Frome Rocks No. 1 Well, which is situated on the northern edge of the Jurgurra Terrace, drilled to 734 feet through Recent sand and the normal sequence of Jurassic sediments of the South Canning Basin and then entered the top of the Frome Rocks Salt Dome. The Frome Rocks Salt Dome consists of l,522 feet of dolomite breccia cap rock and 1,747 feet plus of salt. The base of the salt was not reached. The age of the sediments composing the salt dome is not known, but slender evidence suggests that the age is Devonian to Lower Carboniferous. The intrusion of the salt dome probably took place in post-Permian and pre-Jurassic times.
No signs of hydrocaroons were seen in the well and no formation tests were conducted.
The Frome Rocks No. 2 Well drilled through 206 feet of Recent and Jurassic sandstones, and a standard Permiansequence (3,351 feet thick) to 3,557 feet. A thick sequence of uppermost Devonian (Middle to Upper Famennian) sedimentswas encountered between 3,557 feet and the total depth of 7,504 feet. This thick Devonian succession (3,947 plus feet) has been divided into two formations, one of siltstone and shale with some limestone and sandstone, from 3,557 feet to 6,264 feet, and the other of siltstone, from 6,264 feet to 7,504 plus feet. The primary objective, the Ordovician, was not reached.
Persistent, though slight, hydrocarbon shows occurred in several zones throughout the upper two-thirds of the Devonian in the form of fluorescence associated With limestones, and a spotty oil staining in the uppermost sandstone bed. No hydrocarbons were detected by gas detection equipment. None of the hydrocarbon shows was of sufficient significance to warrant testing.
In the Frome Rocks area, there is an association of a salt dome, a thick impermeable sequence, source rockpotential, and some reservoir potential. Should the same association occur in other areas along the Jurgurra Terrace, particularly if the reservoir potential improves, the petroleum prospects of the area must be upgraded.
