Modelling Primary Hydrocarbon Expulsion in a Synthetic Sedimentary Section: Relative Contributions of Edge Conditions and other Geological Factors

Abstract

The effects of numerical edge conditions and of the relative contributions of geological processes or factors to primary hydrocarbon migration are studied using the GEOPETII program developed by the Basin Modeling Group at USC. Lateral edge conditions can have important effects on the results of modelled sedimentary sections. The absolute mass of an oil or gas accumulation varies with the section length modelled, while more vertical grids (pseudo-wells) along a section result in less lateral continuity of accumulated oil and gas. A modelled section with a highly permeable lithology at the two lateral edges produces lower excess pressure between the three left-edge pseudo-wells and between the three right-edge pseudo-wells, and tends to have less oil and gas accumulated in the section center. If the modelled section has a low-permeability lithology at the two edges, the opposite effect is produced. A modelled section which has a thick basal layer, high permeability, and relatively few horizontal subdivisions produces less excess pressure in a shale source rock above the basal layer, and has smaller oil and gas accumulations in a reservoir unit above the source rock. By contrast, a modelled section with more subdivisions, and a smaller time-step for each layer calculation, produces higher excess pressures and tends to have more oil and gas accumulated in reservoirs above the source rock. Accordingly, the ratio of the thickness deposited in a time-step to the subdivision thickness used in the calculation should be kept small (less than 0.4 for the code used here). Twelve major processes or factors control oil and gas expulsion: (1) buoyancy; (2) the solubility of hydrocarbons in connate waters; (3) the saturation of irreducible water in a source rock; (4) capillary pressure; (5) critical pressure for fracturing a source rock; (6) the viscosity of hydrocarbons; (7) the relative permeabilities of oil and gas; (8) the lithology-dependent permeability ratio in the vertical to horizontal direction; (9) the total organic carbon in the source rock; (10) the kerogen type; (11) the ages of the source-rock strata; and (12) the palaeoheat flux of the area. The Shongliao Basin, China has been intensively drilled, and two volumes of downhole data are available. Those data were used to set limits on the ranges of parameter values and on geological factors. In this way, an evaluation is given of the relative importance of the twelve contributions influencing primary hydrocarbon expulsion. Both separate-phase gas-expulsion efficiency and the absolute mass amount of gas expelled from the source rock increase with increases in: buoyancy, hydrocarbon saturation in irreducible water in the source rock, capillary pressure, relative permeability, total organic carbon, source-rock age, and palaeoheat flux. However, gas expulsion efficiency and the amount of gas expelled decrease with increases in: hydrocarbon solubility in mobile waters in the source rock, critical fracture pressure, the viscosity of the hydrocarbons, and with changes in kerogen type to a more oil-prone variety. The object of the work presented here is to investigate simultaneously all the processes thought to control primary hydrocarbon expulsion. The Shongliao Basin in China was chosen as a case study because copious, basin-wide data are available.