Pretty Hill Formation Research Articles

Reconciling predicted and observed carbon mineralization in siliciclastic formations

The study of natural CO2 storage reservoirs has demonstrated that carbon mineral trapping is the safest mechanism for subsurface CO2 storage. The slow rate of fluid-rock reactions makes it imperative to use reactive transport simulations as a tool for estimating reservoir scale carbon mineral trapping capacities. However, there is often a disparity in the estimations of carbon mineral trapping capacities determined from reactive transport simulations and recorded in natural CO2 storage reservoirs. This might be due to several factors such as the uncertainty around simulations parameters such as mineral reactive surface areas as well as the neglect of sub-core scale lithological heterogeneity in reservoir scale simulations. This study points to the importance of the latter to reconcile the mismatch. Multiphase reactive transport simulations were run on a cm-scale resolution geological model (CRM) of the Paaratte Formation, Otway Basin (Australia), where mm- to cm-scale lithological heterogeneity is incorporated via upscaled rock parameters. Simulations were also run on a m-scale resolution model (MRM) of the same transect to quantify the difference in the predicted secondary mineral composition due to the incorporation of sub-core scale heterogeneity in reservoir scale simulations. The simulation outcomes for both the models were compared to the data from the Pretty Hill Formation, a natural CO2 storage analogue within the Otway Basin with similar primary mineral composition. The results from the CRM compare well with the secondary carbonate mineralogy of the Pretty Hill Formation. In contrast, the MRM strongly underestimates carbonate mineral formation. The results highlight that the incorporation of mm- to cm-scale lithological interfaces in reservoir scale reactive transport simulations can increase carbon mineralization estimates in siliciclastic formations by 3–6 orders of magnitude as these interfaces are sites for mineral precipitation.

New insights into the stratigraphy of the Otway Basin

The Otway Basin is one of the best known and most actively explored of a series of Mesozoic basins formed along the southern coastline of Australia by the rifting of the Antarctic and Australian plates during the Cretaceous. The basin offers a diversity of play types, with at least three major sedimentary sequences forming conventional targets for petroleum exploration in the onshore basin. The Penola Trough in South Australia has enjoyed over 20 years of commercial hydrocarbon production from the sandstones of the Early Cretaceous Otway Group comprising the Crayfish Subgroup (Pretty Hill Formation and Katnook sandstones) and Eumeralla Formation (Windermere Sandstone Member). Lithostratigraphic characterisation and nomenclature for these sequences are poorly constrained, challenging correlation across the border into the potentially petroleum prospective Victorian Penola Trough region. The Geological Survey of Victoria (GSV), as part of the Victorian Gas Program, commissioned Chemostrat Australia to undertake an 11-well chemostratigraphic study of the Victorian Otway Basin. The South Australia Department for Energy and Mining, GSV and Chemostrat Australia are working collaboratively to develop a consistent, basin-wide schema for the stratigraphic nomenclature of the Otway Basin within a chemostratigraphic framework. Variability in the mineralogy and hence inorganic geochemistry of sediments reflects changes in provenance, lithic composition, facies changes, weathering and diagenesis. This geochemical variation enables the differentiation of apparently uniform sedimentary successions into unique sequences and packages, aiding in the resolution of complex structural relationships and facies changes. In this paper, we present the preliminary results of detailed geochemical analyses and interpretation of 15 wells from across the Otway Basin and the potential impacts on hydrocarbon prospectivity.

New insights into the organic geochemistry of the Otway Basin

The Penola Trough of the Otway Basin in South Australia has enjoyed over 20 years of commercial hydrocarbon production from the sandstones of the Early Cretaceous Otway Group with 71.46 PJ sales gas and 65840 kL of condensate produced from five fields until production ceased in 2011. Recent success in Haselgrove 3, where a newly discovered deeper reservoir underlying depleted reservoirs of the Pretty Hill Formation flowed 25 MMscfd on test heralds a new phase of exploration and appraisal with a 2C Contingent Resource of 87 PJ assigned to the discovery. These hydrocarbons are considered to have been principally derived from lacustrine, floodplain and back swamp facies of the Pretty Hill Formation and lacustrine facies of the underlying Casterton Formation. The South Australia Department for Energy and Mining is currently investigating an unnamed Late Jurassic syn-rift sequence of interbedded metasediments, tuffaceous sediments and organically-rich shale, first identified as fractured basement in Sawpit 1, located on the northern flanks of the Penola Trough and which unconformably underlies the Casterton Formation. A low-sulfur medium-gravity paraffinic oil was recovered from drill stem tests from this well and subsequent total organic carbon and Rock-Eval analyses for the same unit in nearby Sawpit 2 identified a source rock containing algal organic matter and thought to be deposited in a deep anoxic lake setting. In this paper, we will present the preliminary results of detailed geochemical analyses of rocks and rock extracts from this previously unrecognised syn-rift sequence and discuss implications for hydrocarbon prospectivity in the deeper portions of the Penola Trough.

