Title: Response to: “Are Tight Gas Resources Overstated?” by Pat Roche in the December 2004 RMAG Outcrop
Author: James E. Fassett, Scientist Emeritus, USGS, and Independent Consulting Geologist
Publication: The Outcrop, March 2005, p. 12-14
As a geologist who has worked in the “Rockies” (an area I prefer to call the Western Interior, by the way) for going-on 45 years, I found the subject article on “basin-centered gas” very interesting. It appears that the discussions, dialogue, and disagreements about this “trapping mechanism” continue to be ongoing and lively. From the moment the concept of “basin-centered gas” first began to be discussed (particularly by some of my USGS colleagues) as a generic kind of gas-trapping entity, I was confused about the general physical principles that supposedly defined this model and how and why it could be considered to be a generic model that could be applied to a number of Western Interior basins. True, there is gas in the centers of many Western Interior basins, but there did not then (and there does not now) appear to exist a set of general, universal, physical principles that applies to the trapping mechanisms for all these gas fields in the “centers” of basins. The overriding concept for “basin-centered gas” seemed to be that the gas was trapped in low-permeability rocks in the centers of these basins because it was “sealed” in these rocks by an up-dip water seal. I have always thought that this concept was counter-intuitive because, in my mind, gas being lighter than water, the gas would move upward through the water in whatever kind of strata it occupied, assuming there was some permeability present. (I tend to think that the up-dip-water-seal concept was not challenged much because many geologists assumed there was some kind of arcane physics at work here that they did not understand and they were thus loathe to challenge those spooky ideas, and thus appear ignorant in the eyes of their colleagues; “the emperor has no clothes” syndrome?) I think that the traditional trap concept we have all studied, where there is normally a three-tiered geometry with a gas cap on top, a layer of oil in the middle, and water at the base, still has much to recommend it. These fluids achieve a stable configuration, through time, based on their densities. (Of course we all know of oil and (or) gas reservoirs where there is little or no water in the system.)
My back yard (the San Juan Basin) has been frequently referred to as a prime example of “basin-centered gas”. In the December Outcrop paper by Pat Roche (what a cool name for a geologic reporter), John Masters’ thoughts on this subject are discussed as follows:
Masters, who now runs his own company in Denver, says there are more than two dozen basin-centered fields in the U.S. He cites the San Juan Basin – the second-largest gas field in North America – as an almost perfect basin-centered accumulation. “And those guys are not ever going to tell me that the San Juan Basin is not a basin-center accumulation in the basin syncline rimmed all the way around by water. The water holds that gas in. There is no question about that.”
Maybe “those guys” won’t, but I will. In reality, the San Juan Basin’s principle sandstone reservoirs, in Masters’ “almost perfect basin-centered accumulation” consist of a sequence of three discrete fractured-sandstone reservoirs in which the gas is contained in stratigraphic traps. (Of course the world-class Fruitland Formation coal-bed methane reservoir in the “center” of the San Juan Basin contains gas that is, for the most part, adsorbed in the organic coal matrix of many hundreds of discrete coal beds and thus can in no way be thought of as a “basin-centered gas” deposit, as defined by earlier workers.) In a paper I wrote on the Oil and Gas Resources of the San Juan Basin (published in 1991 in the volume: Economic Geology, U.S., of the GSA’s DNAG series) I addressed the theory that gas in the San Juan Basin was trapped in the major sandstone reservoirs by an up-dip water seal, as follows:
Natural gas in the three major gas-producing sandstone units, the Dakota Sandstone, Mesaverde Group, and Pictured Cliffs Sandstone, is concentrated largely in broad, overlapping, northwest-trending stratigraphic traps in the south-central part of the basin (Fig. 6). Each of these gas-bearing sandstone units is present throughout the basin, and each crops out around its periphery. The Pictured Cliffs Sandstone is missing in two narrow areas on the east side of the basin. A relatively small amount of gas is present in these three units in smaller stratigraphic and structural traps in the northern and southeastern parts of the central basin. With no apparent trap, why hasn’t all of the gas migrated up dip through the sandstone beds to the outcrop and escaped from this 45-m.y.-old structural basin?
