ABSTRACT An evaluation of an integrated data set collected over the past 12 years designed to identify the parameters controlling reservoir quality and production properties in organic, siliceous mudrocks reveals the key diagenetic processes affecting the development of brittleness in siliceous mudrocks. This work was motivated by the failure of early efforts to correlate brittleness to x-ray diffraction (XRD) mineralogy. The outcome of this analysis has been the recognition of two, often overlapping, pathways to brittleness that are determined at the time of deposition by the relative proportions of clay, detrital quartz, and biogenic silica present in the original sediment and are later affected by burial history. One pathway begins with a phyllosilicate–mud-dominated sediment, and the other begins with a sediment containing common or abundant biogenic silica (opal-A). Both pathways are characterized by compactional porosity loss and both eventually include the generation of authigenic quartz cement; however, the source of that authigenic quartz is different between the two pathways. The authigenic quartz that characterizes the first pathway is developed from the illitization of smectite and is precipitated as a cement within the argillaceous matrix. This authigenic quartz is detectable along with the detrital quartz by XRD analysis. All other factors being equal, the volume of brittle, authigenic quartz cement derived from the alteration of smectite is proportional to the volume of original clay. As a result, the effectiveness of this cement to increase the brittleness of the rock may be impacted by the presence of the ductile clays. In the alternate pathway, authigenic quartz is derived from the transformation of biogenic opal-A and is independent of the amount of clay. Much of the XRD quartz volume in rocks derived from biogenic–silica-rich sediment that contained little or no detrital quartz will comprise a brittle, authigenic cement.

ABSTRACT Kinetic barriers inhibit quartz nucleation and growth at lower temperatures (<50°C [<122°F]). Thus, under ordinary geothermal gradients, the formation of authigenic quartz in fine-grained systems is preceded inevitably by the early stages of compaction. Nucleation sites for quartz precipitation and the abundance and sizes of pores into which quartz cement can be emplaced are limited by the compactional state at the time of precipitation. The two main types of grain alteration that are proposed to yield authigenic quartz, dissolution of biogenic opal and illitization of smectite, occur in different temperature ranges and contrasting compactional regimes. This chapter summarizes petrographic observations on quartz components (grains and cement) by high-resolution cathodoluminescence (CL) and X-ray elemental mapping in 11 mudrock units ranging in age from Ordovician to Oligocene. The amounts of quartz cement observed provide constraints on the sources of silica for the formation of authigenic quartz and mass and volume balances of silica generation and precipitation in mudrock diagenesis. The size (1–3 μm), spatial distribution, and abundance (typically 30–40% of rock volume) of authigenic microquartz that arise from the biogenic opal pathway are consistent with the compactional state of mud in the temperature range of the opal-A to opal-CT transition. Mudrocks that are clearly cemented by authigenic microquartz contain a volume of quartz in excess of amounts potentially generated by illitization (up to about 13% of rock volume). In the absence of abundant biogenic silica and consequent early cementation that inhibits compaction, the most common mudrock in the temperature range of illitization (>~80°C [>~176°F]) have few available nucleation surfaces for quartz precipitation and little available pore space (mostly nanometer scale) to accommodate pore-filling crystal growth. Sites for higher temperature quartz precipitation, synchronous with illitization, are mostly restricted to localized packing flaws at contacts between silt-size particles and constitute a trivial volume of the rock. Thus, tarls tend to feature diagenesis dominated by compaction that dramatically reduces pore space prior to the onset of significant reactions of the grain component such as albitization and illitization. The absence of discernible cementation in most deep basinal mudrocks raises the possibility that mechanical compaction persists as a mechanism of porosity decline to greater depths in mud than in sand.

Abstract: There is no consensus about the rate and style of clay mineral diagenesis in progressively buried sandstones v. interbedded mudstones. The diagenetic evolution of interbedded Miocene sandstones and mudstones from the Vienna Basin (Austria) has therefore been compared using core-based studies, petrography, X-ray diffraction and X-ray fluorescence. There was a common provenance for the coarse- and fine-grained sediments, and the primary depositional environment of the host sediment had no direct effect on illitization. The sandstones are mostly lithic arkoses dominated by framework grains of quartz, altered feldspars and carbonate rock fragments. Sandstone porosity has been reduced by quartz overgrowths and calcite cement; their pore-filling authigenic clay minerals consist of mixed-layer illite–smectite, illite, kaolinite and chlorite. In sandstones, smectite illitization progresses with depth; at 2150 m there is a transition from randomly interstratified to regular interstratified illite–smectite. The overall mineralogy of mudstones is surprisingly similar to the sandstones. However, for a given depth, feldspars are more altered to kaolinite, and smectite illitization is more advanced in sandstones than in mudstones. The higher permeability of sandstones allowed faster movement of material and pore fluid necessary for illitization and feldspar alteration than in mudstones. The significance of this work is that it has shown that open-system diagenesis is important for some clay mineral diagenetic reactions in sandstones, while closed-system diagenesis seems to operate for clay mineral diagenesis in mudstones.

Abstract: A reactive transport model (RTM) is used to simulate the diagenetic evolution of a siliciclastic reservoir with a known burial and thermal history. The diagenetic phenomena occurring in two temperature regimes are simulated: kaolinite/chlorite formation at low/medium temperatures by means of isothermal zero-dimensional (0D) flush models, and smectite illitization and quartz precipitation at high temperatures by means of 0D and 1D non-isothermal models. Zero-dimensional models show that at 30°C kaolinite forms only in freshwater from K-feldspar and quartz. In river or seawater, muscovite is stable instead of kaolinite. Calcite formation depends on pH and total inorganic carbon. At 50°C, seawater promotes Mg-chlorite formation from mica alterations. At 70–110°C, evaporated seawater favours smectite–illite transformation and quartz precipitation. The non-isothermal 1D models are used to simulate the diagenesis of a compacting clay expelling fluid into an underlying sandstone. An sensitivity analysis of the clays’ thermodynamic and kinetic parameters is carried out to assess the possibility of modelling the transformation of smectite into illite in the clay, with the concomitant formation of a quartz cement in the adjacent sandstone. The results of the numerical simulations point out that the extent of the smectite–illite conversion and quartz precipitation is dependent primarily on the availability of potassium: temperature, however, does not seem to play a major role.