Birkhead Formation Research Articles

The Talgeberry Birkhead Reservoir — A Seismic Stratigraphic Study

Recent oil discoveries in the Jurassic-Cretaceous Eromanga Basin of south-west Queensland have been in a variety of zones and many are believed to be controlled mainly by stratigraphic variations. Talgeberry-1 was completed as a dual zone oil producer from the Cretaceous Wyandra Sandstone and mid-Jurassic Birkhead Formation. An appraisal well, Talgeberry-2, drilled at a structurally higher location, did not intersect either of these reservoirs but was completed as a producer from the Early Cretaceous Murta Member. This paper presents a model for the Birkhead reservoir, which has been the most prolific producer to date. At Talgeberry-1 the Birkhead reservoir is interpreted as a fining upwards channel fill sand overlain by a coarsening upward distributary mouth bar sand. The channel fill, with good reservoir parameters, is believed to be the main producing sand. Log analysis and core data support this interpretation. The 10 m sand has a recognisable seismic signature which has been confirmed by modelling. Mapping of this signature has identified a channel approximately 600 m wide meandering from east to west. Structural mapping shows an updip location but it would have poorly developed reservoir sand and should be avoided. Further drilling for development or pressure maintenance should be within the mapped channel.

A STRATIGRAPHIC ANALYSIS OF THE TALGEBERRY OILFIELD

ATP 299P(2) is located in the south-west Queensland portion of the Jurassic Cretaceous Eromanga Basin. Exploration drilling within the permit has resulted in the discovery of several oil pools which are stratigraphically controlled. Appraisal drilling at the Talgeberry Oilfield demonstrated this point when Talgeberry-2, drilled at a structurally higher location, failed to encounter either of the producing sands in Talgeberry-1. Oil is currently being produced from the Wyandra Sandstone and Birkhead Formation in Talgeberry-1 and from the Murta Member in Talgeberry-2.Depositional models are presented for each of the producing reservoirs at the Talgeberry Field, based upon dipmeter interpretation. An integrated seismic stratigraphic study was also undertaken on the Birkhead Formation reservoir.Oil production at Talgeberry is currently confined to distributary channel sands or near-shore distributary mouth bar sands. Distal facies of the distributary mouth bar have been intersected and, while containing oil, have been found to be generally tight.Dipmeter interpretation has proven valuable in determining depositional environment but is limited in defining the a real extent of the reservoir. Seismic-stratigraphic studies are able to define the reservoir geometry better and lead to a more comprehensive understanding of the depositonal environment.

THE HUTTON SANDSTONE-BIRKHEAD FORMATION TRANSITION, ATP 269P(1), EROMANGA BASIN

The Hutton Sandstone and Birkhead Formation are the reservoir and seal, respectively, for most ofthe oil discovered in the Eromanga Basin. The traps are generally simple anticlinal closures; however stratigraphic complexities are common. Development well failures in ATP 269P(1) are commonly caused by stratigraphic variations at the Hutton-Birkhead transition.In ATP 269P(1) two distinct sandstone compositions are present over the Hutton-Birkhead transition, and are end-members of a spectrum of compositions. The first end-member consists of porous and permeable quartzose sandstone which is typical of the Hutton Sandstone. The second end-member is a lithic labile sandstone which is generally not of reservoir quality and is typical of the Birkhead Formation. Mixed and interbedded varieties are common, and are included in the Birkhead. The sandstone composition indicates that the quartzose 'Hutton' variety was derived from a stable, plutonic-metamorphic craton, whereas the lithic 'Birkhead' variety was sourced from a tectonically mobile belt, consisting of acid to intermediate volcanics, regional metamorphics, and metasediments.The Bodalla South Field displays a gradational change from quartzose sandstone in the Hutton to mixed and interbedded varieties in the Birkhead. Substantial range in reservoir quality is seen in the field and is related to sandstone composition and to a fluviodeltaic environment of deposition. In the Kenmore Field lithic sandstone, deposited in channels eroded into the quartzose sediments, provides stratigraphic enhancement of the structural closure.Similar relationships are seen in the Jackson-Naccowlah area and the Surat Basin and are linked to a change from a single provenance during deposition of the Hutton to a dual provenance during deposition of the Birkhead. This change was caused by increased activity in a mid-Jurassic arc system off the present Queensland coast and has important implications for petroleum exploration in the Eromanga and Surat Basins.

