Updated Assessment of Undiscovered Oil and Gas Resources in the National Petroleum Reserve in AK

Updated Assessment of Undiscovered Oil and Gas Resources in the National Petroleum Reserve in AK

 

Alex Demas:  Welcome to the USGS CoreCast, this is your host Alex Demas.

Joining me today are Brenda Pierce, the Director of the Energy Resource Program; and Dave Houseknecht, a geologist who specializes in Alaska.  We will be discussing the new results of the USGS assessment of oil and gas in the National Petroleum Reserve in Alaska. 

Dave, Brenda, thank you for joining me.

Brenda Pierce:  Thank you.

Dave Houseknecht:  Hello.

Alex Demas:  So, why do we do assessments of natural gas and oil resources?

Brenda Pierce:  Well, the USGS has a unique role in assessment of oil and gas potential. The USGS assess undiscovered, technically recoverable resources of the United States and the world. 

Undiscovered, meaning they are not produced yet, they're not known yet. These are the resources that have the potential to be discovered and produced, and technically recoverable in that they can be produced using today's technology and industry practice.

01:01

So, the USGS provides unbiased assessment of this resource potential that no one else provides. We do it with consistent methodologies so that we can compare basin to basin the resource potential of the country, so that policy makers, land resource managers, other scientists have this information for the whole United States and all of the oil and gas resources everywhere.

Alex Demas:  So, I understand that the USGS had done an assessment back in 2002.  Is there a reason that we decided to reassess the same basin?

Brenda Pierce:  Well, I'll let Dave talk about this particular reassessment, but we do routinely reassess areas.  New geologic data come to light, what is "technically recoverable," changes every time because the technologies grows, develops and changes over time.

So, although we conduct resource assessments everywhere, we do reassess routinely as new information comes to light, which was the case for the National Petroleum Reserve, Alaska. 

Dave Houseknecht:  All right.

02:01

Just to put the NPRA, or the National Petroleum Reserve in Alaska in the context, in the mid-90s there was a huge oil discovery just outside the northeast boundary of the NPRA.  It was called the "Alpine Oil Field," and it's the largest onshore oil discovery anywhere in North America over the last quarter century.

As a result of that, there was a huge exploration pushed into NPRA with a number of lease sales. At the very beginning of that process, we did an assessment based on what we knew from the Alpine Oil Field, which contained no free gas at the time of discovery and contained what we now know is going to ultimately be about 750 million barrels of oil. 

So, we did this new assessment recently because as industry explored the NPRA by drilling on the federal leases, they encountered in their wells a very abrupt and unanticipated change or transition from oil near Alpine Field to gas just 15 or 20 miles away from Alpine Field.

03:20

And so we knew as soon as well, results started to be released by industry, that there was more gas and less oil than we have assessed in 2002. However, we had to wait until enough data had been released from those new oils to fully appreciate the scope of that gas-prone nature of NPRA and to do the quantitative update of the assessment.

Alex Demas:  So, why did the USGS decided to reassess the NPRA at this time?  Were we asked to do this or did we decide it to do it on our own?

Dave Houseknecht:  A little bit of each; the USGS monitors exploration activity in domestic and global basins, and we make decisions about when to do assessments or reassessments as new information warrants. 

04:08

In the case of NPRA, we knew very early on when drilling results of the past decade started to become available, that there was more gas than oil that we had anticipated in 2002.  However, most of the well data were proprietary and we had to wait until well data were released. 

So, we knew about six months ago that sufficient well information had been released on which to base the assessment. At about the same time the Bureau of Land Management, which is the administers of the federal land that is NPRA, approached us about doing an assessment for using a new draft integrated activity plan that they are in the process of preparing.

So, it's a little bit of our initiative and a little bit of being asked by the Bureau of Land Management to do the assessment.

Alex Demas:  So you said that six months ago, the propriety knowledge was released or released enough of it to base the assessment. How long did the assessment itself take?

05:17

Dave Houseknecht:  This update only took about six months because we used all of the geology that we had done in 2002, which was the end of a four-year study at that point in time.  

What the new drilling did reveal was that our geological model was correct in that the reservoir sizes that have been encountered, the reservoir ages, the rocks that the oil and gas are in we had correct.

The only difference was the type of hydrocarbon in the reservoir and what we find is when we look at frontier basins, or basins that have been underexplored or not explored at all, the most difficult thing to predict is whether there will be oil or gas in the reservoir. And as it turns out that challenge proves to be accurate in the NPRA because we anticipated oil based on the nearby Alpine Field. 

06:16

It turns out that Alpine is full of oil and a couple of small discoveries closed the Alpine that would have been made in recent years are full of oil. But west of this transition zone, about 15 or 20 miles away west of the Alpine oil field, the reservoirs appear to be mostly full of gas.

Alex Demas:  So, why is that area mostly full of gas instead of oil?

Dave Houseknecht:  That's an interesting question.

As soon as the oil results started to be released in drips and drabs about five years ago, we recognized this transition to gas and so we started doing research to try to explain this transition geologically. Our interpretation is that the oil is reservoired in northern NPRA, about 9o million years ago, roughly in the middle of the cretaceous period.

07:14

Subsequently to that, about 60 to 20 million years ago during the Cenozoic era, there was a broad uplift and widespread erosion in NPRA.  What that did was reduced the pressure on the underground reservoirs and when the pressure is reduced, two things happen. Number one, if oil is filling the reservoir gas will come out of solution from the oil and displace the oil down deep.

You can almost think of it as opening a can of soda pop. The gas that is released is the result of a reduced pressure when you pop open the can. Similarly, on oil field if it is uplifted and the pressure is reduced, the natural gas that is dissolved in the oil will come out of solution.

08:05

Still, further west in NPRA where the uplift was greater, there was a great gas expansion and we believe that the gas simply flushed through most of these reservoirs, and actually displaced the oil so completely that it left the reservoirs all together.

So, ultimately, we believe that this Cenozoic uplift that postdated the accumulation of the oil is responsible for the transition from oil to gas in northern NPRA.   

Alex Demas:  So what are the actual numbers? How have they changed?

Dave Houseknecht:  The new USGS estimate for undiscovered, technically recoverable oil in conventional accumulations is 896 million barrels, and that's less than 10 percent of what we assessed to be present just eight years ago.

09:04

The big reduction is the result of the exploration drilling results, which indicated more gas than oil, and even though that reduction is very substantial, the 896 million barrels that we estimated to be present and discovered still places the NPRA among the top handful of geologic provinces in the United States in terms of undiscovered oil potential.

As far as natural gas, the number is about 90 percent of what it was eight years ago. We estimate nearly 53 trillion cubic feet of natural gas in NPRA.  The reason there's been a bit of a reduction since 2002 is that some of the new drilling encountered reservoir quality that is less than we anticipated.

So, we will be considering some of the reservoirs that have been penetrated as continuous or unconventional accumulations as we go forward, rather than conventional.

10:10

Alex Demas:  Brenda, Dave, thank you for joining me.

Brenda Pierce:  Thank you.

Dave Houseknecht:  You're welcome.

Alex Demas:  For more information on this CoreCast and other CoreCast, please visit our website at www.usgs.gov/corecast.

This CoreCast has been a product of the U.S. Geological Survey, Department of the Interior.

Thanks for listening.