Wireline Log Analysis Research Articles

Regional insights into the sedimentary organisation of the Walloon Subgroup, Surat Basin, Queensland

Despite more than a decade of intensive coal seam gas exploration and development in the Surat Basin, fundamental aspects surrounding the sedimentary organisation of the Walloon Subgroup remain unresolved. While generally agreed to have been deposited on a waterlogged alluvial plain, contradictory paleogeographic models describe the Walloon Subgroup as an internally draining fluvio-lacustrine system or, alternatively, as a southeast-prograding distributive trunk drainage system. The former invokes a radially organised fluvial catchment feeding a basin depocentre, suggesting that rivers within the study area flowed from east to west. In contrast, a prograding trunk drainage system assumes an axial system flowing southeastward towards a distant paleocoastline. Without a unifying conceptual model describing the organisation of the Jurassic depositional system, predictions about the distribution of alluvial subenvironments within the Surat Basin are difficult. This study tests previous hypotheses surrounding the Walloon Subgroup's sedimentary organisation by reconstructing regional variations in alluvial architecture, fluvial styles and sediment dispersal patterns. The workflow integrates borehole image log interpretation, wireline log analysis, core description and numerical modelling to explore the character and occurrence of subenvironments throughout the basin. Synthesis of interpretations suggests a composite meandering to anastomosing fluvial system containing both a south-oriented trunk drainage component and tributary rivers feeding the axial system from the east.

Inorganic control on original carbonate mineralogy and creation of gas reservoir of the Upper Jurassic carbonates in the Kopet-Dagh Basin, NE, Iran

The Upper Jurassic Mozduran Formation (Oxfordian–Tithonian) is the main petroleum reservoir in the Kopet-Dagh Basin, northeast Iran, which consists predominantly of carbonate rocks with subordinate evaporites and siliciclastics deposited in platform to deep marine settings of a subtropical sea. Detailed field surveys, petrographic investigations, facies and wire line log analyses were carried out at eight surface sections and four wells in the Kopet-Dagh Basin. Integration of petrographic and isotopic data suggests primary low-Mg calcite mineralogy of Oxfordian and Tithonian ooids. On the other hand, in the wells, Kimmeridgian ooids and cements are dominantly aragonitic and high-Mg calcite mineralogy (HMC). Marine cements with isopachous, fibrous and isopachous bladed fabrics indicate original aragonite and HMC mineralogy, respectively. The domination of aragonite mineralogy could be related to increased hypersalinity, evaporite precipitation and consequently an increase in Mg/Ca ratio, which resulted in formation of aragonite in preference to calcite. Preserved ooids with radial and concentric cortices in shallow-water settings that are nearby siliciclastic source, together with aragonitic and HMC ooids accompanied by evaporites in the drilled fields, suggest original mineralogy was probably controlled inorganically following local salinity variations. This study suggests that Kimmeridgian pay zones are mainly controlled by depositional facies, aragonitic and HMC mineralogy, and diagenetic processes such as dolomitization and dissolution.

PETROPHYSICAL CHARACTERISTICS OF THE MESSINIAN ABU MADI FORMATION IN THE BALTIM EAST AND NORTH FIELDS, OFFSHORE NILE DELTA, EGYPT

Baltim East and North fields in the offshore Nile Delta produce gas‐condensate from accumulations located in the northern portion of the Abu Madi palaeovalley. The hydrocarbons in the Abu Madi Formation are present in sandstone reservoir units referred to as the Level III Main and Level III Lower. In this paper, the petrophysical characteristics of these reservoir units in the Baltim area are described using data from wireline logs (gamma‐ray, density, neutron, sonic and resistivity) from fourteen wells and core data from one well. Results of wireline log and core analyses indicate that the Level III Main can subdivided into two sandstone‐dominated intervals (both interpreted as sandbar deposits) separated by a shale‐rich interval which is a partial barrier to fluid flow. Effective porosity is 9–18.5% and permeability 40–100 mD. Sandstones in the Level III Lower are interpreted as braided channel facies and have effective porosity of 12.5–22% and permeability of 100–500 mD.Isoparametric maps for the Abu Madi Formation sandstone reservoirs based on log and core interpretations show the influence of depositional facies on petrophysical characteristics and can be used to assess possible targets for future exploration and development.

