Bakken fractures yield the goods

Oil Shale Takes Turn in Spotlight

By LOUISE S. DURHAM, EXPLORER Correspondent

Perspective: To put this quote from Price’s 1999 paper Origins and characteristics of the basin-centered continuous unconventional oil-resource base of the Bakken Source System, Williston Basin, 1999/2000 in perspective, here is a chart containing Price’s low reserve estimate with the proven and potential reserves of the major petroleum producing countries of the world. Click to enlarge slideshow.

Stratigraphic Column: The Bakken Formation has gross thicknesses in the neighborhood of 150 ft and is a “shale sandwich.” Two world-class high TOC marine shales sandwich a brittle dolomitic siltstone with very little native porosity and permiabilities on the order of 0.04 millidarcies.

Parshall Field – Conventional Processing: The Bakken Test line was shot and processed to maximize the seismic thin-bed resolution. This is a segment of the line in Parshall Field that was “dumbed-down” in the processing for comparison. By any standard, this would be considered at or near the top of the quality scale for seismic imaging in this area. However, these data were recorded and processed at 1ms sample rate and the p-wave section above contains additional stratigraphic information that, when properly handled, can point to prospective drill locations.

Parshall Field – VHF Processing: This is the same segment as shown previously, but processed to exploit the high-resolution capacity of the 1ms broad-band data. Vector High Fidelity has successfully separated several thin-bed reflectors and imaged a very subtle fault near the top of the gentle structure. The red trough just under the Bakken top, correlates nicely with the Middle Bakken.

Sanish Field – Conventional Processing: Much farther west on the Vector Bakken Test line, in the Sanish Field, the seismic character is significantly different at the Bakken Level. The Sanish Facies is much sandier.

Sanish Field – VHF Processing: But even here, Vector High Fidelity processing does an excellent job of reducing seismic noise and improving thin bed resolution.

3C Geophones: The 3-component geophones used by Tesla-Conquest to record the Vector Bakken 2D/3C Test Line were made by Sensor and are true geophones. Each unit contains three orthogonal elements permanently mounted in a solid aluminum case – about 2-1/4 inches in diameter and 6.5 inches long. Each phone was buried in a pre-drilled pilot hole.

3C Field Layout: This schematic illustrated how it is critical that each receiver have a common compass orientation in the field. Of course, this is a simple matter but no less important for a 2D line where one of the principal axes is along the line azimuth. It becomes more complex when recording a 3D/3C survey.

3C Processing Layout: I the processing, it is necessary to mathematically orient the geophones into source-centric axes. The radial shear orientation will face away from the shot whereas the transverse, or cross-line polarization will be perpendicular to this azimuth. Notice that the polarization of the geophones on either side of the source are reversed from each other. This has to be accounted for in the processing as well.

P-wave Dry Hole: This is the p-wave version of the seismic profile at the Behm Energy dry hole.

S-radial Dry Hole: This is the radial shear-wave version of the same segment at the dry hole. Note that the Bakken reflection is continuous in the absence of fractures. Note also that there appears to be some evidence of vertical (tectonic) fractures both east and west of the dry hole. This suggests that the tectonic fractures, by themselves, are insufficient to predict hydrocarbons if the well is outside of the thermal-maturity limit for oil expulsion.

P-wave Parshall Field: Parshall Field looks very similar to the dry hole with the p-wave seismic. There is a slightly higher noise level as you progress to the west across the basin. But essentially, there is nothing that would tell an explorationist to drill here and to avoid the Behm location.

S-radial Parshall Field: The radial-shear version of this line segment tells a dramatically different story. Notice that the presence of hydraulic fractures is having a profound effect on the polarized wave field. The pseudo time structure visible on this image is a manifestation of the distortion of the wave front due to fractures and fluids rather than a true mappable time structure. Also notice that the continuous deeper reflection at about 3.400 sec. attests to the fact that the wave field distortion is a localized phenomena. Compare with the deep reflector at the Behm Energy dry hole.

Bakken Edge: The Behm Energy dry hole could have been avoided if the test data generated by Vector had been available. The identification of the limits of the Middle Bakken reservoir in North Dakota and Canada is possible with high-resolution, multi-component surface seismic methods, and at significantly less money that even a single dry hole. To this end, Vector is currently conducting a seismic reconnaissance survey to tag the so-called Bakken Edge and to further identify sweet spots where natural fractures will greatly enhance the economic success of a horizontal well.

Shale gas is in.

Shale gas is out.

Shale oil is in.

No one would argue that the direction of this industry can change on a dime – and fortunately, most players have learned that you just gotta go with the flow.

Following the all-out drilling charge to punch down as many wellbores as possible – often to hold onto leases – in the still-relatively-new shale gas plays across the United States, there now is a massive inventory of clean-burning natural gas.

In fact, you might be tempted to make a buck or two by scouting for new facilities to store the burgeoning supply.

Best to cool your heels.

With all this new natural gas supply and $4/Mcf – give or take – looking tops for now, there’s talk of laying down some rigs. It’s not about resting on laurels and taking time off to chill, but to head for the other new best thing, i.e. oil shales and/or gas shales rich with liquids, such as the Eagle Ford in South Texas.

