A new exploration strategy for a tough play may make life much more profitable for Rocky Mountain explorationists -- as well as hunters throughout the world.
The new strategy focuses on several significant discoveries of "deep," basin-centered gas accumulations in the Rocky Mountains, where companies have found that the tried-and-true exploration techniques can be disappointing and costly.
But help in exploring for this "deep" gas is coming from the Institute for Energy Research at the University of Wyoming, where a new strategy already is substantially reducing the risk associated with basin-centered gas fields.
Field studies have proven the usefulness of these techniques, which may ultimately be applied to similar basins all over the world.
Ronald C. Surdam, former director of the Institute, professor emeritus at the University of Wyoming and a past AAPG Distinguished Lecturer, spearheaded the team of IER scientists (Zun Sheng Jiao, Nicholas K. Boyd III, Sharon Kubichek, Henry Heasler, Peigui Yin, and John Buggenhagen) credited with developing the new exploration methods.
"Drilling in the past several years has proven that the center of the basins in the Rocky Mountains and other regions around the world can be charged with gas. Where present, these accumulations are relatively deep, typically occurring at depths greater that 8,000 feet, and are anomalously pressured," Surdam said.
"Historically, most hydrocarbon exploration in the Rocky Mountains and elsewhere has taken place at shallower depths, where the fluid is predominantly water, the rocks are normally pressured and hydrocarbon accumulations are typically confined by structural closure or conventional stratigraphic traps.
"But deeper in these basins -- below a present-day depth of 7,000 to 9,000 feet -- there is a ubiquitous free gas phase in the fluid, which creates a multiphase fluid-flow system," Surdam continued.
"It is much more difficult for these multiphase fluids to move through a porous medium -- thus, gas tends to stay within the deeper parts of these basins."
A Velocity Slowdown
Typically in these Rocky Mountain basins -- and other basins around the world -- a marine transgression has occurred, resulting in shales and other fine-grained rocks being laid down across the basin, Surdam said.
If these fine-grained, low-permeability rocks contain multiphase fluids, they will act as capillary or "pressure" seals for any hydrocarbons beneath that zone.
"We have known for some time, based on research using sonic velocity logs, that there is a significant velocity slowdown, or inversion, below these regional seals in Rocky Mountain basins, as well as other basins," continued Surdam, a past AAPG Distinguished Lecturer who presented the paper, "A New Exploration Strategy for Unconventional Basin Center Hydrocarbon Accumulations" at the AAPG annual meeting in San Antonio.
"The new exploration strategy we have developed -- in conjunction with Gas Research Institute and the Department of Energy -- focuses on the determination and, if possible, three-dimensional evaluation of the regional pressure boundary between normal and anomalous pressure regimes."
However, identifying the regional pressure seal and what causes the velocity slowdown below the pressure seal is just part of the equation. The really difficult task is finding the hydrocarbon accumulations below that regional seal.
"Once the pressure boundary is delineated, the next step is the detection and delineation of areas of enhanced deliverability and storage (i.e., porosity and permeability), or 'sweet spots' below the pressure boundary, which are more likely to contain large amounts of gas.
"Conventional exploration techniques are not as useful below this boundary," Surdam said, "because these gas accumulations are not necessarily characterized by structural closure or conventional stratigraphic traps -- and in the past 50 years, exploration strategies have focused on finding those kinds of scenarios.
"So, if you're looking for a giant gas field like the Jonah Field in Wyoming, which is not a conventional accumulation, you can't go looking for large anticlines (see related story). To search for these basin-centered gas plays, it's necessary to add some new tools to that toolbox of conventional technology.
"In fact," he added, "delineating the pressure boundary and locating 'sweet spots' is as important to finding gas in the deep, gas-saturated, anomalously pressured section of a basin as structural closure and stratigraphic traps are in a basins's water-saturated, normally pressured section."
New Tools Needed
Conventional wisdom says that the velocity slowdown observed in Rocky Mountain basins is a result of the rocks being undercompacted. And in the Gulf Coast, where the conventional model was developed, this is certainly the case.
