TRACTS OFFERED IN SOUTH AUSTRALIA

South Australia has asked for bids on petroleum exploration licenses (PELs) covering 32,400 sq km in the eastern Officer basin.

PELs are to be assigned to applicants submitting winning bids by Sept. 30 to the oil, gas, and coal division of the Department of Mines & Energy, South Australia (MESA), in Eastwood, S.A.

The Officer basin is a virtually undrilled 375,000 sq km area, about the eastern one-third of which lies in western South Australia.

Most of state's portion of the basin is within areas where surface rights were returned to aboriginal people in the early 1980s. Rights to subsurface minerals remain with the South Australian government.

WHAT'S OFFERED

Areas OF94-A and OF94-B are the northernmost tracts of four on offer.

Area OF94-A is a 6,220 sq km tract with prospective sediments as thick as 4 km on acreage belonging to the Pitjantjatjara aborigines. Two wells have been drilled on the block and 526.6 km of seismic data recorded.

Area OF94-B, a 6,400 sq km tract with prospective formations as thick as 1.5 km, is an east offset to Area OF94-A. Nineteen of the 35 wells drilled on the four tracts on offer are in Area OF94-B, where more than 1,281 km of seismic data have been recorded.

Area OF94-C is a 9,150 sq km tract in the northeast comer of land belonging to the Maralinga Tjarutja aborigines. Thicknesses of prospective sediments across the tract range as high as 2.5 km. Seven wells have been drilled on Area OF94-C and more than 1,419 km of seismic data acquired.

MESA has excluded from the offering a rectangular area within Area OF94-C.

Area OF94-D is the largest and southernmost tract on offer. It covers 10,630 sq km in the southeast comer of Maralinga Tjarutja territory, across which sediments as thick as 1 km have been detected. Seven wells have been drilled on OF94-D and 96 km of seismic recorded.

PETROLEUM POTENTIAL

MESA has tried since 1988 to rekindle international interest in exploring the Officer basin.

With acquisition in 1992-93 of new multifold seismic data, reinterpretation of older seismic data, and a big advance in correlating petroleum reservoirs by using microfossil biostratigraphy the agency has identified 17 structural leads and prospects in the east part of the basin. Those leads are classed into four main trap types: extensional, compressive, salt related, and stratigraphic.

Based on a Monte Carlo simulation of 1992, before new seismic and aero-magnetic data were available, MESA estimated there is a 50% chance the eastern Officer basin's petroleum potential would exceed 5 tcf of gas or 1.8 billion st-tk bbl of oil. The simulation also found a 90% probability that eastern basin reserves would exceed 500 MMcf of gas or 450 million st-tk bbl of oil and 10% that reserves would be more than 18 tcf or 4.89 billion st-tk bbl.

The petroleum potential is focused in widespread, sequential Cambrian to Neoproterozoic clastic and carbonate reservoirs. The basin contains Neoproterozoic and Cambro-Ordovician sediments as thick as 7 km.

Late Devonian Alice Springs orogeny imposed Officer's present geology. The basin's structural highs range from anticlines in troughs to thrust sheets that indicate prominent ridges on seismic and aero-magnetic data. But the basin is mostly hidden beneath the vegetated dunes of south-central Australia's Great Victorian Desert, so there is little topographic expression of subsurface structure.

Because of thicknesses and wide areal extents, sandstone reservoirs are the main apparent exploration targets in the basin. Only five wells in the basin have reached depths below 2 km, and few of the basin's numerous sandstone reservoirs have been penetrated by oil or gas tests.

Shows of oil belonging to four distinct families have been found in six Officer basin formations, implying existence of multiple source rocks.

OFFICER BASIN SANDS

MESA found that darcy permeabilities and porosities of more than 15% are common in seven sands for which data exist from the basin's Mesoproterozoic basement to the early Ordovician. The seven sandstones and summary reservoir characteristics are:

