Showing papers in "GeoArabia in 1996"

The Tectonic Evolution of Interior Oman

Abstract: The evolution of Oman’s onshore sedimentary basins from the Late Precambrian to the Present is reflected by six tectono-stratigraphic units. Unit I, the Precambrian basement, represents continental accretion. Units II and III, Infracambrian to Ordovician, may reflect two periods of rifting, possibly related to Najd movements in western Saudi Arabia. The northeast-southwest trending salt basins formed during this time interval. A classical “steer’s head” basin geometry is developed in North Oman, whereas a less complete rift-sag sequence is preserved in South Oman. Of the entire time-span from Late Silurian to Mid-Carboniferous, only little Devonian (Emsian) sediment is preserved. Unit IV, Late Carboniferous to Mid-Cretaceous, reflects the break-up of Gondwana and the creation of the northeastern and southeastern passive margins of the Arabian Plate. Unit V documents intra-plate deformation related to Late Cretaceous continent-ocean obduction in the north and transpressional movements of the Indian Plate in the east. Unit VI, spanning the Tertiary, represents a return to quiet conditions followed by continent-continent collision in the north. Following Late Eocene uplift, the Gulf of Aden rift developed in the south in the early Oligocene, with sea-floor spreading from the Late Miocene onwards. Salt flow and dissolution, both playing a major role in the configuration of most intra- and post-salt hydrocarbon traps in Oman, are episodic and can be related to tectonic events.

High Resolution Sequence Stratigraphy of the Natih Formation (Cenomanian/Turonian) in Northern Oman: Distribution of Source Rocks and Reservoir Facies

Abstract: The Cenomanian of the Arabian Peninsula comprises a carbonate platform setting with rudists, characterized by gradual lateral facies changes including the interfingering of carbonate reservoirs (Natih and Mishrif formations) and source rocks. In order to be more predictive with regard to the distribution and the geometrical aspects of the reservoirs and source rocks, a high resolution sequence stratigraphic study has been carried out in the Adam Foothills of Northern Oman. Based on detailed field sections a correlation scheme covering a transect of 100 kilometers (km) has been established. Three orders of stacked depositional sequences have been found based on the reoccurrence of facies. During long-term increase of accommodation the depositional environment was separated in basinal and platform facies. In contrast, during longer term sea level fall, i.e. long-term decrease of accommodation space, prograding shelfal units extended platform facies over a large part of the basin. The most heterogeneous facies associations are found in times of minimal accommodation space, when incisions and subaerial exposure produce lateral variable strata (e.g. top Natih E). The organic matter is found at the base of two of the three longer term (3rd order) depositional sequences. The organic carbon is contained in marl-limestone couplets (small-scale cyclicity) with a high abundance of oysters and monospecific brachiopod faunas (coquinas). Rudists are found in the progradational part of these sequences, and occur mostly as reworked rudstone layers in meter to decimeter scale, high frequency cycles. The detailed regional correlation depends on the identification of medium- to small-scale (4th to 5th order) depositional sequences which are bounded by regional shifts of the facies belts. The distinct hierarchical organization of the depositional sequences in the Cenomanian, and the relative stability at that time of the Arabian Peninsula, implies a strong correlation potential and thus a broad regional similarity of the architecture of the petroleum systems at that time.

Structural Elements of Onshore Kuwait

Abstract: Five structural trends are recognized in Kuwait: (1) Three sub-parallel anticlinal trends (005°-015°) occur on the west flank of the Kuwait Arch and trap oil in Lower Cretaceous and Jurassic strata. (2) North-south trending structures, including the Kuwait Arch, are probably founded on basement horsts. These were reactivated from Late Jurassic to post-Turonian time and contain the largest oil pools in Kuwait (e.g. Greater Burgan) in Middle Cretaceous, Lower Cretaceous and Upper Jurassic strata. (3) A northwest trend (320°-340°) in north and west Kuwait reflects the structural grain of the underlying Arabian Shelf and while generally dry in Middle Cretaceous strata has proven oil in Lower Cretaceous and Jurassic strata. (4) East-northeast (030°-050°) anticlines are present mid-flank the Kuwait Arch to the west and north. They contain oil in Jurassic and Lower Cretaceous strata, and Middle Cretaceous strata where north-south trends are overprinted. They may be related to northeast trending shear zones. (5) The Ahmadi Ridge is a rare north-northwest contraction trend probably related to the Zagros orogeny and traps oil where it overprints the Kuwait Arch trend. The apparently simple anticlinal oil field structures are cut by normal faults, which are mapped as radial, with throws up to 50 meters but averaging 15 meters. Structural compartmentalization of reservoirs has not been conclusively identified. The faults are near-vertical and often occur in swarms; the majority deform strata below the Mishrif Unconformity while rare faults reach the surface. Reverse throws are evident on seismic and in one well. Dextral offsets along northwest and northeast trending fault and lineaments indicate strike-slip. Wellbore breakouts, processed borehole imagery data and outcrop joint data define a principal maximum stress field orientation of 040°-050° consistent with regional trends.

