How Much Oil Is Really There?

Alaska Evaluation Due; Offshore California Estimate Grows

AAPG EXPLORER Correspondent

Picture 1 Alaska - a land of great beauty, a land of great potential. An exposure of the Kemik Sandstone - a potential reservoir in the ANWRsubsurface - along the Canning River, with the west-plunging Shublik Mountains in the background. Two hydrocarbon resource assessments, one recently published and one soon to be released, involve the most controversial and politically sensitive prospective areas of the United States.

Those areas: Alaska and offshore California. And to say that the findings of these reports are anxiously awaited may be the year's biggest understatement.

First, the U.S. Geological Survey plans to reveal results of a new oil and gas resource assessment covering part of Alaska's Arctic National Wildlife Refuge (ANWR) at the AAPG annual meeting in Salt Lake City in May.

ANWR tops the U.S. petroleum industry's wish list for leasing, but Congress has prohibited to date drilling in the refuge.

The USGS is prepared for controversy over its new evaluation.

"Naturally, this is a volatile topic," said David Houseknecht, USGS energy program coordinator in Reston, Va. "The results are likely to be met with lots of interest by both advocates of resource development and proponents of environmental conservation."

Also, the U.S. Minerals Management Service (MMS) recently published its "Assessment of the Pacific Outer Continental Shelf Region," part of the 1995 National Assessment of United States Oil and Gas Resources.

That report covers 15 assessed basins in federal waters offshore California, Oregon and Washington. Prolific production from the Monterey formation in the offshore California area, including the Point Arguello oil field, once produced high industry interest.

However, the prospective areas are under a drilling moratorium, and environmental opposition to exploration offshore California remains strong.

Information about the MMS assessment report is available at: and a copy can be requested by e-mail from:

'Thorough Piece of Work'

Ken Bird, research geologist for the USGS in Menlo Park, Calif., served as project leader for the ANWR assessment. The study covers about 1.5 million acres of the north Alaskan coastal plain.

Section 1002 of the federal Alaska National Interest Lands Conservation Act of 1980 authorized evaluation of this area's oil and gas potential by means other than drilling, and it is often referred to as the 1002 area.

No matter how the USGS assessment is received, Bird feels proud of the effort.

"It's by far the best and most thorough piece of work that's been done up there," he said. "We had a very large group of scientists working on this from many different parts of the country."

Specialists in gravity interpretation, paleontology, fluid-flow and many other areas contributed to the study, according to Bird. The USGS team also used data not available in previous assessments and made better use of existing information.

Two seasons of seismic work in 1984-85 produced 1,452 linemiles of data (607 line miles dynamite-sourced and 845 line miles vibraseis) in the 1002 area. For the new ANWR assessment, these data were reprocessed and re-evaluated, Bird said.

In addition, data from wells drilled near the 1002 area became available for the resource study and provided valuable well control, he added.

"I think there are 15-16 additional wells that we had to work with this time that were not available earlier. These are all wells that are within 15-20 miles from the 1002 boundary," Bird said.

Some well data in the area remain proprietary. Bird said the ANWR assessment area includes a 100,000-acre expanse of Native-owned land. An existing 15,000-foot well on that land could have furnished additional data for the resource assessment.

"When you look at the structure maps, you see that this well is located on one of the very biggest structures in the entire area," Bird noted.

But that well is a tight hole. (Bird said the USGS was able to obtain and use information from a well drilled downdip on the flank of the same structure, in federal waters.)

"One of the biggest frustrations in terms of data is that there are still a significant number of wells that are as much as 18 years old that are still proprietary, that are still tight holes, in the vicinity of ANWR," Houseknecht commented.

The new USGS assessment comes on the heels of an ANWR resource report prepared at the request of the Interior Department. Because the request was on a rush basis, the Survey used information from the 1995 national resource assessment and simply did its best to allocate resources to the ANWR area, according to Bird.

"In that 1995 (national) assessment, we did not separate ANWR from the rest of the North Slope. We assessed it in terms of about 11 different petroleum plays. Those plays generally covered large areas, none of them specific to ANWR," Bird said.

Resource allocation led to estimates much lower than expected - or wanted - by proponents of ANWR drilling. Criticism of the study and negative comments followed, Bird recalled.

"There were all sorts of accusations that we were politically motivated to reduce the numbers by the (Clinton) administration," he said, "so, that really stung.

"It caused us to decide that we really should spend the time and the effort to do a complete assessment. We took it upon ourselves to institute a new assessment of the project area. This was strictly on our own."

Ahead of the Curve

Houseknecht offered another reason the USGS undertook its voluntary study of 1002 resources. He said the Survey anticipated future requests for assessment of the area, and wanted time to do the job right.

"We have found that in many cases the federal decision-making process requires technical information, but in a timeframe that does not permit critical science to be done when a report is requested," he explained.

Current seismic data and more well information would have helped in the ANWR assessment, Houseknecht acknowledged. But he feels the existing dataset proved adequate for resource evaluation.

"Any time that you deal with a 2-D seismic network that is more than a decade old, and that has a relatively low density of coverage - naturally, any time you deal with that kind of a dataset there is a resultant uncertainty in the assessment of oil and gas resources," Houseknecht said.

