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Introduction to Petroleum Geology
I. The Basic Overview
a. Petroleum Geology – application of geology (study of Earth, materials and processes) to the exploration and
production of oil and natural gas
i. Geology = interdisciplinary science at the crossroads of Chemistry, Physics and Biology
ii. Petroleum Exploration = focus on oil and gas, part of the hydrocarbon family of organic compounds
b. Petroleum-Related Hydrocarbons (hydrogen–carbon based organic compounds)
i. Three Physical States: solids, liquids, gases (in order of decreasing density)
ii. Gases: Methane, ethane, propane, butane + others
iii. Liquids: oil, crude oil, “crude” (texas tea)
1. Refined to derive petroleum products such as gasoline, diesel fuel
iv. Solids: coal, kerogen, gas hydrates (ices of gas)
1. Visco-plastic solids: tar
c. Petroleum occurrence: natural containment in subsurface reservoir rocks that promotes accumulation of fluids
over geologic time.
i. The petroleum environment exists in the realm of sediments, sedimentary rocks and sedimentary basins.
ii. Petroleum Generation: carbon-based, organic-rich sediments with biogenic materials derived from plants and
animals.
1. Marine Biogenic Sediment Production.
a. Invertebrates, reefs, phytoplankton, zooplankton
b. Biologically rich, high productivity ocean settings
c. Shallow, well oxygenated, sunlit, warm waters
2. Nonmarine Biogenic Sediment Production
a. Plant rich environments
b. Tropical/Subtropical Zones
c. Lakes and Floodplains, deltas
3. Sedimentary Basins: accumulations of sediments in subsiding basins, deltas, continental shelves, tectonic
depressions
a. Water saturated, reducing environments (oxygen deficient) result in preservation of organic materials.
b. Burial of organic rich sediment / sedimentary strata results in increased geothermal heating and lithostatic
pressure.
c. Heat and pressure during burial, combined with biologic activity, chemically and physically transforms
biogenic sediment into liquid oil and natural gas.
d. Geologic time required for biologic lifecycles, biogenic sediment transport, sedimentary basin development,
burial and thermal maturation (time scale on 105 to 106 years of time).
iii. Petroleum Reservoirs: sediments and sedimentary rocks with relatively high porosity and permeability (“oil
aquifers”).
1. Sandstone Reservoirs
2. Carbonate/Limestone Reservoirs
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iv. Impermeable Seals and Cap rock
1. Mudstone, Shale
2. Tightly cemented sandstones, carbonates
v. Traps – containment of petroleum in subsurface: geometric arrangements of strata that result in juxtaposing
permeable reservoir rocks subjacent to impermeable cap rocks to prevent buoyant migration of oil and gas
upward to the earth’s surface.
1. Structural Traps (faults, folds, uncomformities)
2. Stratigraphic Traps (depositional facies, lateral and vertical interlayering).
d. Required Conditions for geologic accumulation of petroleum
i. Organic rich source rock (biogenic sediments) to generate oil and/or gas over geologic time.
ii. Source rock burial and geothermal heating to convert organic sediment to petroleum compounds
iii. Reservoir rock with requisite porosity and permeability to store and transfer hydrocarbon fluids.
iv. Impermeable seal or cap rock to prevent upward escape of buoyant petroleum compounds to the Earth’s
surface.
v. Geometric arrangement of source, reservoir and seal rocks in such a way to trap petroleum in subsurface.
vi. A temporal sequence / geologic history of trap formation and petroleum generation that favors subsurface
accumulation of petroleum.
vii. Subsurface preservation of petroleum accumulation over geologic time until extraction and production
II. Hydrocarbon Composition and Occurrence
a. “Natural Gas” = a mixture of hydrocarbons (‘organic) and other non-hydrocarbons (non-organic)
that exist in a gaseous phase or in solution with crude oil in natural subsurface reservoirs
i. Free vapor phase gas vs. dissolved gas
ii. Dry gas vs. wet gas a function of water vapor content
1. Dry gas = e.g. methane (CH4)
2. Wet gas = e.g. ethane (C2H6), propane (C3H8), butane (C4H10), pentane (C5H12)
iii. Sweet vs. Sour Gas = function of hydrogen sulfide (H2S) content (low vs. high, respectively)
iv. Other naturally occurring Gases
1. Inert Gases via radioactive decay
a. Helium (He), Argon (Ar), Radon (Ra)
2. Other Naturally Occurring Gases
a. Nitrogen (N2) Carbon Dioxide (CO2), Hydrogen (H2), Hydrogen Sulfide (H2S), Water Vapor (H2O)
b. “Crude Oil” = mixture of hydrocarbons that exist in a liquid phase in natural subsurface reservoirs and
that remain liquid at atmospheric surface pressures
i. Liquid phase mixtures of oil and water in subsurface common