Permo-Carboniferous Glaciogenic Al Khlata Formation, Oman: A New Hypothesis for Origin of its Glaciation

Abstract: Al Khlata Formation is an oil-bearing reservoir in Oman. Its origin during the Late Palaeozoic Gondwana glaciation in the southern part of the Arabian Peninsula is demonstrated by the glaciogenic deposits of the formation in Oman. Outcrops of the Al Khlata Formation occur in a belt parallel to the Huqf fold axis, the best outcrops being found in the two wadis Al Khlata North and South. In the southern wadi, glacial deposits rest directly on dolomites of the Precambrian Khufai Formation showing northeast-southwest trending glacial striations. Earlier workers proposed that the direction of ice flow was from the southwest to northeast. This is not accepted in this paper, where evidence is presented to support an ice-flow direction from northeast to southwest. Later fluvial flow towards the northeast possibly resulted from the collapse of the continental margin towards the new Proto-Arabian Sea and its subsequent subsidence. We propose that the Early Permian glaciation in Oman resulted from crustal uplift just prior to the calving of a microcontinent along the northeast Arabia margin of Gondwana and the creation of Neo-Tethys. It is suggested that the triple-junction area where the Neo-Tethys and the Proto-Arabian Sea were later to meet, was a site of sufficient thermal uplift to become a center of mountain glaciation.

Sequential Stratigraphy of Outcropping Strata Equivalent to Arab-D Reservoir, Wadi Nisah, Saudi Arabia

Abstract: Two outcrops in Wadi Nisah of Central Saudi Arabia expose sedimentary successions of the Jubaila and Arab formations which are stratigraphic equivalents of the Arab-D reservoir in Ghawar field. These carbonate exposures consist of lithofacies and stacking patterns similar to the succession found in Arab-D reservoir. Nineteen vertical sedimentary cycles were identified by collectively evaluating disconformable relationships, textural characteristics, and lithofacies assemblages. These, in turn, define 3 depositional cycle types: (1) stromatoporoid, (2) skeletal bank, and (3) thinning-upward. The lateral distribution of lithofacies within the stromatoporoid cycle shows that stromatoporoid grainstones form organic buildups flanked by burrowed mud-dominated carbonates. Stromatoporoid banks locally exhibit thickness increases of about 3 meters (10 feet) over a horizontal distance of 500 meters (1, 700 feet). Conversely, the five foot-thick skeletal grainstone capping the skeletal bank cycle forms a continuous sheet exceeding 2 kilometers (1.2 miles) in width. Thinly bedded carbonate sand units within the thinning-upward cycle exhibit pronounced lateral discontinuity. Beds pinch out against topographic highs formed by stromatoporoid buildups over distances of less than a hundred meters (328 feet). Results of the outcrop stratigraphic analysis enable visualization of lateral and vertical stratigraphic relationships and potential fluid-flow pathways. This study demonstrates that outcropping cycles and their lithofacies components have several important implications for ongoing subsurface reservoir characterization and modeling of the Arab-D. Firstly, cycle definition identifies stratigraphic units whose bounding surfaces describe envelopes that constrain the interwell distribution of lithofacies. Secondly, the lithofacies define geometries for the interwell distribution of petrophysical characteristics. Thirdly, the understanding of lateral relationships of these carbonates aids in reservoir simulation modeling.

