High-energy Shoals Research Articles

Facies Analysis and Depositional Environment of the Euphrates Formation in Selected Sections, Southwestern Iraq

The current study aims to investigate the microfacies, diagenetic processes, and depositional environments of the twenty-three rock samples from the Lower Miocene Euphrates Formation of two outcrops collected at Bahar-Najaf in Najaf governorate and Al-Shinafiya in Diwaniyah governorate, southern Iraq. Thin sections of these samples were prepared; hence, the nature of microfacies and related depositional environments was established. The formation contains abundant benthonic foraminifers and other fossils like miliolids, echinoderm, coral, gastropods, and pelecypods. Seven distinct microfacies were identified: lime mudstone, miliolid packstone, Pelletal Grainstone, Lime Wackestone, Peloidal Grainstone, Oolitic grainstone Microfacies, and fossiliferous packstone packed with bioclasts. Diagenetic processes evidenced in this study are dissolution, neomorphism, cementation, micritization, and dolomitization. The Euphrates Formation was deposited in a shallow marine setting with fluctuating energy levels, ranging from low-energy lagoons to moderate and high-energy shoals and fore-reef environments. Various microfacies indicate a complex depositional environment on a carbonate platform influenced by biota and sedimentary processes. These observations support the inference that the Euphrates Formation accumulated in shallow open marine, inner barrier, and shoal settings, providing a detailed interpretation of its para-depositional and diagenetic development.

Difference of Sedimentary Characteristics and Main Controlling Factors Between Platform Margin Shoal and Ramp Shoal in M Formation of A Oilfield in The Middle East

Abstract The bioclastic limestone reservoirs of M Formation in A Oilfield mainly develop in the ramp shoal and platform margin shoal, but the sedimentary characteristics, spatial distribution and main controlling factors of the two types of shoals are significantly different. Based on the data of seismic, well logging, core, casting thin sections and analytical tests, the comparative analyses of lithology, petrophysics and spatial distribution of the two types of shoals are carried out, and the control mechanism of relative sea level rise and fall and paleogeomorphology on their development is clarified. It is showed that the two types of shoals can be subdivided according to hydrodynamic conditions and bioclastic types. The platform margin shoals are dominated by grainstone and gravel limestone, and the bioclastics are mainly rudist and echinoderms. The logging curve is funnel-shaped, and the seismic is patchy distribution. In the vertical direction, the grain size of shoals becomes coarser, and the sedimentary sequence of medium energy shoal—medium-high energy shoal—high energy shoal is developed. In the horizontal direction, the facies belts of different shoals are narrow, and they are distributed quasi-layered semi-continuously around high energy shoals. The ramp shoals are dominated by packstone and grainstone, and the bioclastics are mainly non-fixed bivalves and algae. The logging curve is micro-tooth box-shaped, and the seismic is sheet distribution. In the vertical direction, the grain size change of shoals is not significant, and the sedimentary sequence of medium energy shoal—medium-high energy shoal is developed. In the horizontal direction, the facies belts of different shoals are wide, and they are distributed layered continuously around medium-high energy shoals. Relative sea level change controls the energy of sedimentary environment and the type of shoal facies, and the high position of paleogeomorphology controls the formation and distribution of high energy shoals. When the relative sea level is high and the paleogeomorphology is gentle, the ramp shoal is developed. As the relative sea level drops, the hydrodynamic condition increases, and the difference in the accumulation speed of different shoals enhances the fluctuation of the paleogeomorphology, the ramp shoal gradually transitions to the platform margin shoal. The above research can provide reference for the exploration and development of such reservoirs.

Facies, Depositional Environment and Reservoir Quality of an Early Cambrian Carbonate Ramp in the Tarim Basin, NW China

