Officer Basin
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The Officer Basin is an intracratonic
sedimentary basin Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock They form when long-term subsidence ...
that covers roughly 320,000 km2 along the border between southern and western Australia. Exploration for
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and Hydrophobe, hydrophobic; their odor is usually fain ...
s in this basin has been sparse, but the geology has been examined for its potential as a
hydrocarbon reservoir A petroleum reservoir or oil and gas reservoir is a subsurface accumulation of hydrocarbons contained in porous or fractured rock formations. Such reservoirs form when kerogen (ancient plant matter) is created in surrounding rock by the presenc ...
. This basin's extensive depositional history, with sedimentary thicknesses exceeding 6 km and spanning roughly 350 Ma during the
Neoproterozoic The Neoproterozoic Era is the last of the three geologic eras of the Proterozoic geologic eon, eon, spanning from 1 billion to 538.8 million years ago, and is the last era of the Precambrian "supereon". It is preceded by the Mesoproterozoic era an ...
, make it an ideal candidate for hydrocarbon production. Along with other nearby sedimentary basins of similar age (
Amadeus Basin The Amadeus Basin is a large (~170,000 km2) intracratonic sedimentary basin in central Australia, lying mostly within the southern Northern Territory, but extending into the state of Western Australia. Origins The Amadeus Basin is name ...
,
Georgina Basin The Georgina Basin is a large (c. 330,000 km2) intracratonic sedimentary basin in central and northern Australia, lying mostly within the Northern Territory and partly within Queensland.Smith, KG (1972). "Stratigraphy of the Georgina Basin. ...
), the Officer Basin is believed to have once been part of the hypothetical
Centralian Superbasin The Centralian Superbasin is a large intracratonic sedimentary basin which occupied a large area of central, southern and western Australia during much of the Neoproterozoic Era (~830–540 Ma). This intra-cratonic superbasin was disrupted by ...
which was fragmented during several episodes of tectonic activity.


Geologic history

Australia is characterized by ancient
craton A craton ( , , or ; from "strength") is an old and stable part of the continental lithosphere, which consists of Earth's two topmost layers, the crust and the uppermost mantle. Having often survived cycles of merging and rifting of contine ...
s that detail the complex tectonic and
geologic history The geological history of Earth follows the major geological events in Earth's past based on the geologic time scale, a system of chronological dating, chronological measurement based on the study of the planet's rock layers (stratigraphy). Ear ...
that has occurred over the course of Earth's evolution. The most notable geologic events that resulted in regional
rifting In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear Fault (geology), downfaulted depression, called a graben, or more commonly ...
of these ancient cratons occurred during
Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...
, when sea floor spreading resulted in the rifting between Australia and
Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean (also known as the Antarctic Ocean), it contains the geographic South Pole. ...
. The breakup of
Gondwana Gondwana ( ; ) was a large landmass, sometimes referred to as a supercontinent. The remnants of Gondwana make up around two-thirds of today's continental area, including South America, Africa, Antarctica, Australia (continent), Australia, Zea ...
resulted in the creation of large onshore and offshore sedimentary basins between the western and eastern Australian cratons. These basins are home to much of the country’s hydrocarbon reservoirs. However, the history of the Officer Basin is distinct from the regional tectonic history in that much of its potential hydrocarbon potential predates the rifting that took place in the Mesozoic. The Officer Basin refers to the sedimentary units that are Neoproterozoic in age, as it is overlain by the younger Gunbarrel Basin which passes north into the Canning Basin. The depocenter of the Officer Basin was along its northern edge, which resulted in an asymmetric accumulation of sediment that gently thins towards the south, with the thickest accumulation in the north. The sediments that constitute the Officer Basin were first deposited upon an unconformable surface over variably
metamorphosed Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...
basement rock In geology, basement and crystalline basement are crystalline rocks lying above the mantle and beneath all other rocks and sediments. They are sometimes exposed at the surface, but often they are buried under miles of rock and sediment. The baseme ...
s. This initial deposition is commonly referred to as "Supersequence 1", and contains the probable reservoir Buldya Group, overlain by a seal sequence containing the Hussar, Kanpa and Steptoe groups. After this depositional sequence, the basin experienced significant folding and
erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...
from the Areyonga movement, which was followed by subsidence during the Marinoan glacial period. The remaining sediments which overlie Supersequence 1 are from other tectonic events, such as the Petermann/Paterson orogeny, which eventually culminate in an unconformable surface at the Delamerian Orogeny (~490 Ma), marking the end of deposition within the Officer Basin. Extensive tholeiitic lava flows characterize the sequences above the unconformity, and mark the basal sequence of the overlying Gunbarrel Basin. The main sedimentary sequence that holds the largest potential for hydrocarbon production and storage is the Buldya Group, which includes coarse siliciclastics overlain by mixed
siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility. Although its permeabil ...
,
shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...
,
carbonate A carbonate is a salt of carbonic acid, (), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word "carbonate" may also refer to a carbonate ester, an organic compound containing the carbonate group ...
and
evaporite An evaporite () is a water- soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as oce ...
succession. The thickest part of this basin sequence is in the north, and thins out to the south and west. Regional tectonism during the Neoproterozoic resulted in the formation of several distinct structural zones that intersect the deepest part of the basin. These zones primarily formed as a result of regional compression, and have been correlated to the Petermann/Paterson Orogeny. The Officer Basin itself likely functioned as a foreland basin during this orogeny roughly 550 Ma, and it shows remarkable preservation of laterally continuous units. Compression from this orogenic event also mobilized salts, resulting in the formation of
diapir A diapir (; , ) is a type of intrusion in which a more mobile and ductilely deformable material is forced into brittle overlying rocks. Depending on the tectonic environment, diapirs can range from idealized mushroom-shaped Rayleigh–Taylor ...
s in the deeper portions of the basin.