Structural evolution of the Early Cretaceous depocentres, Otway Basin, Victoria

The dominant control on the (transition between) depositional settings of the Crayfish Group of the Otway Basin in Victoria, Australia has been determined. The study first involved seismic mapping of six stratigraphic units within the Early Cretaceous, continental Crayfish Group. The resulting 3D structural model was used to identify major Early Cretaceous depocentres, and to determine which Crayfish Group sediments are restricted to individual rift depocentres and which are more widespread as a result of inter-connectivity of the basin. Five structural cross-sections were then constructed across each major depocentre of the basin; these were balanced and restored, and missing section estimated, in order to test the validity of the structural interpretations. This also enabled analysis of differing extensional rates within each depocentre and the calculation of the cumulative displacement of each major bounding fault. Results show that displacement rate, growth and linkage of the faults, as well as the amount of subsidence within the depocentres, had a significant effect on the distribution and development of the major facies within the Crayfish Group. The Casterton Formation and Sawpit Shale equivalent/McEachern Sandstone were restricted to rapidly subsiding, structurally controlled depocentres in the west, while the succeeding Sawpit Sandstone equivalent was deposited within the same depocentres, across the intervening structural highs and in the eastern part of the basin where depocentres had just begun to form. The Pretty Hill Formation shows a similar distribution pattern, while the overlying fine-grained Laira Formation also drapes structural highs but is replaced in the east by coarser-grained sediments of the upper Pretty Hill Formation. Extension was locally up to 21% in the central Otway Basin but was much less in the eastern Otway Basin.

The Pretty Hill Formation as a natural analogue for CO2 storage: An investigation of mineralogical and isotopic changes associated with sandstones exposed to low, intermediate and high CO2 concentrations over geological time

The Pretty Hill Formation of the Otway Basin (Australia) has been studied as a natural analogue for geological storage of anthropogenic CO2 in order to examine the effects that CO2 concentration and reservoir heterogeneity have on CO2-related reactions. New petrographic data are presented, which validate the use of Hylogger™ as a tool to investigate high-resolution vertical changes in reservoir mineralogy. The integrated data set confirms earlier interpretations, showing that chlorite has been altered to kaolinite and siderite/ankerite in reservoir facies exposed to moderate and high CO2 concentrations, while chlorite remains the dominant clay mineral in all parts of the formation where CO2 content is low.Differences have been observed in the degree of CO2-related reaction relative to CO2 concentration and reservoir heterogeneity. Where CO2 content is very high (c. 98mol%) and associated with high water saturations, both chlorite and detrital feldspars have undergone complete reaction in the reservoir facies, resulting in quartzose sandstones with a kaolinite matrix, and with siderite as the dominant carbonate precipitate. Conversely, where CO2 content is moderate (c. 29–57mol%) and within the gas leg of the reservoir, chlorite has undergone significant reaction, but much of the original feldspar is preserved, suggesting relatively minor reaction. Carbonate cements from the moderate CO2 gas-leg comprise calcite, siderite and ankerite, occurring as cemented zones associated with rock heterogeneities and the present-day gas–water contact. Heterogeneities within the gas-leg are likely to have associated pore fluid contacts, whereby relatively high water saturations will be present in the fine-grained baffles and seals. The most advanced feldspar reaction occurs locally at the contact between baffles and reservoir rock, while reactions have been significantly impeded in the finer grained units due to their low permeabilities.Stable isotope data presented for carbonate cements analysed from wells with low and moderate CO2 levels show no clear distinction. Relatively early formed calcite has δ13C values that require an organic carbon source, suggesting precipitation unrelated to the reservoir CO2 in the Otway Basin. In contrast, diagenetically late calcite and siderite samples display two distinct δ13C groups (dependent on carbonate type), where the calculated fluid carbon isotope compositions are similar to documented magmatic CO2 reservoired in the nearby Caroline Field. This suggests that magma-derived CO2 may have been more prevalent through the Pretty Hill Formation than previously thought. Although the CO2 has not been contained over the long term in the low CO2 sites, it may have caused the local dissolution of carbonate and laumontite cement, and also contributed a source of carbon for late-stage calcite cements.These studies illustrate the importance of understanding both the reservoir composition and vertical heterogeneity of potential storage systems. Fluid–mineral reactions are likely to be advanced within stacked reservoir facies and impeded within siltstone layers, while the distribution of carbonate cement may increase the reservoir heterogeneity by the formation of cemented siltstone/sandstone layers, thereby creating impermeable barriers or baffles to CO2.

