Good Report About Stratigraphy Of The Perth Basin And The Economic Geology Of Its Contained Fossil Fuels

Abstract

The Perth Basin is an offshore and onshore sedimentary basin that extends about 1300 kilometers on the southwestern side of Australia. This is a structurally complex basin that covers a total area of over 172,300 square kilometers. The most dominant structural feature in the Perth Basin is the Darling Fault. Over time, the fault has been obscured by the deposition of sediments. Besides the predominant Darling Fault, the architecture of the basin is also characterized by the antithetic, synthetic and northward trending listric and planar faults. It is noteworthy that while these structural features were clearly conspicuous at the time of formation, continuous deposition of sediments in the basin over time and the subsequent formation of sedimentary rocks have obscured the once visible fault lines. The Perth Basin is composed of various stratigraphy units, arranged in a chronological order based on the time in which they were formed. The most uppermost is the cretaceous where the rock formations are mostly sandstone, shale and gingin chalk. The second unit is the Jurassic with rock formations such as quartz sandstone, and coal seams. The lithology of the third unit (Triassic) is made of green-gray sandstone and light-grey sandstone. The final stratigraphy unit is the Permain with a lithology of argillaceous, fluviolacustrine sandstones, basal diamictite and siltstones. The geological evolution of Perth Basin happened in three predominant phases. The phases are the Silurian to Carboniferous phase, the Permian to early Neocomian phase and the Late Neocomian to Holocene phase. The Perth Basin has trapped fossils. These fossils have some economic value. The knowledge of this has led to explorations and extraction. Some of these studies commissioned to explore the prospectivity report six commercially viable hydrocarbon fields, in the northern Perth Basin. The largest of these files is the Dangora hydrocarbon field. The hydrocarbons explored and produced in the Perth Basin include oil and natural gas. The trapping allowing the hydrocarbons to be explored is the fault trap. By today, twenty-six accumulations of petroleum in the Perth Basin have been discovered. Of the twenty-six, thirteen are commercially viable. The production of petroleum and natural gas is mainly done in the Beharra Springs, Dongara, Yardarino, Walyering, Mt. Horner and Woodada

Introduction

The Perth Basin is an offshore and onshore sedimentary basin that extends about 1300 kilometers on the southwestern side of Australia. This is a structurally complex basin that covers a total area of over 172,300 square kilometers. From a geological perspective, the Perth Basin is a region of significance to Australia (Iasky, 1993). The region contains sedimentary rocks among other geological features. There are significant fault lines that also define the region. Besides this, the stratigraphy of the Perth Basin provides more that information on rock formation over time. Many discoveries have been made through explorations carried out in this area (Iasky, 1993). For instance, the onshore sedimentary basin has oil accumulations, gas accumulations and gas fields. The offshore sedimentary basin has oil fields and gas accumulations (Iasky, 1993).
This report explores the stratigraphy of the Perth Basin in Australia. Additionally, the report also explores the economic aspects of the fossil fuels that are trapped in the sedimentary rocks in the Perth Basin. In writing this report, I will use literature from geological surveys in the area. The geological survey conducted by Mory and Iasky (1996) provides useful insights on not only the stratigraphy of the basin, but also the economic components of the fossil fuels trapped in the sedimentary rocks. The report by Causebrook, Dance and Bale (2006) also provides insights on the stratigraphy of the area and the geological model through which carbon dioxide and fossils are trapped and stored in the sedimentary rocks in Perth Basin. Mining and minerals reports, such as the one prepared by Iasky (1993) will also inform the writing of the report. Besides these elements, the report will also delve into the structural settings, stratigraphy, geological evolution and economic geology of the Perth Basin.

