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Using Interpreted Digital Well-Logs and Sequence Stratigraphy to Develop High Resolution Subsurface ModelsMark C. RobinsonDirector of Geological and Geophysical ProductsDrilling Info, Inc.
IntroductionAll geoscientists operate with a
paradigm or working hypothesis on
the nature of the subsurface in their
project areas. Having a working
model is imperative for predicting
the nature and occurrence of oil and
gas reservoirs. The Drillinginfo Earth
Model™ Suite provides a pre-existing
template to which professionals can
readily apply their understanding of
where to drill or how best to exploit
oil and gas reserves.
Taking advantage of a patented
process of high-density well log
correlation and an experienced
team of sequence stratigraphers,
Drillinginfo is uniquely qualified to
create and manage this product and
has launched an ambitious program
to expand coverage to include
virtually every oil and gas basin in
North America. Currently, our focus
is in the unconventional plays such
as the Eagle Ford, Bakken, Niobrara,
Wolf-berry/Wolf-bone and Barnett
trends.
Stratigraphic MethodologyTo build its high resolution
subsurface models, Drillinginfo
utilizes digitized well log data. The
well logs are used to generate
high-density interpretations
according to allostratigraphic
methodology. Allostratigraphy
is a methodology defined by the
North American Stratigraphic Code
that uses bounding discontinuities
to subdivide the sedimentary
section into mappable units.
Bounding discontinuities include
unconformities, disconformities,
discontinuities, and omission
surfaces. The bounding surfaces may
relate back to sequence stratigraphic
boundaries such as Marine Flooding
Surfaces (Bhattacharya and Walker,
1991).
1 Using Interpreted Digital Well-Logs and Sequence Stratigraphy to Develop High Resolution Subsurface Models
Lithostratigraphy maps sedimentary
rocks solely on the basis of their
lithology and does not necessarily
consider that these rocks may have
accumulated over different periods of
time (diachronous lithostratigraphic
units). Lithostratigraphy often ignores
significant breaks in the sedimentary
section, including those caused by
unconformities, omission surfaces,
ravinement surfaces, and flooding
surfaces.
Allostratigraphy maps rock units
on the basis of the timing of their
accumulation. Allostratigraphy
(sequence stratigraphy) uses a
framework based on surfaces
of erosion and non-deposition
(sequence boundaries), and flooding
(transgressive surfaces and/or
maximum flooding surfaces [mfs])
that can be recognized in 2-D and
3-D seismic, well log data, and
outcrops.
The stratigraphic interpretation
that results from a conventional
lithostratigraphic approach versus
an allostratigraphic approach is
shown in Figure 1. Although the
subsurface rocks are the same
the allostratigraphic interpretation
more truly represents the genetic
significance, timing and relationships
between the various depositional
units.
Allostratigraphy and Lithostratigraphy
Using Interpreted Digital Well-Logs and Sequence Stratigraphy to Develop High Resolution Subsurface Models 2
Figure 1. Example of Lithostratigraphic interpretation (A) versus Allostratigraphic interpretation (B). From Prothero and Schwab, 1996 (taken from Dunbar and Rodger, 1957).
Identification of AllomembersIn developing an allostratigraphic
interpretation it is necessary to define
a set of allomembers to be used as
stratigraphic surfaces. The types
of data that can be useful in the
allomember definition process may
include:
Well-logs•
Core description•
Outcrop analysis•
Paleontology•
Sample log/cuttings•
Geochemistry•
Petrophysics•
Ideally allomembers should be:
Regionally correlative•
Have genetic significance•
Figure 2 illustrates how allomembers
are defined from regionally correlative
log responses and how they can
be tied to a sequence stratigraphic
framework. In this case, detailed
core analysis was used to provide the
necessary geologic facies information
required to confidently assign the
allomembers to the appropriate
system tracts.
3 Using Interpreted Digital Well-Logs and Sequence Stratigraphy to Develop High Resolution Subsurface Models
Figure 2. Well log curves used to define allomembers and their sequence stratigraphic significance. From Bhattacharya and Walker, 1991.
Traditional well log displays (as
shown in Figure 3), have been used
for the last 80 years by geologists
as a means to correlate subsurface
depositional units. This method is
still in use today by a large majority
of working geologists. Although
today it is more common to see
scanned well logs being manipulated
on a computer screen rather then
the shuffling of paper logs on a
drafting table, the process between
the two is essentially the same. The
High-Density well log display is a
relatively new and patented method
that facilitates the visualization of
stratigraphic relationships present in
the well logs.