Mixture decompositions and lithofacies clustering from wireline logs

Finite mixture models provide useful methods for modeling a wide variety of natural phenomena. Decomposing a mixture of distributions, however, has many difficulties, including separability of the component distributions, determination of the number of components, predictability of the clusters with realistic spatial patterns, and linkages between the component density functions and underlying physical processes. In this article, we use principal component analysis (PCA) to synthesize multiple wireline logs and decompose mixture populations for lithofacies clustering. The principal components of these logs characterize the rock physics; some of them contain essential information of lithofacies while others represent less relevant information or noise. In many cases, clustering based on one component is effective for decomposing the mixture and classifying lithofacies, although rotating principal components is often necessary to improve the lithofacies discrimination. In more complicated cases, PCA and direct mixture decomposition using histogram can be cascaded to decompose finite mixture models of a rock property. The proposed methodology combines the delicacy of probability theory and simplicity of linear transforms to classify lithofacies. • We present multivariate analysis of wireline logs. • Lithofacies are classified using wireline logs. • Principal component analysis is used to synthesize multiple logs and characterize rock physics of the resource formation. • Probabilistic method of histogram decomposition of rock properties is performed using principal component analysis. • We demonstrate the lithofacies classification and mixture decomposition using wireline logs with a number of examples.

Containment of CO2 in CCS: Role of Caprocks and Faults

The successful commercial scale deployment of carbon capture and storage (CCS) requires assurance of the confinement of the injected CO2 at each potential storage site. The critical elements of the confinement of CO2 are the caprock overlying the storage formation, and any faults or fractures which occur within the caprock. The most significant aspect of containment is the seal potential of the caprock, defined as the seal capacity, seal geometry and seal integrity. The seal capacity refers to the CO2 column height that the caprock can retain before capillary forces allow the migration of the CO2 into and possibly through the caprock. Determination of capacity is achieved primarily through petrophysical analyses such as mercury injection capillary pressure (MICP) tests. For storage in depleted fields, assessments of seal capacity can be made from empirical observations of actual hydrocarbon column heights and converting these to CO2 physical properties (density, temperature, pressure). Where these data sources are unavailable, the use of analogs can be a viable alternative. Seal geometry refers to the thickness and lateral extent of the caprock. The caprock must have sufficient lateral extent to cover whatever structural, stratigraphic or hydrodynamic storage reservoir in which the CO2 is trapped. In addition, its thickness should exceed the throw of any faults that cut so as to maintain an effective barrier despite faults through it. Seal geometry is evaluated through well data (stratigraphic, sedimentological and wireline log analyses) and seismic surveys, which are pre-requisites to any viable storage project. Seal integrity refers to the geomechanical properties of the caprock. These properties are controlled by caprock mineralogy, regional and local stress fields as well as any stress changes induced by injection or withdrawal of water or CO2. The modification of the stress field within a storage formation during and after injection of CO2 can lead to reservoir and caprock mechanical failure. This failure can result in the generation of new faults and fractures, reactivation of existing faults and/or bedding parallel slip. The key parameters determining whether faults might act as conduits or as seals are the juxtaposition relationships of rocks on either side of a fault plane, the properties of the fault zone itself or the reactivation potential of the fault. The greatest likelihood of fluid migration up faults is during or immediately after reactivation. Thus, the mere existence of faults does not automatically rule out a site for geological storage of carbon dioxide. On the contrary, sealing faults commonly trap hydrocarbons and compartmentalize oil and gas reservoirs and could also form suitable confining barriers at CO2 storage sites. Seal capacity, geometry and integrity interpretations must be tempered by the potential geochemical reactions between fluids and rocks and injected CO2 as well as by the hydrodynamic environment above and below the seal which may modify the calculated pressure regimes.

