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W ELCOME T O C RAIN'S   P ETROPHYSICAL  H ANDBOOK   Please be fair to the author. Pay your Shareware fee HERE, and receive the CD-ROM at no extra cost. SPECIAL CASES -- TIGHT GAS RESERVOIRS   Tight Gas Basics Geology Shale Volume Porosity Water Saturation Permeability Lithology Pyrite Corrections Tight Gas Example TIGHT GAS BASICS  Most of us are familiar with traditional tight gas reservoirs   clean, low porosity sandstones or siltstones that look unattractive on log analysis, at least by the conventional wisdom of the 1960s. Porosity averages from 5 to 10% with permeability between 0.01 and 5 mD. The best know play of this type is the Alberta Deep Basin, developed in the 1970's and 1980's, and still an area of interest today.  This particular tight gas play is called a basin-centered gas accumulation - the trapping mechanism is not structural or stratigraphic, but a "water block" above the gas caused by low relative permeability. There is considerable exploration and development effort being expended on such plays toda y, especially in the USA, Europe, and the Middle East. With the steady improvement in massive fracturing jobs, pioneered in the Deep Basin, and in horizontal well placement, even tighter, lower porosity gas zones are now economically feasible. These have porosities that may average 3 to 6% and permeabilities from below a microDarcy to a few milliDarcies. The Doig and Montney in Alberta and northeast BC are examples. Both are radioactive due to uranium and have often been called shales, even though the average grain size is above 4 microns and there is little clay mineral or clay sized particles. The organic content is fairly low (1 to 3% TOC) so there is little adsorbed gas. They do not qualify as "shale g as" until 67% of the particles are less than 4 microns.  Some tight gas plays have a significant liquids component. Such wells are highly desirable due to the price differential between gas and oil. Unfortunately, log analysis cannot partition gas from oil at these low porosities so tight rock core analysis and petrographic analysis are important tools. Mineralogy and clay content are highly variable so XRD analysis is also vital.  <== Alberta Deep Basin strat chart, show ing gas pro duc ing reservoirs, circa 1981. Note that Montney was not amo ng the prod uctive category at that time.  

Tight Gas Reservoirs

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