Helium Exploration - A 21st Century Challenge
HGS North American Dinner, 23 February, 2004
As we enter the first decade of the 21st Century worldwide helium demand is rising as many high tech industries are developing new commercial applications that are dependent on the unique physical properties of helium. The need for continued adequate supplies of helium in the 21st Century will be critical. To fulfill the anticipated future demand for helium, a new approach to helium supply is likely needed soon - the deliberate search for helium-rich gas reservoirs.
In December 2005, the 100th anniversary of the discovery of helium in natural gas will be celebrated in Dexter, Kansas. Up until 1905 helium was a laboratory curiosity, having only been discovered on Earth in 1895. The detection of helium in natural gas at Dexter was followed in the next three decades by many discoveries of small to giant gas fields in Kansas and the Panhandle regions of Oklahoma and Texas that held helium concentrations between 0.5% and 2%. Some contained upwards of 7% helium. In later decades helium-rich oil and gas fields were discovered in Colorado, New Mexico, Arizona and Wyoming, with some holding as much as 10% helium. Helium has been produced in commercial quantities at few other sites around the world. Although Canada, Russia, Poland, and Algeria have produced helium in commercial quantities, none have discovered reserves or helium concentrations comparable to those found in the United States. As a consequence, the United States has been the world's principal source of helium for over 80 years, and both the U.S. petroleum industry and the U.S. government have played key roles in establishing ample supplies of helium throughout the 20th Century.Nearly all helium gas reserves have been discovered serendipitously during the normal course of oil and gas exploration and development. As a result, there has been little need to develop any special geological expertise to target helium-rich gases in order to provide an adequate supply of helium. Scientific and industrial applications that were developed during the 20th Century that required helium were sustained by the abundant and reliable supply of helium associated with high BTU gas production.
Review of the geologic framework of helium accumulations, and recent findings from ancillary fields of study that include isotope and noble gas geochemistry, geohydrology, seismology, volcanology, mineral exploration, and geothermal exploration provide grounds for formulating strategies for the exploration of helium-rich natural gas. The principal source of helium is from the steady rate of radioactive decay of uranium (average: 2.8 ppm) and thorium (average: 10.7 ppm) found throughout the crust. Over geologic time substantial radiogenic, and some nucleogenic helium, will accumulate within the mineral crystal lattices and microfractures in crustal rocks. Degassing of this helium from the crust has been the subject of many studies, including deep crust research drilling. Concentration profiles of helium in ultradeep wells, and that found in confined groundwater systems that hold helium concentrations orders of magnitude higher than can be accounted for by steady-state helium diffusion, have demonstrated the necessity for periodic tectonism to increase fracture permeability of the crust and allow advective transport of helium. Helium soil gas, and helium groundwater surveys adapted for uranium and hydrocarbon exploration have long revealed the association of high helium flux with faults, lineaments, and natural springs. Recent recordings of short-term, voluminous discharges of helium following strong earthquakes have aptly demonstrated the role of seismicity in periodic releases of helium from the crust.
While 4He flux is largely accounted for by degassing of radiogenic helium from the crust, the rare 3He isotope originates from the mantle where the isotopic composition of helium is ~1000 times greater than that found in the Earth's crust (Mantle 3He/4He ratio = ~10-5, Crust 3He/4He ratio = ~ 10-8). The discovery in 1969 of 3He flux from mid-ocean ridges, and subsequently in volcanoes, intra-plate hot spots, and high-temperature geothermal systems, has provided earth science with a highly sensitive tracer of mantle derived gases. He-rich gas fields have variable, but mostly distinctively elevated 3He/4He ratios, providing evidence that degassing and release of mantle volatiles accompanied significant releases of helium from the crust. Commencement of magmatic activity within a volume of long stable, aseismic crust would mobilize long-held helium as levels of seismicity, crustal heat flow and rock temperatures increased.
Formation of economic helium-rich gas deposits can develop when periodic episodes of high He flux are swept up by an overlying regional aquifer, effectively transported, and concentrated in a reservoir trap. Since helium has very high diffusion rates in sedimentary rock, high-density rock seals are needed to effectively slow the relentless flux of helium through the sedimentary column and into the atmosphere where it would eventually be lost into space. Rock types such as anhydrite, salt, or dense (Paleozoic) shale provide the most effective top (or lateral) seals. The first sandstone or carbonate reservoir (commonly Paleozoic, or Mesozoic in age) above the crystalline basement are favorably situated to capture, transport and "temporarily" hold helium-rich gases. Effective helium transport in an aquifer and retention of helium in a trap is further augmented if the reservoir is underpressured. A negative pressure gradient across the cap rock of an underpressured reservoir would be effective in retaining a higher concentration of helium for a longer period of geologic time. Field studies indicate an underpressured reservoir with a cap rock of anhydrite or salt provides the ideal geologic framework for the accumulation of helium gas.
Incorporating recent isotopic studies of noble gases by Ballentine and Lollar (2002), a geologic model is presented to explain the collection, movement and accumulation of over 600 BCF of helium in the Hugoton-Panhandle gas field and adjacent areas. The model recognizes the key sequential geologic events that occurred to form the world's largest helium-rich gas district. These geologic events included: 1) uplift of the Rocky Mountains, 2) development of an east-dipping, underpressured hydrogeologic system of basal Paleozoic clastics and carbonates, 3) deposition of Permian shale, anhydrite, and salt, and 4) Late Oligocene-Early Miocene volcanism of the Spanish Peak region of southeast Colorado.
Not since the helium gas boom years in the Four Corners area of the Southwest in the 1960s has the American petroleum industry given much thought to targeting helium-rich gas fields. As the 21st Century begins the need for the exploration for additional helium reser