S. Shepherd

The University of Texas at Austin, Austin, TX

ABSTRACT: Depositional History and Reservoir Characterization of the Northeast Hardesty Field, Texas County, Oklahoma

The Northeast Hardesty Field in the Oklahoma Panhandle was developed in the late 1950's and produces oil from the Morrow Formation. The Upper Morrow sandstones are fluvial channel fills that form prolific hydrocarbon reservoirs. However, lack of vertical and lateral continuity, grain size variation, rapid facies changes and diagenesis have hindered development projects.

The Upper Morrow "A" was deposited in an incised valley in response to fluctuating base level. The Mid-continent region was tectonically active from late Mississippian to early Pennsylvanian time. Slight tectonic fluctuations initiated changes in stream gradient. The complex style of deposition from both braided and meandering streams is a result of changes in regional structure during deposition.

Distinctions between braid bars deposited by braided streams and point bars deposited by meandering streams are made based on sedimentary structures observed in cores. Braid bars are commonly massive to cross-stratified or structureless. Common features include pebble sheets, mudstone clasts altered to siderite, and carbonized organic debris. Grain size ranges from coarse to very coarse and pebbly with gravel found at the base. These bars are generally poorly organized and poorly sorted. Point bar deposits are characterized by cross-stratified to planar or ripple lamination. Silty, micaceous drapes are common throughout the deposits. Grain size ranges from silt to medium-grained sand with pebbles found at the base. Point bars are generally well-organized and well-sorted.

Deposition can be separated into three stages. Initially, braided streams incised into underlying Morrow shales and braid bars were deposited. Tectonic shifts caused the stream gradient to decrease and more stable, meandering streams deposited point bars over the braid bars or shales. Some cores show evidence of another episode of incision following a tectonic shift and subsequent gradient increase. In the final stage of sand deposition, braided streams incised into the point bars and braid bars were again deposited. Shales encase the incised valley package.

Diagenesis has severely impacted the reservoir quality of the Upper Morrow sandstones in the Northeast Hardesty area. Porosity and permeability have been reduced by cementation and compaction. Natural dissolution of chemically unstable detrital grains and authigenic cements has increased porosity and permeability in the reservoir. Production data and petrographic examination indicate the presence of clays that fill pores and clog pore throats, reducing permeability. Although the Upper Morrow "A" sands appear to be attractive reservoirs, diagenesis has drastically reduced reservoir quality.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado