The Tools of Subsurface Analysis

The Tools of The Tools of Subsurface AnalysisSubsurface Analysis

Resistivity LogsResistivity Logs Spontaneous Potential (SP) LogsSpontaneous Potential (SP) Logs Gamma Ray LogsGamma Ray Logs Neutron LogsNeutron Logs Density LogsDensity Logs Sonic (acoustic) LogsSonic (acoustic) Logs

Delimit of surfaces & identify sediments Delimit of surfaces & identify sediments penetratedpenetrated

Resistivity LogsResistivity LogsSpontaneous Potential (SP) LogsSpontaneous Potential (SP) LogsGamma Ray LogsGamma Ray LogsNeutron LogsNeutron LogsDensity LogsDensity LogsSonic (acoustic) LogsSonic (acoustic) Logs








GEOL 553 Lecture 3; Subsurface Analysis

Well logsWell logs Cores Cores SeismicSeismic Gravity & Gravity &

magneticsmagnetics

Facies analysis of subsurface Facies analysis of subsurface data depends on tools which data depends on tools which delimit of surfaces and provide delimit of surfaces and provide clues as to the sediments they clues as to the sediments they contain:contain:

Well logs Well logs Great vertical resolution Great vertical resolution Delimit bounding surfacesDelimit bounding surfacesEstablish lithology of sediments Establish lithology of sediments

penetratedpenetrated SeismicSeismic

Great lateral continuity and Great lateral continuity and resolutionresolution

Define gross sediment geometryDefine gross sediment geometry

Allostratigraphy: bounding discontinuities including erosion

surfaces, marine flooding surfaces, tuffs, tempestite, and/or turbidite boundaries etc. as time markers

Sequence Stratigraphy: higher level allostratigraphic model which interprets depositional origin of sedimentary strata as products of "relative sea level change"

Well logsWell logs SeismicSeismic




Measures resistance of flow of Measures resistance of flow of electric currentelectric current

Is function of porosity & pore fluid Is function of porosity & pore fluid in rockin rock

Frequently used to identify Frequently used to identify lithologylithology

The most commonly used The most commonly used logs:logs:

Measures electrical current in wellMeasures electrical current in well Result of salinity differences Result of salinity differences

between formation water and the between formation water and the borehole mudborehole mud

Separates bed boundaries of Separates bed boundaries of permeable sands & impermeable permeable sands & impermeable shales.shales.

Next most common Next most common loglog

Records radioactivity of a formationRecords radioactivity of a formation Shales have high gamma radioactive Shales have high gamma radioactive

responseresponse Gamma ray logs infer grain size (and so Gamma ray logs infer grain size (and so

subsequently inferred depositional energy)subsequently inferred depositional energy) Gamma ray logs are most commonly used Gamma ray logs are most commonly used

logs for sequence stratigraphic analysislogs for sequence stratigraphic analysis

Another common logAnother common log



GEOL 553 Lecture 3; Subsurface AnalysisAfter Harris & Saller 1999

Measures porosity of formationMeasures porosity of formation Uses quantity of hydrogen presentUses quantity of hydrogen present Measures lithology when used with Measures lithology when used with

Density LogDensity Log

Another common Another common loglog

Measures formation’s bulk densityMeasures formation’s bulk density Used as a porosity measureUsed as a porosity measure Differentiates lithologies with Neutron Differentiates lithologies with Neutron

LogLog Used with Sonic Logs to generate Used with Sonic Logs to generate

synthetic seismic traces to match to synthetic seismic traces to match to seismic linesseismic lines

A common logA common log

Measures of speed of sound in Measures of speed of sound in formationformation

Tied to porosity and lithologyTied to porosity and lithology Used with Density Logs to generate Used with Density Logs to generate

Synthetic Seismic traces to match to Synthetic Seismic traces to match to Seismic linesSeismic lines

Another common logAnother common log

Well logsWell logs SeismicSeismic


Define geometries of genetic Define geometries of genetic reflection packages that envelope reflection packages that envelope seismic sequences and systems seismic sequences and systems tractstracts

Identify bounding discontinuities Identify bounding discontinuities on basis of reflection termination on basis of reflection termination patterns and continuitypatterns and continuity

Seismic stratigraphic Seismic stratigraphic interpretation used to:interpretation used to:

Toplap termination Toplap termination Truncation of sediment surface Truncation of sediment surface Often channel bottomOften channel bottom

Termination below discontinuity, Termination below discontinuity, or upper sequence boundaryor upper sequence boundary ::

Onlap over surfaceOnlap over surface Downlap surfaceDownlap surface

Above a discontinuity defining Above a discontinuity defining lower sequence boundary:lower sequence boundary:

Below Boundary - Toplap terminationBelow Boundary - Toplap termination

Below Boundary -Below Boundary - Truncation of Truncation of surfacesurface

ChanneledChanneledSurface Surface – – Below Below BoundaryBoundary

Over Boundary - Onlap onto surfaceOver Boundary - Onlap onto surface

Over Boundary- Downlap onto Over Boundary- Downlap onto surfacesurface




Surfaces of erosion & non-deposition (sequence boundaries)

Flooding (trangressive surfaces [TS] &/or maximum flooding surfaces [mfs]) & high stand

condensed surfaces

Subdivision & interpretation of sedimentary Subdivision & interpretation of sedimentary record using a framework surfaces seen in record using a framework surfaces seen in outcrops, outcrops, well logs, & 2-D and 3-D seismicwell logs, & 2-D and 3-D seismic. . Include:Include:

This framework used to predict the This framework used to predict the extent of sedimentary facies extent of sedimentary facies geometry, lithologic character, grain geometry, lithologic character, grain size, sorting & reservoir qualitysize, sorting & reservoir quality

Relative time framework for sedimentary succession

Better understanding of inter-relationship of depositional settings & their lateral correlation

These surfaces subdivide These surfaces subdivide sedimentary rocksedimentary rock &provide:-provide:-

Conceptual models follow that link the processes that formed the sediments and enable the prediction of their gross geometries

Sequence geometries are subdivided and defined byMaximum Flooding Surfaces (mfs) Transgressive Surfaces (TS) Sequence Boundaries (SB)

Define how vertical succession or stacking patterns of unconfined sheets are arranged Prograde (step seaward) Retrograde (step landward) Aggrade (build vertically)

Sheets and unconfined lobes may containNon-amalgamated bodies

Amalgamated, multi-storied bodies Incised topographic fill of valleysUnconfined but localized lobes from point & multiple up dip sourcesUnconfined but localized build ups (carbonates)

Note Incised SurfaceNote Incised Surface







Note Uniform Thickness of LayerNote Uniform Thickness of Layer


Chann

el

Chann

el ShelfShelf

Both have unique Both have unique processes & structures that processes & structures that

can be used to identify can be used to identify their settingtheir setting

Subdivision of section into sequences, parasequences and beds.

Link conceptual models with mix of components of the individual sequence, parasequence or beds

Use these to explain the depositional setting in terms of their lithology, grain size, sedimentary structures, contacts character (gradational, abrupt) etc

This analysis involvesThis analysis involves


Unique ProcessesFlow is in all directions No lateral boundaries, only upper and lower boundaries Velocity changes: high to low

Sediment responsesDecrease in grain size: Fining outward (coarse to fine)Erosional/sharp/gradational contacts Accretion: Downstream, upstream and vertical Decrease in sedimentary structures away from source

GeometriesSheets Thin in direction of flow

Can it be supper time?

The Tools of Subsurface Analysis

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