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The Upper Dalan (Khuff-equivalent) Formation constitutes the principal reservoir of the giant gas fields in the Persian Gulf Superbasin. A sedimentological and sequence stratigraphic analysis was conducted on selected cores from the South Pars, Kish, and Lavan gas fields, offshore Iran, to evaluate a recently proposed method for automatic reservoir zonation, and discuss the predictability of such defined zones. The succession consists of nine evaporite-carbonate lithofacies grouped into three shallow-marine facies associations (shoal, lagoon, and tidal flat) that were deposited on a low-gradient homoclinal ramp. Lithofacies are stacked into two complete long-term (3rd-order?) transgressive-regressive depositional sequences. Sequence boundaries were defined by facies stacking patterns and presence of evaporites and meteoric diagenetic features. The reservoir quality was improved by both early-stage dolomitization and dissolution, whereas pervasive pore-filling anhydrite cementation, compaction, and late-stage over-dolomitization reduced the reservoir quality. Whereas dolomitization overall slightly affected porosity, it significantly increased the permeability in mud-dominated lithofacies. Fibrous and bladed calcite rim cements, as well as micritization of the grain-dominated lagoon and shoal lithofacies, prevented porosity reduction during the early- and late-stage burial by building a stronger framework. Core-plug porosity and permeability measurements were used to calculate the Winland R35, Reservoir Quality Index (RQI), and Flow-Zone Indicator (FZI) values. A novel, fully automated approach, was used to effectively identify the hydraulic flow units (HFUs). The HFUs are sedimentologically distinct units with characteristic combinations of the original rock texture and the subsequent diagenetic overprint, and their subsurface position within the sequence stratigraphic framework may be predicted.

Lower to Middle Eocene organic-rich deep-water limestones of the ∼335-m-thick (1100 ft) Radwany (Thebes-equivalent) Formation represent a source rock and a potential reservoir unit at the October Oil Field in the Gulf of Suez. However, in spite of recent exploration advances, the pore system and diagenetic history of the formation are still poorly understood. This study aims to discriminate porosity types and their vertical distribution, assess the diagenetic processes controlling porosity evolution, and evaluate the unit's overall reservoir potential. To achieve these goals, we utilized an integrated petrographic, mineralogical, geochemical and petrophysical dataset from four offshore wells. At October Oil Field, the Radwany Formation is informally divided into three intervals (from the base up: C, B and A) which are composed of planktonic foraminiferal wackestone, wackestone-packstone and packstone microfacies. Pores include both fabric selective (interparticle, intraparticle and moldic) and non-fabric selective (fracture) types. Visible porosity reaches up to 11.6%, and the highest porosity values are associated with interparticle pores in foraminiferal packstone in interval C. The two overlying intervals have lower porosities but have TOC values of up to 6 wt %. The Nuclear Magnetic Resonance (NMR) analysis yields low amplitudes of short T2 components (max. 52 ms), which suggests the presence of small pores (intraparticle, moldic, fracture, and fissure). Based on the electrical quality index (EQI) rock classification, nine groups were identified, suggesting a high heterogeneity of the formation. The highest EQI values positively correlate with high porosity (visible and estimated) in the lower part of Interval C which is dominated by interparticle/intraparticle porosity, thus reflecting the low tortuosity values of this microfacies. Dominantly low EQI values in Intervals A and B indicate low porosity efficiency and higher tortuosity. Carbonate sediments of the Radwany Formation are interpreted to have undergone a complex series of diagenetic processes. These processes modified the primary pore system either by enhancing the reservoir properties (dissolution and fracturing), or by reducing or destroying porosity (cementation, mechanical and chemical compaction, and minor pyritization, chertification, and asphaltene precipitation). The study highlights the unconventional reservoir potential of the middle and upper parts of the Radwany Formation (intervals B and A) at October Oil Field, and the conventional reservoir potential of interval C. It emphasizes the complexity of diagenetic controls on the porosity evolution in deep-water limestones of the Gulf of Suez, and may provide valuable insight into similar deep-water carbonate systems in rift basin reservoirs elsewhere.