Remote sensing of subsurface fractures in the Otway Basin, South Australia

AbstractNaturally occurring fractures were remotely detected in a 3‐D seismic volume from the Penola Trough in South Australia's Otway Basin and validated through an integrated approach. Identified in image logs are 508 fractures and 523 stress indicators, showing maximum horizontal stress orientation in the Penola Trough is 127°N. Two fracture types were identified: (1) 268 electrically conductive (potentially open to fluid flow) fractures with mean NW‐SE strikes and (2) 239 electrically resistive (closed to fluid flow) fractures with mean E‐W strikes. Core from Jacaranda Ridge‐1 shows that open fractures are rarer than what image logs indicate, due to the presence of fracture‐filling siderite, an electrically conductive cement which may cause fractures to appear hydraulically conductive in image logs. The majority of fractures detected is favorably oriented for reactivation under in situ stresses, although it is demonstrated that fracture fills primarily control which fractures are open. Seismic attributes calculated from the 3‐D Balnaves/Haselgrove survey are mapped to the Pretty Hill Formation to enhance observations of structural fabrics, showing linear discontinuities likely representing faults and fractures. Discontinuity orientations are consistent with natural fracture orientations identified in image logs, striking E‐W and NW‐SE, limited to zones around larger faults. However, it is unlikely that a large proportion of these fractures are open given observations of core and image logs, limiting possible fracture connectivity and therefore significant secondary permeability in the Penola Trough. The integrated methodology presented herein provides an effective workflow for remote detection of subsurface fractures and determining if electrically conductive fractures are also hydraulically conductive.

Comparison of the Mineral Trapping Capacity in three Reservoirs with Variable Mineral Compositions under CO2 Saturated Conditions

Abstract The mineral trapping capacity is considered the most secure long-term CO 2 storage mechanism as it permanently demobilizes injected carbon. Here we compare three different reservoir rock mineral compositions in terms of their mineral trapping capacity using a mass balance approach. The evolution of carbon fractions in the form of residual CO2, CO 2 in the formation water and carbon stored in minerals is predicted over time using reaction path modelling. The two studied siliciclastic reservoirs (Precipice Sandstone, Surat Basin, Australia; Pretty Hill Formation, Otway Basin Australia) have no or only a very small mineral trapping capacity, whereas the volcanoclastic reservoir (Takinoue Formation, central Hokkaido, Japan) has a very large mineral trapping capacity. This difference results from the abundance of reactive Ca-, Mg- and Fe-bearing silicate minerals. It is demonstrated that the mineral trapping capacity of volcanoclastic rocks can exceed residual and solubility trapping. The limitation of the chosen approach is a rather simplistic treatment of mineral dissolution and precipitation kinetics.

CO2 percolation experiment through chlorite/zeolite-rich sandstone (Pretty Hill Formation – Otway Basin–Australia)

