Port Campbell Embayment Research Articles

Preliminary investigation of the hydrogen storage potential in the Port Campbell Embayment, Otway Basin, Victoria, Australia

To understand Victoria’s potential for underground hydrogen storage (UHS), the depleted, unproduced and repurposed fields of the Port Campbell Embayment were assessed for subsurface storage suitability. Additionally, the saline reservoir storage potential of the area was investigated. Data from historic exploration and previous studies in the Port Campbell Embayment were used to undertake geological investigations and storage capacity estimates. The fields investigated in this preliminary study have potential, with the Waarre Formation-Belfast Mudstone reservoir-seal pair highly favourable for UHS. Reservoir injectivity assessments suggest the existing fields are capable of multiple cycles of injection and withdrawal. Seal capacity analysis indicates that the Belfast Mudstone is capable of retaining column heights of hydrogen ranging from 55 to 1267 m, which mostly exceed the greatest known vertical closures in the onshore Otway Basin. The hydrogen storage capacity and injectivity results were used to rank the fields. The Iona field is the most prospective site, with an estimated working gas capacity of 9.1 petajoules (PJ). The total working gas capacity for hydrogen storage in the Port Campbell Embayment fields is approximately 42.7 PJ. Further storage in the area is possible within the Waarre saline reservoir play, with an estimated range of 50.7 to 202.7 PJ.

Structural containment in the Port Campbell Embayment and on the Mussel Platform, Otway Basin, Victoria

As part of the Victorian Gas Program, new geological modelling of the Cretaceous to recent deposits in the Port Campbell Embayment and the Mussel Platform was carried out to investigate fault seal and trap integrity. Structural characterisation of the Late Cretaceous to present-day sedimentary sequence highlights cross-cutting fault trends defining potential structural traps containing Waarre Formation reservoirs. The fault trends are primarily controlled by Cretaceous-Paleogene extension and are reactivated during the Paleogene. Seismic facies in the top seal suggest an N-S environmental shift from open-marine to proximal nearshore marine. The quantification of fault membrane seals suggests that while reservoir-on-reservoir juxtapositions may enable some degree of lateral flow, efficient trapping relying on juxtaposition seal against the Belfast or Skull Creek mudstones is widespread. Fault geomechanics suggests that NW-SE and E-W faults accommodated most of the extensional strain and could have been associated with increased vertical structural permeability; however, there are no leakage indicators to support widespread vertical migration. These results do not support previous assumptions that fault seal integrity and top seal bypass represent a critical and widespread issue within the nearshore Otway Basin.

Quantification of Cretaceous-Cenozoic exhumation in the Otway Basin using sonic velocities and implications for hydrocarbon exploration

Increasing demand for hydrocarbons is prompting increasing exploration in poorly understood and geologically complex basins. These include basins affected by exhumation, where there is often uncertainty regarding the maximum burial depths of source, reservoir and seal horizons. This uncertainty can be reduced by quantifying exhumation magnitudes using techniques such as sonic velocity analysis. The Cretaceous- Cenozoic Otway Basin of the southern Australian margin is an important oil and gas province whose subsidence history was interrupted by exhumation events during the mid-Cretaceous, mid-Eocene and late Miocene. We have used sonic velocity data from 135 wells to quantify the magnitude and extent of exhumation during these events. Here we present estimates of net exhumation based on analyses of Lower Cretaceous Eumeralla Formation shales, which are important source rocks within the basin. Our approach involves measuring the velocities of overcompacted intra-formational shale units identified from gamma ray logs, and determining their displacement on the depth axis from an empirically derived normal compaction trend based on data from wells where the Eumeralla Formation is at maximum burial at presentday. Our results indicate significant values of mid- Cretaceous net exhumation in the Otway Ranges and Colac Trough, with estimates for Olangolah-1 (~2700 m), Stonyford-1 (~1100 m), Tirrengowa-1 (~860 m) and Ingleby-1 (~1000 m) in good agreement with independent estimates of mid-Cretaceous exhumation for these wells based on palaeothermal data. There is also moderate net exhumation around the Merino High (~700 m) but minor (0-200m) net exhumation around the basin?s northern margins and Penola Trough area and moderate (200-600 m) around the Port Campbell Embayment gas fields. Mid-Cretaceous exhumation is interpreted to have been caused by NW-SE directed compression and inversion of the basin. Our results indicate that potential Eumeralla Formation source rocks in wells where large values of mid-Cretaceous exhumation are recorded are most likely over-mature for hydrocarbon generation, in agreement with previous studies of the thermal history of the basin.