The Xiaoerbulake Formation in the Tarim Basin is considered one of the most important deep to ultradeep hydrocarbon reservoirs in the world. The objective of the present study is to analyze the facies, depositional environment and reservoir quality of the Early Cambrian Xiaoerbulake Formation in the Tarim Basin through integrated analysis of 120 m of cores, 3240 m of well cuttings, wireline logs and over 1100 thin sections from 17 exploration wells. Early Cambrian Xiaoerbulake Formation deposits in the Tarim Basin were deposited on a ramp setting. The ramp was occupied by seven facies associations and ten facies types ranging from the inner ramp to the outer ramp. These facies associations include tidal flat, lagoon, high-energy shoal, sabkha, inter-shoal, low-energy shoal and open shelf. Their distribution was controlled by paleogeographic patterns, sea level changes and the paleoclimate. Reservoir quality is considerably affected by facies together with diagenetic processes, including dolomitization and anhydrite cementation. High-quality reservoirs are found in the dolograinstone of high-energy shoal environments, which are favorable exploration facies in the carbonate ramp. Their pore space types consist of interparticle pores, intraparticle pores, intercrystalline pores, vuggy pores and moldic pores, with a porosity of 1.4%–7.5%. This study will help with our understanding of the stratigraphic framework, sedimentary-facies evolution and high-quality reservoir distribution of the Early Cambrian carbonate ramp in the Tarim Basin, facilitating exploration and the production of hydrocarbons from the Xiaoerbulake Formation.

Identification method of thief zones in carbonate reservoirs based on the combination of static and dynamic data: A case study from the Cretaceous Mishrif Formation in the H oilfield, Iraq

Carbonate reservoirs usually have a strong heterogeneity. The zones with relatively high permeability will form a channel through which fluids can easily flow. These channels are called thief zones. Thief zones have notable effects on oil or gas production, for example, high oil recovery rates at the early stage of the exploitation or an early water breakthrough during the later stage of water flooding development. Therefore, it is essential to have a precise identification of thief zones in carbonate reservoirs. In this research study, a simple approach to identify thief zones based on reservoir permeability gathered from well logging is developed. The thief zones are first identified at wells based on the lower limit value of the thief-zone permeability. This value is determined based on the dynamic production data, indicating that the thief zones identified by applying this criterion can reflect the product characteristics. Then, a zonal inter-well recognition method is adopted to identify the connectivity and distributions of thief zones in the regions far away from the well. This method is applied to identify thief zones for the Cretaceous Mishrif Formation in the H oilfield, Iraq. The reliability of the identification results is tested by the well-group injection test. The distributions of thief zones in the study region are discussed. In the study region, 12 members developed thief zones, while two members (i.e., MC1-3 and MC2-2) did not develop thief zones. Specifically, there are five members having a high level of thief-zone development. They are MB1-2C, MB2-1, MB2-2, MC2-3, and MC3-2. Comparing the distribution of thief zones with that of sedimentary microfacies, it is concluded that the thief-zone development is mainly controlled by the sedimentary microfacies and tends to occur in high-energy shoals.

Microfacies Analysis and Depositional Environment of Sarki Formation (Early Jurassic), Rawanduz Area, Kurdistan Region, Northern Iraq

A detail sedimentological analysis of the Sarki Formation (Early Jurassic) in Warte and Zarwan sections in the Imbricated Zone, northern Iraq has been conducted for the first time. The Sarki Formation in both studied sections are consisting of dolomite, dolomitic limestone and recrystallized breccia. The petrographic study of the 80 thin sections analyzed in both outcrops showed the skeletal and non-skeletal components. The skeletal components are including ostracods, bivalves and echinoderms. While the main non skeletal grains are peloids, ooids, extraclasts and intraclasts. The diagenetic processes which affected the carbonate rocks of the Sarki Formation was dolomitization, compaction, cementation, micritization, solution and slicification. The result of XRD and SEM of eight samples of dolomite and dolomitic limestone show that the main minerals are dolomite and calcite. In the carbonate rocks nine main microfacies have been identified, which are classified into three facies groupings that correspond to three depositional environments; peritidal, lagoon and high energy shoal within ramp settings.

Litho- and biostratigraphy of the Upper Jurassic Mozduran Formation, Kopet-Dagh, Iran, and its paleoenvironmental implications

The Upper Jurassic Mozduran Formation at Baghak section, Kopet-Dagh area northeast Iran, characterized by multiple units of carbonates and evaporates intercalated with siliciclastic deposits, yielded benthic foraminifera, calcareous algae, and crustacean coprolites, including Alveosepta jaccardi, Charentia aff. nana, Deloffrella quercifoliipora, Marinella cf. lugeoni, Favreina, and Solenopora sp. The almost consistent occurrence of Alveosepta jaccardi in the studied section, i.e., the Alveosepta jaccardi Zone, indicates late Oxfordian to the Kimmeridgian in age. Facies analysis of the formation reveals depositional settings of a peritidal zone, a lagoon, and a high-energy shoal of a carbonate ramp. Terrigenous and evaporites are found in eastern parts of Kopet-Dagh, representing a more proximal sedimentary environment. These siliciclastic inputs are associated with tectonic events during Oxfordian to Tithonian in the region.

