Lyons W.C. (ed.). Standard handbook of petroleum and natural gas engineering.2001- Volume 1
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Geological Engineering
245
PreCVntdiM
IMamniy igneous arid
msUmrphk
mks:
no
wldmde
wbdivirionr.)
Binh
al
Planet
Eanh
Table
2-28
Geologic Time and Important Events
[26]
-1,CW-
-2.-
-3.m-
Old-1 deled
WkS
-
_.
Riossns
Mi
G
'
CdcadaRiverbe~ins
-
ManfaiMMdbariMh
Tertiary
OliOCCOM
NEVadll
-
Eamm
Yellwnfona
Pah
mkariim
Palgxane
.,"
kky
hntains
-in
Cretaceous
Lowsr
Mimislippi Riwr
tagitm
-1
4,
Jurassic
Trlassic
Permian
tz14
Atlantic
Ocean begins
Carboniferous)
Mississippian
Carnmilsrous)
Devonian
Siiun'an
Cambrian
Later
hOminidS
PrimlNe
hmrnidr
Grasses;
grazing
mammals
Pnmike
hones
Sprsadinp
01
mammals
Dimuis extin3
F7oweric-a
planis
ClimsX
of
dinosaurs
Birds
Cmnlfers. Cycads.
primitive
mammals
Dinosaurs
MamMI-lle
remiles
Coai Iorests. %nsecls,
amphibians. reptiles
Pmphiblans
and plants and land ani-
nab
%nitive Inshes
Manne anmais abundant
'nmitwe rnanne animals
3reen
alpae
3actena. blue
green
algae
The accumulation
of
hydrocarbons occurs when, during migration through a porous
and permeable rock, the migration process
is
impeded by the existence of
an
impermeable rock formation
on
the boundary
(in
the path of the migrating fluid) of
the rock through which the migrating fluid is attempting
to
transverse. Once the
hydrocarbons are trapped, the rock in which they have been trapped becomes the
reservoir rock. Eventually the hydrocarbons accumulate in the reservoir rock and
become stratified in accordance to their fluid phases and the amounts of formation
water within the reservoir. The quantity of gas dissolved in the
oil
depends on the
pressure and temperature in the reservoir. The hydrocarbons
will
accumulate in the
246
General Engineering and Science
highest portion of the reservoirs with the gaseous portion at the top, oil next, and
water at the bottom (see Figure
2-39)
[26-291.
Structural Geology
All sediments are deposited in basically the horizontal plane. Therefore, source
beds and the beds that
will
eventually become reservoir rocks are originally nearly
horizontal. Eventually, after these deposition beds became cemented into rock
formations, tectonic events within the earth applied forces to the bedded rock
formations within the sedimentary basins of the world. The causes of these tectonic
events through geologic time in the historical past of the earth are not entirely
understood. It is speculated that as the earth cooled, the outer surface
of
the sphere
cooled from a liquid to a solid. The cooling process itself released gases which in
turn released condensed liquids (water). These gases and liquids ultimately resulted
in our atmosphere and abundant water at the earth’s surface. The cooling solid crust
of the earth likely cooled differentially, Le., cooled more rapidly on the outer layers
than on the inner layers. Most surface rock materials will shrink as they cool from
the molten state. Therefore, the outer layers of the earth shrank at a greater rate
than the inner layers, causing large cracks to form in the surface outer layers. The
formation of the cracks and the differential cooling of the layers
of
the earth would
Figure
2-39.
Anticlinal hydrocarbon accumulation.
Geological Engineering
247
likely cause a curling effect on the edges
of
large surfaces (plates) that had cracked.
This curling effect at the edges of the plates (or at intermediate positions) could be
partially responsible for the mountain building processes that have occurred
throughout geology time.
The solid plates that resulted from the cooling process at the surface
of
the earth
were able to “float” on the remaining molten inner portion of the earth. Because of
the rotational motion of the earth about its own axis and the earth’s motion in the
solar system, inertial and gravitational forces have produced great interactive forces
between the plates. It is speculated that these interactive forces have led to plate
contact and situations where one plate has slid over another. The great forces created
by plate tectonics are likely responsible for the forces that have resulted in the folding
and faulting of the earth’s crust
[30].
Of interest is the detailed structural configurations that can form in sedimentary
rock formations, because hydrocarbons are nearly always found in sedimentary rocks.
The structural features
of
interest are faults and folds
[26].
1. Faults are breaks in the earth’s crust along which there has been measurable
movement of one side
of
the break relative to the other. Fault terminology is
as
follows (see Figure
2-40):
Dip-the angle the fault plane makes with the horizontal, measured from the
horizontal to the fault plane.
Strike-a line on the horizontal surface represented by the intersection of the
fault plane and the horizontal surface. The strike line is always horizontal,
and since
it
has direction,
it
is measured either by azimuth or bearing. Strike
is always perpendicular to the dip.
Heave-the horizontal component of movement of the fault.
Throw-the vertical component of movement of the fault.
Slip-the actual linear movement along the fault plane.
Hanging wall-the block located above and bearing down on the fault surface.
Footwall-the block that occupies the position beneath the fault, regardless
of whether the hanging wall has moved up or down.
I
+:>.
Dip
slip
\
Hanaing
wall
I
/
I
Figure
2-40.
Fault
terminology
[26].
248
General Engineering and Science
Normal faulting-basically dominated by tension and gravity forces results in
the hanging wall being displaced downward relative to the footwall.
Reverse
faulting-basically dominated by compression forces and, therefore,
the hanging wall is moved up relative to the footwall. The reverse fault that
dips at
30"
or less becomes a thrust fault.
Strike-slip faulting-occurs when the
blocks
slide laterally relative to each other.
2.
