SHORT NOTES ON PETROLEUM SOURCE ROCKS TAREK HAMED, PHD©1

General Petroleum Company

This course is prepared for the undergraduate petroleum geology students from the

faculty of science, Minia University. The course forms the nucleus research to the rout

of petroleum system from source rock accumulation, generation, migration and

preservation into the reservoir traps.

Highlights:

Ancient Egyptians are pioneers to find oil seepage and used it as liquid oil and

Bitumen in mummification, medicine and light.

Ras Gemsa Oil field was drilled in 1886 on the basis of oil seeps and is considered as

the first oil field allover Africa and Middle East.

Hurghada Oil field was the first discovery based on geological studies

Petroleum System

A conventional petroleum system requires four components and two processes.

Elements: source rock, migration fairway, reservoir, rock seal, trap

Processes: thermal maturation, generation, migration, accumulation, preservation

Timing between petroleum migration and creation of reservoir, trap and seal is also

critical. The thorough understanding of the geology, and particularly the time of faulting

and folding, is needed.

1 TAREK HAMED ©All Rights Reserved (2014)

Petroleum System Event Chart

What is an Events Chart?

Events chart is a chart which summaries the major elements forming the petroleum

system of a well developed succession of a specific basin.

What is the critical moment?

The critical moment is defined as the point in time that best represents the generation,

migration, and accumulation of most of the hydrocarbons in of a well developed

succession of a specific basin, where trap must already exist.

GULF OF SUEZ SOURCE ROCKS

EPOCH AGE SOURCE ROCK

MIOCENE

Serravallian Belayim Shale

Serravallian Kareem Shale

Burdigalian - Langhian Rudeis Shale

Aquitanian Nukhul Shale

LATE CRETACEOUS

Campanian - Maastrichtian Brown Limestone

Lower Senonian Matulla Shale

Turonian Wata Shale

Cenomanian Raha Shale

WESTERN DESERT SOURCE ROCKS

EPOCH AGE Source Rock

Upper Cretaceous

Campanian – Maastrichtian Khoman Chalk

Turonian Abu Roash “F” Carbonate

Upper Cenomanian Bahariya Shale

Lower Cretaceous Barremian - Neocomian Alam El Bueib Shale

Middle Jurassic Middle Jurassic Khatatba Shale

NILE DELTA –MEDITERRANEAN SOURCE ROCKS

EPOCH AGE Source Rock

Pliocene Early – Middle Pliocene Kafr El Sheikh Shale

Middle Miocene Sidi Salem Shale

Lower Miocene Qantara Shale

Success Ratio

• The job of exploration geologists is to find oil and gas.

• One measure of their ability is the wildcat success ratio, or the number of dry holes

drilled for each one that produces commercial quantities of petroleum.

• Example: 131,881 NFW wells were drilled in USA 1976-1981 in which 171

significant new fields – those containing more than 3 million barrels of oil, i.e. only

one wildcat in 186 led to significant production.

Number of Significant New Fields 171 1 Success Ratio = --------------------------------------------- = -------- = ------- = 0.00537 Total Number of New Field Wells 31881 186.4

Why Applying Petroleum Geochemistry?

As the exploration for oil and gas prospects grows increasingly complex, more E&P

companies are turning to geochemistry to evaluate a component that is central to the

success of each well: the source rock.

At present, exploration geologists rely very heavily on seismic methods to locate

subsurface features and define traps.

Although any generated prospect based on geophysical procedures offers hope for

identifying the presence of hydrocarbon in some reservoirs, there is no applicable

tool for deciding whether any structure will contain oil, gas or be empty.

The only way of improving odds for success is to use additional exploration

techniques,

“Organic geochemistry is one of these additional exploration techniques”

In most basins the addition of geochemical data to that obtained by the more

conventional methods of geology and geophysics will help improve the chances of

success.

In order to apply geochemistry in exploration we must have methods in recognizing

source rocks and correlating crude oils to each other and to their source rocks.

Identification of source rocks requires understanding the processes by which they

generate crude oils and the way in which the oil migrates. Correlation involves an

understanding of the migration mechanisms.

BASIC PETROLEUM GEOCHEMISTRY FOR SOURCE ROCK EVALUATION

• A SOURCE ROCK can be broadly defined as any fine-grained clastic or carbonate,

organic rich rock that is capable to generate petroleum, given sufficient exposure of

heat and pressure. Its petroleum generating-potential is directly related to its volume,

organic richness and thermal maturity. The volume is a function of thickness and

areal extent. It refers to rocks from which hydrocarbons have been generated or are

capable of being generated.

• Organic richness refers to the amount and type of organic matter contained within

the rock.

• They are organic-rich sediments that may have been deposited in a variety of

environments including deep water marine, lacustrine and deltaic.

• Oil shale can be regarded as an organic-rich but immature source rock from which

little or no oil has been generated and expelled.

• Subsurface source rock mapping methodologies make it possible to identify likely

zones of petroleum occurrence in sedimentary basins as well as shale gas plays.

•

• Every oil or gas play originates from source rock.

• Every Oil and Gas Potential depends on the type of its source rock.

• Source rock can be defined as any fine-grained, organic – rich rock that is capable to

generate petroleum, giving sufficient exposure to heat and pressure.

• Its petroleum-generating potential is directly related to its volume, organic richness

and thermal maturity.

Application of Organic Geochemistry in Exploration

• indicates the source rock potentials and maturities

• Migration fairway along which any structure will be prospective.

• Information from dry holes can help to establish migration path and indicates the

most likely structure for subsequent drilling.

• Seal and/or non-seal faults can be decided.

• Oil shale can be identified and developed as unconventional resource.

• If there are different basin sources, organic geochemistry can tell which one by

correlating crude oil to source rocks and identifying migration pathway(s).

