Tuesday, February 24, 2015

Biodegradation

Biodegradation involves microbes which occur at shallow depths (Connan, 1984), near faults or near unconformities, degrade hydrocarbons (consuming parafins and lighter ends) leaving behind napthenes and asphaltenes (gas chromatograms show parafin spikes missing). Surface (meteoric) water is the main vehicle that brings microbes in contact with pooled hydrocarbons.
Microbes that can metabolize oil include fungi, yeast, and both aerobic and anaerobic bacteria, with aerobic bacteria considered to be the most effective of the group.

Superseals

Superseals are those rocks that have a high ductility (ability to deform without fracturing) under pressure (great depths), and for long time periods. Examples of superseals in order of quality are

  1. Salt (halite). This is the only true superseal.
  2. Anhydrite -containing 30% or more gypsum (water)
  3. Kerogen-rich shales (plastic shales). Usually deep water pelagics.
  4. High pressure shales. May be effective at great depths. 

Growth faults sealing capacity

Growth faults, at shallow depths in soft ductile sediments, may have a better capacity to trap hydrocarbon than post depositional faults at depth in more indurated (brittle sediments).

In Nigeria, hydrocarbons are trapped in the downthrown side of growth faults when they are juxtaposed against an overpressured shale zone on the high side.

Wednesday, February 11, 2015

Instantaneous Velocity

Instantaneous velocity is the speed at a given moment of a wavefront in the direction of energy propagation (Sherrif, 1984) . It is also used to refer to velocities that are derived from sonic or acoustic logs obtained in a borehole, since these are typically taken every few centimeters in the borehole.
In its simplest form, a sonic tool consists of a tranmsitter that emits a sound pulse and at least two receivers at some distance from the transmitter.
The sound propagates from the transmitter through the mud , the formation and through the mud again to the receivers where it is detected as a pressure pulse.
Sonic tools are designed such that sound travels slower through them than through most formations.

The difference in arrival times at two receivers , divided by the distance separating these receivers , provides a measurement of the interval transit time, delta t , or slowness . The velocity of the formation is the inverse of the slowness.