Folds in layered rock formations consist of the deformation of layers without
faulting. Folds are formed by compressional forces within the crust. Often
faulting accompanies extensive folding. Fold terminology is as follows (see
Figure
2-41).
Anticline-a fold with upward convexity.
Syncline-a fold that is concave upward.
Crest
flank
Inflection points
w
Trough
Hinge point
Syncline
Asymmetrical
fold
Symmetrical fold
Figure
2-41.
Folding terminology
[26].
Geological Engineering
249
Hinge point-the point of maximum curvature of a fold. The hinge
surjace
is
the locus of hinge lines within the fold.
Inflection point-occurs when bed curvature in one direction changes
to
bed
curvature in the opposite direction.
Limbs
(orflanks)
of
a
fold-those portions adjacent to the inflection points
of
the fold.
Symmetrical fold-a fold whose shape is a mirror image across the hinge point.
Asymmetrical fold-a fold whose shape is not a mirror image across the huge
point.
Recumbent fold-characterized by a horizontal or nearly horizontal hinge
surface (see Figure
2-42).
Overturned fold-when the hinge surface is depressed below the horizontal
(see Figure
2-42).
Concentric (parallel) folds-rock formations parallel
to
each other such that
their respective thicknesses remain constant (see Figure
2-43).
Figure
2-42.
Recumbent
(A)
and overturned
(6)
folds
1261.
L
J
Figure
2-43.
Concentric
folds
[26].
250
General Engineex-ing
and
Science
Nonparallelfolds-rock formations that
do
not have constant thickness along
the
fold
(see
Figure
2-44).
Similarfolds-folds
that have the same geometric
form,
but where shear
flow
in the plastic beds
has
occurred (see Figure
2-45).
I
Axis
Figure 2-44.
Nonparallel
folds
[26].
Figure 2-45.
Similar
folds
[26].
Geological Engineering
25
1
Disharmonic folds-folds in layered rock that have variable thickness and
competence and, thereby, fold in accordance to their ability (see Figure
246).
Traps
A
trap provides an impermeable barrier to the migration of hydrocarbons moving
through a porous and permeable sedimentary bed. The trap allows the hydrocarbons
to accumulate within the trap. The trap
is
a prerequisite for the formation
of
an
accumulation of hydrocarbons and, therefore, for
a
reservoir
[26].
1.
Structural traps result from deformation of rock formations. Such deformations
are the result of folding or faulting.
Anticline trap-a simple fold that traps hydrocarbons in
its
crest (see Figure
2-39).
Anticlines are some
of
the most common productive structures. Overthrust
anticlines are the most complex of this type of trap (see Figure
2-47).
Such structural traps are associated with complex faulting and are prevalent
in the overthrust belt provinces in the United States and Canada.
Fault traps-involve the movement of the reservoir rock formation to a position
where the formation across the fault plane provides
a
seal preventing further
migration of hydrocarbons (see Figure
2-48).
Salt-related traps-formed when the plastic salt formations deform into dome-
like structures under the overburden forces of the beds above the salt beds.
Such plastic flowing (and bulging) of the salt beds deforms the rock
formations above producing anticline structures and faults in the rock
formation astride the domelike structures (see Figures
2-49
and
2-50).
2.
Stratigraphic traps are formed by depositional and sedimentary factors. In such
traps the depositional process and the follow-on cementing process, which
changes the sediment bed into a rock, create porosity and permeability alterations
in geometric forms that provide traps.
n
Figure
2-46.
Disharmonic
folds
[26].
A
WFCT
PAINTER
RESERVOIR
22-6A
13-6A 33-6A
A‘
CRETACEOUS
,
0
++Aw
FEET
300
METERS
PAINTER RESERVOIR FIELD
6
22-f,4
6’
MI
l1.m
DISCOVERY
33-6,4
It
6.W
-
I
TLL
I
-
2.m
n
Figure
2-47.
Overthrust structural trap (Painter Reservoir, Wyoming)
[26].
Geological Engineering
253
Figure
2-48. Fault structural traps.
Sand body traps-finite sand bodies such as channel sands, river delta sands,
and sea or ocean beach or barrier bar sands. These sand body traps are
deposited over well-defined regions. As deposition continues
on
a wider
regional basis of shale-forming deposits, the sand body becomes enclosed
by shale and thus becomes a trap for fluids, particularly hydrocarbons
(see Figure
2-51).
Ree/
traps-important hydrocarbon-producing geological features. The
porosity and permeability in reefs can be excellent. Again, as in sand
body traps, reef traps are finite bodies that are deposited over well-defined
regions. Continued deposition of silt and clay materials
will
eventually
enclose such features in shale, allowing them
to
trap fluids, particularly
hydrocarbons (see Figure
2-52).
Unconformity traps-the sedimentary rock result of a period of deposition. A
typical example would be when a sequence of horizontal beds
is
tilted, eroded,
254
General Engineering and Science
Figure
2-49.
Salt
dome
structural traps.
and subsequently, overlaid by younger flat-lying beds.
If
flat-lying beds are
impermeable, the permeable tilted beds will be a potential reservoir (see
Figure
2-53).
Combination traps-sedimentary trap features that result from both strati-
graphic and structural mechanisms. There can be many combinations for
stratigraphic and structural traps. An example
of
such a trap would be
a
reef
feature overlaying a porous and permeable sandstone, but in which the
sequence has been faulted (see Figure
2-54).
Without the fault, which has
provided an impregnable barrier, the hydrocarbons would have migrated
further up dip within the sandstone.
Basic
Engineering
Rock
Properties
The petroleum industry is concerned with basic properties of sedimentary rocks.
Although the following discussions can be applied to other rocks, the emphasis will
be
on
sedimentary rocks.