Shoal-to-Strata Ratio Technique and Its Use in Describing Carbonate Rock Sedimentary Facies in the Eastern Sichuan Basin

The requirements for the accurate characterization of shoal sediments have increased in view of the fact that strata are eroded due to uplift and it is difficult to calculate the real thickness of granular shoal. To solve this problem, with the shoal-to-strata ratio, the type and distribution range of carbonate rock shoal facies are described, and the characterization of sedimentary facies is subsequently presented. Taking the P2m2a in Eastern Sichuan as an example, the following conclusions were obtained. (1) The primary rock types of the shoal facies of the P2m2a are bioclastic limestone and micrite limestone. The logging facies of platform margin shoals, intra-platform high-energy shoals, intra-platform low-energy shoals, and sloping low-energy shoals are large funnel-type, small funnel-type, box-type, and finger-like, respectively. The platform margin shoals and intra-platform high-energy shoals are imbricate seismic facies, and the intra-platform low-energy shoals and sloping low-energy shoals are S-type progradation seismic facies and oblique progradation. (2) The shoal-to-strata ratio of platform margin shoals is the highest, followed by intra-platform high-energy shoals, intra-platform low-energy shoals, and sloping low-energy shoals. (3) Shoal facies deposits have a certain controlling effect on gas production, and platform margin shoals have greater exploration potential. (4) This example shows that in the areas where carbonate rock strata are eroded, compared with the method of characterizing the sedimentary facies of carbonate rock using the shoal thickness, the shoal-to-strata ratios are more accurate. (5) The planar distribution diagram of the shoal-to-strata ratio can be used as a supporting map for compiling sedimentary facies maps. This method is worth applying in related research fields.

Tectonic-Lithofacies Paleogeography and Sizeable Reservoir Potential in the Early Cambrian Postrift Depressed Carbonate Ramp, Tarim Basin, NW China

Abstract The carbonate ramps developed at the postrift depositional stage have become a major field of ultradeep hydrocarbon exploration. Taking the Lower Cambrian Xiaoerblak Formation in the Tarim Basin as an example, the paleotectonic framework and lithofacies paleogeography response, reservoir distribution, and internal architecture difference were systematically analysed based on the data of newly processed 42 2D seismic lines, 9 outcrops, and 22 wells. The results show that (1) the palaeotectonic framework with three palaeohighs and two topographic lows (abbreviated to “TPTT”) dominated the differentiation in the ramp system (association) at the Xiaoerblak depositional stage. High-energy grain shoals and mounds occurred around the palaeohighs, and the sediment composition was further complicated by ancient ocean currents. Three subtypes of ramp are recognized: the slope-plateau ramp dominated by grain shoal on the northern margin of the paleohigh of the southwestern Tarim, homocline ramp dominated by the mound-shoal complex on the Keping-Wensu area lower high, and isolated island type in the Lunnan-Yaha lower high. (2) Xiaoerblak reservoirs mainly occur in the high-energy facies belts in the middle ramps around the palaeohighs because original pores were effectively preserved in early dolomitization and reconstructed at the burial stage. Reservoirs are predicted to mainly distribute in the high-energy mound-shoal zones around three palaeohighs (low uplifts) with an area of 9×104 km2. Different ramp systems show different reservoir properties. There are mainly type II reservoirs (porosity range of 1.8-4.5%, permeability range of 0.5-10×10−3 μm2) with sustained distribution and the largest size in the grain shoals at the slope-plateau ramp, which may be the potential favorable hydrocarbon exploration zone. Based on the example anatomy, the differentiation in the carbonate ramp systems forming at the postrift subsidence stage was structurally dominated. This means that it is crucial to deepen the understanding of the presedimentation palaeotectonic framework for the prediction of promising reservoirs and assessment of reservoir properties.