Structural Setting

As highlighted earlier, the Perth Basin in Australia is a faulted trough that is filled with sediments that have been deposited over many historical eras. The most dominant structural feature in the Perth Basin is the Darling Fault. Over time, the fault has been obscured by the deposition of sediments. During the formation of the basin, the Darling Fault was very conspicuous. Presently, the fault is located between one and three kilometers west of the surface expression of the fault. The approximate length of the Darling Fault is 1000 kilometers, with an approximate throw of 15,000 meters. Although the Darling Fault is the most predominant structural feature, the Perth Basin has numerous other faults, most of them located in the north and northwest.
Besides the predominant Darling Fault, the architecture of the basin is also characterized by the antithetic, synthetic and northward trending listric and planar faults (Leyland, 2011). The normal faults in the Perth Basin intersect with transfer structures. The transfer structures in the basin vary in character and throw from the north to the south of the basin (Leyland, 2011). Towards the most northern end of the basin, the faults that are trending northwards are terminated locally by transfer faults that are trending eastwards of the basin. The normal faults in the rest of the Perth Basin are deformed and deflected transfer zones that strike northwards. This structural pattern does not exhibit any continuous fault plane (Leyland, 2011).
It is noteworthy that while these structural features were clearly conspicuous at the time of formation, continuous deposition of sediments in the basin over time and the subsequent formation of sedimentary rocks have obscured the once visible fault lines (Leyland, 2011). Geological data can still locate with near accuracy the location of these fault lines. For instance, the Darling Fault at present is located between one and three kilometers west of the surface expression of the fault (Leyland, 2011). Geological surveys in the Perth Basin keep monitoring any changes in the structural position of these features, as a result of tectonic forces (Leyland, 2011).

Stratigraphy of the Perth Basin

The stratigraphy of the Perth Basin is best understood when examined based on the age at which the various layers of rocks were formed through deposition.
Figure 1 showing the stratigraphy column of the Perth Basin as adopted from Nicholson et al., (2008).
Cretaceous: The most predominant features are the Bunbury Basalt. The rock formations in this layer of deposition are mostly the sandstone, South Perth shale, gingin chalk and formations such as the Warnbro Group and the Pamelia Group (Government of Western Australia, 2014).
Jurassic unit: this is the second layer, and is composed of continental deposits. The thickness of this section is between 429 meters towards the coast and 635 meters towards the eastern side of Bunbury. The lithology of this unit is composed of coarse-grained quartz sandstone that is interbedded with carbonaceous fine-grained clastic rocks and coal seams. The coaly intervals in this stratigraphy unit give an indication that deposition was done in a marshy embayment (Government of Western Australia, 2014). The sandstone formations indicate that they were deposited through distributary channels occurring in a paludal and fluvial braided complex. The Yarragadee formation in the Perth Basin occurs in this stratigraphy unit. The lithology in this formation is composed of sandstone that is fine-to-coarse-grained, feldspathic and poorly sorted. The sandstone is blended with coal and thin layers of siltstone and shale (Government of Western Australia, 2014).
Triassic: This stratigraphy unit is the third in the chronological order in which they appear in the Perth Basin. This unit is composed of various formations. These formations have different lithology. For instance, the Sabina formation is composed of green-grey sandstone that is poorly consolidated, moderately sorted, and micaceous. This formation has a thickness of between 226 meters and 461 meters, although towards the northeaster of Bunbury through, the thickness of the formation is over 500 meters (Government of Western Australia, 2014). This is attributed to the frequent formations of claystone, shale and sitstone beds. The Wonnerup member of the Lesueur formation is composed of light-grey sandstone that is homogenous, coarse and in some sections coarse-grained, poorly sorted and feldspathic. The Yalgorup member of the Lesueur formation predominantly consists of dark-grey sandstone (Government of Western Australia, 2014).
Permain: During this era, the predominant depositions in the Perth Basin were argillaceous, fluviolacustrine sandstones, basal diamictite and siltstones. The Sue group in this stratigraphy unit was comprised of feldspathic sandstone that is poorly sorted. There are subordinate intervals of shale and siltstone (Government of Western Australia, 2014).This layer also is comprised of fossil fuels. The fossil fuels found in the Permain stratigraphy are the sub-bituminous to bituminous coal. This characteristic of the sub-bituminous to bituminous coal included its thin depositional layer, sporadic throughout the basin and its lenticular nature (Government of Western Australia, 2014).

