reply to post by westcoast
reply to post by westcoast
Where to begin? First we need to consider what is the MOHO? It is the 'layer' between the crust and the mantle. This cross-section from
shows the layers.
The Wiki article
on the MOHO gives some background information. At least for this
type of think Wikipedia is quite good. Unfortunately I am old enough to remember Project Mohole and I was very interested at the time. It was a shame
that it got cancelled.
This boundary layer is possibly a temperature gradient and this is quite likely as the layer is different depths depending on where you are, i.e.
under sea or on land. The standard answer is that it is a change in the composition of the rocks, but that may not be the case. The Russians also
drilled a borehole in the 60s, but I did not hear about that one at the time - the Kola
- and this reached about 12,000 metres before it stopped. The most important detail of this project is this to my mind:
However, due to higher than expected temperatures at this depth and location, 180 °C (356 °F) instead of expected 100 °C (212 °F), drilling
deeper was deemed unfeasible and the drilling was stopped in 1992. With the expected further increase in temperature with increasing depth,
drilling to 15,000 m (49,000 ft) would have meant working at a projected 300 °C (570 °F), at which the drill bit would no longer work.
Basically the rock was viscous and the drill bits started to bend, hence they did not work!
This project was not really near the MOHO as such since on land this can be anything up to 100km down. At the Kola bore it is about 35km down.
So what to we have?
- The MOHO is probably a temperature gradient
- The Russians could not drill any deeper because the drill was melting (basically)
- This bore hole did not reach the MOHO
- Several other projects have attempted to get to the MOHO but none have succeeded
- There have been deeper wells than BP which have not hit the MOHO and these have been offshore
- This gives us a clue that BP probably did not drill into the moho!
- If they had drilled into the MOHO the scientists would have been delighted!!!!! - and I think we would have heard about it!
So how deep is the MOHO below the Gulf? I don't know so I went to look.
The Moho beneath the continental shelf has a velocity of 7.4 km/sec at a depth of about 30 km. A normal mantle velocities (8 km/sec) are found at
46 km beneath the continental shelf. The Moho beneath the Sabine Uplift is 40 km deep and has a velocity of 8.0 km/sec.
I cant copy this one (PDF)
but look at the text immediately above the drawing on the right hand side
of the first page. Since the BP rig was not in the centre this would suggest that the MOHO is deepening where the rig is located. (Don't forget that
the BP well is only at 18,000ft or about 5.5km below the sea bed.) Yes it is close and the gas and oil surging upwards is hot, but despite being at
5.5 or so kilometres (I believe it is actually 5.28) I do not believe it has reached or breached the MOHO if only because of the fact that if, as is
strongly indicated, this is temperature gradient then it would seem that it is not technically possible (at present) to drill into the MOHO.
At the end of the day it would still be rock, even if a bit thick and viscous.
Just a small PS.
Abiotic production of oil probably works, but there is no way it will work at MOHO depths. Why? In order to accumulate oil you need a strata of rock
that can hold the reservoir. In a viscous malleable environment there is not place for a reservoir to accumulate.
Whilst I believe that abiotic oil production may well happen, I doubt very much that it will happen at the level of the MOHO.
[edit on 16/7/2010 by PuterMan]