Integrated characterization of ultradeep reef-shoal reservoir architecture: A case study of the Upper Permian Changxing Formation in the giant Yuanba gas field, Sichuan Basin, China

The Yuanba giant gas field in the Sichuan Basin produces from Upper Permian Changxing heterogeneous reef-shoal reservoirs. It is an ultradeep tight gas reservoir with burial depth of 7000 m. This paper proposes an integrated methodology incorporating the reservoir geology description, petrophysics characterization, seismic response analysis, forward seismic modeling, pseudo-acoustic inversion, and reservoir architecture imaging to identify the geologically complex reef-shoal reservoirs and predict the distribution of the producible reservoirs. The facies of reef cap, reef core, reef base and shoal are first identified based on observations and interpretations of cores, thin sections, mud logging, wireline logs, and formation microscanner images (FMIs). High-energy shoals, reef cores, and reef caps are the most favorable reservoir microfacies. Seismic responses of reef-shoal reservoirs are then investigated through forward modeling and well-seismic calibration. Four reef belts are identified based on seismic reflections and edge detection by seismic paleogeomorphologic features. Reservoir distribution and quantitative properties are predicted using the facies-controlled postattack seismic inversion technique through the correlation between reservoir properties and seismic attributes, which is aided by pseudo-gamma acoustic inversion to exclude the shale-bearing non-reservoir and pseudo-neutron acoustic inversion based on its good relationship with porosity. The reef reservoir thicknesses range from 10 m to 130 m, and the thickest reservoir is located in the northwestern Yuanba gas field. Finally, 3-D visualization is used to characterize the spatial variation of the interior architecture, connectivity, and distribution of a good reservoir in the reef system. The accuracy rate of prediction for reservoirs and non-reservoirs reaches 88% using this method. The technique has also been successfully used for the deployment of development wells and well trajectory optimization, and the rate of encountering the predicted reservoirs is 100%. This study provides a practical approach to identify and predict the tight carbonate gas reservoir with similar petrophysical responses of density and acoustic to non-reservoirs in the complex carbonate system.

Depositional and diagenetic controls on reservoir quality: Example from the upper Cretaceous Mishrif Formation of Iraq

The Cenomanian-Turonian Mishrif Formation at the H field in southeastern Iraq displays a wide range of rock fabrics that result in highly variable reservoir quality and flow properties. Reservoir quality in this field is controlled by the complex interplay of depositional and diagenetic factors. Depositionally, seven main productive facies are recognized, which are organized into six sequences; reservoir quality occurs primarily in the grain dominated high energy shoal, shallow open marine, and rudist biostrome facies. A major change in stratigraphic style occurs midway through the reservoir, with a lower portion that is dominated by laterally continuous upward shoaling cycles (that are often shoal or tidal flat capped) and an upper part that is more heterogeneous and dominated by lagoonal and back-shoal facies. While diagenesis has overprinted and altered this initial framework, karsting - resulting in both enhanced reservoir quality due to leaching and reduced reservoir quality due to cementation, brecciation and fines infiltration - associated with sequence boundaries within and at the top of the Mishrif Formation represents the major flow-significant diagenetic effect in the reservoir. Here, diagenesis has followed the stratigraphic pattern established at the time of deposition, and reinforced its impact on reservoir quality. Mishrif reservoir quality is ultimately controlled by sea-level history, in that high energy, grain dominated textures and facies associated with sea-level highstands have the best reservoir quality, while meteoric diagenesis has produced low reservoir quality (and reservoir boundaries) at karsted exposure surfaces that formed during sea-level falls. Recognition that reservoir quality, both good and bad, results from congruent depositional and diagenetic effects ultimately driven by the sea-level history of the succession provides an important key to understanding and predicting reservoir quality in the H field and in other carbonate fields with similar depositional and diagenetic histories.

Contribution to the stratigraphy and sedimentology of the Upper Jurassic – lower Eocene succession of the Mitla–El Giddi stretches, west Central Sinai, Egypt

