DEPOSITIONAL CONTROLS ON RESERVOIR PROPERTIES IN A BRAID-DELTA SANDSTONE, TIRRAWARRA OIL-FIELD, SOUTH AUSTRALIA

The Tirrawarra Sandstone contains 146 million bbl of oil in Tirrawarra field in the Cooper basin of South Australia. We used core, well logs , and petrophysical data to construct a depositional-facies-based flow -unit model of the reservoir, which describes rock properties and hydr ocarbon saturations in three dimensions. Using the model to calculate volumes and residency of original and remaining oil in place, we ident ified an additional 36 million bbl of oil in place and improved unders tanding of past production patterns. The Tirrawarra Sandstone reservoi r was deposited in a Carboniferous-Permian proglacial intracratonic se tting and is composed of lacustrine and fluvial facies assemblages. Th e stratigraphic framework of these nonmarine facies is defined by dist inctive stacking patterns and erosional unconformities. Mudstone domin ated zones that are analogous to marine maximum flooding surfaces boun d the reservoir. At its base a progradational lacustrine-delta system, composed of lenticular mud-clast-rich sandstones enclosed in mudstone , is truncated by an unconformity. Sandstones in these lower deltaic f acies lost most of their porosity by mechanical compaction of ductile grains. Above the unconformity a braid-delta system, dominated by aggr adational bed-load channel-fill sandstones, forms the. core of the res ervoir. Sediment reworking by channel migration and locally by shore-z one processes created quartz-rich, multilateral sandstones, which reta ined the highest porosity and permeability of all the reservoir facies and contained most of the original oil in place. The braid delta is e rosionally truncated and overlain by a more proximal braid-plain syste m. Braided-channel sandstones, however, are overlain by lenticular mea ndering-channel sandstones, which in turn grade upward into widespread mudstones and coals. Thus, this uppermost part of the reservoir displ ays a retrogradational stacking pattern and upward-decreasing reservoi r quality. On the basis of the systematic arrangement of facies within this stratigraphic framework, as well as facies-related differences i n composition and texture, we identified reservoir flow units that hav e characteristic petrophysical properties. Our results demonstrate tha t depositional variables are the primary controls on reservoir quality and productivity in the Tirrawarra Sandstone.