Depositional environments of the Kalhur Member evaporites and tectonosedimentary evolution of the Zagros fold–thrust belt during Early Miocene in south westernmost of Iran

The Oligocene–Miocene deposits of the Asmari Formation are the most important oil reservoirs in the Zagros fold–thrust belt in southwest Iran. The Aquitanian evaporites of Kalhur Member of the Asmari Formation were deposited in a depression situated nearly between the Mountain Front Fault and the Dezful Embayment Fault, which borders northern and southern margins, respectively. Detailed field, petrographic studies and wireline log analyses were used to investigate the depositional facies and sedimentary environments of the Kalhur Member. The occurrence of evaporites between pelagic facies and the presence of slump folding, considerable continuity of laminated gypsum/anhydrite along with progradation of clinoforms towards depositional side of evaporites, suggest the presence of relatively deep and isolated depressions during deposition of Kalhur Member. Different facies and thickness variation along with various geometries were formed as the result of overthrust loading event related to the Zagros deformation during Rupelian–Chattian. As the result of this tectonic event, depocentres shifted laterally and relatively narrow deep depressions were filled by evaporites under relative sea-level lowstand and arid conditions and sea-level falls during Aquitanian. Sea-level rise during Burdigalian resulted in sedimentation of the Asmari Formation carbonates throughout the Lurestan and Dezful Embayment, meanwhile with deeper facies towards north. This was along with basin reconfiguration and creation of carbonate ramp.

Depositional architecture and sequence stratigraphic correlation of Upper Ordovician glaciogenic deposits, Illizi Basin, Algeria

Abstract Upper Ordovician glaciogenic deposits are profoundly important as hydrocarbon reservoirs across North Africa, such as within the Illizi Basin of SE Algeria. In this study we present a new sedimentological and sequence stratigraphic model for Upper Ordovician glaciogenic deposits based on the analysis of core descriptions and wireline logs from 25 wells in the Tiguentourine Field. Within the glaciogenic succession, two ice advance–retreat cycles can be defined, consisting of glaciomarine ice-contact fan deposits and tillites. Deposits of the marine ice-contact fan systems generally show a retrogradational stacking pattern from ice-proximal to ice-distal deposits. This pattern is attributed to the deposition in front of a retreating ice sheet. The proximal marine ice-contact fan deposits consist of massive or low-angle cross-bedded pebbly sandstone. They are interpreted as the deposits of turbulent, high-energy plane-wall jets, emerging from subglacial meltwater conduits. These jet-efflux deposits are up to 60 m thick and interbedded with deposits of cohesive and non-cohesive debris flows. The jet-efflux deposits are overlain by fine-grained, thick-bedded massive sandstone. These mid-fan deposits build up the bulk of the glaciomarine fans and are interpreted as deposits of underflows, generated at the point of flow-detachment, where marine meltwater jets become buoyant and large volumes of sediment fall-out from suspension. In the upper part of the fan succession massive sandstones pass upwards into mud-prone massive sandstones, interpreted as deposits of cohesive sandy debris flows. The most ice-distal deposits are muddy sandstones and mudstones deposited by waning low-density turbulent flows and suspension fall-out. The best reservoir properties within the glaciogenic succession are attributed to the proximal and medial deposits of the ice-contact fans such as coarse-grained jet-efflux deposits and sustained high-density turbulent flow deposits. However, the mud content within the massive sandstones is highly variable and influences the reservoir quality. Both glacial depositional sequences infill 60–175 m deep, elongated depressions, which are interpreted as subglacial tunnel valleys. These tunnel valleys acted as depocentres for the glaciomarine fan deposits. After final deglaciation and post-glacial transgression, organic-rich shale was preferentially deposited in underfilled tunnel valleys.

A novel model for assessing the pore structure of tight sands and its application

Pore-structure poses great influence on the permeability and electrical property of tight sand reservoirs and is critical to the petrophysical research of such reservoirs. The uncertainty of permeability for tight sands is very common and the relationship between porestructure and electrical property is often unclear. We propose a new parameter δ, integrating porosity, maximum radius of connected pore-throats, and sorting degree, for investigating the permeability and electrical properties of tight sands. Core data and wireline log analyses show that this new δ can be used to accurately predict the tight sands permeability and has a close relation with electrical parameters, allowing the estimation of formation factor F and cementation exponent m. The normalization of the resistivity difference caused by the porestructure is used to highlight the influence of fluid type on Rt, enhancing the coincidence rate in the Pickett crossplot significantly.

