AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Back to the Rocks: A New Petrophysical Model for Siliciclastics Engaging Old Petrological Techniques
(1) Santos and The Australian School of Petroleum - The University of Adelaide, Adelaide, SA, Australia.
Providing reliable petrophysical interpretations in reservoirs with complex mineralogy can be particularly challenging. These challenges are exacerbated when the sediments are rich in lithics and subsequent diagenesis has modified the pore structure by filling the pores with a variety of clays and other cements. In order to constrain the petrophysical models for the oil reservoirs in the Cooper and Eromanga basins, a petrological study was proposed, incorporating thin sections analysis, X- Ray diffraction and Scanning Electron Microscopy to quantify the mineralogy and pore structure, document provenance evolution and redefine the petrophysical model used in the area. The target sandstones were in the Hutton, Birkhead, Westbourne, Namur and Cadna-Owie formations. Following the QFR classification from Folk (1974), results from the ternary plots suggested the analysed samples fall in the arkose, subarkose and lithic arkose categories but the Birkhead samples, which are feldsphatic litharenites. XRD analyses and SEM photomicrographs revealed the presence of smectite, kaolinite, illite, chlorite and mixed clays layers as the main authigenic clays. Also, siderite and calcite were identified as the main cements in the Westbourne and Birkhead formations, whilst silica and kaolin cements were present in the Namur and Hutton formations. The new petrophysical model changed the clay minerals through the formations and now included smectite and chlorite. A framework mineral, calcite, was substituted by chlorite (an authigenic clay) in the new model. In general, an increase in the volume of clays was observed within the formations as a consequence of the modifications in the clay mineralogy, with a slight decline of the total porosity. Yet, an increase in effective porosity and water saturation was noticed. The replacement of chlorite for calcite in the Petrophysical model had only a minor impact on total porosity due to the very similar properties for the two minerals. However, the change in effective porosity and water saturation was significant. This change was due to the effects of the bound water associated with the chlorite. The chlorite, being a clay, will have some volume of associated water (clay bound water CBW), and this water will contribute to the overall water saturation. However, while the total water saturation will increase, this water is not mobile, and consequently the only impact of the increased water saturation will be in a reduction in reserves.