Hydrocarbon prospectivity within PEL 423, Stansbury Basin lying within Gulf St Vincent
William, Stella N.
Master of Science Degree, 2009
University of Adelaide
Petroleum Exploration License (PEL) 423 area of Stansbury Basin lies within Gulf of St. Vincent. The geology of this region comprises of three successive basins. The Middle Eocene St Vincent Basin overlies the Late Palaeozoic Troubridge Basin. The Troubridge Basin overlies the Early to Middle Cambrian Stansbury Basin. The prospective areas for hydrocarbons of the Stansbury Basin are the Yorke Peninsula and the area lying within the Gulf of St. Vincent.
The aim of the project was to build an up to date database of the Stansbury Basin in order to determine hydrocarbon prospectivity of the study area. The scope of the study was limited to area of Stansbury Basin lying within Gulf of St. Vincent. There has been less assessment of the Gulf St Vincent Basin sediments. Out of 75 wells drilled within the Stansbury Basin, two wells were drilled offshore Gulf of St. Vincent. Troubridge-1 well was drilled on a sandbank. Both wells (Enchilada-1 and Frijole-1) did not intersect the prospective Early to Middle Cambrian sediments tested onshore Stansbury Basin. Enchilada-1 well terminated in Precambrian while Frijole-1 terminated within Permian sediments.
In order to understand the hydrocarbon potential of PEL 423, a database from previous work including drilling, geophysical, magnetic and gravity studies was compiled. The information on wells which are open files was put together into a Microsoft Excel file. The formations intersected while drilling the wells and analyses performed on the samples were compiled. The analyses included porosity and permeability, vitrinite reflectance and Rock Eval Pyrolysis studies. GeoFrame 22.214.171.124 software was used to interpret seismic data from PEL 423 area to assess possible structures for hydrocarbon exploitation. The tops of Middle Cambrian Yuruga Formation and Middle Eocene Maslin Sand were mapped on all the seismic lines. In addition the top of the Early Cambrian “Red Beds” unit was also mapped. Furthermore, in order to determine maturity levels of potential source rocks in Enchilada-1 well burial history was determined using PetroMod 11 1D Express software.
Seismic interpretation mapped the undeformed sediments of Gulf St. Vincent which appear as sub-horizontal strata within the central area of the St. Vincent Basin. Due to flexural loading by the Late Cambrian Early Ordovician Delamerian Orogeny, up to 6000 m of Kanmantoo Group rocks were deposited within the Gulf of St. Vincent. The Kanmantoo Group rocks were deposited within the Stansbury Basin during late Early Cambrian and early Late Cambrian. The main faults marking the boundary of the Gulf of St. Vincent Basin were also mapped. Normal faults, which developed during Early Cambrian extension, were later reactivated during the Late Cambrian Early Ordovician Delamerian Orogeny. These thrusts border the St Vincent Basin on both sides from Yorke Peninsula in the west and the Mt. Lofty-Flinders Ranges in the east. In mapping the faults, anticlines which form potential traps were mapped. Thermal maturity modelling shows the Early Cambrian “Red Beds” are within the oil window and the Middle Cambrian Yuruga Formation is in the early oil window, using three periods of uplift and erosion to constrain the model. These periods are indicated in tectonic evolution of the three successive basins and in onshore Apatite Fission Track Analysis (AFTA) studies. The three periods of uplift and erosion are associated with the Late Cambrian to Early Ordovician Delamerian Orogeny, Late Permian Alice Spring Orogeny and Miocene compression.
Based on limited data available from core samples analyses in the PIRSA records, the tested Early Cambrian Parara limestone on Yorke Peninsula is a potential source rock for the Stansbury Basin and is presumed to extend beneath Gulf of St. Vincent to the Early Cambrian Heatherdale shale on the Fleurieu Peninsula. The Early Cambrian Kulpara Formation which crops out on the Yorke Peninsula and extends beneath Gulf of St. Vincent is a dolomitised limestone and is the principal reservoir within the Stansbury Basin. The Early Cambrian Koolywurtie limestone Member is thought to extend from Ardrossan to Kangaroo Island a distance which is over 100 km hence can form a potential reservoir for basin. The potential seal for the entire basin is the Middle Cambrian Yuruga Formation. This is a fine-grained feldspathic sandstone and siltstone which is red-brown in colour. Early Cambrian Sellick Hill Formation on the Fleurieu Peninsula is considered to extend beneath the Gulf St. Vincent onto the north coast of Kangaroo Island, acting as a potential regional seal. Potential traps are considered to occur within the anticlinal structures along the NNE striking Ardrossan fault. The offshore Early to Middle Cambrian units lying within the Gulf of St Vincent need to be tested by drilling and the results compared to the same units tested in the onshore Stansbury Basin.