The successful fracture stimulation and production test of the Amungee NW-1H well placed the Velkerri Shale play and the Beetaloo Sub-basin on Australia’s energy radar. The Velkerri Shale dry gas play is currently Australia’s most promising shale gas prospect; however, it is not the only prospect in the Beetaloo Sub-basin. Four additional potential plays have been identified, each with their own specific risk profile and relative benefits. These are the Velkerri Shale liquids rich gas play, the Kyalla Shale and hybrid liquids rich gas plays, and the Hayfield Sandstone oil/condensate play. Appraising each of these opportunities requires special attention to ensure efficient and appropriate deployment of capital. A framework approach allows for the high-level assessment and comparison of each of the discussed opportunities within the Beetaloo Sub-basin portfolio.
Origin Energy completed exploration campaigns in 2015 and 2016 in the Beetaloo Sub-basin of the Greater McArthur Basin in the Northern Territory, drilling three vertical wells and one horizontal well. These wells have provided a wealth of technical data to assist in the characterisation of the primary source rock reservoirs or ‘shale gas’ plays in the Basin – the Velkerri Formation Play and the Kyalla Formation Play. In this paper we demonstrate the presence of thick, gas saturated and over-pressured source rocks across the sub-basin. In addition to the drilling campaign, the multi-stage hydraulic fracture stimulation of the Amungee NW-1H horizontal well was completed in 2016 – this operation was unique as it represents the first horizontal stimulation operation in the Beetaloo Sub-basin and the longest ‘plug and perf’ type horizontal completion in Australia. Data from the extended production test of the Amungee NW-1H are critical from a technical and, potentially, economic and political perspective. In addition to the technical work program, Origin has undertaken preliminary environmental baseline studies and substantial stakeholder engagement. Ensuring environmental baseline data are available is key to demonstrating that onshore gas developments can be undertaken without adverse environmental outcomes, and also for gaining social acceptance. However, data and facts alone are not sufficient to build community confidence. Origin has engaged extensively with pastoralists, local communities and Traditional Owners to build direct relationships and partnerships that encourage acceptance of the gas industry’s ability to coexist and deliver mutual benefits to the businesses and communities of the Barkly region and the Northern Territory more broadly.
Stable isotope composition of gas is widely used in hydrocarbon exploration to determine the composition and thermal maturity of source rocks. Many isotope classification systems used for gas to source rock correlation and thermal maturity determination are primarily based on empirical observations made in conventional reservoirs and the kinetic isotope effects observed during pyrolysis experiments performed on source rocks. However, such relationships may not be readily applicable to onshore unconventional reservoirs due to the strong molecular and isotope fractionation that occur during extensive gas expulsion associated with basin uplift and depressurization.Degassing studies of freshly recovered core samples can provide useful insight into the behaviour of gas molecules in unconventional reservoirs during basin uplift. The analyses of Australian coal and marine shale samples demonstrate that during desorption both molecular and isotopic compositions of gas change at variable rates. Gas initially desorbed from the samples is mostly CH4, whereas later desorbed gas becomes increasingly enriched in C2H6 and higher hydrocarbons. Hydrocarbon molecules also fractionate according to their isotopic composition, where the early released gas is enriched in 12C causing the remaining gas in the reservoir to be enriched in the heavier 13C isotope. During the release of gas from the Bowen Basin coals the C isotope ratio of CH4 (δ13C1) changes by up to 21‰ (VPDB), whereas that for C2H6 (δ13C2) and C3H8 (δ13C3) changes by <6‰. Similar changes in the isotope composition can be seen during the release of gas from marine source rocks of the Beetaloo Sub-basin. In a fully gas-mature middle Velkerri shale sample, δ13C1 changes by up to 28‰ and δ13C2 by up to 3‰ with no appreciable change occurring in δ13C3.The extent of molecular fractionation during gas flow through carbonaceous rocks is primarily related to the adsorption–desorption properties of organic matter and diffusivity through the overall rock matrix. Using the current dataset, the magnitude of the contributions exerted by the desorption and diffusion processes cannot be readily distinguished. However, both Bowen Basin coals and Beetaloo Sub-basin shale show similar fractionation effects during gas flow, where the heavier alkane molecules, including those containing more 13C, desorb and move slowly compared with the lighter components, in particular CH4. Different rates of isotope fractionation between hydrocarbon molecules during gas flow cause the shape of compound-specific-isotope (CSI) curve to change with time. Early released gas is characterized by a normal CSI trend where the short-chain hydrocarbons are isotopically lighter compared with the longer-chain hydrocarbons. Because CH4 and C2H6 molecules enriched in 12C desorb and diffuse more readily than the heavier hydrocarbons (including those enriched 13C), the gas remaining in the coal and shale samples after extensive desorption shows a reversed CSI trend where CH4 and C2H6 are isotopically heavier compared with the longer chain hydrocarbons. Reversed isotope trends may also develop over geological time, particularly where the source rock is fully gas-mature and has expelled hydrocarbons due to prolonged degassing. As seen in the Beetaloo Sub-basin, the CSI trend in the dry-gas-mature Velkerri shale is reversed, possibly due to the loss of a large proportion of originally generated CH4 during post-Cambrian basin uplift.
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