Latrobe group

As a first example, we consider the diagenesis of clastic sandstones in the Gippsland basin, southeastern Australia, basing our model on the work of Harrison (1990). The Gippsland basin is the major offshore petroleum province in Australia. Oil production is from the Latrobe group, a fluvial to shallow marine sequence of Late Cretaceous to early Eocence age that partly fills a Mesozoic rift valley. 

Harrison (1990) proposed that the diagenetic alteration observed in the Latrobe group resulted from the mixing within the formation of two types of groundwaters. Table 25.1 shows analyses of waters sampled from two oil wells, which she took to be representative of the two water types as they exist in the producing areas of the basin. 

The first water is considerably less saline than seawater and hence is termed a fresh water, although it is far too saline to be potable. This water is apparently derived from meteoric water that recharges the Latrobe group where it outcrops onshore. The water flows basinward through an aquifer that extends 60 km offshore... 

Table 25.1. Analyses of formation water sampled at two oil wells producing from the Latrobe group, Gippsland basin (Harrison, 1990)...

Fig. 3. Van Krevelan diagram outlining maturation pathways of Types I, II, and III kerogens. The dashed line represents onset of liquid hydrocarbon generation. Samples are from the Latrobe Group, Gippsland Basin, Australia...

Fig, 10. Mineral composition of the Latrobe Group sandstones. The sandstone classification was adapted from Folk (1974) data were derived from point-counting thin sections (300 points per thin section). It should be noted that most of these sandstones have suffered significant dissolution of feldspar and rock fragments... 

Table 5. Chemical and isotopic composition of dolomite cement, Latrobe Group, Gippsland Basin...

Early precipitation of dolomite cement in the Latrobe Group is supported by petrographic observation. In most of the dolomite-cemented sandstones. 

Figure 16 is a diagram showing porosity/depth relations for sandstones of the Latrobe Group. The compaction/porosity loss curves are from Pittman and Larese (1991). The 50 and 75% quartz curves of porosity loss vs. depth (Fig. 16) establish a porosity baseline, based on the quartz content of each sandstone. These porosity loss vs depth curves are used to define porosity anomalies. 

Fig. 17. Photomicrograph illustrating quartz overgrowths (Qc) developed on quartz (Q) in deep Latrobe Group sandstones. Note dust rims beneath overgrowths (open arrow) as well as euhedral terminations (solid arrows). Marlin-1, 2572m (8438ft). Scale bar =...

---