## 1 Introduction  
The petroleum industry generates substantial amounts of oily sludge during the exploitation, storage and refining of crude oil [1]. Oily sludge is a potentially hazardous material [2; 3; 4], which requires safe containment or treatment to minimize its adverse impacts on the environment [5; 6; 7]. Beneficial uses allow cost-effective management of oily sludge to be achieved [8]. To assess the environmental impacts of oily sludge and develop sound management strategies, it is necessary to understand the characteristics of oily sludge. [PERSON] et al. [9] investigated the characteristics of oily sludge from an oil refinery using parameters such as saturated alkanes, aromatics, non-hydrocarbons, resins and asphaltenes. [PERSON] et al. [10] characterized sludge deposits in crude oil storage tanks using the parameters of water, free oil and emulsion fractions. [PERSON] et al. [11] assessed the characteristics of oily sludge from a refinery using parameters such as the total petroleum hydrocarbon (TPH), total volatile solids, oil and grease, dissolved oxygen (DO), five-day biological oxygen demand (BOD5) and chemical oxygen demand (COD). Oilfield sludge pits are temporary storage facilities used for the disposal of oily wastes (drilling mud and cuttings) generated from drilling oil wells [4] or collecting crude oil due to emergency drill operation [12]. These facilities are exposed to an open environment. Unlike oily sludge generated in oil tanks or from crude oil processing, oily sludge in oilfield waste pits is formed via a strong interaction of the disposed of oil wastes with natural environmental factors such as solar radiation, atmospheric deposition and underlying soils. Volatile short-chain petroleum hydrocarbons in the oil wastes tend to evaporate rapidly. Photocatalytic degradation of petroleum hydrocarbons caused by solar radiation could also take place [13]. So far, detailed research work to characterize the oily sludge in oilfield sludge pits has been limited. Crude oil contains various petroleum hydrocarbon species with different chemical structures and carbon numbers. At an equivalent carbon number, an aromatic hydrocarbon tends to be more toxic compared to its aliphatic counterpart [14]. For the same type of petroleum hydrocarbon, the smaller the carbon number, the more toxic it tends to be [15]. Therefore, the hazard of a given oily sludge is related to the composition of petroleum hydrocarbon compounds. For this reason, characterization of oily sludge based on parameters that provide no information on petroleum hydrocarbon speciation or fractionation--such as TPH, saturated alkanes, aromatics, resins, asphaltenes, free oil and emulsion fractions does not allow sensible assessment of the environmental risk associated with exposure to oily sludge. During the storage of oil wastes in the oilfield waste pits, downward movement of oil fluid takes place, causing contamination of the underlying soils and potentially shallow aquifers [16]. Assessing the environmental risk of the contaminated soils and developing soil remediation strategies are also important parts of oilfield waste pit management. Investigation of soil contamination by petroleum has so far been focused on those formed following one-off oil spills [17; 18]. The penetration depth of oil into soils depends on many factors such as characteristics of the penetrating oil, soil type, water input, etc. [19]. Oilfield waste pits are purpose-built structures frequently with a layer of compacted clay on the bottom to minimize the infiltration of oil fluid into the underlying soils. Unlike oil spill incidences, oilfield waste pits receive continuous oil input, and the soils beneath the oil wastes could experience oil penetration for an extended period. Therefore, it is expected that the characteristics of contaminated soils in oilfield waste pits differ from those caused by accidental oil spill events. However, there has so far been no detailed investigation into petroleum-contaminated soils in oilfield waste pits.