Integration of the sedimentological and petrophysical properties of mudstone samples

Abstract

Mudstones are of considerable scientific and economic importance as they are the dominant sedimentary rock type, forming the main repository of Earth history and having significance to numerous aspects of petroleum exploration and production, and many other industries. This study investigated the sedimentological characteristics of 150 diverse mudstone samples. The novel integration of grain size analysis combined with petrographic observations lead to a framework in which six mudstone grain size distribution (GSD) types are defined. The grain size types proposed are remarkably consistent in their form and characteristics and can be understood in terms of well constrained physical processes of deposition. The basis for this definition reflects largely the relative contributions of a flocculated, clay-rich component and an unflocculated silt/sand-rich grain size component. Integration of grain size data, pore size data and petrographic observations suggests a critical division between: (a) flocdominated mudrocks whose structure is supported by the clay matrix; and (b) silt-rich mudrocks whose structure is supported by a silt/fine sand framework. Floc-dominated mudrocks with clay matrix support develop low permeabilities and become very good capillary seals at relatively shallow depths. In contrast, silt-rich mudrocks with framework support only become low permeability units and very good capillary seals at much greater levels of compaction. The framework proposed here can form the basis of predictive flow and seal capacity models for mudrocks. A combined PCAcluster analysis approach to the grain size based classification of mudstones showed that of the six types defined in Chapter 2, types 1 — 4 (floc — silt mixtures) were consistently partitioned from types 5 — 6 (silt or sand rich mixtures). An attempt was made to quantify the distribution of key pore parameters, such as mean pore size, by grouping the data to reflect the matrix (grain size types 1 — 4) and framework (grain size types 5 — 6) support regimes and dividing into 5% porosity bins. The statistical distribution of pore network properties could not be verified, principally due to a combination of sparse sample numbers and highly variable nature of this data. This work illustrates that variability in mudstone pore size distributions is not constrained solely by lithology (support regime) and porosity, and thus that other factors must be taken into account if their evolution during compaction is to be understood.