ABSTRACT: Diagenetic Controls on Sandstone Reservoir Characteristics, Surat Basin, Queensland, Australia
H. M. Hawlader, P. J. Conaghan
Sandstones of the Surat basin range from quartzose through sublabile to labile varieties and their present-day porosity is diagenetically controlled. Compaction has significantly reduced primary depositional porosity but cementation locally halted further compaction and preserved moderate to good precement bulk volume. Some of these cementing phases were later dissolved creating secondary intergranular porosity accompanied by variable amounts of framework-grain dissolution porosity. Compaction is strongly influenced by detrital composition and is more pronounced in the lithic sandstones.
Authigenic minerals occur either singly or in preferred association in all stratigraphic units and there is a first-order control of sandstone detrital composition on the mineralogy of the authigenic species. The relatively quartz-rich petrofacies is characterized by quartz overgrowth and kaolinite cements with minor smectite, mixed-layer clays, and chlorite. The quartz-poor labile facies is characterized by zeolite, smectite, mixed-layer smectite-illite, and some kaolinite. Authigenic carbonates are present in varying amounts in sandstones of all mineralogic composition. Porosity and permeability are drastically reduced by the presence of these diagenetic minerals and the extent of this reduction depends on the amount, mineralogy, crystallographic habit, and geometry of distribution f the authigenic species within the pore system. Moreover, many of these authigenic species, especially the clays, contributed significantly to the creation of ineffective microporosity.
The evolved porosity of the Surat Basin sandstones is a combination of both primary porosity and secondary dissolution porosity, the former being volumetrically marginally more prevalent than the latter. In specific formations secondary porosity averages 80% of the total thin-section porosity. Primary porosity is prevalent in the younger stratigraphic units, presumably reflecting relatively minor compaction compared to the more deeply buried formations. Nevertheless, they also show variable amounts of secondary porosity created mainly by meteoric flushing after inception of the Great Artesian System. Secondary porosity is ubiquitous throughout the whole stratigraphic section of the basin which averages 1.5 km and attains 2.5 km maximum thickness. However, significant amounts of second ry porosity occur within specific depth intervals where we believe such enhancement to be the result of the action of organic maturation products emanating from the intercalated mudrocks. The quality and quantity of organic matter, its level of thermal maturation, and the timing of hydrocarbon generation all played a vital role in the creation and preservation of secondary porosity at depth.
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990