Diagenetic controls on the phyllosilicate fabric of mudstones

Abstract

Changes to the orientation of phyllosilicates in mudstones in the Podhale Basin and the Northern North Sea have been quantified using High Resolution X-ray Texture Goniometry (HRXTG). Samples were studied from four wells: Chocholów PIG-1 and Bukowina Tatrzanska PIG-1 from the Podhale Basin, and Magnus 211/12-2 and Rhum 3/29a4 from the Northern North Sea. The samples cover a temperature range of ~60 to 170°C. Porosity and pore size distributions were quantified by mercury intrusion porosimetry. Changes in mineralogy with depth have been addressed by Quantitative Xray Diffraction (XRD). Analytical Transmission Electron Microscopy (ATEM) was performed on illite-smectite from selected samples from the Podhale Basin. All wells were subjected to 1-D maturity modelling using Genesis 4.8®. Maximum burial temperatures of the Podhale samples range from ~60 to ~170°C (3 and a km). XRD reveals that the sample sets straddle part of the smectite to illite RO to R1 ordering transition zone and that illitization terminates at ~8O% illite layers. An increase in chlorite and a decrease in both kaolinite and K-feldspar occur over the same depth/temperature interval. Quartz contents increase with increasing temperature. At the top of the sequence, porosities are. close to 10%, indicating very substantial compaction. However, HRXTG shows that there is only a modestly preferred alignment of both iIIite-smectite and chlorite-kaolinite. A strongly aligned fabric is developed through the smectite to illite transition in Chocholów, with a less marked but readily measurable increase beyond the transition in Bukowina Tatrzanska. Samples from Chocholów document a strong relationship between millimetre-scale fabric and clay mineral transformations, but the relationship is more complex in Bukowina Tatrzanska. The trend in %1 in illite-smectite shows a gradual increase with depth from ~37% layers to ~75% layers in Chochol6w, with a fabric change from ~3.4 m.r.d. to 5.2 m.r.d. The trend in the %1 in illite-smectite is constant at ~75% of layers in Bukowina Tatrzanska, but with a fabric increase from ~4.8 m.r.d. to ~6.2 m.r.d. ATEM has been performed on end member samples to determine chemical variation in illite-smectite. ATEM reveals significant differences in the octahedral sheet Fe concentrations of samples from Bukowina Tatrzanska with increasing temperature, pointing to continued crystallite change beyond the termination of smectite illitization. A relationship between SADP and structural formulae has been found. Single crystal patterns dominate where structural formulae reveal little or no substitution in the octahedral sheet. Samples from Magnus (~3200m; ~120°C) and Rhum (~4800m; ~150°C) consist of Tertiary, Cretaceous and Jurassic sediments of varying thicknesses and maturity. Porosity reduction in both wells is from relatively uncompacted sediments (~45%) to tight muds (~5%). Preferred alignment of phyllosilicates is weak in shallow samples from both wells (m.r.d. ~2.3) and only shows a modest increase in Magnus as compaction and burial diagenesis (smectite illitization) proceed. Stronger phyllosilicate alignments are developed at the base of Rhum (6.1 m.r.d.). Generally, a high correlation between the two mineral peaks exists, the exception being the Jurassic of Rhum where low kaolinite and chlorite abundance as measured by XRD explains the low fabric development by HRXTG. XRD also reveals an increase in the %1 in illite-smectite, 60% in Magnus and 65% in Rhum, with increasing depth and temperature. XRD reveals differences in mineralogy between the Jurassic, Cretaceous and Tertiary reflecting differences in depositional environment and sediment input. 1-D maturity modelling has established a correlation between, vitrinite reflectance, %1 in illite-smectite and the development of preferred alignment of phyllosilicates. Little change in preferred alignment at porosities >15% has been noted, with the major changes occurring between ~15% and 5% porosity; this coincides with the change between RO and R1 ordering in I-S and sits in a temperature range between 75°C and 125°C. The mechanism for preferred orientation change is therefore postulated to be dependent on the dissolution-precipitation reaction mechanism in smectite to illite formation in a high effective stress environment.