Part One: Write short notes on the following: (20 Marks)
1) The Paleozoic and Jurassic Petroleum Systems of Saudi
Arabia.
2) Effects of diagenesis on carbonate reservoirs.
3) Unconformity traps.
4) How does primary migration differ from secondary
migration?
Model Answer:
1) The Paleozoic and
Jurassic Petroleum Systems of Saudi Arabia.
The Paleozoic Petroleum System of the central
and northern provinces of Saudi Arabia contains one of the
most prolific-oil prone source rocks of Silurian age, world wide, the Qusiba
shale of the Qalibah Formation.
-
The petroleum prospectivity and potential of the Paleozoic sequence in Saudi Arabia
was the results of the coincidence of several factors:
-
The development and the preservation of organic rich material at the base of
the Qusaiba resulted from a major marine transgression.
-
As time progressed, the overlying strata (Devonian) were significantly eroded
during the hercynian event.
-
Following this erosive period, good quality reservoir rocks were deposited
above the source rocks which allowed for efficient expulsion and migration.
- Paleozoic
reservoirs include Khuffm Unayzah, and Jauf Formations.
-
Thick, regionally extensive seal (Khuf carbonate and evaporate) trapped large
volumes of oil in large, low0relief closures.
The Jurassic Petroleum System
-
Most of the oil is derived from the thermally mature Jurassic (Callevonian and
Oxfordian) Hanifa and Tuwaiq Formation.
- These source rocks are laminated organic-rich lime
mudstone units.
-
The most important reservoirs are to be found in the Arab Formation, where they
are lettered A to D from top tobottom.
-
The Jurassic source rocks and reservoirs are capped by regionally extensive
evaporities of the Hith Formation, which prevented extensive migration.
-
We can notice that all the necessary components to form an efficient and
productive petroleum system are present in Saudi Arabia.
2) Effects of Diagenesis
on Carbonate Reservoirs
The
various routes that may be taken by a lime sand as it is buried and undergoes
diagenesis can be summarized as follows:
-
At time of deposition,
initial porosities are as high as 50% (a).
-
If burial takes place very
quickly without early diagenesis, porosity may be reduced by compaction as the
shells and grains are crushed (b).
-
Residual porosity may then
be infilled by a sparite cement (c).
-
In some environments, early
diagenesis takes place with a rime cement of sparry calcite crystals (d).
-
If hydrocarbons invade the
reservoir, further porosity loss by cementation is prevented (e).
-
At any time of its history,
even if compaction and cementation have destroyed all porosity, secondary
solution porosity can form (f).
-
This later secondary
porosity can also be invaded by hydrocarbons preventing any further cementation
of the secondary pores (g).
-
If petroleum invasion does
not occur, the secondary pores may be infilled with a spary calcite cement (h).
3) Unconformity traps
Oil
and gas accumulations can associate with unconformity, which involves
combination of stratigraphy, deformation, and erosion and subsequent
deposition.
Angular Unconformity
Form
giant oil and gas traps when a reservoir rock is truncated under an angular
unconformity and overlain by a seal.
Disconformity
Deposition
over an erosion surfacein flat-lying strata produces a disconformity. Oil and
gas accumulation in the high portions of the old topography below the
unconformity. Strata above and below erosional surface are parallel.
Nonconformity
Sediments
that unconformably overlie an eroded igneous or metamorphic surface produce a
non-conformity.
If
the crystalline rocks below the unconformity are fractured or weathered, they
can act as a reservoir.
4) Migration of oil and
gas.
·
It is the movement of oil
and gas from its fine-grained source rocks to coarse-grained, permeable
reservoir rocks.
·
Primary migration is the
movement of oil and gas from the source rock into a carrier bed (sandstone or
limestone). It is of limited distance of a few hundred meters.
·
Secondary migration is the
movement of oil or gas through the carrier beds to accumulate in a trap. Can
involve distances of 10 to 100
km or more.
Paleozoic Petroleum System of Saudi Arabis
Jurassic Petroleum
System of Saudi Arabia
Part
Two: Define the following terms:
(10 Marks)
1) Kerogen
2) Secondary
dolomites
3) Atypical
reservoir
4)
Source rocks
5)
Cap rocks
Model
Answer:
1)Kerogen
The
disseminated organic matter of sedimentary rocks that is insoluble in
non-oxidizing acids, bases, and organic solvents. When heated, saporpelic
kerogens yield oil and gas, and humic kerogen
yield mainly gas. Kerogen includes both marine and land derived organic
matter.
2)Secondary dolomites: form by
dolomitization, the replacement of a pre-existing calcium carbonate deposit.
They are often coarsely crystalline, with porosity that may exceed 30%.
3) Atypical reservoir:
Any rock can seve as a reservoir, providing that it has the two properties,
porosity and permeability. Atypical reservoir include:
Shales,
granites, and other igneous and metamorphic rocks. Generally, porosity that
occurs in these is due to fracturing.
4) Source
rocks
- Organic matter rich rock unit which under the influence of time
and temperature has generated and expelled hydrocarbons (effective source rock).
-Potential source rock: is one which has not yet generated
significant amounts of hydrocarbons due to immaturity.
5) Cap rocks
An impermeable rock layer that forms the seal on top of an oil or
gas reservoir.
Part Three: (10
Marks)
1) Methane is lightest natural
gas and has a formula of CH4. What are the next 3 heavier
hydrocarbon gases, and what are
their formulas?
2) Circle one
Paraffins are/are not pure hydrocarbons.
Resins/Asphaltenes are/are not Non-hydrocarbons.
Aromatics are/are not pure hydrocarbons.
Naphthenes are/are not Non-hydrocarbons.
3) Match each of the “Gas” terms
above with its definition below by placing the correct letter in the space
provided.
A. Free Gas B. Dissolved Gas C. Associated Gas D. Dry Gas
E. Wet Gas F. NGL G. LPG
H. LNG
1- _____ is hydrocarbon gas
(usually methane) which is compressed into liquid for storage
and transportation.
2- _____ is in a gaseous phase in
the subsurface reservoir and remains in a gaseous phase
when produced.
3- _____ are the hydrocarbon liquids which are
separated from the produced-gas stream.
4- _____ is in solution within
crude oil in the subsurface reservoir, but bubbles out of
solution when pressure and temperature
drops.
Model
Answer:
Part Three:
(10
Marks)
1) Ehane (C2H6,)
Propane (C3H8) and Butane (C4H1010).
2) Circle one
Paraffins are/are not pure hydrocarbons.
Resins/Asphaltenes are/are not Non-hydrocarbons.
Aromatics are/are not pure hydrocarbons.
Naphthenes are/are not Non-hydrocarbons.
A. Free Gas B. Dissolved Gas C. Associated Gas D. Dry Gas
E. Wet Gas F. NGL G. LPG
H. LNG
1- LNG (H) is
hydrocarbon gas (usually methane) which is compressed into liquid for storage
and
transporation.
2- Free Gas (A) is
in a gaseous phase in the subsurface reservoir and remains in a gaseous
phase
when produced.
3- NGL (F) are the hydrocarbon liquids which are
separated from the produced-gas stream.
4- Dissolved Gas (B)
is in solution within crude oil in the subsurface reservoir, but bubbles out of
solution when
pressure and temperature drops.
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