The present study deals with the stratigraphy, petrography and facies analysis of the stratigraphic succession of Mitla-El Giddi stretches (west Central Sinai, Egypt). Lithostratigraphically, the study area is covered by a thick succession extending from Late Jurassic to early Eocene time. This succession is subdivided into the Masajid Formation (Upper Jurassic (Oxfordian) at the base followed upwardly by the Risan Aneiza Formation (Lower Cretaceous (? Aptian- Albian), the Galala Fm. (Cenomanian), the Abu Qada Formation (lower Turonian), the Buttum Formation (middle Turonian), the Wata Formation (middle–upper Turonian), the Themed Formation (Coniacian–Santonian), the Sudr Chalk (Campanian–Maastrichtian), the Mitla Formation (= Esna Shale) (Paleocene–lower Eocene), the Thebes Formation (lower Ypresian) and the Minia Formation (upper Ypresian) at top. The Aptian – Albian Risan Aneiza Formation is first recorded at G. El Hamra in the study area. As well, a new formational name (the Mitla Formation) is used to describe the Paleocene – lower Eocene unit equivalent to the Esna Shale due to variation in lithology mainly represented by chalky limestone and argillaceous limestone. Many unconformities, represented by variable sedimentologic features, i.e., red paleosols, conglomerates, crustified laminated limestone and glauconitic facies were investigated and mostly attributed to the Syrian Arc System.The petrographic investigations especially for the lithified rocks revealed eighteen limestone microfacies, three dolostone microfacies and four sandstone microfacies, besides the non-lithified shale, sandy shale, clay and marl lithofacies. The recognized facies and their related paleoenvironments document lateral and common vertical transition between inner, middle and outer ramp setting. These facies could be subdivided into eight associations that are related to six depositional environments: peritidal flat/beach clastics, peritidal flat carbonates, lagoonal clastics, lagoonal carbonates, high energy shoal of ooids and patch reefs (oolitic shoal), intertidal-subtidal open marine, storm influenced subtidal open marine (mid-ramp) and hemipelagic outer ramp facies.The study indicates that, the area was controlled by a long-term transgressive phase and several higher order sea level fluctuations during the deposition of the studied succession. The main factors controlling the ramp deposition and the described events are; structure control mainly Syrian Arc deformation, eustatic sea level fluctuations combined with environmental influences such as autochthonous carbonate productivity-siliciclastic supply and paleorelief conditions.

Active Ooid Growth Driven By Sediment Transport in a High-Energy Shoal, Little Ambergris Cay, Turks and Caicos Islands

Ooids are a common component of carbonate successions of all ages and present significant potential as paleoenvironmental proxies, if the mechanisms that control their formation and growth can be understood quantitatively. There are a number of hypotheses about the controls on ooid growth, each offering different ideas on where and how ooids accrete and what role, if any, sediment transport and abrasion might play. These hypotheses have not been well tested in the field, largely due to the inherent challenges of tracking individual grains over long timescales. This study presents a detailed field test of ooid-growth hypotheses on Little Ambergris Cay in the Turks and Caicos Islands, British Overseas Territories. This field site is characterized by westward net sediment transport from waves driven by persistent easterly trade winds. This configuration makes it possible to track changes in ooid properties along their transport path as a proxy for changes in time. Ooid size, shape, and radiocarbon age were compared along this path to determine in which environments ooids are growing or abrading. Ooid surface textures, petrographic fabrics, stable-isotope compositions (δ^(13)C, δ^(18)O, and δ^(34)S), lipid geochemistry, and genetic data were compared to characterize mechanisms of precipitation and degradation and to determine the relative contributions of abiotic (e.g., abiotic precipitation, physical abrasion) and biologically influenced processes (e.g., biologically mediated precipitation, fabric destruction through microbial microboring and micritization) to grain size and character. A convergence of evidence shows that active ooid growth occurs along the transport path in a high-energy shoal environment characterized by frequent suspended-load transport: median ooid size increases by more than 100 μm and bulk radiocarbon ages decrease by 360 yr westward along the ∼ 20 km length of the shoal crest. Lipid and 16S rRNA data highlight a spatial disconnect between the environments with the most extensive biofilm colonization and environments with active ooid growth. Stable-isotope compositions are indistinguishable among samples, and are consistent with abiotic precipitation of aragonite from seawater. Westward increases in ooid sphericity and the abundance of well-polished ooids illustrate that ooids experience subequal amounts of growth and abrasion—in favor of net growth—as they are transported along the shoal crest. Overall, these results demonstrate that, in the Ambergris system, the mechanism of ooid growth is dominantly abiotic and the loci of ooid growth is determined by both carbonate saturation and sediment transport mode. Microbes play a largely destructive, rather than constructive, role in ooid size and fabric.