New Insights into Regional Correlation and Sedimentology, Arab Formation (Upper Jurassic), Offshore Abu Dhabi

Abstract: Correlation uncertainties within the Arab Formation reservoirs of the Ghasha-Bu Tini field were resolved with a combined biostratigraphic and lithostratigraphic study. A chronostratigraphic framework was established based on the type and reference localities elsewhere on the Arabian Peninsula. Construction of a regional biozonation scheme for the entire Arab Formation resulted in a downward shift of the Arab C-D unit boundary by approximately 100 feet in the field, which was reconcilable with a lithological change from anhydrite to dense limestone at that stratigraphic horizon. As a result of the modified Arab C-D boundary, younger boundaries in the Arab Formation were adjusted. These secondary adjustments were assisted by the identification of lithological markers such as thrombolitic horizons, charophytic bands and distinctive anhydrite layers. These horizons were constrained (in a broad sense) by the new biozonation scheme. Ghasha-4 is introduced as a candidate reference well for the Arab Formation of western Abu Dhabi and could be used to adjust reservoir boundaries in other fields. In this manner, not only could regional consistency be ensured, but further insights in exploration could be gained. Evidence for the occurrence of subaqueous sulphate deposits is discussed. Subaqueous anhydrites comprising palisades of subvertically orientated nodules exist at all levels within the upper Arab Formation, but the case is made for thicker, nodular anhydrites interbedded with subtidal stromatolites also having formed subaqueously.

A New Bioevent Stratigraphy of Late Jurassic Arab-D Carbonates of Saudi Arabia

Abstract: A series of fine-scale biofacies cycles are recognised by the application of semiquantitative micropalaeontological analysis of cores in eight wells from the reservoir carbonates from the Arab-D and upper Jubaila formations in Saudi Arabia. They are of Kimmeridgian - Tithonian age, and form the primary producing reservoir of the world’s largest single continuous reservoir located in the Ghawar field of Saudi Arabia. Three new composite biozones are identified that include a basal, deep marine Biozone D3 with fining-upwards cycles which equate, in part, with the Jubaila Formation. Biozone D2 was deposited in shallower conditions of the lower Arab-D carbonate, and Biozone D1 deposited in a very shallow marine environment of the upper Arab-D carbonate; both zones contain multiple coarsening-upwards cycles. Numerous additional bio-events, of potential subzonal value, are detected within these broad zones, and are considered to be related to discrete shoaling-upwards responses to transgressive episodes. The proposed scheme differs significantly from that erected for the Arab-D Formation in offshore Abu Dhabi. Carbonates of the Arab-D member are concluded to be composed of a series of high frequency palaeobathymetric changes which were superimposed upon an extensive carbonate platform. Gradual shallowing of the platform resulted possibly from a reduction in subsidence rate, a fall in sea level, rise in carbonate productivity rate or a combination of these. The latest episode of very shallow marine carbonate sedimentation terminated with the deposition of the Arab-D evaporites.

Rift Basin Sequence Stratigraphy: Some Examples from the Gulf Of Suez

Abstract: Unconformity-bounded sequences within the Miocene strata of the Suez Rift reflect a complex interplay between tectonism and sea level fluctuations. Analyses of Miocene outcrops along the Sinai margin of the Gulf of Suez provide new insights into the sequence stratigraphy of this basin. The Miocene strata can be subdivided into seven major sequences separated by biostratigraphically defined time breaks. These lacunae represent depositional sequence boundaries, transgressive surfaces and condensed sections. These basinwide time breaks were related to major tectonic events from rift initiation through rift climax, and post-rift stages. These events include regional sag and fault initiation, fault linkage, footwall uplift, shallowing of detachment depths and increased fault block rotations, regional isostatic uplift, and thermal subsidence. Superimposed on this complex structural evolution were Miocene sea level fluctuations of a magnitude of several tens of meters to a hundred meters. The Sinai outcrops expose the four oldest Miocene biostratigraphic sequences which correspond to two depositional sequences. The lower sequence consists of the Nukhul Formation which was deposited during a transgression (with the higher frequency events recorded as local erosional surfaces, flooding surfaces, and ravinements) and the Mheiherrat Formation which was deposited during a relative high stand. The upper sequence includes the Asl Formation which was deposited during a low stand and the Ras Budran Member of the Ayun Musa Formation which was deposited during the ensuing high stand.

