Operators have produced more than 53 million barrels of oil/condensate and 845 billion cubic feet of gas from Mississippian-age Leadville limestone in seven fields in the northern Paradox Basin region, referred to as the Paradox fold and fault belt, of Utah and Colorado.
Even so, only 100 wells have penetrated the Leadville over the entire 7,500 square-mile Paradox Basin. This equates to about one well per township, according to David Seneshen, vice president of Vista Geoscience (formerly Direct Geochemical).
The reason for the sparsity of wells is pretty straightforward.
“Exploration for Mississippian Leadville-hosted hydrocarbon reservoirs in the Paradox Basin is high risk in terms of cost and low documented success rates of about 10 percent, based on drilling history,” Seneshen said.
“But the potential for more hydrocarbon reserves is enormous,” he added.
However, money looms as a major obstacle to stepped-up exploration efforts, despite today’s supercharged industry environment.
For the most part, it’s independents who operate the Leadville wells, and they don’t have the deep pockets required for big exploratory 3-D seismic shoots in this environmentally sensitive region.
Thanks to low cost surface geochemical technology, this may cease to be an
issue.
Looking for Clues
Simply put, this technology focuses on various organic and inorganic geochemical anomalies in surface soils, outcrop fracture-fill soils and vegetation, and six-foot deep free gas that result from migration of hydrocarbon molecules from subsurface reservoirs up to the surface along fractures, joints and bedding planes.
The Utah Geological Survey (UGS), along with Direct Geochemical, recently completed a geochemical study in the Paradox Basin, which was funded by the U.S. Department of Energy – a report will be available near-future via the UGS.
The program was implemented over the Lisbon and Lightning Draw Southeast fields
in Utah. It was designed to test the effectiveness of several conventional and unconventional surface geochemical methods for predicting productive andnon-productive Leadville carbonate reservoirs.
“The main objective in testing these techniques is to find low-cost, non-invasive geochemical exploration methods that could be used to pre-screen large areas for subsequent geophysical surveys and lease acquisition,” Seneshen said, “and also act as follow-up to classify geophysical anomalies as (either) productive or barren.”
“Most of the samples we collected were surface soils,” he continued, “but because there’s so much outcrop, we also sampled soil and vegetative tissue (bryophyte moss and lichen) from joints in sandstone outcrops to assess organic and inorganic compositional signatures of ascending seepage.
“Both the surface soils and outrcrop fracture-fill media work well for distinguishing between productive and non-productive areas.”
Free = Good
Free gas sampling also produced good results.
“I don’t think anyone has ever tried free gas over a Leadville reservoir,” Seneshen said, “drilling down six feet and extracting air from the soil pores and analyzing that gas.
“We only did this over Lightning Draw Southeast field, and it worked especially well,” he noted. “We got a good, distinct high-contrast hydrocarbon anomaly right over the gas cap of the field.”
In fact, the free gas sampling technique is recommended as a follow-up to anomalies identified in a regional soil geochemical survey.
“Once you’ve found areas to focus in on and have shot 3-D and found structures,” Seneshen said, “then you go over this with a Geoprobe and punch holes in the overburden and extract free gas out of the soil to better define the anomaly as to where you want to drill.
“You start big and then gradually focus in with the geophysics and more detailed geochemistry using free gas instead of the soils,” he said, “the reason being the anomalies we see are quite narrow, like 700 feet across. This means you must take samples at short intervals.
“Because you’re using a Geoprobe drill, it gets a little costly, but it’s still a fraction of what seismic costs,” Seneshen added. “And it’s an excellent technique for following up seismic anomalies to see if there are hydrocarbons in the reservoir or just carbon dioxide and nitrogen – those reservoirs are only about a third hydrocarbons, a third carbon dioxide and a third nitrogen.”
High Impact
Results from the recent study conceivably could have a major impact on further Leadville exploration in the Paradox Basin.
The main conclusion derived from the effort is that certain low-cost, non-invasive geochemical methods are effective as pre-screening and follow-up tools for exploring Leadville hydrocarbon reservoirs, according to Seneshen.
He noted they can see both of the project’s target fields using combinations of hydrocarbons and trace metals in the soils and vegetation.
“We can use a regional technique taking samples every 1,500 feet,” Seneshen said, “to focus further geophysical surveys or lease acquisitions.
“This conceivably could be applicable to other basins,” he added, “but you would have to test there first, because with surface geochemistry every area is different.”