Jilh Formation Research Articles

The triassic formations of Kuwait: From geological concepts to numerical forward stratigraphic modelling applications

The Triassic exploration campaign onshore Kuwait initiated during the 1980s was mainly based on the evaluation of regional 2D seismic data. To date, few scattered wells have penetrated this succession reaching the Minjur, Jilh and Sudair mixed clastic and carbonate formations. The latter reservoirs are expected to hold potential gas and condensates that are yet to be proven. This communication aims at deciphering the spatial and temporal evolution of sedimentary architectures and associated Triassic depositional environments and facies along Kuwait. An integrated G&G assessment utilizing multi-disciplinary and multi-scale dataset allowed to identify the major petroleum system elements and develop sequential conceptual depositional models. The proposed models are then tested using numerical process-based forward stratigraphic applications. The seismic interpretation points to dominantly south to north oriented progradations centered on the Dibdibah low with thicknesses reaching around 1300 ft in the Sudair. In addition, progradations oriented from East to West highlight the location of the basin margins around the Kuwait Arch and its northern extension. These prograding tendencies are also identified in the overlying Jilh Formation. Onlapping reflectors point to transgressional systems related to flooding events that are expected to deposit marine fine-grained carbonatesand thus act as potential seals. Minjur Formation on the other hand, shows thick siliciclastic deposits along the southern sector of the Kuwait Arch. These fluvio-deltaic deposits are 150 feet thick and thin towards the west. Stratigraphic modelling allowed to test and constrain the paleobathymetric evolution of the system. The latter ranges between 0 and 20 m (inner ramp) and 20–40 m (Middle ramp) and reaches up to 80 m in the deeper sectors of the basin. The resultant facies at 3rd- 4th stratigraphic order highlights the evolution of lithologies and facies with regards to the various system tracts leading to a better understanding of the timing and spatial extent of mixed carbonate systems, evaporites and siliciclastics. Future works will focus on a higher resolution mapping of geobodies and prospects using integrated shared earth approaches including seismic multi-attribute assessment and 3D inversion and characterization driven by novel artificial intelligence workflows.

The Middle to Late Triassic of Central Saudi Arabia with emphasis on the Jilh Formation. Part I: lithostratigraphy, facies and paleoenvironments, palaeontology and biostratigraphic age calibration from outcrop studies

The Middle to Late Triassic of Central Saudi Arabia with emphasis on the Jilh Formation. Part I: lithostratigraphy, facies and paleoenvironments, palaeontology and biostratigraphic age calibration from outcrop studies

The Middle to Late Triassic of Central Saudi Arabia with emphasis on the Jilh Formation. Part II: sequence stratigraphy, depositional and structural history, correlations and paleogeography

The Middle to Late Triassic of Central Saudi Arabia with emphasis on the Jilh Formation. Part II: sequence stratigraphy, depositional and structural history, correlations and paleogeography

Early Triassic flora from Jilh Formation (Anisian to Ladinian), Al-Qassim, Saudi Arabia

Early Triassic flora from Jilh Formation (Anisian to Ladinian), Al-Qassim, Saudi Arabia

Sedimentology, sequence stratigraphy, palynology and diagenetic evaluation of the Triassic Jilh Formation. New insights from Saudi Arabia

