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ABSTRACT: Rose City: A Synergistic Approach to Proration of a Prairie du Chien Field, Ogemaw County, Michigan

Catherine D. McRae

Proration of gas fields in Michigan is required by law and is regulated by the Michigan Public Service Commission (MPSC). The recent discovery of Prairie du Chien (Ordovician) gas reservoirs necessitate redefining proration rules, formulas, and assumptions. Existing proration standards predicated on the Niagaran (Silurian) Pinnacle reef play are not appropriate for the Prairie du Chien. Shell Western prepared a proposal for proration of the Rose City field, Ogemaw County, Michigan, that was accepted by the working interest owners and presented to the MPSC. The proposal is based on a comprehensive, synergistic subsurface study that will help set the standards for proration of other Prairie du Chien fields.

To accomplish our goal of presenting the first thorough proration proposal to the MPSC, a multidisciplinary approach was used to analyze the extensive data available at Rose City. Examination of cores and thin sections from three of the seven wells in the field led to the definition of five lithofacies. Petrophysical responses could be related to lithofacies type. The lithofacies distributions and calibrated log responses were combined with seismic structural interpretations to characterize the reservoir system and map the distribution of hydrocarbon-bearing pore volume (SgOH). A computer mapping program was used for manipulating data, calculating volumetrics, and preparing display. Volumetrics and material balance calculations of original gas in place agreed within 10%, which is cons dered excellent. As a result, proration methods proposed for Rose City are based solely on underlain hydrocarbon pore volume (ASgOH). This approach is being applied to other multiwell Prairie du Chien fields to ensure equity and improve ultimate recovery.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990