1. Investigating the link between provenance and reservoir quality, in the Slyne Basin, Offshore Western Ireland.
Jessica Franklin and Shane Tyrrell
Earth and Ocean Sciences and Irish Centre for Research in Applied Geosciences (iCRAG), National University of Ireland, Galway
Introduction
The key aims of this study are to identify potential causes of provenance-controlled reservoir sandstone heterogeneity from a micro- to a macro depositional system scale. Provenance analysis endeavours to identify the sources of sediment and to unravel the series of processes experienced by a sediment. A range of sedimentary rocks can be derived from similar sources and conversely very similar sedimentary rocks can be produced from diverse sources. This is due to a multitude of factors, including climate and tectonics, which influence and modify sediment during generation, transport, storage and diagenesis. This modification ultimately affects reservoir sandstone quality. Therefore through the investigation of sedimentary provenance we can reconstruct a “source to sink” model for sedimentary basins, which then allows for better prediction of the characteristics of reservoir sandstones in the sub-surface.
Study Area
Background and Rationale
This project will initially focus on a well characterised, though heterogeneous, sedimentary system – the Lower Triassic in the Slyne Basin
Diagenesis is a key controlling factor on sandstone reservoir quality.
To what extent is this controlled by primary detrital mineralogy?
Burial of a quartz arenite (left) may lead to the development of quartz overgrowths which negatively affects reservoir quality by reducing porosity and permeability
In this example we can see that primary detrital mineralogy has a definite control on reservoir quality.
Arkosic sediments present a more complicated scenario however.
Burial of arkosic sediments (left) can lead to a variety of diagenetic modifications.
Processes which positively impact porosity and permeability include, the presence of feldspar hindering the pervasive overgrowth of quartz, and the dissolution of grains creating secondary pore space.
Triassic Palaeogeography (Tyrrell et al. 2012)
Location of targeted wells in the Slyne Basin
Results
3D plot of feldspar data
Discussion and Future Work
Results from the Pb isotopic analysis of K-feldspars reveal mixed Archaean - Proterozoic sources with minor contributions from the Annagh Gneiss Complex and Irish Caledonian granites. U-Pb geochronology of zircon and apatite also show these Archaean - Proterozoic ages, with a significant peak at Ga, corresponding to the Laxfordian of the Lewisian Complex. Grains yielding this Laxfordian age potentially correspond to population 2 in the feldspar data, with population 1 representing older components of the Lewisian, Nagsuggtoqidian and equivalents. Population 3 feldspar may be derived from 1) Grenvillian terranes; or 2) Caledonian granites. Caledonian aged zircons and apatites are relatively under-represented.
There are younger U-Pb ages which do not correspond to any previously identified source. The contrast in feldspar and zircon/apatite data may be related to feldspar type and/or burial depth, diagenesis, or preservation, recycling of zircons, or perhaps mineral fertility issues in the sourcelands. These results emphasise the importance of a multi-proxy approach in provenance studies.
The next phase of this project will involve further Pb isotopic analyses of feldspar and U-Pb geochronology. Detailed EDS mapping will also be carried out in order to establish a detailed characterisation of diagenetic phases. These data will be linked to reservoir characteristic parameters in order to evaluate fully the relationship between provenance and reservoir quality.
This publication has emanated from research supported in part by a research grant from Science Foundation Ireland (SFI) under Grant Number 13/RC/2092 and is co-funded under the European Regional Development Fund and by PIPCO RSG and its member companies.
References:
Tyrrell, S., Haughton, P. D. W., Souders, A. K., Daly, J. S. & Shannon, P. M Large-scale, linked drainage systems in the NW European Triassic: insights from the Pb isotopic composition of detrital K-feldspar. Journal of the Geological Society, 169,
Acknowledgements:
Travel to this conference was funded by the Thomas Crawford Hayes Travel Fund, NUIG