Diagenetic and Depositional Impacts on the Reservoir Quality of the Upper Jurassic Arab Formation in the Balal Oilfield, Offshore Iran

AbstractThe Kimmeridgian‐Tithonian aged Arab Formation, as the main reservoir of the Jurassic succession in the Balal oilfield, located in the offshore region of the Iranian sector of the Persian Gulf, is investigated in this study. The formation is composed of dolomites and limestones with anhydrite interbeds. Based on detailed petrographic studies, six microfacies are recognized, which are classified in four sub‐environments including supratidal, intertidal, lagoonal and the high energy shoal of a homoclinal carbonate ramp. The main diagenetic features of the studied succession include dolomitization, anhydritization, cementation, micritization, fracturing and compaction. Based on stable isotope data, dolomitization of the upper Arab carbonates is related to sabkha settings (i.e. evaporative type). In terms of sequence stratigraphy, three shallowing‐upward sequences are recognized, based on core and wireline log data from four wells of the studied field. Considering depositional and diagenetic effects on the reservoir quality, the studied facies are classified into eight reservoir rock types (RRT) with distinct reservoir qualities. Dolomitization has played a major role in reservoir quality enhancement, whereas anhydritization, carbonate cementation, and compaction have damaged the pore throat network. Distribution of the recognized RRTs in time and space are discussed within the context of a sequence stratigraphic framework.

Lower Jurassic Bahamian-type facies in the Choč Nappe (Tatra Mts, West Carpathians, Poland) influenced by paleocirculation in the Western Tethys

The Lower Jurassic (upper Sinemurian) of the Hronicum domain (Tatra Mts., Western Carpathians, Poland) represents typical tropical shallow-water carbonates of the Bahamian-type. Eight microfacies recognized include oolitic-peloidal grainstone/packstone, peloidal-bioclastic grainstone, peloidal-lithoclastic-bioclastic-cortoidal grainstone/packstone, peloidal-bioclastic packstone/grainstone, peloidal-bioclastic wackestone, spiculitic wackestone, recrystallized peloidal-oolitic grainstone and subordinate dolosparites. The studied sediments were deposited on a shallow-water carbonate platform characterized by normal salinity, in high-energy oolite shoals, bars, back-margin, protected shallow lagoon and subordinately on restricted tidal flat. Some of them contain the microcoprolite Parafavreina, green alga Palaeodasycladus cf. mediterraneous (Pia) and Cayeuxia, typical of the Early Jurassic carbonate platforms of the Western Tethys. The spiculite wackestone from the upper part of the studied succession was deposited in a transitional to deeper-water setting. The studied upper Sinemurian carbonates of the Hronicum domain reveal microfacies similar to the other Bahamian-type platform carbonates of the Mediterranean region. Thereby, they record the northern range of the Lower Jurassic tropical shallow-water carbonates in the western part of the Tethys, albeit the thickness of the Bahamian-type carbonate successions generally decrease in a northerly direction. The sedimentation of the Bahamian-type deposits in the Hronicum domain, located during the Early Jurassic at about 28°N, besides other specific factors (i.e., light, salinity, and nutrients) was strongly controlled by the paleocirculation of warm ocean currents in the Western Tethys.

Characteristics of depositional environment and evolution of Upper Cretaceous Mishrif Formation, Halfaya Oil field, Iraq based on sedimentary microfacies analysis

As one of the most important carbonate targets in the Middle East, Upper Cretaceous Mishrif Formation has been highlighted for a long time. Although consensus has been reached on the overall sedimentary background, disputes still exist in understanding the sedimentary environment changes among sub-regions due to relatively limited research, rare outcrop, and incomplete drilled core, which hinders the analysis on sedimentary environment and thus the horizontal and vertical correlation. In this study, taking the Halfaya Oil Field as an example, the sedimentary microfacies analysis method was introduced to comprehensively characterize the cored interval of Mishrif Formation, including Single Layers MC1-1 to MA2. A total of 11 sedimentary microfacies are identified through system identification of sedimentary microfacies and environmental analysis, with reference to the standard microfacies classification in the rimmed carbonate platform. Then three kinds of environments are identified through microfacies assemblage analysis, namely restricted platform, open platform, and platform margin. Systematic analyses indicate that the deposits are mainly developed in the open platform and platform margin. Meanwhile, rock-electricity interpretation model is established according to the electricity response to cored intervals, and is then employed to interpret the uncored intervals, which finally helps build the sedimentary evolution pattern through horizontal and vertical correlation. It is proposed that the Single Layers MC1-1 to MB2-3 were deposited in the open platform featured by low water level, including sub-environments of low-energy shoal within platform and inter-shoal sea; Single Layers MB2-2 to MB1-2B were deposited in the open platform and platform margin, including sub-environments of high-energy shoal on the platform margin, low-energy shoal within platform, inter-shoal sea, and open sea; and Single Layers MB1-2A to MA2 were again deposited in the open platform with high water level, and the circumstance of open sea was dominant. The deposition of Single Layers MC1-1 to MA2 actually corresponded to a retrogradation-progradation process. Results of this study will not only provide significant guidance to the exploration and development of Mishrif Formation, Halfaya Oil Field, but also support that the theory of sedimentary environment correlation with adjacent areas is reliable.

