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Earth Structure and Plate Tectonics

Overview

Petroleum means "rock oil". Oil and gas are stored in rocks, so an understanding of how rocks are formed and how they control the accumulation and flow of hydrocarbons trapped in them is necessary to identify and produce hydrocarbon reservoirs.

The geological processes that control how reservoir rocks form and are charged with hydrocarbons occurs over millions of years of geological time. Geological processes control the accumulation and transmissivity of reservoir fluids.

In this section, you will learn about:

Earth structure. Continental and oceanic crusts. Plate tectonics and continental drift.

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Petroleum Geology Core ═════════════════════════════════════════════════════════════════════════

© PetroSkills, LLC. All rights reserved._____________________________________________________________________________________________

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Earth Structure

Geology tells us that the Earth formed approximately 4.5 billion years ago as gravity brought together the gas particles formed during the Big Bang. Over time, as these particles coalesced into more and more dense and larger objects, the Earth as we know it today was formed.

Reviewing our diagram, we see that the crust is actually composed of both oceanic and continental plates. The oceanic crust is denser than the continental crust and together they form the Lithosphere. The oceanic crust thickness varies from as little as 5 km at the mid-oceanic ridge, while the continental crust averages 30 km. Where mountains such as the Alps are seen, it can be up to 70 km thick. With increasing depth the Mantle density increases. Also with increasing depth, both pressure and temperature increase. The mantle, like the crust, is solid and is approximately 2900 km thick. The iron-rich core consists of a liquid outer core and a solid inner core. As the Earth rotates, the liquid outer core spins, creating the Earth's magnetic field.

By recording the energy generated by earthquakes at seismographs positioned around the globe, we are able to understand the Earth’s structure, composition, and the thickness of each layer.

There are three principle types of energy waves generated during an earthquake:

The compressional (P) wave which propagates, or travels, through both solid and liquid materials. The P wave speed/velocity changes as the composition of the rock changes. This then allows us to identify the minerals that make up the rock.

The shear or "S" wave will not travel though liquids, which allows us to differentiate between the solid inner and liquid outer core.

The third wave is the surface (shake) wave that travels on the earth’s surface.

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By recording the total travel times and changes in travel time as the waves move through the earth we determine the Earth’s structure and composition.

The crust and upper mantle are of most interest to us. That is because that’s where we find the oil and gas.

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Earth Structure: The Crust

Continental and Oceanic Crusts

The crust is composed of both oceanic and continental plates. The oceanic crust is denser than the continental crust and together they form the lithosphere.

The oceanic crust thickness varies. The continental crust averages 18 miles (30 kilometers), and the mid-oceanic ridge can be as little as approximately 3 miles (5 kilometers) thick. In mountain regions, such as the Alps, it can be up to 43 miles, or 70 kilometers thick.

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About 35% of the Earth’s surface is continental crust and the remainder is oceanic. The sedimentary rocks that form part of the petroleum system are of continental origin. They are not present on the deep ocean floor. We look for oil and gas where sediments accumulate on the continents and on the continental shelf, whether those rocks are above or below sea level today.

Reservoirs are still found on the continental crust, onshore (above sea level) and offshore (below sea level), even if running marine seismic or drilling offshore.

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Earth Structure: The Crust

Abyssal Plains

Sea level is the reference point for elevation of the solid surface of the Earth.

Over 70% of the Earth's surface is below sea level. However, the ocean floors are not composed of sedimentary rocks and therefore are not considered petroleum provinces.

While the abyssal plains make up a significant percentage of the Earth's surface, it is the surface regions that have primary hydrocarbon potential: the continents, the continental shelves, continental slopes, and continental rises. COPYRIG

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Geological Time Scale

Geological Time Scale

PangaeaCOPYRIG

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Pangaea~500 million years ago

Permian Period~290 million years ago

Modern World

Continental Drift

Ocean Cross-section

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Ocean Cross-section

SouthAmerica

SouthernAfrica

ConvergentBoundary

Crustal Plates Mid-Oceanic Ridges

Crustal and Ocean Plate Boundaries

DivergentBoundary

TransformBoundary

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ConvergentBoundary

San Andreas Fault

Crustal Plates Mid-Oceanic Ridges

Crustal and Ocean Plate Boundaries

DivergentBoundary

TransformBoundary

Pacific Plate

PacificPlate

PacificPlate

PacificPlate

PacificPlateCOPYRIG

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Review

In this section, you learned about:

Earth structure. Continental and oceanic crusts. Plate tectonics and continental drift.