1. Light Oil vs. Heavy Oil: density less than or more than water, respectively
ii. Range of Elemental Composition of Crude Oil by Weight Percent
iii. Molecular Composition – complex variety of over 200 organic compounds due to the versatile nature
of carbon bonding with 4 electrons in outermost valence shell
1. Hydrocarbon Compunds
a. Paraffins
i. General formula: CnH2n+2
1. n < 5 gaseous at normal temp and pressure
2. n ranging from 5-15 liquid at surface temp. and pressure
3. n > 15 solid waxes and viscous liquids
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b. Napthenes
i. General formula: CnH2n
ii. Liquid at normal temperatures and pressures
iii. Comprise ~40% of crude oil composition
c. Aromatics (ring structure molecules)
i. E.g. benzene C6H6
2. Mixed compounds
a. Crude oil contains low percentages of oxygen, nitrogen, sulfer, trace metals
3. Average Organic Composition of Crude Oil
4. Average Crude Oil Distillates in Order of Increasing Density and Boiling Point Temperature
III. Stages of Petroleum Exploration, Production and Processing
a. Land acquisition / mineral rights
b. Petroleum Exploration and Geologic Prospecting
i. Surface Geology / Geologic Mapping
ii. Remote Sensing
iii. Subsurface Geology
1. Geophysics/Geochemistry
2. Drilling/Exploration
c. Petroleum Production
i. Production Wells / Petroleum Engineering
ii. Conveyance, Extraction and Storage Systems
iii. Transportation
d. Refining
i. Chemical Processing
ii. Distillation
e. Marketing/Sales
IV. Historical Overview of Petroleum Production and Exploration
a. Early History
i. Liquid oil and semi-solid asphalt derived from natural outcrops and seepages
1. Hand excavation in pits, collection from seeps and drainage at Earth’s surface
2. E.g. La Brea Tar Pits of Southern California
ii. 200-400 B.C. Greeks used petroleum compounds to water proof ships and for medicine
iii. 1600-1800 A.D. Europeans used petroleum products for manufacturing, water proofing
and fuel
1. Pitch, oil, tar, brine water
2. Refined and separated through boiling and heating
iv. 1800’s refineries developed for fuel products
1. Paraffin wax
2. Liquid paraffin (kerosene, coal oil)
3. Replaced whale oil for lamps and lighting
b. Modern History
i. 1859 First successful / commercial oil well drill
1. Cable tool techniques
a. Simple impact / chisel drilling
4. Oil Creek, Pennsylvania (Titusville) “Colonel Drakes Well”
ii. Early 1900’s
1. Advances in drilling and exploration techniques
a. Exploration of anticlines
b. Stratigraphic occurrence, paleo shorelines, diagenesis
c. Advanced drilling techniques
2. Advances in petroleum refinement and processing
a. Lighter density derivatives (light gases, petrol, gasoline, diesel fuel)
3. 1920’s birth of the modern petroleum industry
a. British Petroluem, Shell (Dutch)
b. Esso (Exxon), Gulf Oil, Texaco, Mobil, Socal (Chevron)
c. Middle East and South American petroleum exploration
i. Arabian-American Oil Company (ARAMCO)
iii. Mid-1900’s / 1960’s
1. Organization of Petroleum Exporting Countries (OPEC)
a. Iraq, Iran, Kuwait, Saudi Arabia, Venezuela + North African Countries added in
2. Advances in exploration techniques
a. Geophysics, geochemistry, remote sensing
i. Gravity/magnetic surveys
ii. Seismic reflection
b. Advanced drilling technology, enhanced recovery (e.g. Hydraulic fracturing)
iv. 2000’s
1. 3-D Seismic, GIS/GPS technology, offshore-deep water drilling
2. Enhanced recovery (“fracking”), thermal processing (oil shale)
3. Major Oil Producers
a. OPEC Countries, U.S., China, Russia, Brazil, Malaysia
V. Thematic Topics in Petroleum Geology (crossroads of chemistry, physics, geology)
a. Physical and Chemical Properties of Petroleum
i. Chemical Composition
ii. Physical Properties
b. The Subsurface Geologic Environment
i. Geologic Materials
ii. Fluids
iii. Temperature and Pressure
c. Generation and Migration of Petroleum
i. Biogenic Production of Organic Matter
ii. Preservation and Diagenesis of Organic Matter
iii. Thermal Maturation
iv. Petroleum Migration
d. Reservoir Characterization
i. Geologic Materials
ii. Porosity
iii. Permeability
iv. Diagenesis
v. Reservoir Examples
vi. Reserve Estimates
e. Traps and Seals
i. Structural Traps
ii. Stratigraphic Traps
f. Methods of Exploration
i. Drilling Techniques
ii. Well Logging and Petrologic Analysis
iii. Surface Geophysics
1. Gravity
2. Magnetic
3. Seismic
iv. Borehole Geophysics
v. Remote Sensing
g. Production Methods
i. Petroleum Engineering / Production Wells
ii. Primary vs. Secondary Recovery
h. Sedimentary Basins and Petroleum Systems
i. Basin Styles and Tectonic Setting
ii. Petroleum Occurrence
i. Nonconventional Petroleum Resources
i. Tar Sands
ii. Oil Shale
iii. Tight Oil
iv. Coalbed Methane
v. Shale Gas