Depositional Processes and Multi-Scale Variability Within the Mudstone-Dominated Early Jurassic Mudstones, Cleveland Basin, United Kingdom

Abstract

The effectiveness of production from shale reservoirs depends on the understanding of the variability of the characteristics in such reservoirs, including organic carbon content, mineralogy, mircro-texture, porosity and permeability. This study aims to understand the vertical and lateral variability in the early Jurassic shales of Cleveland basin and which sedimentological, depositional and diagenetic processes are responsible for this variation. This study is using data from previously undescribed core from the Cleveland basin, UK. , as well as outcrops on the Yorkshire coast. Detailed lithofacies characteristics have been defined using optical microscopy, high resolution SEM images, and characterised for composition by TOC and XRD analysis. Eight mudstone lithfacies have been identified in the examined Grey Shales and the Mulgrave shale Member in the Felxikirk and Brown Moor Boreholes: (A) Homogeneous, argillaceous-siliceous, fine to medium mudstone, (B)Ripple-Laminated, argillaceous- siliceous, fine to medium mudstone, (C) Diagenetic detritus-derived, argillaceous – calcareous, fine to medium mudstone, (D)Silt-rich, argillaceous, fine mudstone, (E)Algal-macerals bearing, normal-graded, Carbonaceous, fine mudstone, (F)Homogeneous, argillaceous-calcareous, medium to coarse mudstone, (G)Homogeneous, calcareous, fine mudstone, (H)Fracture rich, argillaceous, fine to medium mudstone. At millimetre to centimetre-scale, these facies include fine, medium, and coarse mudstones that are composed of different proportions of argillaceous, calcareous, and siliceous minerals whose depositional fabrics have been variably affected by burrowing and diagenesis. These mudstones contain significant quantities of total organic carbon (from 1.3% to 8.9%), and cements (including kaolinite, illite, calcite and dolomite). Many diagenetic features are recognized within these rocks and include clay and carbonate cement as well as the development of pyrite framboids associated with organic matter. Overall, we interpret these mudstones to have been deposited in a shallow, high energy shelfal environment, with much evidence for dynamic sediment transport.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018