FACIES ANALYSIS AND DEPOSITIONAL SEQUENCES OF THE UPPER JURASSIC MOZDURAN FORMATION, A CARBONATE RESERVOIR IN THE KOPET DAGH BASIN, NE IRAN

Upper Jurassic carbonates of the Mozduran Formation constitute the principal reservoir intervals at the giant Khangiran and Gonbadli gasfields in the Kopet Dagh Basin, NE Iran. These carbonates were investigated using detailed field studies and petrographic and wireline log analyses in order to clarify their depositional facies and sequence stratigraphy. Facies were interpreted to reflect deep basin, fore‐shoal, shelf margin, lagoon, tidal flat and coastal plain depositional systems.The Mozduran Formation is composed of six depositional sequences. Thickness variations were controlled by differential subsidence. Aggradation on the platform margin and reduced carbonate production in the deep basin together with differential subsidence resulted in the creation of a narrow seaway during the late Oxfordian. Petrographic studies suggest that Mozduran Formation carbonates had a low‐Mg calcite mineralogy during the Oxfordian, and an aragonite to high‐Mg calcite mineralogy during the Kimmeridgian. Reservoir pay zones are located in highstand systems tracts within the lower and middle Kimmeridgian depositional sequences. The rapid lateral thickness variations of these sequences were controlled by tectonic factors, leading to compartmentalization of the Mozduran Formation reservoir with the possible creation of stratigraphic traps, especially at the Khangiran field.

Spectral trend attribute analysis: applications in the stratigraphic analysis of wireline logs

S. Djin Nio1, Jan Brouwer3, David Smith2, Mat de Jong1, and Alain Bohm1 present what they claim to be an entirely new methodology for the stratigraphic interpretation of wireline logs.

Fractal and cumulative trace analysis of wire-line logs from a well in a naturally fractured limestone reservoir in the Gulf of Mexico

Hemos demostrado que los registros de pozo medidos en un yacimiento carbonatado naturalmente fracturado tienen un carácter fractal, estando sus parámetros fractales correlacionados con las propiedades petrofísicas de los depósitos. Los registros de porosidad neutrón, densidad, rayos gamma, tiempo de transito de Ia onda P y resistividad somera y profunda, se relacionaron bien con cinco estratos: Jurasico Superior Tithoniano y Kimmeridgiano (JST, JSK), Cretácico Superior, Medio e Inferior (KS, KM, KI). Cada una de estas capas tiene dimensiones fractales diferentes. Utilizando el software BENOIT se ajustaron las trazas de los registros como movimientos Brownianos fraccionarios (fBm) obteniéndose dimensiones fractales promedio para el registro de porosidad D=l.70 (KS), D=1.66 (KM), D=1.75 (KI), D=1.84 (JST), D= 1.67 (JSK). Establecemos que la geometría fractal de los poros controla las fluctuaciones y dimensiones fractales de los registros en cada unidad geológica. El análisis de las trazas acumuladas ha proporcionado una mejor comprensión del sistema, incluyendo las cinco unidades principales.doi: sin doi

A Completion Solution for the Paddy Reservoir in Northeast British Columbia: A Case Study

Abstract The Paddy reservoir, located near the Foothills of the Rocky Mountains in northeast British Columbia, Canada, is a fluvioestuarine valley-filled sandstone that has moderate to high permeability. Evidence of tectonic stress has been observed during hydraulic fracture treatments in the form of excessive wellbore friction and net pressures. Historically, attempts to fracture-stimulate these wells has been difficult due to the inability to generate sufficient fracture width necessary for proppant placement. This paper presents a completion solution for the Paddy reservoir in northeast British Columbia that enabled successful placement of propped fracture treatments. Pressure response of past fracture treatments were analysed to understand the cause of past failures, and to quantify the influence of tectonicallyinduced stress. A recommended completion solution is presented which includes orientated perforating to reduce near-wellbore friction and a process for designing fracture treatments to enable successful placement of proppant. Field examples of the new completion technique are presented which show that near-wellbore friction was reduced by 50%, and that up to four times the amount of proppant was placed in the formation compared to previous treatments. Pressure match simulation results from the fracture treatments are also presented. Geologic Summary The Paddy Member forms the upper portion of the Peace River Formation and is distributed over the Peace River area of northwest Alberta and northeast British Columbia. It is underlain by dominantly coarse-grained shoreface deposits of the Cadotte Member and overlain by marine shales of the Shaftesbury Formation. The Paddy Member is made up of thinly bedded sandstones and shales with occasional thin coals. In the study area, the Paddy is approximately 40 m thick. Reservoir quality, incised valley sandstones located in the upper part of the unit are up to 18 m thick. They are dominantly fine to medium and occasionally coarse grained, fairly quartz-rich sandstones with 10 - 30% chert and other lithic grains. Beds are from centimetre to metre scale in thickness and show small to large-scale trough cross lamination. Where bedding thickness is reduced, millimetre scale wavy shale laminae are common and are often incorporated into stylolitic concentrations, which greatly reduce vertical permeability. The cleaner sandstones were probably deposited under fluvial conditions with some tidal influence where thin shale laminae are common. The dominant cement in these sandstones is quartz, which substantially occludes intergranular porosity in the study area. Small amounts of carbonate cements also occur but are rarely volumetrically important. In addition to intergranular porosity, a significant amount of microporosity has been generated through the dissolution of detrital grains, primarily chert. The abundance of microporosity in these sandstones, together with low water saturations indicated by wireline log analysis, may be the origin of the widely held concern that the Paddy reservoir is susceptible to formation damage through water blocking. Core from the study area indicates that porosity in the Paddy reservoir ranges up to 20%, with associated permeability to air at minimal confining pressure of several hundred millidarcies. A minimum porosity cutoff for pay is taken at approximately 8 - 9%, which is typically associated with 1 md permeability.

