Reading the hieroglyphics of maturation

One-Run Kinetics Give a ‘Quick History’

Under the right circumstances, single-run kinetics can be a home-run method of understanding source-rock maturity.

The technique was developed by AAPG member Doug Waples, consultant in Evergreen, Colo.

Kinetics analysis gives the exploration geologist a view of the maturation history of source rocks in a basin. It’s done by pyrolysis, a technique that involves heating rock samples to determine their maturity and the type of kerogen they contain.

If several pyrolysis runs are carried out on a single source-rock sample at different heating rates, kinetic parameters can be determined for the sample.

But this traditional method of determining kerogen kinetics is slow and expensive, and thus only small amounts of kinetic data are normally acquired.

Waples devised a method of conducting kinetic analysis using one Rock-Eval pyrolysis run per sample – a lower-cost method that can either enable one to measure kinetics even on a limited budget, or on a normal budget to acquire much more data.

At the recent AAPG International Conference and Exhibition in Calgary, Waples presented a paper in which he described an extension of one-run kinetics concept to extract kinetic parameters from existing pyrolysis data without performing new laboratory analyses.

In addition to providing even greater cost savings, the new approach has obvious logistic advantages, since it doesn’t require access to rock samples.

Call it a super-efficient way to develop kinetics information.

Waples said he became interested in geochemistry after studying chemical kinetics at Stanford University. He was working at Chevron’s laboratory in La Habra, Calif., when he began applying kinetics to geochemical processes.

The key to one-run kinetics, according to Waples, is to use a fixed A frequency factor. Not only does that simplify the analysis, it also eliminates values that are “thermodynamically impossible,” he said.

“By defining the activation energy, you accomplish two important things. You can do the kinetic analysis in one run and you don’t have an A value that can hurt you,” Waples said.

“One danger of traditional kinetics is that the A values are chosen mathematically, with no regard for the laws of chemistry and physics, and are thus sometimes quite wrong,” he added.

Applied to an area of the Mexican Gulf of Mexico, one-run kinetics allowed Waples and his Pemex colleagues Alfredo Vera and Jorge Pacheco to challenge the prevailing geological interpretation.

“Everybody had concluded there was only one organofacies but two lithofacies with an inner ramp and an outer ramp. What we discovered was there actually were two organofacies,” he said.

With a set of existing pyrolysis runs from the Bakken Shale in the Williston Basin of North Dakota, Waples had the opportunity to extend one-run kinetics to archived data.

“A database fell into our hands and we were wondering what to do with it,” he said. “The caution is, not every database is as carefully preserved as this one was.”

Working the Bakken

That dataset of several thousand pyrolysis analyse was given to Platte River Associates in early 2000 by the U.S. Geological Survey. Platte River’s intention was to utilize these data in both software testing and better Bakken understanding for modeling.

AAPG member Robert Coskey, president of Rose Exploration Inc. in Denver, described the Bakken kinetics work during AAPG’s Calgary meeting, in a talk titled “Spatial and Temporal Maturity Variations of the Bakken Shale Using True Kinetic Parameters,” prepared by AAPG members Jay Leonard and Mohamed Said along with himself and Waples.

Coskey, in describing the exploration potential of the Bakken kinetics, said the existing data set included about 200 pyrogram records, out of which about 180 were used in the kinetics analysis.

“Platt River Associates was just very fortunate to salvage that data set from work that Leigh Price at the USGS was doing at the time,” he said.

Using Waples’ one-run kinetics analysis, the authors were able to identify and characterize the more thermally mature and less mature areas of the shale play.

“You could see how the activation energies increased as you moved from the less mature parts of the basin to the more mature part,” Coskey noted.

“Observations of this rate through both space and time provides valuable insights to identification of future Bakken sweet spots,” Leonard commented. “We believed that the kinetics information would highlight the subtle differences of generation.”

