Abstract This chapter includes 11 cross-sections and one well log profile to show the depositional geometry and setting in specific areas around the basin: the Saudi Arabia outcrop belt; the Rimthan Arch; and the eastern and central areas of the intrashelf basin in Saudi Arabia, Bahrain, Qatar, Abu Dhabi and Oman. These cross-sections are used to demonstrate the similarity and degree of continuity of the upper Dhruma Formation, the Tuwaiq Mountain Formation, the source rock, the Hanifa, Jubaila–Arab and Arab–Hith formations and depositional sequences in these different locations in the basin. They show the manner in which the underlying platform formed, the rim developed, the source rock was deposited and the basin progressively filled. The blanket deposition of the Arab-D anhydrite was followed by the Arab-C to Arab-A and Hith carbonate and evaporite sequences. The cross-sections provide the framework used in subsequent chapters to make a series of facies maps and other interpretative diagrams and cross-sections that summarize and, for some intervals, revise the interpretation of the settings and geological events that formed the Arabian Intrashelf Basin.

Abstract: Microporosity in carbonate reservoirs is generated by the complex interplay between depositional and diagenetic processes. This petrographical, SEM, fluid-inclusion and isotopic study of a Lower Cretaceous carbonate reservoir, Abu Dhabi, UAE, revealed that: (1) micritization of ooids and skeletal fragments, which resulted in spheroidal (rounded) micrite, accounts for most microporosity in peloidal packstones and grainstones; and (2) transformation of spheroidal micrite into subhedral/euhedral micrite and microspar, known as aggrading neomorphism, could happen via precipitation of syntaxial calcite overgrowths around micrite (micro-overgrowths) and not only, as suggested previously in the literature, by recrystallization involving the dissolution (of micrite) and reprecipitation (of microspar). Precipitation of calcite cement around micrite (i.e. destruction of microporosity) is more extensive in the water zone than in the oil zone, which is possibly contributing to the lower porosity and permeability of the carbonate reservoir in the water zone. Similarity in bulk oxygen isotopic values of micritized packstones and grainstones in the water and oil zones (average δ 18 O V-PDB = −7.2‰ and −7.8‰, respectively) is attributed to: (1) a small difference in temperatures between the crest (oil zone) and the flanks (water zone); and (2) calcite precipitation around micrite occurred prior and subsequent to oil emplacement. Bulk carbon and strontium isotopic compositions of micritized packstones and grainstones in the water and oil zones (average δ 13 C V-PDB = +3.7‰ and average 87 Sr/ 86 Sr ratios = 0.707469) indicate that calcite cement was derived from marine porewaters and/or dissolution of the host limestones. The minimum formation temperatures of bulk micrite/microspar, which are inferred based on paragenetic relationships, fluid-inclusion microthermometry and oxygen isotope data, are around 58–78°C.

Abstract A sedimentary bed is classically defined as a distinct layer of sedimentary rock that has a relatively uniform composition. Several outcrop-based studies have shown that facies within individual beds can vary laterally on a scale of around 100 m. As facies transitions are important criterions used as a proxy for the depositional environment and often to infer sequence stratigraphic trends in subsurface studies where data is limited to one-dimensional wells, this observed facies heterogeneity has implications for both paleoenvironmental studies and sequence stratigraphy. In this study, we investigate whether sedimentary and facies heterogeneities known to occur at the hundred meters scale are also present at the meter to tens of meters scale in a well-preserved facies mosaic deposited on a carbonate ramp (Wadi Naqab, Izhara Formation, Lower Bajocian, northern UAE), where a robust, outcrop-based sequence stratigraphic framework exists. A bed set was logged and mapped across a 120 m long curving cliff face; combined with thin section analysis, the data allowed the reconstruction and quantification of facies heterogeneity at this location. Results reveal a large amount of lateral facies transitions at the meter scale. Lithofacies types have a probability of less than 70% of being laterally continuous over 12 to 18 meters, representing the highest amount of lateral facies heterogeneity so far reported in an ancient example. The case study reveals intra-bed facies transitioning attributed to spatially heterogeneous biogenic carbonate production as well as to syndepositional homogenizing and sieving processes occurring within shallow-marine depositional environments in ancient as well as in modern analogous systems. A series of continuous hardgrounds, previously interpreted as flooding and exposure surfaces, provide an independent sequence stratigraphic framework that demonstrate that the existence of small-scale lateral facies heterogeneities complicates interpretation of the vertical stacking pattern of facies. This confirms that the best practice is to limit sequence stratigraphic interpretations based on facies trends to larger stacking patterns (>10 meters). Meter-scale vertical patterns in carbonates often do not represent a proxy for base level changes, as illustrated in our study; therefore, fine-scaled, high-resolution sequence stratigraphy or lateral correlations are not attainable in carbonate sequences based on lithostratigraphy.