# Top 10 Landmark Papers in Petrophysics and Formation Evaluation

##### Stephen Prensky (Chair)

###### Sub-committee:

Richard Bateman, Bob Cluff, John Doveton, Darwin Ellis, Mauro Gonfalini, Terry Hagiwara, David Kennedy, Pat Lasswell, Brian Moss, Don Oliver, Philippe Theys, E.C.Thomas, Paul Worthington

###### Approach:

Different members of the committee volunteered to do the write-ups for the nominated papers. The committee member who developed the write-up for a particular paper is listed below the title of the paper.

## Landmark Papers

##### Archie, G. E., 1942, The electrical resistivity log as an aid in determining some reservoir characteristics: Paper SPE-942054-G, *Transactions AIME, *v. 146, p. 54–62.

###### E.C. Thomas

Gustave Archie had university training in Geology and Mining Engineering. He also had practical experience working in his father's quarry. Thus, he had the unique knowledge to both appreciate the complexity of rocks and the mathematical acumen to devise laboratory measurements on the electrical properties of rocks and to interpret the experimental results into simple equations useful to both geologists and petroleum engineers. He gave examples of actual borehole electric logs and how to use his new equations to obtain water saturation and connate water resistivity. He also published graphs showing all the experimental data points.

One measure is the citation history of this publication. The number of citations is too large to state with accuracy. It is usually the first citation listed in the references of any paper concerning field resistivity log interpretation and laboratory-derived measurements. Even today, the equations provide quicklook techniques that are good enough for preliminary evaluation of many rock types. Subsequent equations that have found widespread use when Archie's equations are insufficient, are in fact Archie's equation with a correction factor.

This paper began a revolution in resistivity well-log interpretation that led us out of the dark forest of guesswork or hunches about the value of a given bed's worth in a wellbore into the open sunshine of the possibility of a quantitative evaluation of the fractional hydrocarbon saturation surrounding the wellbore. When this datum is combined with geologically derived formation parameters, we can estimate a value for hydrocarbons in place.

This paper has experienced acceptance and use worldwide.

Until this paper was published the general belief was that reservoir rocks were too complex to ever be understood well enough to use a mathematical equation to predict hydrocarbon volume in place. Today it is the norm.

We rate the paper's readability as 10 being extremely well written and easy to follow.

##### Archie, G. E., 1950, Introduction to the petrophysics of reservoir rocks: AAPG Bulletin, v. 34, p. 943–961.

###### Richard Bateman

In this seminal paper, Archie sets out not only to give a name ("petrophysics") to the then nascent discipline of the study of the physics of rocks and fluids but also meticulously sets down on paper the intimate relationships between the key factors of pore-size distribution and fluid distribution of each phase (oil, gas and water) within the pores of a rock system. This opened the door to changing an art into a science.

The fact that today we routinely refer to the study of rocks and fluids as "petrophysics" and to those that conduct the studies as "petrophysicists" is witness to the lasting impact that this publication had on subsequent generations of earth scientists.

Prior to this publication by Archie, the interrelationships between the key factors had been "Balkanized" through unconnected studies and publications from the likes of Purcell, Leonardon, Wyllie, Fearon and Doll. For the first time, Archie set out clearly the waft and weave of the complex fabric that is petrophysics. He logically traced the interconnections between rock type, porosity, permeability, fluid saturations, water salinity, hydrogen content, capillary pressure, natural radioactivity, response to neutron bombardment, spontaneous potential, and electrical resistivity. The paper includes two timeless figures (12 and 13) that stunningly encapsulate an entire study guide for a modern petrophysics course.

Archie included in the paper detailed experimental data obtained from a wide variety of hydrocarbon producing fields. His rock samples came from both sandstones and carbonates. The trends that he richly illustrated with copious plots underline the universality of his petrophysical relationships that endure up to the present.

The analysis of cores and well logs prior to Archie's publication was a somewhat ineffective process due to the fragmented approach, which he so aptly described by "In actual practice, further complications arise due to practical difficulties, economic considerations and the personal equation." The publication of his "petrophysical road map" clarified the workflow needed for a logical and semiquantitative approach to formation evaluation. In the day when the tools available were somewhat primitive compared to those available today his paper laid out a practical method for a log analyst to integrate many different threads to form a working formation evaluation fabric. That was new and innovative.

The paper is well written, easy to read and follow and deserves a 10/10.

##### Clavier, C., G. Coates, and J. Dumanoir, 1984, Theoretical and experimental bases for the dual-water model for the interpretation of shaly sands: Paper SPE-6859, *SPE Journal*, v. 24, p. 153–168.

