Introduction To Petroleum Geology And Geophysics

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GEO4210Introduction to PetroleumGeology and GeophysicsGeophysical Methods inHydrocarbon Exploration

About this part of the course Purpose: to give an overview of the basicgeophysical methods used in hydrocarbonexploration Working Plan:– Lecture: Principles Intro to Exercise– Practical: Seismic Interpretation excercise

Lecture Contents Geophysical Methods Theory / Principles Extensional Sedimentary Basins and itsSeismic Signature Introduction to the Exercise

Geophysical methods Passive:Method using the natural fields of the Earth, e.g. gravityand magnetic Active:Method that requires the input of artificially generatedenergy, e.g. seismic reflection The objective of geophysicsis to locate or detect the presence of subsurfacestructures or bodies and determine their size, shape,depth, and physical properties (density, velocity,porosity ) fluid content

Geophysical methodsMethodMeasured parameter“Operative” physicalpropertyGravitySpatial variations in thestrength of thegravitational field of theEarthDensityMagneticSpatial variations in thestrength of thegeomagnetic fieldMagnetic susceptibilityand remanenceElectromagnetic Response toElectric(SeaBedelectromagnetic radiation conductivity/resistivityLogging)and inductanceSeismicTravel times ofreflected/refractedseismic wavesSeismic velocity (anddensity)

Further reading Keary, P. & Brooks, M. (1991) An Introduction toGeophysical Exploration. Blackwell ScientificPublications. Mussett, A.E. & Khan, M. (2000) Looking into the Earth –An Introduction to Geological Geophysics. CambridgeUniversity Press. McQuillin, R., Bacon, M. & Barclay, W. (1984) AnIntroduction to Seismic Interpretation – ReflectionSeismics in Petroleum Exploration. Graham & Trotman. Badley, M.E. (1985) Practical Seismic Interpretation. D.Reidel Publishing php

Gravity Gravity surveying measures spatial variations inthe Earth’s gravitational field caused bydifferences in the density of sub-surface rocks In fact, it measures the variation in theaccelaration due to gravity It is expressed in so called gravity anomalies (inmilligal, 10-5 ms-2), i.e. deviations from apredefined reference level, geoid (a surface overwhich the gravitational field has equal value) Gravity is a scalar

Gravity Newton’s Universal Lawof Gravitation for smallmasses at the earthsurface: Spherical Non-rotating HomogeneousG M mG MF mg g 22RR––––G 6.67x10-11 m3kg-1s-2R is the Earth’s radiusM is the mass of the Earthm is the mass of a smallmassg is constant!

Gravity Non-sphericalEllipse of rotation RotatingCentrifugal forces es in the acceleration

NEllipse ofrotationEarth surfacecontinentEllipse of rotationGeoidoceanGeoid main sea-levelSphereGeoidgav 9.81 m/s2gmax 9.83 m/s2 (pole)gmin 9.78 m/s2 (equator)Anomaly

NGU, 1992

Magnetics Magnetic surveying aims to investigate the subsurfacegeology by measuring the strength or intensity of the Earth’smagnetic field. Lateral variation in magnetic susceptibility and remanencegive 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-2In 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

Magnetics Magneticsusceptibility, ka dimensionlessproperty which inessence is ameasure of howsusceptible amaterial is tobecomingmagnetized 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

Magnetics Magnetic Force, H Intensity of inducedmagnetization, Ji Ji k · H Induced andremanentmagnetizationH Magnetic anomaly regional - residualJiJresJr

NGU, 1992

ElectromagneticsElectromagnetic methodsuse the response of theground to the propagationof incident alternatingelectromagnetic waves,made up of twoorthogonal vectorcomponents, an electricalintensity (E) and amagnetizing force (H) in aplane perpendicular tothe direction of travel

ElectromagneticsPrimary fieldTransmitterReceiverPrimary fieldSecondary fieldConductorElectromagnetic anomaly Primary Field – Secondary Field

Electromagnetics – Sea Bed LoggingSBL is a marine electromagnetic method that has the ability to map thesubsurface resistivity remotely from the seafloor.The basis of SBL is the use of a mobile horizontal electric dipole (HED) sourcetransmitting a low frequency electromagnetic signal and an array of seafloorelectric field receivers.A hydrocarbon filled reservoir will typically have high resistivity compared withshale and a water filled reservoirs.SBL therefore has the unique potential of distinguishing between ahydrocarbon filled and a water filled reservoir

Reflection SeismologyMarine multichannel seismic reflection data

Reflection Seismology

Reflection Seismology

Reflection SeismologyIncident rayAmplitude: A0Reflected rayAmplitude: A1Layer 1ρ1, v1Layer 2ρ2, v2ρ2, v2 ρ1, v1Acoustic Impedance: Z ρ·vReflection Coefficient: R A1/A0ρ 2 v2 ρ1v1 Z 2 Z1R ρ 2 v2 ρ1v1 Z 2 Z1Transmission Coefficient: T A2/A02 ρ1v1T ρ 2 v2 ρ1v1Transmitted rayAmplitude: A2-1 R 1R 0All incident energy transmitted (Z1 Z2)no reflectionR -1 or 1All incident energy reflectedstrong reflectionR 0Phase change (180 ) in reflected wave

Reflection Seismology Shotpoint interval 60seconds 25-120 receivers Sampling rate 4milliseconds Normal seismic lineca. 8 sTWT

Reflection Seismology

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

Extensional Sedimentary Basins Offshore Norway – VikingGraben, Central GrabenLate Jurassic – EarlyCretaceousMature HydrocarbonProvince

Basin AnalysisPRE-RIFTSYN-RIFTPOST-RIFT

Syn-RiftRotated Fault BlocksIncreasing Fault Displacement

Seismic Signature of ExtensionalSedimentary Basins

INTRODUCTION TO EXERCISE

Seismic Signature of ExtensionalSedimentary Basins – Offshore Norway

Stratigraphy – Offshore Norway

Summary Offshore Norway Main Rifting Event: Late-Jurassic – EarlyCretaceous Structural Traps – Fault bounded Main Reservoir: Upper Triassic – MiddleJurassic, containing Tarbert, Ness,Rannoch, Cook, Statfjord and Lunde Fms. Source Rock: Upper Jurassic, Heather Fm Cap Rock: Early Cretaceous

Exercise Interprete seismic line NVGTI92-105 Interprete pre-, syn- and post-riftsequences Interprete possible hydrocarbon traps Point out source-, reservoir, and cap-rock

geomagnetic field Magnetic “Operative” physical property Method Measured parameter. 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 .

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