Drinking deep in east Africa

15/06/2007 Petroleum geologists from the NPD have helped to find potable water at a depth of 600 metres in Tanzania. The seismic data suggest that large volumes are available.

text: Fridtjof Riis / photo: Henning Dypvik

The NPD’s geologists had probably never looked at this area of the geosciences until hydrogeologist Fridtjov Ruden in the Tanzanian project being run by groundwater specialist AGWA got in touch.

Since that initial contact last summer, they have discovered both similarities and differences between exploring for petroleum and water.

For a start, petrogeologists focus on a non-renewable resource while the hydrogeologist wants to find something renewable – in both cases located in the pores between the rock grains.

While the NPD specialists are looking for sealed traps, their opposite numbers on the hydro­logical side aim to find trough-shap­ed structures which lead water down to the well.

The levels involved also differ. Petroleum deposits are well sealed and usually located deep down, while water is sought in shallow strata, perhaps with some help from seismic surveys.

All sedimentary basins with porous rocks contain waterlogged pores, but the water is usually saline and unsuitable for drinking purposes.

How deep you have to drill to find fresh water in sedimentary basins on land depends on levels of precipitation, water run-off and the depth of rainwater penetration.

Some fresh water reservoirs lie deep-buried, beyond the reach of rain. But such deposits would eventually be depleted if they were tapped for consumption.

 

Seismic data from the NPD have helped drilling for water in Tanzania.
Seismic data from the NPD have helped drilling for water in Tanzania.

Sandstones
An area around the Tanzanian capital of Dar-es-Salaam contains sandstone deposits from ground level to deep below the surface. Rainwater penetrates deeply, and run-off is low.

The whole area, which supports more than three million people, is short of potable water. Mr Ruden’s idea was to see whether drinkable supplies could be found below the usual depths.

In addition, he wanted to find out where it would be appropriate to position wells to ensure that they were refilled by rainwater – even well below ground.

Some deep exploration wells were drilled for oil and gas in the area during the 1970s and 1980s, and data from these indicated that fresh water might exist at some depth.

Mr Ruden had been in touch with Tanzanian oil company TPDC, and acquired information both from the wells drilled and from seismic surveys.

The NPD has a collaboration agreement with TPDC, and has assisted the African nation with advice in the petroleum sector since the 1970s.

Its geologists accordingly knew something about the area, and could call up in-house data from seismic surveying and old exploration wells after Mr Ruden had got in touch.

Since the water-filled reservoirs lie so deep, both seismic data and well logs which actually derived from an oil hunt could be used for a different search.

Anticipation
The NPD’s geologists do not often find themselves waiting in anticipation for the results of a water well, but they were very gratified when the first drilling operation proved positive.

Sandstone formations contain­ing fresh water were proven at depths right down to 600 metres, and Mr Ruden’s calculations show that refilling from the surface is good.

Brine from the sea normally penetrates deep groundwater deposits. But the potability of these resources probably reflects a slow drainage of fresh water through the sub-surface sediments to the Indian Ocean.

The project is now under way with its second well, closely watch­ed by the NPD.

Updated: 14/12/2009