Prolific deep-marine slope channels of the Nile Delta, Egypt

The Nile Delta offshore is rapidly emerging as a major gas province. High-quality three-dimensional (3-D) seismic data, coupled with data from 13 consecutive successful deep-water exploration and appraisal wells, have highlighted clear phases of erosion and deposition within the upper Pliocene deep-marine slope channels. The gross reservoir architecture is spectacularly imaged by 3-D seismic techniques, both in time sections and through a variety of amplitude extractions, while an extensive program of core and wire-line log acquisition and analysis has enabled high-resolution definition of the channel-fill sediments. The channels were initiated by the introduction of coarse sediments to the shelf edge possibly at times of relative sea level fall. Initially, there was significant erosion, especially in areas up depositional dip, creating what we term slope valleys. Subsequent valley infill commonly commenced with debris flows, slumps, and slides, sometimes overlying basal, bypass-related sands, and progressed to amalgamated or stacked channels in packages of upward-decreasing net-to-gross sand ratios. This pattern was commonly repeated following reincision, which may have occurred several times. The different stages of channel development can be considered in terms of slope equilibrium with a reduction in slope gradient promoted by increases in flow size and density and decreases in grain size.

A Sequence Stratigraphic Analysis of the Late Permian Succession in the Dural Area, Central Sydney Basin, New South Wales

This paper presents the results of an integrated core, wireline log and seismic sequence stratigraphic analysis, and structural interpretation, of the Late Permian Illawarra Coal Measures, within the central coalfield, Sydney Basin, New South Wales, using data obtained from an AGEX Pty Ltd exploration programme. Five stratigraphic sequences were correlated from the southern coalfield into the central coalfield, using well control, and were then tied into a local grid of reprocessed single-fold, and more recently acquired multi-fold, seismic data. Time-structure form maps within the Illawarra Coal Measures are presented, together with isochron (time-thickness) maps of five sequences recognised within the upper part of that stratigraphic interval. The structural interpretation and trap validity of the previously explored Dural Dome, within the central coalfield of the Sydney Basin, were revisited during this study and the remaining hydrocarbon potential of that feature has been demonstrated to be relatively small.

ABSTRACT: Using Well-Cutting and Wireline Log Analysis to Better Understand the Tectonic and Eustatic Controls of Reservoir Development in the Subsurface Appalachian Basin, West Virginia, USA

ABSTRACT: Using Well-Cutting and Wireline Log Analysis to Better Understand the Tectonic and Eustatic Controls of Reservoir Development in the Subsurface Appalachian Basin, West Virginia, USA

Abstract: Subsurface high resolution sequence stratigraphy for Mississippian mixed carbonate-siliciclastics using well-cuttings and wireline log analysis Appalachian Basin, West Virginia

Abstract: Subsurface high resolution sequence stratigraphy for Mississippian mixed carbonate-siliciclastics using well-cuttings and wireline log analysis Appalachian Basin, West Virginia

