The Application Of Capillary Pressures In Determining The Seal Capacity Of Eromanga Basin Cap-Rocks.
Solomon, Catherine J
Degree of Master of Science, 1992
University of Adelaide
The seal capacity, measured in terms of the height of oil column a seal can hold, has been determined for several Eromanga Basin cap-rock samples from mercury injection capillary pressure measurements. The threshold or breakthrough pressure has been estimated from the mercury saturation versus injection pressure curves and converted to reservoir conditions. Using an equation that relates the threshold pressure to the reservoir and oil densities, the theoretical oil column height has been calculated.
Calculated oil column heights for the Eromanga Basin samples range from 288 feet (88m) up to 5,550 feet (1692m) indicating good to excellent sealing capacity. These silty mudstones from the Murta and Namur Members of the Mooga Formation and the Birkhead Formation are composed primarily of quartz, illite and kaolinite, with minor amounts of chlorite, siderite and feldspar.
Microporosity constitutes 90 to 100 per cent of the measured porosity and occurs between oriented platy clay and mica grains, as observed under Scanning Electron Microscope. Although measured porosities range from less than one per cent up to 14.9 per cent, permeabilities are consistently very low. All samples were reported to have values less than 0.09 millidarcies.
Calculated oil column heights were compared to the actual columns intersected in the Eromanga Basin to determine the role of seal capacity in controlling the size and distribution of oil pools.
Many Eromanga Basin structural closures are only a few hundred feet or less from the crest to the spill point. Most oil pools contained in these structures are small, with oil columns far less than those predicted by the experimental results. Oil columns rarely exceed 50 feet (15m) and are generally less than the structural closure. Although the experimental results show that the mudstone layers can be excellent seals, only a few Eromanga Basin oil pools are interpreted to be filled to structural spill. The occurrence of multiple, stacked oil pools within the sequence indicates that either quantities of oil are leaking through the seals or that there could be multiple sources for the oil.
Although some oil pools may have received a contribution from Eromanga Basin source rocks, current evidence from both well and regional data suggests that most of the oil trapped within the Eromanga Basin sequence is being sourced from the underlying Permian Cooper Basin sequence and is migrating into suitable Eromanga Basin traps where the lower regional seals are eroded or faulted.
The size and distribution of oil pools within the Eromanga Basin can be explained in part, by poor lateral continuity of the seals, which allows leakage of oil from one horizon to another. This is probably caused by rapid facies changes within the finely interbedded sealing intervals. Mudstones grade into siltstones or sandstones which have lower sealing capacities. Another major factor in disrupting the lateral seal continuity is faulting and fracturing caused by either burial compaction or later tectonic events.