Alteration, charging history play role

All Crudes Are Not Created Equally

Scientific knowledge about the origins of oil in the Gulf of Mexico Basin and the resulting impact on oil quality has evolved over a long period of time.

The biggest changes probably took place 20 to 25 years ago, according to AAPG member Barry Katz, Chevron Fellow and team leader for hydrocarbon charge at Chevron.

He noted that’s when a step-change occurred.

“Before that, people thought every sand had its own source,” he said. “It’s a very different working model that we have today; it has been a gradual evolution.”

And it’s one that continues today – something Katz will talk about during a presentation at the upcoming GCAGS meeting in Veracruz, Mexico.

He attributes the evolution of understanding of the origins of the oils in the Gulf basin to evolving thoughts on petroleum geochemistry and the significant expansion of the available geochemical database.

Katz works the deepwater, although not the GOM. He has a particular interest in oil quality.

He emphasized that oil quality is of increasing economic importance as exploration shifts to deeper water and as reservoirs become more challenging, owing to the impact of the quality of the oil on producibility and price.

Unlocking GOM Origins

Years of study established that multiple petroleum systems ranging in age from the Jurassic to the Eocene occur in the GOM and are geographically limited. These systems produce oils of differing initial quality, depending on their lithology and depositional setting.

Source rock attributes of these systems have largely been inferred via hydrocarbon geochemistry instead of detailed source rock characterization and definitive correlations.

Katz noted there were three keys to unlocking current understanding of the GOM’s oil origins:

♦ Acceptance of the disassociated nature of the Gulf’s petroleum systems.

It’s now recognized that instead of being source-related, variances in oil character are caused by either post-charging alteration, level of maturation and/or charge timing differences.

♦ Agreement that source rock volume was not an effective substitute for source rock quality.

General acceptance that low concentrations of organic matter do not allow efficient expulsion.

♦ Overall acceptance of the importance of vertical migration in deltaic systems and where halokinesis has occurred.

Katz emphasized that the nature of the source rock is a very strong component of what is controlling oil quality in the Gulf, just like anywhere else.

“In addition to the nature of the source rock, the alteration and charging history play an important role in establishing oil quality,” he noted. “Alteration processes include biodegradation, water washing, phase segregation and de-asphalting.

“The charging history includes both the actual time of emplacement, late versus early generation, as well as whether multiple charging events might have occurred, introducing the potential for oil mixing,” Katz added.

“Although most oil quality discussions focus on the degradation of oil quality,” he said, “increasing thermal maturity of the source or thermal stress on the oil-bearing reservoir results in higher quality crude oils.

“Another means of improving oil quality is through hydrocarbon recharge and the introduction of fresh hydrocarbons into a biodegraded pool,” he added.

The Value of Quality

Oil quality is a key component in establishing flow rates, flow assurance, producibility and the character of refined products. This is of particular importance in reservoirs where permeability is low and oil quality might limit flow, such as the Wilcox in the deepwater Gulf.

When queried as to the impact of this knowledge on the leasing process, Katz noted that identification of which systems are working in different parts of the Gulf is taken into consideration, because this drives the nature of crude being generated.

That, in turn, controls price at the wellhead, API gravity and how well it will flow.

“You take what you know and put it into the framework,” he said. “You want to come up with a reasonable economic model, and part of that is oil quality.

“We all see Brent and WTI quoted daily,” Katz noted. “But there’s a big range of values daily in the crude oil market as things are being discounted because of sulfur presence, lower API or higher metals.

“You like to know as close to upfront as you can what you’re going to get,” he emphasized. “You don’t want to wind up with high sulfur content you’re not prepared for, because there won’t be any space on the platform to deal with it.

“But you don’t overbuild to deal with something that doesn’t exist.”

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Katz Talk Set for GCAGS Meeting

AAPG Honorary Member Barry Katz will present his paper “Evolving Thoughts on the Origin of Oil in the Gulf of Mexico Basin and Its Impact on Oil Quality,” at the next Gulf Coast Association of Geological Societies annual meeting, set Oct. 16-19 in Veracruz, Mexico.

The meeting’s theme is “Sharing Knowledge to Add Value,” and the event marks the first time a GCAGS annual meeting has been held in Mexico.

Katz’ talk will be offered at 1:30 p.m. Tuesday, Oct. 18, as part of an oral session on Petroleum Systems and Oil Quality Controls in the Gulf of Mexico.

Information and registration details for the GCAGS meeting can be found online