Project Summary

Knowledge of abnormal fluid pressure (overpressure) is an important aspect of petroleum exploration with respect to drilling hazard, and as a potential control on open natural fractures and fluid flow.

Fluid pressure is balanced while drilling by varying drilling mud weight. It is important to use a mud weight in the 'window' between pore pressure and fracture pressure to prevent blowouts (during underbalanced drilling), and prevent mud loss and formation damage (during overbalanced drilling). In overpressured formations, the 'window' is narrow and knowledge of the pore pressure is vital for safe and efficient drilling. The project focuses on overpressure in two Australian basins: the Cooper and Carnarvon Basins. The former is an 'older' onshore basin and the latter a 'younger' offshore basin. Overpressure has been encountered in both basins and is a hazard for ongoing petroleum exploration and development. The aims of this project are to:

• Provide an improved model of the distribution of overpressure within permeable sediments (e.g. sandstones) using direct pressure measurements and mud weights;
• Quantify wireline log and physical characteristics of the overpressured sediments;
• Identify the origin of overpressure in each basin, and;
• Provide a basis from which overpressure can be detected prior to drilling using seismic velocity data.

Direct pore pressure measurements and mud weight data have identified overpressure in the Nappamerri Trough region of the Cooper Basin, and extensive overpressured strata in the Carnarvon Basin.

A wireline log analysis of the Cooper and Carnarvon basins has focussed on empirically determining pore pressure in low permeability lithologies (e.g. shales) using the Eaton and equivalent depth methods. There is a complex acoustic velocity-depth relationship in the Cooper Basin that cannot be simply explained by variations in pore pressure and compaction. A detailed sonic log analysis incorporating uplift and other wireline logs was needed to gain pore pressure estimates that accurately reflect direct pressure measurements and mud weights. There is a 30–45 ms/m sonic anomaly associated with the overpressured seiments that may be detectable using seismic velocity data.

In the Carnarvon Basin pore pressure estimates in thin shale sequences, derived from sonic logs, accurately reflect direct pressure measurements in adjacent sandstones. However, log-derived pore pressure estimates were far in excess of mud weights in thick shale sequences. It is unclear whether the sonic log-derived pressure estimates in these thick shale sequences accurately reflect pore pressure (i.e. the formations were drilled underbalanced) or whether the sonic anomaly is unrelated to pore pressure. The variation in sonic anomaly associated with the overpressured sediments would need to be considered in the pre-drill seismic detection of overpressure in the Carnarvon Basin.

The origin of overpressure in the Cooper Basin and Carnarvon Basin has been investigated using a combination of wireline log analysis and basin history analysis. Wireline log analysis involves comparing the physical properties, especially porosity, of the normally pressured and overpressured sediments (e.g. porosity-effective stress analysis). Additionally, the burial, thermal and tectonic histories of each basin have been studied to identify the timing and mechanism of overpressure generation. The Cooper Basin has not been subjected to significant sedimentation since the Late Cretaceous (90 Ma), and reached maximum paleotemperature before 75 Ma. Hence, the origin of overpressure in the Cooper Basin is not easily explained by the commonly cited burial or temperature driven processes, and is most likely related to an increase in horizontal stress acting since the Late Cretaceous. The origin of overpressure in the Carnarvon Basin is predominantly disequilibrium compaction related to Tertiary burial, with minor contributions from hydrocarbon generation and lateral transfer.