The Jilh Formation (late Anisian to Carnian) was deposited on the southwestern margin of the Neotethys Ocean recording the flooding of a broad shallow marine platform, and is characterized by restricted clastic-prone mudrocks and sabkha anhydrite with interbedded shallow marine dolomitized carbonates. Within the Lower Jilh Formation, deposits encompass tidal flat to low-energy inner ramp thrombolytic microbial deposits with local anhydrite nodules and fenestrae structures interbedded with ft-scale intraclast-rich floatstones with desiccation surfaces. Within the interbedded mudrocks, land-derived miospores (including common Lunatisporites spp. and other taeniate bisaccate pollen) and clastic material are common. During the Carnian, the higher sea level resulted in a change to tidal flat bioturbated and microbially induced mudstones, along with inner ramp subtidal peloidal sand banks and skeletal wackestones with benthic foraminifera within the Upper Jilh Formation. Terrestrially derived miospores are still common (assemblage including common Camerosporites secatus, Conipollenites arabicus, Duplicisporites spp., Samaropollenites speciosus), but an increasing marine influence is also noted during the Carnian from the presence of marine palynomorphs such as acritarchs and prasinophytes (including acanthomorphic acritarchs, Veryhachium spp. and Dictyotidium spp.). The backstepping of the platform is marked by the progressive flooding of the tidal flats and the deposition of open marine facies culminating in a regional 2nd order maximum flooding surface. Petrographic and geochemical analyses show the development of fabric-replacive planar-s to planar-e dolomite fabrics associated with negative δ18O values and positive δ13C values, suggesting precipitation of the dolomite on top of 3rd and 2nd order sequences from possible transgressive reflux syn or post deposition of the Jilh Formation. Zoned dolomite rhombs lining macropores might be related to burial diagenesis and pore-filling anhydrite and calcite are locally common. The local presence of native sulfur replacing primary anhydrite nodules, along with the minor calcitization of the anhydrite also suggest the onset of thermochemical sulfate reduction during the burial history at possibly high temperature.

Pressure and thermal regimes and systems in the sedimentary sequence of central and eastern Saudi Arabia

Reservoir pressure data in central and eastern Saudi Arabia reveals the presence of two normally pressured (NPR 1and NPR 2) and two overpressured (OPR 1 and OPR 2) regimes. The NPR 1 comprises the sequence from the Saq through the lower Jilh Formations, from outcrops to 15,000 ft below ground. The NPR 2 comprises the sequence from the upper Jilh through the Dammam Formations and extends from outcrops to 14,000 ft. The OPR 1 comprises the sequence from the Saq through the Khuff Formations at depths from 12,000 to 19,000 ft. Overpressures in the OPR 1 are a result of reduced porosity and permeability, and gas generation from the Qusaiba shale. The OPR 2 comprises the lower Jilh Formation at depths between 10,000 and 13,000 ft. Compaction and/or hydrocarbon charging from the Qusaiba shale could be the reason for overpressures in the OPR 2. The NPR 1, NPR 2, and OPR 1 are divided into pressure systems by stratigraphic seals, but these are locally breached by unconformities or faults forming local anomalies. Geothermal gradients illustrate shallow and deep thermal regimes (STR and DTR) separated by a high thermal conductivity zone (HTCZ). The STR extends from land surface to 8000 ft., with low and high gradients in the up-dip and down-dip areas, respectively. The DTR extends from 10,000 to 19,000 ft., with no differences in gradients between the up-dip and down-dip areas. The HTGZ lies between 8000 and 10,000 ft., with low gradients.

Managing clastic reservoir heterogeneity II: Geological modelling and reservoir characterisation of the Minjur Sandstone at the Khashm al Khalta type locality (Central Saudi Arabia)

