Page 72 - Petrophysics 2E

46 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

Figure 2.7. Unconfomzity, showing the uplaped, eroded strata overlain in an
unconfomzhg pattern by younger sediments.

formations that developed under very different environmental conditions
(Figure 2.7). The rocks immediately below an unconformity are likely to
be porous and permeable because an unconformity is a zone of erosion
that is on top of a weathering zone where water is percolating through
the rocks causing solution of some minerals and precipitation of others
as cementing agents. This is especially true of carbonate formations
underlying unconformities. In addition, the mixed debris deposited on
top of an unconformity can form permeable conduits for migration of oil
from source rocks to geologic traps [ 121.

ORIGIN OF PETROLEUM

The biogenic origin of petroleum is widely accepted on the basis
of geochemical studies. Petroleum contains compounds that have
characteristic chemical structures related to plants and animals such
as porphyrins, isoprenoids, steranes, and many others. In addition,
the source rocks where the precursors of petroleum were originally
deposited are the fine-grained sediments that are deposited in shallow
marine environments during the low-energy transgressive phases of geo-
logic basin formation. Particulate organic matter is not much denser than
water and, therefore, sedimentation along with clay and line carbonate
precipitates will take place slowly in low-energy environment. Depletion
of oxygen takes place in quiet water leading to an anaerobic condition
and preservation of organic matter. Anaerobic bacteria tend to reduce
organic compounds by removal of oxygen from the molecules in some
cases, but they do not attack the carbon-to-carbon bond of hydrocarbons.
The evidence for the origin of petroleum in low-energy, anaerobic
environments is supported by the fact that in the opposite condition
(high-energy, aerobic environments) aerobic bacteria decompose
organic matter to carbon dioxide and water [9, 13, 141.