Fifth EAGE Arabian Plate Geology Workshop 2015

The Middle to Upper Triassic Jilh Formation is a thick predominantly carbonate-evaporite succession extending across much of the central Arabian Basin in eastern Saudi Arabia. The Jilh Dolomite Member shows the best reservoir potential forming a 90-100 ft. thick blocky and laterally persistent unit in the upper part of the Jilh succession. It is dominated by very fine to medium grained fabric destructive dolomite originally deposited in lagoon to shoal settings following a period of transgressive flooding and backstepping of the depositional facies. Thinner beds of dolomite, anhydrite, shale and dolomitic limestones occur above and below the Jilh Dolomite suggesting deposition within restricted peritidal settings. Measured porosities in the Jilh Dolomite range from poor to excellent with permeabilities ranging from poor to very good. Macropores are dominated by relatively large mouldic/vuggy pores and finer intercrystal pores that show a complex patchy or non-uniform distribution. The origin of the pores is difficult to determine; some pores may be relicts of the precursor limestone, some may have formed during the dolomitisation process and some may postdate dolomitisation. The Jilh Dolomite was probably dolomitised during relatively early burial by descent of magnesium-rich fluids from overlying anhydrites. Growth zonation, seen mainly within euhedral dolomite rhombs closely associated with macropores, suggests multiple phases of dolomite precipitation. This has led to a significant loss of porosity within the Jilh Dolomite Member. In contrast, facies within the rest of the Jilh Formation are likely to have been dolomitised by seepage reflux due to their close association with laminated anhydrites. This type of dolomite tends to be fabric preserving.

This assessment focuses on regional 3D basin modeling through integration of a stratigraphic framework, newly defined gross depositional environments (GDEs) and lithofacies maps of Triassic and Jurassic source rocks, reservoirs, and seals that were revealed by integrating seismic interpretations and geological and petrophysical mapping in northeastern Saudi Arabia. The Triassic Jilh and Minjur formations represent two separate third-order composite sequences, consisting of proven source rocks, reservoirs (e.g., Jilh Dolomite and Minjur Sandstone), and seals. The Lower Jurassic Marrat Formation and Middle Jurassic Dhruma Formation represent a third-order and a second-order composite sequence, respectively. Proven Dhruma source rocks (equivalent of Sargelu source rocks) and a variety of grainier carbonate reservoir rocks, spanning the Marrat, Faridah, Sharar, and Lower Fadhili reservoirs, were taken into account as inputs. Well-defined depth grids, derived from regional seismic and well-based mapping and integrated GDE/lithofacies maps of Triassic and Jurassic source rocks, reservoirs and seals, have been utilized for constraining this newly built 3D basin model. This integrated 3D basin modeling has resulted in significant insights for hydrocarbon migration and charge of the Triassic and Jurassic petroleum systems. The Triassic hydrocarbon accumulations are primarily self-sourcing from Jilh source rocks, with exceptions where hydrocarbons are charged from Paleozoic source rocks due to breaching at high-relief structures (e.g., Northeastern Arabian Offshore). The Dhruma source rocks (Sargelu equivalent) primarily charged a variety of Lower and Middle Jurassic reservoirs. Additionally, basin modeling results support insights on HC migration and charge for exploration concepts related to stratigraphic traps and new play fairways.

The Triassic Middle and Upper Mahil Members in the Oman Mountains represent outcrop equivalents of the Sudair and Jilh Formations. This study focuses on sedimentologic descriptions, sequence stratigraphic interpretations and correlations of 8 outcrop sections around the area of al Jabal al-Akhdar in the Sultanate of Oman. In the Oman Mountains, the facies were deposited in a predominantly restricted inner ramp setting. In more central parts of the Arabian Plate, the Sudair and Jilh Formations are traditionally known to form important seals for hydrocarbon accumulations in underlying Permo-Triassic sequences. However, along the former basin margin, represented by the outcrops in the Oman Mountains, several potential reservoir units could be identified in the equivalent outcrop formations: Meter-thick oolitic/peloidal shoal grainstones in the Middle Mahil (Sudair equivalent) and intraclastic-rich peritidal grainstones in the Upper Mahil (Jilh equivalent) present new potential targets for future exploration. The cyclic nature of vertical facies changes is well reflected in the outcrop weathering profile permitting sequence stratigraphic interpretations in a hierarchical manner. In order to capture lateral facies and thickness variations within the sequence stratigraphic framework, various wall panels and correlations were constructed on small (meters) and large scale (hundreds of meters/kilometers) and integrated into a 3D facies model.

