--> --> Virtually eliminate dry or sub-economic wells in over-pressured and normal-pressured fields: Jonah Field and Anadarko Basin case studies

AAPG Pacific Section and Rocky Mountain Section Joint Meeting

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Virtually eliminate dry or sub-economic wells in over-pressured and normal-pressured fields: Jonah Field and Anadarko Basin case studies

Abstract

In the Jonah field in the Green River Basin in Wyoming it was realized, through the drilling of multiple sub-economic wells, that over-pressured zones were critical to gas storativity, porosity preservation, and field economics while normal pressured areas were sub-economic. The question became how to differentiate prospective over-pressured areas from sub-economic areas. An ultrasensitive surface hydrocarbon survey was employed to map areas of elevated hydrocarbon richness, enhanced porosity, and over-pressured zones. A direct correlation was found between the survey hydrocarbon probability values and OGIP (original gas in place) times reservoir pressure. The difference in geochemical signatures at over-pressured versus normal-pressured wells was related to intensity and not composition. Thus, the ultrasensitive hydrocarbon survey was able to map the areas of over-pressure versus normal-pressure, which was useful in mapping field prospectivity. The Anadarko basin, in western Oklahoma, contains numerous charged horizons (oil and gas) throughout the Paleozoic section, including carbonate, sand and shale intervals. Of particular interest are over-pressured Pennsylvanian Red Fork Channel sands. This geochemical survey technique was utilized to differentiate and map Red Fork fluvio-deltaic sands. Analysis of the survey samples showed strong correlation between effective reservoir porosity, net pay thickness, and the surface geochemical expression. A plot of phi-h versus surface probability values resulted in an r2 value of 0.87. The data correctly differentiated dry wells, sub-economic wells, and productive wells. Twenty-two wells were drilled on positive post-survey hydrocarbon anomalies. Twenty-one wells were commercial and 1 was dry. Of eight wells drilled on dry survey anomalies, five were plugged and abandoned, and one was sub-economic. Thus the two case studies show that ultrasensitive hydrocarbon surveys can be used effectively to define field sweet spots and optimize field production.