###### Phillipe Theys

The interpretation of shaly sand formations has been recognized early as a challenge for petrophysicists. Many saturation equations introduce a shale component and a shale resistivity. In 1968, after extensive laboratory work, Waxman and Smits proposed a saturation-resistivity relationship involving the cation exchange capacity of the shale portion. Unfortunately, a direct measurement of CEC is rarely available. The dual-water model was developed by Clavier et al. as a practical solution that is based on three premises:

- The conductivity of clay is due to its CEC.
- The CEC of pure clays is proportional to the specific surface area of the clay.
- In saline solutions, the anions are excluded from a layer of water around the surface of the grain.

The thickness of this layer expands as the salinity of the solution (below a certain limit) decreases, and the thickness is a function of salinity and temperature.

In the dual-water model, clay is modeled as two components: bound water and clay minerals. The clay minerals do not contribute to conductivity and the clay electrical conductivity only originates from the clay-bound water. The amount of bound water varies according to the clay type, being higher in montmorillonite, and lower in kaolinite. All parameters (clean formations and shaly zones) can be directly identified on the logs and a quicklook interpretation can be completed in a short time at the wellsite.

30 years after its conception, the dual-water model is still one of the favorite options in computer-processed interpretation.

Before the dual-water model, the interpretation of shaly sands was considered an art, not a science.

The dual-water model is extensively used in the interpretation of all shaly sands reservoirs. The dual-water model has been implemented in Schlumberger's Cyberlook computer processing and is still an option in modern interpretation software.

The paper definitely introduced some real physics to solve the challenge of interpreting shaly formations. Other methods were mostly empirical, and the Waxman-Smits method was missing a key item, namely "how to derive the CEC of the formation."

The paper is well written, considering that it covers fundamental physics, complicated petrophysical concepts and laboratory experiments. It does not offer any difficulty to a person with some background in physics.

##### Klein, J. D., P. R. Martin, and D. F. Allen, 1997, Petrophysics of electrically anisotropic reservoirs: The Log Analyst, v. 38, p. 25–36.

###### David Kennedy

Resistivity anisotropy in earth formations had been recognized from the days of surface electrical prospecting, and had been mentioned as an effect on electrical logs from the earliest logging literature. However, early models in formation evaluation treated reservoir rocks as massive and isotropic; according to this model anisotropy was a property recognized in, and confined to, reservoir seals. Analysis of anisotropy was confined to explanations of anomalous log responses in these, and other, shale beds. No petrophysical analysis of the effects of anisotropy in a reservoir was attempted. By the last quarter of the 20th century, laminated shale-sand, low-resistivity reservoirs had become recognized as significant sources of hydrocarbon storage and production. Drilling technology from offshore platforms and Arctic drilling pads necessitated an increase in the number of deviated wells penetrating these formations. Resistivity log responses in deviated wells are different from the responses of the same instruments in the same formations in vertical wells. This paper was the first analysis that tackled the problem of analyzing log responses in laminated shale-sand formations in terms of the resistivities and thicknesses of the individual laminations comprising the shale-sand packages. Making the assumption that Archie's model for isotropic sands would apply in the sand layers individually, and with the volume fraction and conductivity of the conductive shale laminations known from log analysis, then the water saturation in the sand fraction can be estimated. The Klein et al. model is formulated in terms of macroporous sandstones interbedded with microporous mudstones, the water saturations in the components determined by the capillary pressure—water-saturation functions of the respective layers. Completing the petrophysical model, Klein et al. offer estimates of the anisotropic permeability components in terms of the anisotropic resistivity components. The authors anticipated the introduction of triaxial induction resistivity instruments by four years. With the introduction of these new instruments, the tensor components of resistivity could be estimated directly from the instrument responses. The interpretation methods developed to use the data provided by the new instruments, are all built upon the foundational principles set out in this paper.

A search of OnePetro on the phrase "resistivity anisotropy" yielded a total of 382 hits; the same search restricted to the years 1996–2015 yields 353 papers. Thus, before the initial publication of Klein, Martin, and Allen in 1996, which appeared as a conference paper in the SPWLA Transactions, only 29 papers on the topic had appeared. Since that date, 353 papers, or an average of 17 papers per year. Without having looked at each of the 353 papers to check for citations of Klein et al. the prima facie (and I believe correct) conclusion is that their paper opened the floodgate to a deluge of research concerning resistivity anisotropy and related topics.