The Adriatic Platform, Croatia is a large isolated carbonate platform that developed under a warm greenhouse climate interspersed with cooler episodes in western Tethys. However, the δ13C record and high-resolution sequence stratigraphy of the Middle and Upper Jurassic portion generally is poorly known. The ~1900 m thick (30 Myr duration) mainly Middle to Upper Jurassic succession of the platform was studied bed-by-bed to track the fluctuations of δ13C, facies stacking into bundles, superbundles, and sequences and potential sea-level changes within ongoing studies of evidence of astronomical forcing of the units. The study interval consists of cyclic subtidal intervals (upper Toarcian-lower Aalenian, lower Callovian, Oxfordian, lower Kimmeridgian, and lower Tithonian) interspersed with highly cyclic peritidal units (upper Bajocian-lower Bathonian, upper Kimmeridgian, and upper Tithonian-basal Berriasian). Ages were constrained by biostratigraphy and δ13C chemostratigraphy. The absolute δ13C values obtained from the shallow-marine bulk carbonate matrix are similar to those from coeval pelagic carbonates and exhibit similar trends, are characterized by overall little data spread (

Despite the vast research on various aspects of its stratigraphy, relatively little is known about the source-rock potential of the Upper Jurassic Adriatic platform, southern Croatia. Here a case palynological study has been provided to determine the kerogen type, quality, and thermal maturity, and assess its relative hydrocarbon generative potential. Palynofacies characteristics indicate a low-energy, shallow-marine oxygen-depleted setting. The deposited mud-supported carbonates contain organic matter that is almost entirely composed of a fluorescent amorphous organic matter of planktonic and bacterial origin. A high phycomata/dinocyst ratio suggests relative hydrographic stability in an environment favorable for motile-stage dinoflagellate community with reduced production of dinocysts. The lack of anaerobic degradation and very good preservation of lipoid palynomorphs indicates dysoxic conditions ; the latter also is suggested by a low proportion of algal-phytoplanktic carbohydrates. The lack of oxygen-deficient zone within the water column, coupled with well-preserved lipoid sediment components, suggests that the boundary between the oxygen-rich and oxygen-poor levels was likely located at/near the sediment-water interface. The palyno-organic facies analysis suggests that the organic matter corresponds to the type II (possibly I/II) kerogen (VRo ∼0.3–0.4%) that was protected from degradation before, during and after sedimentation. Well-preserved lipoid components suggest a good hydrocarbon-generative potential, but the low TOC values (

The Lower Triassic Kangan Formation of the Persian Gulf Superbasin forms one of the largest reservoirs in the South Pars/North Field, the world's largest gas field. The upper Khuff-equivalent, gas-hosting carbonate-evaporite succession was formed on a gently-sloping, homoclinal carbonate ramp in a warm, tropical aragonite sea under a hot-and-arid greenhouse climate. This study explores the impact of vertical variations in depositional facies and diagenetic features on the reservoir quality to improve the prediction of the subsurface facies and hydraulic flow unit distribution. Detailed core logging and petrographic analysis resulted in the recognition of twelve distinct carbonate-evaporite facies that are grouped into four major facies associations, from shallowest to deepest: supratidal to restricted subtidal, lagoon, carbonate shoal, and shallow mid-ramp. The facies associations are stacked into two long-term sequences KS2 (58 m) and KS1 (77 m) bounded by anhydrites and/or unconformities; three higher frequency sequences (KS1a, KS1b, and KS1c) can be identified within the KS1. Based on the petrophysical properties, sedimentary facies characteristics and their diagenetic modifications, five hydraulic flow units (HFUs) are identified, with the best porosity-permeability values and flow- and storage capacities in the late transgressive (TSTs) and the earliest highstand systems tracts (HSTs) of the two long-term sequences. The reservoir quality and the distribution of hydraulic flow units are a product of interactions between primary mineralogy, depositional facies and their stacking, and the early- and late-diagenetic alterations. The reservoir quality decreases in landward direction, being the lowest in the mud-supported textures of the lagoon and supratidal to restricted subtidal facies associations. The seaward improvement in reservoir quality reflects a change to grain-supported textures that formed along the tide-and wave-agitated ooid-bioclastic shoals, whereas the best reservoir quality characterizes the shallow mid-ramp open-marine facies association, where secondary porosity was improved by dissolution and dolomitization. Both the early and late diagenetic dolomitization overall improved the reservoir quality, with the dolomitization pathways controlled by the porosity and permeability heterogeneity in the original depositional facies. The results may be useful in reservoir modeling, recognition of the productive zones, and the further development of the South Pars/North Dome Field.

