Oilfield

MAP

OPENING THE GIFT

OIL DERRICK CONSTRUCTION

The fourth construction level is the upgrade
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OIL DERRICK BARTER

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Oil Derrick are limited to 50 ressource reserve.   If you don't build the second level of oil derrick, you get:
25 x 50 = 1250 oil (8750 for 7 oil derricks)
But, if you build the second level, you get:
50 x 50 = 2500 oil (17500 for all all derricks)