In addition, the company said oil was recovered during wireline sampling and added that this classic pre-salt sequence overlay a thick sand interval with good reservoir quality.

Spudded on November 21, the well is the first in Roc's program to specifically target a pre-salt structure containing most of the oil reserves in the adjacent area offshore Cabinda.

The two earlier wells were plugged and abandoned for being non-commercial, while the first well, Massambala-1 - representing the first drilling effort in onshore Angola for more than 30 years - was found to have made a heavy oil discovery.

Roc was upbeat about the latest drilling result, saying it had provided some valuable information about the region.

"While the well is judged to be non-commercial, Milho-1, which is located 12 kilometres inland, is only the second well in this part of the block to penetrate a full pre-salt sequence to basement and, as such, it is a very important data point," the company said.

"It provides the first modern sub-surface evidence that the pre-salt petroleum system, which is so prolific in the adjacent offshore area, underlies a considerable portion of the onshore Cabinda South Block.

"The lack of shows in the pre-salt reservoir may relate to the well being down-dip from the top of the structure."

When it is finished with Milho-1 later this month, the Simmons 80 rig will move to the Coco-1 well site about 1.5km north.

Drilling is expected to start in early April, with total depth to be reached by the end of May.

Following this operation, the rig will move to drill the next well, Sesamo-1, to test a large pre-salt prospect in the eastern part of the block.

Roc and its co-venturers are also considering an optimal field appraisal program for the Massambala-1 heavy oil discovery, made last August.

A subsurface resistivity survey is scheduled to begin in the second quarter, followed by the possibility of a shallow well appraisal drilling program and a final investment decision in April.

Roc estimated the Massambala field contained between 50 million and 500 million barrels of in-place oil resource.

"As a result of reprocessing 3D seismic in order to clarify the shallow seismic events, the areal and vertical closures of the four-way dip Massambala structure have been confirmed as 24 square kilometres and 24 metres respectively," Roc said.

"Laboratory studies of core material … have confirmed the heavy nature of the oil and the excellent quality of the reservoir sands, which have porosities of up to 35 percent and permeabilities of up to 10 Darcys."