Petroleum County, Montana
Shrink-swell clay limits houses without basements on 57% of Petroleum County's mapped land, a higher share than 93% of US counties and the 2nd highest of 53 Montana counties. A shallow water table limits basements on under 1%.
None of the county is in drought on the October 13, 2026 Drought Monitor map, after 1.3% a week earlier. FEMA's flood data list no flood losses here since 2016. 46% of homes were built before 1980, against 50% nationally; the median home dates from 1983.
Data as of October 13, 2026 · sources
expansive clay: shrink-swell limits slab homes
high water table: limits basements
in drought, October 13, 2026
of homes built before 1980
Soil under Petroleum County
| Expansive clay (limits slab homes) | 57.3% |
| High water table (limits basements) | 0.1% |
| Septic fields very limited | 73.1% |
| Clay swell (LEP, by area) | 6.3% |
| High-swell land (LEP 6%+) | 56.8% |
| Mapped soil | 1,071,500 acres |
| Soil series | Share here | Series nationally: expansive clay |
|---|---|---|
| Neldore | 15% | n/a |
| Cabbart | 7% | under 1% |
| Gerdrum | 7% | n/a |
| Bascovy | 6% | n/a |
| Delpoint | 5% | n/a |
| Yawdim | 4% | n/a |
How we count · survey areas MT069 (2025-08-28)
What the numbers mean here
Clay swell averages a linear extensibility of 6.3% by land area here, and 57% of the land reaches USDA's high class (6% or more). Clay like that swells when wet and shrinks when dry, under 57% of the mapped land.
Rain over the 12 months to August 2026: 12.62 in, 1.68 in below the 1991-2020 average year of 14.30 in.
USDA rates 73% of the land very limited for septic absorption fields.
Drought
0% of the county is in drought (D1 or worse) on the October 13, 2026 map. Part of the county was in drought in 44 of the last 52 weeks; the most was 100% of its area, in 6 weeks, most recently on the June 23, 2026 map.
| Category, October 13, 2026 | Share of county |
|---|---|
| D0 abnormally dry or worse | 1.5% |
| D1 moderate drought or worse | 0% |
| D2 severe or worse | 0% |
| D3 extreme or worse | 0% |
| D4 exceptional | 0% |
Flood losses since 2016
FEMA's flood data list no flood losses in Petroleum County since 2016.
Rain
| Average year, 1991-2020 | 14.30 in |
| 12 months to August 2026 | 12.62 in (-1.68) |
| 2025 | 15.13 in (+0.83) |
| Wettest year since 1895 | 2011: 21.82 in |
| Driest year since 1895 | 1931: 5.86 in |
| Days with 1 inch or more, normal at VALENTINE (19 mi away) | 1.2 |
Homes
| Housing units | 311 |
| Median year built | 1983 |
| Built before 1980 | 46% |
| Built 2000 or later | 18% |
| Owner-occupied | 73% |
| Median value, owner-occupied | $201,400 |
| Single-family detached | 77% |
What repairs cost
Published ranges from cost guides, not quotes. More sources, and where they disagree.
Nearby counties
| Place | Distance | Expansive clay | High water table |
|---|---|---|---|
| Musselshell County, MT | 45 mi | 24% | 1% |
| Fergus County, MT | 48 mi | 51% | 3% |
| Garfield County, MT | 58 mi | 22% | under 1% |
| Golden Valley County, MT | 69 mi | 26% | 2% |
| Treasure County, MT | 78 mi | 44% | 4% |
| Phillips County, MT | 78 mi | 64% | 2% |
| Yellowstone County, MT | 83 mi | 23% | 3% |
| Wheatland County, MT | 89 mi | n/a | n/a |
Sources: USDA SSURGO (August 28, 2025) · US Drought Monitor (October 13, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status