Depositional facies and extent of the late Neogene Sandringham Sandstone in southern Victoria, Australia

The late Neogene sedimentary rocks in the Port Phillip region have in the past been subdivided into a lower shallow marine unit, Black Rock Sandstone, overlain by a fluvial unit, Red Bluff Sandstone. Re-examination of the type section of these units at Sandringham shows that it is entirely of paralic origin, with no evidence for fluvial deposition. Criteria for interpreting a shallow marine origin are sedimentary structures including a planar bedding style and swaley cross-stratification, and highly rounded clasts in conglomerates. Using these criteria it can be demonstrated that all named late Neogene sedimentary rock units in the Port Phillip region are paralic deposits, and that the same applies to the Hanson Plain Sand of the Port Campbell Embayment. Because these were deposited as a continuous sheet, the multiplicity of names used hitherto is not justified and should be unified under the single name Sandringham Sandstone. This formation was deposited on a strandplain that extends across western Victoria to the southern fringe of the Western Uplands. The same lateral continuity applies to the underlying Miocene marl formation, which is unified under the name Gellibrand Marl.

CO2CRC Otway project–Soil gas baseline and assurance monitoring 2007–2010

A four-stage soil gas monitoring study was conducted from March 2007 to March 2010 in the western Port Campbell Embayment of the onshore Otway Basin, SE Australia. This study evaluated existing vadose zone soil gases over the CO2CRC Otway Project’s site for CO2 storage, and nearby deep natural CO2 accumulations. Carbon dioxide, methane and helium were the main gases analysed and much of the time CH4 concentrations were too low for accurate analysis. Carbon dioxide concentrations ranged from atmospheric background levels of 0.033% up to about 10% of the total soil gas. The δ13CCO2 values are typical for organic matter decomposition as the main source of the CO2 in the soil gas. Modern radiocarbon ages, determined from 14C analysis, and low helium concentrations support the conclusion that CO2 in the soil gas is predominantly from modern organic sources. There was a relative increase in CO2 concentration measured in locations where the Port Campbell Limestone is near the ground surface, which implies additional CO2 has been produced from the weathering of limestone.

Fault reactivation in the Port Campbell Embayment with respect to carbon dioxide sequestration, Otway Basin, Australia

Abstract The Naylor structure in the Port Campbell Embayment, Otway Basin, South Australia is proposed as a demonstration site for the subsurface geological storage of carbon dioxide (CO 2 ). The Naylor structure is a fault‐bounded high with normal faults to the north and west to SW. Seismic interpretation shows evidence of recent fault reactivation in the Otway Basin. It is postulated that residual hydrocarbon columns (accumulated and leaked prior to present day) in the Otway Basin leaked due to fault reactivation. Thus, a critical issue in the geological storage of CO 2 in the Port Campbell Embayment is the potential for the reactivation of faults bounding the Naylor structure. The propensity of faults to be reactivated is assessed by determining the in‐situ stress field, the mechanical properties of the fault rock and the orientations of the existing faults. The in‐situ stress field lies on the boundary of a strike‐slip and reverse faulting regime in the Port Campbell Embayment. The vertical, minimum horizontal and maximum horizontal stress gradients are 21 MPa km −1 , 19 MPa km −1 and 38 MPa km −1 respectively and the pore pressure gradient is hydrostatic. The maximum horizontal stress in the Port Campbell Embayment is oriented at 150°N. One planar and two curviplanar faults were identified within the Naylor structure. Two fault segments act to trap accumulations at the crest of the structure. These fault segments have relatively low propensities to reactivate near the crest of the structure. The intended migration pathway of the CO 2 plume does not intersect the identified faults until it reaches the crest of the Naylor structure. However, reservoir heterogeneities such as sub‐seismic faults may cause the migrating CO 2 plume to move towards identified fault segments which are not intended to trap the injected CO 2 and have a relatively high propensity to reactivate.