With the publication of this paper, the formation evaluation community realized that the incorporation of transversely isotropic resistivity and permeability anisotropy in routine interpretation models was mathematically quite tractable. Prior literature had treated resistivity anisotropy in terms of electromagnetic instrument responses, rather than petrophysics, and appeared to be quite complicated. With the appearance of this paper, journeymen petrophysicists and log analysts could embrace anisotropy into their interpretation workflows.

Klein et al. anticipated the introduction of triaxial induction resistivity instruments and the generalization of their interpretation method (to which Klein continued to contribute) to explicitly include the deviation of wellbore axes with respect to bed boundaries. Their interpretation method (and its extensions) is part of every commercial log analysis program, and is used each time a triaxial induction instrument is logged.

In 1997, times were ripe for a new understanding of formation resistivity incorporating anisotropy. If the paper did not initiate a paradigm shift in resistivity interpretation, it can certainly be said to have been the first rolling stone of what turned out to be an avalanche of understanding that followed in the next decade.

This paper, like all truly significant scientific papers, is written in a tutorial style. Setting out new ideas for the first time, the reader is taken step-by-step from the standard methods in use from the time of Archie, to the new method, which accounts for anisotropy.

##### Luffel, D. L., and F. K. Guidry, 1992, New core analysis methods for measuring reservoir rock properties of Devonian shale: Paper SPE-20571, Journal of Petroleum Technology, v. 44, p. 1184–1190.

###### Patrick Lasswell

Don Luffel and his team published three papers in 1992 and 1993, concentrating on physical property determinations in gas shales. This work became the Gas Research Institute method for shale property determinations, abbreviated as the GRI method. The first paper in this effort, SPE-20571, established the groundwork outlining the main challenges and concepts facing shale property investigations. Primarily, it asked the question, can legitimate laboratory-based porosities be determined in shales? The question was answered by offering an analytical alternative; crushing the material to yield greater surface area, facilitating access to the pore structure.

Over the last 20 years, almost without exception, crushed property determination (the basis of the GRI process) is put forward as the method best suited for porosity and fluid saturation determinations in shales and other tight unconventional reservoirs. In a search of publications using OnePetro, the method was referenced a total of 2,179 times.

In shales, the crushed-properties (GRI) method provided a means whereby defensible porosity and fluid saturations were obtainable. Without this new process, the basic volumetrics would not have been properly assessed or realistically understood. However, the crushed method did not show that the plug-based method was entirely suspect or ill-suited for use in shales. Rather, the experimental work showed that property determinations in shales were variable from a play or interval perspective.

While the Luffel investigations dealt with North American Devonian shales, the process has been successfully applied to worldwide use. In addition, the method, with relatively minor adjustments, has been adapted for use in more complicated shale/tight unconventional oil plays.

The analytical method put forward by Luffel's team, became the new industry standard for shales and other tight unconventional plays. This was not by accident. True, the solution proved to be insightful and novel, but more importantly, it was robustly tested and verified. The team started with the plug-based method and built a direct bridge to their new crushed-properties method. Experimentation was done sequentially on a large set of intact shale plugs obtained from different shale plays. They provided the understanding that method-based property response was related directly to the specific shale play. The two methods might provide comparable results in one play, yet in another play, the porosities might be significantly different. In all cases, if a porosity difference was observed, the crushed porosities were the better numbers.

The paper is well written and the concepts are clearly outlined. In addition, the paper addresses the assumptions and potential weaknesses in the methodology in a transparent and professional manner. After 20+ years, the paper, the method and the process have stood the test of time.

##### Pickett, G. R., 1973, Pattern recognition as a means of formation evaluation: The Log Analyst, v. 14, p. 3–11.

###### Dan Krygowski

This paper provided a detailed explanation, with examples, of a method introduced in an earlier paper (Pickett, 1966). This graphical ("pattern recognition") technique minimizes the need to calculate water saturation (*Sw*) via Archie's equation (in a time before calculators). It also provides the means to quickly determine *Sw* without the knowledge of porosity matrix values, or values for Archie equation parameters porosity (cementation) exponent (*m*), and formation water resistivity (*Rw*). The method, under appropriate conditions, can predict those parameters from the data directly.

The method has survived the introduction of computer software, the output of which was initially text only, became one of the plots available with early alphanumeric printer-based graphics, and is implemented at this time in many technical software packages as an interactive application. In addition, extensions to the method (e.g., Greengold, 1986; Aguilera, 2004; and Krygowski and Cluff, 2012, 2015) have enhanced its use.

It helped moved petrophysical technology from algorithm-based interpretive techniques, which required knowledge of calculation parameters, to graphical techniques that provide both qualitative answers and quantitative results from parameters predicted from the method.

