Diagenesis of Upper Carboniferous sandstones: southern North Sea Basin

Abstract

Summary Upper Carboniferous sediments from the central part of the southern North Sea Basin were deposited on a broad fluvio-deltaic plain. Potential reservoir rocks occur within distributary channel facies sandstones, but porosity and permeability have been reduced by authigenic mineral growth and improved by at least two separate phases of porosity enhancement: firstly by the infiltration of meteoric waters during post-Carboniferous uplift, and secondly by the action of acidic fluids generated during burial. The depth of invasion of meteoric fluids is highly variable, ranging from a few metres to hundreds of metres, and the best porosity is preserved in sediments which have undergone both phases of porosity enhancement. Virtually all the observed porosity in Carboniferous sandstones can be interpreted as having a secondary origin. Integrating the deduced diagenetic history with a burial and temperature history for specific wells allows the timing of diagenetic events to be estimated.

Origin of reddening and secondary porosity in Carboniferous sandstones, Northern Ireland

“…This suggests that meteoric fluids penetrated rocks to depths of hundreds of metres. Cowan (1989) reported similar observations in the southern North Sea Basin.…”

“…For instance, the source of gas discovered in the Rotliegendes and Sherwood Sandstone Groups in the southern North Sea Basin is in deeply buried organic-rich 252 W.H. WANG mudrocks and coals of Carboniferous age, and gas shows have frequently been recorded from intervening Carboniferous sandstones (Cowan 1989). However, there are no oil/gas shows in the Ballycastle district because the source rocks are not mature (Parnell 1991).…”

The Schooner Field, Blocks 44/26a, 43/30a, UK North Sea

“…Porous reservoir generation then post dates the development of reddening and is thought to have taken place during deep Mesozoic burial (Besly et al 1993). Elevated temperatures and aggressive (low pH) formation waters suggested as being associated with maturation of organic matter, are the possible cause of this enhanced porosity generation (Cowan 1989).…”

An integrated approach to provenance studies: a case example from the Upper Jurassic of the Central Graben, North Sea

“…Kaolinite is recorded in Westphalian mudstones from Wells 31/.27-1 and 39/2-1 and kaolinite is a common cement in Westphalian sandstones (e.g. Cowan 1989). An alternative cause of a kaolinite influx could be a climatic change to conditions favouring intense sub-tropical leaching on exposed highs, although persuasive arguments against this idea were forwarded by Hurst (1985).…”