See discussions, stats, and author profiles for this publication at: https://www.researchgate.

net/publication/228459666

A New Method to Help Identify Unconventional Targets for Exploration and

Development Through Integrative Analysis of Clastic Rock Property Fields

Article · January 2004

CITATIONS READS

6 238

1 author:

Frank Walles
Baker Hughes Incorporated
4 PUBLICATIONS   23 CITATIONS   

SEE PROFILE

Some of the authors of this publication are also working on these related projects:

Multi-Space representation of Data View project

All content following this page was uploaded by Frank Walles on 19 November 2014.

The user has requested enhancement of the downloaded file.

 A New Method to Help Identify Unconventional
Targets for Exploration and Development Through
Integrative Analysis of Clastic Rock Property Fields
by Frank Walles P.G. #1980
Advanced Interpretation Consultant

model and data visualization framework for clastic rock (point count) and geochemical data (total organic carbon) are
A properties, Clastics Graphic Synthesis Model (CGSM), was
developed to cost-effectively identify
also integrated into the CGSM model.

fractured/fracturable, unconventional Cuttings are often available and The lack of full utilization of X-ray data
targets previously missed within active for petroleum exploration and develop-
and inactive wellbores, fields and plays.
now, through the application of ment has been due in part to the
the Clastics Graphics Synthesis absence of a proper visualization frame-
CGSM provides the framework to work that integrates interrelated rock
empirically interpret under-utilized, Model, can be readily used to properties data. CGSM is the initial step
relatively inexpensive X-ray diffraction of a process to define fractured, tight
data (matrix and cements) from well help identify potential fractured sand and shale producibility models for
cuttings, sidewall cores and cores. individual wells, fields and plays.
Porosity data, thin section information completion zones. A New Method continued on page 36

Figure 1.

October 2004 Houston Geological Society Bulletin 35

A New Method to Help Identify Unconventional Targets
A New Method to Help Identify Unconventional Targets continued from page 35 ___________________________

Petroleum geoscientists are increasingly faced with identifying provide this framework. Another factor is that the project geosci-
unconventional/overlooked targets within active and inactive entist often overlooks XRD data because it is often requested by
fields and plays, sometimes with complex data sets, or with another project team member, such as the reservoir engineer,
limited data sets. These targets are often fractured carbonates, petrophysicist or petrographer, for observational determinations—
fracturable, tight sands, and fractured shales. such as reservoir fluid compatibility or capillary entry pressure
inferences.
Identifying open fracture systems or fracturable zones within
reservoirs can be difficult and expensive, and many new tech- The CGSM approach
nologies ranging from borehole imaging to 4-D seismic are now The CGSM provides a technique to assist in the identification of
utilized. brittle zones occurring within the reservoir. The purpose of this
rock properties model is to help identify zones with the highest
The conventional approach for direct/indirect detection of fractures potential for fractured reservoir development. The focus is on
utilizes wellbore wireline tools including video, image logs (FMI, physical rock properties and their susceptibility to brittle rock
FMS), whole cores, sidewall cores, full wave sonic and tempera- deformation.
ture logs. Each of these tools has limitations.
The CGSM (Figure 1) graphically illustrates the multi-dimen-
X-ray diffraction (XRD) data from well cuttings, sidewall cores and sional fields for fractured reservoir potential through the rock
cores is an often overlooked and under-utilized for several reasons. property inter-relationships with derived axes of percent and
A primary factor is the lack of a rock properties field framework type of cementation, rock composition (through ternary-based
to synthesize and analyze this detailed data set. This article will QFL diagrams), and by percent A New Method continued on page 41

Figure 2.

36 Houston Geological Society Bulletin October 2004

 A New Method to Help Identify Unconventional Targets
A New Method to Help Identify Unconventional Targets continued from page 36 ___________________________

fine matrix material. Capillary entry pressure and rock mechani- the derived data can be readily used to help identify potential
cal data are also directly inferred from this model. completion zones.

