Pore Pressure Prediction

[hoangduy7696]

Can sb help me with some question below?
In this, and indeed many,
petroleum basins it is
observed that at
shallower depths pore
pressures are mostly
hydrostatic. Why?
Deeper in the basin high
overpressures are
commonly seen. Why?
At intermediate depths
overpressure
development is more
variable. Why?
Note that the
overpressure appears to
be limited by the fracture
pressure as represented
by the LOT’s. Why?

Thank you.

[aows51]

for the first question, i think it is because of the centroid effect

[petrogeo_69]

Q1 - in most cases these are younger sediments, where time, overburden pressure and temperature have not allowed for over-pressure to develop. Also, there is rarely the sealing or confining capacity of over laying sediments to trap building fluid pressure.
Q2- a general rule is that with increased depth = increased compaction = increased pressure and increased temperature.
Q3- many factors effect over pressure build-up 1st & foremost is rate of deposition & compaction.
Q4- LOT is a test of Rock Strength, rock strength governs confining pressure. so, if a LOT is tested to 15.0 ppg (which means it took the equivalent of 15.0 pounds per gallon pressure in a wellbore's annulus to progagate fluid into the rock or to break it down) this test is done at the shallowest open hole point in order to determine a safe drilling margin between over-pressure (pore pressure) and rock strength (fracture gradient).