PETROLEUM EXPLORATION OVERVIEW
Petroleum
exploration is a process that used a science and technology to find where to
drilled the first well. In this book we are going to describe and illustrate
the techniques and their tools that will allow petroleum engineers and
petroleum geoscientists to see deep underground into what we call the sub
surface. With good tools and proper interpretation we can increase chances of
finding profitable accumulations of oil and gas, and how do we pinpoint the
exact location that will maximize or chances of finding producible oil.
Oil and Gas Field Life Cycle
Before
we pinpoint the exact location of oil and gas accumulation area, let's examined
what is called the life cycle of a well or Oil and gas field.
Oil and gas Life cycle
Exploration Phase
The
first stage in getting oil and gas to market is called exploration; this is
where the geoscientist and the petroleum engineer’s work together to locate and
drill into a location that they think will produce oil.
Petroleum Exploration phase
PETROLEUM EXPLORATION PROCES
In
petroleum exploration they studied the surface and sub surface of the earth by looking
for evidence of the lithificasion of organic material in the rock, the
diagenesis of hydrocarbons, the migration of oil into a reservoir rock and the
entrapping and accumulation of these hydrocarbon molecules.
Petroleum
Exploration – Surface, Subsurface, Hydrocarbon generation
Petroleum Origin
By
using the organic theory of how oil is created, they pinpoint possible
formations were hydrocarbons could accumulate. Geologists and scientists study
the surface of the earth, it’s mountains it's oceans it's, and valleys it's
plains, they studies satellite images of the earth, they look for signs of oil,
places where oil seeps to the surface, they examine the rock types, formations,
and structures called outcrops, and they use the process of elimination, once
to eliminate or cross out areas where they cannot get access or areas that they
know through experience will not contain oil, like areas all made up of igneous
or metamorphic rock.
Organic theory in Petroleum exploration
Surface and subsurface of the earth
Oil seeps
In
the first method they looked for signs of oil on the surface, and oil seep is
an area where the oil trap has been broken and oil is now seeping naturally to
the surface. For thousands of years people have been able to use the oil that
seeps in pools and tar pit on the surface of the earth because this Oil was as easy to find, as you can see in the
illustration, gas use components in crude oil have evaporated, believing these
heavier tars.
Oil seep in Surface
Nowadays
Geologist still look for these seeps because they suggests that trap oil maybe
present nearby, that the black marine shale other source rock might be nearby
in the sub surface, in this image Of a hypothetical oil seep you can see that
entrapped oil is still present nearby, here is another example oil is entrapped
but is overflowing because there is so much, this is called a spillover of a
trap.
Hypothetical oil seep and nearby rock
Unfortunately
most oil seeps have been discovered and exploited but they're still maybe some
undiscovered ones in remote unexplored areas. Since the earth is dynamic and
changing as we know new oil seeps can also suddenly appear after recent
earthquakes that weaken the cap rock allowing Oil to migrate to the surface.
Although new ones are rare we still must keep finding out for them.
Dynamic of Earth weaken the caprock
Formation Outcrops
Another
method the geologists and geoscientist used to study the surface of the earth's
geology is to focus on formation outcrops, the surface exposure of underlying
rocks, they endeavor to estimate what might be underneath by studying rock
structures that have been pushed to the surface, and how can the geologists
know what happens to the outcrop formations that disappeared underground into the
sub surface.
Wild Cat Drilling
To
encourage the investments of money and time needed to drill a wild cat well,
the first well that is drilled in particular area geologists and geoscientists
must offer more evidence or prove then gues as to what formations of the rocks
are in the sub surface. they must
struggle to better the odds that oil will be found on average in only one out
of four wildcat wells.
Surface and subsurface evidence
They
use technology to push these averages in their favor, nevertheless the oil
business is still about risk and convincing others to invest large sums of
money in what will most likely be a dry hole, a term we used to describe it
drilled well with no commercial oil or gas.
The
symbol used on geological maps or wellmap to symbolized a dry hole, so why do
they take these risks? if they are lucky
they will find a rich field that will not only pay for the other three wells,
but also produce a large profit. they're looking for the bonanza, in any of them the geologists and go
scientists meticulously compiled there's surface data to convince others that
there are signs of oil and that a closer look maybe in order.
Wellmap and exploration wells
Basin Analysis & Petroleum Play
To
find a good petroleum play or hydrocarbon play, geologists start by looking for
sedimentary basins, where there is sedimentary rock. The yellow and orange areas on the map,
demonstrate sedimentary basins the brown color symbolizes known areas of oil, in
this study the geologists have colored the map to show that the Middle East is
a sedimentary basin with large oil deposits. In the larger map the sedimentary
basin where some of the large oil fields have been found, here we can see the
location of sedimentary basins containing of the world's largest oil reserves.