ABSTRACT The increasing demand on geological reservoirs, whether for developing geothermal energy or for CO2 geological storage, raises questions on how reservoir heterogeneity might increase or decrease reservoir performance. To address this issue we selected the Minjur Sandstone Formation, a groundwater-bearing formation of Triassic age in Central Saudi Arabia, for complex reservoir modelling, simulation and prediction of the spatial distribution of sand bodies in a fluvio-deltaic system. This paper builds on a previous study that focused on the facies, stratigraphy, and reservoir characterisation of the Minjur Sandstone at the Khashm al Khalta type locality. Its purpose is to construct a deterministic 3-D model for (1) studying the connectivity of the Minjur Sandstone, and (2) illustrating a typical fluvio-deltaic reservoir and its associated heterogeneity. A first model simulates the spatial distribution of the depositional environments, which were further coded into relative proportions of sand, shale, evaporites and carbonates. This leads to a second model that contributes to reservoir applications through estimating the reservoir’s volume and storage capacities. Sequences 1 to 4 of the succession (Upper Jilh Formation–Lower Minjur Member), with a net-to-gross sand/shale ratio (NG) of ca. 8%, consist of poorly connected sandstone reservoir bodies. In contrast, sequences 5 to 9 (Upper Minjur Member), with an average NG of ca. 42%, consist of well-interconnected sandstone reservoir bodies. The NG depends on the tectonic influence and on relative sea-level variations. The best Minjur Sandstone reservoir bodies are at the base of each sequence, where limited available space favours a stack of deposits: interconnected fluvial channels which form wide spreads of coarse sandstone showing little diagenesis. The greatest potential is in the Upper Minjur Member. The effective reservoir volume was isolated using a sand content of > 85%. Rock volume and pore volume for an average porosity of 17% were subsequently calculated from the outcrop model. A representative block of 600 m x 600 m x 144 m was selected in order to simulate a fraction of the reservoir with the same properties as the whole. The block’s CO2 storage capacity was 57,000 tonne (in the International System, ‘SI’) for an arbitrary CO2 density of 0.7 (supercritical). This result was then transposed to the aquifer in the Riyadh area where similar conditions are assumed to exist. To obtain a ‘reservoir scale’ estimation, the block dimensions were upscaled to 20 km x 20 km x 80 m (the last figure being the effective thickness given by hydrogeological studies). The inferred storage capacity here was 30.5 Mt (million tonnes, International unit System, ‘SI’), which is an excellent figure when one considers the large-scale projects of Europe (Sleipner: 20 Mt) and Canada (Weyburn: 14 Mt).

Facies, sequence stratigraphy and reservoir/seal potential of a Jilh Formation outcrop equivalent (Wadi Sahtan, Triassic, Upper Mahil Member, Sultanate of Oman)

ABSTRACT The investigated Middle to Upper Triassic Upper Mahil Member, representing a Jilh outcrop equivalent in the Northern Oman Mountains, illustrates the proximal portion of a flat epeiric carbonate ramp. A sedimentological study of well-exposed outcrops in Wadi Sahtan may serve as a reference section for a sequence-stratigraphic framework and detailed facies description of the Upper Mahil Member. It also provides an insight into the seal and reservoir potential of carbonates in a low-accommodation inner ramp setting. Outcrop observations and thin section analyses yielded 14 different lithofacies types ranging from a supratidal marsh to high-energy subtidal shoal environment. Vertical facies stacking patterns show three basic small-scale cycle motifs (fifth-order). While mud-rich backshoal cycles with claystone intercalations and rooted/bioturbated mud-/wackestones illustrate potential baffles and seal units around the center of the Upper Mahil, potential reservoir units occur stratigraphically in the upper part of the formation. There, a few meter-thick trough cross-bedded oolitic-/peloidal-rich grainstone depicts maximum accommodation within backshoal to shoal cycle types below the erosional base-Jurassic unconformity. The investigated outcrop section in Wadi Sahtan was subdivided into nine almost complete third-order sequences. Two to four of these sequences are further stacked into three second-order super-sequences which are well reflected in the gamma-ray pattern. The highest reservoir potential occurs around second-order maximum floodings. Internal seals can be observed at third-order sequence boundaries where shales and muddy carbonates are up to 20 m thick. A regional correlation with subsurface data from Yibal and Lekhwair in Oman shows that the apparent thickness changes in the Upper Mahil (Jilh) are mainly determined by the Late Triassic/Early Jurassic erosional truncation. The occurrence of thick anhydrite units in the subsurface indicates a more proximal setting towards the southwest.

