Only sedimentary rocks made at a certain time contain
petroleum. Scientist look for tiny fossils and pollen that are trapped in
the rock millions of years ago to determine a rock's age. Petroleum was greatly
produced 23 million years ago.
What are fossils?
Preserved remains or traces of plants and
animals are fossils. We would not know what prehistoric plants and animals looked
like or when they lived without fossils.
Where are organic sediments most likely
to accumulate and be preserved?
Quiet areas of continental shelves that do
not receive much oxygen and are low enough to allow some organic matter to be
buried without being decomposed by bacteria are good places for organic sediments
to accumulate and be preserved.
Why are petroleum geologists interested
in sediments?
Petroleum found in sedimentary rock takes
millions of years to form. As rock particles accumulate into layers, they pile
up microscopic plant and animal remains, trapping them in the sediment. It then
undergoes tremendous heat and pressure. This process generates petroleum.
How do you make rock from rock?
The earth's crust is constantly worn down
by rain and wind. Eroded rocks are transported and settled in low areas and
offshore. Over time, strata forms where the sediments are deposited into layers.
The sedimentary rock is formed when the strata is compacted and cemented.
Where do sediments accumulate?
Rock layers of sedimentary basins hold petroleum.
Most are located along coastal areas and continental shelves where marine life
and sediment supplies from rivers are plentiful. Most sedimentary basins are
only about 25 million years old. The oldest sedimentary basin is about 40 million
years old.
Why are some offshore rich in petroleum?
At one time sea levels were lower than today.
Continental lakes and shallow seas collect the organically rich sediments, which
later formed petroleum.
Why are sedimentary rocks so important
to petroleum formation?
The organic material trapped in it was converted
into petroleum, which contain the resource of petroleum.
You have to know how to "read" the rocks
to find petroleum
Over time, rocks that appear hard and solid
changed. Everyday rocks are broken down by wind, water and temperature. In a
process called the rock cycle, these fragments formed new rocks over millions
of years ago. Geologist can tell how and where a rock was formed by studying
it. The best clue to identify a rock is the texture. The texture helps the geologist
to describe the rocks. The texture of a rock is determined by the size and arrangement
of the particles.
Metamorphic rocks - a crystal-like texture
Garnet Schist - metamorphic rock look layered
Hornfels - metamorphic rocks appear homogenous
Igneous rocks - crystaline and non porous
Brown Granite - solidified slowly underground
and cooled igneous rocks, have large grain size, classified as plutonic
Basalt - quickly cooled igneous rocks, have
small grain size, classified as rockanic
Shale - made of pro-existing rock cemented
together, called clastic
Limestone - formed from shells and fossil
fragments, called carbonates
-
Energy rich rocks
Off the coast 1200 meters beneath the ocean floor, sandstone
is found in petroleum basins. 25 million years ago, when fragments of quartz
and other minerals were eroded from large rock masses and carried into lakes
and rivers which eventually transported some sediments into the sea, sandstone
was formed. The oil trapped in this rock was formed in another rock located
even deeper. The remains of plants and animals in this source rock were deposited
together with rock particles and became trapped with the newly formed rock.
The buried organic materials are then converted into petroleum through heat
and pressure. The oil in this 'kitchen' migrated upwards into the sandstone
reservoir through time. Shells of ancient marine organism form limestone.
Off the coast located 2000 meters beneath the ocean floor, limestone comes
from a reservoir of natural gas. Layers of ancient seashells that were compressed
formed the limestone about 20 million years ago.
The energy cycle
Petroleum is solar energy captured by plants.
There would be no petroleum if plants could not capture and process the sun's
energy. Solar energy is used by green plants to convert carbon, oxygen and hydrogen
into organic molecules also known as carbohydrates in the process called photosynthesis.
This converted energy is passed onto animals when the plants are eaten. Eventually
petroleum is generated by the plants and animals which becomes organic matter.
There are 3 types of kerogen that produce
oil and natural gas.
Type 1 kerogen
It is made from organic matter that is deposited
in fresh water enviroment. This includes lakes, beaches and areas covered over
by rising sea levels. It produces large amounts of oil as it is rare.
Type 2 kerogen
It originates from marine enviroment. Mixture
of microscopic plants, animals and bacteria deposited in a low-oxygen enviroment
produces this type of kerogen. In this type of kerogen, there is a medium to
high concentration of sulphur. This type of kerogen forms most of the world's
oil and gas fields.
Type 3 kerogen
It comes from the deposits of land plants
such as woody materials. It is found usually along the coast. It can produce
substacial natural gas resources but has only a moderate potential for generating
oil.
