--> Compositional, Textural, and Diagenetic Controls on Porosity Distribution and Evolution in Permian Tirrawarra Sandstone, Cooper Basin, South Australia, by S. P. Dutton and H. S. Hamlin; #90986 (1994).

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Abstract: Compositional, Textural, and Diagenetic Controls on Porosity Distribution and Evolution in Permian Tirrawarra Sandstone, Cooper Basin, South Australia

Shirley P. Dutton, H. Scott Hamlin

The Lower Permian Tirrawarra Sandstone, a major oil reservoir in South Australia, was deposited by progradation of a bed-load fluvial system into a glacial lake basin. The oil reservoir in Tirrawarra field can be divided into four depositional facies that comprise six flow units, each having distinct petrographic and petrophysical properties. Despite extensive diagenesis at temperatures reaching 140°C during burial to as deep as 3 km, depositional facies remain the major control of porosity and permeability in Tirrawarra sandstones.

Point counts of 81 Tirrawarra Sandstone thin sections indicate that these sandstones are medium- to fine-grained sublitharenites (Q84F0R16); metamorphic and sedimentary rock fragments are the most abundant lithic grains. The major diagenetic events were compaction, precipitation of quartz and siderite cements, and dissolution and alteration of rock fragments, biotite, and feldspar into secondary porosity, illite, and kaolinite. Mechanical compaction of ductile grains and chemical compaction by intergranular pressure solution and stylolitization are the major causes of porosity loss in Tirrawarra sandstones. The average sandstone lost about 28 percent porosity by compaction and an additional 11 percent by cement precipitation in primary pores.

The main controls on porosity and permeability in these sandstones are grain size and volume of ductile grains, which are both functions of the original depositional environment of the sandstones. The best reservoir quality occurs in facies 3 (braid-delta plain composed of broad, shallow channels) and facies 4 (braidplain evolving into meandering stream system) sandstones that have coarser grain size, relatively quartz-rich detrital mineral composition, and a high proportion (>60 percent) of macroporosity. Poor reservoir quality occurs in facies 1 (delta-front) and facies 2 (deltaic distributary-channel and channel-mouth-bar) sandstones having poor sorting, an abundance of ductile rock fragments, and a low proportion (<25 percent) of macroporosity.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994