The upper Katian (Upper Ordovician) Red River Formation is a carbonate-evaporite supersequence that formed in the epicratonic Williston Basin of western North America. This study focuses on the upper part of the formation in the southern, US part of the basin. The studied succession is made up of three transgressive-regressive sequences (RR1, RR2, and RR3), each ∼0.5–∼1 m.y. in duration and composed of relatively few meter-scale high-frequency cycles (parasequences). Sequence boundaries are characterized by intraclastic breccia-laminated dolomite successions that have been truncated and capped by evaporite deposits. Subaqueous anhydrites within each sequence represent evaporite Lowstand Systems Tracts (LSTs) that formed during relative sea-level lows that lead to the precipitation of gypsum (later recrystallized into anhydrite) in a quiet, salina-type environment. Transgressive (TST) and early Highstand Systems Tracts (HST) are composed of subtidal facies of predominantly skeletal mudstone to wackestone-packstone with abundant burrow mottling. Late HSTs are predominantly composed of peritidal, porous laminated dolomite. The study illustrates how the sequence stratigraphic framework of carbonate-evaporite deposits from the Williston Basin relates to the Late Ordovician glacio-eustasy, transitional climates, and low basin subsidence in an arid setting.

The Lower Cretaceous shallow-marine succession of Mljet Island in Croatia records the geological history of the southern part of the Adriatic Platform during its last tectonically quiet period, prior to the Late Cretaceous collision processes between the Adria Microplate and the Eurasian Plate. We studied the Early Cretaceous biostratigraphy of benthic foraminifera and calcareous algae in order to establish a precise, combined benthic biozonation for the Adriatic, which in turn facilitates a better understanding of the global distribution of these microorganisms during the Early Cretaceous. Thirty-four species from 26 genera of benthic foraminifera and 32 species of calcareous algae, distributed among 11 genera, were recovered from the Lower Cretaceous shallow-water carbonate rocks of Mljet Island in southern Croatia. Nine biostratigraphical units, eight of which are based upon the benthic foraminifera and calcareous algae, and one upon the microencruster Bacinella irregularis, have been distinguished within this interval: the Clypeina parasolkani-Humiella catenaeformis Interval Zone (Berriasian–earliest Valanginian), Epimastopora cekici-Pseudoclypeina? neocomiensis Interval Zone (Late Valanginian), Montsalevia salevensis Taxon-range Subzone (Late Valanginian), Clypeina? solkani Abundance Zone (late Early and Late Hauterivian), Salpingoporella melitae-Salpingoporella muehlbergii Interval Zone (Barremian), Bacinella irregularis Assemblage Zone (Early Aptian), Salpingoporella dinarica Abundance Zone (Late Aptian), Orbitolina (Mesorbitolina) texana-“Valdanchella” dercourti Interval Zone (Early Albian) and “Valdanchella” dercourti Taxon-range Zone (Late Albian). The stratigraphical position of the main genera and species of benthic foraminifera and calcareous algae within the Berriasian–Albian interval is discussed, and the established biozones are correlated within the Adriatic Platform domain. The Early Cretaceous diversification of benthic foraminifera in the area investigated can be shown to follow the sea-level curves for that period, and the major foraminiferal turnovers coincide with global sea-level rise and fall. During relative sea-level rises, coupled with reduced oceanic circulation and expansion of nutrient-poor, shallow tropical waters, these organisms were able to diversify into various euphotic habitats, particularly within shallow subtidal environments of the platform interior. Regressive episodes resulted in the reduction of oligotrophic habitats and a decrease in species richness.

The mid Cretaceous shallow-water limestones on the islands of Cres and Losinj contain abundant and diversified orbitolinid foraminifera. A biostratigraphy, based on the orbitolinid taxa and their chronostratigraphic interpretation, is proposed for the Aptian, Albian and Cenomanian deposits in the area investigated. Four orbitolinid biozones have been recognized: (1) Palorbitolina lenticularis Taxon-Range Zone in the Lower Aptian; (2) Orbitolina ( Mesorbitolina ) texana Assemblage Zone in the Lower Albian, the body of strata being characterized by a distinctive assemblage of Orbitolina ( Mesorbitolina ) texana , O. ( M .) subconcava , O . ( M .) parva and O. ( M .) pervia ; (3) ‘Valdanchella’ dercourti Taxon-Range Zone in the basal Upper Albian; and (4) Orbitolina ( Conicorbitolina ) conica Abundance Zone in the Lower–Middle Cenomanian, a body of strata in which the abundance of the species Orbitolina ( Conicorbitolina ) conica is significantly greater than is usual in the adjacent parts of the section, regardless of either association or range. The established biozones on the islands of Cres and Losinj are correlated with orbitolinid biozonations and orbitolinid stratigraphic distributions in the adjacent areas, with emphasis on the Karst Dinarides.