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Exploration Geophysics ppt

geophysics in petroleum exploration ppt and geophysics lecture notes ppt
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RyanCanon,United Arab Emirates,Teacher
Published Date:21-07-2017
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GEO4210 Introduction to Petroleum Geology and Geophysics Geophysical Methods in Hydrocarbon ExplorationAbout this part of the course • Purpose: to give an overview of the basic geophysical methods used in hydrocarbon exploration • Working Plan: – Lecture: Principles + Intro to Exercise – Practical: Seismic Interpretation excerciseLecture Contents • Geophysical Methods • Theory / Principles • Extensional Sedimentary Basins and its Seismic Signature • Introduction to the ExerciseGeophysical methods • Passive: Method using the natural fields of the Earth, e.g. gravity and magnetic • Active: Method that requires the input of artificially generated energy, e.g. seismic reflection • The objective of geophysics is to locate or detect the presence of subsurface structures or bodies and determine their size, shape, depth, and physical properties (density, velocity, porosity…) + fluid contentGeophysical methods Method Measured parameter “Operative” physical property Gravity Spatial variations in the Density strength of the gravitational field of the Earth Magnetic Spatial variations in the Magnetic susceptibility strength of the and remanence geomagnetic field Electromagnetic Response to Electric (SeaBed electromagnetic radiation conductivity/resistivity Logging) and inductance Seismic Travel times of Seismic velocity (and reflected/refracted density) seismic wavesFurther reading • Keary, P. & Brooks, M. (1991) An Introduction to Geophysical Exploration. Blackwell Scientific Publications. • Mussett, A.E. & Khan, M. (2000) Looking into the Earth – An Introduction to Geological Geophysics. Cambridge University Press. • McQuillin, R., Bacon, M. & Barclay, W. (1984) An Introduction to Seismic Interpretation – Reflection Seismics in Petroleum Exploration. Graham & Trotman. • Badley, M.E. (1985) Practical Seismic Interpretation. D. Reidel Publishing Company. • Gravity surveying measures spatial variations in the Earth’s gravitational field caused by differences in the density of sub-surface rocks • In fact, it measures the variation in the accelaration due to gravity • It is expressed in so called gravity anomalies (in -5 -2 milligal, 10 ms ), i.e. deviations from a predefined reference level, geoid (a surface over which the gravitational field has equal value) • Gravity is a scalarGravity • Newton’s Universal Law • Spherical of Gravitation for small • Non-rotating masses at the earth • Homogeneous surface: G × M × m G × M F = = mg → g = 2 2 R R -11 3 -1 -2 – G = 6.67x10 m kg s – R is the Earth’s radius à g is constant – M is the mass of the Earth – m is the mass of a small massGravity • Non-sphericalà Ellipse of rotation • Rotatingà Centrifugal forces • Non-homogeneousà Subsurface heterogeneities à Disturbances in the accelerationN Ellipse of Earth surface rotation continent Ellipse of rotation Geoid ocean Geoid = main sea-level Sphere Geoid 2 g = 9.81 m/s av Anomaly 2 g = 9.83 m/s (pole) max 2 g = 9.78 m/s (equator) minNGU, 1992Magnetics • Magnetic surveying aims to investigate the subsurface geology by measuring the strength or intensity of the Earth’s magnetic field. • Lateral variation in magnetic susceptibility and remanence give rise to spatial variations in the magnetic field • It is expressed in so called magnetic anomalies, i.e. deviations from the Earth’s magnetic field. -2 • The unit of measurement is the tesla (T) which is volts·s·m -9 In magnetic surveying the nanotesla is used (1nT = 10 T) • The magnetic field is a vector • Natural magnetic elements: iron, cobalt, nickel, gadolinium • Ferromagnetic minerals: magnetite, ilmenite, hematite, pyrrhotiteMagnetics • Magnetic • Sedimentary Rocks susceptibility, k – Limestone: 10-25.000 – Sandstone: 0-21.000 a dimensionless – Shale: 60-18.600 property which in • Igneous Rocks essence is a – Granite: 10-65 measure of how – Peridotite: 95.500-196.000 susceptible a material is to • Minerals becoming – Quartz: -15 magnetized 7 – Magnetite: 70.000-2x10Magnetics • Magnetic Force, H • Intensity of induced magnetization, J i • J = k · H i • Induced and remanent magnetization J i H • Magnetic anomaly = J J r res regional - residualNGU, 1992Electromagnetics Electromagnetic methods use the response of the ground to the propagation of incident alternating electromagnetic waves, made up of two orthogonal vector components, an electrical intensity (E) and a magnetizing force (H) in a plane perpendicular to the direction of travelElectromagnetics Primary field Transmitter Receiver Primary field Secondary field Conductor Electromagnetic anomaly = Primary Field – Secondary FieldElectromagnetics – Sea Bed Logging SBL is a marine electromagnetic method that has the ability to map the subsurface resistivity remotely from the seafloor. The basis of SBL is the use of a mobile horizontal electric dipole (HED) source transmitting a low frequency electromagnetic signal and an array of seafloor electric field receivers. A hydrocarbon filled reservoir will typically have high resistivity compared with shale and a water filled reservoirs. SBL therefore has the unique potential of distinguishing between a hydrocarbon filled and a water filled reservoirReflection Seismology Marine multichannel seismic reflection dataReflection Seismology