LARGE areas of shale in Victoria’s Otway Basin are highly brittle in the same way as prolific unconventional basins in the US, according to a pioneering Australian study.
The research is the basis of a PhD study at the Australian School of Petroleum in Adelaide by David Tassone, who has subsequently taken up a position at Woodside.
The PhD supervisor and co-author was ASP’s Dr Simon Holford. Ian Duddy and Paul Green of Geotrack International in Melbourne and Richard Hillis of the Deep Exploration Technologies CRC were also co-authors.
A paper on the research was the cover story in last month’s Bulletin of the American Association of Petroleum Geologists.
Holford told Energy News the study was believed to be a world-first in looking at the effects of basin uplift or exhumation on the prospectivity of unconventional targets.
“As we put together the paper, it became clear that most though not all of the big unconventional basins in the US contained shales and other rocks that have been more deeply buried and uplifted to shallower depths,” he said.
“It’s been quite well known that if you have mud-dominated rocks, they deform in a ductile manner at quite shallow depths.
“If you bury them deep enough, their density increases and they become more brittle. If there is subsequent uplift, these rocks don’t become any less brittle and they are easier to break by hydraulic fracturing.
“That can be a bad thing for conventional plays because it has implications for seal integrity, but conversely for unconventional targets this makes it much easier to create permeability.
“That led us to think about what are the implications of uplift and erosion for prospectivity.
“Quite a lot of research has been done in the past decade on the effects of uplift and erosion on conventional basins, but not shale formations and other unconventional targets.”
Holford said many petroleum basins simply subsided, such as the Niger Delta and the Gulf of Mexico, but many basins had rocks that were brought back up.
“Any onshore basin with marine rocks has logically been through a period of uplift, whether due to inversion of a normal fault or mantle plume activity,” he said.
The study selected a technique known as sonic transit times to measure the density of over-compacted shales and estimate the amount of uplift or exhumation.
The technique has been used widely in the North Sea and around the British Isles, and in a more limited way in Australian studies of the Eromanga and Canning Basins.
Holford said exhumation estimates had been made in the past for parts of the Otway Basin using vitrinite reflectance and apatite fission track data.
“Some great work had been done in the Otway Basin using these palaeo geothermometers, but these are quite expensive techniques and are restricted to certain locations,” he said.
“The advantage of sonic transit time analysis is it can obtain information on compaction wherever you have wireline log data, so you can get a better idea of spatial patterns of exhumation of a basin.”
The study used sonic transit time data from 110 wells to generate the first basin-wide picture of exhumation in the Otway Basin.
Holford said the results of sonic transit time analysis were consistent with exhumation estimates from other techniques.
“That validates both methods, and gives us confidence we can use sonic transit times to map out exhumation across the Otway Basin,” he said.
The study found net uplift in the Otway Ranges of more than 1500 metres over the past 90 million years, including significant uplift in only the past 10 million years. This confirmed previous estimates.
This uplift has shaped the development of Port Campbell’s 12 Apostles and other famous features along the Otway Ranges coastline.
Coincidentally, the prolific Barnett Shale in the US has also experienced net uplift of about 1500m.
However, the study also points out major differences in the depositional environment, source rock quality and mineralogy of the Otway Basin and the US shales.
The area of greatest uplift is localised around the Otway Ranges, but is still significant around the Merino High (more than 690m), Colac Trough and Torquay Subbasin (more than 560m).
Further west, there has been only minor exhumation (less than 200m) in the Port Campbell embayment, the Shipwreck Trough and the around the Penola gasfields in South Australia.
Holford said the study highlighted the potential for tight gas plays in the Eumeralla Formation, which outcropped in the Otway Ranges and extends across most of the basin.
“We looked at the history of wells in the area particularly those in the eastern Otway basin,” he said.
“Many of the early wells penetrated the entire stratigraphy and recorded gas and condensate shows in the Eumeralla formation, but explorers consistently found there was little primary porosity or permeability.
“As exploration progressed, all the gas fields were discovered around Port Campbell in the shallower stratigraphy in the Waare sandstones. The potential of the Eumeralla was overlooked subsequently.
“We’re not making any claims about the actual prospectivity of these shales, but there have been a lot of gas and condensate shows. If porosity and permeability was an issue in the past, fraccing could unlock that potential.”
The study has the potential to increase pressure on the Victorian government to lift its ban on fraccing.
The ban is due to expire in July 2015, about six months after the next state election.
While there has been significant opposition to fraccing among local communities, there could be an opportunity for energy explorers to position tight gas in the Otway Ranges as one of the antidotes to closure of major industries in nearby Geelong.