As a graphical solution to Archie's saturation equation, it has global application to all "Archie reservoirs," and may be of help in the identification and interpretation of "non-Archie reservoirs" as well.

Prior to this method, determination of water saturation was largely numerical or algorithmic, with required knowledge of calculation parameters. Graphical methods (e.g., Tixier et al., 1959; Hingle, 1959) while in some use, required specialized graphical displays which were not a versatile as the full logarithmic plot used here, nor could those plots commonly predict all the parameters that could be predicted from this method.

With this graphical technique, determination of water saturation became much quicker, required knowledge of fewer parameters, and could overcome several types of calibration errors in porosity and resistivity measurements.

The paper provides clear explanations, with pictures of resistivity-porosity plots (now commonly referred to as "Pickett plots") and log plots, of the behavior of the method in determining water saturations, predicting parameter values, and how the method can be used to detect log measurement calibration errors. Some of the nomenclature and equations, expressed properly for the time of publication, have been modified or are used in different or combined forms. (9 to 10 of 10)

###### References Cited

Aguilera, R., 2004, Integration of geology, petrophysics, and reservoir engineering for characterization of carbonate reservoirs through Pickett Plots: AAPG Bulletin, v. 88, p. 433–446.

Greengold, G. E., 1986, The graphical representation of bulk volume water on the Pickett crossplot: The Log Analyst, v. 27, no.3, p. 21–25.

Hingle, A.T., 1959, The use of logs in exploration problems: paper presented at the SEG 29th Annual Technical Meeting, Los Angeles, California.

Krygowski, D.A., and Cluff, R. M., 2012, Pattern recognition in a digital age: a gameboard approach to determining petrophysical parameters: Paper E, *Transactions, SPWLA 56th Annual Logging Symposium, Cartagena, Colombia*.

Pickett, G. R., 1966, A review of current techniques for determination of water saturation from logs: Paper SPE-1446, Journal of Petroleum Technology, v. 18, p. 1425–1433.

Tixier, M. P., R. P. Alger, and C. A. Doh, 1959, Sonic logging: Paper SPE-1115-G, Transactions, AIME, v. 216, 1p. 06–114.

##### Thomas, E. C., and S. J. Stieber, 1975, The distribution of shale in sandstones and its effect upon porosity: Paper T, Transactions, SPWLA 16th Annual Logging Symposium, New Orleans, Louisiana, 4–7 June.

###### E.C. Thomas

Thomas and Stieber demonstrate the necessity to handle the effects of anisotropy on the determination of porosity. Since porosity enters into the computation of water saturation, permeability and hydrocarbons-in-place as well, it follows these computations are effected as well. The earth is manifestly not isotropic; anisotropic formations are the norm.

One measure is the enormous citation history of this publication. The equation is now programmed into every commercial digital well-log evaluation package and is a standard evaluation step for all shaly sands to first make a Thomas-Stieber correction to make a first-order correction for anisotropy, then apply the Waxman-Smits equation to take care of the shaliness effects of a now isotropic formation.

This paper demonstrates that one should always consider an anisotropic model first when interpreting shaly sands. Only after one can determine that the bed or formation is homogeneous can we take the step to use homogeneous saturation models.

This paper has experienced worldwide acceptance and application.

Until this paper was published, the usual evaluation methodology only used isotropic, infinite-medium approximations in formulating the transform needed to convert logging tool responses into the values presented on the log as well as the transform from log readings into reservoir properties.

We rate the paper's readability as 10 being extremely well written and easy to follow due in part to the use of simple, linear equations and graphical solutions.

##### Timur, A., 1968, An investigation of permeability, porosity, and residual water saturation relationships for sandstone reservoirs: The Log Analyst, v. 9, no.4, p. 8–17.

###### Richard Bateman

This paper reported on an exhaustive project undertaken by Turk Timur at the Chevron Research Center at La Habra, California, that established an empirical formula to predict formation permeability from porosity and irreducible water saturation.

One hundred fifty-five sandstone cores from three different fields covering a wide range of porosities, permeabilities and residual water saturations were painstakingly analyzed to measure *f*, *K* and *Swr* (in today's nomenclature we would write *Swir*). The result was an empirical fit between the measured parameters and permeability based on knowledge of porosity and irreducible water saturation. The iconic equation, still in use today is:

The paper concisely and logically reviewed the prior work by Leverett, Tixier, Wyllie and Rose, Purcell, and Koseny, all of whom had attempted to find acceptable permeability predictors using different measurable parameters from drill cuttings and the like. Timur, who was also working on early nuclear magnetic resonance (NMR) logging tools, understood the vital link between the free-fluid index (FFI) that would later to become a readily available quantity from NMR logging tools, and the much sought after permeability. Since the FFI is a direct link to the irreducible water saturation the Timur fit provided the industry with a predictor of permeability from logging measurements without the need for expensive core retrieval and lab analysis.

