Spraberry channel chasers aim carefully

More ‘Silver Bullets’ Helpful

Spraberry Stratigraphy
Spraberry Stratigraphy

When 3-D seismic evolved into the hot technology for E&P during the 1990s, it quickly gained a reputation as a kind of Holy Grail of the industry.

Prospect-generators trying to peddle a drilling prospect without 3-D were shunned for the most part.

Yet 3-D seismic frequently needs a bit of help, particularly when it comes to exploring for stratigraphic traps.

In these cases, traditional exploration techniques – including regional play concepts, subsurface well control, 2-D seismic and early 3-D seismic surveys – often yield marginal results.

The geoscientists at Midland-based Fasken Oil and Ranch recognized this fact early on in their effort to explore in the Permian-age Spraberry trend in Texas.

“The Spraberry trend of the Midland Basin poses significant challenges to extracting known reserves,” said AAPG member Glenn Winters, chief geophysicist at Fasken. “Poor recovery factors are due to low permeable, mud-rich, distal fan deposits, basinward of the Horseshoe Atoll.

“Proximal fan deposits in the northern half of the Midland Basin often contain better reservoir properties,” Winters said, “but are much more discontinuous and difficult to delineate.”

3-D’s Exploration Potential

Fasken’s attention is focused on specific intervals in the trend.

“We’re looking for conventional zones within the Spraberry, isolated sandstones that are conventional pays in the unconventional Spraberry, which overall is a tight sand,” said Fasken geologist and AAPG member Stonnie Pollock.

The company has several waterfloods producing from the Gin sand interval and other zones close to the Gin at roughly 7,700 feet deep. These fields were discovered by operators drilling to deeper targets as much as 11,000 feet, according to Pollock.

“Historically, it was hard for people to drill and then successfully offset and find a field,” Pollock noted. “When they would try to complete an entire field they would drill a lot of dry holes.”

An example is the Key West Field near La Mesa, Texas, where 30 wells were drilled in and around the field, and only 13 are producers.

“This is one of the first areas where 3-D seismic was used as an exploration tool,” Winters said. “In the middle ’90s, people shot a lot of 3-Ds out here looking for deeper targets.

“They really didn’t utilize their 3-D seismic for Spraberry channels,” he noted. “Part of the reason is the data quality probably wasn’t good enough to delineate the fields.

“The premise we started off with was when you find it, it could be good, it’s not that deep and land was relatively inexpensive here,” Winters said.

Nice Recovery
Los Ybanez Field. Dawson County, TX
Los Ybanez Field. Dawson County, TX

In spring 2004 Fasken drilled an industry-generated prospect in Dawson County, where the Spraberry was initially discovered in 1948, according to Pollock; the prospect was developed from subsurface well control.

The company had two early 1980s vintage seismic lines that were near the proposed well location, and a geophysical signature thought to be associated with the target reservoir was interpreted from those lines.

The initial well was a duster.

“After we drilled the dry hole, we said what can we do to better identify a field,” Pollock noted, “and when we do, how can we better delineate it?”

To explore specifically for the Gin interval in Dawson County, the Fasken team first completed field studies over several producing fields containing many Spraberry reservoirs.

They reprocessed and mapped their widespread database of 2-D seismic and obtained and reprocessed old 3-D data sets. Several lead areas were identified, and a high-effort 3-D survey was acquired.

Ultimately, conventional 3-D seismic interpretation techniques were integrated with advanced technologies, including Gore-Surface Geochem Analysis and Geotrace’s RockRes seismic processing techniques.

As a result of this all-out effort, two fields – Los Ybanez and Mati Rae – were discovered in 2006 adjacent to the Key West Field that Fasken used as an analog.

Seven of eight wells drilled were economic. Additionally, the exploration team predicted several dry or uneconomic wells that were drilled by competitors in the area, Winters noted.

To date, the two fields have produced over 600 Mbo and a Bcf of gas from nine wells. Current daily production tallies 440 bopd and 1 MMcf/d.

There’s more action to come.

“We acquired a second 3-D seismic in early 2008,” Winters said, “and several anomalies have been identified.

“Once again, Geotrace’s RockRes and Bandwidth Extension processes were applied,” he said, “and this past January Fasken completed a large geochem survey deplolying several hundred modules.

Fasken plans to commence a multi-well drilling program in May.

Winters noted there is no one silver bullet, emphasizing that the use of three or four silver bullets will reduce risk.

“We’re trying to advocate you can’t just look at conventional seismic data,” he said. “Using the old 2-D data, the old vintage 3-D seismic data, it will give you an area to focus on – but then we go to the next level and apply several of these technologies together.”

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