Rock-Eval pyrolysis is used to determine several properties of source rock, including:

  • S1, the amount of free hydrocarbons in a rock sample.
  • S2, the amount of hydrocarbons generated through thermal cracking of kerogen (nonvolatile organics) in the sample.
  • S3, the amount of carbon dioxide produced during pyrolysis of the kerogen.
  • Tmax, the temperature of maximum S2 production, or the temperature at which the maximum release of hydrocarbons from cracking of kerogen occurs.
  • The Hydrogen Index or HI, a derived measurement of the hydrogen richness of the rock, used for distinguishing kerogen type.

With Waples’ method, “you could under the right circumstances get a double bang for your buck. You get the geochemical data, the S1-S2-S3 data, and by running the program you get a look at the kinetics,” Coskey said.

In this case, Coskey described the most mature Bakken Shale as having a HI of less than 100, Tmax of greater than 445 and total organic carbon (TOC) of less than 8 to 10 percent.

Some findings were surprising. In thermal evaluation, the Tmax range of the mature oil zone is usually greater than 435 degrees to 450 degrees Celsius.

“What we found out – and the ground truth seems to prove this out – is that even at a Tmax of 430 you’re generating substantial amounts of oil” in the Bakken, Coskey said.

Geologists generally see high TOC as a favorable sign in shale plays. But lower TOCs can be expected in the mature areas of a play, as found in the Bakken example.

“Some of the TOCs I’ve seen are well over 30 percent by weight, which is 50 percent by volume. By the time you get to the mature part of the basin, it’s 8 to 10 percent,” Coskey said.

Since kinetics involves not only the effects of temperature but also time, Coskey was able to develop a better understanding of the basin’s burial history.

“It appears that the cooking pot is wider than we originally thought,” Coskey said.

“We never had any real Bakken kinetics before,” he added. “To me, the new kinetics Doug came up with show that the Bakken matured earlier than we thought, and more rapidly.”

Leonard points out that the PRA geologic burial history modeling shows that the additional exposure to heat during rapid burial, coupled with the thermal blanketing effect of the Pierre Shale, override the lower activation energy prediction of the new kinetic model. The net results were that the timing of oil generation did not change significantly between the new single run kinetics and the older Lawrence Livermore Type II kinetics.

However, with a different, slower rate of burial the effect could be quite different, Leonard said – and the generation time issue may result in a structure being charged, or not, depending on the timing of structures and oil generation.

A Valuable Tool

Waples developed the idea of one-run kinetics 10 years ago and introduced the concept in a paper published in 2001-02. The industry response was ...

Underwhelming.

“No one other than me has used archived data, to my knowledge. And few people have done one-run kinetics,” he said.

“It’s kind of leading edge,” Coskey acknowledged.

“What he’s doing differently is processing that S2 curve to come up with a mean activation energy for each of those samples,” he said.

As of now, Waples said, the only lab with a history of conducting his one-run kinetics evaluation is StratoChem Services in Cairo (the one in Egypt, not Illinois).

And there are some caveats involved in the approach.

One-run kinetics makes the biggest contribution “if you’ve got something where there’s a significant variation in maturity across your area,” according to Waples.

When using existing data, the records have to be well-preserved with accurate pyrolysis temperature readings and pyrolysis yields as a function of time.

And kinetics gives the geologist a basis for forming a concept of maturity, not an exact duplication of a basin’s thermal history.

“We do these Rock-Evals fast, in 15-20 minutes. These are only an approximation of what goes on in the Earth – but we don’t have a couple of million years to run the tests,” Coskey said.

Still, Waples sees one-run kinetics as highly valuable for maturity indication, organofacies analysis and large databases of kinetics and even avoiding an adjustment problem in the standard attrition-model approach to kinetics. The application of one-run kinetics to the Bakken even revealed a possible weakness in the analysis.

“It’s like you were given a library of hieroglyphics and suddenly you can read them,” Waples said.

“It’s a tool, and you’re silly if you’ve got a problem and you don’t consider all your tools,” he added.

For Leonard et al., the ability to use kinetics information in the Bakken Shale was more than worthwhile.

“As for exploration geologists,” Coskey said, “it gives you a far better understanding of the thermal and generative history of the basin.”

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