Geological Controls on Overpressure in the Northern Carnarvon Basin

Abstract

A small, but significant fraction of wells drilled in the Northern Carnarvon Basin have encountered problems with overpressure: better pore pressure prediction would improve safety and economy for drilling operations. In the Northern Carnarvon Basin the occurrence of overpressure and likely mechanisms are under investigation as part of the Australian Petroleum Cooperative Research Centre (APCRC) Research Program on Pore Pressure Prediction. Previous workers have proposed a number of mechanisms as the main cause of overpressure including undercompaction, hydrocarbon generation, horizontal stress and clay reactions.A preliminary regional study was undertaken incorporating over 400 well completion reports which identified approximately 60 wells with mud weights greater than 1.25 S.G. A subset of these wells was investigated and more reliable but much scarcer pressure indicators such as kicks or direct pressure measurements were examined. Depth-pressure profiles of wells across the region are variable and commonly show pressure compartmentalisation. Using a range of indicators, it was observed that overpressured strata in the Barrow Subbasin:occur over a wide depth range (2,500 to 4,000+ mbsl);occur over a wide stratigraphic range (Late Triassic to Late Cretaceous);are not regionally limited by major structural boundaries;are associated with sequences dominated by finegrained sediments with variable clay mineralogy; and in depositionally, or structurally, isolated sandstones; andmainly to the west of the Barrow and Dampier Subbasins around the Alpha Arch and Rankin Trend, coinciding with thickest Tertiary deposition.Previous published work in the study area has tended to support hydrocarbon generation as the primary cause of overpressure, though more recent publications have emphasised compaction disequilibrium. The log response (DT, RHOB and NPHI) of overpressured clay-rich strata has been investigated to constrain the type of overpressure mechanism. A normal compaction trend has been derived for four stratigraphic groupings; Muderong Shale, Barrow Group, Jurassic and Triassic. All overpressure occurrences were accompanied by an increase in sonic transit time. Not all wells have suitable log data for evaluation, but all stratigraphic groups show some evidence of elevated porosity associated with overpressure consistent with disequillibrium compaction as a dominant mechanism. Overpressures in the Barrow Group in Minden-1 and the Jurassic section within Zeepaard–1 do not have accompanying porosity anomalies suggesting a different overpressure mechanism model is needed.

Overpressure transmission through interconnected igneous intrusions

“…Tingay et al (2007) demonstrated the likely vertical transfer of overpressure up normal faults within the inner shelf of the Baram Delta, Borneo. Other notable areas where such pressure transfer is documented include that of the Northern Carnarvon Basin, northwest Australia shelf (van Ruth et al, 2000;Dodds et al, 2001;Tingate et al, 2001;Hoskin et al, 2015), and the Qaidam Basin, northwest China (Fan et al, 2016).…”

Overpressure transmission through igneous intrusions: An unrecognized drilling hazard in volcanic affected basins?

“…Tingay et al (2007) demonstrated the likely vertical transfer of overpressure up normal faults within the inner shelf of the Baram Delta, Borneo. Other notable areas where such pressure transfer is documented is that of the Northern Carnarvon Basin, Northwest Australia Shelf (van Ruth et al, 2000;Dodds et al, 2001;Tingate et al, 2001;Hoskins et al, 2015) and the Qaidam Basin, northwest China (Fan et al, 2016).…”

The Effect of Unconformities and Subsequent Loading on Pore Pressure Profiles

“…The wells we have analysed in this paper are cited as being mainly affected by disequilibrium compaction (Nyein et al, 1977, Vear, 1998, Swarbrick and Hillis (1999, Tingate et al, 2001, van Ruth et al, 2004and Sagala and Tingay, 2012. The Barrow wells also have not had significant Tertiary re-burial (Tingate et al 2001). Swarbrick (2012).…”

“…The loading model in the Madeleine Trend and Kendrew Trough areas will not work in the Barrow sub-basin, where wells such as Barrow Deep-1 and Barrow L35J and F-24J are similarly uplifted (Dodds et al, 2001;Tingate et al 2001, van Ruth et al, 2004and Sagala and Tingay, 2012. These wells have been influenced by fluid expansion processes rather than purely disequilibrium compaction (Zaunbrecher, 1994, van Ruth et al, 2004and Sagala and Tingay, 2012.…”