Reservoir quality of sandstones is mainly derived from their permeability and porosity. As a result, porosityreducing processes need to be understood in order to evaluate and model reservoir quality in sandstones. This case study from a Rotliegend gas reservoir in the Northern German Basin utilizes petrophysical measurements in conjunction with petrography in order to assess reservoir qualities and define rock types. The most significant diagenetic factors influencing the development of the IGV (intergranular volume) are quartz cementation due to low illite grain coating coverages on grain to IGV interfaces and chemical compaction due to pronounced illite grain coating coverages on grain to grain interfaces. Where large proportions of the interface between adjacent grains are coated by illite, stronger chemical compaction (pressure dissolution) was observed to occur. This chemical compaction reduces the IGV, and thus open pore space.Permeabilities measured under decreasing confining pressures from 50 to 2 MPa were used to determine the pressure sensitivities of permeability (David et al., 1994), which ranged from 0.005 to 0.22 MPa −1 . The pressure sensitivity of permeability, porosity and permeability were linked to the petrographic texture, implying three different major rock types: Type A is characterized by an uncemented petrographic texture with high porosities (avg.: 9.8%), high permeabilities (avg.: 126 mD), and low pressure sensitivities of permeability (avg.: 0.019 MPa −1 ). Type B is intensely cemented with reduced porosities (avg.: 4.0%), reduced permeabilities (avg.: 0.59 mD), and increased pressure sensitivities of permeability (avg.: 0.073 MPa −1 ). Type C is characterized by intense chemical compaction leading to the lowest porosities (avg.: 1.8%) and permeabilities (avg.: 0.037 mD) in concert with the highest pressure sensitivity of permeability (avg.: 0.12 MPa −1 ). The heterogeneity induced by diagenesis will have an impact on recoverable resources and flow rates in both hydrocarbon and geothermal projects in similar siliciclastic reservoirs.

Authigenic minerals formed during diagenesis in conjunction with compaction by burial have long been known to lead to porosity-loss of sandstones, and a subsequent deterioration in reservoir quality. The diagenetic impact on reservoir quality and permeability heterogeneity measured horizontal and vertical to bedding was characterized in three fluvio-eolian Lower Permian Rotliegend outcrops from the Flechtingen High, the northern Hesse Basin (both Germany) and the Vale of Eden (UK) using point-counting, polarized light-microscopy, helium pycnometry and permeability measurements. Results show significant porosity (10 to 35%) and permeability (0.01 to 10,000 mD) ranges largely independent of depositional environment. The major control on reservoir quality in Cornberg Sandstones are dolomite and siderite cementation in conjunction with illitization and illite and kaolinite cementation, leading together with quartz cementation to a mostly cemented IGV and poorest reservoir quality (avg. horizontal permeability: 0.96 mD). Flechtingen Sandstones are most intensely compacted due to the lack of significant early diagenetic cement phases and continuous illitic grain-to-grain coatings, which inhibited intense quartz cementation but enhanced chemical compaction at quartz grain contacts, resulting in intermediate reservoir quality (avg. horizontal permeability: 34.9 mD). Penrith Sandstones lack significant authigenic phases besides quartz due to carbonate dissolution during uplift. They show the least amount of detrital feldspars and clay minerals, leading to no major reservoir quality reduction by burial diagenetic clay mineral alterations, resulting in the highest reservoir quality (avg. horizontal permeability: 5900 mD). Additional results highlight higher horizontal to vertical permeability ratios kh/kv in less homogeneous sandstones of < 10 mD of 10, and in more homogenous, higher permeable sandstones > 1000 mD of 1. Although detrital and authigenic sample compositions vary throughout the studied areas, the general effect of grain coatings coverages on syntaxial cement inhibition and chemical compaction can be delineated. This study increases the understanding of porosity reduction in sandstones, as it confirms the necessity to differentiate between the illitic grain-to-grain coatings and illitic grain-to-IGV coatings. As a result, the enhancing effect of illite on chemical compaction on quartz grain-grain boundaries can be better constrained, as well as the effect of grain coatings on quartz cementation. This is relevant for reservoir quality and risk assessment in hydrocarbon and geothermal plays as well as in storage.

Initial detrital composition and authigenic alterations during diagenesis of three sandstone types are related to their mechanical properties. Sandstones were prepared for geotechnical standard tests [density, uniaxial compressive strength (UCS), Young’s modulus (E), strain at failure (ε)] and thin sections for petrographic analyses (point counting). UCS ranges from 3 to 62 MPa and positively correlates with density (1.75–2.35 g/cm3) and E (0.3–12.7 GPa). Optical porosity is controlling these mechanical parameters and was linked to diagenetic alterations. Diagenetic alterations affecting porosity reduction are the abundance of clay minerals, and the intensity of mechanical and chemical compaction. The latter is controlled by clay mineral coatings on contacts between detrital grains, and the occurrence of authigenic quartz and dolomite. Horizontal contact lengths of grains normalized to their respective particle diameter (effective contact ratio, ECR) and porosity are identified as a control on the mechanical properties UCS and E, reflected by the rock strength index SR. The results of this pilot study suggest that SR is able to predict UCS and E based on petrographic information obtained from the studied samples. These results enhance the understanding of the coupling between mineralogy and geomechanics and highlight the impact of diagenesis on geomechanical behavior.

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