Compaction and microfabric rearrangement of fine-grained siliciclastic sediments

Abstract

The main topic of this thesis is the mechanical and non-mechanical compaction and fabric rearrangement of fine-grained siliciclastic sediments. Part A concentrates on the mechanical compaction of shallow «1000mbsf), fine-grained marine sediments, a majority of which was provided by DSDP/ODP. The used porosities and stresses were based on measurements by DSDP/ODP on fresh sample material. Results reveal, that the reduction of porosity with increasing effective stress is mainly controlled by lithology. Clay fraction (% material <2um) was used as parameter for the grain size of the sample material and found the single most important control on the relationship between porosity and effective stress. However, for a given clay fraction the porosities of samples containing significant amounts of microfossils exceeded those of the fossil-poor sediments. Larger amounts of grains between 2J.1m and 8J.1m in the fossil-rich material presumably led to these differences in pore space. Deviations between the porosities measured on fresh (wet) sample material by DSDP/ODP with those determined by mercury intrusion porosimetry on the partly dried out samples used in study highlighted the sensitivity of soft, clay-rich sediments to air drying. Experimental drying of two clay samples confirmed these observations and revealed the changes in total porosity and pore size distribution during air and oven drying. Furthermore, two case studies, one located in the Mid-Norway area and one offshore New Jersey, underlined the importance of a thorough assessment of lithology and confirmed the influence of microfossils on shallow mudstone compaction. In Part B of this thesis, a novel approach was applied to correlate the petrophysical changes of fine-grained sediments during compaction and early diagenesis to alterations of the sediment microfabric. High Resolution X-ray Texture Goniometry (HRXTG) was used to quantify the alignment of phyllosilicates and results reveal, that mechanical compaction does not necessarily lead to a Significant alignment of platy minerals. The analysis of the present mineralogies and grain size/shape distributions led to the conclusion, that only if both the clay and silt-fractions are dominated by platy minerals, mechanical compaction results in higher degrees of phyllosilicate alignment. On the other hand, if the silt-fraction is dominated by spherical grains (e.g. quartz, feldspar), increasing effective stress simply results in tight, poorly-aligned grain packing. Advanced stages of clay mineral diagenesis were only observed in two Gulf of Mexico wells. Here, the onset of the smectite-to-illite conversion was delayed until temperatures of at least 115°C, presumably due to the high sedimentation rates (>1000m/Ma). Although observed in other studies, an obvious influence of diagenesis on the clay mineral fabric of this sample set was not evident. It can therefore be assumed, that the mud- and siltstones from the Gulf of Mexico are still too immature to reflect diagenetic reactions in their phyllosilicate fabric.