The advantage of the CGSM is integration of XRD data (actual Most unconventional, fractured plays are not simple petroleum
physical rock properties) with porosity and permeability (P&P) systems. In these types of plays, industry often implements pilot
data and thin section point count data. XRD data and thin- programs that are utilized to gather data as well as to experiment
section point count data can be readily obtained from cuttings, with the most effective completion programs.
cores or sidewall cores. Because well cuttings are often available, A New Method continued on page 43

Figure 3a.

Figure 3b.

October 2004 Houston Geological Society Bulletin 41

 A New Method to Help Identify Unconventional Targets
A New Method to Help Identify Unconventional Targets continued from page 41 ___________________________

A number of controlling factors (ellipses of focus) make a well, prioritize the understanding of those drivers or combination of
field or play economically viable. This approach will build upon drivers. Understanding the rock property heterogeneity and how
the initial focus of this article—the investigation of the inferred it is controlled is also an element of the producibilty model.
rock properties fields that can be derived from the traditional
cost- effective data sets such as thin sections, XRD (matrix and Within combination fractured shale gas and fractured/
cement), porosity and basic geochemical data. fracturable tight sand systems the identification of brittle zones
as well as non-brittle zones is important. Non-brittle zones often
The Producibility Model Perspective form the seals that retain the economically recoverable gas satu-
As a first-order understanding for the basis and origin of rations occurring in the brittle zones. Seal zones are also critical
unconventional targets, the field and play data sets need to be for managing the fracture stimulation programs whereby vertical
integrated and synthesized to determine the primary driving fracture growth is inhibited and horizontal fracture growth is
factors that define a field or play’s hydrocarbon producibility. developed within the more brittle reservoirs.
A useful approach involves the building of producibility mod-
els for shale gas and tight gas sands. A producibility model Seal zones (typically more ductile shales) within shale and tight
defines the ellipses of critical drivers within a well, field or sand targets may have a simple key XRD derived factor such as
hydrocarbon play. calcite percent being greater than 5%. Reviewing the XRD data
carefully and calibrating to log information is an important part
An illustrated producibility model (Figure 2) can be useful of understanding particular drivers in wells, fields and play
because primary drivers are visually highlighted and therefore trends. The CGSM is designed to visually bring out these compo-
prioritized within the petroleum systems analysis. Developing a sitional variations from the XRD data with inferences to
competitive edge within a field or play requires recognition of the potential seal or reservoir rock.
underlying driver and therefore requires an increased effort to A New Method continued on page 45

Figure 4.

October 2004 Houston Geological Society Bulletin 43

 A New Method to Help Identify Unconventional Targets
A New Method to Help Identify Unconventional Targets continued from page 43 ___________________________

Another critical factor within shale gas systems is the variability long-lived, basic siliciclastic classification system. To the QFL
of adsorbed gas. Adsorbed—or bound gas (vs. free gas)—is often ternary diagram Dott added a percent fine matrix axis. The
a function of total organic carbon within the shales. The CGSM builds upon this original classification by adding another
methane or longer chain hydrocarbons preferentially adsorb axis: the cementation axis
(through weak Van der Waal forces) to the surfaces of available
carbon atoms within the system. Adsorbed gas content within Many sandstone classification systems have since been proposed (50
shales often varies from 10 to 100 standard cubic feet/ton since 1955). However, the Dott/Raymond series appears to be the
(scf/ton) depending upon percent TOC. most basic and compelling for siliciclastic sediments. Lindsey (1999)
published an evaluation of many such classifications and included
Increased percent TOC typically influences the brittleness of the an analysis utilizing variation scattergrams for classifications.
shale section inversely. Therefore within the producibilty model
it is located on the opposite side of the ellipse associated with Another reason the QFL ternary diagram has been utilized by
brittleness. The percent TOC is included in the CGSM as part of geoscientists is that it can be used to interpret provenance of the
the percent fine matrix axes. clastic sediments. Understanding provenance helps predict
subsurface diagenesis and maturity level of the clastic sedimentary
Building the CGSM rock sample. The ternary diagrams (Figures 3a and 3b) illustrate
The initial data focus for building the framework of the CGSM two concepts—the first, the provenance inferences, and the
includes developing knowledge of where a particular sample fits second, the subsurface diagenesis models illustrating the effects
within the standard quartz, feldspar. lithics or labiles (QFL) associated with subsurface diagenetic fluids (carboxylic- and
ternary diagram. The QFL ternary diagram and rock classifica- carbonic acid enriched fluids).
tion framework were initially developed by Dott (1964) and
further refined by Raymond (1995) and others. The basic Dott The position of the data within the CGSM is referenced within
framework for sedimentary rocks is still valid and serves as a the ternary diagram with respect A New Method continued on page 46
Figure 5.