Depositional architecture and sequence stratigraphy of the Upper Jurassic Hanifa Formation, central Saudi Arabia

To document the depositional architecture and sequence stratigraphy of the Upper Jurassic Hanifa Formation in central Saudi Arabia, three composite sections were examined, measured and thin section analysed at Al-Abakkayn, Sadous and Maashabah mountains. Fourteen microfacies types were identified, from wackestones to boundstones and which permits the recognition of five lithofacies associations in a carbonate platform. Lithofacies associations range from low energy, sponges, foraminifers and bioclastic burrowed offshoal deposits to moderate lithoclstic, peloidal and bioclastic foreshoal deposits in the lower part of the Hanifa while the upper part is dominated by corals, ooidal and peloidal high energy shoal deposits to moderate to low energy peloidal, stromatoporoids and other bioclastics back shoal deposits. The studied Hanifa Formation exhibits an obvious cyclicity, distinguishing from vertical variations in lithofacies types. These microfacies types are arranged in two third order sequences, the first sequence is equivalent to the lower part of the Hanifa Formation (Hawtah member) while the second one is equivalent to the upper part (Ulayyah member). Within these two sequences, there are three to six fourth-order high frequency sequences respectively in the studied sections.

Paleoecology and paleoenvironment of the Middle–Upper Jurassic sedimentary succession, central Saudi Arabia

Four Middle–Upper Jurassic sections from central Saudi Arabia have been investigated to evaluate microfacies types and macro-invertebrate paleocommunities and to interpret their paleoecology and paleoenvironments. The studied Jurassic successions are part of the Middle–Upper Callovian Tuwaiq Mountain Limestone and the Middle–Upper Oxfordian Hanifa Formation. Three main facies were recorded, including mud-supported microfacies, grain-supported microfacies and boundstones. A data matrix comprising 48 macrobenthic species in 35 samples collected from four sections were grouped into fifteen assemblages and one poorly fossiliferous interval by means of a Q-mode cluster analysis. The recorded macrofaunal assemblages have been subdivided into low-stress and high-stress on the basis of hydrodynamic conditions, substrate type, nutrient supply and hypoxia. The low-stress assemblages occur in (a) high-energy paleoenvironments with firm substrates; (b) high-energy shoals with unstable substrates of low cohesion and in (c) low-energy open marine environments with soft-substrates. The moderate- to high-stress assemblages occur in (a) oligotrophic environments with reduced terrigenous input in shelf lagoonal or in restricted inner ramp settings; (b) low-energy, soft substrate environments with hypoxia below the sediment–water interface; and, in (c) high-energy shoals and shelf lagoonal environments. The temporal distribution patterns of epifaunal and infaunal bivalve taxa are controlled by variations in water energy, substrate characteristics and productivity level. The reported litho- and biofacies confirmed that the Callovian Tuwaiq Mountain Limestone and the Oxfordian Hanifa Formation were deposited across wide spectrum of depositional environments, ranging from restricted lagoon to moderately deeper open marine basin, and providing the perfect conditions for macrofossils.

