ABSTRACT: Late Proterozoic Source Rocks Associated with Diapirs in the Central Flinders Ranges, South Australia
LEMON, NICHOLAS M., and ANDREW McG. GOOPER, National Centre for Petroleum Geology and Geophysics, Adelaide, South Australia, Australia, and DAVID M. McKIRDY, University of Adelaide, Adelaide, South Australia, Australia
Recent drilling in the late Proterozoic Adelaidean sequence of the central Flinders Ranges in South Australia has discovered methane in a 400 m-thick, fractured, siliciclastic source rock of surprisingly low thermal maturity. Frontier Exploration's Blinman-2 was a test of sediments on the flank of the Blinman Diapir. The hole was collared in gray siltstone of the middle Adelaidean Tapley Hill Formation and terminated at 2031 m in the underlying glaciogenic sequence. The source of the gas is the basal Tindelpina Shale Member of the Tapley Hill Formation, a black pyritic shale (TOC = 0.2-1.4%) which was deposited during a major transgression that followed the extensive Sturtian glaciation. Local facies variations were induced by syndepositional movement of the diapir. Slumps and turbidi es with diapiric debris are common, particularly in the lower half of the Tapley Hill Formation, where the gas-filled fractures occur. Another organic-rich unit (TOC ~1%), intersected in the upper part of the Tapley Hill Formation, correlates with a major pulse in the upward movement of the diapir. Evidently the associated depositional sink was deep enough to become periodically anoxic; and the shoal above the rising salt plug is likely to have been a site of enhanced algal and bacterial productivity. Locally organic-rich shales occur near diapirs at other stratigraphic levels elsewhere in the Flinders Ranges. The normally redbed Bunyeroo Formation is markedly carbonaceous (TOC ~2%) adjacent to the Mucatoona diapir. Thus syndepositional diapirs provided an ideal environment for the growt and preservation of sedimentary organic matter.
The Adelaidean sequence was folded by the late Cambrian to Ordovician Delamerian orogeny. Associated metamorphism was most intense in the eastern and northeastern parts of the geosyncline where kerogen in the Tindelpina Shale is graphitic (atomic H/G <0.10). Along the western side of the fold belt the stratigraphically equivalent kerogen is subgraphitic (atomic H/C = 0.10-0.25), although our results indicate a much lower degree of thermal alteration in Blinman-2 The gently deformed central zone of the Flinders Ranges has been protected from intense folding and metamorphism to such an extent that the Tindelpina source rock is still within the gas-generation window (equivalent vitrinite reflectance = 1.9%).
AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)