October 2004 Houston Geological Society Bulletin 45

A New Method to Help Identify Unconventional Targets A New Method to Help Identify Unconventional Targets continued from page 45 ___________________________

to the total percent of the rock sample of each of the QFL com- Figure 3b illustrates the most common diagenetic pathways for
ponents. The data utilized should be consistently used between specific fields of rock suites with subsurface diagenesis. The
wells or field areas. A good approach is to utilize thin-section prevailing mechanism for this alteration is the introduction of
point count data if possible. If that is not available, the XRD data acids, both direct and indirect, from kerogen catagenesis and
can be utilized to determine related cement volume among QFL metagenesis.
percentages. For quartz, silica cement would be added. For
feldspar, feldspar-related cements (i.e. illite and smectite, kaolin- The acidic character of subsurface diagenetic fluids are most
ite) would be included. Most other cements (i.e. pyrite, calcite, often influenced by inorganic and organic acids created from
dolomite, ankerite, and siderite) would be included within the kerogen maturation (Surdam et al., 1984). Each kerogen type
lithics proportion. produces a specific suite of carboxylic and carbonic acids for each
maturation level. Each of these acids degrades specific rock com-
Silica cement is most susceptible to brittle failure without rapid ponents. (Surdam. et al., 1993). Therefore, timing of occurrence
re-precipitation and re-cementation. Enrichment in primary and of these acids within the subsurface system affects the timing of
secondary quartz or silica cement is often associated with the rock brittleness characteristics. The full producibility model
optimized fractured shales and tight sandstones reservoirs. should take into account these critical timing elements

In the subsurface, multiple processes can affect the precipitation Degree of cementation within a clastic rock is a critical compo-
of secondary quartz cement. Styolite surfaces are often good indi- nent of the rock properties associated with its strength. The
cators of significant alteration and re-precipitation of these CGSM utilizes this cementation component as a separate axis.
quartz cement fabrics within normal-pressured environments. The cementation axis is defined by the percent reduction of the
Within hydrocarbon-generated geopressured environments the original pore fabric by cements. Figure 4 illustrates this axis and
inhibition of significant grain-to-grain contact and resulting lack the empirical formula utilized.
of dissolution often reduce the volume of silica cementation.
This will be reflected within the CGSM and will indicate a posi- Thin-section analysis can also provide information about cemen-
tion within a less ideal brittleness field. tation history and sequence timing for the development of
brittleness rock property characteristics. Cementation history

Figure 6.