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Rock Classes and Depositional Environments

Overview

To understand these reservoir rocks and other rocks important in a successful petroleum system, you need a basic understanding of geology. In this section, you will learn about:

Clastic rocks. Biogenic Chemical sedimentary rocks.

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Rock Classification

Successful petroleum systems are comprised of rocks with void space where hydrocarbons and water can be stored over geological time. There are three categories of rock types, but not all of them are potential reservoir rocks:

Igneous Rocks

Igneous rocks (igneous means "fire" in Latin), are formed from liquid rock called magma. Whether they intrude from the mantle into the crust as, or flow to the surface as lava, the magma cools, crystalizes and forms an igneous rock. Some common igneous rocks include granite, obsidian, and pumice.

Origin – Cooled from a molten state.

Texture – Igneous rocks have a massive, crystalline or glassy texture; and because there is no void space available for fluids to accumulate, they are not potential reservoir rocks.

Laccolith Lava Crystalline Texture

Sedimentary Rocks

Sedimentary rocks derived from the Latin word "seated" are composed of sediments. The three predominant sources include:

Clastic – rocks formed by the processes of weathering and erosion creating fragments that are then

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transported, deposited and into rocks.

Biogenic – rocks of nature, resulting from deposition of plant and/or animal residue. Examples include coal, coral reefs, and chalk.

Chemical – rocks formed by mineral and salt precipitation from mineral-rich surface water evaporation.

Origin – From pieces of other rocks or chemical solution. Some common examples include shale (47%), sandstone (31%), and carbonates (22%). Texture – Fragmental or crystalline, often porous. Because there is void space or between the fragments or crystals in the rock, they are considered potential reservoir rocks. They are also potential hydrocarbon source rocks and seal or cap rocks.

Metamorphic Rocks

Metamorphic rocks, from the Greek for “change form”, are existing rocks that are physically changed by heat and pressure over geological time. As sediments accumulate, they exert increasing pressure on the rocks below. As more sediment builds up, it forces the rocks deeper into the ground, where over geologic time, the increased temperature and pressure affect and transform the rocks. Common metamorphic rocks include quartzite, marble, and anthracite (coal).

Origin – through changing of other rocks by heat, pressure, and solution. Because the grains have been recrystallized, the void space is filled in and the grains are aligned with each other. There is no porosity, so it is not a reservoir rock. Texture – Massive, aligned crystals.

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The Rock Cycle

Pressure

Heating

Pressure

Heating

Erosion

Dep

osition

Igneous RockIgneous RockSedimentary RockSedimentary Rock

SedimentSediment

MagmaMagma

Metamorphic RockMetamorphic Rock

Heating

Melting

The Rock Cycle

Pressure

Heating

Pressure

Heating

Erosion

Dep

osition

Igneous RockIgneous RockSedimentary RockSedimentary Rock

SedimentSediment

MagmaMagma

Metamorphic RockMetamorphic Rock

Heating

MeltingCOPYRIG

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Geological Time Periods

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Sedimentary Rocks: Clastics

Fragmental, or clastic sedimentary rocks include sandstone, siltstone, and shale. They are formed by weathering, erosion, transport, and deposition of fragments of pre-existing rocks into sedimentary basins, which are lithified over geological time. The process can be described as follows:

Uplift creates Mountains...

For example, the Rocky Mountains were created by two continental plates colliding and then folding up into mountains.

Add weathering and erosion...

The rocks are weathered by rain, wind, and freeze-thaw action, rock fragments are created.

Then transportation...

