Oil and gas companies look to find the “sweet spot” in their current unconventional fields, aided by hundreds, if not thousands of papers written over decades on the definition and characterization of such hydrocarbon-rich zones. The term generally refers to that portion of a field where the optimum combination of various parameters come together to maximize either production, profitability, or both. Yet the term “sweet spot” has been used in the oil and gas lexicon well before the unconventional revolution and was not originally coined for oil and gas usage.
People in the oil and gas industry often speak of, and always seek to find, the “sweet spot” of a given play, field or reservoir. Ask anyone in the oil patch, “What is a sweet spot?”, and you will often get an answer which is long on colorful recollections and short on hard details. Ask a follow-up question about the history of sweet spots and you will probably receive only a baffled look.
When was the concept of sweet spots introduced in oil and gas?
Where was it introduced?
When was the first published reference to sweet spots?
How does the definition of sweet spot vary between geoscientists and engineers?
How has the definition of sweet spots changed over the years?
Sometimes we will find multiple answers, some conflicting each other. While this brief history of sweet spots won’t help you find your next one, we hope it will add some perspective to your search.
History of the Term
The Oxford Dictionary has “sweet spot” as a noun with two meanings:
- Informal: The point or area on a bat, club, or racket at which it makes most effective contact with the ball.
“A bigger sweet spot forgives off-center hits.”
- Formal: An optimum point or combination of factors or qualities.
“The market may have reached its sweet spot, with prices high enough to encourage sellers but still low enough to promise a good return.”
One of the earliest known published usages of “sweet spot” was in John Atkins,’ “A Voyage to Guinea, Brasil, and the West-Indies,” on page 206, referring to Barbados, published in 1737. In Scientific American, Oct. 12, 1872, page 224, there is mention of a topographic sweet spot 40 miles from Denver, on the Palmer Divide. Another early usage is found in Robert Louis Stevenson’s 1883 classic ,“Treasure Island,” in which the pirate Long John Silver tells the narrator, “Ah … this here is a sweet spot, this island – a sweet spot for a lad to get ashore on. You’ll bathe, and you’ll climb trees, and you’ll hunt goats … Why, it makes me young again.”
The informal usage shows up in many sports writings, from early tennis and baseball accounts, and in this headline about golf: “Trigonometry Finds ‘Sweet Spot’ for the Golf Club to Meet the Ball” in The New York Times of March 16, 1957. It may have come into newspaper usage much earlier when in 1874 Major Walter C. Wingfield registered his patent for a wooden strung lawn tennis racquet that revolutionized the English lawn tennis game.
One of the earliest published usages for the oil industry apparently came in the work of a pulp fiction novel, “Radio City,” by Hartzel Spence, published in 1941. In a New York Times review of the book, the protagonist is described as an advertising man who had a client company named “Sweetspot Oil.” It may be that oil men spoke of a field of sweet spots where wells were not dry holes! Perhaps this novel foresaw the world of unconventionals? Kudos to the late William Safire of the Times for finding this obscure reference.
History of “Sweet Spots” in Technical Usage: the 1970s and ‘80s
Mature members of our industry often have careers that span a half century. Our admittedly biased, totally unscientific poll of a few of those older members indicates the term “sweet spot” was in general usage in the industry early in their careers. Consequently, as a lower bound, the origin of sweet spots must date back more than half a century. As the global oil and gas industry has existed for almost two centuries, we have an upper bound on the introduction of sweet spots to our industry. But exactly when and where was the concept of a sweet spot introduced?
The first published reference to “sweet spots” in the AAPG Datapages is Hunter Yarbrough’s 1977 Special Publication Course Notes “Continental Margin Types Related to Plate Tectonics and Evolution of Margins” in “Geology of Continental Margins” where he refers to the “’cold” sweet spot – a Miocene “salt lid” on top of a “Nubian kettle.”
A search of the petroleum engineering literature in English (we have not searched in other languages but suggest that this might be useful) gives the first reference to sweet spots in a 1981 paper by Kuuskraa and co-authors. The paper discussed the impact changes in technology and economics might have on developing the tight gas resource in the United States. Sweet spots were defined as those relatively small areas with permeabilities higher than the 0.1 millidarcy permeability used to define tight gas reservoirs. No methodology for finding these permeability-defined sweet spots was presented.
The third occurrence of a paper dealing with sweet spots is the 1984 work by Schrider and co-authors, which focused on exploration and development of the Trenton tight gas reservoir in northwest Ohio. A three-year evaluation program, which included both geological studies and active well drilling and operations, concluded that further exploitation would not be commercial under the then-current economic conditions, except in sweet spot areas. Although no explicit definition of a “sweet spot” or how to locate one was presented, this paper intuitively presented three concepts repeatedly seen in the sweet spot literature: First is the concept that sweet spots are identified by geological evaluation, second are stimulation treatments to enhance production from a sweet spot, and third are economic returns which meet company hurdles.
