Seismic Expressions of Igneous Sills in San Juan Sag, South-Central Colorado: Humps or Bumps, Tricks or Traps?: ABSTRACT

Abstract

Petroleum exploration in the San Juan sag of south-central Colorado has been an exciting but frustrating experience. Thick surface volcanics cause seismic data to be fair to poor quality and often make mapping of the stratigraphy difficult. Some of the strongest reflections are igneous sills intruded parallel to the bedding planes of prospective horizons. Sills are dense, high-velocity (13,000-18,000 ft/sec) layers which stand out as high-contrast reflections in the lower velocity Cretaceous shales and to a lesser extent in the similar velocity Dakota, Morrison, and Junction Creek sandstone sections. Almost every well drilled in the San Juan sag has had at least one sill present in the Cretaceous or Jurassic section. Sills vary in thickness from 6 to 1,036 ft, with most common thicknesses from 200 to 500 ft. On seismic data the strong reflections caused by the velocity contrast of the igneous rock make the sills stand out as amplitude anomalies. Since the sills are additive to the section, they can cause bumping or thickening which may appear as a structural bump. Several wells have been drilled on these apparent structures. Color displays of seismic amplitude and phase can help detect igneous sills. Ironically, the only productive well inmore » the San Juan sag to data has an igneous sill as its reservoir. The Del Norte field produced approximately 1,500 bbl of oil from fractured sill intruded into the Mancos Shale. The sill is 58-155 ft thick and is visible on seismic. Although rare, sills can be hydrocarbon traps as illustrated by the Dineh-bi-Keyah field of northeastern Arizona, which has produced more than 15 million bbl of oil.« less