1. Oil and gas are found trapped within structural or stratigraphic traps underground, not freely flowing. 2. Structural traps are formed by deformation of the earth's layers through folding, faulting, or salt domes. Stratigraphic traps occur where permeable rock layers are enclosed by impermeable layers. 3. When drilling for oil and gas, potential problems include missing the reservoir by drilling at its edge, encountering a depleted reservoir, or hitting a structural trap like a fault that separates the reservoir. Careful analysis by petroleum geologists is needed to plan successful wells.

1. Technical College Of Engineering
Petroleum and Energy Engineering Department
1st
stage
(2016-2017)
(second semester)
(oil trap)
Prepare By:Zanyar nawzad hama ali
Date : 29/5/2017
Group :

2. Oil and gas traps
All oil and gas deposits are found in structural or stratigraphic traps.
You may have heard that oil is found undergroundin “pools,”“lakes,”
or “rivers.”Maybe someone toldyouthere was a “sea”or “ocean” of
oil underground. This is all completely wrong, sodon’t believe
everything youhear. Beneaththe earth's surface, oil will ooze through
rocks if there is enoughspace betweenthem, but this oil will not
accumulate intolarge quantities unless something traps it ina
particular place. There are a variety of geologic traps, which
themselvescanbe broken intocategories:
 Structural traptypes:anticline, fault, salt
 Stratigraphic traptypes:unconformity, lens, pinch-out
It is important to remember that oil is not all by itself insome sort of
undergroundcave, but is, instead, containedwithinsolidrock - which
has enoughroom withinit toactually soak up (or become soaked in,
however you look at it) oil.
Structural Traps
These traps hold oil and gas because the earthhas beenbent and
deformedin some way. The trap may be a simple dome (or big bump),
just a “crease”in the rocks, or it may be a more complex fault trap like
the one shownbelow. All pore spaces inthe rocks are filledwithfluid,
either water, gas, or oil. Gas, being the lightest, moves tothe top. Oil
locates right beneaththe gas, and water stays lower.
Once the oil and gas reachan impenetrable layer, alayer that is very
dense or non-permeable, the movement stops. The impenetrable layer
is calleda “cap rock.”

3. Stratigraphic Traps
Stratigraphic traps are depositional innature. This means they are
formedin place, oftenby a body of porous sandstone or limestone
becoming enclosedinshale. A stratigraphic trapaccumulates oil due to
changes of rock character rather thanfaulting or folding of the rock.
The term"stratigraphy"basically means "the study of the rocks and
their variations". One thing stratigraphy has shownus is that many
layers of rock change, sometimes over short distances, evenwithinthe
same rock layer. As an example, it is possible that alayer of rock which
is a sandstone at one locationis a siltstoneor a shale at another
location. In between, the rock grades betweenthe tworock types.
From the sectiononreservoir rocks, we learnedthat sandstones make
a good reservoirbecause of the many pore spaces containedwithin.
On the other hand, shale, made up of clay particles, does NOTmake a
good reservoir, becauseit does not containlarge pore spaces.
Therefore, if oil migrates intothe sandstone, it will flow along this rock
layer until it hits the low-porosity shale. Voilà, astratigraphic trapis
born!

4. Here are four traps. The anticline is astructural type of trap, as is the
fault trap and the salt dome trap.

5. The stratigraphic trapshownat the lower left is a cool one. It was
formedwhen rock layers at the bottom were tilted, thenerodedflat.
Then more layers were formedhorizontally ontop of the tiltedones.
The oil moved up through the tiltedporous rock and was trapped
underneaththe horizontal, nonporous (cap) rocks.
Another Stratigraphic Trap
This hole has beendrilledinto a sandstone that was depositedina
streambed. This type of sandstone follows awinding path, and can be
very hard to hit witha drill bit!The plus is that old streambeds make
excellenttraps andreservoir rock, andsome of these fields are tens of
miles long!
This type of sandstone is usually enclosedinshale, making this a
stratigraphic trap.
Just because youdrill for oil or gas does not mean that you will find it!
Oil and gas reservoirs all have edges. If youdrill past the edge, youwill
miss it ! This might explainwhy your neighbor has a well on his land,
and you do not!

6. Stratigraphic Problems When Drilling
When you drill, you may find a producing reservoir very near the
surface. But many other things can happen:
You might drill intoa reservoir that has beendepleted(all the oil and
gas removed) by another well. There may be a new infill reservoir
betweentwowells that couldbe developedwitha thirdwell. Or one
that was incompletely drained. Maybe if youdrill a little deeper you
might hit a deeper pool reservoir!Youmight be able to back up and
produce a bypassedcompartment. The petroleumgeologisthas to
think of all these things whenplanning a new well!

7. Structural Problems When Drilling
Finally, structuresinthe earthcangive the PG many challenges. Look
at this diagram. Imagine you first drilledthe hole onthe left intothe
greenlayer whichrepresents anice oil and gas-bearing rock. YES!You
have a great well, producing lots of oil and gas!
Then you drilledyour secondhole to the east (right) of the first one.
What happenedto that hole?
Answer:The oil reservoir has beensplit intwoby the fault, whichis
nothing but a place in the earthwhere rock layers break in two. The
arrows on the diagramshow that the rocks moved DOWN on the LEFT
side of the fault and UP on the RIGHTside of the fault. This createda

8. GAP in the oil field……right whereyoudrilledyour secondhole!
Incredibly bad luck!Or, bad seismic!Your secondhole is a DRY HOLE.