Underground CO2 sequestration is highly recommended as an effective means of significantly decreasing CO2 concentration in the atmosphere. Mineral storage is the more secure technology, but requires the presence of high concentration of divalent cations in the pore-fluid. Results from CO2 percolation experiments through chlorite/zeolite-rich sandstone samples from the Pretty Hill Formation (Otway Basin, Australia) are presented. The dissolution of the laumontite (7wt.%) and chamosite (7wt.%) are the potential sources of calcium, iron and magnesium required for carbonate precipitation. The percolation experiment was setup to reproduce, at laboratory scale, the in situ temperature and pressure conditions (T=95°C and P=10MPa). The fluid injected at constant flow rate is a rock-equilibrated brine subsequently enriched in CO2 up to partial pressure of 6MPa.We observe feldspars, laumontite and chamosite dissolution, kaolinite and silica precipitation and a noticeable sink of CO2 in the sample which is attributed to the precipitation of both amorphous carbon due to the reduction of CO2 and Fe-rich carbonate. Permeability decreases of about one order of magnitude due to the localization of the kaolinite precipitation in the main flow paths, while porosity increases.The high reactivity of this sandstone makes this reservoir a valuable target for CO2 mineralization, but the associated permeability decrease may limit the injection rate and the spreading of the CO2 in the reservoir.

The Ladbroke Grove–Katnook carbon dioxide natural laboratory: A recent CO 2 accumulation in a lithic sandstone reservoir

The Ladbroke Grove and Katnook Gas Fields, within the western Otway Basin, southeastern South Australia, provide a natural laboratory to study the effects of CO 2 on siliciclastic reservoirs. CO 2 degassing from nearby volcanics has migrated into the methane accumulation of the Ladbroke Grove Field within the Pretty Hill Formation. CO 2 levels in the Ladbroke Grove Field range from 26 to 57 mol% while Katnook has less than 1 mol%. In Ladbroke Grove, the CO 2 has altered or dissolved most of the reactive minerals, somewhat constrained by the Pleistocene to Recent age of the CO 2 influx. The developed late-stage kaolinite, quartz and less soluble carbonate are products of the reactions involving CO 2 and reactive minerals. The major formation water types were identified using the geochemical code EQ3NR. Ladbroke Grove waters above the gas–water contact (GWC) have bicarbonate levels an order of magnitude higher than the other waters analysed. Below the GWC, Ladbroke Grove and Katnook formation waters have similar ionic compositions, however, pH levels in Ladbroke Grove are low relative to Katnook. The Ladbroke Grove Field has efficiently stored CO 2 in a gaseous and aqueous phase since the influx began in the Pleistocene. In addition, due to the high amount of reactive minerals within the reservoir, mineralisation of ferroan carbonates has also occurred as a more permanent form of mineral storage of some of the CO 2.

CARBON DIOXIDE AND CARBONATE CEMENTS IN THE OTWAY BASIN: IMPLICATIONS FOR GEOLOGICAL STORAGE OF CARBON DIOXIDE

Understanding CO2 source and carbonate cements in natural gas accumulations is important for predicting the behaviour of anthropogenic CO2 in a reservoir system. The Otway Basin offers an excellent opportunity to examine late CO2-derived cements as an analogue for mineralogical storage of CO2. Understanding Otway Basin diagenesis and carbonate cement distribution is also of great significance to petroleum production in the region.Elemental and textural examination of Otway Basin cements has identified five carbonates in reservoir rock from CO2-rich gas accumulations. These carbonates show an overall increase in Fe2+ and Mg2+ relative to the calcites in CO2-free reservoir rock, indicating cation derivation from CO2 interaction with labile minerals.δ13C isotopes of 2.18‰ to -6.7‰ PDB from late carbonate cements in reservoirs containing CO2, confirm an inorganic CO2 origin. 3He/4He gas isotopic ratios of R/Ra > 1 indicate a predominantly mantle input for the CO2-rich accumulations.Degassing of magma associated with Pleistocene to Recent volcanics is suggested as the dominant, CO2 source for the existing CO2 accumulations. CO2 influx from the magmatic source was rapid, and is the most analogous scenario to injection of anthropogenic CO2. Natural influx of CO2 and the opportunity for mineralisation of CO2 is variable, with CO2 dissolving some original carbonate and precipitation dependant on pH, ρCO2, and available cations. Positive mineralogical CO2 storage occurs in the Pretty Hill Formation, due to a higher content of labile lithic minerals, with ~36 kg/m3 of CO2 (~48 kg/m3 carbonate) stored in the Ladbroke Grove Field from the current CO2 phase. The Waarre Sandstone has negative mineralogical storage of CO2, with less carbonate than similar reservoir rock without CO2, and therefore more CO2 being released from dissolution of early carbonates.

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.