Paleoenvironmental evolution of the southern Neuquèn basin (Argentina) during the Tithonian-Berriasian (Vaca Muerta and Picún Leufú Formations): a multi-proxy approach

The Tithonian-Berriasian interval in the southern part of the Neuquén Basin is represented by the Vaca Muerta and the Picún Leufú Formations. Facies analysis and correlation of the Vaca Muerta Formation and the lower part of the Picún Leufú Formation in the Picún Leufú Anticline allow us to characterize the evolution of successive facies belts representing siliciclastic shelf and mixed ramp environments. Shoreface and offshore facies are developed on the siliciclastic shelf in the western and southern parts of the Picún Leufú Anticline. The offshore transition domain is characterized by storm beds and slump features, whereas the offshore domain corresponds to black to grey shales and turbidites. The mixed siliciclastic-carbonate ramp is characterized by the development of a lagoon and high-energy shoal in the proximal part of the inner ramp, whereas the distal part comprises a tidal complex. The mid-ramp zone is characterized by storm influence and the outer ramp by fine-grained deposits. Two major transgressive-regressive sequences and five high-frequency transgressive-regressive sequences are recognized. The high-frequency transgressive-regressive sequences make up three progradational sequences, an aggradational-progradational sequence and an aggradational sequence. The geochemical characteristics and clay mineralogy of the Tithonian-Berriasian interval in the southern Neuquén Basin indicate that (1) climate played a key role in the evolution of the sedimentary environment, (2) a “normal marine” depositional environment with oxic sea water and sediment pore waters, rapidly changing to suboxic conditions at shallow depth below the seabed and (3) the occurrence of episodically restricted water-mass circulation at the onset of deposition of the Vaca Muerta Formation.

Combination and distribution of reservoir space in complex carbonate rocks

This paper discusses the reservoir space in carbonate rocks in terms of types, combination features, distribution regularity, and controlling factors, based on core observations and tests of the North Truva Oilfield, Caspian Basin. According to the reservoir space combinations, carbonate reservoirs can be divided into four types, i.e., pore, fracture–pore, pore–cavity–fracture, and pore–cavity. Formation and distribution of these reservoirs is strongly controlled by deposition, diagenesis, and tectonism. In evaporated platform and restricted platform facies, the reservoirs are predominately affected by meteoric fresh water leaching in the supergene–para-syngenetic period and by uplifting and erosion in the late stage, making both platform facies contain all the above-mentioned four types of reservoirs, with various pores, such as dissolved cavities and dissolved fractures, or structural fractures occasionally in favorable structural locations. In open platform facies, the reservoirs deposited continuously in deeper water, in an environment of alternative high-energy shoals (where pore–fracture-type reservoirs are dominant) and low-energy shoals (where pore reservoirs are dominant).

Microfacies and depositional environments of Jurassic (Callovian) Tuwaiq Mountain Formation in central Saudi Arabia

Three stratigraphic sections of the Callovian Tuwaiq Mountain Formation at Shaib El-Hisyan, Khashm Al-Giddayah and Dirab, near Riyadh City, central Saudi Arabia have been studied to distinguish their microfacies features and the equivalent probable depositional environments. Lithostratigraphically, the Tuwaiq Mountain Formation is subdivided into two lower and upper Tuwaiq members. The part that informally is known as upper Tuwaiq Member is subdivided from base to top into T1, T2, and T3 members, respectively. The microfacies analysis of the Tuwaiq Mountain Formation led to the recognition of 12 microfacies types (FT1–FT12) that are grouped into three associations: (1) open platform facies association (FT1–FT5); (2) high-energy shoals of ooids and patch reefs facies association (FT6–FT9); and (3) restricted carbonate platform facies association (FT10, FT12). The lower part of the Tuwaiq Mountain Formation is characterized by relative abundance of wackestone and wacke/packstone facies that are capped by thin peloidal molluscan floatstone facies. The former indicates a quiet open platform environment with periodic agitation, while the later indicates deposition in a sand shoal environment. The facies of basal unit of the upper Tuwaiq Mountain Formation, T1, are enriched with dense micrite matrix and shell fragments of epifaunal and deep infaunal organisms (e.g., Pholadomya spp.). The microfacies pattern of Baladiyah Member (T1) indicates a quiet open platform with low and sometimes high water circulation. The Maysiah Member (T2) is composed of hard limestone with abundance of coralline microfacies in framestone, grainstone, and packstone textures suggesting a high-energy shallow reefal environment with periods of quiet open platform conditions. This shallow reefal environment becomes more restricted southward, where shales and algal dolomitic grainstones were formed. The Daddiyah Member (T3) that made up of chalky limestone with chert intercalations is predominated by matrix-support lime mudstone, wackestone, and packstone facies. These microfacies reflect open platform with slightly low circulation below fair-weather wave base with period of restricted marine conditions forming rudstone microfacies. Southward, these environments became shallower and subjected to considerable quartz influx in an agitated sand shoal.