Depositional environment and facies as first-order controls on unconventional reservoir properties: examples from siliciclastic, carbonate, and mixed mudstone systems

Unconventional reservoir quality is often attributed to burial history, hydrocarbon (HC) generation, compaction, and related geomechanical–geochemical evolution. This view underemphasizes the first-order role of depositional environment and facies in establishing original texture and organic matter (OM) type/distribution that ultimately controls hydrocarbon deliverability. We present examples from siliciclastic, carbonate, and mixed mudstone systems to show that depositional setting governs initial grain assemblage, detrital clay type and abundance, and the sorting and grain-size distribution (GSD) of silt- and sand-sized fractions. Where depositional fabrics are preserved and detrital clays are limited, better-sorted, narrower GSD facies retain larger, better-connected pore-throat networks and higher effective permeability. In contrast, poorly sorted, broader GSD facies promote early pore-filling, tighter fabrics, and limited connectivity that are difficult to overcome through maturation-related processes. Where diagenesis substantially overprints or destroys depositional textures, reservoir quality is instead controlled by the extent, timing, and mineralogy of early diagenetic cements.

OM type commonly exerts a stronger control than total organic carbon (TOC) alone on reservoir quality. The marine–terrestrial distinction remains fundamental and is commonly coupled with extrabasinal versus intrabasinal mineral grain assemblages. Quantifying bitumen occlusion is critical, as pore clogging can strongly reduce effective porosity and permeability regardless of TOC. Kerogen type adds further controls: Type I-like algal OM tends to generate waxier oils, whereas Type IIS kerogen can generate oil earlier, enabling hydrocarbons to interact with and potentially modify early diagenetic pathways. Climate-state influences also matter: mudstones deposited under greenhouse versus icehouse periods can exhibit markedly different facies stacking patterns and 3D heterogeneity.

Finally, silica diagenesis driven by dissolved biogenic silica, commonly co-developing with marine algal productivity, yields the most favorable coupling of source and reservoir attributes (higher TOC, greater conversion efficiency, improved oil quality, and increased brittleness). Detrital-dominated facies commonly degrade reservoir potential by diluting and oxidizing OM and shifting kerogen toward terrestrial dominance. Together, these relationships provide a predictive framework for sweet-spot identification and stratigraphic targeting across unconventional plays.