Page:Niger Delta Ecosystems- the ERA Handbook, 1998.djvu/150

The Resources of the Niger Delta: Minerals 14.3 OIL GEOLOGY AND PRODUCTION

14.3.1 THE GEOLOGY OF THE NIGER DELTA

As explained in chapter three there have been two Niger Deltas over geological time. The first delta was deposited in tertiary times when the Niger flowed into the tertiary sea, some 60 to 150 million years ago, at least 30m higher than the modern sea. The second delta is made up of deposits left from the lower quaternary era to the present day.

The drop in sea level in quaternary times (commencing about 2 million years ago) caused the Niger to erode a wide flood plain through the tertiary deposits between Yenegoa and Onitsha. The sudden drop from the tertiary to the quaternary deposits is obvious through much of the region, creating two distinct terraces. This is particularly obvious in Port Harcourt at the Bonny River, where a cliff rises thirty feet high above seventy feet of deep water.

This picture is further complicated by what is known as Localised Uplift caused by earth movements. This is why, for instance, the Sombrero River/Ogoni Terrace, at 1518m above sea level, is a few metres higher than the rest of the tertiary terrace.

Because the delta is made up of deposits carried down by the vast Niger/Benue basin it covers a huge area: about 75,000 km² onshore, 75,000 km² offshore on the continental shelf, and about 90,000 km² beyond the continental shelf on what is known as the Guinea Abyssal Plain. This makes a total area of about 240,000 km², only slightly smaller than the entire United Kingdom. The immense weight of the delta (where, at its thickest, the sediments are over 15 km thick) compresses the lower layers (strata) of sediment and also depresses the earth's crust, which periodically subsides as a result. Each time this subsidence occurs, the front (seaward) portion of the delta slumps downwards and forwards creating a fault, introducing sea water (and thus marine conditions) to be brought inshore. Following the subsidence there is a period of stability when fresh riverine deposits are laid over and beyond the older slumped part of the delta, until there is another subsidence.

As a result of these alternating processes, a cross-section of the delta shows an inter-leaving of strata of marine and river sediments. These are roughly horizontal (the older strata altered by compression and subsidence, and by any geological uplift which may occur from time to time), but complicated by obliquely vertical fault lines caused by the slumping of the delta during subsidence.

The areas between the faults are known as Depobelts, which run parallel to the coast, merging at the flanks of the delta where deposition rates are less. Thus the lens shaped depobelts succeed one another in an oceanward direction, becoming older inland, where they are eroded by the modern rivers to provide some of the material for their successors.

It should be remembered that they take place over tens of millions of years.

14.3.2 THE GEOLOGY OF OIL

Oil (and other hydrocarbons associated with it) is formed in the earth when decayed fish, other animals and plants are subjected, over millions of years, to pressure and heating caused by the subsequent deposition of sediments (sedimentation) on seabeds from neighbouring landmasses. Temperatures of between 100 and 150 degrees centigrade are required, which frequently occur between 4 and 10 kilometres below the surface of 148