Let's look at a lake:
2F2C
When your first worker is done, you theoretically will work this tile over a 2FxH (you'd probably work a 3F tile over 2F2C and most definitely 3F1C over 2F2C) tile for max 5 turns (1 turn movement+5 turn improvement is longest for first improvement) so you get 10F10C commerce, 12F from next pop, about 3 turns, meaning you have another 3 turns for the lake before the next improved tile = 6F6C ... each new pop might get to work the lake at most 3/4 turns depending on trees and hills.
You're looking at about 30 commerce, which with discounts can be 1/2 a tech.
However, consider that you need Fishing for this... meaning you don't have one of the useful techs you'd normally get (ex. Agriculture or TW or...).
Oasis is much better as you will work it for the first 20~21 turns and then again another 3~4 turns per new pop... about 1 free tech and doesn't require Fishing.
Still, it's not comparable to gold in the capital or second city... 10 turns of gold will match the Oasis in worker/settler production+commerce.
Don't get me wrong, it's totally doable to grab Oracle without a good commerce start and no Mysticism, it's just riskier and can affect your expansion.
2F2C
When your first worker is done, you theoretically will work this tile over a 2FxH (you'd probably work a 3F tile over 2F2C and most definitely 3F1C over 2F2C) tile for max 5 turns (1 turn movement+5 turn improvement is longest for first improvement) so you get 10F10C commerce, 12F from next pop, about 3 turns, meaning you have another 3 turns for the lake before the next improved tile = 6F6C ... each new pop might get to work the lake at most 3/4 turns depending on trees and hills.
You're looking at about 30 commerce, which with discounts can be 1/2 a tech.
However, consider that you need Fishing for this... meaning you don't have one of the useful techs you'd normally get (ex. Agriculture or TW or...).
Oasis is much better as you will work it for the first 20~21 turns and then again another 3~4 turns per new pop... about 1 free tech and doesn't require Fishing.
Still, it's not comparable to gold in the capital or second city... 10 turns of gold will match the Oasis in worker/settler production+commerce.
Don't get me wrong, it's totally doable to grab Oracle without a good commerce start and no Mysticism, it's just riskier and can affect your expansion.