MAPPING HYDROCARBON CHARGE HISTORIES: DETAILED CHARACTERISATION OF THE SOUTH PEPPER OIL FIELD, CARNARVON BASIN

Abstract

The South Pepper Field is a small hydrocarbon accumulation located in the Barrow Sub-basin, North West Shelf. In this study, an integrated approach has been adopted which uses new analytical approaches, and combines fluid inclusion, micro and conventional GC-MS, stable isotope, mineralogical and petrological data, to allow the accurate reconstruction of the charge history of this field. The micro GC-MS work on oil-bearing fluid inclusions in the reservoir section allows, for the first time, a comparison to be made between the geochemical characteristics of the presently reservoired oil with samples of the palaeo-r eservoired oils, as preserved within the fluid inclusions.

Oil inclusion abundances (OIA) recorded within the reservoir section in South Pepper-1 show two distinct abundance peaks, which are interpreted to correspond to zones of high oil saturation. The first occurs near the top of the reservoir, in sands that are presently gas saturated and represents a palaeo-oil column. The oil inclusions occur primarily within pre-existing ankerite cement that crystallised from highly saline fluids, probably derived from Palaeozoic evaporites. Carbon isotope compositions measured on the ankerite are isotopically depleted, consistent with the derivation of CO2 from the maturation of organic matter. Geochemical characterisation of the oil contained within these inclusions shows the presence of biomarkers such as 30-norhopanes and methylhopanes which are indicative of a calcareous source rock. This excludes the Jurassic Dingo Claystone as a source for this early oil charge and migration of oil from a pre-Jurassic source rock is inferred. Subsequent uplift and sub-aerial exposure of the basin margin during the Middle Miocene allowed fresh waters to dilute formation waters in the Barrow Group and biodegrade this early oil charge. However, the abundance of highly water-soluble compounds in the fluid inclusion oil suggests that biodegradation occurred subsequent to initial oil emplacement.

The second oil inclusion abundance peak corresponds to the present-day oil zone and, although absolute oil inclusion abundances are similar to those recorded in the upper zone, the fluorescence colours are significantly different, suggesting a different source for the second oil charge. Geochemical characterisation of oil recovered by DST confirms this later oil charge was derived from a more clay-rich oxic source and has previously been typed to the Upper Dingo Claystone. The low abundance of this type of oil inclusion in samples from above the present GOC indicates that this later oil charge failed to accumulate at the top of the reservoir and so it cannot have been displaced by a later gas charge. Rather, the oil inclusion data suggest the emplacement of gas, probably derived from Triassic source rocks, occurred either prior to, or coincident with, the second oil charge.

High OIA in the presently water bearing reservoir of South Pepper-3 suggests that high oil saturation originally extended beyond the present OWC. However, the absence of stratigraphically equivalent high OIA samples in wells from the east of the field suggests that late stage westerly tilting of the structure resulted in a reduction in closure and loss of oil across the eastern spill point. The presence of high gas readings and associated amplitude anomalies in the Windalia Radiolarite suggests that hydrocarbons are continuing to leak up the bounding fault. However, the absence of significant residual oil zones in other wells suggests that liquids were largely retained and that leakage involved the progressive bleeding of gas across a soft seal.