Berry, John Shaw; (1995) Diagenesis of Westphalian sediments from the southern North Sea Gas Basin. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This diagenetic study of Westphalian fluvio-deltaic sediments from the southern North Sea Gas Basin has revealed two distinct and complex paragenetic sequences for the sandstones and mudrocks. Periodically these parageneses are convergent in time space, documenting periods of mudrock mediated diagenesis within the sandstones. These paragenetic sequences were constructed on the basis of a detailed petrographic, fluid inclusion and stable isotope study. Eogenesis within the sandstones and mudrocks was characterised by extensive siderite cementation which was mostly sourced from the mudrocks, and reflects the meteoric composition of the depositional pore fluids. This period of siderite cementation provided framework support during burial. A significant volume of kaolinite cement, also sourced from the mudrocks, partially occluded some of the remaining primary porosity during eogenesis. Petrographic evidence from the mudrocks indicates that discrete periods of overpressure developed during eogenesis. Release of overpressured fluids from the mudrocks promoted the discharge of fluids from the mudrocks into the sandstones. The rapid end Carboniferous inversion event elevated the Westphalian sediments through a column of hypersaline Permian formation waters, and displaced the modified meteoric waters from the residual primary porosity (background porosity) of the Westphalian sandstones. These highly saline brines were out of equilibrium with siderite and would have initiated its dissolution. With a resumption of burial during the Triassic, compaction driven "connate-meteoric" fluids from the mudrocks were expelled (perhaps periodically, related to the release of fluid overpressure generated during rapid burial, in the presence of well cemented sandstones aquicludes) into adjacent sandstones and initiated mixing corrosion of the siderite. This was the main period of secondary pore generation. Quartz cementation was initiated at a temperature of 100°C within the secondary porosity and improved the framework stability. Petrographic data strongly supports a mudrock source for the silica. The remaining sandstone paragenesis comprises secondary pore occluding silicate, phyllosilicate, carbonate and sulphate cements. Mesogenetic cementation was mostly mediated by internal reactions such as siderite dissolution, albitisation and the breakdown of argillaceous lithic fragments. Mudrock diagenesis was essentially terminated after the expulsion of silica rich fluids. Flushing of sequences by late stage (130°C) low salinity fluids, apparently introduced via large basin margin fault zones, restricted the development of further cements, therefore preserving better reservoir conditions. The peak hydrocarbon (methane) generating window overlapped with the mesogenetic cementation events. A majority of diagenetic events have been explained in terms of processes requiring a minimum of fluid transport, usually over distances of 10's of metres. This is an important consideration within Westphalian sediments that contain abundant stratigraphic (thick mudrock units) and diagenetic (heavily cemented sandstones) barriers to prolonged flow of large fluid volumes. Reservoir quality is determined by depositional processes and the cumulative sequential operation of several diagenetic processes during eogenesis and mesogenesis.

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