THE KIMMERIDGIAN MARL IN THE TIMOR SEA: RELEVANCE TO REGIONAL GEOLOGICAL EVOLUTION AND POSSIBLE HYDROCARBON PLAYS

Abstract

Lower Kimmeridgian marls in the Sahul Syncline have a distinct seismic character and can be used to define the limits of depocentres, but the reflector is difficult to map in the Vulcan Sub-basin. The top of the carbonate is an unconformity in shelfal areas and on uplifted structures, but in the depocentres, where a distinctive and thin limestone is present at the top of the marls, there is no evidence of missing section. The limestone, which is interpreted to reflect the paraconformity of the 139 Ma Type 1 sequence boundary, consists of the amalgamated condensed highstand overlying the 139.5 Ma condensed section, and the condensed transgressive systems tract of the overlying C. perforans shelf margin wedge.

The underlying Upper Jurassic section is an important source interval in the Vulcan Sub-basin and the regional extent of the reflector may approximate the area in which Oxfordian source beds were deposited. Regional isochore mapping may also delineate areas of potential silled, fault-bounded depocentres within the greater Sahul Syncline that were in existence during latest Callovian to early Kimmeridgian time. Structures located within the fetch of these depocentres should be ideally placed to trap migrating hydrocarbons sourced from restricted marine shales of Oxfordian age.

Erosion accompanying the 138 Ma lowstand has removed some or all of the marls and the C. perforans sediments from structures uplifted during the contemporaneous mid-Kimmeridgian tectonism. Major sea level falls, probably also associated with tectonic movements at the 136, 135 and 134 Ma sequence boundaries, could have led to further erosion or non-deposition on high blocks. Given sufficient time and suitable lithology, fresh water diagenesis and leaching of these marls may have led to the development of secondary porosity with later sealing by post-Kimmeridgian shales, especially where the interval contains coarser clastics as along the flank of the Flamingo High.