Gradient Geophysics

After the Grand Daoui horizon was exhausted, the great  Oulad Abdoun Basin exploitation of a new phosphate deposit began. Some so-called ‘disturbances’ such as sterile hardpan are hard to detect and can interfere with the phosphate extraction. The geophysical surveys indicated several prospective anomalies within the possible Paleozoic basement rock. 

Representing more than 75 percent of the world’s reserves in phosphate, Morocco has an annual output of 19 million tons and a reserve in excess of 35 billion cubic meters.

In the Sidi Chennane deposit, it is sedimentary stratiform and contains several distinct phosphate-bearing layers. Found in contact with alternating layers of calcareous and argillaceous hardpan, these layers, in this field were extracted after Grand Daoui deposits were exhausted.

Locally known as “derangements’ or disturbances, the new deposits contains many inclusions and lenses by extremely tough hardpan found throughout the phosphate-bearing sequence. Normally, hardpan pockets can only be detected at the time of drilling. They interfere with field operations and introduce a severe bias in the phosphate reserves. 

Well logging or surface geology – direct exploration methods – is not particularly effective. Although, the chemical changes which are detectable at the hardpan and phosphate rock interface produce an important resistivity contrast. Factors such as changes in clay content and lithofacies and the consistence appear to account for some additional resistivity difference.

Located within the Oulad Abdoun basin, the Sidi Chennane phosphate deposit is about 33 kilometers South-East of Khouribga. West meridian 372500 – Lambert – South meridian 22800 – Lambert – East highway RP22 and North are its boundaries, having the outcrops of the basement of the phosphate-rock sequence.

Vegetation is mostly sparse dwarf palm trees due to the areas arid climate of phosphate plateau. November to May is the rainy season and it’s usually kept below 400 mm. The rural population exists on ranching cattle and seasonal agriculture in the small villages or dour. Ground water is increasingly in short supply.

The geologic section rest uneasily on Paleozoic schist and quartzites and the basement is well sedimentary covered in fairly thick Maestrichtian and Eocene which contain the phosphate-bearing strata which are 30 to 50 millimeters thick.

Below the phosphate-bearing strata, you can find up to 70 meters of Senonian marls and limestone marls; 20 to 60 meters of Turonian limestones; a Cenomanian formation of alternating limestone marls and gypsum marls and finally, 10 to 60 meters of red marls and mudstone of pre-Cenomaniage age.

Differentiated by size of the pocket or inclusion, the disturbances may be a type of material, clay content, hardness or type of contact with the phosphate rock. There are usually two main types of disturbances found, the first type is throughout the mineral deposit and it appears to be a random mixture of limestones, clays, cherts, marls and a low-grade phosphate with large amounts of cherty limestone.

The second type is a highly disturbed and lacks any dominant facies. It appears as an accumulation of low-grade phosphate limestone blocks with large nodules of marl, chert, some fragments of chert and phosphate rock.