Lithofacies Variability in Fine-Grained Mixed Clastic Carbonate Successions: Implications for Identifying Shale-Gas Reservoirs

Joe Macquaker¹ and Sarah Davies^2
1School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, United Kingdom
²Department of Geology, University of Leicester, Leicester, United Kingdom

Lithofacies variability in fine-grained, organic carbon-depleted interbedded carbonate clastic, sedimentary successions is poorly known relative to that present in other sedimentary strata. In most existing models, changes in the balance of clastic input, predominantly deposited from suspension settling, to primary productivity linked to subtle changes in pore water oxygen concentrations and substrate firmness are considered the main forcing mechanisms that underpin facies variability. In order to investigate the validity of these models, a fine-grained, organic matter poor, mixed carbonate-clastic, mud-dominated succession (Brigantian) exposed at Streedagh Point (Sligo, Ireland) has been investigated using a combination of field, optical, electron optical and geochemical methods.

Analyses reveal that there is a great deal of cryptic lithofacies variability preserved on <10^-2 m scale in these strata. Individual depositional beds are very thin (<10^-2 m), commonly upward-fining, exhibit low angle ripple lamination, contain widely differing proportions of both detrital and productivity derived components (both silica and carbonate-dominated faunas), and have been subjected to varying bioturbation and early diagenesis.

Existing models used to explain lithofacies variability underestimate the dynamism and episodic character of this sedimentary system. In particular sediment dispersal by high energy, advective processes operating close to the sediment water interface and changes in productivity from dominated by organisms with calcareous tests to that dominated by organisms with siliceous tests has been missed. The implications of this variability for shale gas play exploration strategies are discussed.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas