OBVIOUS GAS SAND WITH GAS SHALE (SILT
This case history shows a number of obvious and not so obvious features of petrophysical analysis in sands and shaly sands. The obvious is the huge amount of crossover on the density neutron log in the clean sand. Combined with the high resistivity, there is no doubt that this is a gas zone. Even the sonic log shows gas effect in the clean sand.

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.

Above the obvious sand, there is a silty / sandy section up to a few meters below the top of the log segment. There is no seal on the top of the obvious gas sand, so gas must be present in any pore space within the silt. This interval is typical of many so-called "gas shales". Many gas shales are not really shale, they are silts with some porosity and very low permeability. With modern hydraulic fracturing techniques using slick water, these zones can make commercial gas wells if they are thick enough.

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 clean gas zone is obvious because of its high resistivity and density neutron crossover. The sonic log tracks the density log and shows the same amount of crossover. The shalier sands above and below the obvious sand are less clear - are they wet, or just wetter because they are finer grained? The tests prove that there is no moveable water below the obvious gas and there is producible gas in the low porosity silt above the obvious gas. The silty interval above he clean sand is typical of what is now called "gas shale". Mud gas logs (not shown) indicate the presence of gas well above the tested intervals.

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.