The fourth paper to appear in the sweet spot literature was a 1986 paper by Newman on basin evaluation. Two important concepts appeared in this paper which are still relevant to sweet spots today. The first is the process of finding sweet spots, however they might be defined, by mapping the key sweet spot elements such as thermal maturity, porosity, permeability and net pay, then overlaying the maps to locate any sweet spots. Second is the difference in sweet spots between rank exploration plays and developed plays. Exploration sweet spots often cover large areas with approximate bounds and rely heavily on intuition. In contrast, sweet spots in developed plays are typically smaller in size and better defined (faults, oil-water contact and other features). Data already acquired in the play are combined with analogs to locate sweet spots more precisely. Similar to the Schrider article, Newman noted the economic component to defining sweet spots, using finding costs as an exploration sweet spot metric.
Since those early papers, the term has come into common use, and abuse, to mean many things to many different people, just as there is no one accepted meaning for the term “unconventional.” One of the more comprehensive definitions comes from the current (2020) Schlumberger Oilfield Glossary, which defines a sweet spot as a “colloquial expression for a target location or area within a play or a reservoir that represents the best production or potential production. Geoscientists and engineers attempt to map sweet spots (to) enable wellbores to be placed in the most productive areas of the reservoir. Sweet spots in shale reservoirs may be defined by source-rock richness or thickness, by natural fractures, or by other factors, using geological data such as core analysis, well log data, or seismic data.”
Sweet spots are like most good golf or tennis swings: once you get that perfect connection one time, you know it when you achieve it.
As seen in figure 1, the 1980s saw five sweet spot papers published, bringing the cumulative to a half dozen papers. In addition to the one paper from the ‘70s and the three papers above, one of the ‘80s sweet spot papers, by Walbe in 1989, discussed using a personal computer to map lineament densities in naturally fractured reservoirs. Another, by Tucker in 1989, dealt with modelling a group of Triassic gas fields connected through a common aquifer in the Bowen Basin of Queensland, Australia. These two papers are the beginning of a trend still evident in today’s sweet spots literature. Both papers define sweet spots in narrow, rigorous, technical perspectives. One dealt with identification of sweet spots by lineament density, regardless of other reservoir characteristics or reservoir fluids. One dealt with gas fields connected by an aquifer, neglecting differences in gas composition and reservoir quality. Neither paper considered economic influences on sweet spots.
The 1990s and the First Decade of the 21st Century
The annual sweet spot paper count rose steadily through the 1990s from a few per year to more than a dozen per year at the turn of the century. The cumulative sweet spot paper count reached 79 in 2000. Papers in this decade discussed sweet spots in developed reservoirs and fields more than in exploration plays. They covered well drilling and completions rather than production optimization, and gas over oil and condensates.
Breaking this decade into two five-year intervals, the first interval from 2001 to 2005 saw 76 sweet spot papers appear in the literature, while in the second five years, from 2006 to 2010, the number of sweet spot papers nearly tripled, to 203 papers. This decade saw a total of 279 papers defining sweet spots, addressing various aspects of them and discussing how to find them. Cumulative production of all sweet spot papers through 2010 was 358, thus this decade saw three-fourths of all sweet spot papers published to that time. Not only did the number of papers dramatically increase in this decade, but the range of topics also expanded enormously. From detailed engineering papers, such as one that refers to optimizing coalbed methane well treatments presented by M.M. Reynolds and J.C. Shaw at the 2005 Canadian International Petroleum Conference, to play-level studies by Strecker and co-authors at the 2004 Society of Exploration Geophysicists conference, to economics by Stabell and co-authors in paper 108081 published by the Society of Petroleum Engineers, workers throughout the oil and gas industry pursued uniquely defined sweet spots.
Topics became increasingly specialized, likely to the point that the sweet spot definition utilized by a given discipline was probably unintelligible to those outside the community.
Recent Sweet Spot Papers: 2011-20
Again, breaking the decade into two five-year periods, the first period, from 2011 through 2015, saw a total of 651 sweet spot papers published – double the count of the previous five years and almost twice the total number of sweet spot papers previously published. As the shale revolution swept through the oil and gas industry during these years, it should be no surprise that the majority of the papers focused on exploration and geoscience topics such as tools and techniques to identify sweet spots in unconventional reservoirs. In contrast, one of the more provocative papers by Haskett for SPE in 2014 discussed the “Myth of Sweet Spot Exploration.” Noting that the time and resources required to identify sweet spots can be enormous, often yielding unreliable results, the paper suggests a better exploration strategy is to evaluate the entire opportunity. Rather than seeking an elusive sweet spot, the paper embodied the old oilfield advice that we should try “to play the hand we’ve been dealt.”
