Understanding Valley Fill Heterogeneity: a New Depositional Model for the Upper Morrow “a” Sands from Postle Field, Texas County, Oklahoma

Jobe, Tiffany D.¹; Wiley, Mark ²; Heris, Ayyoub E.^3
1 Department of Geology, Colorado School of Mines, Golden, CO.
² Department of Geophysics, Colorado School of Mines, Golden, CO.
³ Department of Petroleum Engineering, Colorado School of Mines, Golden, CO.

Pennsylvanian aged Morrow valley fill sandstones are prolific producers of oil and gas in the mid-continent. Upper Morrowan reservoirs are particularly attractive because of their abundant reserves, good production history, and shallow depths; but are often poorly developed because of severe heterogeneity caused by rapid and extreme changes in facies and grain size.

Postle Field, Texas County, OK produces from the Upper Morrow “A” sands and has over 300 million barrels of original oil in place. Today only 40% of the OOIP has been produced. The relatively poor recovery on waterflood and CO2 miscible flooding over part of the field is due to the extreme heterogeneity in the field.

In order to understand the field complexities, a detailed field study was done which integrated cores, logs and seismic data. The study area within the field is the Hovey Morrow Unit over which a 6.25 square mile multi-component 3-D seismic survey was shot in March of 2008. The area includes 63 wells from which log data were available as well as 4 cored wells.

Detailed core descriptions and facies interpretations as well as well-log correlations were integrated with seismic interpretation to produce a new depositional model for the area. Previous workers have attributed sand deposition to deltaic or fluvial dominated deltaic deposits similar to the Morrow in southeast Colorado and Kansas. The lithology, grain size and texture of the Postle sediments indicate a fluvial braided system which is proximally sourced form the west. Seismic data supports this interpretation and indicates there may be sand in previously unexplored areas.

This new interpretation has significant impact on future well placement and field development. Modeling within the framework of a fluvial braided system will help to capture the reservoir heterogeneity we see in the field. Also understanding reservoir connectivity will lead to better fluid flow modeling and simulation ultimately resulting in increased recovery in the field.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009