ĐỊA CHẤT DẦU KHÍ ( PETROLEUM GEOLOGY ) - CHƯƠNG 3 docx

Chapter 3: TRAP
Chapter 3: TRAP
3.2 Classification:
3.2 Classification:
four major
four major
types:
types:
Structural,
Structural,
Stratigraphic,
Stratigraphic,
Hydrodynamic and
Hydrodynamic and
Combination
Combination
3.1 Definitions and Concepts
3.1 Definitions and Concepts
3.1.Definitions and Concepts
3.1.Definitions and Concepts
•
•
A trap is subsurface configuration of reservoir
A trap is subsurface configuration of reservoir
rock and cap rock or seal that has potential to
rock and cap rock or seal that has potential to
concentrate petroleum in the pores of a reservoir
concentrate petroleum in the pores of a reservoir
rock
rock
•

•
A trap is a geological feature of a reservoir rock
A trap is a geological feature of a reservoir rock
that restricts the flow of fluids
that restricts the flow of fluids
•
•
A trap can content one or more reservoirs
A trap can content one or more reservoirs
• The highest point of the trap is the crest or
culmination.
• The lowest point is the spill point. A trap may or
may not be full to the spill point.
• The horizontal plane through the spill point is
called the spill plane.
• The vertical distance from the high point at the
crest to the low point at the spill point is the
closure.
• The productive reservoir is the pay.
• Its gross vertical interval is known as the gross pay.
This can vary from only one or two meters in Texas to
several hundred in the North Sea and Middle East.
• Not all of the gross pay of a reservoir may be
productive. For example, shale stringers within a
reservoir unit contribute to gross pay but not to net pay
• Net pay refers only to the possibly productive reservoir.
(Figure 2, Facies change in an anticlinal trap, illustrating
the difference between net pay and gross pay).
Figure

Figure
1
1
:
:
Nomenclature of a trap using a simple anticline as an example
Nomenclature of a trap using a simple anticline as an example
Figure 2
• A trap may contain oil, gas or a combination of the two. The oil-
water contact, OWC, is the deepest level of producible oil within
an individual reservoir
• ( Figure 3a , Fluid contacts within a reservoir in an oil-water
system).
• It marks the interface between predominately oil-saturated rocks
and water-saturated rocks. Similarly, either the gas-water contact,
GWC ( Figure 3b , Fluid contacts within a reservoir in a gas-water
system),
• or the gas-oil contact, GOC ( Figure 3c , Fluid contacts within a
reservoir in a gas-oil-water system) is the lower level of the
producible gas. The GWC or GOC marks the interface between
predominately gas-saturated rocks and either water-saturated rocks,
or oil-saturated rocks, as the case may be.
Figure 3
• Source rock chemistry and level of maturation, as well
as the pressure and temperature of the reservoir itself,
are important in determining whether a trap contains
oil, gas or both.
• In some oil fields (e.g. Sarir field in Libya), a mat of
heavy tar is present at the oil-water contact.
Degradation of the oil by bottom waters moving

beneath the oil-water contact may cause this tar to form.
Tar mats cause considerable production problems
because they prevent water from moving upwards and
from displacing the produced oil.
• Boundaries between oil, gas and water may be
sharp ( Figure 4a , Transitional nature of fluid
contacts within a reservoir- sharp contact
• Gradational ( Figure 4b , Transitional nature of
fluid contacts within a reservoir- gradational
contact). An abrupt fluid contact usually
indicates a permeable reservoir. Gradational
contacts usually indicate low permeability
reservoirs with high capillary pressure.
Figure 4
• Directly beneath the hydrocarbons is the zone
of bottom water ( Figure 5 , Nomenclature of
underlying reservoir waters).
• The zone of edge water is adjacent to the
reservoir.
Figure 5
• Fluid contacts in a trap are almost always planar but
are by no means always horizontal.
• Should a tilted fluid contact be present, its early
recognition is essential for correct evaluation of
reserves, and for the establishment of efficient
production procedures.
• One of the most common ways in which a tilted
fluid contact may occur is through hydrodynamic
flow of bottom waters ( Figure 6 , Tilted fluid
contact caused by hydrodynamic flow)

Figure 6
• There may be one or more separate
hydrocarbon pools, each with its own fluid
contact, within the geographic limits of an oil
or gas field ( Figure 7 , Multiple pools within
an oil and gas field). Each individual pool may
contain one or more pay zones.
Figure 7
3.2.Classification
3.2.Classification
Basically, traps can be classified into four major
Basically, traps can be classified into four major
types:
types:
structural,
structural,
stratigraphic,
stratigraphic,
hydrodynamic and
hydrodynamic and
combination
combination
TRAP TYPES CAUSES
Structural Traps
Fold Traps:
Compressional Folds
Compactional Folds
Diapir Folds
Tectonic processes
Depositional / Tectonic

processes
Tectonic Processes
Fault Traps Tectonic Processes
Stratigraphic Traps Depositional morphology or
diagenesis
Hydrodynamic Traps Water flow
Combination Traps Combination of two or more of
the above processes
BASIC HYDROCARBON TRAPS
BASIC HYDROCARBON TRAPS
UNCONFORMITY
ANTICLINAL
SUB-SALT
SEDIMENT
TRUNCATION
FAULT
BURIED REEF
PINCH-OUT
GRADING
SAND LENS
SALT DOME
Figure 8
3.2.1 Structural Traps
3.2.1 Structural Traps
• "A structural trap is one whose upper boundary
has been made concave, as viewed from
below, by some local deformation, such as
folding, or faulting, or both, of the reservoir
rock."
Fold Traps

Fold Traps
Fold Traps (Compressional )
Fold Traps (Compressional )
• Anticlinal traps which are due to compression
are most likely to be found in or near
geosynclinal troughs (mangs).
Examples of Compressional Fold Traps
Examples of Compressional Fold Traps
• 01-The Wilmington oil field in the Los
Angeles basin ( Figure 9 , Oil fields of the Los
Angeles basin) is a giant anticlinal trap with
ultimate recoverable reserves of about 3 billion
barrels of oil.