OBVIOUS GAS SAND WITH GAS SHALE (SILT
However, as we go below the obvious gas crossover, we see lower resistivity, no crossover, and the zone gets somewhat shalier according to the gamma ray log. So is the zone wet because we have penetrated the free water level, or is it wetter just because the grain size has become smaller, giving higher irreducible water saturation in a gas zone? The best way to find out is to test the zone and see what you get. After a year of drilling and mapping these wells, I finally convinced management to test the lower part of the sand and we got gas. In the absence of test data, the log analysis could be pushed either way by moving RSH; lower gives more gas, higher gives less gas. A depth plot of shale corrected density and shale corrected neutron also helps, as some gas crossover exists throughout the interval.
In addition to free gas in the effective pore space, there is adsorbed gas attached to the surfaces of any cleats or fractures in the shale. Adsorbed gas is not part of the hydrocarbon volume determines by PHIe * (1 - Sw);- it is found by empirical methods based on the assumed surface area of the cleats, temperature, and pressure. Adsorbed gas can add from a few percent to 100 percent more gas in place than the free gas alone.
This example is about 50% clay and 50% non-clay minerals. (above the obvious gas sand) The mud log on the well shows gas throughout the interval. I managed to coax a test in the best porosity of the silty interval and proved the presence of gas. This was done back in 1975, long before gas shales hit the news. In addition to free gas in the porosity, there will be adsorbed gas in the clay fraction, which adds to the reserves for this field. This drilling project proved up 13 Trillion cubic feet of gas in the obvious gas sand. How much more lives in the shaly sand below and the silty shale above? All of the gas is still there because none of it has been produced due to difficulties in connecting to markets.
The older computed log analysis (right) illustrates the thinking of the era, namely that the upper silty shale could not hold much gas, even though the mud log showed gas throughout.
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