SALT DOME DELINEATION WITH BOREHOLE GRAVITY

The Williamson 20-13 #1 well was drilled on the flank of a salt dome in Covington County, Mississippi. 2D seismic data allowed for the possibility of reservoir beneath a salt overhang at 16,000 feet. The well was drilled into salt at approximately 10,000 feet and was still in salt at 15,000 feet, despite sidetracking. A Borehole Gravity (BHGM) survey was run to determine the salt geometry.

Figure 1. Final interpreted shape of salt dome and surrounding geology with the field and model surface and borehole gravity responses. Vertical exaggeration 2.5:1.

Several gravity stations were made in open hole in the salt to a depth of 15,000 feet. Other stations were made in casing in salt and in the sediments before salt entry. The stations in the sediments included formation tops.

A 2.5D gravity model was constructed on site during the logging operation. The model BHGM density response (red trace) was too low when an overhang was introduced (Figure 2.) but matched the field response (black trace) when the model salt flank sloped slightly outward with increasing depth. It was concluded that a salt overhang was not present beneath the current well location. The well was subsequently abandoned, saving further drilling expense.

Figure 2. BHGM response to slight salt overhang beneath the well.