Successful Short-Radius Re-Entry Well in Deep-Gas Drilling in Saudi Arabia

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 139855, ’The First Successful Short-Radius Re-Entry Well in Deep-Gas Drilling in Saudi Arabia,’ by Shaker Al-Khamees, SPE, and Hope Okwa, SPE, Saudi Aramco, and Jaywant Verma, SPE, and Sukesh Ganda, SPE, Schlumberger, prepared for the 2011 SPE/IADC Drilling Conference and Exhibition, Amsterdam, 1-3 March. The paper has not been peer reviewed. Successful short-radius sidetracking in a deep gas area in Saudi Arabia is presented. Well ANDR-XYZ was sidetracked from vertical at 12,242 ft and built inclination to 90°, with maximum dogleg severity (DLS) recorded as 39°/100 ft. Thereafter, a 2,400-ft-long lateral was drilled in the Khuff-B reservoir. The sidetracking and lateral were drilled because of deteriorating production. The time required for this work was 53 days vs. the planned 60 days. The short-radius sidetrack was one of three options studied and resulted in saving 45 rig days compared with the other options. The well also extended the frontier of drilling and workover options and enabled opportunities to re-enter several other wells in deep gas areas in Saudi Arabia. Introduction Most deep gas wells in Saudi Arabia are drilled in the southern Ghawar field. The Late Permian Khuff-B and Khuff-C stacked carbonate reservoirs are the main gas-producing zones at depths of 11,500- to 12,500-ft true vertical depth. Two types of casing designs are used in these deep gas wells: “K1” and “K2.” In the conventional K2 profile, 133/8-in. casing is set from the surface to 30 ft into the base of Jilh dolomite formation. A 12-in. section is drilled into the Khuff-C carbonate, and 95/8-in. casing is set. Thereafter, 83/8-in. hole is drilled across Khuff or pre-Khuff reservoir. In a typical K1 design, in place of the 133/8-in. casing, 95/8-in. casing is set 30 ft into the base of the Jilh dolomite formation. An 83/8-in. section is drilled into the Khuff-C carbonate, and a 7-in. liner is run. After setting the 7-in. liner, 57/8-in. hole is drilled to the target formation. Occasionally while drilling these wells, higher pressures are encountered in the base of the Jilh formation. Therefore, the K2 design is preferred over the K1 design because it provides the opportunity to set an additional casing string. At planning stages, off-set wells are studied to finalize casing design. Because drilling a slimmer hole is faster and less costly, the K1 design is preferred when there is no history of high pressure in offset wells. While planning workover operations on Well ANDR-XYZ, various options were studied from the perspectives of cost, time, and technical feasibility. The short-radius-sidetrack technically was the only economically acceptable approach. The full-length paper details the conventional long-radius drilling approach, drilling options, and planning and execution of the first successful short-radius well in deep-gas drilling in Saudi Arabia. The objective of the Well ANDR-XYZ L1 re-entry was to sidetrack from the original vertical well in the Khuff-B formation and drill a 57/8-in. lateral targeting the Khuff-B reservoir.

A review of aquatic vertebrate remains from the Middle–Upper Triassic Jilh Formation of Saudi Arabia

A recent field survey of the Middle–Upper Triassic (upper Anisian to lowermost Carnian) paralic marine deposits of the Jilh Formation in central Saudi Arabia has yielded large quantities of vertebrate fossils. These finds prompt a revision of the existing faunal list and include at least one novel stratigraphical occurrence for the Arabian Peninsula. The remains comprise sauropterygian marine reptiles (Psephosauriscus sp., Nothosaurus cf. tchernovi, Nothosaurus cf. giganteus, Simosaurus sp.), a lungfish (Ceratodus sp.), hybodontiform sharks (Hybodus sp.) and saurichthyform actinopterygians (Saurichthys sp.). Palaeobiogeographical assessment reinforces Tethyan affinities for the assemblage and reflects the close proximity of the Arabian region to the ‘Sephardic Realm’, a compositionally distinct circum-Mediterranean faunal province characterized by hypersaline Muschelkalk facies.

