by Bea Ferrigno

Results of a recent hydrologic study in Rio Grande county raise concerns that oil and gas exploration or development there could lead to contamination of subsurface waters.  The Conejos Formation, which is an aquifer and includes oil-bearing shales, is complex and quite permeable to its considerable depth. It also lacks any barrier, such as the blue clay layer present in the San Luis basin, that could prevent contact between subsurface waters and oil or drilling fluids.  Oil is definitely present, as some wells in the area have produced cores with oil-bearing deposits, and oil seeps also occur on the surface.

The study also determined that there is deep water circulation in the Conejos formation; consequently, its authors recommend that exploration and development wells be cased to the bottom of the Conejos formation which in some areas lies nearly 6000 feet below the surface.  This provision is considerably more stringent than those lately recommended by the Colorado Oil and Gas Conservation Commission (COGCC).

The study further recommends that exploration or development wells not be placed within 1,000 feet of an alluvial flood plain or intermittent or ephemeral streambed unless measures are in place to prevent the escape of drilling fluids; and that all water wells within a mile of an oil or gas well be monitored for water quality. The COGCC presently requires that only two water wells within ½ mile be monitored.

Although there has been sporadic exploration in the mountain areas around Del Norte,  recent applications for drilling and increasing concern about its environmental effects prompted the county to declare a moratorium on new drilling until this study could be completed.  Two drilling applications, however, were already in the approval process prior to the moratorium so the study conseqently focused on their sites near San Francisco and Old Woman creeks, which are settled mountain areas with numerous domestic water wells.  Creeks in this part of Rio Grande County flow to the Rio Grande; the area is also a recharge source for the aquifers of the San Luis basin.

Lying in the western two-thirds of Rio Grande County, the area of interest is a formation known as the San Juan Sag, a northeastward extension of the San Juan basin which is a petroleum-producing area. In Rio Grande County,  oil-bearing deposits lie west of the Cimarron Fault which runs roughly north-south at the longitude of Del Norte.  East of the fault, the oil shales have been removed by erosion, thus there is no potential for oil extraction in the eastern third of the county.

The study was sponsored by Rio Grande County and funded through the Rio Grande Interbasin Roundtable.  It was conducted by Davis Engineering Service and GeoLogical Solutions, both of Alamosa, and HRS Water Consultants of Lakewood.  The complete 126-page + appendices study is available in print for $30, on CD for $10, and online for free at the Rio Grande County website, under Land Use.  For the complete report, please go to:


See related story on page B-5