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.



• 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



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



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



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.



"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.



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.



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.



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



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.



• 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).



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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