Composition of the earth, Geologic Time, and Plate Tectonics Layers of the earth Chemical vs. Mechanical Chemical : Mechanical: 1) Core: Ni and Fe 2) Mantle: Mostly Peridotite 3) Crust: Many different kinds of rocks (for the most part, continents composed of granite and ocean basins composed of basalt) 1) Inner Core: Solid 2) Outer Core: Liquid and convective source of earth s magnetic field 3) Mesosphere (lower mantle): Solid 4) Asthenosphere: Ductile and weak, behaves plastically 5) Lithosphere: Brittle and strong, uppermost layer of crust 1
Rocks of the crust: 1) Igneous: solidified rock from liquid magma or lava. 2) Metamorphic: rock that has been re-crystallized under extreme T s and P s; source rock can be igneous, metamorphic, or sedimentary. 3) Sedimentary: rock formed from consolidated sediments. The breakdown, consolidation, and perhaps recrystallization of old rocks to form new rocks is known as the rock cycle. Properties of the crust: Geothermal Gradient, G T : A product of the heat flux from the mantle and the local surface temperature. Is dependent on thermal conductivity of subsurface formations. Average G T for the earth is 18.2 ο C/km (1 ο F/100ft). Can be used to calculate temperature of a formation in the subsurface: T f : Temperature of formation T s : Local, mean annual surface temperature G T : Geothermal Gradient D : Depth from surface to formation T f = Ts + G Bottomhole temperature = T f if measured accurately Lithostatic Pressure Gradient: 1 lb/psi (22.7kPa/m) Lithostatic pressure is the difference between the overburden pressure and the fluid pressure T D p = p + ob l p f p ob : Overburden pressure p l : Lithostatic pressure : Fluid pressure p f Hydrostatic Pressure Gradient: 0.465 lb/psi (10.53kPa/m) Greater than fresh water gradient due to brines which contain dissolved solids Is affected by temperature and dissolved gas decreases hydro. gradient 2
Plate Tectonics: The crust is divided into plates Convection in the mantle moves these plates relative to one another Overarching theory to explain mountain ranges, basins, and movement of continents The zones of contact between plates are known as boundaries; boundaries can be either continental or oceanic depending on geographic location and do not necessarily define the earth s continents. 3
Plate boundaries are either convergent, divergent, or transform depending on the direction of motion of the plates on either side of the boundary. Divergent boundaries Rising liquid mantle emerges at the crust to form spreading centers and new crust Crust moves AWAY from divergent margins Mid-ocean ridges (oceanic, e.g., Atlantic Ocean), Rift Zones (continental, e.g., East Africa) Extensional Stress Associated with basins (e.g., oceanic basins) Normal and Transform faulting 4
Convergent boundaries Oceanic crust is much denser that continental crust Older and Colder oceanic crust is denser that younger and warmer oceanic crust Oceanic crust moving AWAY from a divergent margin will eventually collide with continental crust or older and colder (hence, denser) oceanic crust The denser crust at a collision zone (convergent margin) will subduct beneath the other crust and travel back down into the mantle and the mantle convection system Western North and South America, East side of Japan Compressional Stress Associated with Mountain Ranges (e.g., Andes) Reverse faulting and heavy folding and deformation Convergent Boundaries Three Cases Oceanic continental convergence Continental continental convergence Oceanic oceanic convergence 5
Transform boundaries Plates move laterally and in opposite directions to one another Shear Stress Mid-ocean ridges, and San Andreas Fault Zone, California Transform or Strike-slip faultin 6
Evidence for Plate Movement Pattern of magnetic stripes in ocean rock (basalt) on ocean floor that reflect earth s magnetic reversals Nasa Satellite GPS tracking movement of plates Plates move at average speed of 3cm/yr 7
The Three Basic Fault Types Normal and Reverse Faults Transform or Strike-Slip Fault 8
PETR 571 Geologic Time Geologic time scale: 9
