Draken's Get Rich Quick Scheme
The Easiest 1-2-3 Way to Make Steady Income Without the Need to Cut Trees or Kill Animals!
1. Restart New Game until multiple Oil Deposits. If you find a map with 2 or more Oil Wells, Save it. They are rare.
2. Complete the Deconstruction, Delivery and Ancients Quests.
3. Build Your Oil Rigs and Purchase 2 Gas Cans and 6 Oil Drums for Each Rig.

Oil wells are located on flat stone textured ground surfaces. They have a shiny black puddle appearance when you find a good well.

There are much better descriptions with pictures and video available online. Feel free to do a search for them. But below you can find a quick list of the circuit I take when looking for the wells.

Locations
1. As you leave the ranch, the first well is on the left side of the road across from the 3 fields about 20 meters off the road.

2. As you head from the Ranch to the Gas Station, you will pass a field on the right side of the road. The second oil well is located in the corner closest to the Ranch, just off the road.

3. Travel up to the Gas Station and then keep going until you have just started to go down hill again. Find the third well, on your right about 10 meters off the road.

4. As you travel further down the hill, you will see a dirt road on your left and the first farm house. Take a left. Just as you pass the farm you will find two well locations on the right side of the road.

5. If you keep going towards the lake, but before you get to the dock-house, there are two locations in the trees just off on the right side of the road.

6. Travel up to the lookout tower and then down the hill behind the tower to the opposite side of the lake from the first farm house. Here you will see the second farmhouse, this one has a wheat field. On the right side of the road as you leave the farm, you can find two more Oil Well locations in the trees.

7. The next one you need to start at the Hardware Store. Take a left from the store driveway and, just as you pass the fenced in store drop area there is a hill with some trees. You will find a well up there as well.

8. The next well is on the left side of the road across the street from the General Store.

9. As you leave the drive from the General Store go up the hill away from the Ranch. The road turns to the right. Up about 10 meters off the road is another well.

There may be more but these are the ones I found with 16 new game starts and scouring the map.

If you read through my well locations list (Location 6.), the second farm with the wheat out front is where you pick up the quests that give you the funds to build your wells.

There are good guides with pictures and videos for the Ancients Quest. Make use of them.

This may not be complete. But it is the best I could do.