Triassic and Jurassic carbonates across the Middle East host large amounts of hydrocarbons and might serve as reservoirs for CO2 storage in the future. The Triassic-Jurassic transition was marked by global changes, including carbon-cycle perturbations and the opening of the Atlantic Ocean. These changes were accompanied by one of the major extinction events of the Phanerozoic. In this study we investigate the evolution of a carbonate ramp across the Triassic-Jurassic boundary on the Musandam Peninsula in a series of wadis approximately 35 km apart (Wadis Naqab, Ghalilah and Al-Ghabbah) in the United Arab Emirates. In the proximal part of the ramp the Triassic benthic carbonate factory dominated by automicrite and microbialites was replaced by an ooidal-bioclastic ramp during the Jurassic. In the more distal part of the ramp, microbialites with abundant siliciclastic detritus are overlain by muddy carbonates with few oolites. The very abundant clastic components are being sourced most likely from the Arabian Craton. The change in the carbonate factory across the Triassic Jurassic transition, especially evident in the proximal part of the studied carbonate ramp is the result of an important change in palaeoenvironmental conditions. The carbon-cycle perturbations across the Triassic-Jurassic boundary have been recorded in carbon isotope curves from bulk carbonates, organic carbon and fossil wood in several Tethyan locations and have been used for chemostratigraphic purposes. Our site was located at the equator and on the southeastern margin of the Tethys throughout the Late Triassic and the Early Jurassic, and this study provides the first constraints of environmental changes at the low-latitudes for the Triassic-Jurassic boundary. Our stable isotope results on micrite show a prominent negative shift in carbon isotope values of approximately 2 ‰ just below the inferred position of the Triassic-Jurassic boundary. A similar isotopic trend is also observed across the Tethys with amplitudes ranging from ~2 ‰ to ~4 ‰. These results seem to indicate that the neritic carbonates from our studied section can be used for chemostratigraphic purposes. We gratefully acknowledge funding from Qatar Petroleum, Shell and Qatar Science & Technology Park.

Jilh carbonate succession represents Middle to Late Triassic stratigraphy in Kuwait. It is deposited in shallow marine inner ramp and restricted settings. Three major lithofacies units are recognized based on well data: Jilh Lower Carbonates which is overlying the Early Triassic Sudair Formation, Jilh Dolomite in the middle and Jilh Upper Carbonates. A thin salt layer, near absent at places, separates the bottom unit from the upper ones. The entire sequence from top to bottom is characterized by numerous thin anhydrite layers, exhibiting prevalence of arid climate. Whereas Jilh Dolomite and Upper Carbonate sequences are regionally consistent with near uniform thickness, the bottom unit occurs with highly varying thickness ranging from 300’ to 800’. Increase in thickness is seen mainly in the western parts of Kuwait where it is also referred to as ‘Kra AlMaru’ Unit. It comprises mainly dolowackestone and dolomudstones. Good amount oil and gas flowed from these reservoirs during initial testing in a couple of wells. Therefore, the depositional pattern may be interesting from the oil exploration point of view.

The Middle Jurassic Dhruma Formation is of Bajocian-Bathonian age, and represents a composite second-order sequence in Saudi Arabia. An integration of well log correlations, 3D seismic interpretations, and sedimentological studies has been undertaken to unravel the sequence stratigraphic framework. The Lower Dhruma Member is interpreted as a transgressive systems tract, with the maximum shale content of the “Dhruma Shale” representing the maximum flooding surface (MFS) of the second-order Dhruma sequence. The highstand systems tract of this second-order sequence consists of an overall shallowing-up interval comprising the Faridah, Sharar and Lower Fadhili members, with each member representing a third-order composite sequence. The Faridah grainy carbonate reservoirs that were deposited in the highstand systems tract of the third-order Faridah sequence can be subdivided into the following fourth-order sequences: Lower-, Middle- and Upper-Faridah subunits. The grainier facies of the Faridah highstand changes laterally to muddy limestone and organic-rich mudstone facies (Sargelu-equivalent source rocks) in basinal areas. The top of the Faridah sequence represents a subaerial exposure surface across most of the Arabian carbonate platform, in response to a significant base-level fall. The Sharar third-order sequence comprises a basal lowstand wedge (reworked sediments from carbonate shoaling complexes), transgressive deposits with muddy facies and possible organic-rich facies in the distal and basinal setting; and also a highstand grainy reservoir facies. The third-order Lower Fadhili sequence is dominated by widespread grainier carbonate facies, indicating infilling of most of the accommodation space. The top of the Dhruma Formation (i.e., the top of the Lower Fadhili Member) represents a significant subaerial exposure surface (hiatus). Gross depositional environment (GDE) maps of the Dhruma Formation have been reconstructed by integrating core/drill cutting sedimentology, well log electrofacies, and seismic attributes. These GDE maps depict evolving depositional environments that span the Faridah, Sharar and Lower Fadhili third-order sequences. A variety of new exploration opportunities have emerged through building a robust stratigraphic framework and reconstruction of gross depositional environments, and detailed play fairway mapping.