These fragments can be transported by wind or water for example, which transport large and small fragments. As the river velocity and energy declines, so does the ability to carry the fragments, and deposition of the largest fragments takes place.

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Leads to lithification.

Lithification is the process of transforming sediment into rock. Over geological time, as the sediments accumulate on top of each other, the sediments compact and the original sea or lake water "connate water" is expulsed.

As depth of burial increases, the pressure and temperature increases, and the rock lithifies. During this process, the rock will often be cemented by the dissolution of calcite from sea shells in the original sediment, which at greater depth recrystallizes to cement the grains together.

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Grain Size

Clastic or fragmental rocks are classified using grain size. A gravel has a predominant grain size between 2 mm and 16 mm. When lithified it becomes a conglomerate, which is a reservoir quality rock. For example, in the North Sea, Brae field is reservoir rock that is a conglomerate.

When we go to a sandy beach, we are actually describing sediments that range in size from 2 mm to 1/16 mm. When this sand is lithified over millions of years, we call it sandstone. Likewise, as we walk out from the beach into the sea, we eventually find muddy sediments with less than silt-sized particles and clay minerals, which over geological time become shale.

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Clastic Rocks

Clastic rocks are formed by weathering and erosion of larger rocks. Common types of clastic sedimentary rocks include:

Sandstone

This sedimentary rock is comprised of predominantly sand-sized fragments composed of quartz (silica) and other minerals. Sandstones typically are regarded as excellent reservoir rocks as there is void space between the sand grains.

When rocks are deposited in a marine environment for example, water (brine) fills the space between the grains. This space between the grains is referred to as porosity and is a measure of the rock's ability store fluids.

Siltstone

Siltstone is a sedimentary rock with a predominant grain size in the silt range. The primary difference then between a siltstone and a sandstone is the predominant grain size and presence of a wider range of other minerals. Siltstones are considered potential reservoir quality rock and are often referred to as fine-grained dirty sandstones.

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Shale

Shale is a combination of very fine-grained particles with a significant percentage of clay minerals and feldspars. Until the last ten years, shale was not considered a reservoir quality rock because the void space was so microscopically small that while oil and gas could be stored, they could not be produced economically.

It is now considered a potential reservoir rock. It has always been highly valued as a potential hydrocarbon source rock and as a seal in a successful petroleum system.

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Environment of Deposition

Accumulation of sediments results in wind ripples orsand dunes

The major geological structure or setting of deposition


Accumulation of sediments results in wind ripples orsand dunes

The major geological structure or setting of deposition

The shape and size of reservoir

rock

Its abilityto transmit

fluids

Itsrecoveryefficiency

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Deposition

• Horizontal sorting of sediments

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Sediment, Depositional Environment, and Rock Type

Here we summarize the type of sedimentary rock and the sediments it is formed from. For example, CaCo3 produced by marine plankton in a deep sea forms chalk; whereas, shells and a lime-rich mud in a shallow sea forms limestone.

Sediment Sedimentary

RockEnvironment of

Deposition

Gravel Conglomerate Alluvial fans, river channels, wave-swept coastlines

Sand Sandstone Desert dunes, river channels, shorelines, deltas, shallow seas

Mud Shale Lakes, rivers, floodplains, tidal flats, distal deltas, deep seas

Shells and lime mud Limestone Warm shallow seas

CaCO3 produced by marine plankton

Chalk Deep seas

SiO2 produced by marine plankton

Chert Deep seas

Woody plant matter: peat

Coal Lagoons or marginal seas in horse latitudes

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Sedimentary Rocks: Chemical and Biogenic

Carbonates

Let's review non-clastic sedimentary rocks. Firstly, chemical rocks.

Limestone is a sedimentary rock characterized by being primarily calcium carbonate. You will hear the word calcite used in reference to calcium carbonate. Limestones of chemical origin commonly have (but not always) a crystalline rock structure; therefore, the pore structure where oil and gas accumulate and flow from in carbonates is very different from clastics.

Visually limestones are classically light-colored and easily recognized by the matrix crystalline structure.

