Dolomite cement in the Gippsland basin

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 

To test whether the mixing hypothesis might explain the diagenetic alteration observed, we begin by equilibrating the fresh water, assuming equilibrium with the potassium feldspar ( maximum microcline in the database), quartz, and muscovite (a proxy for illite) in the formation. In REACT, we enter the commands 

To model fluid mixing, we will use the fresh water as a reactant, titrating it into a system containing the saline water and formation minerals. To do so, we pick up the fluid from the previous step to use as a reactant  

TABLE 19.1 Analyses of formation water sampled at two oil wells producing from the Latrobe group, Gippsland basin (Harrison, 1990) 

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