Influence of anoxia on source rock preservation in the carboniferous of northern England
File(s)
Author(s)
Sims, Michael James Edward
Type
Thesis
Abstract
The Lower Carboniferous basins of northern England acted as sinks for marine and terrestrially derived organic matter. By understanding the relative contributions of biomarkers, aspects of the depositional setting and history of geological formations may be understood. The work included in this thesis particularly focussed on determining evidence of anoxic to dysoxic conditions in the Widmerpool Gulf and Craven Basin. Reducing conditions were determined throughout the water column. Highly reducing conditions were stratified in the water column along redoxclines associated with either productivity or thermoclines. The influence this had on the preservation of Rock-Eval source rock quality indicators like Total Organic Carbon (TOC) and Hydrogen Index (HI), was evaluated finding a poor to good quality source rock despite the encouraging environmental conditions. Diasteranes were identified through GC-MS analysis, indicating that bitumen was interacting with clay minerals catalytically influencing maturity indices.
The disparity between the bulk geochemistry and biomarker evidence enabled further exploration into the organic geochemical character of macerals in the appendix chapters. Samples were found to be comprised of residual carbon through exploration of oxidative and pyrolysis Rock-Eval curves. Deltaic sourced sediments contained fossilised inertinite macerals, while marine sediments contained altered or matured organic matter forming the ‘residual carbon’ measurement. Kerogen isolates from the Widmerpool Gulf were studied revealing abundant heterogenous granular amorphous macerals. These were correlated with published identifications as being bacterial in origin associated with deltaic deposition. The amorphous organic matter is considered in the current study to be oil prone, with capabilities for early generation. This justifies the extensive hydrocarbon fields found across the East Midlands, despite its lower maturity than the Craven or Cleveland Basins. Vitrinites were commonly observed in the sample material, along with prasnophyte algae. The kerogen type that the Lower Carboniferous may be classified as ‘mixed’ between type II, III and I.
The disparity between the bulk geochemistry and biomarker evidence enabled further exploration into the organic geochemical character of macerals in the appendix chapters. Samples were found to be comprised of residual carbon through exploration of oxidative and pyrolysis Rock-Eval curves. Deltaic sourced sediments contained fossilised inertinite macerals, while marine sediments contained altered or matured organic matter forming the ‘residual carbon’ measurement. Kerogen isolates from the Widmerpool Gulf were studied revealing abundant heterogenous granular amorphous macerals. These were correlated with published identifications as being bacterial in origin associated with deltaic deposition. The amorphous organic matter is considered in the current study to be oil prone, with capabilities for early generation. This justifies the extensive hydrocarbon fields found across the East Midlands, despite its lower maturity than the Craven or Cleveland Basins. Vitrinites were commonly observed in the sample material, along with prasnophyte algae. The kerogen type that the Lower Carboniferous may be classified as ‘mixed’ between type II, III and I.
Version
Open Access
Date Issued
2023-10-06
Date Awarded
01/12/2024
Copyright Statement
Creative Commons Attribution Licence
License URL
Advisor
Fraser, Alastair
Sephton, Mark
Sponsor
Natural Environment Research Council (Great Britain)
Grant Number
NEM00578X/1
NE/R018065/1
Publisher Department
Earth Science & Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)