Geophysical methods
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The document provides an overview of geophysical methods used in hydrocarbon exploration, including gravity, magnetic, and electromagnetic techniques, and their applications in identifying subsurface structures and properties. It discusses the principles and theories behind these methods, as well as specific exercises related to seismic interpretation in the context of extensional sedimentary basins, particularly in offshore Norway. Further reading materials are also suggested for a deeper understanding of geophysical exploration and seismic interpretation.
Geophysical methods
- 1. Introduction to Petroleum Geology and Geophysics Geophysical Methods in Hydrocarbon Exploration GEO4210
- 2. About 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 excercise
- 3. Lecture Contents • Geophysical Methods • Theory / Principles • Extensional Sedimentary Basins and its Seismic Signature • Introduction to the Exercise
- 4. Geophysical 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 content
- 5. Geophysical methods Density Spatial variations in the strength of the gravitational field of the Earth Gravity Seismic velocity (and density) Travel times of reflected/refracted seismic waves Seismic Electric conductivity/resistivity and inductance Response to electromagnetic radiation Electromagnetic (SeaBed Logging) Magnetic susceptibility and remanence Spatial variations in the strength of the geomagnetic field Magnetic “Operative” physical property Measured parameter Method
- 6. Further 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. http://www.learninggeoscience.net/modules.php
- 7. Gravity • 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 milligal, 10-5 ms-2), i.e. deviations from a predefined reference level, geoid (a surface over which the gravitational field has equal value) • Gravity is a scalar
- 8. Gravity • Newton’s Universal Law of Gravitation for small masses at the earth surface: – G = 6.67x10-11 m3kg-1s-2 – R is the Earth’s radius – M is the mass of the Earth – m is the mass of a small mass • Spherical • Non-rotating • Homogeneous g is constant! 2 2 R M G g mg R m M G F × = → = × × =
- 9. Gravity • Non-spherical Ellipse of rotation • Rotating Centrifugal forces • Non-homogeneous Subsurface heterogeneities Disturbances in the acceleration
- 10. N Sphere Ellipse of rotation gav = 9.81 m/s2 gmax = 9.83 m/s2 (pole) gmin = 9.78 m/s2 (equator) Earth surface continent ocean Ellipse of rotation Geoid Geoid = main sea-level Geoid Anomaly
- 11. NGU, 1992
- 12. Magnetics • 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. • The unit of measurement is the tesla (T) which is volts·s·m-2 In magnetic surveying the nanotesla is used (1nT = 10-9 T) • The magnetic field is a vector • Natural magnetic elements: iron, cobalt, nickel, gadolinium • Ferromagnetic minerals: magnetite, ilmenite, hematite, pyrrhotite
- 13. Magnetics • Magnetic susceptibility, k a dimensionless property which in essence is a measure of how susceptible a material is to becoming magnetized • Sedimentary Rocks – Limestone: 10-25.000 – Sandstone: 0-21.000 – Shale: 60-18.600 • Igneous Rocks – Granite: 10-65 – Peridotite: 95.500-196.000 • Minerals – Quartz: -15 – Magnetite: 70.000-2x107
- 14. Magnetics • Magnetic Force, H • Intensity of induced magnetization, Ji • Ji = k · H • Induced and remanent magnetization • Magnetic anomaly = regional - residual H Ji Jres Jr
- 15. NGU, 1992
- 16. Electromagnetics 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 travel
- 17. Electromagnetics Transmitter Receiver Primary field Secondary field Conductor Primary field Electromagnetic anomaly = Primary Field – Secondary Field
- 18. Electromagnetics – 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 reservoir
- 19. Marine multichannel seismic reflection data Reflection Seismology
- 22. Reflection Seismology 1 2 1 2 1 1 2 2 1 1 2 2 Z Z Z Z v v v v R + − = + − = ρ ρ ρ ρ Incident ray Amplitude: A0 Reflected ray Amplitude: A1 Transmitted ray Amplitude: A2 ρ1, v1 ρ2, v2 ρ2, v2 ≠ ρ1, v1 Acoustic Impedance: Z = ρ·v Reflection Coefficient: R = A1/A0 R = 0 All incident energy transmitted (Z1=Z2) no reflection R = -1 or +1 All incident energy reflected strong reflection R < 0 Phase change (180° ) in reflected wave Layer 1 Layer 2 Transmission Coefficient: T = A2/A0 1 1 2 2 1 1 2 v v v T ρ ρ ρ + = -1 ≤ R ≤ 1
- 23. Reflection Seismology • Shotpoint interval 60 seconds • 25-120 receivers • Sampling rate 4 milliseconds • Normal seismic line ca. 8 sTWT
- 25. Sedimentary Basins • Hydrocarbon provinces are found in sedimentary basins • Important to know how basins are formed • Basin Analysis – Hydrocarbon traps – Stratigraphy of • Source rock • Reservoir rock • Cap rock – Maturation of source rocks – Migration path-ways
- 26. Extensional Sedimentary Basins • Offshore Norway – Viking Graben, Central Graben • Late Jurassic – Early Cretaceous • Mature Hydrocarbon Province
- 28. Syn-Rift Rotated Fault Blocks Increasing Fault Displacement
- 29. Seismic Signature of Extensional Sedimentary Basins
- 32. Seismic Signature of Extensional Sedimentary Basins – Offshore Norway
- 33. Stratigraphy – Offshore Norway
- 35. Summary Offshore Norway • Main Rifting Event: Late-Jurassic – Early Cretaceous • Structural Traps – Fault bounded • Main Reservoir: Upper Triassic – Middle Jurassic, containing Tarbert, Ness, Rannoch, Cook, Statfjord and Lunde Fms. • Source Rock: Upper Jurassic, Heather Fm • Cap Rock: Early Cretaceous
- 36. Exercise • Interprete seismic line NVGTI92-105 • Interprete pre-, syn- and post-rift sequences • Interprete possible hydrocarbon traps • Point out source-, reservoir, and cap-rock