Precambrian Frontier Opportunity in Australia's Beetaloo Sub-Basin

Iain Clementson
Pacific Oil & Gas Pty. Ltd.
Box Hill, Australia

A previously unexplored sub-basin of the Precambrian McArthur basin in northern Australia is developing into an exciting new petroleum province. Exploration by Pacific Oil & Gas Pty. Ltd. the past 7 years has indicated that vast amounts of hydrocarbon have been generated from thick, highly organic rich shales present throughout the Beetaloo sub-basin.

Several potential sandstone reservoirs are conveniently situated to be charged from the source units. Pacific has recovered oil and gas from these sandstones in several exploration wells demonstrating that maturation, migration, and preservation of hydrocarbons have been effective within the sub basin. With only a few poorly defined targets drilled to date, more exploration is required to assess fully the hydrocarbon potential of the Beetaloo sub-basin.

In addition to exploration for conventionally trapped oil and gas, Pacific has also been evaluating innovative methods to recover the immense resource of unmigrated live oil present within the source units.


REGIONAL GEOLOGY
The McArthur basin lies in the northeast of the Northern Territory, Australia (Fig. 1) and comprises Precambrian sediments divided into four groups separated by regional unconformities. The petroleum potential of the uppermost Roper group has been the focus of recent exploration activity.

The upper Roper group (Fig. 2) consists of five sandstone shale cycles deposited in an extensive intracratonic basin from a variety of environments ranging from fluviodeltaic through littoral to deep marine. Two thick shale sequences have excellent source rock qualities and are adjacent to sandstones with variable reservoir characteristics.

The Roper group is extensively distributed throughout the McArthur basin but principally within the Maiwok sub-basin to the north and the concealed Beetaloo sub-basin to the south (Fig. 3). The Beetaloo sub-basin contains the thickest known Roper group sequence.


PACIFIC'S EXPLORATION
Recognizing that older basins are capable of hosting giant oil and gas accumulations, Pacific conducted a study of older Australian basins seeking petroleum opportunities.

Encouraged by the association of thick organic rich shales and potential sandstone reservoirs in the Roper group and by reported oil occurrences, Pacific took tenement over parts of the Maiwok sub-basin and St. Vidgeon inlier in 1986.

A mapping, seismic, and drilling program confirmed source quality and located live oil and residual oil occurrences within the upper Roper group. Pacific subsequently shifted attention towards the concealed Beetaloo sub-basin which was at that time only inferred from regional gravity data.

The 1 Jamison stratigraphic well in the central Beetaloo sub-basin flowed small quantities of oil and gas during drillstem tests in 1990 from the "Jamison sandstone" and a sandstone interval within the "Hayfield mudstone." These flows provided considerable encouragement, indicating that maturation, migration, preservation, and potential deliverability were favorable within the Beetaloo sub-basin.

Since 1990 Pacific has conducted semi-regional and some detailed seismic within the sub-basin and has drilled several more wells. Shows have been common, and low rate oil and gas flows have been recorded from four of these wells. To date however, Pacific has not tested an unambiguously robust target and considers that the true potential of the Beetaloo sub-basin remains untested.


SOURCE AND MATURITY
Source characteristics of the Roper group within the Beetaloo sub-basin are excel-lent.

Two thick source units are known over virtually the entire sub-basin. The Velkerri formation contains a central portion up to 350 m thick with TOC generally 3-10%. The Kyalla member is an interbedded shale and subordinate siltstone sequence where the shales have 1-3% TOC over intervals of up to 300 m.

The kerogens in both source units are of algal origin. The generative potential of immature to marginally mature Velkerri and Kyalla source materials is very good to excellent. Hydrogen indices for the Velkerri and Kyalla are up to 600 and 680, respectively. Both units can be found at all levels of maturity in the Beetaloo subbasin from pre-generation through to gas generation. Recovered oils are low sulphur and light (35-39 gravity).