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Biogenic

Besides clastics, the other primary types of sedimentary rocks are of biogenic sources:

Limestones can be a chemical or biogenic origin. Biogenic means created from of plant and animal remains, such as corals, mollusks (oyster shells), and diatoms; again, the present informs the past. Limestones full of oyster shells or coral reef limestones from Cretaceous time (millions of year ago) can be prolific reservoir rocks. They were created in the same environment where we see massive oyster beds today, or a coral reef like the Great Barrier Reef.

The oyster beds or coral reef are buried over time and lithified to become limestone rocks. While carbonates are reservoir rocks, their rock texture is very different from sandstones.

Chalk is also of biogenic origin. It is composed of the calcium carbonate skeletons of diatoms and was created in in deep seas, as opposed to limestones created in shallow seas (fossiliferous and reef limestones).

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Carbonates Evaporite

In the sedimentary sequence, we find rocks such as salt (Halite), which are formed by precipitation from mineral rich seawaters or brines where the water replenishment rate is less than the evaporation rate.

Anhydrite and gypsum are other examples. The carbonate molecules are derived from the solution of atmospheric carbon dioxide in seawater.

As the carbon dioxide dissolves in the seawater, it combines with the calcium to form calcium carbonate. This occurs when there is an abundance of CO2 in the atmosphere. When CO2 leaves the atmosphere to form massive accumulations of limestone, as seen during Cretaceous times, we see a corresponding decrease in global temperatures in the rock record.

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Coal

Coal is formed from the accumulation of plant debris in a swampy, warm, carbon rich environment. It is no accident that coal was found during the carboniferous period – a time of a very warm, wet, global climate.

As plants such as trees and ferns accumulated in the swampy environment, they decomposed; and over time lithified through an ever-increasing depth of burial to more carbon rich forms, such as peat moss, and to high quality coal we call bituminous. Coal is an unconventional resource; we refer to it as coalbed methane (CBM) or coal seam gas (CSG). It can also be an important source rock for natural gas reservoirs. We see this in the southern sector of the North Sea.

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Review

In the industry, sedimentary rocks are important because they are part of a successful petroleum system that is developed over geological time. They are the rocks in which the fluids are stored – reservoir rocks. They can also be the source of the hydrocarbons that then migrate into the reservoir rock.

Once that reservoir rock is charged with oil and/or gas from the source, they can act as the seal or cap rock, ensuring a successful petroleum accumulation.

Sandstone is a classic reservoir quality rock; historically shale was seen as a source rock or cap rock, but today and in the last 10 years by the application of fracturing and horizontal drilling technologies, it is now also considered a very valuable unconventional reservoir rock. Carbonates are prolific reservoir rocks and to a lesser extent can be source and seals.


Types of rock. Clastic sedimentary rocks. Environment of deposition. Biogenic and chemical sedimentary rocks.

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Historical Geology

Overview

Oil and gas are stored in rocks, so an understanding of how rocks are formed and how they control the accumulation and flow of hydrocarbons trapped in them enables us to identify and produce hydrocarbon reservoirs.

The earth's structure and the geological processes that control how reservoir rocks form and are charged with hydrocarbons occurs over millions of years of geological time. So, while oil may be found today in Saudi Arabia or the North Sea geologically, both provinces produce millions of barrels of oil a day from a specific geologic time period-the Jurassic.

In this section, you will learn about:

Historical geology. Relative age of rocks and index fossils. Global and regional vs. local stratigraphy.

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Historical Geology

While it may seem that oil and gas are often found in specific locations, such as the Gulf of Mexico, or the Arabian Peninsula, oil and gas is found in geological formations that were formed many millions of years ago during a global time period. Knowing the time period when oil and gas accumulations likely occurred is essential to finding oil and gas accumulations.

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The Principle of Superposition

During the development of historical geology, the principle developed that the deeper the rock strata the older the rock. This is called the principle of superposition, where young rocks are laid down on top of the older rocks.

However, with uplift and mountain building such as in the Rocky Mountains or the Pyrenees in response to tectonic pressures, older rocks have been thrust on top of younger rocks; so, it's important to understand the processes that take place once rocks have been formed over geological time in a sedimentary basin.

