Seeing is Believing; High Resolution Electron Imaging and Mineral Mapping Shows Trace Minerals can Control Reservoir Properties

Abstract

Studies of conventional reservoirs routinely apply optical petrography to determine the association of minerals, pores, and occluding cement. This approach is not feasible in shale reservoirs because the resolution of optical petrography does not allow imaging of the nano-to microscale grains and pore systems characteristic of shale. Given the common view that shale is largely homogenous, bulk measurements such as powder X-ray diffraction or gas sorption are most commonly employed to characterise unconventional reservoirs. While these techniques often reveal the presence of trace minerals and microscale porosity, insitu studies of pores using electron imaging has revealed results that could not have been predicted on the basis of bulk properties, such as the presence of pores in specific phases such as organic matter. Here we show an analogous refinement in understanding of mineralogical reservoir properties using electron mineral mapping to locate trace minerals identified in bulk powder analysis. Electron image based mineralogical data are compared between two late Permian aged shale reservoirs in South Australia; the lacustrine REM interval of the Cooper Basin and adjacent marine Stuart Range Formation in the Arckaringa Basin. Both show systematic distributions of early carbonate cements. The low sulfur REM typical of lacustrine settings resulted in early diagenetic Fe carbonate (siderite) cements that preferentially formed in coarser grained laminae. These cements parse the reservoir, restricting migrating hydrocarbons to finer grain size intervals. By contrast, S sourced from seawater in the Stuart Range led to sequestration of Fe in pyrite, barring the formation of siderite and allowing sulfurization reactions that preserved lipid-rich type II organic matter. Pyritic intervals alternate with Mn-carbonate cemented intervals dominated by type III (refractory terrestrial) OM in in varves resulting from oscillations in basinal redox conditions. The dominance of one cycle over the other influences hydrocarbon potential as well as brittleness and reservoir compartmentalization where Mn-carbonate intervals increase. While Mn-carbonate and siderite were present in trace amounts in many of the samples analysed by powder X-ray diffraction, the spatial data from the insitu technique provided the environmental significance and the ability to better understand basinal trends in source, reservoir, and rock properties.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016