However, he added, "for a frontier area like ANWR this is a relatively high-quality dataset. And it's one that certainly gives us the kind of constraints we need to evaluate the petroleum resources."

Basic methodology for the ANWR assessment was very similar to the approach used for the 1995 national resource assessment, Bird said. The USGS chose a different methodology for projecting economically and technically recoverable resources, however. It also presented its methodology for review and comment prior to releasing results.

Picture 2

Photos courtesy of David Houseknecht
A detailed view of the Kemik Sandstone, a potential reservoir of Alaska's ANWR.

"We have attempted to make this assessment project one where we have collaborated with as many people as possible," Houseknecht said. "We have held review meetings in Anchorage and in Houston, where we presented the geological basis for our results as well as our methodology."

That review process included a presentation to the AAPG Committee on Resource Evaluation (CORE) in January. Ben Hare is CORE chairman and chief geologist for Vastar Resources Inc. in Houston.

Pending favorable analysis, CORE will express it's evaluation of the USGS technical process and assessment methodology to the AAPG Executive Committee, Hare said.

This is not the first time the committee has contributed to an assessment review, he added.

"The CORE committee participated in the 1995 U.S. (national oil and gas) assessment by making AAPG members available," Hare said. "The committee chairman at that time was Tom Barber. Through Tom's effort we made some 75-100 AAPG members across the country available to them, to make sure they were on the right scientific track."

California Surprises

The MMS Pacific OCS report documents the oil and gas commodities, resource categories, data and methodologies of the assessment of the federal offshore area of California, Oregon and Washington.

Its significant findings include:

  • Nearly 11 billion barrels of undiscovered oil and 19 trillion cubic feet of undiscovered gas in the region may be recoverable using existing technology.
  • Relatively large volumes of undiscovered oil may exist offshore central and southern California, due largely to the presence of Monterey-type strata, which are potential source and reservoir rocks.
  • Half of the undiscovered, conventionally recoverable oil and gas in the region may be economically recoverable under existing conditions.
Cathie Dunkel, MMS geologist in Camarillo, Calif., served as the report's principal editor. She said the apparent extent of Monterey-type plays and the possible large volume of undiscovered oil in these plays offshore central California were the assessment's biggest surprises.

"Bottom-line, the volume of potentially fractured siliceous rocks offshore California is much larger than we previously thought," she said. "Only nine of the 46 plays we assessed contain these Monterey-type reservoir rocks, but those nine plays, all offshore central and southern California, are estimated to contain more than half of the undiscovered oil and about a third of the undiscovered gas."

Monterey-type plays offshore central California and in the Santa Barbara-Ventura Basin comprise the region's top five plays, based on estimated undiscovered oil resources, she noted.

Dunkel, a 16-year MMS veteran, found pursuing a geologic hypothesis the most intriguing part of the assessment process. She cited suggestions that the onset of petroleum generation in the area's Neogene siliceous rocks (Monterey formation or correlative) coincides with the temperature at which the diagenetic quartz facies forms.

"For many years it's been recognized, or at least speculated, that siliceous rocks of increasing diagenetic grade are more brittle and have greater potential to be fractured, and of course to have more fracture porosity," she added.

Using newly acquired mineralogic data from well samples, MMS was able to identify the stratigraphic location of the diagenetic facies in many of the wells and to correlate them with structurally anomalous reflectors on seismic profiles, Dunkel said.

The resulting improved understanding of the stratigraphic and areal extent of the siliceous rocks and their facies led to the conclusion that larger-than-expected volumes of Monterey-type rocks - including potentially generative source rocks and potentially fractured reservoir rocks - exist offshore California.

In conducting its assessment, MMS was able to draw on data and interpretations from many of the nearly 1,100 wells and 200,000 miles of seismic-reflection profiles in the region.

"We also used available geologic and production data from onshore and state offshore areas to assess some plays that extend onshore. This is an improvement from past assessments," she noted. "Also, we received valuable information and feedback from many members of government, geological organizations, industry and academia."

A Conceptual Approach

Resource estimates in the report are larger than previous estimates, Dunkel said, and she offered several possible reasons for the growth. MMS assumed a lognormal size distribution of oil and gas accumulations, which led to the inclusion of additional pools in many plays.

"Because we used a play-based approach for this assessment, we included consideration of conceptual plays," she noted. "Those are plays where no discoveries have been made - in fact, where we don't have any hard data confirming the presence of hydrocarbons, but for which data suggest that hydrocarbons may have formed."

Also, "some of the rocks that are included in these plays probably weren't given suitable consideration in prior assessments," she continued, "and that may be one of the reasons the numbers have increased."

Virtually all of the Pacific OCS is under oil and gas leasing moratoria status through the year 2000, and the assessment region is not included in the 1997-2002 OCS leasing program, Dunkel said. In addition, some of the region's prospective areas are in national marine sanctuaries.

Digital information from the assessment is being compiled for release on the Internet or a CD-ROM later in 1998. This information will include a browsable version of the report and tabular data regarding plays and resource estimates.


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