Sedimentary basin map and sedimentary rocks
These
large oil fields are located here in Russia, in the Gulf of Mexico, Alaska,
Canada, the North Sea, the Middle East, africa and Venezuela. They know there
is oil there so they continue to explore the region for new fields, some even
hidden below existing field.
Hydrocarbon
provinces are found in sedimentary basins. Important to know how basins are
formed, Basin Analysis used to identify Hydrocarbon traps, the Stratigraphy
(Source rock, Reservoir rock, Cap rock), the Maturation of source rocks, and
Migration path-ways some literatures called Petroleum system.
Sedimentary
basins are the subsiding areas where sediments accumulate to form stratigraphic
successions. The plate tectonic setting is the premier criterion to distinguish
different tyes of sedimentary basins. There are several main type of
sedimentary basin, as follows:
-
Rif-type Basins from at extensional plate,are associated
with increased heat flow due to mantle plumes. They occur, for example at
continental margins.
-
Foreland-type basins form at compressional plate
boundaries in front of migrating fold and thrust belts
-
Transtensional basins occur where plates move in a
strike-slip fashion relative to each other
Types of
sedimentary basins
We
sometime can see the sedimentary basin in seismic profile, the seismic profile
interpreted in regional scale (hundreds to thousands of KM long)
The
sedimentary basin in seismic profile example
Petroleum
system can be define as the essential elements and processes and all
genetically-related hydrocarbons that occur in petroleum show, and
accumulations whose provenance is a single pod of active source rock.
Petroleum System
Elements
The
Petroleum System Elements, consist of
following:
1.
Source Rock - A
rock with abundant hydrocarbon-prone organic matter
2.
Reservoir Rock – A rock in which oil and gas accumulates,
it has porosity (space between rock grains or fractures in which oil
accumulates), permeability (passage-ways between pores or pore fractures through which oil and gas
move).
3.
Seal Rock- A rock through which oil and gas cannot
move effectively (such as mudstone and claystone).
4.
Migration Route – Avenues in rock through which oil and gas
moves from source rock to trap.
5.
Trap – The structural and stratigraphic configuration that
focuses oil and gas into an accumulation.
The
Petroleum System Elements
Petroleum System
Processes
The
Petroleum System Processes consist of
following:
1.
Generation – Burial of source rock to temperature and
pressure regime sufficient to convert organic matter into hydrocarbon
2.
Migration – Movement of hydrocarbon out of the source
rock to ward and into a trap
3.
Accumulation – A volume of hydrocarbon migrating into a
trap faster than the trap leaks resulting in an accumulation.
4.
Preservation – Hydrocarbon remains in reservoir and is
not altered by biodegradation or “water-washing”
5.
Timing – Trap forms before and during hydrocarbon
migrating.
The
Petroleum System Processes
Area Elimination
In
addition geologists and geoscientists use what is known as a process of
elimination, in preparing their data they look for reasons why an area can be
excluded for instance they keep track of regions where it may be too dangerous
to drill or areas where the rigs could not get permission to enter and drill.
Political and non-geological reasons helped eliminate large as of the earth's
surface that is not open to oil companies to drill.
In
any event, once the geologists and geoscientists have exhausted all the ways where
they can require data from the surface, they turned their attention to the sub
surface. Here they must rely on other types of tools and instruments to help
them visualize what is down below in the sub surface, of course they want
actually be able to see into the sub surface but they're tools will allow them
to gather data that will enable them to plot the structures, rock types, and
presence of fluids in the sub surface.
SUBSURFACE DATA ACQUISITION
There
are three basic tools that they use in the sub surface to acquire data, they
are the gravity meter the magnetic meter and the seismic, and Electromagnetics
as addition tool. Surprisingly the technology used in these tools were
perfected many years ago, even so this technology is still widely used today,
because the technology used in these three tools still allows the petroleum
geoscientists to envision the big picture of underground structures.
The gravity Survey
The
gravity meter measures the earth's gravity, the second the magnetic meter
measures the earth's magnetic field, the third is the seismic survey measures
sound waves refracting and reflecting of different layers of rock.
Let’s
look at each in more detail the gravity
meter first measures that changes in rock density in the sub surface. The
standard acceleration gravity constant (G) was first developed by isaac newton
in the seventeenth century, today scientists have precise calculated G to equal
9.80665 m/s2 (nine pointing eight zero six six five meters per second squared). G represents the gravitational constant of
the entire earth .
