Pore Structure and Mobility of Pore Fluids Within Source Rocks of an Unconventional Middle Cambrian Petroleum System, Southern Georgina Basin, Central Australia

Abstract

The Georgina Basin is a Neoproterozoic to Lower Devonian sedimentary basin straddling the Northern Territory and Queensland border. It is a northwest-southeast-trending extensional basin, in which exploration drilling has evaluated both conventional and unconventional petroleum plays with the middle Cambrian lower Arthur Creek Formation being the main source unit. Industry has estimated that there are technically recoverable resources in the order of 5 Bbbl oil and 18 Tcf gas within this play. Ultra small and small angle neutron scattering (USANS and SANS) techniques are well established non-invasive standard tools for investigating properties of the pore space in sedimentary rocks on the scale from 0.001 to about 20 micrometers. SANS and USANS measurements were performed on 37 oriented cores of lower Arthur Creek Formation source rocks originating from five wells drilled in the Dulcie and Toko synclines and in the Undilla Sub-basin. These samples cover the main depocentres of the southern Georgina Basin with their thermal maturity ranging from immature, through the oil window and into the gas window. For each rock sample the following properties were determined; pore space anisotropy, specific internal surface area, pore size distribution, calculated mercury injection curve, and calculated gas sorption isotherm. Trends in the evolution of the pore space with depth and hydrocarbon maturity were observed. In addition, for three rock samples the SANS and USANS measurements were performed under simulated reservoir conditions using pressurised deuterated methane confined in a stainless steel vessel. Results indicate that a significant fraction of pores are not accessible to methane, the actual value being both sample-dependent and pore-size-dependent. Comparison of the results with similar measurements on cores from hydrocarbon-producing wells in other Australian basins, such as in the Cooper Basin, suggests that micro- and nanopore accessibility may be a decisive factor controlling well producibility. These data imply that it will be difficult to produce tight gas upon fracturing the lower Arthur Creek Formation. Such a conclusion is also conveyed by the disappointing results of recent drilling of this unconventional play. Therefore, although large gas-in-place reserves are estimated, it may be difficult to develop such a resource in the southern Georgina Basin.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015