Significant amounts of live but non-migrated oil are present within the source shales of the Velkerri formation. If an appropriate technique can be developed for its recovery this may prove to be a very large oil reserve.


RESERVOIR
Three sandstone units, each located immediately above or below the two major source intervals, are the principal conventional reservoir targets within the Beetaloo sub-basin. They are the Bessie Creek sandstone, the Moroak sandstone and the "Jamison sandstone." All are thick ( 80 m) with good primary porosity and permeability which have been variably affected by diagenesis. Interformational sands within the Kyalla member and Hayfield mudstone are secondary reservoir targets. Fair to good reservoir characteristics (10% porosity) can be found in all of these sandstones. Flow rates (of formation water) have exceeded 3,000 b/d.

The Velkerri and Kyalla source units may themselves be unusual potential reservoirs. They have many similarities to the gas producing Devonian shales of the Appalachian basin and to the oil and gas producing Bakken shale of the Williston basin. Naturally fractured environments in either the Kyalla or Velkerri source intervals are believed to exist in the Beetaloo sub-basin and could be important targets for an adventurous explorer.


STRUCTURE
The upper Roper group in the Beetaloo sub-basin is generally only weakly dEformed. Wrench related fault zones tens of kilometres wide are separated by large expanses of virtually flat lying strata. Strike-slip and reverse faults with enechelon positive flower structures are developed within these fault zones. Though never frequent or intense, such fault zones and associated flower structures are best developed towards the margins of the sub-basin and are attractive exploration targets.

These flower structures have at least some early component. Evidence suggests some were being formed during deposition of the uppermost Roper group and would have been available to be charged with hydrocarbons being generated from the Velkerri formation as it was loaded by the later Roper group sediments.

For much of the Phanerozoic it appears that the Beetaloo sub-basin has been stable and has not suffered any major structural event likely to have breached or dramatically modified these early structures.


THE OPPORTUNITIES
The Beetaloo sub-basin shows great potential to be a productive petroleum basin, but may be suffering an "age" problem.

If a large Mesozoic or younger basin, traversed by existing pipelines, was shown to have a superabundance of mature source rocks, acceptable sandstone reservoirs, suitable structures from which initial work had already yielded many shows and minor oil and gas flows, then it is probable that competition for exploration acreage would be strong. That would lead to a variety of ideas and techniques being applied and overall a great exploration effort would develop.

Unfortunately this is lacking in the Beetaloo sub-basin because few explorers are currently aware of, or excited by the potential of this type of older basin.

Pacific's exploration has scratched the surface of the opportunities which lie in the Beetaloo sub-basin. Structural, stratigraphic and fracture plays and leads for oil and gas exist which, in time, will be followed up.

Pacific intends to focus the next round of exploration over a broad basinal arch identified from the regional seismic and confirmed by wells drilled in 1993. This arch is believed to be an early feature and should have been a focus for oil migrating updip from the deeper parts of the sub-basin. Pacific is developing new approaches to overcome problems in seismic data quality caused by the cavernous Cambrian limestone and basalt cover and is hopeful that these approaches will allow identification of unambiguous targets for drilling.

An unconventional opportunity within the Velkerri formation is currently being assessed by Pacific's parent company, CRA Ltd. The source facies of the Velkerri formation is very thick and laterally extensive. It contains hundreds of billions of barrels of live oil generated but not expelled from the Velkerri formation.

Studies indicate that both the porosity and permeability of the source shales could be increased, in-situ, using an innovative thermal technique which may allow production of some of that oil. The technique is called Torbit (Tight Oil Recovery By Induced Thermal pressurization) and is intended specifically to address oil recovery from low permeability, clay-rich rocks. While still very much a research project, Torbit may prove an important spinoff from Pacific's exploration in the Beetaloo sub basin.


Copyright 1994 Oil & Gas Journal. All Rights Reserved.
Oil & Gas Journal June 27, 1994. Volume 92, Issue 26


Source: O&G Journal