SILICA DIAGENESIS AND PORE DEVELOPMENT IN PERMIAN SANDSTONES, SOUTHERN COOPER BASIN

Abstract

As the Cooper Basin is a mature petroleum province, more work is being directed towards further understanding the characteristics of the main reservoirs to maximise exploitation and recovery.

To investigate the reservoir quality of Permian sandstones in the Cooper Basin, a study of the diagenesis and pore geometry has been carried out on the Toolachee and Patchawarra Formations in the Daralingie, Moomba and Della Fields. Following detailed petrography, scanning electron microscopy and cathodoluminescence observations, representative samples were selected for petrophysical analysis via mercury injection. The diagenetic studies show that early silica diagenesis has had a major impact on reservoir quality. Early quartz cement has helped rocks resist compaction so that primary porosity is preserved and allowed later porosity enhancement via dissolution.

From petrophysical analysis, pores of different origin can be recognised and classified into three groups: inter-granular pore, kaolin framework pore and micropore groups. These groups can be distinguished quantitatively by their controlling pore throat radii. The radii for the various groups are ≥ 2.5 (µm (inter-granular), 2.5 − 0.5(µm (kaolin framework) and < 0.5 µm (micropore).

In the samples studied the relationship between the controlling pore throat radius (rpk) and permeability (k) is approximately

k=rpk2

Using this relationship, the calculated permeability associated with rocks containing only microporosity is less than 0.25 mD, making them unsuitable reservoirs. Permeability associated with the kaolin framework pore group ranges from approximately 0.25 to 6.25 mD, showing that it offers significant effective storage space for hydrocarbons, especially gas. The inter-granular pore group has variable permeability, ranging from near 6 to over 2000 mD. The causes for this variability are mainly diagenetic and include variable quartz cementation and different types of dissolution.

Using the pore group division outlined above, different types of reservoirs can be distinguished. In addition, the results emphasise the importance of porosity within kaolin aggregates in Cooper Basin reservoirs and stress the need for adequate testing of initially unpromising horizons and careful choice of reservoir stimulation methods.