Willouran Pindyin is a widespread, clean, aeolian sandstone encountered by 1 Giles wildcat on Area OF94-C with outcrops on the north margin of the Birksgate subbasin. Pindyin porosity ranges from 3.8% to 22.5% with a mean of 11.8% and permeability from 0.04 to 1,538 md with a mean of 48 md.
Ediacarian Tarlina, an alluvial-tidal-shallow marine sandstone, disconformably overlies the Alinya formation on the Murnaroo platform. Tarlina porosity ranges from 9% to 19.6% with a mean of 15.9% and permeability from 0. 16 to 4. 5 md with a mean of 1.2 md.
Ediacarian Murnaroo occurs on the Murnaroo platform and east of the Manya trough, cutting southwest to northeast across areas OF94-A and OF94-B. Feldspathic, micaceous, and locally glauconitic, the formation's porosity ranges from 1.2% 18.8% with a mean of 14% and permeability from 0.01 213 md with a mean of 20 md.
Early Cambrian Relief sandstone disconformably overlies Proterozoic intervals in the Manya trough, with aeolian, fluvial, and tidal facies related to marine lowstand-highstand cycles. Relief porosity is related to burial depths reached during the Alice Springs orogeny and averages 2.9% in footwall traps, 6.7% in hanging wall traps, and 16.4% in areas far from the Marla overthrust zone detected in the northern parts of areas OF94-A and OF94-B. Permeability as great as 8 darcies has resulted from dissolution of clay and carbonate cements in the formation.
Cambrian Arcoeillinna, an immature, muddy micaceous arkose with widely varying reservoir properties, extends through the Manya and Munyarai troughs and onto the Murnaroo platform. The sandstone's porosity averages more than 13%, with permeability ranging from 0.1 to 1,700 md.
Cambrian Trainor Hill sandstone is finely grained and usually quartzose and extends through the Manya and Munyarai troughs and onto the Murnaroo platform. The sand's porosity averages 15%, with permeability measured as high as 100 md.
Ordovician Mount Chandler is a clean, quartzose sand with good reservoir potential. The formation's porosity ranges from 12.7% 19.7% and permeability from 0.26 to 238 md. It presents an attractive footwall trap target in overthrust zones. However, depending on its stratigraphic position, there is a risk that Mount Chandler sands lack seals because of Permian erosion.

BASIN ACTIVITY

Seismic surveying and exploratory drilling near Marla in Area OF94-B by a unit of Conoco Inc. in the 1960s and by MESA in the 1970s found small amounts of oil, the first reported discoveries in the basin. The work by Conoco and MESA also began to reveal the Officer basin's complexity.

A more coherent, integrated picture of eastern Officer's potential began emerging when South Australia's $40 million exploration initiative in 1992 began funding studies of the basin by MESA, the University of Adelaide, National Center for Petroleum Geology & Geophysics, and oil and gas companies. Using funds from the initiative, MESA undertook a $2 million program to expand knowledge of South Australia's portion of the 1,100 km by 770 km wedge-shaped basin, based on reinterpretation of Comalco seismic and mineral drillhole data and acquisition of 378 km of seismic data on Pitjantjatjara land. The exploration initiative also funded new studies of Conoco, Comalco, and MESA wildcat well cores that yielded well preserved acritarch microfossils. Other studies concentrated on sandstone reservoir properties and on a new possible hydrocarbon source rock near the base of Officer's sedimentary sequence.

Under an access agreement with the Pitjantjatjara Council, Geosystems Pty. Ltd., Perth, in August and September 1993 collected another 900 miles of seismic data, creating an acoustic picture that revealed the basin's large horizontal faults that form major structures capable of trapping hydrocarbons.

APPLICATION PROCEDURE

The government of South Australia has no set form for PEL applications. However, state officials stress the government solidly supports exploration for and development of natural resources within its jurisdiction. Similarly the basin's aboriginal people welcome geophysical surveying on their lands. Each PEL application requires a $2,000 fee and a proposal for a 5 year work program, including details of the applicant's technical and financial resources and a map of the application area. A bond of at least $15,000 is required to assure compliance with PEL conditions.

PEL initial terms are for 5 years, and as many as three optional 5 year renewals are allowed. Yearly rentals escalate in each subsequent 5 year renewal term from 24/sq km in the initial 5 years to 36/sq km in the first 5 year renewal, 48(t/sq km in the second 5 year renewal, and 60/sq km in the third and final 5 year renewal. Applications for renewals require a $1,000 fee. Yearly minimum spending required during the first 2 years of the initial PEL term is $16/sq km and $24/sq km in the next 3 years. Yearly spending of $100/sq km is required in the first 5 year renewal period, $125/sq km in the second 5 year renewal period, and $150/sq km in the last 5 year term. South Australia requires petroleum exploration licensees to relinquish 25% of a PEL area at the end of each 5 year term, in addition to areas given up voluntarily.