Deepwater Reservoir Analogue - Bunkers Sandstone, Donkey Bore Syncline, Flinders Ranges, Australia.
Reilly, Mark Robert William
Honours Degree, 2001
University of Adelaide
AbstractOutcrop analogues provide a detailed 2D view of the sedimentary structures and the geometries of sand-prone units that would otherwise be below seismic resolution and below the resolution of conventional wireline logs. The Donkey Bore Syncline in the Northern Flinders Ranges of South Australia hosts an overall fine-grained, mud-rich, second order, turbidite system of Early Cambrian age that outcrops as ridges on three sides of a syncline with a topographically low centre. The semi-srid climate of the Northern Flinders Ranges limits substantial vegetation cover, allowing individual beds to be correlated for kilometres.
The turbidites of the Donkey Bore Syncline comprise over 400m of section and flank an active salt diapir to the east that was tectonically active during initial deposition of the turbidite facies. The basal part of the turbidite deposits are commonly sand-prone and massive, with only rare occurrences of Bouma sequences and are interpreted to occur in the lowstand to the upper transgressive systems tract. These deposits are overlain by more mud-rich, thinly interbedded classical turbidites. The turbidite facies in turn overlie micritic limestone deposited during the previous highstand. Lenticular geometries of massive sands suggest depsition in an outer fan.
The geometries of deepwater depositional elements were resolved by mapping key stratigraphic surfaces across the study area. Eight sections were measured through the unit with accompanying spectral gamma logs and four alternative correlations were constructed using different aspects of the gamma log combined with the stratigraphic logs. Preliminary petrographic studies give a qualitative assessment of the reservoir potential for various stratigraphic units and facies associations indicating a variety of shelf sediments including allochthonous debris from the Wirrealpa Diapir.
The eight stratigraphic logs intersect a basal, multistorey, amalgamated sand-prone unit (40m thick stratigraphically, 5 km exposed in outcrop) identified as a basin floor fan and overlying a mass transport complex. The basin floorfan is overlain by turbidite facies consisting of thin turbidite units, massive snady debris flows, and siltstone. The basin floor fan is interpreted as a lowstand systems tract, and represents the principle reservoir analogue in terms of reservoir quality and net to gross. The total framework grain content of the Basin floor fan commonly consists of 95% quartz grains and up to 26% visual porosity, whereas sandstone units of the overlying transgressive systems tract turbidite deposits consist largely of cemented lithic calcareous grains with little to no visual porosity.
Vertical and horizontal connectivity values for 5th order turbidite sub-units calculated from the correlation panels indicate a high degree of heterogeneity within the basin floor fan. This heterogeneity is not likely to be resolvable using conventional wireline logsand is below seismic resolution. Results from this study provide a high-resolution analogue for understanding the internal architecture of hydrocarbon reservoirs that were deposited in deepwater depositional environments.