capacity to produce will have declined by 25% to 35% over that time
the largest volume fraction in a reservoir is comprised of solid (minerals such as quartz, feldspar, or in non-detrital sedimentary reservoirs, carbonate). The oil and gas resides in pores spaces and fractures. If you throw a bunch of marbles in a box, the spheres meet at points and there will be a lot of gaps filled by air. The same thing happens with sand where the sand grains are the marbles and oil takes the place of air. If the sediment is capped by an impermeable rock, e.g. shale, salt, carbonate, etc., and it overlies an oil source rock, then the pores can become filled with oil, natural gas, and water.
The pore spaces and the 'throats' connecting them can be very small (very small fractions of a millimeter). Size of pore throats and the simultaneous presences of multiple fluid phases (e.g. multiple liquids and gases) matters to the ability of the oil and gas to flow in the reservoir. The interfaces between the different fluid phases can effectively act like bridge arches, e.g. they can support a lot of force and prevent flow even when there is a substantial pressure gradient between the well and the reservoir.
When a reservoir is produced the system is disturbed and it will try to 're-equilibrate' in response. The least viscous fluids will move most freely down the pressure gradient as part of this 're-equilibration' process. Water is frequently less viscous than the oil so it will move ahead of the oil. This can effectively cause the oil to become 'locked up' behind or between water filled pores. It's also possible that 're-equilibration' process can involve precipitation of solids, e.g asphaltenes and inorganic solids, in the pore throats.
Reservoirs are dynamic systems and when they've been produced that dynamicism can be accelerated. Like an automobile engine, it can be detrimental to use it and then let it sit around un-used for a long period of time. In the case of an automobile engine, the oil and fuel evaporate between the cylinder and piston and leave a varnish which will wreck the engine. The same sort of process can take place in a reservoir.
On a cheerier note: it appears that these sorts of processes may be reducing the flow of oil from the Macondo well. When a reservoir is 'over-produced' water can also move ahead of the oil and do what i've described above.