Stratigraphy - Supersequence 1


Browne formation

The most notable formations in the Officer basin, in terms of thickness and hydrocarbon potential are the Browne, Hussar and Kanpa. The Browne formation, much like the other formations within the Officer, is deepest in the central portion of the basin, up to several kilometers. This is typical of the Officer basin, as it is deepest in the central portion in the east, and generally shallows westward. The Browne formation constitutes the most identifiable and thickest basal unit of the Officer, and shows remarkable lateral continuity, with limited change in
facies In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with distinctive characteristics. The characteristics can be any observable attribute of rocks (such as their overall appearance, composition, or con ...
across the basin. The
lithology The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lit ...
of the Browne is largely shale and
mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...
, but can vary from dolomite to
dolomitic Dolomite () is an anhydrous carbonate mineral composed of calcium magnesium carbonate, ideally The term is also used for a sedimentary carbonate rock composed mostly of the mineral dolomite (see Dolomite (rock)). An alternative name sometimes ...
limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...
, to thick evaporitic sequences. Based on this, it is likely that the Browne formation's provenance was an
intertidal The intertidal zone or foreshore is the area above water level at low tide and underwater at high tide; in other words, it is the part of the littoral zone within the tidal range. This area can include several types of habitats with various sp ...
to lagoonal environment. Due to the prevalence of evaporites within the Browne, diapir formation is common and there are many instances of
breccia Breccia ( , ; ) is a rock composed of large angular broken fragments of minerals or Rock (geology), rocks cementation (geology), cemented together by a fine-grained matrix (geology), matrix. The word has its origins in the Italian language ...
ted or contorted strata proximal to diapirs. The Browne has little potential as a hydrocarbon reservoir or source, as it likely reached maximum hydrocarbon production early in the basin’s history. Thus, much of the hydrocarbon production was exhausted during the Neoproterozoic.


Hussar formation

The Hussar is the next youngest layer above the Browne, albeit a much thinner layer in comparison. The Hussar’s evaporitic content is almost nonexistent; instead this formation is predominantly
sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...
and shale, sometimes interbedded with dolomite. The lack of evaporites in the Hussar contrasts sharply with the
halite Halite ( ), commonly known as rock salt, is a type of salt, the mineral (natural) form of sodium chloride ( Na Cl). Halite forms isometric crystals. The mineral is typically colorless or white, but may also be light blue, dark blue, purple, pi ...
-rich underlying Browne formation. In addition, the basal unit of the Hussar is a very distinct, near 100 m-thick mudstone unit that has been traced in seismic logs, making it a significant
marker horizon Marker horizons (also referred to as chronohorizons, key beds or marker beds) are stratigraphic units of the same age and of such distinctive composition and appearance, that, despite their presence in separate geographic locations, there is no do ...
. Several
progradation In sedimentary geology and geomorphology, the term progradation refers to the growth of a river delta farther out into the sea over time. This occurs when the volume of incoming sediment is greater than the volume of the delta that is lost through ...
al sequences have been identified within the Hussar, suggesting that its
depositional environment In geology, depositional environment or sedimentary environment describes the combination of physical, chemical, and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will b ...
was a transitional one, ranging from shelf and
shoreline A coast (coastline, shoreline, seashore) is the land next to the sea or the line that forms the boundary between the land and the ocean or a lake. Coasts are influenced by the topography of the surrounding landscape and by aquatic erosion, su ...
to
tidal flat Mudflats or mud flats, also known as tidal flats or, in Ireland, slob or slobs, are coastal wetlands that form in intertidal areas where sediments have been deposited by tides or rivers. A global analysis published in 2019 suggested that tidal ...
and
fluvial A river is a natural stream of fresh water that flows on land or inside caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of its course if it ru ...
.


