Geochemical Prediction of Volumes and Composition of Trapped Petroleum

A. S. Mackenzie, T. M. Quigley

Predicting the potential volumes and composition of petroleum trapped in a structure or region before drilling is an integral part of petroleum exploration. Our methodology for such calculations is based on a fundamental understanding of the geologic processes that create commercial accumulations of petroleum.

To calculate the mass of petroleum expelled from the mature source rocks that feed a region or structure, we used three methods: (1) a mass balance with immature equivalents of the source, which assumes constant organic matter type; (2) an Arrhenius kinetic scheme that relates masses of petroleum generated to source rock thermal history; and (3) calibrations between molecular maturity parameters and kerogen to petroleum conversion. Iteration between the three methods gives us the most likely answers for masses of expelled petroleum, source rock thermal history, and organic matter type. By estimating the gas/oil ratio of the expelled petroleum, based on the organic matter type, we can calculate the phase relations of the expelled petroleum and, hence, the subsurface volumes of expelled oil and gas. We combine a one-dimensional model of the pore-fluid pressure history with the effects of capillary pressure and buoyancy, to predict the direction and range of migration for the oil and gas phases.

Losses incurred by migrating oil and gas are expressed as a function of the pore volumes through which they can migrate. Hence, by subtracting these losses from the expelled volumes, we obtain volume estimates for the oil and gas remaining to be trapped in a structure or region. After evaluating the sealing capacity of the cap rock, we compare the volumes of oil and gas remaining with the volume of the structure or structures to calculate our final answer--volume and composition of petroleum trapped. We also use probability theory to estimate errors in the prognosis.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.