Penola Trough Research Articles

AVO as an Exploration Tool in the Penola Trough, Otway Basin

In 1988, Katnook-1 discovered gas in the basal Eumeralla Formation in the Penola Trough. All subsequent commercial discoveries have been within the underlying Crayfish Group. Amplitude-versus-offset (AVO) analysis performed on single gather records correctly predicted the subsequent discovery of gas within the Crayfish Group in the Katnook-2 and Ladbroke Grove-1 wells. Seismic recorded since 1990 has generally better quality data which are more suitable for AVO analysis. Two lines were recorded in 1993 over the Katnook Field, tied to the three Katnook wells. Only a small AVO effect was predicted at some locations from modelling using the well data, which was probably due to the poor quality well logs. The seismic AVO analysis, however, using all gather records, produced clear anomalies at both reservoir levels which approximately matched the predicted extent of the gas in the field from structural mapping. The subsequent Katnook-4 appraisal well confirmed the presence of gas at these levels, but unfortunately was not commercial. AVO and inversion were also run over the Haselgrove Gas Field prior to its discovery in 1994. AVO anomalies are seen at both Katnook reservoir levels, but the strong basal Eumeralla Formation anomaly is interpreted to be due to a clean, water saturated sand rather than gas. The anomalies at the top Pretty Hill Sandstone level are more consistent with structure although they extend below mapped closure. Subsequent reprocessing using pre-stack migration prior to AVO analysis produced anomalies which closely match the actual gas column height, and are within the zone of strong seismic amplitudes at this level. AVO anomalies in the area appear to be caused by both clean, water filled sands and the presence of hydrocarbons. Seismic inversion can help the prediction of sand quality variations, and the mapped extent of structural closure can be used to identify those AVO anomalies more likely to be related to hydrocarbons. Pre-stack time migration prior to AVO processing is essential in most cases. Unfortunately, AVO anomalies also occur in reservoirs with low gas saturations or low reservoir permeability. Despite these drawbacks AVO with inversion has proven to be a valuable exploration tool in the Penola Trough. No gas discoveries have yet been made which are not associated with an AVO anomaly.

THE SAWPIT STRUCTURE—EVALUATION OF A FRACTURED BASEMENT RESERVOIR PLAY IN THE OTWAY BASIN

The 1992 exploration well Sawpit-1, drilled in the Penola Trough of the western Otway Basin, recovered a minor amount of oil (1.5 BBL) from fractured basement. This oil recovery, the first in the basin from rocks older than the Eumeralla Formation, has opened up a new oil play. Fractured basement reservoirs, which can be prolific producers, require prediction of fracture orientation and intensity to be appraised and developed successfully. Such information was sought from a detailed structural analysis of fault attributes (including dip, vertical displacement and heave) using 3D seismic data acquired over the structure in 1993.This 3D seismic data set, acquired at twice the group interval and less than one-quarter the fold of 2D data in the area, has provided the best imaging to date of the complex rifting events of the Sawpit region. Structural analysis of fault attributes indicates an initial south-southwest direction of extension in the Late Jurassic/Early Cretaceous for the Penola Trough, rather than oblique extension as suggested by some authors. This south-southwest extension orientation favours the formation of moderate to high angle fractures trending west-northwest to northwest. Present day minimum horizontal stress is favourably oriented approximately northeast-southwest for these fractures to be open. Interpretation of the 3D seismic data indicates Sawpit-1 probably intersected northeast trending fractures that are oriented perpendicular to the maximum horizontal compressive stress direction and therefore likely to be closed.A deviated well drilled from the northeast would be optimally oriented to intersect northwest trending basement fractures and test a play that has significant oil potential.

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.

WESTERN OTWAY CRAYFISH GROUP TROUGHS

Commercial volumes of hydrocarbons have been discovered in Early Cretaceous synrift sediments of the Crayfish Group of the Penola Trough, western Otway Basin. Four other such depocentres are recognised using available seismic and drilling data. A standard nomenclature is proposed for the major structural elements observed in this part of the Otway Basin.Various tectonic models proposed for the Early Cretaceous rift history of the Otway Basin are reviewed and tested using true dip analysis of seismic events and sandbox modelling experiments. Results support a generally north-south direction for rift extension in the Tithonian-Barremian. This implies that the ENE trending Robe Trough is the headwall rift compartment (although with 20°obliquity), whereas the NW trending Penola Trough is a more oblique or transtensional rift compartment.

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.