SOUTH-WEST QUEENSLAND GAS — A RESOURCE FOR THE FUTURE

Natural gas has been discovered in 22 fields in the Central Eromanga and Cooper Basins of southwestern Queensland in the area comprised by ATP 259P. Proved, probable and possible reserves in excess of 36 × 109 m3 (1.27 TCF) are located in four discrete structural provinces. Fluvial sandstones of the Early Permian Patchawarra Formation and Late Permian Toolachee Formation contain the majority of the reserves. Minor amounts of gas are reservoired in the Early Permian Epsilon Formation, the Early-Middle Triassic Nappamerri Formation and the Early Jurassic Hutton Sandstone and Birkhead Formation. Considerable gas-liquids reserves are also found in these reservoirs.Existing reserves are located primarily in structural traps although lithofacies variations are widely recognised, particularly in the Patchawarra Formation, indicating both new play opportunities and difficulties in assessing the undiscovered gas potential of the permit. Additional gas potential is identified in flank areas of the more prominent structural axes such as the Jackson-Wackett-Innamincka Trend in fault-bounded, pinchout and sub-unconformity trapping configurations.More than 200 prospects and leads are identified with the potential to entrap approximately 51 × 109 m3 (1.80 TCF) of gas on an unrisked basis. When combined with reserves from established fields, the ultimate potential of the ATP is assessed as 87 × 109 m3 (8.10 TCF).Despite the very high success rate of previous exploration and appraisal programs, the ultimate gas potential of the Queensland portion of both the Cooper and Eromanga Basins has been only partially addressed. Exploration and appraisal programs providing future additions to proved and probable reserves are considered low risk and are dependent upon an agreement with the Queensland government that would enable the ATP holders to produce and sell gas interstate.

RECENT ORGANIC GEOCHEMICAL EVALUATION OF THE CENTRAL EROMANGA BASIN

Systematic analyses of bulk properties and of molecular composition, by gas chromatography (GC), fluorescence spectroscopy, gas chromatography-mass spectroscopy (GC-MS), reflectance photometry and 'Rock-Eval' pyrolysis of core samples from three stratigraphic bores from the central and northern-central Eromanga Basin, reveal the following:The organic facies variations in the central Eromanga are considerable; however, the biofacies can be grouped into three major types.The sediments do not enter the mature zone of oil generation until a reflectance level of 0.65 per cent Rv max. or higher is reached. The Birkhead Formation, with a 'hydrogen-rich' type III kerogen, is only marginally mature and the amounts of hydrocarbons already generated are minor. In the Early Jurassic sedimentary unit (Basal Jurassic Shale Unit), however, there is a distinct increase in the level of S1/Organic Carbon indices and in the maturity of organic matter. While the extracts from younger Jurassic sequences of the basin display a substantial waxy character, the bitumens derived from the Basal Jurassic Unit are highly aromatic in character and contain few saturated hydrocarbons.

POROSITY AND PERMEABILITY OF RESERVOIRS AND CAPROCKS IN THE EROMANGA BASIN, SOUTH AUSTRALIA

When effective porosity and permeability are measured at simulated overburden pressure, and grain size variation is taken into account, two distinct relationships are evident for Eromanga Basin reservoirs. Reservoirs in the Hutton Sandstone and Namur Sandstone Member behave such that significant porosity reduction can be sustained with retention of high permeability, whereas permeability of reservoirs in the Birkhead Formation and Murta Member is critically dependent on slight porosity variations. Logging tool responses are compared with core-derived data to show in particular the effects of grain size and clay content on the gamma ray, sonic, and density tools, where clay content is assessed from cation exchange capacity measurements. Sonic and density crossplots, constructed to provide comparison with a water-saturated 'reference' reservoir, are advantageous in comparing measured effective porosity from core plugs at overburden pressure with porosity calculated from logs. Gamma ray and sonic log responses of the Murta Member in the Murteree Horst area are clearly distinct from those of all other reservoirs, perhaps partly due to differences in mineralogy and shallower depth of burial compared with other formations.