ONSHORE OTWAY BASIN CARBON DIOXIDE ACCUMULATIONS: CO2-INDUCED DIAGENESIS IN NATURAL ANALOGUES FOR UNDERGROUND STORAGE OF GREENHOUSE GAS

The study of natural carbon dioxide (CO2) accumulations, such as those found in the onshore Otway Basin, is necessary for the validation of underground long-term storage technology as an option for decreasing greenhouse gas emissions.The investigation of natural CO2 occurrences is being investigated as part of the Geological Disposal of Carbon Dioxide (GEODISC) research program. This study identifies the effects of CO2 on reservoir rock’s mineralogy through time as well as its porosity and permeability. The Otway Basin CO2 accumulations display variations in reservoir type, CO2 concentration and time of injection. A range of typical reservoirs types for the CO2 accumulations occurs in the Otway Basin, including feldspathic litharenites, subfeldsarenite and quartz arenite. CO2 concentrations in the Otway Basin vary greatly in the accumulations studied, ranging from 10 mol% within the Port Campbell Field to 99 mol % in the Caroline Field. The source of the CO2 is degassing of the deep-seated magmas of the Newer Volcanics, with CO2 influx occurring between ~2 million to as recently as 5,000 years ago. This study investigated three areas of the Otway Basin;Penola Trough—Ladbroke Grove, Katnook (non-CO2)Port Campbell Embayment—Boggy Creek, Langley, Port Campbell; andGambier Sub-Basin—CarolineDue to their close proximity and similar geological history prior to CO2 influx, the Ladbroke Grove-Katnook gas accumulations are particularly useful for examining differences between a CO2-rich (Ladbroke Grove) and a CO2-absent field (Katnook) and for developing a post- CO2 diagenetic history. Variation in grain size and CO2 concentration affects the degree of reaction of CO2 with the reservoir rock. Petrology and formation water chemistry of these fields indicate that CO2 has modified the rock properties. In all CO2-rich reservoirs examined (>10 mol % CO2), dissolution and alteration of lithic and felsic framework grains has occurred (e.g. albite dissolution). Clays and cements throughout most of the Otway Basin CO2 accumulations are modified to minerals more stable in the changed gas compositions (e.g. chlorite to kaolin). The change in mineralogy after the recent CO2 influx shows that the Pretty Hill Formation with high amounts of reactive minerals and smaller grain size is an effective reservoir unit for mineral storage of CO2. Longterm storage in the Waarre Sandstone quartz-rich reservoirs also displays the effectiveness of CO2 storage in pore space.This study of natural accumulations of CO2 has demonstrated that geological storage of CO2 is a viable option. Understanding of the mineral reactions involved with CO2 in reservoir rock is vital for selection of storage sites and modelling the behaviour of CO2 in the subsurface.

Stratigraphic relations of australites in the Port Campbell Embayment, Victoria

Abstract— In the Port Campbell Embayment of Victoria, australites have been found in situ in channel deposits of the Hanson Plain Sand of Pliocene and Pleistocene age. The large majority of the australites, however, occur as a lag deposit at the basal contact of the Sturgess Sand of late Pleistocene and Holocene age and are spatially correlated with ferruginous sandstone clasts that are derived from the Hanson Plain Sand. Some of the tektites are imbedded in or bonded to the ferruginous sandstone clasts, but most are found as individual tektite fragments. A few percent of the tektites have nearly perfectly preserved, complete aerodynamically shaped forms. The sandstone clasts and associated tektites have been reworked from the much older underlying Hanson Plain and have been locally concentrated in the lag deposit. Some tektites also occur at higher levels in the Sturgess Sand, almost invariably in association with stone flakes, exotic stones transported by the aborigines and, locally, with middens of mollusc shells. Circumstantial evidence indicates that the aborigines transported the tektites found in the upper part of the Sturgess, particularly at Stanhope Bay. As Port Campbell australites unequivocally occur in strata much older than the late Pleistocene and Holocene Sturgess, there is no longer any conflict between the apparent stratigraphic age of the tektites and the middle Pleistocene ages obtained by various chronometric methods.

Marine‐derived dolomite in the shallowly buried temperate Port Campbell Limestone (Miocene), Otway Basin, Australia

