The Assessment of Grain Size and Mineralogy on Reservoir Quality for the Mungaroo Formation; Rankin Platform, North West Shelf, Australia
Tombra S. Brossa - 2012
Master of Science (Petroleum Geoscience)
Australian School of Petroleum
The University of Adelaide
The Late Triassic Mungaroo Formation contains significant hydrocarbon reservoirs in the Rankin Platform of the Northern Carnarvon Basin. This study aims to assess the controls that grain size and mineralogy exert on reservoir quality of sandstones from the Mungaroo Formation, in order to improve reservoir understanding and to further optimise development opportunities. A detailed grain size analysis of 340 samples and a compositional analysis of 69 samples from cores of ten wells in the Mungraoo Formation was carried out. The techniques applied in determining grain size in this study are commonly used to determine grain-size distributions for sand-dominated sediments. However, the degrees of consistency and resultant differences in interpretation when using a combination of grain-size methods (Laser, Sieve, Thin section and Core) had not yet been assessed for sand-dominated sediments in the study area. Results obtained from these methods show a compelling correlation between mean grain size thin section results and laser diffraction results. However, a consistent relationship is observed between the laser diffraction results and the sieve results in the determination of grain size distribution and therefore these two methods are comparable for determination of grain size in the sand dominated sediments.
Although it is well known that reservoir quality is controlled by a range of parameters, grain sorting is seen to have the most impact in the Mungaroo Formation as porosity and permeability are seen to vary directly with a change in sorting. No direct adequate relationship can be established between grain size and reservoir quality in the Mungaroo Formation. Grain composition is observed to influence reservoir quality to a lesser extent as porosity and permeability are seen to increase with an increase in percentage of quartz. However no constant relationship can be established between reservoir quality and felspathic grains or rock/lithic fragments.
Determined from earlier studies, reservoir quality in the studied sandstones ranges from poor to excellent. Most samples contain modest to high amounts of clean, interconnected primary intergranular porosity, supplemented by scattered secondary pores (He injection porosity: mean 17.3 %, range 0.0-29.1 %; visual porosity: mean 12.9 %, range 0.0-24.3 %). Permeabilities are mostly fair to excellent and are generally higher in the TR 29 (mean 4087.78 mD, range 0.0-21280.0 mD) compared with other stratigraphic intervals investigated.
Mungaroo Formation sandstones are mostly subarkosic and quartzarenitic with minor sublitharenite and rare arkose, lithic arkose, feldspathic litharenite and litharenite. Sands are typically fine- to coarse-grained and moderately- to well-sorted. Framework grains are dominated by monocrystalline quartz, with minor polycrystalline quartz, feldspar and rock/lithic fragments (mean Q90.7 F7.4 R1.9). Mica, heavy minerals, organic matter and detrital clay are rare. Authigenic phases are dominated by kaolin clays (and less common illite) and quartz overgrowths. Other locally important minerals include pyrite, siderite, calcite and ankerite.
Channelized (fluvial and distributary) depofacies are observed to exhibit best reservoir quality with planar cross bedded and rippled sandstone (Spr) lithofacies displaying highest mean porosity and permeability values.