HYDROCARBON GENERATION, MIGRATION AND ENTRAPMENT IN THE JACKSON-NACCOWLAH AREA, ATP 259P, SOUTHWESTERN QUEENSLAND

The northern part of the Naccowlah Block, situated in the southeastern part of the Authority to Prospect 259P in southwestern Queensland, is a major Eromanga Basin hydrocarbon province. The Hutton Sandstone is the main reservoir but hydrocarbons have been encountered at several levels within the Jurassic-Cretaceous sequence. In contrast, the underlying Cooper Basin sequence is generally unproductive in the Naccowlah Block although gas was discovered in the Permian at Naccowlah South 1. Oil and gas discoveries within the Eromanga Basin sequence are confined to the Naccowlah-Jackson Trend. This trend forms a prominent high separating the deep Nappamerri Trough from the shallower, more stable northern part of the Cooper Basin.The Murta Member is mature for initial oil generation along the Naccowlah-Jackson Trend and has sourced the small oil accumulations within this unit and the underlying Namur Sandstone Member. The Birkhead Formation is a good source unit in this area with lesser oil source potential also evident in the Westbourne Formation and 'basal Jurassic'. Source quality and maturation considerations imply that much of the oil discovered in Jurassic reservoirs along the Naccowlah-Jackson Trend was generated from more mature Jurassic source beds in the Nappamerri Trough area to the southwest. Maturation modelling of this deeper section suggests that hydrocarbon generation from Jurassic source units commenced in the Early Tertiary. Significant oil generation and migration has therefore occurred since the period of major structural development of the Naccowlah-Jackson Trend in the Early Tertiary. This trend, however, has long been a major focus for hydrocarbon migration paths out of the Nappamerri Trough as a result of intermittent structuring during the Mesozoic. Gas reservoired in Jurassic sandstones at Chookoo has been generated from more mature Jurassic source rocks in the deeper parts of the Nappamerri Trough.Permian sediments in the Nappamerri Trough area are overmature for oil generation and are gas prone. Gas generated in this area has charged the lean Permian gas Field at Naccowlah South, along the Wackett-Naccowlah- Jackson Trend. North of this trend Permian source rocks are mainly gas prone but more favourable levels of maturity allow the accumulation of some gas liquids and oil. However, geological and geochemical evidence suggests that Permian sediments did not source the oil found in Jurassic-Cretaceous reservoirs in the Jackson- Naccowlah area.

BIRKHEAD REVISITED: FETROLOGICAL AND GEOCHEMICAL STUDIES OF THE BIRKHEAD FORMATION, EROMANGA BASIN

Significant discoveries of oil have been made in Middle Jurassic sandstone reservoirs of the Birkhead Formation, Eromanga Basin, Australia.Some, or all, of this oil has been generated within the Birkhead Formation. A large proportion of the oil appears to be in relatively subtle traps. The source rock characteristics of the Birkhead Formation have been rated as fair to very good. In an attempt to improve the success rate in searching for less obvious reservoirs, a rating system based on the factors that control the location of hydrocarbon source rocks has been applied to many wells within the Birkhead Formation. The variables measured, which relate to source potential and maturation, were (1) thickness of shale units; (2) volume and type of dispersed organic matter; (3) volume and type of coal occurrences; and (4) rank.High values of source potential as measured by this rating system occur in the Birkhead Formation overlying the southwestern and northeastern parts of the Nappamerri Trough of the Cooper Basin. These areas include, or are close to, the known discoveries within the Birkhead Formation.In addition, investigation of the nature of the hydrocarbons produced from the Birkhead Formation using pyrolysis-gas chromatography (Py-GC) of whole rock samples and GC analysis of solvent extracts from the rocks indicates a terrestrial origin for the organic matter in most cases. Waxy oils could be expected from these samples under appropriate maturation conditions.