It’s a matter of simple math: oil continues to fetch a price generally in the upper $70/bbl range.

The Big Dude shale in the oil game is the Bakken shale oil play in Montana and North Dakota, which is becoming increasingly popular following a period of successful yet relatively low profile action.

Adding to the allure of this play is the U.S. Geological Survey assessment that revealed the Bakken harbors about 3.65 billion barrels of undiscovered technically recoverable oil along with 1.85 Tcf of associated/dissolved natural gas and 148 mbo of natural gas liquids.

The widespread Upper Devonian-Lower Mississippian Bakken formation is comprised of an upper and lower shale member and a mixed siliciclastic carbonate middle member, which is ordinarily referred to as a dolomitic sand or sandy dolomite.

This middle section is the target of the drill bits that ordinarily go down about 10,000 feet vertically before veering horizontally into the brittle dolomite, where multi-stage fracing is used to more efficiently produce the oil.

Not all wells are created equal.

“When you spend maybe $7 million on a horizontal well and bring it in at 200 to 300 barrels a day, that’s economic failure,” said AAPG member Scott Stockton, executive vice president of Vector Seismic Data Processing in Denver. “You need at least 1,000 barrels a day to be able to smile when you leave the wellhead.”

Oh, So Sweet

Enter multi-component seismic data to help ID the sweet spots.

In early 2009, Vector Seismic formed a consortium to evaluate the seismic signature of fractured reservoirs in the Middle Bakken. This ultimately led the company to determine that differences in the seismic image of shear waves over producing wells vs. dry holes in the Bakken formation are key for drilling success.

The Middle Bakken has proved elusive when it comes to detailed imaging from conventional surface seismic applications, for two reasons:

♦ With a thickness typically between 15 and 60 feet at a depth of 8,000 or so, it’s below resolution of conventional seismic methods.

♦ The P-wave response of seismic energy in the fractured vs. non-fractured rock is virtually identical.

Stockton noted that companies have acquired significant amounts of conventional seismic data in the play and are getting a great structural picture – but they haven’t been able to ID the fractures, which are required for reservoir to exist in the tight siltstone having little or no native porosity or permeability.

“We took a high resolution approach, going in very broadband,” Stockton said. “This does image the thin beds and small faults that are potentially indicative of the presence of fractures, but it doesn’t get you all the way home.”

They decided to use converted-wave recording, given there’s only one working set of shear wave vibrators available in the continental United States, according to Stockton. He noted these were unavailable at the last minute.

“In retrospect, I was glad,” he said. “If you do a converted wave (3-C) seismic survey, it means you have available to you all kinds of P-wave sources, such as dynamite and Vibroseis. If you can get as good an image with vibrators, you can save a lot of money.

“We recorded a high resolution line twice over the area of interest, once with vibrators and once with dynamite,” he noted.

A high resolution converted wave seismic profile tied the dry-hole Behm Energy well in Mountrail County in northwestern North Dakota with Bakken producing wells to the west in Parshall and Sanish fields. The seismic signature of the waveform on the converted-wave image shows marked differences that can be correlated to natural fractures in the Bakken formation and better production.

Hot and Haute

The MO in the Bakken play thus far has been to chase after tectonic fractures. Even though deep underground, e.g. 8,500 to 12,000 feet, they tend to “pop” on the surface showing up pretty much as straight lines, or lineaments.

The other fracture mechanism is hydraulic, which Stockton thinks is key to really prolific wells in the Middle Bakken.

It’s all about the Bakken petroleum system, which is a closed, self-sourced system.

The combo of a uniquely closed petroleum system, a high thermal gradient and volumetric expansion of the Upper and Lower Bakken kerogen into oil has resulted in high potential for creating in situ fractures parallel to bedding planes.

“When kerogen cooks out of the Bakken shale it experiences an intense volumetric increase of about 114 to 170 percent,” Stockton said. “There’s great energy stored in that volume increase and it wants to fracture the rock, mainly along bedding planes.”

He noted that the horizontal fractures can be a huge factor in terms of where the reservoir is and where it’s best.

“Where the tectonic fractures intersect the hydraulic, you get the best wells,” he emphasized. “You get great wells where you have both, good wells where you have hydraulic fractures, okay to good wells where you have vertical (tectonic) fractures.”

It’s all mighty hot and haute.

But the oil won’t do anyone any good if it just sits on site in tanks.

“The current interest in the Bakken might be called a frenzy,” Stockton exclaimed. “Now that we’ve shown that seismic can tell where the oil is, the big problem is the infrastructure – like, how do you get oil out to the market?”

In the advanced technology milieu of shale drilling and production, the current transport solution is so low-tech one is tempted to laugh.

But, hey, it works.

Plus, it’s a fine example of good old oil patch can-do.

Scott noted some of the companies bought a bunch of old rail lines and rail cars and basically have tanker trains that they load up with oil to transport to Oklahoma and elsewhere for refining.

“This is American ingenuity at its best,” he exclaimed. “It was one heckuva idea.

“This is a massive transportation issue,” he said, “and there are a lot of abandoned rail lines up there in North Dakota.”

Given the potential for so much more production in this play, perhaps some enterprising investors will figure out a way to go long on old rail cars.