• The presence of two unrelated crude oils shows that there must be at least two source

rocks in the area.

• It must be stressed that organic geochemical data should not be used alone and is

most useful when integrated with information from other sources.

• In most basins the addition of geochemical data to that obtained by the more

conventional methods of geology and geophysics will help improve the chances of

success.

Organic Carbon and Inorganic Carbon

Inorganic Carbon can be obtained in large quantity from inorganic source as minerals.

They include carbonates, bicarbonates and some other carbon-containing compounds.

Organic carbon compounds derived from plants or animals and usually contain carbon

and hydrogen. For example: glucose: C6H12O6) and proteins. They are able to dissolve in

organic solvent (such as methanol, ether and chloroform) and unable to dissolve in

inorganic solvents (such as water, acids and alkalis).

Total Carbon includes both organic carbon and inorganic carbon (mineral and those

shells formed by animal beings).

Organic Material and Organic Matter

Organic Material includes the fossil remains including organic carbon and skeleton

shell formed by inorganic or mineral carbonates as Aragonite and Calcite.

Organic Matter is a matter composed of organic compounds that has come from the

remains of organisms such as plants and animals and their waste products in the

environment. Basic structures are created from cellulose, tannin, cutin, and lignin,

along with other various proteins, lipids, carbohydrates and hydrocarbons. Organic

matter excludes skeletal parts.

SOURCE ROCK IDENTIFICATION

Effective source rock must have three features:

Quantity of organic matter

Quality capable of yielding moveable hydrocarbons

Thermal maturity

DETERMINING SOURCE ROCK POTENTIAL

The quantity of organic matter is commonly assessed by a measure of the total organic

carbon (TOC by weight %) contained in a rock. Quality is measured by determining the

types of kerogen contained in the organic matter. Thermal maturity is most often

estimated by using Vitrinite Reflectance measurements and data from pyrolysis

analyses.

The following table shows the common methods used to determine the potential of a

source rock.

To determine… Measure…

Quantity of source rock Total organic carbon (TOC) present in the

source rock

Quality of source rock

Proportions of individual kerogens

Prevalence of long-chain

hydrocarbons

Thermal maturity of source rock Vitrinite reflectance

Pyrolysis Tmax

Total Organic Carbon (TOC)

The amount of organic carbon present in a rock is a determining factor in a rock's ability

to generate hydrocarbons. The following table is a guideline for assessing richness:

Generation potential Wt % TOC, shales Wt % TOC, carbonates

Poor 0.0-0.5 0.0-0.2

Fair 0.5-1.0 0.2-0.5

Good 1.0-2.0 0.5-1.0

Very Good 2.0-5.0 1.0-2.0

Excellent > 5.0 > 2.0

KEROGEN

• It is defined as the fraction of large chemical aggregates in sedimentary organic

matter, representing 90% of the organic carbon, complex, disseminated in sediments.

• Insoluble in both non-polar solvents (benzene/methanol, toluene), insoluble in non-

oxidizing acids (HCL and HF)

• In contrast, the fraction that is soluble in organic solvents is called Bitumen.

• major starting material for most oil and gas generation

• When sediments including kerogen are subjected to heating in the subsurface, oil and

gas is generated.

• most abundant form of organic carbon on earth (1000 x more than coal)

• made up from altered remains of marine and lacustrine microorganisms, plants and

animals - with differing amounts of terriginous debris

• structured, terriginous portion of kerogen is similar to coal elemental composition.

• Kerogen does not migrate, so sediment matrix and „kerogen‟ are from same

depositional and thermal history. It contains info about the depositional, geological,

and geothermal history of sediments.

Determination of Kerogen Types

Kerogen type is necessary to find out the Petroleum Generating Potential using the

following techniques:

• Optical (microscopic) investigation : - work well for structured kerogen

• Chemical methods : - work well for amorphous organic matter OM (usually present

in greater abundance than structured)

• Analytical methods

– pyrolysis techniques

– nuclear magnetic resonance spectroscopy (NMR)

– Electron microscopy (diffraction)

VAN KREVELEN DIAGRAM

The most commonly utilized scheme for classifying organic matter in sediments is based

on the relative abundance of elemental carbon, oxygen, and hydrogen plotted graphically

as the H/C and O/C ratio on a so called Van Krevelen diagram.

Geothermal Gradient is the rate of increasing temperature with respect to increasing

depth in the Earth’s interior.

Bottom Hole Temperature – Mean Surface Temperature Geothermal Gradient = -------------------------------------------------------------------------------

Total Depth (TD) (Km)

Example:

A well was bottomed at depth 2000m and the bottom hole temperature (BHT)

measured 80oC and the surface temperature (S.T) was 25oC, then:

G.G. (oC/m) = (BHT-ST)/T.D. = (80 – 25) / 2000 = 0.0275 (oC/m)

OR: G.G. (oC/100m) = (BHT-ST)/T.D. = (80 – 25) X 100 / 2000 = 2.75 (oC/100m)

OR: G.G. (oC/Km) = (BHT-ST) X 1000/T.D. = (80 – 25) X 1000 / 2000 = 27.5 (oC/Km)

To determine the Temperature at depth 1200m from last example, then:

Formation Temperature= [Formation Depth (m)XG.G (oC/m)]+Surface Temperature (oC)

= 1200 X 0.0275 + 25 = 58oC

N.B.

1. Conversions must be done before calculation

To convert oC to oF oF = (oC X 9/5 ) + 32 example: 25oC = 77oF

To convert oF to oC oC = (oF – 32) X 5/9 example: 25oC = 77oF

To convert Depth from meter to Foot multiply by 3.281

To convert Depth from Foot to meter divide by 3.281