First Triassic lungfish from the Arabian Peninsula

Triassic lungfish (Dipnoi) have been extensively documented from the Gondwanan continental and marine shelf deposits of Africa and Madagascar (Teixeira, 1949; Lehman et al., 1959; Beltan, 1968; Martin, 1979, 1981; Kemp 1996), Australia (Kemp, 1993, 1994, 1997a, 1998), India (Jain et al, 1964; Jain, 1968), and Antarctica (Dziewa, 1980). Numerous records also exist from Laurasian landmasses including Europe (Agassiz, 1838; Schultze, 1981), North America (Case, 1921) and central and eastern Asia (Liu and Yeh, 1957; Vorobyeva, 1967; Martin and Ingavat, 1982). By comparison, nothing is known of contemporary lungfish fossils from the Middle East. Thus, the recent recovery of a single tooth plate representing a new geographic occurrence of the genusCeratodusAgassiz, 1838 from paralic marine deposits of the Jilh Formation, a latest Anisian to lower Carnian unit that crops out along the eastern margin of the Proterozoic Arabian Shield in central Saudi Arabia (Fig. 1), is significant because it provides the stratigraphically oldest record of dipnoans from the Arabian Peninsula.

A Middle Triassic Vertebrate Fauna from the Jilh Formation, Saudi Arabia

A Middle Triassic vertebrate fauna from the Jilh formation in Saudi Arabia is described. Although the material is very fragmentary, the occurrence of a wide taxonomic variety of taxa can be documented, including hybodontiform sharks, actinopterygians, and sauropterygian reptiles such as a pachypleurosaur, Simosaurus, nothosaurs and cyamodontoid placodonts. The fauna compares well with other Middle Triassic faunas from the Germanic and Alpine Triassic, and from the northern Gondwanan shelf, in particular with the fauna from Makhtesh Ramon, Negev, Israel.

The stratigraphy, depositional environments and periods of deformation of the Wajid outcrop belt, southwestern Saudi Arabia

The Wajid outcrop belt in southwestern Saudi Arabia is an area of over 22,000 square kilometers where strata of Middle(?) Cambrian through to Jurassic crop out. The Early Palaeozoic sediments in southwestern Arabia have been placed in the Wajid sandstone. This paper elevates this unit to group status and recognizes its former members as formations. The remaining Palaeozoic successions are placed in the Qalibah and Khuff formations. The Middle(?) Cambrian through to Early(?) Ordovician Dibsiyah Formation, Wajid Group, correlates with the Saq sandstone of central and northern Saudi Arabia. The ‘lower’ Dibsiyah buries a topography developed on the Arabian shield during the Late Precambrian Pan-African and/or the late Early Cambrian periods of uplift. The late Early Cambrian period of deformation is documented in southwestern Saudi Arabia and adjacent Oman by an angular unconformity which separates the Dibsiyah Formation correlative from the underlying Late Precambrian-Early Cambrian Lower Haima Group. The ‘lower’ Dibsiyah was deposited in a northward prograding fluvial apron. The ‘upper’ Dibsiyah was deposited in intertidal to shallow subtidal marine settings during a period of coastal onlap. The Late(?) Ordovician Sanamah Formation overlies the Dibsiyah Formation in angular and/or erosional unconformity. This unconformity was developed during a period of regional uplift which began near the close of the Middle Ordovician. The Sanamah Formation was deposited in broad valley complexes excavated in fluvial settings. The Early Silurian Qusaiba Member of the Qalibah Formation unconformably overlies the Sanamah. The Qalibah Formation was deposited in shallow marine to mid-shelf settings during a major period of coastal onlap. The angular unconformity to disconformity, which separates the Qalibah Formation from underlying units, resulted from diastrophism of Late Ordovician to earliest Silurian age. The Early Devonian to Middle(?) Carboniferous Khusayyayn Formation unconformably overlies the Qalibah Formation. The Khusayyayn Formation was deposited in shallow marine, lagoonal, eolian and fluvio-deltaic settings. The unconformity separating the Khusayyayn Formation from the Qalibah Formation documents a period of regional diastrophism of late Early Silurian to late Early Devonian age. Unconformably overlying the Khusayyayn is the Middle Carboniferous through Early Permian Juwayl Formation. This unit was deposited in broad valley complexes excavated in fluvial settings. The base Juwayl unconformity was produced by the Hercynian orogeny. The Permian Khuff Formation unconformably overlies the Juwayl and the former deposited in shallow marine settings. The base Khuff Formation unconformity resulted from a period of major coastal onlap. The Early Triassic Sudair shale succeeds the Khuff and the former was deposited during a period of coastal offlap in lagoonal to coastal plain settings. Near the end of the Early Triassic the Arabian Peninsula underwent another period of tectonism. In central and southwest Saudi Arabia this event is reflected by the erosion of the Sudair shale and older units from the higher portions of reactivated fault blocks and the syndepositional thickening of the Jilh Formation on the down thrown sides of these fault blocks.