High-Density Well Log Interpretation
Using Interpreted Digital Well-Logs and Sequence Stratigraphy to Develop High Resolution Subsurface Models 4
Figure 3. Traditional well log display of four wells. From Rigg, 2009.
The High-Density well log display
shown in Figure 4 is composed of
gamma-ray, spontaneous potential
and deep resistivity logs. Facies
shading has been applied to the
gamma-ray and spontaneous
potential curves to emphasize the
presence of sand. Many regionally
significant depositional patterns can
be easily discerned in this display.
It would be virtually impossible to
use paper or scanned well logs to
create the cross-section shown in
Figure #4 let alone try to visualize the
stratigraphic relationships.
Figure 4. High-Density Cross Section from Webb County, Texas. Datum is Lower Wilcox. Wells are equally spaced over a distance of 65 miles.
5 Using Interpreted Digital Well-Logs and Sequence Stratigraphy to Develop High Resolution Subsurface Models
Sequence StratigraphyBy carefully correlating key
allostratigraphic surfaces in a basin
and understanding their relationship
to identified sequence boundaries the
sequence stratigraphic model for a
basin can be developed. Major 1st
and 2nd order sequence boundaries
frequently correspond to major
flooding surfaces, unconformities
and named formation boundaries.
The 3rd and 4th order sequence
boundaries are mapped where
they can be identified in well logs.
The focus of the EarthModel is
to generate regionally significant
correlations over a large area that
can be integrated with seismic data
or used independently to understand
local geology in a regional context, as
well as to easily visualize productive
facies, their lateral extent, and
potentially identify analogs.
This methodology is the foundation
for Drillinginfo’s EarthModel™
Suite, one of many Knowledge
Base Products under development.
Drillinginfo’s EarthModel provides
a means to assimilate knowledge
concerning the subsurface and
present results in a useable format
to professionals working in the oil
and gas industry. Components
of the EarthModel Suite include
interpreted stratigraphic surfaces,
digital well logs and petrophysically
derived rock properties all integrated
with Drillinginfo’s completion and
production data, as well as well test
and show information. In addition,
the digitized well logs used to create
the EarthModel are available so
that workers can verify, infill, and
extend the existing knowledge
base as necessary for their unique
applications.
Visualizations of the subsurface
using the EarthModel can lead
to a better understanding of the
relationship between geology and
known production. All Knowledge
Base Products (KBP) exist as layers
accessible from Drillinginfo’s webapp
providing a flexible means to evaluate
how a particular acreage position
may relate to mapped geologic
trends. Among the KBPs that are or
will be available are structure maps
generated for each of the interpreted
flooding surfaces or sequence
boundaries, isopach maps of each
gross interval, petrophysical property
maps for each interval and a pre-
selected grid of high-density well log
cross-sections.
The EarthModel™ Suite and Drillinginfo Knowledge Base Products
Using Interpreted Digital Well-Logs and Sequence Stratigraphy to Develop High Resolution Subsurface Models 6
Figure 5. This is an example of how the EarthModel correlations can be seen to match interpreted seismic data. Original figure is from Li and Zang, 2008 as shown in Robinson, 2010.
7 Using Interpreted Digital Well-Logs and Sequence Stratigraphy to Develop High Resolution Subsurface Models
Structure maps are generated for
each correlated surface in a basin or
trend. Many areas will have 10 to 20
structure maps detailing the unique
characteristics associated with each
depositional layer. Because of the
correlation methods and data density
the presence of even relatively minor
faults can be frequently seen by their
effect on the structure maps.
Structure MapsBased upon the interpreted results
of thousands of well logs it is
possible to created highly accurate
and very detailed structure maps
of the subsurface. Because the
EarthModel™ correlations focus
upon flooding surfaces and sequence
boundaries the structure maps can tie
back to 3D and 2D seismic data and
be used to better control the time/
depth relationships.
Using Interpreted Digital Well-Logs and Sequence Stratigraphy to Develop High Resolution Subsurface Models 8
Gross Isopach MapsPerhaps the most remarkable and
valuable KBP derivable from the
EarthModel Suite are the gross
isopach maps of the intervals
between the interpreted surfaces.
Gross isopach maps can be extremely
valuable tools in defining the paleo-
structure in a basin. They have the
ability to identify a thicker region that
may indicate a zone of subsidence
allowing for greater accumulation
of sediments. This ability is largely
a function of the EarthModel™
correlations being sequence
boundaries and therefore more likely
to capture the significant depositional
packages.
Taking the vertical difference
between two adjacent surfaces in
each interpreted well creates the
gross interval isopach maps. Gross
isopach maps are being created for
each interval of the EarthModel™.
