Fine-grained lithologies control the rates and pathways of migrating fluids in sedimentary basins, and as such, play an important role in the trapping and sealing of hydrocarbons in the subsurface. However, the capacity for mudstones to retain large hydrocarbon column heights is highly variable and closely linked to grain-size distribution. For petroleum explorers, bedding-scale variations in grain-size distributions diminish the potential capacity of seal units and may be instrumental in compromising otherwise effective sealing sequences. Studies presented here utilise exceptional outcrop exposures and Ocean Drilling Program (ODP) sediment cores to investigate the sedimentology and heterogeneity of mudstone seal analogues. Quantitative data regarding the thickness, interconnectivity and lateral continuity of coarser beds have been collected and evaluated in the context of specific basin settings. Based on these findings, a range of depositional and post-depositional processes which are thought to govern the distribution of coarser material in deep-water environments are discussed. Utilising ODP data archives, the impact of lithological and textural heterogeneities on diagenesis and mechanical compaction of mudstone sequences are also addressed and evaluated from the perspective of seal integrity. Outcrop studies carried out in the UK and Taranaki, New Zealand, have identified heterogeneous slope mudstone facies containing upto 40% coarse thin-beds. These discrete layers of silt and sand are often laterally continuous, ranging in thickness from millimetre- scale laminations to beds commonly 1-4 cm thick, and are shown to be below the resolution of outcrop gamma-ray spectrometry. The examination of over 7000 m of hemipelagic slope sediment from the ODP core archives indicates that discrete coarse thin-beds comprise upto 6% of the 24 sequences examined, and that background sediments vary, reflecting differing proportions of clay, silt, sand, and biogenic material. Data presented from 255 ODP and Integrated Ocean Drilling Program (IODP) wells suggests that lithology - primarily grain-size, plays a significant role in determining the depositional porosities of mudstone sediments and the rates at which they undergo compaction during shallow (<500 m) burial. It is suggested that the use of 'default' initial porosity values for hemipelagic sediments in basin models and decompaction analysis ignores a large range of natural variability, and could lead to significant analytical errors. Porosity profiles of buried slope mudstones also indicate that in shallow sequences, hemipelagic sediments do not consistently conform to trends of exponentially decreasing porosity values with depth. Deviations from well constrained trends often appear to correlate with shifts in bulk lithology, suggesting that the inherent physical properties of particular intervals are responsible for varying porosity values with depth. The diagenetic processes associated with the conversion of volcanic ash during burial are also investigated and proposed as a post-depositional mechanism capable of altering the physical properties of mudstone units. Studies presented here highlight several depositional and post-depositional processes which may be detrimental to mudstone seal quality. This work represents a series of 'pilot studies' which aim to show that a seal evaluation and risking approach which considers these factors, will better predict the character of subsurface mudstone units.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:584684 |
| Date | January 2009 |
| Creators | Praeger, Thomas |
| Publisher | Cardiff University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://orca.cf.ac.uk/55182/ |
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