Stoneburner’s key 2013 paper, “The Exploration, Appraisal, and Development of Unconventional Reservoirs: A New Approach to Petroleum Geology,” earned an AAPG Distinguished Lecture tour and the Norman H. Foster Outstanding Explorer Award.
In the past five years, a total of 765 papers discussing sweet spots have appeared in the literature. With the fading of the shale frenzy and the collapse of oil prices, it’s not surprising that these recent papers concentrate on drilling, completions and production of unconventional wells with relatively few studies focusing on the geoscience aspects of sweet spots.
Economic Influences on Sweet Spots
Economics plays an important and often unrecognized role in sweet spots. Sweet spots expand and contract in response as prices and costs increase and decrease. For instance, “sweet spots” defined under the assumptions of $80 per barrel oil and $4 per million British thermal units gas have shriveled in today’s prices of $40 oil and $3 gas.
Our search revealed no papers specifically focused on the economic aspects of sweet spots. Geoscience and engineering factors dominated sweet spot considerations, with economic factors treated as secondary controls at best. Quantifying the impact of decreasing prices and costs on sweet spots may likely be the topic of many future sweet spot papers, catalyzed by the present low oil and gas prices.
One such example is a 2018 paper by the authors of this article, which presented two key ideas. The first is that defining the subsurface factors as the intersection of three clear quality indicators – organic quality, rock quality and mechanical quality – and that each is composed of several key characteristics. The second key idea is that the sweet spot is also controlled by a variety of surface factors – fit for purpose operations, economics, and environmental, social and governance considerations – which change over time. Thus, the areas of “sweet spots” are dynamic. This alignment of the surface factors with the reservoir factors can enlarge or decrease the “sweet spot” through time.
An example of this dynamic nature of the sweet spot is shown in figure 3 with the outline of the main productive area of the Niobrara Formation in the DJ Basin of Colorado taken in two snapshots. The first is from 1970 when the only technology was vertical completions and during a period of relatively low oil price. The second is from 2018 at the height of the period of horizontal drilling with long laterals and relatively high oil prices. During that nearly 50-year period, the geologic understanding greatly increased, although there was no substantial change in the subsurface conditions. Changes in technology, operations, regulations and the fiscal environment allowed expansion of the “sweet spot.” Future restrictions based on environmental regulations may in turn lead to smaller area in the future.
Another example of the dynamic nature of sweet spots can demonstrated by an examination of two studies of the Middle Bakken Formation in the U.S. portion of the Williston Basin. In the first study, we evaluated all horizontal wells drilled in the Middle Bakken Formation in 2015 and calculated the cumulative oil recovery, normalized for lateral length. We chose 2015 as an index year, as it was far enough past the first commercial horizontal oil production that operators had sufficient data to understand key productivity trends. The year also saw an average drop in oil prices from around $96 per barrel to about $50 per barrel, which would concentrate drilling in the highest-grade acreage. The second study came from a presentation from E. MacDonald presented at the 2020 AAPG Annual Convention and Exhibition, which evaluated every Middle Bakken well drilled for both reservoir factors (rock, organic and mechanical) and for completion style (proppants, lateral length, and amount of fluid, among other factors) to calculate a three-tiered sweet spot (see figure 5).
While both of these studies captured a similar eastern area of the Middle Bakken, the first study also identified an additional area on the Montana\North Dakota border.
Neither of these studies looked at the economics of the wells or the plays, and neither considered flaring considerations or other environmental, social and governance factors. One of the important take-aways is that the metric or yardstick that is chosen for determining the sweet spot will have the determining influence on the location of the sweet spot. Therefore, it is very important to understand what you are including and excluding from the measurement.
Summary and Conclusions
It appears the term “sweet spot” first occurred in the petroleum literature almost 40 years ago. More than 1,700 papers have been published on the topic of sweet spots since 1977, an average of just under one paper per week.
There are some key learnings: sub-surface productivity factors can vary by basin, shale or field but can be subdivided into organic quality factors, rock quality factors and mechanical quality factors. Due to above-ground factors of commodity price, drilling and completion efficiency, regulation and societal challenges, “sweet spots” are not static but change over time. Additionally, the choice of how one measures or quantifies the sweet spot will determine its area.
The term “sweet spot” has different meanings to different people. The sweet spot of a geophysicist is probably not the same as the sweet spot of an operations engineer or the sweet spot of a banker. We suspect that all the different oil field disciplines would agree that we probably can’t improve on the definition of “sweet spot” given by an old oilfield hand who noted that a sweet spot is like true love: “You’ll know it when you find it.”
Acknowledgements:
The authors thank Ray Sorensen for finding several of the earliest usages of the term “sweet spot” and to Shawna Harrison and Jessica Davey for their support in research and drafting.