Sedimentary facies analysis of the subsurface triassic and hydrocarbon potential in the United Arab Emirates

The Triassic sediments in the subsurface of the United Arab Emirates has been divided into three formations (from bottom to top): Sudair, Jilh (Gulailah) and Minjur. The Sudair Formation consists of four lithofacies units composed mainly of limestones and minor dolomites interbedded with terrigenous shaley mudstones and anhydritic dolomitic limestones. These were deposited in shallow marine supratidal to subtidal settings. The Jilh (Gulailah) Formation has five lithofacies units dominated by anhydritic dolomitic limestone, fine terrigenoclastic sediments and bioclastic and intraclastic limestones. The formation was laid down under lagoonal to supratidal sabkha conditions with little normal marine influence. The Minjur Formation is composed of three lithofacies units characterized by argillaceous quartzitic sandstones, shales, mudstones, dolomitic and ferruginous limestones with thin coal seams. These facies represent deposition in prograding delta lobes, reflecting humid continental to marginal-marine conditions.

Jurassic ammonites from Jebel Tuwaiq, central Arabia with stratigraphical introduction

The Jurassic system as developed along a strike of some 500 miles in the Jebel Tuwaiq, central Saudi Arabia (Nejd), is described and divided into formations, of which the names are here published for the first time, with a synopsis of their lithology and fossil contents. Apart from small collections,'including only one ammonite species, made on camel journeys by H. St J. B. Philby, nothing was hitherto known of these formations; their age (except for the Middle Callovian date given by the ammonite), and the existence of most of them, were unknown until the area began to be mapped and explored geologically about 1940 by geologists of the Arabian American Oil Company, whose results are here summarized. The total thickness of marine Jurassic rocks described exceeds 1000 m., all of it in neritic facies. This sequence is divided, in ascending order, into Marrat, Dhruma, Tuwaiq Mountain, Hanifa, Jubaila and Riyadh formations, above which follows the Cretaceous system. Below the marine Marrat is the Minjur Sandstone (315 m.), of 'continental’ facies, with only obscure plant remains, which possibly corresponds to the supposedly Rhaetic-Lower Lias plant-bearing Kohlan Sandstones of the Yemen. The underlying Jilh formation of Saudi Arabia contains marine Middle Triassic fossils near the top. Collections of ammonites establish the presence of marine Jurassic stages from Lower Toarcian (Lower Marrat) to Lower Kimeridgian (Jubaila); the Riyadh has not yet yielded ammonites at outcrop. The ammonites described are of extraordinary interest from the points of view of palaeogeography, correlation, phylogeny and systematics. While most of the faunas contain just enough links with other parts of the world to establish broad correlations, many are entirely new, or confined, outside Arabia, to Sinai. In the Toarcian there was faunal continuity with Madagascar and Baluchistan, in the Bajocian only with Sinai, from the Middle Bathonian onwards with western Europe. Most of the commonest cosmopolitan genera are missing from central Arabia, where there lived a succession of highly peculiar forms characterized at several successive horizons by unstable suture-lines. It has been found necessary to make eight new genera, besides three already named but hitherto not found outside Sinai, Madagascar and Baluchistan, although employing a taxonomic scale so large that some palaeontologists will consider it old-fashioned. This is believed, however, to be more useful at this stage of our knowledge than a proliferation of generic names based on inadequate material; for the area from which the collections were made is about equal to that of all the Jurassic outcrops of England. The paper concludes with a discussion of problems of evolution, speciation, correlation, and palaeogeography raised by the material. In an appendix are described relevant Bajocian ammonites from Sinai, with a hitherto unpublished section of the strata.