Geological Evolution

Perth Basin was formed during the Palaeozoic and Mesozoic era along the western margin in Australia. According to Cockbain (1990), the geological evolution of Perth Basin happened in three predominant phases. The phases are the Silurian to Carboniferous phase, the Permian to early Neocomian phase and the Late Neocomian to Holocene phase. During the Silurian to Carboniferous phase, there was deposition of the Tumblagooda Sandstone towards the north of the Perth Basin (Iassky, 1993).
Seismic and drilling data show that some of the oldest sediments in the Perth Basin, especially towards the south were in the Early Permian. The second phase ion the geological evolution of the Perth Basin involved the rifting of the inferior structure. The third and final phase of the geological evolution of the basin is called the extensional-transtensional stage. One of the most significant determinants of the geological evolution of the Perth Basin was the Darling Fault. The most significant nominal movement of the fault was experienced between the Early Triassic period and the Middle Jurassic period. In the northern offshore of the Perth Basin, tectonism that is the result of rifting happened in the Early Permain period. During the same time, there was a deposition of glacial sediments all over the Perth Basin. There was little to no faulting experienced in the Perth Basin during this time (Iassky, 1993).
However, there was extended sedimentation across the basin. A tectonic event that occurred in the Late Permian resulted in faulting; arguably causing the deep graben that is located in the Perth Basin. The Neocomian breakup event instigated the last significant faulting within the Perth Basin. However, there still is continued subsidence to date. As a result of the favorable warm shallow environments and the oceanic circulation the length of the continental margins, during the Tertiary and Late Cretaceous, there was the deposition of carbonates. During the Tertiary period, there was significant carbonate and clastic sedimentation offshore. This period also saw considerable erosion and lateralization in the onshore Perth Basin. The Perth Basin has been formed through deposition, sedimentation and erosion over long periods of time (Iassky, 1993).

Economic Geology

As highlight above, the Perth Basin has trapped fossils. These fossils have some economic value. The knowledge of this has led to explorations and extraction. Numerous studies have been commisioned to determine the prospectivity of hydrocarbons, particularly petroleum and gas in the Perth Basin (Owad-Jones & Ellis, 2000). Some of these studies report six commercially viable hydrocarbon fields, in the northern Perth Basin (Owad-Jones & Ellis, 2000). The largest of these files is the Dangora hydrocarbon field (Owad-Jones & Ellis, 2000). There are many gas fields in the Perth Basin. Most of them are constituted differently, and as such have different properties. This section of the paper will offer an overview some of the gas fields (Owad-Jones & Ellis, 2000).
One of the predominant gas fields is the Beharra Springs gas filed. The major type of hydrocarbon extracted from this gas field is natural gas (Owad-Jones & Ellis, 2000). The reservoir upon which this gas field is located is of the Upper Permian Beekeeper Formation (Mory & Iasky, 1996). This formation is made up of a unit of sandy limestone above which is a sequence of sandstones and a layer of claystones (Owad-Jones & Ellis, 2000). The deposition in this formation took place in the marine shelfal environment. This hydrocarbon reserve relies gravely on a lateral fault seal (Owad-Jones & Ellis, 2000).
This is because of the positioning of the Lower Triassic Kockatea Shale adjacent to the reservoir. The trap in this reservoir is the fault trap (Owad-Jones & Ellis, 2000).This is the result of the movements rocks the length of an existing fault line. In this case, the reservoir rock moved opposite the Lower Triassic Kockatea Shale that is adjacent to the reservoir (Owad-Jones & Ellis, 2000). The Lower Triassic Kockatea Shale prevents the oil in the reservoir from escaping, by that keeping it trapped for extraction. With regards to the source rock geochemistry, the average porosity of the rock is 13%. The average water composition is between 17.8% and 32.3%, especially in the gas columns (Owad-Jones & Ellis, 2000).
As highlighted earlier, the Dongara oil and gas filed is the most predominant in the Perth Basin. The major reservoir for the Dongara oil and gas field is the Upper Permian Dongara Sandstone (Mory & Iasky, 1996). This is composite reservoir that also consists of the Yardarino Sandstone, Wagina Sandstone, Basal Triassic Sandstone and the Dongara Sandstone (Owad-Jones & Ellis, 2000). They type of hydrocarbon in this field is natural gas and oil. The oil contained in the reservoir comes from mixed terrestrial and marine organic matter (Owad-Jones & Ellis, 2000).
This organic matter is located in the basal section in the Lower Triassic Kockatea Shale (Owad-Jones & Ellis, 2000). The natural gas that is extracted from this field is sourced in part from carbonaceous claystone and coal from the Lower Permian and Carynginia Formation (Owad-Jones & Ellis, 2000). With regards to the geochemistry of the source rocks, the predominant diagenetic materials include 15% of the trace to common kaolinite, 14% of the quartz overgrowths. The trap in this gas field is the structural trap type, particularly the fault trap (Owad-Jones & Ellis, 2000).
The exploration of hydrocarbons in the Perth Basin varies in terms of status from one area of the basin to another. Exploration in the northern onshore area is submature while in most offshore areas, it is immature to frontier (Crostella & Backhouse, 2000). Nonetheless, the exploration began late in the 1940s. During this time, an onshore filed survey was carried out. Offshore drilling of hydrocarbons (oil and gas) in the Perth Basin commenced in 1965. However, offshore exploration and production remains under-exploited (Cadman, Pain & Vuclovic, 1994), with as little as thirty two wells drilled by 2003 (Crostella & Backhouse, 2000).

