Unconventional Gas Traps: Low Permeability Sands and Gas Accumulations
Authors: Williams K.E. , Lerche I. , Maubeuge F. 
Published: November 2022

This paper documents the presence of commercial gas production from tight, unconventional, pervasive gas accumulations in North America. The work consisted of two main phases: (i) library research and data collection; and (ii) application of both a reservoir engineering model and a basin analysis model to determine the origins of tight gas sands. The Wattenburg Field in the syncline of the Denver Basin, the Almond Sand trend in the Eastern Green River Basin, and the Canyon Sands of the Ozona-Sonora trend of the Val Verde Basin, all represent recent development projects in the USA in tight gas sands, while the Deep Basin trend of the Western Alberta Basin, including the giant Elmworth Field which was discovered in 1976, yielded a rapid development of this trend in the early 1980's.

The main products of the first phase of this work are the Field Summary Sheets of Appendix A, and the Spreadsheets of Appendix B, which collect and categorize the data about each of the fields and reservoirs. The Field Summary Sheets gather the data on fields and allow the comparison of their main characteristics, with concentration on the geological characteristics of the 40 typical reservoirs examined in the study. This database was used to try to make sense of the often conflicting values of the parameters that these fields have in common. The main text of the paper attempts to summarize and synthesize this database.

In the application to the Elmworth Field, the largest gas field of the Western Canada sedimentary Deep Basin, gas found in stratigraphic traps lies downdip from water, with no known permeability barrier between. A two dimensional quantitative analysis of the basin was undertaken in order to simulate such unconventional entrapment. The modeling shows how it is possible to accumulate gas in the low permeability sand reservoirs of the Lower Cretaceous. It appears that the Elmworth basin is a system in dynamical evolution, where gas loss due to migration is partly compensated for by contemporary gas generation. While the on-going gas generation is highly influenced by the recent paleoheat flow, the early migration has been controlled by cementation which, by lowering the permeability, reduces the gas loss with time, so that significant gas trapping can be obtained today.

Overall, the accumulations are described in terms of an impedance trap where the presence of gas is a transient phenomena in terms of geologic time. The free flow of gas from the source rock to surficial escape is impeded by the tight sands, to the point where a commercial volume of gas is trapped in the basinal syncline or on monoclinal dip on the basin flanks. This paper describes how and why this trapping mechanism works, and what the accumulations have in common.

The work presents a set of conditions leading to such unconventional gas accumulations and discusses the parameters influencing the total amount of gas accumulated at present day; characteristics of the sediments, such as porosity and permeability behavior through time, cementation parameters, amount of sediments removed during erosion events, or heat flow variation with time, are also bracketed in order that gas accumulations predicted are in agreement with those observed today. The underpressure due to gas saturated formations is also considered.