EXPERIMENTAL

Solvent extraction

Analyzed samples were well cuttings collected specifically for organic geochemical analyses. About 10-20 g of a hand-pulverized sample was used for extracting the solvent-soluble organic matter. The extraction was carried out for 24 hours in a Soxhlet apparatus using approximately 350 ml of an azeotropic mixture of 87% chloroform and 13% methanol. After the solvent was removed in a rotary evaporator (temperature set at 35-40oC), the extracts were dissolved in chloroform and treated with colloidal copper to remove elemental sulphur, which is considered to be an artifact resulting from the oxidation of pyrite during sample handling. The mixture was filtered through glass fibre filter paper to remove the copper sulphide and excess of copper, then the filtrate was rotary-evaporated, dried and weighed until a constant weight was obtained. The total yields were normalized to TOC and expressed as milligrams of total extract per gram of organic carbon. Total extracts were then dissolved in a minimal amount of chloroform, treated with pentane to precipitate asphaltenes and then vacuum filtered to remove the precipitate. The asphaltenes were dissolved in chloroform, collected to a separate tared flask, rotary-evaporated and weighed to constant weight.

Liquid chromatography

A mixture of 28-200 mesh Silica Gel (MCB) and 80-200 mesh alumina (ALCOA) (1/3:2/3 by weight respectively) was used as a support for the column. The support is activated by heating at 120o-150oC for 12 hours. A glass wool plug is placed at a bottom of the column and covered with a 1 cm thick layer of sand. The support, weighed as 1 g of support/10 mg of deasphalted sample, is slowly settled in pentane and any air trapped is released by gentle tapping on the column. A deasphalted sample, dissolved in a minimal amount of previously measured pentane, is then added to the column. Saturates are recovered by eluting with pentane (3.5 ml/g support), aromatics with a 50:50 mixture of pentane and dichloromethane (4 ml/g support), resins with methanol (4 ml/g support) and any remaining asphaltenes with chloroform. The solvents are rotary-evaporated, separate fractions transferred to tared 1 dram vials, dried in a slow stream of nitrogen and weighed to constant weight.

Gas chromatography

Saturate fractions were analysed using gas chromatography (GC). A Varian 3700 FID gas chromatograph was used with 30m DB-1 column with helium as the carrier gas. The temperature program was 60oC to 300oC at a rate of 6oC/min and then isothermal for 30 min. Some earlier analyses were conducted using 25m OV-1 column and temperature programmed from 60oC to 260oC at a rate of 4oC/min. The eluting compounds were detected and quantitatively determined using a hydrogen flame ionization detector.