Saturday, June 12, 2010
June 11/2010-6:35 PM EST.
By: Isaac N.
Where exactly was the Horizons borehole drilled ? The Macondo Project. Not a sub-salt prospect as are other deep-water wells but actually in between allochthonous salt bodies that have not formed a continuous canopy like what exists further west. The Allochthonous salt layer is overlying part of its overburden, which is a sheet like salt body tectonically emplaced at stratigraphic levels overlying the Autochthonous salt layer.
Traditionally, deep salt in the G.o.M. has been referred to as autochthonous salt to distinguish it from the shallow canopy of allochthonous salt. It lies within stratigraphically younger strata.The near-surface sediment is young and unconsolidated muck. Shallow water-flow is a big issue here (that is overpressured unconsolidated sands, sometimes with biogenic gas or just overpressured brine — a very unstable near-surface section.
An oil prospect in this area is called a Gumbo Zone.
Gumbo Zone is the name given in the Gulf of Mexico to the structural disrupted zone, directly under allochthonous salt, which works as a permeability pathway. Fluid flushing through this zone develops hyper-pressures, in the section immediately below the salt, which create big problems during drilling. The Gumbo zone was discovered when oil companies tested the hydrocarbon potential of sub-salt prospects. They found major drilling problems at the bottom of the salt layer due to the over-pressures of the Gumbo zone. On the other hand, seismic interpreters soon realize the seismic marker associated with the bottom of the salt, was actually induced by the Gumbo zone rather than by the salt. So, when an obvious bottom of allochthonous salt is recognized on a seismic line, the chances that a Gumbo zone is present are quite high.
Since a Gumbo Zone is what produces the pressure in a well-field, it's a perfect place to prospect for oil.
Unless you happen to be the company known for topping the accident list, operating with faulty equipment in a seismically active area, with the deepest ever sub-sea borehole, drilled through a fluctuating pressure zone, and topped with non-working blow-out preventers containing dead batteries.
Below, are several graphics depicting sub-floor shear in salt formations, the cause of volume problems.
(Goguel, J., 1965) The Second Law of Thermodynamic when applied in Geology. The Goguel's law can be summarized as follows: "Throughout deformation, volume is conserved, but for that due to a loose porosity or dissolution".
This sketch represent a geologic interpretation of a seismic line shot in an area where strike slip movements are absent. If that is so, the seismic interpreter did not respect the Goguel's law, since, in this interpretation there is a major volume problem (see below).
A critical discussion of the above geological interpretation clear indicates the interpreter did not respect the Goguel's law. Indeed, such a interpretation puts away a so large volume of sediments that the interpretation becomes unlikely.
Here is a satellite depiction of the earth without water.
Below is a paragraph from a paper written by several industry geologists, BP's Eric Liedtke and the International Centre for Geohazard's Farrokh Nadim, concerning the Sigsbee .
" The Gulf Coast stress province ( The Sigsbee Escarpment where BP was drilling ) is characterized by active listric growth faulting. This faulting is steeper near the surface and shallows toward horizontal at depth. The state of stress within the province appears to be uniform with the greatest principal stress vertical and the least principal stress perpendicular to the continental margin. The state of stress is probably not associated with tectonic processes but due to sediment loading. The state of stress within the underlying bedrock is unknown. " from Onepetro paper
" Tectonics has played an active part in molding nearly every part of this planet we live on. A postulated long-term passive situation (no tectonic activity) existing in the large Gulf basin since mid-Jurassic would be very unusual. During this same period of time the adjoining areas around the Gulf experienced major orogenic disturbances (Wilson, 1993). There should be tectonic activity in this basin with accompanying plate movement, and, if so, would categorize it as a tectonically “active” basin (Reed, 1994, 1995, 2001 and Shirley and Reed 2002). This active basin building theory for the GoM reveals several new exploration trends across this basin. This paper will discuss one of these trends. " --- Explorer magazine.
SOURCE: Offshore Magazine
SOURCE: University of Portugal