Berry (1959) was the first worker to offer a solution to this conundrum; he suggested that the gas in these sandstone units had somehow been prevented from escaping by hydrodynamic forces. The argument was that these gas-bearing rocks were underpressured, principally as the result of uplift of the Colorado Plateau in Miocene time and the rapid erosion of a few thousand meters of overlying rock. The resultant pressure release on the gas-bearing sandstone beds caused dilation of the pore space in these rocks and a decrease in temperature. With this pressure drop, meteoric water moved centripetally down the pressure gradient through the three gas-bearing sandstone units, sweeping the gas toward the deepest part of the basin. Berry’s (1959) hydrodynamic-trap model was embraced by most subsequent workers. The hydrostatic-gas-trap hypothesis was recently evaluated by Cumella (1981, p. 168-175). Cumella discounted the hypothesis to some extent, but did not completely dismiss it; further, he suggested that secondary kaolinite cement in the southwestern part of the basin, along with stratigraphic trapping of gas in the stair-steps of the Point Lookout and Pictured Cliffs Sandstones (Fig. 3), are also factors contributing to the trapping of the gas in these rocks.
A comprehensive review of all available publications on the stratigraphy of the three sandstone units, personal observations of these rock units on the outcrop and throughout the subsurface on geophysical logs, and my own detailed studies of the Pictured Cliffs Sandstone throughout the basin lead me to the conclusion that most of the gas trapped in the Dakota, Point Lookout, and Pictured Cliffs is stratigraphically trapped and that the hydrodynamic gas-trap [Basin-centered-gas] hypothesis, however elegant in a theoretical sense, is a chimera. On the basis of all available information, it seems apparent that the gas is trapped in the three sandstone units by permeability barriers. Reservoir characteristics of these formations are clearly structurally enhanced by fracturing, but little has been published on the subsurface fracture pattern of the basin. Even though these three gas-producing sandstone bodies were considered in the past to be sheet-like sandstone layers with interconnected permeability basinwide, subsequent studies have shown that each of them consists of a complex of individual sandstone beds separated by impervious mudstone layers. Berry’s (1959) concept of a hydrodynamic trap for these sandstone units was proposed before the stratigraphy of these rock units had been worked out in detail. More recent work on these rocks, discussed below, does not support the concept of interconnected permeability of these rock units throughout the basin.
At the time I wrote those words, I suspected that further investigation of “basin-centered gas” in other Western Interior basins would yield similar conclusions, and the work of Shanley, Robinson and Cluff in their studies of the oil and gas fields of the Greater Green River Basin (discussed in the December Outcrop) support those earlier suspicions. I also find it interesting to learn that the author of the Outcrop piece seems to find that the Shanley and other’s conclusions are so surprising when he writes:
“If the conclusions are correct, it turns a quarter century of tight-gas thinking on its head.”
Not really, in my opinion. Oil and gas prospects that first sell and second end up producing hydrocarbons in commercial quantities are almost always grounded in sound, basic geologic principles and not on speculative, arm-waving academic concepts. Some plays, admittedly, are pretty far out and sometimes (oftentimes?) a successful prospect produces oil or gas for reasons other than those conceived of by the prospect developer. (Think of the giant Aneth field of the Four Corners area that was first drilled by Texaco as a structural trap (and coincidentally as leases were about to expire) and ended up producing from an “algal reef”.) Or, even more fun, the Dineh-bi-Keyah oil field in the Chuska Mountains in the northeastern-most part of Arizona discovered by Kerr McGee drilling on a small anticlinal structure and discovering a nice little oil field that produced from a Tertiary syenite sill intruded into Pennsylvanian black shales! We must all be increasingly ingenious in trying to find untapped oil and gas deposits in times when all the “bird-nests-on-the-ground” have long since been discovered. In my opinion, however, playing an up-dip-water-seal concept will never find us a spoonful of oil or a puff of gas. “Basin-centered gas”, has been an interesting topic of conversation over the last couple of decades, but it is now time to put it to bed along with such concepts as “crickology” (which actually, in a few cases, had geologic merit), or the elusive search for hydrocarbons in igneous rocks. These may be fun subjects to talk about in your spare time, but not ones you could take to the bank.
It is always discouraging (and startling) for any author to learn that his “words of wisdom”, published years ago in a reputable peer-reviewed journal (quoted above), have apparently gone unread for so long. Perhaps if I had published my San Juan Basin oil and gas paper in an AAPG publication (or better still, The Mountain Geologist!), rather than in the more “academic” GSA, the word would have gotten out sooner, who knows? I will conclude by suggesting that John Masters (and the author of the Outcrop article) do their homework a bit more carefully in the future.