2014 Rocky Mountain Section AAPG Annual Meeting

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Natural and Artificial Cracking of Oil Source Rocks and Unconventional Reservoirs


Source rocks expel oil and gas when the internal pressure generated by volume increase reactions due to the conversion of kerogen to solid bitumen and then to oil and gas exceeds the geostatic pressure. The rocks naturally rupture, oil and gas are expelled or pushed out, and the pressure drops below geostatic. This process is repeated many times as the source rock passes through the oil and gas maturity “windows”. Primary oil and gas expulsion is very inefficient with less than twenty percent of the oil and gas generated ever leaving the source rock. Unconventional reservoirs are most commonly suspended oil source rocks. They were once active but stopped generating prior to becoming spent due to cooling associated with overburden removal. Internal pressure is between hydrostatic and geostatic depending on sealing capacity and other factors. Hydrocracking artificially ruptures the source rock and some additional oil and gas are pulled out by differential pressure between the source rock and the well bore. Like primary oil migration, the process is very inefficient and less than ten percent of the oil and gas remaining in the source rock is produced. Many factors control the efficiency of both natural oil expulsion and artificial fracking oil and gas production. Some of these include the concentration and distribution of organic matter in the source rock, the anisotropy of both the organic matter and lithological units within the source rock, the brittleness variations of source rock lithologies, natural and artificial frac barriers, oil and gas composition at various maturities, differential permeability of produced products, and a host of other factors which affect the efficiency of both source rocks and unconventional reservoirs.