46 Houston Geological Society Bulletin October 2004

 A New Method to Help Identify Unconventional Targets
within fracture zones is especially useful. Fluid inclusion analyses, field and the combination of axes, creating the framework of the
as well as isotope evaluations, are additional data sets that can be CGSM.
incorporated into the producibility model.
Illustrating the visual position of data points within the CGSM is
The percent fine matrix axis of the CGSM (Figure 5) is utilized to best handled by use of “tadpoles” (Figure 7). Geoscientists have
differentiate the mud matrix (percent fines) and the grain size successfully utilized the tadpole concept in dipmeter logs.
matrix. When a clastic sedimentary rock varies from a pure grain However, within the CGSM, the tail of each of the tadpoles is
(0% fine matrix) composition to a mixed composition (varied placed at the tie point to the plane created from the cementation
percent fine matrix) to a pure fine matrix rock (pure shale – axis and the percent fine matrix axis. The “head” of the tadpole
100% fines), the rock changes from an isotropic material (by lies in the position referenced within the associated QFL ternary
grains) to an anisotropic material (mixed grains) and then to an diagram that is placed at the end tail location. Figure 7 illustrates
isotropic material (all fine grains), respectively. This does affect the position of a series of tadpoles within the CGSM model and
the strength of the material. The apparent change in rock how their position are illustrated with respect to the Best
strength is characterized by varying values of Young’s modulus E. Properties Field for Fractured Reservoirs.

The need to characterize changes in rock strength as a function Summary

of anistropy is the basis for the importance of percent fine matrix The GCSM is a novel visualization tool that utilizes data previ-
axis of the CGSM. ously not effectively utilized in identification of targets within
fractured shales and fracturable tight sands. The purpose of this
When the three basic elements of the CGSM are integrated (the rock property model is to help visually identify zones with the
ternary QFL diagram, the cementation axis and the percent fine highest potential for fractured reservoir development.
maxtrix axis), the expected brittleness field can plotted with
respect to each of these axes. The “Best Properties Field for The model graphically illustrates the multi-dimensional fields
Fractured Reservoirs” defines the rock property suite that has the for fractured reservoir potential through the rock property inter-
greatest inferred rock strength and resulting potential for main- relationships with derived axes of percent and type of
taining brittle behavior in the subsurface. Figure 6 illustrates this cementation, by rock composition A New Method continued on page 49

Figure 7.

October 2004 Houston Geological Society Bulletin 47

A New Method to Help Identify Unconventional Targets

A New Method to Help Identify Unconventional Targets

continued from page 47 ___________________________

(through ternary based QFL diagrams) and by percent fine Bibliography

matrix material. This model utilizes QFL ternary diagrams in Dott, R.H. Jr., 1964, Wacke, greywacke, and matrix—what approach to
multi-dimensional space so that primary inter-related data can immature sandstone classification? Journal of Sedimentary Petrology, v. 34, p.
first be mapped and then layered into additional planes. 625-632.

The advantage of the CGSM is integration of XRD data and thin Lindsey, D.A., 1999, An evaluation of alternative chemical classifications of
section data with P&P data from cuttings, cores or sidewall cores. sandstones, USGS Open File Report, 99-346.
Cuttings are often available and now through the application of
CGSM, can be readily used to help identify potential fractured Raymond, 1995, Petrology: The Study of Igneous, Sedimentary, Metamorphic
completion zones. Alternatively, rock mechanical data usually Rocks, Wm. C. Brown Communications Inc., Chicago, 742 p.
require whole core or sidewall core material and are often much
more expensive to obtain. ■ Surdam R.C., Boese, S.W. and Crossey, L.J., 1984, The chemistry of secondary
porosity, in McDonald, D.A. and Surdam, R.C. eds., Clastic Diagenesis, Amer.
Biographical Sketch Assoc. Petrol. Geol. Memoir 37, p. 127–150.
FRANK WALLES is a geological consultant specializing in advanced
interpretation techniques encompassing petroleum systems Surdam R.C, Jiao, Z.S. and MacGowan, D.B. 1993, Redox reactions involving
evaluations, producibility model development, and identification hydrocarbons and mineral oxidants, a mechanism for significant porosity
of missed completions. International and domestic corporate enhancement in sandstones, American Association of Petroleum Geologists
experience has included Tenneco Oil Company, British Gas E&P, Bulletin, v 77, p. 1509–1518.
Union Pacific Resources, Anadarko Petroleum, Kerr McGee,
as well as joint venture teams with Shell and Mobil. He can be
contacted at fjwalles@earthlink.net.

October 2004 Houston Geological Society Bulletin 49

View publication stats