ABSTRACTThe Miocene Port Campbell Limestone in the Otway Basin (Port Campbell Embayment), south‐eastern Australia, is a shallowly buried (<350 m), temperate carbonate grainstone which consists primarily of benthonic foraminifera, bryozoans, brachiopods, echinoids and planktonic foraminifera. Volumetrically insignificant calcite cements include scalenohedral, blocky and syntaxial overgrowths. Dolomite is present in variable amounts (1–25%), scattered throughout the unit as euhedral rhombs, usually comprising <2% of the whole rock volume. The dolomite post‐dates the calcite cements and is mainly an interparticle cement with crystal size varying between 10 and 150 μm (mean 50 μm). Under cathodoluminescence the dolomite rhombs have a dull core and a nonluminescent outer rim.The dolomite is nonstoichiometric, Ca‐rich (Ca54–62Mg38–46), with high trace element concentrations. The Mn concentrations range from 0 to 310 p.p.m. in the crystal cores (mean 140 p.p.m.) and 80–240 p.p.m. in the crystal rims (mean 140 p.p.m.). The Fe concentrations increase from the crystal cores (range 640–5690 p.p.m.; mean 2030 p.p.m.) to the crystal rims (range 2840–9440 p.p.m.; mean 6040 p.p.m.), whereas the Sr concentrations decrease from the crystal cores (range 690–1510 p.p.m.; mean 1280 p.p.m.) to the crystal rims (620–1240 p.p.m.; mean 930 p.p.m.). The δ13CPDB values of the dolomite range between +2.5 and +2.6%, whereas the δ18OPDB values range from +0.3% to+0.6%.This dolomite occurrence supports the idea that marine or near‐marine dolomite can form not only syndepositionally, but also in the shallow subsurface of temperate units, soon after sediment deposition, under reducing conditions. The fine‐grained, low‐permeability nature of the Port Campbell Limestone contributed to the reducing conditions at shallow depth, the high trace element concentrations of the dolomite (especially in Fe) and the near marine composition of the dolomitizing fluids, as large volumes of meteoric water were inhibited.

A Reappraisal of the Petroleum Prospectivity of the Torquay Sub-basin, Offshore Southern Victoria, Australia: ABSTRACT

The Torquay Sub-basin is located in the offshore part of the eastern Otway Basin, some 50km southwest of Melbourne. Three wells, all dry holes, were drilled between 1967 and 1992. Nerita-1 drilled in 1967 tested Eocene and Early Cretaceous reservoirs in a Miocene anticline. Snail-1 drilled in 1972 was not a valid structural test, and Wild Dog-1 drilled in 1992 tested Late Eocene sands in an Oligocene inversion faulted anticline sourced from Early Cretaceous coals. The area was assessed by previous explorers as lacking effective source. Work currently underway indicates these wells were dry because of lack of migration pathways to the Tertiary. To the west, significant gas has been discovered in Late Cretaceous reservoirs offshore at Minerva-11 and LaBella-1, and onshore in wells in the Port Campbell Embayment. In the Bass Basin to the south, there have been consistent oil, condensate and gas shows. Geochemical analysis of the Early Cretaceous Eumeralla Formation and Casterton beds throughout the Otway Basin demonstrate they contain source rocks capable of generating both oil and gas. Our study indicates that early Cretaceous sandstones with porosities better than 20%, may be present at depths of less than 2000m in the Torquay Sub-basin in tilted faultmore » blocks. Source would be from down-dip lacustrine shales of the Casterton Beds. The general basement high area in which this play is developed is some 15km by 15 km with up to 400m of relief.« less

PORT CAMPBELL REVIEWED: METHANE AND CHAMPAGNE

Gas fields in the Port Campbell Embayment cur­rently supply all the natural gas markets (non-LPG) in western Victoria as well as commercial quanti­ties of carbon dioxide (C02) to industrial markets. Initial discoveries made between 1979 and 1981 were brought on-stream in 1986 with production from the North Paaratte field. Another substantial discovery was made in 1988, the Iona gas field, followed by the Boggy Creek C02 field in 1991, then the My lor and Langley fields in 1994. Discovery of Mylor marked the first recovery of oil from the Late Cretaceous Waarre Formation. Extensive 2D seis­mic data sets have been recorded in the region since 1979, and the first 3D seismic survey in the Otway Basin was carried out in 1993 extending beyond the area of the initial discoveries. No data on the fields have been published for nearly a decade and little detail about the structural and stratigraphic geol­ogy of the Late Cretaceous in the area has been documented. Summaries of the fields are presented incorporating many insights gained from interpre­tation of the 3D seismic data and its verification by the 'rotary lie detector'.

