So far in my career I have seen some characterisations of reservoir fluids, but cannot remember that anyone included alkenes.

Normally cracking of hydrocarbons is the method of producing alkenes.

gas reservoirs by the petroleum reservoir engineering definition exist in the gas phase in their native state, i.e. pressure and temperature. and we know this from a laboratory study of the reservoir fluid at reservoir temperature commonly called a pvt(pressure-volume-temperature) study. the fluid is studied at constant temperature to determine its physical properties with depleting pressure. a pvt study will normally include a separator test, or series of separator tests to model production through surface equipment and determine (at least) initial liquids yield. the other important results of such a study are the composition of the separator gas and liquid.

gas which is produced from an oil reservoir under pressure depletion will have largely the same characteristics. gas produced from an oil reservoir was originally disolved in a liquid phase with the smaller c through c6 molecules occupying intersticies between the larger liquid hc molecules.

and wrt alkenes, we dont normally analyse any h-c components heavier than c6 as individual components, they are usually lumped as one fraction.