Relative dating: Geologists long noted a pattern in the superposition of sedimentary layers and the mineralogy as well as flora and fauna that occurred in those layers. Absolute dating: Chemists and Physicists noted that elements decay, some faster than others, but follows exponential decay function. Noting the number of daughter isotopes and fitting it with the exponential decay function, the age of the minerals composing different rocks can be calculated. This is known as radiometric dating and is used to calculate the age of rocks. The earth has been calculated to be 4.6 billion years old. All this while, plate tectonics has been changing the face of the earth as it moves the continents together and apart again, drastically changing the climate and biota So how old is Petroleum? Petroleum has been found in formations of widely different ages (millions of years) reflecting different depositional environments at different times for the same location 10
Glossary*: Absolute time: Specific geologic time in years. Asthenosphere: Upper mantle earth layer below the lithosphere. Basalt: Dark, basic, aphanitic, extrusive igneous rock of pyroxene, calcium feldspar, and olivine. Basement rock: Crustal rocks, usually crystalline (igneous, metamorphic), below the sedimentary section of any area. Basin: A low area with no exterior drainage. Often an area of sedimentary deposition: lake basin; marine basin. Compressive stress: Pressure that pushes material together: operates in opposition toward a common point of plane. Continental crust: Crust of silicon and aluminum minerals which underlies continental masses: SIAL. Convection (mantle): Heat exchange by heating and cooling circulation patterns in the earth s mantle. Core: The solid and liquid center of the earth. It is about 7000km in diameter, composed mostly of iron and nickel, and has an average specific gravity of approximately10.5. Crust (earth): The outer shell of the earth comprising oceanic and continental components with a specific gravity of 2.6-2.65. Convergent boundary: The leading edge of an advancing continental margin. Usually associated with subduction, deformation and volcanism. Daughter product: A nuclide formed by radioactive decay of a parent mineral. May be an isotope of the parent or a different product. Deformation: Changing shape by stress application: folding, shearing. Displacement: Relative movement of two sides of a fault. Divergent plate boundary: The margin of a crustal plate that moves away from a spreading center: the trailing edge of a plate. Eon: The longest geologic time unit: Phanerozoic Eon includes Paleozoic, Mesozoic and Cenozoic eras. Epoch: A geologic time unit longer than an age and shorter than a period. Era: A geologic time unit smaller than an eon. Extrusive: Pertaining to igneous rock which flows or is ejected onto the Earth s surface to cool. Fault: A break in the Earth s crust along which there has been movement. Fauna: Animal population of an area or geologic time increment. Faunal succession: The evolutionary sequence of fauna through geologic time. Flora: The plant population of an area or geologic time increment. 11
Fold: A bent or curved stratum, cleavage plane or foliation. Fossil: Preserved plant or animal material. Geologic time scale: The chronologic sequence of geologic events which correspond to divisions of geologic time. Geology: The study of the history of the earth, its processes, and properties. Geothermal gradient: The increase in temperature with a depth below the Earth s surface: on average, 1 ο F/100ft. Granite: An acidic plutonic igneous rock containing quartz, potassium feldspar, amphibole and mica. Half-life: The time required to radioactively break down a radioactive material to half its original radioactivity. Igneous rock: Rock that has crystallized from magma (molten material). Lava: Mobile, extrusive igneous rock. Lithosphere: The outermost layer of the Earth which includes continental and oceanic crust in its upper layers and the uppermost part of the mantle. It is up to 100km thick. Magma: Molten rock. Mantle: The second layer of the earth below the lithosphere, 2900km thick, and with a specific gravity of 4.5-5.0. Metamorphism: The process of mineralogically, physically, chemically, and structurally changing rock by temperature, pressure, shearing, and chemical means. Oblique-slip fault: Fault that is a combination of dip-slip and normal/reverse. Period: An increment of geologic time smaller than an era and larger than an epoch, e.g., the Cambrian period. Plate-tectonics: A theory of crustal deformation involving the motions of lithospheric plates. Radiometric age: A geologic age in years determined by measurement of elemental half-lives, radioactive emission rates and daughter product formation. Transform boundary: A lithospheric plate boundary characterized by strike-slip motion/displacement. *Link, Peter K., Basic Petroleum Geology, Oil & Gas Consultants international, Inc., Tulsa, Oklahoma. 1987 12