Reservoir lithofacies study of the Toolebuc Formation, Eromanga Basin, Australia as a potential unconventional target

Abstract

Open file data and new laboratory data were integrated with different lithofacies from gamma-ray (GR) response and core to interpret reservoir properties that would lead to a hydrocarbon potential targeting model for the Toolebuc Formation (Fm). The Toolebuc Fm is divided into an upward deepening cycle (UDC) followed by an upward shallowing cycle (USC), in which silica-rich argillaceous mudstone (Facies 1), mixed argillaceous mudstone (Facies 2) and interbedded calcareous mudstone and shelly thin beds/horizons (Facies 3) occur. Facies 1 is enriched in clay and silica component with relatively low Total Organic Carbon (TOC). Facies 2 has a mixed mineralogical composition with high TOC. Facies 3 is enriched in carbonates (coccoliths and shelly debris). The TOC value is high and shows large variation. The organic matter (OM) in the Toolebuc Fm corresponds to Type II kerogen and exhibits variability in maturity indicators but has arguably entered the early oil window. The dominant pore types of the Toolebuc Fm are intraparticle mineral pores and interparticle mineral pores while OM pores are rare. Samples collected from the basin centre show evidence of hydrocarbon generation.Vertically, the three lithofacies show a cyclic stacking pattern that corresponds to deepening and shallowing in a marine environment. Facies 1 is the transitional interval between the Toolebuc Fm and its adjacent units, the Wallumbilla Fm below and the Allaru Fm above. Facies 2 dominates the upward deepening cycle while Facies 3 dominates the upward shallowing cycle. The stacking of lithofacies corresponds to an upward increase and a following decrease in GR response, TOC, Brittleness Index (BI), and an upward decrease and a following increase in clay content.Facies 3 in the Toolebuc Fm shows the greatest unconventional potential due to the coincidence of the high TOC and BI. Facies 2 shows moderate potential with the high TOC and moderate BI, while Facies 1 is less favourable with the lower TOC and low BI. The target zone is located at the basin centre due to the higher thermal maturity relative to the marginal areas.