THE LA BELLA AND MINERVA GAS DISCOVERIES, OFFSHORE OTWAY BASIN

In the past 100 years of hydrocarbon exploration in the Otway Basin more than 170 exploration wells have been drilled. Prior to 1993, success was limited to small onshore gas fields. In early 1993, the La Bella-1 and Minerva-1 wells discovered significant volumes of gas in Late Cretaceous sandstones within permits VIC/P30 and VIC/P31 in the offshore Otway Basin. They are the largest discoveries to date in the basin and have enabled new markets to be considered for Otway Basin gas. These discoveries were the culmination of a regional evaluation of the Otway Basin by BHP Petroleum which highlighted the prospectivity of VIC/P30 and VIC/P31. Key factors in this evaluation were:geochemical studies that indicated the presence of source rocks with the potential to generate both oil and gas;the development of a new reservoir/seal model; andimproved seismic data quality through reprocessing and new acquisition.La Bella-1 tested the southern fault block of a faulted anticlinal structure in the southeast corner of VIC/P30. Gas was discovered in two Late Cretaceous sandstone intervals of the Shipwreck Group (informal BHP Petroleum nomenclature). Reservoirs are of moderate to good quality and are sealed vertically, and by cross-fault seal, by Late Cretaceous claystones of the Sherbrook Group. The gas is believed to have been sourced from coals and shales of the Early Cretaceous Eumeralla Formation and the structure appears to be filled to spill as currently mapped. RFT samples recovered dry gas with 13 moI-% CO2 and minor amounts of condensate.Minerva-1 tested the northern fault block of a faulted anticline in the northwest corner of VIC/ P31. Gas was discovered in three excellent quality reservoir horizons within the Shipwreck Group. Late Cretaceous Shipwreck Group silty claystones provide vertical and cross-fault seal. The hydrocarbon source is similar to that for the La Bella accumulation and the structure appears to be filled to spill. A production test was carried out in the lower sand unit and flowed at a rig limited rate of 28.8 MMCFGD (0.81 Mm3/D) through a one-inch choke. The gas is composed mainly of methane, with minor amounts of condensate and 1.9 mol-% C02. Minerva-2A was drilled later in 1993 as an appraisal well to test the southern fault block of the structure to prove up sufficient reserves to pursue entry into developing gas markets. It encountered a similar reservoir unit of excellent quality, with a gas-water contact common with that of the northern block of the structure.The La Bella and Minerva gas discoveries have greatly enhanced the prospectivity of the offshore portion of the Otway Basin. The extension of known hydrocarbon accumulations from the onshore Port Campbell embayment to the La Bella-1 well location, 55 km offshore, demonstrates the potential of this portion of the basin.

SOURCE, MIGRATION AND ENTRAPMENT OF HYDROCARBONS AND CARBON DIOXIDE IN THE OTWAY BASIN, VICTORIA

Salinity distribution, structure, carbon dioxide occurrences, facies and thermal gradient variations and gas isotope studies in the onshore Victorian Otway Basin indicate that the migration of fluids from the Eumeralla Formation source rocks, and the entrapment of hydrocarbons in the Waarre Formation reservoir sands of the Port Campbell Embayment, are closely associated with the presence of three major fault systems. Hydrocarbon accumulation appears to take place where high salinities and normal rift-valley faults are in closest proximity to a structural trap. The large carbon dioxide occurrences are thought to be associated with volcanism and deep vertical fault conduits.Although there are, as yet, considerably fewer exploration wells in the Tyrendarra Embayment/Portland Trough, present studies are nevertheless indicating that broadly similar migration and salinity/fault relationships probably exist in this western portion of the basin. Drilling and analytical results also show this end of the basin to be prospective for not only gas but also oil.

WAARRE SANDSTONE DEVELOPMENT WITHIN THE PORT CAMPBELL EMBAYMENT

The Late Cretaceous Waarre Formation is recognised as the principal reservoir unit throughout the Port Campbell Embayment, where a number of small gas fields were discovered and developed in the late 1970s and the early and mid- 1980s.The Waarre Formation can be subdivided into four units, identified as Unit A, a basal fining upward sequence; Unit B, a medial siltstone with interbedded calcareous sandstones; Unit C, a coarse- grained porous sandstone (the primary gas reservoir); Unit D, a ferruginous siltstone/sandstone sequence.The Upper Waarre, Units C and D, represents a variety of environments unique to a beach barrier- system, including back- water lagoons, swamps, tidal channels, tidal deltas, and beach sands. The development of an Upper Waarre sandstone- beach- barrier model, the identification of various facies, and the construction of regional palaeogeography leads to an understanding of sedimentary deposition during Waarre times.Recent drilling has shown the Upper Waarre to extend laterally in an easterly direction and, as proposed by the depositional model, development of the prospective Waarre Unit C gas sands in a restricted linear east- west zone.By combining the complex structural history of the Port Campbell Embayment and the resultant structures developed with the depositional model of the Waarre Formation, major migration pathways, and thick (>50 m) overlying seals, exploration throughout the embayment can be directed towards prospective gas reservoirs within the Waarre, Unit C, sandstone bodies.