Oh boy was this damn hard to crack.

Ok, I believe before it was established before that Aperture Science headquarters are in Cleveland, OH.

Source: HL2EP2

Though, this has been found.

Source: Portal 2

It can be assumed that the Enrichment Center is there, as there is an underground mine in the game.

Now here's the catch: Not only are there no salt mines in the Upper Peninsula, the surface as seen at the end of Portal 2 is flat with wheat growing. The UP is very mountainous, so it is impossible that it is flat for miles and miles. Also, no wheat is going to naturally grow there, it is too damn cold. Also, Michigan only has ever had one salt mine. (Source: Michigan State University, Lansing MI) We can either call paradox or assume the newspaper fudged up and meant to say Lower Michigan. Taking that into consideration, Aperture Science bought said mine from the Detroit Salt Company in January 1944. (Source: That Newspaper article from the game. The number 1944 is very hard to read.)

So, see this map from Detroit Free Press.

That is the location of the Salt Mine. Unless Valve wants to call a sensible location, I am the first person to pinpoint the exact location of where Portal takes place.

So, is the field of wheat at the end of the game possible? Yes. Although this is Downtown Detroit, enough time has taken place for all the standing buildings to be gone.

First of all, Half-Life takes place in 2003. (Proof below)

Source: Half-Life

A December like that only happened in 03 and 08. Also, in the book Half-Life 2: Raising the Bar, a note by the series's writer mentions the date March 31, 2003.

HL2 takes place 20 years later, making that 2023. Portal happens around the same time as HL2. The minimum amount of time that Chell could have been in hibernation is 2.4 years, so Portal 2 takes place during 2025 or later. (The wake up call tries to specify the amount of days in hibernation, but just repeats 9, so 900 days is reasonable. No facility will decay in 90 days) 29401 AD is also plausible. (9999999/365.25+2029) For many possibilities, it is long enough for the buildings to just plain fall apart. Even for 2025, The Combine may have turned Detroit into scrap metal.

Though, Valve really needs to do the research to make this make more sense.