Assessing Unconventional Resource Potential of the Mesoproterozoic Roper Group, Beetaloo Sub-Basin Region, Australia

Abstract

In the Northern Territory of Australia, the Beetaloo and associated Gorrie, Broadmere and OT Downs Sub-basins are potential targets for unconventional hydrocarbon exploration. Nine wells within this region were investigated in this study to assess hydrocarbon prospectivity of key formations within the Mesoproterozoic Roper Group (~1.4 Ga) using source rock geochemistry. The Roper Group comprises a regionally extensive, upward-coarsening cyclic succession of mainly marine mudrocks, including laminated organic-rich shales, alternating with sandstones and other minor lithologies. Within this sequence, five intervals composed of dominant shale lithology were evaluated, including the Kyalla, upper, middle and lower Velkerri, Corcoran and Mainoru formations. The geochemical data set includes historical data compiled by the Northern Territory Geological Survey and more recent analyses performed by Weatherford Laboratories.

The two main intervals with greatest hydrocarbon potential are the Kyalla and middle Velkerri formations. Geochemical characteristics of these shales compare favorably with producing unconventional plays in the USA. The Kyalla has an average total organic carbon (TOC) content of 1.08 wt.% and the middle Velkerri averages 3.76 wt.%. These source rocks were originally composed of oil-prone Type II marine kerogen, which has been thermally altered due to hydrocarbon generation and expulsion. Although geochemical maturity data from organic petrology (R_o) and programmed pyrolysis (T_max) indicate the presence of oil-mature and gas-mature rock, thermal maturity may be a limiting factor in shallower parts of the basin. The Kyalla Formation is generally within the oil-window; whereas the middle Velkerri has reached the dry-gas window in the deeper sub-basin regions. Spatial and depth based trends in the geochemical data within the Beetaloo Sub-basin will be reviewed and compared to previously published results.

Hydrocarbon yield calculations were utilized in conjunction with other methods such as core based shale rock properties and pyrolysis S1 yields to evaluate original oil and gas in-place contents (OIP, GIP). Comparative volumetric assessment figures for the OIP and GIP are presented that indicate a range of P50 values, from 94–722 MMbbl oil in the Kyalla Formation, to 9–83 MMbbl oil and 202–752 TCF gas in the middle Velkerri Formation. The sources of uncertainty associated with these estimations of hydrocarbons in place will also be discussed.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018