Cracking kinetics of crude oil and alkanes determined by diamond anvil cell-fluorescence spectroscopy pyrolysis: technique development and preliminary results

Non-conventional diamond anvil cell (DAC) pyrolysis techniques have been developed to monitor in situ the cracking of oils and hydrocarbon compounds using fluorescence spectroscopy (FS). The experiments were conducted under closed-system conditions at five programmed heating rates up to a maximum temperature of 650°C. The fluorescence of oil reveals two major stages of intensity change during cracking pyrolysis. This appears to be the first reported in situ observation of fluorescence behavior of oil during cracking. The first stage of fluorescence change, which shows an abrupt decrease in intensity of the liquid phase and a simultaneous increase of intensity of the coexisting vapor phase of oil samples, occurs in a temperature range close to that for the cracking of heavy fractions of oils. The kinetic parameters calculated based on the fluorescence intensity change are close to those for the cracking of black oils to light oils, thus we have proposed a potential method for estimating the relative depths or temperatures for cracking of different black oils in the subsurface. The second stage, which shows a steep decrease in fluorescence intensity of the vapor phase, occurs at higher temperatures and is probably attributed to the conversion of light hydrocarbon to gases. The experimental results using pure saturate compounds, including normal alkanes and a diamondoid, also show significant fluorescence intensity changes in the temperature range corresponding to the decomposition of the compounds. The convenient technique may be used as a screening tool to study the effects of geological parameters, such as pressure, water, and redox conditions, on the cracking kinetics of liquid hydrocarbons.