"However, in the Rockies, where we've examined virtually every one of the basins, we see no undercompacted rocks," Surdam said. "You can't explain the velocity slowdown using the conventional model -- however, every one of the Rocky Mountain basins we've studied has a gas phase below the regional pressure seal that likely accounts for the inversion.
"In fact, research has shown that a very significant velocity slowdown occurs if there is 20 percent or more free gas in the fluid phase."
Surdam notes that in the Rockies, it is much more likely that any observed velocity inversions have resulted from gas being present in the multiphase fluid-flow system below the pressure seal than from undercompaction. This theory has been proven by observing natural phenomena, as well as by a recent study in cross well tomography that tracked the advance of a CO2 flood.
This test demonstrated that the addition of CO2 (i.e., the addition of a gas phase to the fluid), caused a significant velocity slowdown in the rocks. Scientists were able to study the sonic velocity before and after the CO2 experiment, and discovered that the addition of the CO2 gas phase to the fluid caused a velocity slowdown of 10 to 20 percent.
Goal Setting -- And Attaining
So, in response to the petroleum industry's request for a new and better way to search for and exploit these accumulations, the Institute for Energy Research, GRI and the DOE undertook a study of the pressure compartmentalization of deep gas with four goals in mind.
The exploration techniques developed as part of that study provide the diagnostic tools needed to uncover additional gas reserves -- and to eliminate a great deal of the risk currently associated with developing basin-centered unconventional gas plays.
These methods give operators the ability to determine the position of the regional pressure boundary and allow for evaluation of the 3-D aspects of the pressure boundary surface, with special emphasis on areas characterized by positive relief, Surdam said.
Topographic highs on the regional pressure boundary, or velocity inversion surface, are a result of gas rising into the stratigraphic column along conduits and into porous reservoir units. Commonly, natural fractures play a major role in providing these conduits for the gas to reach a reservoir.
"The techniques mainly involve using well log and seismic data to create visualizations that can give us an idea of what is happening in the subsurface. The diagnostic tools ... first determine the position of the pressure boundary by establishing a velocity anomaly in individual wells using sonic logs," Surdam said. "These sonic logs are then combined to form a profile along a line of interest in order to evaluate the two-dimensional aspects of the boundary.
"The depth of the onset of the velocity inversion indicates the pressure boundary and the top of an anomalously pressured compartment," he continued. "If additional velocity data are available, the next step is to extend the analysis of the velocity inversions to three dimensions by adding the additional coordinate to the spatial analysis.
"Because previously drilled wells are irregularly spaced, well log profiling is plagued by significant data voids over which the velocity data are linearly interpolated from well to well. Seismic interval velocities can be used to greatly reduce this interpolation distance, thereby significantly improving spatial sampling.
"The velocity inversion is apparent on both sonic well logs and seismic data, but because well data provide a wealth of independent subsurface measurements, these data can best identify the cause of the inversion. Therefore, well data are used to constrain the interpretation of the seismic velocities.
"By using sonic velocity profiles to constrain the seismic data," he concluded, "we have more confidence in using seismic velocities."
The 2-D and 3-D models created using these data are then used to help establish which depositional facies below the pressure boundary constitute porosity and permeability "sweet spots."
"We have observed areas of locally extreme seismic velocity inversions between wells and in wildcat settings in two and three dimensions," Surdam noted. "Often these inversions correspond to porosity and permeability 'sweet spots' below the pressure boundary."
These models also are used to help document the potential determinative elements, such as fractures, overpressuring, dissolution or early migration of liquid hydrocarbons that have controlled the development of "sweet spots" in the targeted lithofacies.
Surdam hastened to add that "ProMax seismic processing software and Landmark interpretation software provided to IER by Landmark Graphics Co. have greatly expedited the development of these concepts. Moreover, Geodynamics Geographic EarthVision™ software has been used extensively in our geospatial modeling and has made possible the vastly improved visualizations of the anomalous velocity distributions."
Wind River Basin Test
Surdam and his colleagues recently tested their new exploration techniques in Wyoming's Wind River Basin. Tom Brown Inc., came to the Institute for Energy Research to help unravel the mysteries on a large concession on the Wind River Indian Reservation.
