Technological advances

Explorers Focus on Deepwater Reservoirs

American Association of Petroleum Geologists (AAPG)

Even though shale has become the figurative prima donna in the E&P world, there’s action aplenty focused on other types of plays – in widely diverse regions.

Over the past several years a number of successful giant oil discoveries have occurred around the world in conventional reservoirs, no less, in the deep water.

John Dribus
John Dribus

Three of these currently are in the forefront, attracting focused exploration attention along the Atlantic Margin, according to AAPG member John Dribus, global geosciences adviser at Schlumberger.

“In the last 10 years, exploration around the world has discovered there are a couple of really key reservoirs where the big accumulations are being found,” Dribus said. “The reservoirs are in pre-salt carbonates underneath salt in Brazil and Angola, and in deepwater turbidites like the Gulf of Mexico and over in Ghana.

“Everyone always talks about how big they are,” he noted, “but no one has ever talked about the reservoir characteristics of the rock.”

Dribus stepped forward to address that void.

He has assembled an array of info, allowing him to paint the big picture for the E&P community.

He even has condensed his findings into a quick, crisp overview of the three main reservoirs:

  • Pre-salt in Brazil.
  • Cretaceous Ghana Jubilee abrupt margin turbidite and its equivalents.
  • Passive margin turbidites in the GOM.

“It’s important to know how the Atlantic Margin opened up and created these particular depositional environments into where these reservoirs were deposited,” Dribus said.

“I’ve looked at how each was formed and what their basic rock properties are and what distinguished them as excellent reservoirs.”

Breeds Apart

Dribus, who will be giving a talk on this at the upcoming AAPG/DPA Playmaker Forum in Houston, noted that even though the GOM and Ghana are turbidites, they are breeds apart.

“The one in the Gulf is an unconfined broad passive margin turbidite, and the one in Ghana is a much steeper abrupt margin, higher energy turbidite,” Dribus said. “So they create different types of reservoirs.

“The other thing that distinguishes the reservoirs in the Gulf is that many are subsalt reservoirs,” he added, “and Ghana doesn’t involve any salt.”

He goes on to describe Brazil as a completely different environment, noting that Brazil is old salt, so it’s actually pre-salt as opposed to subsalt.

Underlying the pre-salt is a special carbonate environment known as microbialites.

Petrobras is long recognized as the big-name in exploration offshore Brazil, where it has found a number of pre-salt carbonate plays in different basins, such as Santos and Campos.

What sets these areas apart is the amount of salt, which acts as an effective seal to the reservoir, allowing large oil columns to accumulate underneath, according to Dribus.

Adding to the unique environment is the exceptional porosity and permeability in the microbialites.

Stand-Alone Region

In fact, Cretaceous microbialite reservoirs and associated facies also are being actively explored across the Atlantic from Brazil along western Africa in Kwanza and Benquela basins in Angola.

Dribus noted the Cretaceous Ghana Jubilee abrupt margin turbidite and its equivalents are being explored in the deep waters off Ghana, Liberia and across the conjugate margin to French Guiana and equatorial Brazil.

“We found (a reservoir) in Jubilee and followed the transform fault across the Atlantic, where the Zaedyus fan was discovered offshore French Guiana,” he said.

“The Gulf of Mexico is different because it stands alone – there’s no big conjugate margin like the Atlantic Ocean,” Dribus emphasized. “In the Gulf, you’re getting these great, vast Lower Tertiary fans sourced from the North American continent and deposited in deep water, later covered by the salt migrating over them and providing part of a seal to the system.”

The passive margin turbidites in the GOM include the long-familiar Tertiary Wilcox and Frio formations, currently being produced at Shell’s Perdido field and the Petrobras Cascade-Chinook field.

Dribus emphasized that producing these type fields required that the industry develop tools that are at the vanguard of technology.

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