Stratigraphers perform the work of describing the succession of different rock strata and the context within the geological time record. By understanding which fossils exist at a specific time, it’s possible to correlate rocks on a global basis by the plant and animal fossil life that is preserved in the rocks, these are called index fossils.

Dating the rocks can be accomplished using:

Fossil records to determine the relative age. Radiometric dating, which accurately dates the rocks by specifically

testing the volcanic ash in the sedimentary sequence. COPYRIGHT

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Index Fossils

Geologists also study the depositional environment for different types of plant and animal fossils within the sediments. If significant evidence of plant and animal remains are found in the sedimentary bed of a determined geologic age, then those particular plants and animals existed at that time in geologic history. If these were of a species that evolved considerably over short periods of geological time, then the fossils become index or marker fossils.

If the same fossils are found elsewhere, in other sedimentation, it means that sedimentation was occurring at the same time in geologic history as was the sedimentation bed where these particular fossils were originally found, so both sedimentation beds should be the same age. In this way, index fossils are used as a geological clock to determine the time of geological events.

Since the present informs the past, geologists can infer the environment millions of years ago by looking at the environment of similar modern-day organisms. For example, if you go to the seashore today, where there are plenty of shallow water marine fossils, such as oysters, starfish, and snails,

and then see those as fossils in the rock record, you can assume that they were formed in a shallow marine environment.

Likewise, in the massive chalk deposits of the Permian time period in the North Sea, there are silica-rich fossils, such as coccoliths and sponges.

Today those fauna live in deep water environments, which means those chalk reservoirs were formed in a deep water environment. And, because coral live in a narrow temperature and salinity band, you can assume that reef limestones were laid down in warm, relatively shallow depths of water, because that is where reefs are formed today.

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Historical Geology

Present Day

Origin of EarthApprox. 4.5 billion years

The Earth

Last 1/10 of the age of earth

Sufficient quantities of plantsand animals to provide organicmatter to generatehydrocarbons

Most important time period tothe oil and gas industryCOPYRIG

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Global vs. Regional Stratigraphy

On a global basis, we have the historical record; however, regionally and locally, it depends on whether sedimentation actually happened at that specific time in the geological history of the earth to determine whether the rocks are present or not.

Likewise, the rocks in a specific sedimentary sequence in a specific location will have a local name. If we take a look at this geological sequence in Trinidad, we see that on this small island there are two distinct sedimentary basins (Northern and Southern), in Tertiary time where sedimentary rocks were laid down separately.

The petroleum geologist searches for oil and gas at different times in different locations on the island. In the southern part of the island, the rocks can be correlated with those seen in Venezuela, yet they have different names. So, as we search for oil and gas at different geological times and at different locations, we encounter different sedimentary sequences with different names that may or may not be present. We can however, correlate these horizons globally by geological age using index fossils and other supporting data.

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Regional Stratigraphy – North Sea

Looking at the generalized stratigraphy of the North Sea, note the global time periods and then the local geological sequence or series. In the northern part of the North Sea, there is a very different series of oil and gas fields, compared to those in the southern sector.

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Sequence Through Geological Time: The North Sea

Carboniferous Period:

Known locally as the coal measures in the southern sector of the North Sea – they are an abundant source of natural gas that charges (fills up) the reservoir rocks laid down during the Permian.

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Permian Period:

During the early Permian time, northern Europe became a wide-ranging desert, in contrast to the swampy environment of Carboniferous time. Not only the fossils, but the dune-like stratification of the iron-rich, red rocks (the name Rotliegendes means “red layers”) indicate a non-marine environment. The presence of dunes in the rock record confirm a desert setting.

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Triassic Period:

After a period of uplift in the early Permian, there was subsidence and establishment of a deep warm sea, indicated by the silica-rich fossils found in the chalk deposits. These chalk deposits form both gas and oil reservoirs in the late Permian, southern sector of the North Sea province.

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Review


Historical geology. Relative age of rocks and index

fossils. Global and regional vs. local

stratigraphy

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