3D FAULT MODELLING AND ASSESSMENT OF TOP SEAL STRUCTURAL PERMEABILITY—PENOLA TROUGH, ONSHORE OTWAY BASIN

Many of the commercial hydrocarbon accumulations discovered to date within the Pretty Hill Formation in the onshore Otway Basin of southeastern Australia rely on a semi-brittle top seal and fault seal. Therefore a detailed and integrated fault, stress field and fracture analysis is fundamental to prospect evaluation.A syn-kinematic interpretation of the 3D seismic data set, using variance cube and visualisation technology was augmented with interpretation of the dip-meter and high-resolution borehole images. This resulted in the interpretation of a more complex fault history than previously inferred from 2D seismic mapping and dipmeter analysis alone.There are two major prospect/field bounding fault sets within the Penola Trough. Northwest-trending faults are associated with two commercial fields and several palaeo-accumulations. East-west trending faults are associated with three major fields, two uneconomic fields and two possible palaeo accumulations.Hydrocarbon leakage is probably caused by the creation of structural permeability across the regional seal. The location of leakage depends on the interaction between the seal, associated faults, and the regional stress field. Faults deflect regional stress trajectories within the top seal, creating local areas of high differential stress which enables brittle failure and the development of structural permeability. Predicting stress trajectories, the magnitude of differential stress and thus the location of structural permeability within the top seal to the underlying Pretty Hill Formation reservoirs, will reduce exploration risk uncertainty.

GAS DETECTION IN THE ONSHORE OTWAY BASIN USING ARRAY SONIC WAVEFORM DATA

The Otway Basin and in particular the onshore Penola Trough has recently become a focus for gas exploration. Gas identification in sandstones of the Lower Cretaceous Pretty Hill Formation, the primary reservoir target in this trough, has proved difficult using traditional wireline log analysis. A resistivity contrast associated with gas and water sands is not observed on well logs and poor hole conditions affect pad tools suppressing the gas cross-over response on density-neutron logs. Identification of gas sands has historically relied on mud logs, pressure and test data.The array sonic tool records the entire waveform and records shear and Stoneley waves in addition to compressional waves. Obtaining 'slowness' from shear and Stoneley waveforms opens up a new range of applications including gas detection. Wave theory predicts that a small percentage of gas in the pore space significantly increases the compressional slowness whereas the effect of gas on the shear slowness is much less pronounced. The shear to compressional slowness ratio (Vp/Vs) is therefore a powerful tool for gas detection.Acoustic array sonic waveforms were recorded over the Pretty Hill Formation in the recent onshore Otway Basin wells Katnook-4, Haselgrove-1 and Haselgrove-2 using Schlumberger's Sonic Digital Tool (SDT). A Vp/Vs ratio value less than 1.7 was established as representative of sands containing gas. The productivity of these gas sands was confirmed by wireline, open hole and cased hole tests in Haselgrove-1 and Haselgrove-2. The uppermost reservoir section in Katnook-4 was gas-bearing but proved to be tight on test.In a reservoir such as the Pretty Hill Formation, where hydrocarbon identification is rendered difficult by conventional logging suites, acoustic array sonic waveform data help to distinguish gas sands from water sands.

THE HYDROCARBON POTENTIAL OF THE PENOLA TROUGH, OTWAY BASIN

The Penola Trough is an elongate, Late Jurassic to Early Cretaceous, NW-SE trending half graben filled mainly with synrift sediments of the Crayfish Group. Katnook-1 discovered gas in the basal Eumeralla Formation, but all commercial discoveries have been within the Crayfish Group, particularly the Pretty Hill Formation. Recent improvements in seismic data quality, in conjunction with additional well control, have greatly improved the understanding of the stratigraphy, structure and hydrocarbon prospectivity of the trough. Strati-graphic units within the Pretty Hill Formation are now mappable seismically. The maturity of potential source rocks within these deeper units has been modelled, and the distribution and quality of potential reservoir sands at several levels within the Crayfish Group have been studied using both well and seismic data. Evaluation of the structural history of the trough, the risk of a late carbon dioxide charge to traps, the direct detection of gas using seismic AVO analysis, and the petrophysical ambiguities recorded in wells has resulted in new insights. An important new play has been recognised on the northern flank of the Penola Trough: a gas and oil charge from mature source rocks directly overlying basement into a quartzose sand sequence referred to informally as the Sawpit Sandstone. This play was successfully tested in early 1994 by Wynn-1 which flowed both oil and gas during testing from the Sawpit Sandstone. In mid 1994, Haselgrove-1 discovered commercial quantities of gas in a tilted Pretty Hill Formation fault block adjacent to the Katnook Field. These recent discoveries enhance the prospectivity of the Penola Trough and of the Early Cretaceous sequence in the wider Otway Basin where these sediments are within reach of the drill.