"Tom Brown requested that we do a study over their acreage and try to develop some exploration leads," Surdam said. In addition to Tom Brown and partner Belco Oil and Gas, GRI collaborated in the study.
Tom Brown provided the IER scientists with data over an area covering 1,500 square miles -- including 58 wells with sonic logs, 20 2-D seismic lines covering about 160 miles, and two 3-D seismic surveys covering 60 square miles.
"The first step we took was to study the velocity field associated with five commercial gas fields in the area," he said. "All five were producing from below the regional velocity inversion surface, and all were characterized by velocity anomalies.
"Now we had a clue," Surdam continued. "We knew that if we were going to develop new leads in the area, we needed to look for anomalously slow velocity domains below the regional inversion surface."
The team members were able to do that by putting together an orthogonal grid with the data and modeling the region, using both sonic velocity and seismic velocity, thereby allowing them to define the regional velocity inversion surface and to look sweet spots below the inversion.
At the same time Surdam and his colleagues were conducting the study for Tom Brown, the DOE was looking to support work demonstrating new and innovative ways to look for hydrocarbon accumulations. As a result, the Institute for Energy Research formed a research partnership with the DOE, Snyder Oil and Belco Oil and Gas to examine an area on the southern edge of the Wind River Basin, where Snyder had acquired two 3-D seismic surveys.
"The better resolution provided by the 3-D data allowed us to go from an exploration lead to a prospect level in this area," Surdam said. "Using our techniques, we determined the regional inversion surface, looked at the topography of the inversion surface, and searched for anomalously slow velocities below the inversion, and, consequently, we were able to identify some prospective velocity anomalies."
Snyder has recently drilled five wells into one of the velocity anomalies identified in the study. The wells within the heart of the anomaly had initial production of two to four million cubic feet of gas per day; wells at the edge of the anomaly had initial production of about one million cubic feet of gas per day.
One well drilled outside the anomaly had initial production of less than one million cubic feet a day and was shut in.
The Word is Spreading
At least seven operators working in the Rocky Mountain Laramide basins and four operators working internationally have examined the potential of these new exploration techniques.
"Our method has been tested in 24 basins around the world," Surdam said. "In some basins, it's worked extremely well -- and in others not quite so well, but in every basin the new techniques have been beneficial in determining the top of the regional velocity inversion, or the anomalous pressure boundary."
In addition to North America, Surdam and his colleagues have tested their technologies in China, Indonesia, Colombia, offshore West Africa and Argentina.
"We identified some potential leads for YPF, Argentina, in the Neuquen Basin, and the first well drilled off-structure and based on our work is a success, producing both gas and condensate," Surdam said.
Huge discoveries like McMurry Oil's Jonah Field in the Greater Green River Basin of southwestern Wyoming, where the best wells have initial production rates of up to 16 million cubic feet of gas per day, continue to fuel the search for basin-center gas.
"The Standard Draw-Echo Springs Field in the Washakie Basin of the Greater Green River Basin is another example of a large, basin-centered gas accumulation," Surdam noted. "Ultimate reserves for the field likely will exceed one trillion cubic feet of gas before it's depleted.
"There is little doubt that basin-centered gas is one of the hottest exploration play types in the world today," Surdam said. "Natural gas is becoming a more attractive commodity.
"About 90 percent of the gas discovered in the world to date has been found above the regional velocity inversion surface in most sedimentary basins, but likely 90 percent of the hydrocarbons to be discovered in the future will occur below this surface."
Surdam and his associates at IER are enthusiastic about the future of the new exploration strategy. Recently, McMurry Oil of Casper, Wyo., and IER have agreed to test extensively the newly developed strategy for searching for and exploiting basin center gas accumulations in Rocky Mountain Basins.
The IER methodology will be applied to exploitation of the giant Jonah gas field, and to the search for new, basin-center gas prospects in the Rocky Mountains.
"What makes this project so unique," Surdam said, "is that, unlike the case with so many scientific theories, we will have the opportunity to rigorously test our exploration paradigm in a real-life exploration situation."