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Reservoir zonation is one of the fundamental processes conducted on permeability-porosity core data to classify a reservoir thickness based on its productivity potential. In all the methods currently used, human decision plays a prominent role at every stage, from basic reservoir rock clustering to final flow zone determination. This study aims to reduce the often biased human decision-making role at each stage of the reservoir zonation by utilizing simple mathematical algorithms, and enhance the reservoir rock clustering approach. To do so, we propose the application of the elbow algorithm as a straightforward yet effective method to optimize the cluster numbers, and the Pruned Exact Linear Time (PELT) algorithm to effectively determine the zones with identical fluid flow potential (in this paper regarded as Hydraulic Flow Unit – HFU). Both the elbow and PELT algorithms are employed to effectively classify reservoir thickness based on the commonly used flow zone indicator (FZI), reservoir quality index (RQI), and flow unit speed (FUS). The efficiency of the suggested mathematical algorithms was tested against the results of the sedimentological and petrophysical study of the Upper Dalan (Khuff) gas reservoir, the Persian Gulf. Three lithofacies associations (sabkha and tidal flat, lagoon, and shoal) with 11 distinctive lithofacies plus crystalline dolomite have been identified and the major diagenetic processes influencing their petrophysical characteristics have been assessed. Based on the automated approach, the studied Upper Dalan succession can be subdivided into five HFUs with consistent poroperm values and distinct lithofacies and diagenetic modifications: barrier unit (HFU 1; permeability (k) < 0.1 mD), baffle unit (HFU2-3; mean k < 0.1 mD), normal unit (HFU4; k = 10–100 mD, mean k 17 mD), permeable unit (HFU5-6; k = 10–100 mD), and highly-permeable unit (HFU7; k >100 mD). The study suggests that the automated approach may be an effective alternative to a conventional reservoir zonation of similar mixed carbonate-evaporite reservoirs.

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.

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 (

Patch reefs and their surrounding skeletal–peloidal–intraclast grainstone facies are very abundant on modern (sub-) tropical carbonate platforms. Compared to the barrier and fringing reef facies, the processes influencing composition, mineralogy, and stable-isotope record of peri-reefal sediment have received much less attention, despite a potentially significant volume of such sediment in many Phanerozoic carbonate platforms. To better understand the evolution and sedimentary record of patch-reef settings, this study investigates the patch-reef biota (scuba diving surveys) and composition (petrography), mineralogy (X-ray diffraction) and stable-isotope record (mass spectrometry) of peri-reefal sediments at East Beach on the northeastern, windward margin of San Salvador Island, the Bahamas. The macroalgae-dominated ecology is assessed, and a decadal decline in stony coral coverage with a shift to more opportunistic, stress-tolerant species is documented. The most common stony corals on these shallow-water (3–5 m) patch reefs are Diploria strigosa, Porites astreoides, and P. porites, and Halimeda lacrimosa dominates among green algae. Peri-reefal sediment is pure carbonate sand composed of roughly equal volumes of aragonite and high-Mg calcite (HMC), with mean MgCO3 content in HMC of 15.2%. The bulk-sediment isotope record shows very limited variation (δ13C between + 2.3‰ and + 3.2‰, and δ18O between − 0.8‰ and − 0.6‰) and this is consistent with similar modern settings. A lack of very fine particles and an abundance of intraclasts suggest that the environment is strongly influenced by bio- and physical erosion. Fast-growing algae have likely reached an abundance beyond the grazing capacity, which has drastically reduced the space for coral settlement and recruitment, and thus has compromised the patch-reef resilience in this part of the Bahamian Archipelago.

The Jurassic-Cretaceous dolomites, Adriatic platform, Croatia resulted from climate-influenced post-depositional reflux dolomitization (as opposed to synsedimentary peritidal- and deeper burial dolomitization), and subsequent stabilization within a Mesozoic, “calcite sea” isolated platform. The dolomites are stratiform (10 to 200 m thick), fabric destructive (20 to over 500 μm crystals), nonluminescent, and zoned with respect to Ca. Bulk dolomites have low Mn (10 to 30 ppm), moderate Sr (60 to over 200 ppm), positive δ13C and δ18O values, only moderate ordering (0.25 to 0.6) and single-phase fluid inclusions (temperatures +3‰ VPDB and Sr>100 ppm) composes most Jurassic and 40% of Cretaceous dolomites, making up turbid dolomite cores and initial clear dolomite rims. A first generation of low-Ca dolomite (LCD-1 ; 50 to 53 mol% Ca ; δ18O +1 to >+3‰ VPDB ; 100-180 ppm Sr) forms cement and variably replaces (stabilizes) earlier HCD cores. HCD and LCD-1 formed in refluxing marine-dominated pore waters under semi-arid climate (55 mol%) phases. A second generation of much younger, low-Ca dolomite (LCD-2 ; fracture-associated, more negative δ18O from -1.4 to +1‰ VPDB, Sr

Clypeina lagustensis n.sp. has been found in the Lower Tithonian deposits of the Lastovo Island (Dalmatia, Croatia). It is visually similar, obviously related and in some sections appearing almost identical, to Clypeina jurassica FAVRE, from which it differs by visible swellings and thinning of the central cavity, more pronounced distance between neighbouring whorls of fertile branches, and shape and structure of the interverticillate thallus parts, characterized by having well developed, hairy, sterile branches. These, after emerging from the exit pore, divide into several bundles which form a common turf with a calcareous envelope in the proximal part.

The shallow marine carbonate deposits on the island of Cres, overlyingdeeper-water Cenomanian–Turonian limestones, are characterizedby an assemblage of rudists, benthic foraminifera, and associatedmicrofossils. The paucispecific character of the fossil association suggestsdeposition in shallow areas of a carbonate platform, with lowcurrent-energies and restricted circulation. Similar assemblages indicatingsimilar palaeoenvironments, are common in the Upper Cretaceousdeposits of the Adriatic Carbonate Platform and adjacent areas.The assemblage of rudists (hippuritids) and microfossils indicatethe Turonian to (?)Coniacian age of the investigated carbonate succession.The biostratigraphic importance of the so-called “primitive”hippuritids within the micropalaeontologically poorly defined biostratigraphyof deposits of this age, is accentuated.