Constraining the oil charge history of the South Pepper oilfield from the analysis of oil-bearing fluid inclusions

Abstract

The South Pepper oilfield, located in the Barrow Sub-basin on the NW margin of the Australian continent, has experienced a multi-phase charge history. Abundant oil-bearing fluid inclusions are present in samples from within the current gas cap, suggesting that an oil column existed prior to gas. This palaeo-oil (gas-leg FI oil) has Ts/Tm and C 29/C 30 αβ hopane ratios of ∼1 and the C 35 homohopanes are a significant proportion of the extended homohopanes. It has lower Pr/Ph and diasterane/sterane ratios than the currently reservoired live oil and contains gammacerane, a series of peaks tentatively identified as C 30 to C 34 17α(H)-30-norhopanes and a large amount of 2α-methylhopanes. Collectively, geochemical analysis of the gas-leg FI oil suggests that it was generated from a less mature, more calcareous source rock, deposited under more reducing conditions than the Upper Jurassic Dingo Claystone, the main source of the live oil. In addition the presence of C 30 dinosteranes in the gas-leg FI oil provides a Triassic or younger age constraint. This makes Palaeozoic carbonates an unlikely source. Possible source intervals for the gas-leg FI oil are thin, Lower Jurassic limestones and marls which occur at the base of the Lower Dingo Claystone, or a thin limestone unit (the Cunaloo Member) at the base of the Triassic Locker Shale. Samples from within the present oil-leg also contain abundant oil inclusions, consistent with high oil saturations at the present day. However, these oil inclusions exhibit different fluorescence colours, suggesting they represent a second oil charge. Geochemically the oil-leg FI oil has an intermediate composition between the live oil and the gas-leg FI oil, suggesting that gas charge displaced the first oil charge, samples of which were preserved as fluid inclusions in the oil-leg sample. Biodegradation of the first oil charge, indicated by the presence of 17α(H)-25-norhopanes in the currently reservoired live oils, can be attributed to the ingress of meteoric waters, probably during sub-aerial exposure of the basin margin during Miocene wrenching. Changing environmental conditions curtailed bacterial activity and allowed unaltered oil sourced from the Dingo Claystone to accumulate below the gas cap and mix with the biodegraded residues of the first oil charge to achieve the live oil composition. The biodegradation event must have preceded the second charge as the live oil contains compounds such as n-alkanes which would have been removed had alteration occurred after the second charge. Complex charge histories are common and the analysis of palaeo-oils trapped within fluid inclusions provides the opportunity to achieve a more comprehensive assessment of hydrocarbon charge