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Exploration Expense
 12 Months Ended
Dec. 31, 2020
Extractive Industries [Abstract]  
Exploration Expense Exploration Expense
A summary of our exploration expense for the years ended December 31, 2020, 2019 and 2018 is as follows:
Years Ended December 31,
202020192018
 ($ in millions)
Impairments of unproved properties$411 $32 $59 
Dry hole expense7 25 37 
Geological and geophysical expense and other9 27 66 
Exploration expense
$427 $84 $162 
Unproved oil and natural gas properties are periodically assessed for impairment by considering future drilling and exploration plans, results of exploration activities, commodity price outlooks, planned future sales and expiration of all or a portion of the projects. The exploration expense charges during 2020 are primarily the result of non-cash impairment charges in unproved properties, primarily in our Brazos Valley, Haynesville, Powder River Basin and Mid-Continent operating areas. The decrease in geological and geophysical expense in 2019 and 2020 was due to fewer exploratory geological and geophyscial projects.