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

Abstract

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.