Geochemistry of dew point petroleum systems, Browse Basin, Australia

The Browse Basin is located offshore on Australia's North West Shelf and is a proven hydrocarbon province hosting gas with associated condensate; however, oil reserves are small. The assessment of a basin's oil potential traditionally focusses on either the presence or absence of oil-prone source rocks. However, light oil can be found in basins where the primary hydrocarbon type is gas-condensate and oil rims form whenever these fluids migrate into reservoirs at pressures below their dew point (or saturation pressure). The relationship between dew point pressure and condensate-gas ratio (CGR) depends on the liquid composition and is therefore a petroleum system characteristic (Fig. 1). By combining geochemical studies of source rocks and fluids with petroleum systems analysis, the four Mesozoic petroleum systems identified by their geochemical fingerprints (Rollet et al., 2016) can be correlated with several gas-prone (dew point) petroleum systems:


Gas-condensates generated and reservoired within the Lower-Middle Jurassic Plover Formation are derived from terrestrial organic matter in fluvio-deltaic to pro-deltaic environments. Such gas is dominated by methane (gas dryness* = 91%), with ideal gas condensate ratios (CGRs) ranging between 7 and 35 bbl/MMscf. Liquids recovered from wells tested along the Scott Reef Trend (e.g. Calliance, Brecknock and Torosa) comprise pale yellow condensates (49-53° API gravity), as do those from the deepest (Plover) reservoirs within the Ichthys gas accumulation (e.g. Gorgonichthys). These liquids plot on Figure 1 as dew point fluids. The molecular and carbon isotopic signatures of these condensates are similar, classifiying them into a single family (W1_1BRO) in Figure 2. The biomarkers providing the strongest discrimination are the high relative abundances of C29 sterane and C19 tricyclic triterpane, coupled with an enrichment in  δ13C of their saturated and aromatic hydrocarbon fractions testifies to their terrestrial organic origin.


Fluids with similar bulk properties (gas dryness = 91%; ideal CGRs 27-52 bbl/MMscf) to those of the aformentioned Plover-sourced fluids are found in the greater Crux accumulation in the Heywood Graben. The pale yellow condensates (47°API gravity) also exhibit similar biomarker assemblages as the W1_1BRO family. However, due to their greater enrichment in δ13C, the condensates plot as a separate family (W1_2BRO) in Figure 2. A difference in thermal maturity is also noted, with the Crux accumulation having lower maturity (calculated vitrinite reflectance# [Rc] = 0.77%) relative to the condensates on the Scott Reef Trend (av Rc = 1.18%). The most likely source rocks for the Crux fluids are the terrestrially-dominated Plover Formation coals and shales, but shaly coals also occur within the thick Upper Jurassic section in the northern part of the basin. These fluids are categorised as a separate dew point system within the Heywood Graben (Fig. 1).


Gas-condensates reservoired within the Brewster Member of the upper Vulcan Formation in the Ichthys/Prelude and Burnside accumulations have a greater liquid content than the aformentioned gases, with ideal CGRs of 22-151 bbl/MMscf at Titanichthys 1 and a gas dryness of 84%. The pale yellow condensates have API gravities of 55° and are potentially a separate intraformational dew point petroleum system within the central Caswell Sub-basin. Their biomarker and isotopic signatures indicate derivation from mixed marine and land-plant organic matter and plot as another family (W2W3_1BRO; Fig. 2). The source of these fluids is probably the organic-rich shales of the Upper Jurassic-Lower Cretaceous Vulcan Formation that encase the Brewster Member sandstone reservoir. PVT data for Brewster-reservoired fluids is affected by synthetic mud contamination, which has an impact on the measured dew point pressures. In the absence of measured values, a similar phase behaviour to North Sea (UK) gas-condensates (England, 2002) is assumed.


The Cretaceous reservoir in Caswell contains an unbiodegraded brown 'light oil' (47°API gravity) but PVT data are not available. Biomarker and isotope signatures show that the liquids were generated from source rocks containing both marine and terrigenous organic matter lying within the early oil window (Rc# = 0.75%). They correlate with the Yampi Shelf biodegraded oils (W3_1BRO, Fig. 2), gas at Adele, and with extracts of the Lower Cretaceous Echuca Shoals Formation (Boreham et al., 1997). However, these marine shales have low hydrogen indices (~200 mg hydrocarbons/gTOC) and hence may only be able to expel sufficient hydrocarbons to sustain migration over short distances. Since biodegraded solution gases in the Yampi Shelf accumulations contain neo-pentane - a highly resistant compound - with isotopic affinity to Plover Formation generated fluids, it is possible that Cretaceous-sourced liquids were mobilised and carried to the shelf edge by co-migrating Plover-derived gas.


gas dryness = 100 [C1/(C1+C2+C3+iC4+nC4+neoC5+iC5+nC5)] 
where C is carbon number, i = iso-alkane, n = normal-alkane, and neo = neo-alkane

Tags: Earth Sciences, Published_External, petroleum geochemistry
Organization: Geoscience Australia Data