Kanpa formation

The Kanpa formation overlies the Hussar, and again the trend of younger formations being less thick than the previous formation continues. The Kanpa formation is unique because it contains a very compact sequence of basaltic flows, called the Kenne Basalt, which has minimal interbedded sediments. Generally, the Kanpa is a mixed siliciclastic-carbonate sequence of interbedded dolomite, with shale, evaporites, and
chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a prec ...
. Detrital zircons within the uppermost sandstone layer of the Kanpa were dated using U-Pb dating, and provide a maximum age constraint of 725 Ma. The prevalence of
stromatolite Stromatolites ( ) or stromatoliths () are layered Sedimentary rock, sedimentary formation of rocks, formations (microbialite) that are created mainly by Photosynthesis, photosynthetic microorganisms such as cyanobacteria, sulfate-reducing micr ...
s within the Kanpa suggest that its depositional setting was a carbonate-dominated shallow marine to tidal flat setting.


Steptoe formation

The Steptoe formation overlies the Kanpa, and constitutes the final member of Supersequence 1. Its thickness is not well constrained, as it has only been observed in 2 cores to date, but can be up to ~500 m thick. The lithology of the Steptoe is largely sandstone and dolomite, which typically grade into siltstone. The age of this formation is also unconstrained, but it must be younger than 725 Ma, the age of the top of the Kanpa. Due to the lithologic similarities between the Kanpa and the Steptoe, it is likely that their depositional settings were similar, although the Steptoe’s was likely more restricted. More in-shore coastal settings are hypothesized for the depositional environment of the Steptoe.


Hydrocarbon potential

Exploration of this basin in the search of hydrocarbons has been limited, with less than 20 exploration wells being drilled to 2020. Despite this, many of the exploratory wells do show potential for a petroleum reservoir at depth, mostly in the northern portions of the Officer Basin. Exploration wells haven’t been drilled in this area since the late 1990s, but some of the more promising results show that a reservoir could exist within Supersequence 1, specifically within the Kanpa and Hussar formations, which show good to excellent organic richness. Modelling has shown that hydrocarbon production varies greatly across different formations within Supersequence 1. For example, in the ~800 Ma Browne formation, peak hydrocarbon production was reached approximately 750 Ma, but in the younger Kanpa and Hussar formations, peak production shifts to around 300 Ma. This drastic difference in
source rock In petroleum geology, source rock is a sedimentary rock which has generated hydrocarbons or which has the potential to generate hydrocarbons. Source rocks are one of the necessary elements of a working petroleum system. They are organic-rich sedim ...
maturity is attributable to the complex tectonic history the Officer Basin has experienced. The seven major tectonic events that the Officer Basin experienced extended the maturation window to up to 1 km in some areas, allowing for production to occur much later in time in younger units compared to older ones.


Reservoirs and trapping

Due to the age of this basin and when peak hydrocarbon production was reached, much of the source rocks have been depleted, leaving only reservoir rocks as potential areas of hydrocarbon extraction. Supersequence 1 represents the most prospective portion of the Officer Basin for petroleum exploration, as many of the formations are likely interbedded seals and reservoirs. Within Supersequence 1, the best reservoir rocks are within the Hussar formation, as it is mainly sandstone, and the best seal rocks are in the Browne (below the Hussar) and the Paterson formation (above the Hussar). Reservoir properties are estimated at porosities greater than 20% and permeabilities in the hundreds of millidarcies. Thin, organic rich beds are present within much of Supersequence 1, with varying degrees of maturity. These beds can show excellent hydrocarbon-generating potential and despite their disperse nature, the diverse trapping mechanisms present within Supersequence 1 may allow for significant accumulations of hydrocarbons. The total organic content of these beds is generally within the range of 1 – 3%, but some samples go as high as 21%. To date, there have been 6 oil shows within core samples taken from Supersequence 1. The
salt tectonics upright=1.7 Salt tectonics, or halokinesis, or halotectonics, is concerned with the geometries and processes associated with the presence of significant thicknesses of evaporites containing rock salt within a stratigraphic sequence of rocks. This ...
present within the Browne could allow for a wide range of trapping mechanisms, from anticlinal to fault-bound, due to deformation of overlying strata, making more local hydrocarbon accumulations likely in the deeper, central portion of the basin.


Thermal evolution

During early basin formation, sediments which constitute the Browne formation reached peak hydrocarbon maturity and thus hydrocarbon generative potential was exhausted during the Neoproterozoic. However, the subsequent deposition of the Hussar, Kanpa and Steptoe formations were not as deep as the Browne, allowing for hydrocarbon generation from these units to extend into the Phanerozoic. After deposition of Supersequence 1, roughly 700 Ma, the basin was affected by at least seven major tectonic events. The effect these events had on hydrocarbon generation and maturation of source rocks is unclear, as the region is grossly understudied. However, a 2000 m thick section of Neoproterozoic rock is currently within the oil generation window within the central portion of the Officer basin. The main type of hydrocarbon production that occurs within the source beds of the Browne, Hussar, Kanpa and Steptoe formations is oil and gas generating type II kerogen. The presence of bitumen and oil within exploratory cores suggests the presence of a petroleum system within Supersequence 1.


References

{{Reflist Sedimentary basins of Australia Geology of South Australia Geology of Western Australia