Almost without exception, modern computer-aided log analysis routines have a built-in option for the log analyst to generate a permeability curve from a pulldown menu that offers "the Timur permeability", a lasting tribute to this paper, the man and the work that generated it.

##### Waxman, M. H., and L. J. M. Smits, 1968, Electrical conductivities in oil-bearing shaly sands: Paper SPE-1863-A, SPE Journal, v. 8, p. 107–122.

###### E.C. Thomas

The most significant contribution made by this paper was to demonstrate that the mathematical analysis of water-bearing shaly sands in conductivity space results in a linear equation while the traditional analysis in resistivity space results in a nonlinear, power-law equation. Using conductivity, these authors present a simple, logical explanation for the observed behavior of shaly sands when exposed to an electrical field gradient, expressed as an imposed voltage drop. The explanation uses physical chemical principles supported by a large body of theoretical and experimental conductivity data. The paper introduces a new parameter, *Qv*, the cation exchange capacity per unit pore volume of a unit of rock, and explains how *BQv* is the correct parameter to explain and predict observed electrical behavior of shaly sands rather than *Vclay* or *Vshale*. An equation developed from data obtained at ambient temperature defines the parameter, *B*, the cationic equivalent conductance of the hydrated sodium ion in aqueous solutions of sodium chloride. Lastly, the authors propose a new equation using *BQv* to extend the water-bearing equation for shaly sands into one for hydrocarbon-bearing shaly sands.

One measure is the huge citation history of this publication. The equation is now programmed into every commercial digital well log evaluation package and is the standard evaluation for all shaly sands.

This paper spelled the death knell of all shaly sand equations using *Vshale* to correct for the extra conductivity of clay mineral hydrated sodium cations

This paper has experienced worldwide acceptance and application.

Until this paper was published the general belief was that *Vshale* was the parameter to use to make shaly sand resistivity corrections. Everyone now knows the correct parameter is *BQv*.

We rate the paper's readability as 9 being extremely well written but heavy on theoretical physical chemistry, thus some-what difficult to follow for one not having the opportunity to take a university physical chemistry class.

##### Wyllie, M. R. J., A. R. Gregory, and L. W. Gardner, 1956, Elastic wave velocities in heterogeneous and porous media: Geophysics, v. 21, p. 41–70.

###### John Doveton

This paper reported on results of laboratory measurements of acoustic velocity for a variety of rock samples with a focus on their relationship to porosity. Up to the time of this paper, the sonic velocity log had been recorded primarily as an aid to the interpretation of seismic surveys. While recognizing nonlinearities of the transit time (or velocity "slowness") at higher porosities due to a variety of factors, the authors concluded that a linear interpolation between matrix and fluid transit times was a good approximation in consolidated reservoir rocks. This relationship is expressed by the Wyllie time-average equation and provided the first log prediction of porosity other than inferences made from resistivity measurements.

The Wyllie time-average equation is today still the most widely used function to predict interparticle porosity from sonic velocity logs. Some improvements have been proposed by nonlinear models that consider the entire porosity range, of which, the most popular is the Raymer-Hunt-Gardner transform. However, the Wyllie equation should be considered as a robust first-order model for interparticle porosity estimation in reservoir formations that are not confounded unduly by significant issues of lack of consolidation, shaliness, or matrix mineral variability.

The introduction of a method to estimate pore volumes over extensive intervals that had limited core samples provided a new source of information to add to textural properties observed in drill cuttings. In carbonates, it was recognized that the time-average estimate was restricted primarily to interparticle pores, so that vuggy pores were not accounted for. However, when used in combination with density and/or neutron log measurements this limitation could be turned into an asset by allowing the subdivision of a dual-porosity system by pore type.

When first introduced, the sonic porosity transform was considered to be the principal method to estimate porosity from logs. However, density and/or neutron logs, particularly in combination, are now generally preferred in the evaluation of total porosity. Nevertheless, sonic velocity logs are still commonly recorded in connection with geophysical applications and the Wyllie time-average equation routinely used to predict interparticle porosity.

The paper demonstrated that a logging measurement that was introduced to aid geophysicists in their interpretation of seismic traces could be transformed easily into a viable measurement of porosity, and so with major geological implications.

This paper is well written and has a wealth of data and interpretation that deserves reading, particularly by those who misstate what Wyllie and his coauthors actually said. 10/10.