir sandstones from underlying mudstones. The contact represents incision during a lowstand, and the reservoir was deposited through fluvial aggradation within the valley during transgression. Coastal processes influenced upper Morrow B deposition during continued transgression. A ravinement surface separates coarse sands from fine sands and muds of the overlying Morrow shale.

The Morrow shale is comprised of three lithofacies. Lowermost is a fine sand to fine mudstone deposited in an estuarine environment during transgression. This terminates at a laminated, fine-to-medium mudstone (Mfml), and the transition represents a flooding surface and change from estuarine to marine deposition. The middle of facies Mfml represents maximum flooding, above which deposition occurred during regression. The uppermost facies is a calcareous mudstone deposited in a shallow marine setting. The top of the Morrowan section and base of the Atokan series is a sequence boundary.

The Atokan interval alternates between two distinct lithofacies: a coal bearing, fine to medium mudstone facies, and a carbonate cementstone facies. Coals were deposited in coastal swamps during lowstands and cementstones formed under restricted marine conditions during highstands.

Detailed core description and interpretation of depositional environments has enabled construction of an improved 3D reservoir/caprock model for use in SWP’s monitoring and verification efforts.