SEQUENCE STRATIGRAPHIC INTERPRETATION OF CARBONATE WIRELINE LOG MOTIFS: AN EXAMPLE FROM THE NORTH WEST SHELF OF AUSTRALIA

An integrated biostratigraphic, wireline and seismic sequence stratigraphic study has been conducted to constrain the timing and evolution of Late Cretaceous to Tertiary depocentres along the North West Shelf of Australia. During this study a model for the sequence stratigraphic interpretation of wireline logs in this carbonate-dominated regime has been developed.A series of readily identifiable, lowstand clastic deposits interspersed within the predominantly carbonate passive margin section of the North West Shelf provide well-defined correctable events with which to divide the section. Biostratigraphic data have indicated the presence of missing section at the base of these clastic deposits and their shelfal equivalents. These events have been correlated to define sequence boundaries that are represented on wireline log data by a sharp increase in the gamma signature. Lowstand systems tracts exhibit an irregular sonic and upwardly increasing gamma signature. Transgressive systems tracts show characteristically upward-decreasing gamma and sonic profiles. Maximum flooding surfaces have been identified as the point of cleanest carbonate sedimentation represented by gamma minima on wireline logs. Log motifs exhibiting little character have been interpreted as highstand systems tracts. On seismic these sequence stratigraphic events are represented by stratal geometries that would be expected for these systems tracts.The model has enabled the definition of a higher resolution chronostratigraphic framework for the Mid Cretaceous to Recent section of the North West Shelf than has previously been possible. Forty basin-wide events have been identified from the biostratigraphic and wireline log analysis, thirty of which can be tied throughout the Barrow, Dampier and Roebuck basins.

FRACTALS AND THEIR APPLICABILITY IN GEOLOGICAL WIRELINE LOG ANALYSIS

A fractal analysis method (rescaled‐range analysis) was applied to wireline logs through the fluvio‐deltaic Brent Group (Middle Jurassic) in the Dunlin field, northern North Sea, to test the applicability of fractal analysis in geological interpretation of subsurface data. This analysis reveals that the fractal dimensions of gamma‐ray logs range from 1.0025 to 1.1033. A higher fractal dimension may indicate more heterogeneous lithologies or reservoirs. Analysis of the procedures adopted suggests, however, that fractal concepts in wireline log analysis should be treated with caution. Changes in the calculation procedure can cause larger variations in the estimate of the fractal dimension for a single well than is seen between wells. The validity and suitability of fractals in geological wireline log analysis need to be considered carefully.

THE STAG OILFIELD

The Stag Oilfield is located approximately 65 km northwest of Dampier and 25 km southwest of the Wandoo Oilfield near the southeastern margin of the Dampier Sub-basin, on the North West Shelf of Western Australia,.The Stag-1 discovery well was funded by Apache Energy Ltd (formerly Hadson Energy Ltd), Santos Ltd and Globex Far East in June 1993 under a farmin agreement with BHP Petroleum Pty Ltd, Norcen International Ltd and Phillips Australian Oil Co. The well intersected a gross oil column of 15.5 m within the Lower Cretaceous M. australis Sandstone. The oil column intersected at Stag-1 was thicker than the pre-drill mapped structural closure.A 3D seismic survey was acquired over the Stag area in November 1993 to define the size and extent of the accumulation. Following processing and interpretation of the data, an exploration and appraisal program was undertaken. The appraisal wells confirmed that the oil column exceeds mapped structural closure and that there is a stratigraphic component to the trapping mechanism. Two of the appraisal wells were tested; Stag-2 flowed 1050 BOPD from a 5 m vertical section and Stag-6 flowed at 6300 BOPD on pump from a 1030 m horizontal section.Evaluation of the well data indicates the M. australis Sandstone at the Stag Oilfield is genetically related to the reservoir section at the Wandoo Oilfield. The reservoir consists of bioturbated glauconitic subarkose and is interpreted to represent deposition that occurred on a quiescent broad marine shelf. Quantitative evaluation of the oil-in-place has been hampered by the effects of glauconite on wireline log, routine and special core analysis data. Petrophysical evaluation indicates that core porosities and water saturations derived from capillary pressure measurements more closely match total porosity and total water saturation than effective porosity and effective water saturation.A development plan is currently being prepared and additional appraisal drilling in the field is expected.

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.