References

Cadman, S., Pain, L. and Vuclovic, V. (1994). Perth Basin, WA. Australian Petroleum Accumulations Report 10, Bureau of Resources Sciences, Canberra
Causebrook, R., Dance, T. and Bale, K. (2006). Southern Perth Basin site investigation and geological model for storage of carbon dioxide. Retrieved from http://www.co2crc.com.au/dls/pubs/regional/perth/PerthBasin_mainreport_RPT06- 0162.pdf
Crostella, A. and Backhouse, J. (2000). Geology and petroleum exploration of the central and southern Perth Basin, Western Australia. Western Australia Geological Survey, Report 57, 85p
Government of Western Australia. (2014). Summary of petroleum prospectivity: Perth Basin. Retrieved from http://www.dmp.wa.gov.au/documents/basin_summary%281%29.pdf
Iasky, R. (1993).A structural study of the Southern Perth Basin Western Australia. Retrieved from http://www.dmp.wa.gov.au/documents/gsdrpt31.pdf
Kempton, R., Liu, K., Boreham, C., Bradshaw, B.E., Eadington, P. and Passmore, V. (2002). Oil migration and accumulation in the offshore Perth Basin, Western Australia. CSIRO Confidential Report 02-005 to Geoscience Australia, 71p
Leyland, L. (2011). Hydrogeology of the Leederville aquifer Central Perth Basin, Western Australia. Retrieved from http://www.water.wa.gov.au/PublicationStore/first/101457.pdf
Mory, A. and Iasky, R. (1996). Stratigraphy and structure of the onshore northern Perth Basin Western Australia. Retrieved from http://www.dmp.wa.gov.au/documents /5._gsdrpt46.pdf
Nicholson, C., Borissova, I., Krassay, A., Boreham, C., Monteil, E., Neumann, V., di Primio, R. and Bradshaw B. (2000). New exploration opportunities in the southern Vlaming Sub- basin, Australian Petroleum Production and Exploration Association Journal, 48 (1): 371-379.
Owad-Jones, D. and Ellis, G. (2000). Western Australia atlas of petroleum fields, onshore Perth Basin. Petroleum Division, Department of Minerals and Energy Western Australia. Retrieved from http://www.dmp.wa.gov.au/documents/ PERTH_FIELDSBKMK%285%29.pdf