22 Basin Anal Techniques '14
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Transcript of 22 Basin Anal Techniques '14
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the tools and techniques
GEOL 202: Earth Surface Processes and Deposits
Practical Basin Analysis
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Mostly - adding the subsurface dimension
drilling
remote sensing
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three-dimensional visualization and analysis
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Applicability: Energy resources: oil, gas, coal, nuke,
geothermal,
Minerals
Groundwater
Seismic hazards
Engineering applications
Environmental studies
?
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measured section (review) what it will show
lithology (symbols)
grain size of
individual beds
grain-size trends
in beds
among beds
interpretation
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what about subsurface data? mudlog
core
geophysics
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lithologic log - mud-logging
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data aquired as the drill-bit goes down
cuttings
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subsurface measured sections from wells well logs and well
data
sample cuttings
and core
drop electrical
probes down the hole
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how does this work?
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Reverse-circulation drilling
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subsurface measured sections from wells remember cuttings
are returned from the top, down!
data will be
gathered in reverse order of stratigraphy
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how does this work?
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and core samples lots of information
and very expensive
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down-hole electric logs pull the drill-string and lower a variety of
probes to determine whats down there
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basic electric log response SP - spontaneous
potential
voltage between
the surface and the depth
resistivity - between two probes in the hole
gamma radiation
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combined well logs
the lithology log here is symbolic (to make it all digital)
based on well ...
cuttings brought
up in the circulating mud
core
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Kansas Geological Survey
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geophysical logs
left - gamma ray log
SGR = total
CGR = w/o uranium
right - resistivity log
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Kansas Geological Survey
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example of the end product, delivered by contractors
this is all the exploration geologist sees, most of the time
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logging other aspects
composition
cuttings
geophysical
properties
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% composition plots
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down-hole imaging Schlumbergers latest flashy
technology - virtual core
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what other kinds of data can we gather?
from outcrop
from geophysical attributes
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High-resolution stratigraphy - very detailed logs of outcrop or well data
usually only a few (or one) components at a time
Systematic sampling strategy - e.g. a regular interval like every 10cm, or ?
advantages
disadvantages
practicality?
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high-resolution stratigraphy (cont.)
Choosing the data suite
geochemical
climate, biological activity, alteration
geophysical
best for cryptic sections?
petrological
component sources
other component
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chemo-stratigraphy
an example:
carbon isotope
ratios in limestone record changes in sea-level
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next slide
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close-up detail - carbon isotope fluctuations reflect sea-level change
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another example - gamma-ray
log of outcrop
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sedimentology-based data: Paleocurrent analysis
detailed information about environmental systems based on paleocurrent indicators
key issues
identifying type of indicator
data distribution
statistically significant numbers
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how to display data? rose diagram
a circular
histogram of directions
length of petal shows number of measurements
direction of petal is orientation
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paleoflow scale hierarchy: example from one riverEach hypothetical rose averages the box area. Which one is outcrop scale?
smallest scale: crevasse in river bar face
largest scale: delta system
largest area
smallest area
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Interpret data spread Meandering river with
laterally migrating bars
Average is not far o, but
Range is wide, some are
upstream
Importance of enough data!
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Stratigraphic analysis with paleocurrents
Fluvial section, shows different trends for:
in-channel sand
laterally aggrading bars
bar tops
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K Gallup Ss.: offshore to backshore prograding beach system
Note: Oscillatory indicators (NE-SW) in shoreface (shallow, fair weather) and in offshore (deep, storms)
Longshore drift indicators (NW-SE) in upper shoreface
Harms et al., 75
Stratigraphic analysis with paleocurrents
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Regional analysis - paleocurrents
Devonian conglomerate clastic section with stratigraphically AND regionally consistent paleoflow to the WSW
(Nilsen et al. 1981)
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Three-dimensional correlation Placing stratigraphic sections on a map
defining axes
x and y axes are often north and east
z axis is almost always thickness
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Fence diagrams 3-dimensional correlation of sections
on a map base
tie together two or more cross-section
false perspective view
(unless map is tipped, too!)
allows analysis of spatial distribution
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example - parking lot substrate, an engineering study
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Tina Marja Hariu
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with many fence posts... better 3D
distribution
problems:
perspective
hidden panels
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Maps that interpret stratigraphy - isopleth maps
Structure contour maps
Isopach maps
Isochore maps
Isolith maps
Facies maps
Paleogeographic maps
isopleth maps defined - any map that represents data at geospatially located points in a continuum that can be contoured
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Structure contour maps a contour map on a surface, usually a
lithosome (formation) boundary
datum is sea-level
negative elevations possible and common
data are surface elevation
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visualize topography on the
top of a key bed
slopes = bedding
dip
hills & valleys =
anticlines and synclines
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example: structure contours on a doubly plunging syncline Salem, KY
note:
closed
contours
parallel
fold limbs
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more complex subsurface shapes
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anticline
syncline
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Isopach maps contour maps of stratigraphic
thickness
USGS
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isopach maps, the rules: datum is base, or (usually) top of unit
(why?)
data are measured *true thickness
*corrected for dip
special features
no negative values (i.e. no thickness less
than zero)
zero line is pinch-out
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stratigraphic vs. vertical thickness problems if dips are steep
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thickness for isopach maps
cross section
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using isopach maps to define an impact structure in California Jared Morrow and Bennett Spevak (2007)
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isopach maps on crater fill
seismic section across crater 3D reconstruction
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regional isopach maps Pennsylvanian
tectonic basins, thickness indicates
depth
accommodation
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interpreting this map?
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Ron Blakeys paleogeography
same scales
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Isochore maps drilled vertical thickness
thickness for isochore maps
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tephra thickness isopach Rio Blanco tephra (3500ybp), Costa Rica
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structure and isopachs note thickening over structure high
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Isolith (lithofacies) maps Percentage maps
contours of equal percentage of some value
e.g. % mud, % plagioclase feldspar
Ratio maps
contours of ratio values
commonly thickness ratios
favorite version - clastic ratio map
ratio of siliciclastic to carbonate rocks within
a given section
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lithofacies map example
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facies are determined from a ternary plot (Young et al., 1976)
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example of a clastic ratio map the ratio is
contoured
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Facies distribution maps Interpretive facies maps
Paleogeographic (time-
slice) maps
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geologic subcrop maps pick a unit or an age, and remove
everything above/younger
e.g. a Triassic subcrop map includes all
Triassic and older rocks.
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example: Mississippian in Kansas
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Kansas state map of major structural elements
Mississippian subcrop map
Schaben oilfield
Kansas Geol. Survey
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zoom in to the Schaben oilfield
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cross section based on well logs
Miss. subcrop map
shows a west-dipping angular unconformity
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Newer geophysical analyses seismic exploration
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layers are acoustic events and thickness is really time.
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Newer geophysical analyses 3D seismic
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combine 3D seismic with side-scan sonar
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