Using Interpreted Digital Well-Logs and Sequence...

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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.


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


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.


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


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.


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.


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.


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.


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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