Saharan fields Hassi Messaoud (10 Gb reserves, discovered in 1956) and Hassi R’Mel (100 Tcf gas plus 2.4 Gb condensate reserves, discovered in 1957) are by far the largest oil field and the largest gas field in Africa.
Both were found thanks to refraction.
In 1947 SNREPAL (50 percent BRP and 50 percent Algerian government) decided that the Algerian northern portion of the Sahara looked sufficiently promising and attractive to launch an initial exploration program. A joint geological survey with CFP (now Total) followed, and in 1952 both companies agreed to share exploration on 12 Algerian government-issued licenses, covering 200,000 square kilometers, on a 51/49 percent basis.
The terms were tough and called for 50 percent surrender of the acreage in 1957 and another 25 percent in 1962. The choice of the operator of the checkerboard permits was decided among the companies by the roll of a dice: SNREPAL got the even numbers and CFP the odd numbers!
At that time I was a young geophysicist working for CFP.
Results from the initial gravimetric surveys in the northern part of the Sahara were poor and difficult to interpret – and the first reflection seismic surveys were essentially useless, with poor penetration because of the large thickness of dry sand on the surface and of evaporites in the Triassic formations.
At the recommendation of H. de Cizancourt, refraction seismic was then chosen, and high velocity markers (about 6,000 m/s = basement) were mapped.
The CFP exploration team was headed by Claude de Lapparent, with Gilbert Pommier, my boss, as chief geophysicist. CGG (Compagnie Generale de Geophysique) was the contractor chosen to carry out this difficult refraction survey and to interpret the data.
I sometimes have to remind today’s young explorers that 60 years ago there was no GPS, and surveying was a very important part of geophysics, starting with an astronomical sun point to establish a net of beacons.
Surveying the Sahara was possible thanks to 4x4 trucks left over from World War II and aerial photos.
And despite all of these seeming limitations we found oil – and sometimes lots of it!
For refraction shooting in the Sahara we used fertilizer mixed with fuel and prime, on surface or in hole at an offset of 15 kilometers. I measured the anisotropy of the sediments in Om81 with the geophone in the basement at 3,700 meters, shooting four tons on surface. The high anisotropy in the seal of salt and anhydrite needs correction to interpret the results.
After drilling 20 deep dry stratigraphic holes in January 1956, with only one having oil shows, SNREPAL and CFP began drilling wildcat well Md1 at Hassi Messaoud on a refraction seismic structure. They needed a well, because 50 percent of the permit areas had to be surrendered the following year.
The structure of Hassi Messaoud, colloquially known among the interpreters then as Pommier’s egg, was mapped from several refraction lines centered on the water well Hassi Messaoud, also the site of the refraction party’s main camp.
In July 1956 the first Hassi Messaoud wildcat discovered light oil (43 degrees API) in a sandstone below thick Triassic salt. The discovery was confirmed by well Om1, and the magnitude and historic significance of the find were highlighted by General De Gaulle’s visit to Hassi Messaoud in March 1957.
Further drilling in the area proved the sandstone was Cambrian in age, forming a large dome draped over a granite porphyry basement. Closure is structural on the flanks and on the crest a pre‐Triassic unconformity. The field has an area of about 500 square miles (1,300 square kilometers) and an oil column of 900 feet (270 meters). Sandstone porosity, and especially permeability, is very variable and not all production wells, drilled later, were successful, even though some had initial production rates as high as 6,000 b/d.
A strange thing on Hassi Messaoud is found in Wikipedia about the origin of the name.
For the 50 years since it was drilled it was well known that the name came from the 10-meter deep camel water well. “Hassi Messaoud” in Arabic is translated as “lucky well,” because Hassi means well and the surname Messaoud means fortunate, prosperous or happy. This water well for camels, first shown in a 1927 map, was dry by the 1950s, and the French military forces built a new one 50 meters away (with a door with an arch). This well has now disappeared and was replaced by a fake well, shown by Wikipedia.
Wikipedia asserted that the first water well was drilled in 1917 by Messaoud Rouabeh, a regional well digger. Another site says that he found “a strange liquid,” which was analyzed in France then and determined to be oil, drilling a few meters with a goat horn – a bad joke!
Nowadays 600 heirs of the so-called oil finder claim rights over the field owned by Sonatrach!
Hassi R’Mel (2,400 square kilometers in Triassic sandstones) also was found on a refraction high. Its gross production of gas is high because:
- Condensate is not concerned by OPEC quotas.
- R’Mel condensate is produced at high rate (300 kb/d in 2010) with a large amount reinjected in the field, which is the base of all Algerian gas production (in 2009, 7 Tcf gross production and 3.4 Tcf reinjected).
Using Sahara experience, I did a refraction survey in the Canada Northwest territories in 1958 with dog teams, skidoos, fertilizer plus fuel explosive on surface and one helicopter to check a surface anticline at depth.
After the success in the Sahara, CFP decided to explore Australia’s Simpson desert, looking for Paleozoic reservoirs.
We shot a long line (300 kilometers with 1,000 dunes) across it in 1963 (it is now used 50 years later by tourists to cross the Simpson desert, and named on the maps as the French line). We used reflection geophones to shoot long offsets in order to see the reflection events as refraction first events.
FPC Australia drilled two dry wells on the Simpson desert and decided to relinquish the permits. Later, about 20 wells were drilled in the Pedirka and Simpson basins, and they all were dry.
Our exploration of the Simpson desert was a failure as finding oil, but was a success as finding quickly and cheaply that the oil potential of these basins was very poor, thanks to refraction.
In my 2000 French paper (there’s an updated 2005 translation of my “Memories and Thoughts On 50 Years of Oil and Gas Exploration”) I offered lunch in the best Paris restaurant to anyone able to show me the large Hassi Messaoud structure on a reflection profile without the help of wells.
One CGG manager told me that he was going to Algiers soon and that he hoped to bring back such a profile. But few months later he told me that he was unable to find such data.
My bet is still good!
A more detailed paper with references is available on Aspofrance.org