Diagnetic modelling in Middle Jurassic clastic sediments from the Ravenscar group, Yorkshire, and the Brent group, northern North Sea

Kantorowicz, John Duncan

Abstract

Diagenesis is the sum of those processes whereby an originally sedimentary assemblage attempts to reach and maintain an equilibrium with its environment. Numerous factors influence and affect the diagenesis of sedimentary elastic assemblages - fundamentally unstable when deposited but never reaching equilibrium ith their environment. The interrelationship of these factors, however, precludes the identification of any single factor as wholly controlling the diagenesis of elastic sediments. The multifactorial nature of diagenesis is illustrated here by reference to the Middle Jurassic Dogger Formation and Ravenscar Group which outcrop on the North Yorkshire Moors and the Brent Group from Wells 3/3-1, -2, and -3 in the Ninian Field, and 210/15-2, also in the Northern North Sea.The Ravenscar Group is interpreted as being deposited in a fluvio-deltaic complex. Here, attempted equilibration of non-marine sediments with their pore waters resulted in a variety of diagenetic modifications. These are interpreted as being influenced strongly by bacterial degradation of organic matter, which lowered pH and then reduced Eh. This reduction of pH caused feldspar dissolution and muscovite neomorphosis to kandites throughout. Similarly, in the texturally and inineralogically mature sandbodies, quartz overgrowths and veriniform kandites precipitated from oxygenated pore waters, whilst chlorite and overgrowths formed in anoxic pore waters. This resulted in complete reduction of porosity in, and cementation of, finer and texturally less mature overbank sands. Conversely, channel sands were cemented into rigid but porous quartzose frameworks. In addition, soil horizons of sphaerosiderite developed as standing water on the floodplains stagnated. Bacterial ferric iron reduction throughout the water table then raised the pH and extensive siderite cements were precipitated. During burial, calcium carbonate saturated formation waters migrated into the remaining porous sandbodies and precipitated replacive ferroan calcite. In marine elastics, meanwhile, illite and potassium-feldspar overgrowths precipitated before in situ bacterial processes lowered pH here also. This resulted in dissolution of feldspars, muscovite neomorphism and precipitation of vermiform kandites. Subsequently, pH rose and ferroan calcite cementation occurred.It is suggested that "aggressive fluids" migrated into the larger connected sandbodies during burial. They dissolved the carbonate cements and precipitated dense pockets of blocky kandites. These sediments were little affected during continued burial. However, their Recent weathering may have dissolved carbonates and feldspars as well as neomorphosing chlorite to vermiculite.The reservoir rocks of the Ninian Field have formed from mature quartz-rich elastic sediments which accumulated in a transgressive sequence above Liassic mudrocks which was subsequently incised into by a fluvial system. Diagenetic modifications of the original sediments are similar to those of the Ravenscar Group, although neither chlorite nor soil horizons were observed in the wholly non-marine sediments. Porosity in marine sediments is occluded by extensive authigenic illite within a generally quartz framework. The effects of the fresh water table are also seen in the marine sediments over which the delta prograded. Then, during burial, ferroan calcite cementation and subsequent leaching and blocky kandite precipitation occurred here also. Hydrocarbon maturation and migration into the reservoir was preceded by alteration of pre-existing carbonates to ankerite, and. minor illitisation of blocky kandites. However, the only effort which can be related to oil emplacement is the widespread pyritisation around the oil-water contact.The sandstones from 210/15-2 are interpreted as formed from coarse elastic sediments which accumulated in a shallow-marine nearshore environment, possibly incised into by distributary channels. Initial marine connate water precipitated potassium-feldspar overgrowths before bacterial processes lowered pH and caused widespread kandite formation. Subsequently, these sediments were affected by ferroan calcite cementation, then leaching and blocky kandite precipitation. Although oil has migrated into these sediments, no other effects were observed.In addition to the factors which have been proposed previously as influencing diagenesis, I should like to propose that the climate of both the source area and of the depositional basin was of fundamental importance to diagenesis and many of the features observed in these rocks may be related to the original tropical climate. Moreover, as a result of the fundamental stability of the quartzose frameworks established during eogenesis, this climatic "finger print" may be recognised in all these sediments despite their subsequent diverse history.

Citation

Kantorowicz, J. D. Diagnetic modelling in Middle Jurassic clastic sediments from the Ravenscar group, Yorkshire, and the Brent group, northern North Sea. (Thesis). University of Hull. https://hull-repository.worktribe.com/output/4210773

• Thesis Type: Thesis
• Deposit Date: Aug 15, 2011
• Publicly Available Date: Feb 22, 2023
• Keywords: Geology; Mineralogy; Sedimentology
• Public URL: https://hull-repository.worktribe.com/output/4210773
• Additional Information: Department of Geology, The University of Hull
• Award Date: Sep 1, 1982