Field work required

Microseeps Can Indicate Mega-Secrets

American Association of Petroleum Geologists (AAPG)

Containing costs and reducing risks are good basics for any project, but for smaller exploration companies – like many in the U.S. mid-continent region – they can be go-or-no factors.

Data often is abundant, but often, too, it is old or spotty – and at those times, oil-seekers might find better searching with geochemistry, according to AAPG member Daniel Hitzman of Geo-Microbial Technologies of Ochelata, Okla.

In other words, mid-continent operators typically comb through old 2-D seismic and well data for overlooked or under-explored trends – a venture’s size, location and budget can easily take a new 3-D seismic survey off the table.

But adding microseepage data to the mix can be a quick way to narrow the search at a fraction of the cost, Hitzman said.

Microseeps occur over oil and gas deposits as light hydrocarbon gas molecules leak through reservoir seals and percolate up through a “gas chimney” to the surface, creating a signature detectable in soil and rock samples.

Microseeps differ from “Jed Clampett-type” macroseeps, which can be great confidence builders in some areas, Hitzman acknowledged. Because the larger, heavier, macroseep molecules can’t pass through the seal rock, they may travel along gross faults to the surface far from the source, he said.

In microseeps, the lighter gases move vertically to create not a halo, but an apical signature, Hitzman said.

An Effective Approach

Hitzman and co-author Brooks Rountree presented the paper “When Seismic is Not Available: Hydrocarbon Microseepage Surveys Focus Drilling Strategies for Mid-Continent Operators” at the recent AAPG Mid-Continent Section annual meeting in Oklahoma City.

Hitzman said he and his colleagues use two methods to evaluate microseeps:

  • Sorbed soil gas tests, which requires collecting free gas molecules trapped in the surface soil or rock, digesting the samples in a lab and measuring with a gas chromatograph.
  • Microbial Oil Survey Technique, or MOST, in which soil samples are tested for the presence of specific hydrocarbon-munching microbes.

(Both techniques were developed by Phillips Petroleum scientists, including Hitzman's father, Donald Hitzman, who held over 40 patents with the company.)

Compared to modern seismic, both methods are cheap, simple and quick – but Hitzman said the MOST method is less expensive and has become their primary tool.

A well-equipped microseepage expedition, he said, typically requires a vehicle and two people with GPS, shovels and cans.

"It's about as green as you can get," he said.

"Samples are a handful of soil at shovel depth – six to eight inches, typically about a tenth of a mile apart,” he continued. “There are no damages to be paid because there's no damage to fences or land or anything.”

In "reconnaissance mode," surveyors can drive section roads, sampling on public rights-of-way up to the fence lines. A vehicle and two-man crew might cover 30-40 linear miles in a day.

"We can narrow down areas for further examination,” he said. “That may mean geochemistry, seismic or looking at the wells logs again.”

An "exploration mode" survey is more detailed – two crew members on foot with the same gear sample a given area in a grid pattern, covering up to two square miles a day, he said.

The low-tech collection methods means surveys can be made in just about any terrain the surveyors can tramp through.

"In a virgin reservoir, the lighter molecules move up to form an apical anomaly ... and generally don't drift beyond the structural limits of the feature," Hitzman said.

"Once you start drilling, you get a reduction in pressure,” he said. “It no longer goes through the gas chimney, but migrates to the wellbore ... we see a depletion in our seepage figures.”

Surveys might range for "400 square miles in Yucatan down to quarter-section operations in Osage County (Okla.)," he said.

Seeps can be identified with the type of source material by specific microbes "attacking" the gas chimney.

"I've been asked, 'How specific?' In one case study we identify a thermogenic butane map based on microbial populations.

"We can't tell you how deep,” he continued, “or whether its a structure or a trap, but we know there's gas there.”

Seepage surveys may spot anomalies that escaped structure-focused seismic interpretations.

"A geoscience team may see our anomalies and send up a 'doubt flag,'” he said. “We all hate to throw out prospects, but seepage is a very strong discrimination tool for risk reduction. I'd say it has about a 90 percent accuracy rate in saying where not to drill," he said.

A Little Respect?

Microbial analysis is a helpful location tool while soil gas tests allow geochemists to characterize the findings as oil-, gas- or condensate-prone, Hitzman said.

“Seismic is very good at finding traps and structures, but they don’t all hold hydrocarbons ... or we wouldn’t have dry holes.”

And at about $700 to $800 per linear mile, a seepage survey can be an efficient way to narrow choices, he said.

Hitzman said about a half-dozen U.S. companies, operating mostly in the mid-continent, provide microseepage surveys.

Admittedly, the method can have drawbacks.

A quick survey with too few data points might tend to “over-promise” results, he said – a single-line survey compared to a grid sampling is like 2-D seismic vs. 3-D.

Also, a geo-team comparing seepage data to a 3-D seismic set may resist findings that don’t confirm all their prospects, he said.

“Or we may have an anomaly not seen in the seismic that puts other parts of the set in doubt – even though its a perfectly good stratigraphic feature,” he said.

Raw microseepage data is “noisy,” and soil gas collection can be something of an art, affected by such things as low- or high-pressure weather systems, he said.

The needed expertise can be developed, however.

Hitzman and Brooks, for example, are both geologists who “hung out” with geochemists, he said, and who also work with well-known experts in the field.

In an industry that prides itself on integrating data and disciplines, geochemistry occasionally suffers from something like a Rodney Dangerfield syndrome, he suggested.

Geochemical analyses may be seen as icing on the cake, and many managers do not require them.

“It’s not so much rivalry among disciplines,” Hitzman said with a chuckle, “it’s just that we get ignored.”

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