Core Facies, Petrology, and Permeability of Tirrawarra Sandstone, Moorari Field, Cooper Basin, South Australia

J. M. Bever, P. G. Carroll, E. W. WILD, B. P. J. Williams

The oil gas-bearing Tirrawarra Sandstone lies in the basal section of the Cooper basin sequence, which is largely Permian in age. The sandstone is characteristically thick, but both interfingers with and conformably overlies glacio-lacustrine diamictites and varvites of the late Carboniferous-Early Permian Merrimelia Formation. The Tirrawarra Sandstone has previously been interpreted as being deposited in a glacio-fluvial braided river environment. The sandstone produces high gas:oil-ratio oil at the Moorari field, from depths of 9,400 ft below sea level. Appraisal and development of the field has been hampered by the patchy distribution of reservoir quality sandstone.

This study investigated the cause of reservoir quality variations. For seven cored wells, core facies analysis, core plug porosity/permeability, petrology, and wireline logs were all matched and compared. The results are as follows. (1) Facies states (grain size and bed form) largely control permeability distribution in the Tirrawarra Sandstone at the Moorari field, such that horizontally bedded medium-coarse sandstones are consistently more permeable than cross-bedded equivalents. (2) Diagenesis levels are high and include extensive silica cement and patchy kaolinite and siderite cements. However, diagenesis rarely operates independently of original depositional fabric. (3) An association between depositional environment and permeability is recognizable, with medial bars in particula providing better reservoir quality.

Predicting the areal distribution of specific environments is not yet possible within the Tirrawarra Sandstone at Moorari, due to the complexity of environments in glacial settings and the absence of suitably refined fluvial and glacio-lacustrine facies models.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.