Integrating Geochemical and Petrographic Analyses to Better Understand Proximal to Distal Variations in Source Rocks, Using an Example From the Bashkirian in the UK

Abstract

Mudstone lithofacies are now known to be highly variable, impacting on all aspects of their source, seal and reservoir potential in different locations within the basin. Previously, much of this variability has been interpreted by examining chemical proxies and their subtle variability in redox sensitivities (particularly the presence of anoxia). However, some of the interpretations from geochemistry appear to be at odds with conclusions reached from the petrographic and detailed logging studies. The aim of this paper is investigate these discrepancies and refine stratigraphic models to provide a clear insight into how a source rock varies laterally within a basin. This is achieved by integrating detailed (sub-mm to 10s m scale) petrographic analyses of lithofacies (grain size, mineralogy, fabric), total organic carbon and inorganic geochemical data acquired from a well-constrained proximal to distal succession of Bashkirian-aged mudstones (beds enclosing the Bilinguites gracilis horizon) in the UK Pennsylvanian basin. Seven lithofacies have been identified in this study (using the combined approach of petrographic and geochemical analysis), which are either bioturbated or organized into thin graded beds. In proximal locations, facies are mainly silt-bearing, clay-rich mudstones with up to 2% TOC and contain <4.5 ppm U and <1.8 ppm Mo. In more distal locations the facies are broadly similar, but contain more clay and TOC (up to 8.9%), with higher concentrations of the redox sensitive elements, up to 25.1 ppm U and up to 205 ppm Mo. When integrated, as in this example, the datasets appear to provide a relatively consistent story, indicating that there are systematic differences in the grain size down the sediment transport path (reflected in compositional variability in the chemical data, and grain size in the petrology) and that typically more organic matter was preserved downdip in conditions that may have been prone to developing anoxia (high trace element concentrations). Somewhat counterintuitively, slower sediment accumulation rates updip (perhaps accommodation limited) may be the primary cause of the lateral differences. Sediment accumulation rates downdip were more continuous and faster, enabling a higher proportion of organic matter to be preserved by relatively continuous burial. This study demonstrates the need to integrate geochemical and petrographic methods when seeking to understand controls on source rock facies variability in basins.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015