Geochem Case Histories

Micro-Seeps Tell Reservoir Tales

By LOUISE S. DURHAM
EXPLORER Correspondent

Graphics courtesy of Deet Schumacher

 
Mid-Continent Section Annual Meeting
October 12-14, 2003
Tulsa, Oklahoma

Deet Schumacher will present his paper, "Geochemical Exploration for Oil and Gas: New Life for an Old Technology" at 10 a.m. Monday, Oct. 13, at the AAPG Mid-Continent Section meeting in Tulsa.

The paper is part of a session chaired by Schumacher on "Case Histories of Successful Application of Geochemistry."

That session also will feature another paper co-authored by Schumacher and presented at 10:40 a.m. by Daniel Hitzman titled "When Seismic is Not Enough: Improving Success by Integrating High-Resolution Surface Geochemical Data With Seismic Data."

Both are with Geo-Microbial Technologies in Ochelata, Okla.

In their ongoing effort to wrest the new from the old, oil finders are discovering that more often than not it takes a mix of different technologies to achieve success.

Much of the technology used to find new pay in old plays and bypassed pay in old fields is what you would expect, such as 3-D seismic. Some of it, however, smacks of the more esoteric, such as geochemical exploration, which focuses on the search for analytically-identifiable surface or near-surface occurrences of hydrocarbons as clues to the location of undiscovered oil and gas.

High resolution hydrocarbon microseepage surveys have come to be recognized as a flexible, low risk and low cost technology that complements the more traditional geologic and seismic methods.

Advances in geochem technology and the results of some recent applications will be recounted by Deet Schumacher of Geo-Microbial Technologies (GMT), Ochelata, Okla., in a paper at the AAPG Mid-Continent Section meeting this month in Tulsa. Schumacher also presented a geochem paper at the recent AAPG International Conference and Exhibition in Barcelona.

It has been documented that hydrocarbon microseepage from petroleum accumulations is both widespread and vertical except in the case of very complex geology, according to Schumacher. This microseepage is also dynamic in that it responds quickly to changes in the reservoir.

These characteristics make surface geochemical surveys suited for a whole new array of applications beyond the more established uses for high-grading leases, leads and prospects, including:

• Early delineation of field limits.
• Finding by-passed oil and gas.
• Evaluation of infill and step-out locations.
• Contributions to reservoir characterization studies.
• Documentation of hydrocarbon drainage over time.
• Monitoring of waterflood and CO₂-flood operations.

"When we do a well-designed survey and outline an anomaly," Schumacher said, "it can approximate the distribution of the producing reservoir at depth.

"If there's one productive zone in the area it makes it simple," he said, "but multiple zones make it much more difficult. If there's stacked pay, you must look at other information to see which zone seems to best fit the anomaly."

DOWNLOAD FULL IMAGE (90 kb)

Global Examples

Schumacher cited the example of an old field in Argentina with multiple pay zones, where a geochem survey was used to look for bypassed pay and for other applications.

"A couple of nice anomalies showed up," he said, "and when you compare a map of the anomalies with the three productive zones, only one matches. It's a nice structural closure, and contours on the structure show the closure matches nicely with the geochem.

"But until we drill, we won't have proof that's the source.

"That's the limitation of these methods," Schumacher added. "We can't tell depth, so we usually compare maps on different horizons."

Schumacher also cites the results of some recent GMT geochem surveys in western Venezuela -- one of which, in an old, nearly depleted field, focused on searching for areas of by-passed pay.

There, an area of high microseepage was identified next to a trapping fault, representing a part of the field that hasn't been effectively drained by existing wells. Schumacher noted there are opportunities here for several new wells and/or recompletions.

Hydrocarbon microseepage profile and seismic section across a recent discovery in western Venezuela. This is an example of a surface geochemical anomaly associated with an oil field that is just beginning to be developed. Geochemical samples were collected at 300m intervals along seismic lines to identify the probable limits of the productive reservoir. The most prospective area for future drilling occurs west (left) of the wells. Also note the low seepage values in the immediate vicinity of the 2 producing wells, possibly due to depressurization of the reservoir due to production. DOWNLOAD FULL IMAGE (67 kb)

Another geochem survey of a nearby field indicated a strong hydrocarbon seepage anomaly.

"The extent of the anomaly indicates the probable extent of the productive reservoir at depth," Schumacher said. "Such information can help guide placement of development wells and/or placement and extent of a post-discovery 3-D survey.

"The geochem profile also shows two well-defined geochem lows in the immediate vicinity of the producing wells, which is seen commonly," he said. "Seepage is dynamic, and within weeks to months of a new well going on production, the magnitude of the seepage anomaly decreases dramatically within 100 to 200 meters of the well location."

And Closer to Home ...

A documented GMT success story that occurred closer to home is the Park Springs Conglomerate field in Montague County, Texas, where a client initially identified an Ellenberger high as his drilling prospect using 3-D seismic.

A geochem survey was implemented using the Microbial Oil Survey Technique (MOST). Developed by Phillips Petroleum and modified by GMT, MOST is based on the presence of hydrocarbon microseepage above buried accumulations. Active microseepage is detected by measuring the concentrations and distributions of hydrocarbon-indicating microorganisms found in shallow soils.

Over the course of the Montague County survey, 167 shallow soil samples were collected in accordance with MOST procedures to evaluate the hydrocarbon microseepage patterns of the roughly 3.5-square-mile area previously characterized by the 3-D survey. Because this area had no producing wells prior to this project, it represents a wildcat exploration program, according to Schumacher.

The geochem survey revealed only a minor seep anomaly associated with the high, which harbored the selected drilling target. However, it showed a strong, extensive seepage anomaly over the low on the Ellenburger surface.

A well drilled on the crest of the structure turned out to be non-commercial.

"The operator then re-evaluated the geology and considered other play possibilities that might be consistent with the geochem lead," Schumacher said. "They said there must be a good geologic reason for seepage to be focused over the low and postulated there might be Pennsylvanian sands and conglomerates in that area."

They drilled and made a field discovery, which has turned into a 14-well field in an old, highly-explored area.

Four dry holes also were drilled in the conglomerate feature, but they were located in areas outside of the microbial microseepage anomalies, Schumacher noted.

"Geochem can be pointing the way, but you still have to evaluate it," he cautioned. "It has to make geologic sense."