THE FATE OF HYDROCARBONS ASSOCIATED WITH DRILLING FROM THE NORTH RANKIN 'A' GAS AND CONDENSATE PLATFORM, WESTERN AUSTRALIA.

Abstract

Drilling operations from the North Rankin 'A' platform ceased in August 1991. At that time two sets of triplicate samples of seabed sediments for hydrocarbon analysis were collected using a remotely operated vehicle. One set of sediments was collected along a transect up to 10 km from the platform in the direction of the prevailing current; the other was collected along a perpendicular transect to a distance of 1.2 km. The total hydrocarbon concentrations of these samples were 75 000 mg/kg (dry weight of sediments) beside the platform, decreasing rapidly with distance from the platform. At approximately 800 m in the direction of the current levels were 40 mg/kg, and at approximately 2000 m the concentrations found were 2 mg/kg.

Detailed examination revealed the presence of aromatic hydrocarbons in the sediments. This indicates that they must be derived, at least in part, from formation fluids; however, a substantial contribution from the low toxicity base oil (hydrogenated kerosene) component of drilling fluids is present, judging from the molecular weight distribution of the sediment hydrocarbons.

Near the cuttings chute samples contained approximately five per cent aromatic hydrocarbons. Beyond approximately 400 m in the direction of the prevailing current, and along the perpendicular transect, aromatic hydrocarbons made up approximately 20 per cent of the sediment hydrocarbons. As the distance from the cuttings chute increased, the extent of biodegradation of the hydrocarbons in the sample also was found to increase.