Abstract:The porosity and permeability of sandstone and carbonate reservoirs (known as reservoir quality) are essential inputs for successful oil and gas resource exploration and exploitation. This chapter introduces basic concepts, analytical and modelling techniques and some of the key controversies to be discussed in 20 research papers that were initially presented at a Geological Society conference in 2014 titled 'Reservoir Quality of Clastic and Carbonate Rocks: Analysis, Modelling and Prediction'. Reservoir quality in both sandstones and carbonates is studied using a wide range of techniques: log analysis and petrophysical core analysis, core description, routine petrographic tools and, ideally, less routine techniques such as stable isotope analysis, fluid inclusion analysis and other geochemical approaches. Sandstone and carbonate reservoirs both benefit from the study of modern analogues to constrain the primary character of sediment before they become a hydrocarbon reservoir. Prediction of sandstone and carbonate reservoir properties also benefits from running constrained experiments to simulate diagenetic processes during burial, compaction and heating. There are many common controls on sandstone and carbonate reservoir quality, including environment of deposition, rate of deposition and rate and magnitude of sea-level change, and many eogenetic processes. Compactional and mesogenetic processes tend to affect sandstone and carbonate somewhat differently but are both influenced by rate of burial, and the thermal and pressure history of a basin. Key differences in sandstone and carbonate reservoir quality include the specific influence of stratigraphic age on seawater composition (calcite v. aragonite oceans), the greater role of compaction in sandstones and the greater reactivity and geochemical openness of carbonate systems. Some of the key controversies in sandstone and carbonate reservoir quality focus on the role of petroleum emplacement on diagenesis and porosity loss, the role of effective stress in chemical compaction (pressure solution) and the degree of geochemical openness of reservoirs during diagenesis and cementation. This collection of papers contains case study-based examples of sandstone and carbonate reservoir quality prediction as well as modern analogue, outcrop analogue, modelling and advanced analytical approaches.Gold Open Access: This article is published under the terms of the CC-BY 3.0 license.Porosity and permeability (reservoir quality) exert fundamental controls on the economic viability of a petroleum accumulation (Blackbourn 2012).They need to be quantified from basin access and exploration, via appraisal and field development through secondary and tertiary recovery in order to

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Discontinuity surfaces in shallow‐marine carbonate successions may represent significant time gaps in the geological record of ancient epeiric‐neritic seas. Understanding the hidden geological information contained in major discontinuities is thus of key significance in palaeo‐environmental analysis, sequence stratigraphy, reconstructions of sea‐level change and basin evolution. In the present paper, the Aptian top Lower Shu’aiba Formation discontinuity in the Sultanate of Oman is taken as a prominent example of a regionally extensive (>100 000 km2) surface with a long (up to 10 Myr) and complex geological history. The top Shu’aiba discontinuity formed on the topographically elevated domain of the Oman platform and represents in essence the Late Aptian time interval. Coeval carbonates in the intrashelf Bab Basin and oceanic rim indicate forced regression and sequence‐wise, gradual down‐stepping. Available regional, sedimentological, sequence‐stratigraphic, petrographic, palaeontological and geochemical evidence from outcrops and cored wells in Oman is summarized, in part complemented by new data, and reviewed in a process‐oriented context. In the field, the discontinuity is expressed as a low relief, stained surface with evidence for a marine hardground stage being dominant. Indistinct features that indicate a transient meteoric precursor stage (isotope shifts, meteoric cements, circumgranular cracks, etc.) are present but their interpretation requires careful and detailed work. This feature is remarkable, as a series of relative sea‐level falls with amplitudes of up to several tens of metres from the Early to Late Aptian boundary to the end of the Aptian are reported from the Middle East and elsewhere. Despite the palaeogeographic position of the study area in the tropical climate zone, evidence of deep‐cutting karst features, characteristic for many long‐term exposure surfaces worldwide is scarce. Acknowledging the fact that the modern world offers no genuine analogues for the Lower Aptian carbonate system in Oman, morphological similarities between actualistic, wave‐eroded coastal terraces and the top Shu’aiba discontinuity are discussed critically. This discussion may imply that, during an exposure time of several million years, the top Shu’aiba discontinuity experienced repeated stages of shallow flooding and emergence, with each transgression removing portions of the underlying rock record. The data shown here exemplify the complexity of hiatal surfaces in epeiric‐neritic carbonates and may serve as a case example for other major discontinuities.

The Pleistocene speleothems of Sa Bassa Blanca cave, Mallorca, are excellent indicators of palaeoclimate variations, and are samples that allow evaluation of the products and processes of mixing-zone diagenesis in an open-water cave system. Integrated stratigraphic, petrographic and geochemical data from a horizontal core of speleothem identified two main origins for speleothem precipitates: meteoric-marine mixing zone and meteoric-vadose zone. Mixing-zone precipitates formed at and just below the water-air interface of cave pools during interglacial times, when the cave was flooded as a result of highstand sea-level. Mixing-zone precipitates include bladed and dendritic high-Mg calcite, microporous-bladed calcite with variable Mg content, and acicular aragonite; their presence suggests that calcium-carbonate cementation is significant in the studied mixing-zone system. Fluid inclusion salinities, d 13 C and d 18 O compositions of the mixing-zone precipitates suggest that mixing ratio was not the primary control on whether precipitation or dissolution occurred, rather, the proximity to the water table and degassing of CO 2 at the interface, were the major controls on precipitation. Thus, simple two-end-member mixing models may apply only in mixing zones well below the water table. Meteoric-vadose speleothems include calcite and high-Mg calcite with columnar and bladed morphologies. Vadose speleothems precipitated during glacial stages when sea level was lower than present. Progressive increase in d 13 C and d 18 O of the vadose speleothems resulted from cooling temperatures and more positive seawater d 18 O associated with glacial buildup. Such covariation could be considered as a valid alternative to models predicting invariant d 18 O and highly variable d 13 C in meteoric calcite. Glacio-eustatic oscillations of sea-level are recorded as alternating vadose and mixing-zone speleothems. Short-term climatic variations are recorded as alternating aragonite and calcite speleothems precipitated in the mixing zone. Fluidinclusion and stable-isotope data suggest that aragonite, as opposed to calcite, precipitated during times of reduced meteoric recharge.

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