The Upper Cretaceous to Lower Paleocene Chalk Group in the North Sea comprises important reservoir targets for hydrocarbon and energy storage. The quantitative impacts of mineralogy, chalk diagenesis and oil saturation on petrophysics are often based on general trends that cannot be directly applied to reservoir engineering. The present study aims at identifying quantitatively the individual effects of mineralogy, compaction and cementation, and oil saturation on porosity and permeability by means of detailed samplings and mineralogical and petrophysical analyses from base to top of the Kraka reservoir. Special attention is brought to the visual inspection of the rock fabric and its variations across the reservoir in order to better constrain interpretations and establish relationships within an otherwise scattered dataset. Porosity and permeability variations are categorised into two trends, a mineralogical and diagenetic trend, which can both form tight rock. For chalk containing >7% quartz and > 4% clay, porosity decreases linearly from 35% to 21% and permeability from 0.5 mD to 0.07 mD with increasing quartz content. The amount of quartz rather than clay appears as an effective tool to estimate the petrophysical properties of impure chalk. Below 7% quartz and 4% clay, compaction and cementation decrease porosity and permeability in two steps along transects from oil- to water-bearing intervals. A first step of predominantly mechanical compaction reducing porosity and permeability from >41% to 37% and from 3 to 3.5 mD to 1.5–2.5 mD, respectively, is followed by a phase of chemical compaction and cementation that further deteriorates petrophysical properties. In Ekofisk Fm., stylolitised chalk and deposits located adjacent to chert horizons display singular petrophysical properties that do not fit the observed trends. The outliers likely result from phases of local chemical compaction and early cementation associated with chert formation. Porosity-permeability functions and uncertainty analyses are also proposed for the different controlling factors and at different scales of observation. The proposed petrophysical relationships are instrumental in building more realistic reservoir models and can assist geoscientists in interpreting and upscaling the large amount of geological data collected in chalk fields.

北海的上白垩统至下古新世垩系群是油气和能源存储的重要储层目标。矿物学，白垩岩成岩作用和含油饱和度对岩石物理学的定量影响通常基于无法直接应用于油藏工程的总体趋势。本研究旨在通过详细采样以及从克拉卡储层底部到顶部的矿物学和岩石物理分析，定量确定矿物学，压实和胶结作用以及油饱和度对孔隙度和渗透率的个体影响。为了更好地限制解释并在原本分散的数据集中建立关系，应特别注意岩层及其在储层中的变化的目视检查。孔隙度和渗透率变化可分为两个趋势，即矿物学和成岩学趋势，两者均可形成致密岩石。对于含> 7％石英和> 4％粘土的白垩，随着石英含量的增加，孔隙率从35％线性降低至21％，渗透率从0.5 mD线性降低至0.07 mD。石英而不是粘土的量似乎是估计不纯白垩的岩石物理性质的有效工具。低于7％的石英和4％的粘土，压实和胶结作用会沿着从含油到含水间隔的断面分两步降低孔隙率和渗透率。第一步，主要是机械压实，将孔隙率和渗透率分别从> 41％降低到37％，从3到3.5 mD降低到1.5-2.5 mD，随后是化学压实和胶结作用的阶段，这进一步恶化了岩石物理特性。在Ekofisk Fm。，针状白垩和与石层位相邻的沉积物显示出不符合观测趋势的奇异岩石物性。异常值可能是由于局部化学压实和与石形成有关的早期胶结作用的结果。还针对不同的控制因素和不同的观测规模，提出了孔隙度-渗透率函数和不确定性分析。拟议的岩石物理关系有助于建立更现实的储层模型，并可以帮助地球科学家解释和扩大在白垩田中收集的大量地质数据。针状白垩岩和石层位附近的沉积物显示出奇异的岩石物理特性，不符合观察到的趋势。异常值可能是由于局部化学压实和与石形成有关的早期胶结作用的结果。还针对不同的控制因素和不同的观测规模，提出了孔隙度-渗透率函数和不确定性分析。拟议的岩石物理关系有助于建立更现实的储层模型，并可以帮助地球科学家解释和扩大在白垩田中收集的大量地质数据。针状白垩岩和石层位附近的沉积物显示出奇异的岩石物理特性，不符合观察到的趋势。异常值可能是由于局部化学压实和与石形成有关的早期胶结作用的结果。还针对不同的控制因素和不同的观测规模，提出了孔隙度-渗透率函数和不确定性分析。拟议的岩石物理关系有助于建立更现实的储层模型，并可以帮助地球科学家解释和扩大在白垩田中收集的大量地质数据。还针对不同的控制因素和不同的观测规模，提出了孔隙度-渗透率函数和不确定性分析。拟议的岩石物理关系有助于建立更现实的储层模型，并可以帮助地球科学家解释和扩大在白垩田中收集的大量地质数据。还针对不同的控制因素和不同的观测规模，提出了孔隙度-渗透率函数和不确定性分析。拟议的岩石物理关系有助于建立更现实的储层模型，并可以帮助地球科学家解释和扩大在白垩田中收集的大量地质数据。