Australia isn’t commonly thought of as a global mecca for exploration and production, but the continent holds a great deal of potential.
Marita Bradshaw, an AAPG member and retired geologist with Geoscience Australia, has spent her career helping to uncover that potential and is now pleased to see recent large discoveries on the continent and the expansion of the export LNG industry – endeavors to which her work has contributed over the decades.
A symposium will be held in her honor this month at the AAPG-SEG International Convention and Exhibition (ICE) in Melbourne. There, she will speak about the history of paleogeography in Australian exploration and where she expects the science to go in the future.
“It has come a long way from the cartoon maps we used to draw with colored pencils,” she said, “and still has a long way to go.”
She explained that continent-wide paleogeography maps were produced in the 1980s in a cooperative project between government, universities and the petroleum and minerals industries. Draft versions were hand-colored paper maps, though the final published series became the precursors for computer-assisted map production in Australia.
Today, the light table and dusty well completion reports have been replaced by the computer screen linked to digital databases and visualizations of ancient geomorphologies extracted from 3-D seismic data volumes.
Despite changes in technology, paleogeography maps still depend on data with robust time control, and their value remains in their predictive ability, whether interpolating at the reservoir field scale with 3-D- and 4-D-seismic control or extrapolating out hundreds of miles from the proximal basin edge into undrilled deepwater frontiers.
Uncovering Australia
Bradshaw became interested in geology as a child, growing up in a time when exciting missions to the moon caught the attention of the world.
“I noted that when the astronauts went to the moon, it was to collect rocks,” she recalled.
Her own interests in investigating plants, animals, rocks, aboriginal carvings and caves sealed her professional fate.
Bradshaw spent more than 30 years in petroleum geology, working for Geoscience Australia, the country’s equivalent to the U.S. Geological Survey. She also worked in the industry with Esso Australia and Western Mining Ltd.
One of her first assignments in the 1980s was to work with a team to produce paleographic maps for 70 separate time slices from the Cambrian to the Holocene periods.
“The science was fascinating, looking at the whole continent, ignoring state borders and the current coastline, and instead seeing its evolution over hundreds of millions of years,” she said.
The roughly 10-year effort generated detailed digital databases for Australian well data and biostratigraphic correlation charts.
“The synthesis of all the facies, age and geochemistry information led to the development of a classification of Australian petroleum systems,” Bradshaw said.
In the early 1990s, Bradshaw got the chance to go to work on the deepwater edge of the North West Shelf, dredging up information from 2.5 to 3.7 miles down along the margin of what is now Australia’s main hydrocarbon producing area.
“We were able to get rock samples showing that the Triassic sandstones, which now are the main gas reservoirs, extended all the way to edges of the Exmouth Plateau,” Bradshaw said of the large, mid-slope continental margin plateau lying at water depths of 0.5 to more than 1.8 miles off northwest Australia. “We also found a Late Triassic-Early Jurassic carbonate belt out on the margin in places. One of these Late Triassic reef prospects has been drilled in recent years.”
“Now, 25 years later, it is satisfying to see successful exploration on the outer margins of the North West Shelf, and that there are still key questions to be answered,” she said.
Some of these questions include:
- The origin of the Exmouth Plateau – Is it underlain by Paleozoic oceanic crust as some have recently proposed?
- What is the extent to the Early Triassic oil play recently indicated by the Phoenix South discovery?
In 2007, Bradshaw had the opportunity to facilitate another dredging program in the highly underexplored Great Australian Bight off the coast of the south side of the continent.
“The funds, vessel, equipment, people, skills, science concepts and even the weather all lined up so that Geoscience Australia was able to collect the predicted oil prone source rocks from canyons incised into the sedimentary section along the margin of the Bight Basin,” she recalled.
From paleogeography reconstructions and the geochemistry of stranded bitumens found along Australia’s southern beaches, it was believed that mid-Cretaceous oil source rocks were present in the Bight Basin, Bradshaw said. Dredging targets were identified from seismic data and further pinpointed with detailed seafloor imaging so that Turonian oil prone shales were recovered in 2007, she added.
“Now we await the outcome of exploration where several companies have taken acreage positions and have drilling commitments,” she said.
Getting On the Map
The fact that ICE is being held in Australia this year brings utter delight to Bradshaw, who has long believed in the continent’s great potential for hydrocarbons and is now seeing her predictions fulfilled.
“Australia has great E&P potential,” she said. “It already is a major exporter of LNG, both from conventional giant and super-giant offshore gas fields along the North West Shelf, and from the newly developed onshore CSG-LNG – the world’s first major LNG project sourced from coal seam gas.”
Bradshaw sees Australia as a continent with a great diversity of proven petroleum systems – “all the way from ‘the oldest oil in the world,’ recovered from Mesoproterozic rocks at 1.4 billion years old, right through to Cenozoic petroleum systems.”
However, the continent doesn’t get the exploration efforts required to find what is there, she said.
The scale of the industry is such that most effort is concentrated in the proven producing basins, and in recent years the focus has been on development and monetizing the giant gas resources, rather than exploration into frontier basins, she explained.
Yet, the investment climate in Australia is very favorable with low above-ground risk, easy access to data, and profit-based petroleum taxation.
“Some government initiatives – such as well and seismic data that is collected and eventually becomes open file for other explorers to use – help,” she said. “But a big oil discovery in a new area such as the Great Australian Bight would really help put us on the map.”