DETRITAL AND AUTHIGENIC MINERALOGY OF THE PRETTY HILL FORMATION IN THE PENOLA TROUGH, OTWAY BASIN: IMPLICATIONS FOR FUTURE EXPLORATION AND PRODUCTION

The Pretty Hill Formation in the Penola Trough is a productive gas reservoir in the Katnook, Ladbroke Grove and Haselgrove fields. Thin sections, X-ray diffraction, scanning electron micros­copy and electron microprobe analyses have been used to characterise the mineralogy of core samples from eight wells. The reservoir sandstones are typically fine to medium grained, moderately sorted feldspathic litharenites. Framework grains comprise detrital quartz, feldspars (albite, microcline and anorthite), lithics (dominantly volcanic), mica and accessory minerals. Authigenic minerals of chlorite, laumontite, carbonate, quartz, feldspar, sphene, anatase, glaucony and illite are present in all wells. Kaolinite is restricted to Ladbroke Grove-1. Chlorite, laumontite and carbonate are volumetrically the most important authigenic minerals.There is a wide range in core plug porosity (one to 23 per cent) and permeability (10"3to 103 md) in the reservoir sandstones. In samples with high per­centages of authigenic clays microporosity is im­portant. Regional trends indicate reservoir quality decreases with increasing depth but superimposed on this trend is the influence of the detrital and authigenic mineralogy. Cleaner, coarser sublitharenites and subarkoses have good reservoir char­acteristics but where lithics concentrate in the finer feldspathic litharenites and litharenites deformation of these ductile grains has limited porosity and permeability. Authigenic minerals have both reduced and enhanced reservoir quality. Chlo­rite rims with associated microporosity have decreased the impact of mechanical compaction and inhibited silicification. Pore filling cements of laumontite and carbonate have occluded intergranular pores and replaced grains. Secondary porosity produced by the dissolution of these cements in the gas zones has significantly improved reservoir quality.Other information gained from the mineralogical study could influence future exploration and production. Lack of contrast on resistivity logs between gas and water zones is not due to the mineralogy of the Pretty Hill Formation. However, the restriction of early diagenetic laumontite to the water zones of gas producing wells does indicate the location of the gas-water contact. Laumontite was dissolved from the gas zone by an increase in C02 prior to hydrocarbon migration. Use of acids to enhance permeability in the Pretty Hill Formation should take into account the probable formation damage caused by reactions with the clays. Kaolin- ite could dissolve to produce a silica gel and the high Fe3+ content of the chlorite will result in a gel unless iron chelators are used in the mud acid. The depositional environment of the Pretty Hill Forma­tion has historically been interpreted as braided fluvial stream deposits interfingering with finer grained lacustrine shales and siltstone. However, this model can not explain the presence of glaucony grains, unless the glaucony has been reworked, but there is no unequivocal evidence to support this hypothesis.

The use of organic fades for refining palaeoenvironmental interpretations: A case study from the Otway Basin, Australia

Four depositional settings interpreted from sedimentary fades associations are present in the non‐marine Early Cretaceous sequence of the Otway Basin: high energy fluvial, low energy fluvial, lacustrine and fluvio‐deltaic facies associations. Organic‐petrological investigation of Otway Group coals delineated four coal types with distinct assemblages of maceral subgroups and individual macerals. The coal types, designated A to D, occur with distinct assemblages of dispersed organic matter (dom) in associated clastics. Specific coal type assemblages in the Otway Group are closely related to changes in sedimentary facies, depositional energy exerting a primary control on the organic facies. High energy deposits of the Pretty Hill Formation and the upper part of the Eumeralla Formation are characterized by a single organic facies. Organic facies in the lower energy lacustrine deposits of the Casterton Formation and the lower part of the Eumeralla Formation appear to be sensitive to subtle changes in environme...