Additionally, isopach maps can
be created by combining multiple
intervals. These combined isopachs
are necessary in order to visualize
larger deposition systems.
Petrophysical Property MapsBecause the EarthModel is dynamic
and derived from the interpretation
of an ongoing and ever increasing
collection of digitized well-logs, it
becomes immediately possible to
calculate and map petrophysical
properties for each interpreted
interval.
Common well-log derived property
maps such as feet of gamma-ray
above or below a certain cut-off
provides insight into the distribution
of sand, limestone or organic shale in
a basin. The value of this information
is even greater when workers can flip
through a series of layers and observe
how the depositional thickness may
migrate over time.
More advanced petrophysical
properties or even geochemical
measurements can be associated
with an EarthModel interval. The
trend of these properties can then be
mapped and followed through each
succeeding depositional sequence.
9 Using Interpreted Digital Well-Logs and Sequence Stratigraphy to Develop High Resolution Subsurface Models
A powerful feature of the EarthModel
Suite is the ability to match
interval data with well completion
and production information. As
a result workers are no longer
dependent upon the operators to
provide accurate reservoir names
for completed intervals. All well
completions are matched to actual
EarthModel™ intervals that are
consistent throughout the entire Basin
or Trend.
With completions matched to a
consistent set of intervals it becomes
possible to relate the EarthModel-
derived property maps to known
production trends. This powerful
tool is extremely useful in the
generation of opportunity maps for
each potentially productive interval.
Figure 6 is an example of the types
of visualizations that can be created
using data and information from the
Drillinginfo EarthModel Suite.
Integration with Production and Completion Data
Figure 6. This is a perspective view of a gross isopach map of a Cherokee interval in the Anadarko Basin with interval production bubbles (red) and petrophysical calculated net sand (yellow) superimposed.
The EarthModel Suite is delivered
in various standard Geological/
Geophysical workstation formats,
such as Petrel, Geographix,
Kingdoms/SMT, and Petra.
Using Interpreted Digital Well-Logs and Sequence Stratigraphy to Develop High Resolution Subsurface Models 10
SummaryDrillinginfo has undertaken the
development of the EarthModel™
Suite, a comprehensive geological
model for each of the primary oil
and gas basins and trends. This
geological model is created through
the application of a patented process
that utilizes high-density digital well
log displays. The EarthModel Suite
generates a large and expanding
array of knowledge-based products
(KBP), principally composed of
structure maps, isopach maps,
various petrophysical property
maps and high-density well log
displays. Drillinginfo is integrating its
production and completion database
as a component of the EarthModel™
Suite to provide an invaluable tool for
the identification and validation of
opportunity trends.
About the AuthorMark C. Robinson joined Drillinginfo
in 2010 as the Director of
Geological/Geophysical Products. He
started his career as a ground water
geologist participating in studies in
California, Tennessee, and Puerto
Rico. In 1982 he joined UNOCAL,
where he developed techniques
for the application of computers to
support exploration and development.
Subsequently, Mark applied his
talents at OXY, Pennzoil, Landmark,
and Schlumberger and on five
different continents. For the last
ten years Mark has focused on the
creation and sequence stratigraphic
interpretation of high-density digital
well log data bases. He has made
numerous presentations at AAPG and
GCAGS and is an outspoken advocate
of this emerging methodology. He
received his B.S. in Geology from
UCLA in 1978 and his M.S. in
Geology from The University of Texas
of the Permian Basin in 1990. He
continues to pursue graduate studies
at the University of Houston as time
permits.
www.drillinginfo.com888.477.7667 Opt 1.
ReferencesBhattacharya, J. P., and R. G. Walker, 1991, Allostratigraphic subdivision of
the Upper Cretaceous Dunvegan, Shaftesbury, and Kaskapau formations in the
subsurface of northwestern Alberta: Canadian Bulletin of Petroleum Geology, v. 39.
Escalona, Alejandro, and Paul Mann, 2006, Sequence-stratigraphic analysis of
Eocene clastic foreland basin deposits in central Lake Maracaibo using high-
resolution well correlation and 3-D seismic data: AAPG Bulletin, v. 90, p. 581-623.
Li, Xiupeng, and Zeng, Hongliu, 2008, Seismic sedimentology and depositional
systems in the Upper Cretaceous Olmos Formation, Gold River North field, Webb
County, south Texas, in Gulf Coast Association of Geological Societies Transactions,
v. 58, p. 623–634.
Robinson, M. C., 2010, Delineating parasequence boundaries with high-density
digital well-log displays in the Olmos Formation, Webb County, Texas: Abstract, in
Gulf Coast Association of Geological Societies Meeting, San Antonio, Texas.
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