geo chem exp
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7/21/2019 Geo Chem Exp
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Vitrinite Reflectance (VR)
Measurements
• Vitrinite Reflectance (VR) is the mostcommonly used organic maturation indicatorused in
• the petroleum industry. This is mainly becauseit is accurate, quick, non-destructive and
• inexpensive. Vitrinite, because it is not
strongly prone to oil and gas formation, iscommon as a
• residue in source rocks.
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• Liptinite ('Exinite') Maceral Group (Type I & II Kerogen)
• UV Fluorescence = strong yellow or green
• Reflectances = low
•• Type I Kerogen; waxy, lipid-rich and resinous parts ofplants.
• • Type II Kerogen; green algae and blue-green algae;common in marine anoxic shales where
• vitrinite is very rare.• Liptinite-rich rocks have a high oil and gas producing
potential
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Vitrinite Maceral Group (Type III
Kerogen)
• UV Fluorescence = none or poor
• Reflectance = moderate
• • the most common maceral (organic
component) in most humic coals• • a common consitituent of organic source rocks.
• • remains of cell lumens (cell walls), woody tissueof stems, branches, leaves and roots of plants
• and the precipitated gels from these materials.• Vitrinite-rich rock tends to be prone to gas
generation
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Inertinite Maceral Group (Type IV
Kerogen)
• UV Fluorescence = no
• Reflectance = very high
•
• peats that have been oxidised early in theirformation
• • bark, stems, leaves, roots
•
inertinites are not prone to oil and gasgeneration
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Surface indications of oil and gas seepage have been noted for
thousands of years, and such seeps have led to the discoveryof many important petroleum producing areas.
Over the past sixty years, numerous geochemical and non
seismic geophysical surface exploration methods havebeen developed.
The fact that hydrocarbons migrate to the surface in
detectable amounts, but many remain uncertain of how
such information can best be integrated into conventional
exploration and development programs.
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"A look at the exploration history of the important oil
areas of the world proves conclusively that oil and gasseeps gave the first clues to most oil-producing regions.
Many great oil fields are the direct result of seepage
drilling.
Seepages are most numerous in the youngest sediments,
especially where they have been folded, faulted, and
eroded, and on the margins of basins. Exceptions are
easily explained by a comparatively calm geological
history as depicted by the Gulf Coast region, West Texas,
the Mid-Continent, and areas bordering stable masses.
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Seepage Activity
The term active seepage refers to areas where subsurface
hydrocarbons seep in large concentrations into shallow
sediments and soils and into the overlying water column.
Active seeps often display acoustic anomalies on conventional
or high-resolution seismic profiles. Active seepage occurs in
basins now actively generating hydrocarbons or that contain
excellent migration pathways. These seeps are easily detected
by most sampling techniques.
Anomalous levels of hydrocarbon seepage may only be
detectable near major leak points or below the zone of
maximum disturbance.
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Passive seepage
The zone of maximum disturbance, defined by Abrams
(1992, 1996a), is a near-surface zone of variable depth and
thickness in which sedimentary and biological processes
alter or destroy volatile hydrocarbons. Anomalous
concentrations of hydrocarbons may not be detectable if
samples are not obtained from below the zone of maximum
disturbance. Deeper sampling may be required in areas of
passive seepage.
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Macroseepage vs. Microseepage
Microseepage is defined as high concentrations of analytically
detectable volatile or semi volatile hydrocarbons in soils,
sediments, or waters. These invisible seeps are recognized
only by the presence of anomalous concentrations of thefollowing:
• Light hydrocarbons
•Volatile or semi volatile high-molecular-weight hydrocarbons
(such as 2 –4 ring aromatics)•Hydrocarbon-oxidizing microbes • Hydrocarbon-induced
alteration products
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High-molecular-weight hydrocarbons may be present in ever-
wet or intermittently wet environments; however, only
volatile or semi volatile hydrocarbons are expected above the
water table.
Most surface geochemical methods, including both direct
and indirect methods, were developed to detect
microseepage.
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• MACROSEEPAGE:
• The term macroseepage refers to visible oil
and gas seeps. Macroseeps are very localizedareas containing large concentrations of light
hydrocarbons as well as, if available, high-
molecular-weight hydrocarbons. They are
localized at the termination of faults,
fractures, and outcropping unconformities or
carrier beds. These visible seeps have led to
the discovery of many of the world’s
important oil and gas producing areas (Link,
1952; Macgregor, 1993).
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BIOLOGICAL MARKAR (CHEMOFOSSILS):
• steranes: c19 – c31, 3-4 ring alkanes, derivedfrom plants and animals
• hopanes: c27 – c35 pentacyclic alkanes, derived
from bacteria• diterpanes: c20 bi-tri cyclic alkanes, derived
from plants
•
isoprenoids: chain alkanes, e.g. pristane andphytane, various sources
• aromatic steroids: related to the steranes
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