The gravity meter measurements
However
small deviations from this number can be measured at the surface of the plane
using a gravity meter it indicates small variations in the rock density below,
the last number five for example can vary between plus or minus five digits
depending on the density of the rock below, this allows the geologists to
measure density changes.
Gravity meter - small deviations
This
is important to get G or the gravity constant of a local area, a gravity meter
is put on the ground to measure whether the sub surface has a high or low
density, igneous and metamorphic rocks have slightly higher densities than the
average density of the earth, so when igneous and metamorphic rocks are
presents, they will measure a slightly larger G reading. With sedimentary rock,
the gravity meter measured slightly lower G that the average G of the earth, by
taking several measurements over an area on the surface of the earth, petroleum
geoscientist can begin to construct maps illustrating the positions of the
different rock types below they then plot this data on a grid making countor
maps, high G are usually colored red
indicating harder high density rock structures, low density rock or sedimentary
rock is usually colored blue, it is these sedimentary rock formations and
basins in blue that the geologist is interested in.
Local gravity measurements and results
Gravity maps example
The Magnetic Survey
The
second tool is the magnetic meter has a compass that points to magnetic north, where
in the earth we find large deposits of igneous rocks made up of iron or
magnesium, these large structures can be very magnetic which allows the
magnetic meter to deflect slightly from true north. by identifying these
smaller magnetic fields, the magnetic meter like the gravity meter allows the
Geoscientist to use the recorded measurements over a large areas to build
contour maps to illustrate the presence of small magnet variations. Areas of
high magnetic variations or colored red and those with low magnetic variations
or colored green, the red areas our associated with igneous rocks and the green
areas are associated with sedimentary basins. This example of how geoscientists
use the process of elimination, they eliminate the areas of igneous rocks from
further exploration consideration.
Magnetic survey and magnetic measurements
Topography, Gravity, and Magnetic model
These
tools while useful to view the big picture, can only suggest were to look
further, Survey Magnetic by plane. More detailed today is still needed to
determine if the sedimentary basins contain traps and hydrocarbon
accumulations.
There
are some examples of models of computer generated maps of formations that
contain different structures like an declines and faults with their depths and
heights, by also studying countor maps we can see what is called across profile
of a structure underground. It is what we might see if we could cut into the
surface, like we would have a layer cake. Here we can see what looks like a
mountain underground these negative numbers indicate that you are below
surface, from raw data to Contour. We can see what might be and anticline using
how powerful computer tools we can rotate the 2D and 3D visual contour maps to
look at structures from different directions and angle from the top to the
bottom, from any perspective what we are looking for is closure.
The Seismic Survey
The
third and most useful most powerful tool in the geoscientist’s tool now is the
seismic survey. This tool uses sound waves that allow petroleum geoscientists
and petroleum engineers to interpret what is in the sub surface. Seismic
surveys are made from sound waves that travel through the earth, like the
science used to understand gravity and magnetism, the technology for acquiring,
processing and interpreting seismic waves has been around for a long time.
What
make seismic surveys so useful today and why we still use this technology is
because powerful computer processing capabilities. A coupled with it advance
software development allow us to record pinpoint and interpret with ever
increasing accuracy where oil and gas might accumulate. But how do we pinpoint
the exact location that will maximize our chances of finding produce able oil.
The seismic survey
The Electromagnetics Survey
Electromagnetic
methods use the response of the ground to the propagation of incident
alternating electromagnetic waves, made up of two orthogonal vector components,
an electrical intensity € and a magnetizing force (H) in a plane perpendicular
to the direction of travel.
Two
orthogonal vector components
Electromagnetic
method use Transmitter and receiver, where the electromagnetic anomaly is the
difference of Primary field and secondary field
Electromagnetic
anomaly
Electromagentics – Sea Bed Logging
SBL
(Sea Bed Logging) is a marine electromagnetic method that has the ability tomap
the subsurface resistivity remoterly from the seafloor. The basis of SBL is the
use of a mobile horizontal electric dipole (HED) source transmitting a low
frequency electromagnetic signal and an array of seafloor electric field
receivers.
A
hydrocarbon filled reservoir will typically have high resistivity compared with
shale and a water filled reservoirs. SBL therefore has the unique potential of
distinguishing between a hydrocarbon filled and a water filled reservoir.
SBL (Sea
Bed Logging)
For
further read, you may get the book from amazon.
PETROLEUM EXPLORATION OVERVIEW
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