| Definition: | Last year gas was discovered in a particular county, color coded by 10 year intervals. |
| Light Pink - (1850's) | |
| Pink - (1860's) | |
| Red - (1870's) | |
| Pale Green - (1880's) | |
| Light Green - (1890's) | |
| Green - (1900's) | |
| Pale Blue - (1910's) | |
| Light Blue - (1920's) | |
| Dark Blue - (1930's) | |
| Pale Sky Blue - (1940's) | |
| Light Sky Blue - (1950's) | |
| Sky Blue - (1960's) | |
| Pale Purple - (1970's) | |
| Light Purple - (1980's) | |
| Purple - (1990's) | |
| Gray - No Gas Production |