Field development in complex carbonate evaporite reservoirs has been a challenge for geoscientists by using simple sedimentology based geological models. A high resolution sequence stratigraphic framework has been designed from a conceptual 3D depositional model to a deterministic predictable model. In this process framework boundaries have been fine-tuned with carbon and oxygen isotope signals and defining ichno-facies associations to map lateral continuity of member formations within Marrat sequence. As dolomites and porous grain stones are the key reservoir quality rock types for Marrat Group of formations, position of precursor lithofacies in sequence stratigraphic frame work is found to be crucial. Dolomite samples are analyzed for d13C and d18O. Upper and Lower Marrat members are characterized by thinly laminated, micro crystalline exposure dolomites and high in d18O signal which are associated to low stand packages, whereas Middle Marrat dolomites are relatively low in d18O and high in d13C are associated to high stand precursor facies. Moreover, typical ichnofacies association of porous dolomites defines vertical and lateral connectivity of flow zones in Middle Marrat reservoirs. The reliability of this model has been established through recent tested intervals, with PLT and MDT data integration demonstrates predictability of flow zone connectivity.

This core- and log-based analysis provides a depositional and stratigraphic synthesis of the Faridah compartment in the area. It brings together data from a number of cored wells, totaling 1950 feet in length. These wells are located in the onshore and adjacent coastal waters. The Middle Jurassic Dhruma Formation is stratigraphically positioned between the Tuwaiq Mountain and Marrat formations, respectively, and consists predominantly of carbonates and a minor amount of shale. The formation has been subdivided into six members; D1-D6, with D-4 containing the Farida layers. The type section of the Faridah in the study area is the FRDH-A well, where the upper part of the section comprises four stacked and distinct layers, termed the Faridah-D, -C, -B and -A, in ascending stratigraphic order. The grainy-rich layers are considered to represent the upper part of the Middle Dhruma D4 Member. The sedimentologic analysis and lithologic breakdown of the Faridah sediments resulted in the identification of twelve primary lithofacies. These are designated LF1 to LF12 in broadly shallowing upward order. The lateral lithofacies distribution suggests that environments of deposition are in line with platform interior to shelf margin to basin. The stratigraphic architecture and facies stacking patterns suggest that the Faridah spans parts of at least two long-term depositional sequences, together forming a large-scale northward-prograding and thinning shelf-margin succession. This thinning trend is accompanied by a gross change in depositional facies, from (a) thick platform-top peloid-oncoid packstone and grainstone, to (b) shelf-edge micro-gravels, coral boundstones and slumped foreslope facies, to (c) thin bituminous basin-floor laminites in the northern parts. The Dhruma D4 stacking patterns comprise four correlatable high-frequency depositional cycles (Faridah D-A layers), representing a mid- to late-highstand systems tract that is genetically-linked to the underlying Dhruma D3 TST and early HST. These four cycles progressively thin basin-ward, and their correlatability becomes insecured and less distinguishable. The best quality rocks recorded in the area are associated with the Faridah shelf-margin grainstone facies at Faridah, Ash Shihiyah and southern Juraybi’at areas, with excellent porosities and fair to good permeabilities. The northward transition from clean porous shelf-edge facies to tight basin-floor laminites occurs over a distance of just a few tens of kilometers, and provides opportunities for stratigraphic trapping in the area.

Digital outcrop models provide important high resolution and quantitative sedimentological data unavailable or difficult to be obtained from the subsurface formations. The Jurassic succession contains several important reservoirs that have excellently exposed stratigraphically equivalent units in central Saudi Arabia. This study provides an outcrop-based case study dealing with Late Triassic to the Middle Jurassic of Minjur, Marrat and Dhurma formations. The study was carried on selected outcrops around Riyadh in central Saudi Arabia. The main objective of the study is to investigate the sedimentological heterogeneity, establish the stratigraphic hierarchy and the reservoir zonation and eventually to build up digital outcrop model. The study was based on sedimentological field observations, Gamma Ray measurements, terrestrial laser scanning or LiDAR data and laboratory analysis. The products of this study are high resolution digital outcrop models for these formations. This final integrated model was capable to reveal detailed description of the sedimentology, the stratigraphic hierarchy, the spatial stacking pattern, distribution of the lithofacies and cyclicity within each formation. The model also helped to characterize and correlate the reservoir and the non-reservoir units and to reveal types of depositional heterogeneity and their lateral continuity and architecture. The models might contribute to understand and reveal micro to macro inter-well sedimentology and heterogeneity and help to and resolve any related uncertainties. The final digital models could act as a geological database and if integrated with subsurface data would enhance our understanding and prediction for the reservoir quality and architecture of these formations.

This paper describes the planning, execution, data acquisition and successful completion & well test of a horizontal well into a ultra thin Mafraq carbonate reservoir in the north of Oman. The reservoir spreads laterally like a blanket. Five vertical well tests into the same reservoir had a production range from 0.05 to 0.5 MMm3/d. This range could not be explained. A horizontal well was the only way to access the volume. The learning from a unique outcrop analogue and the vertical appraisal wells have been used to place a successful horizontal well with a stable production at a rate which succeeded the expectations. The key to the success was the integration of all disciplines during the planning and real time monitoring during the execution phase. Without constant interaction and challenging between the disciplines, this would not have been possible. Special core test for stimulation planning, newest LWD tools for steering and formation evaluation are the key novel technologies.