The role of hydrogeology in Petroleum Development Oman

Abstract: A hydrogeological study of the South Oman Salt basin in particular, and the Oman basin in general, was carried out by Petroleum Development Oman to support basin modelling. The study focused on major aquifer systems: the Tertiary Umm Er Radhuma aquifer and the Paleozoic Haima-Haushi aquifers. Temperature, water quality, radiocarbon age dating and potentiometric data was used to determine the flow, thermal and salinity regimes of the systems. The resulting models were then used to define areas where oil biodegradation due to meteoric water influx can be expected. A review of oil chemistry data confirmed that fields in these areas have the highest API gravity oil demonstrating that hydrogeological modelling can be beneficial in predicting oil quality for prospect evaluation. An additional benefit of this study was the development of a formation water resistivity model for enhancing log analysis techniques.

Fluid Saturation Predictions in a “Transition Zone” Carbonate Reservoir, Abu Dhabi

Abstract: The field is a low amplitude structure with a chalky, Lower Cretaceous, Thamama reservoir characterised by a large hydrocarbon transition zone. Porosity generally decreases with depth within the trap although porosity versus depth trends are skewed by tilting. Porosity and permeability mapping was therefore achieved using templates based on seismic amplitudes. Special core analysis data were used to construct algorithms of Leverett J functions versus saturation for a variety of rock types mapped throughout the 3-D geological model of the field. The templated poroperms were then combined with capillary pressures to predict fluid saturations from these algorithms. The modelling of fluid distributions was therefore dependent upon heterogeneities imposed by the rock fabrics. Calibrating the model-predicted saturations against log-derived saturations at the wells involved regression techniques which were complicated by: notional structural tilting of the free water level, imbibition, hysteresis and permeability averaging procedures. Filtered “stick displays” proved useful in assessing the quality of the calibrations and were invaluable tools for highlighting and investigating data anomalies.

Geostatistical Model for the Arab-D Reservoir, North ’Ain Dar Pilot, Ghawar Field, Saudi Arabia: An Improved Reservoir Simulation Model

Abstract: The North ‘Ain Dar 3-D geocellular model consists of geostatistical models for electrofacies, porosity and permeability for a portion of the Jurassic Arab-D reservoir of Ghawar field, Saudi Arabia. The reservoir consists of a series of shallow water carbonate shelf sediments and is subdivided into 10 time-stratigraphic slices on the basis of core descriptions and gamma/porosity log correlations. The North ‘Ain Dar model includes an electrofacies model and electrofacies-dependent porosity and permeability models. Sequential Indicator Simulations were used to create the electrofacies and porosity models. Cloud Transform Simulations were used to generate permeability models. Advantages of the geostatistical modeling approach used here include: (1) porosity and permeability models are constrained by the electrofacies model, i.e. by the distribution of reservoir rock types; (2) patterns of spatial correlation and variability present in well log and core data are built into the models; (3) data extremes are preserved and are incorporated into the model. These are critical when it comes to determining fluid flow patterns in the reservoir. Comparison of model Kh with production data Kh indicates that the stratigraphic boundaries used in the model generally coincide with shifts in fluid flow as indicated by flowmeter data, and therefore represent reasonable flow unit boundaries. Further, model permeability and production estimated permeability are correlated on a Kh basis, in terms of vertical patterns of distribution and cumulative Kh values at well locations. This agreement between model and well test Kh improves on previous, deterministic models of the Arab-D reservoir and indicates that the modeling approach used in North ‘Ain Dar should be applicable to other portions of the Ghawar reservoir.

Geophysics in Oman

Abstract: To date Petroleum Development Oman has acquired approximately 200,000 kilometers of 2-D and 20,000 square kilometers of 3-D seismic data. Five 3-D and one 2-D crews are operated by contractors. For 3-D surveys PDO uses 2 vibrator source groups recording a “double zig-zag”. The vibrators have GPS systems. The standard spread consists of 4 receiver lines of 120 active channels, spaced 200 meters apart, with receiver spacing of 50 meters. Bin size is 25 by 25 meters, multiplicity is 60 fold and the active spread is 6 kilometers. Most of the seismic data is processed inhouse by CGG. Additionally PDO uses proprietary software for bench-marking, training and attribute studies. Recent lines recorded with vibroseis, rather than thumper, and long offsets (4 to 6 km compared to 2 km) have improved both shallow (1 to 1.5 second) and deep reflections including sub-salt events (>2.5 second). Improved data quality has better imaged exploration targets (e.g. post-salt Haima and intra-salt Athel) and reservoir geometry (e.g. Alkhlata and Gharif). Proper imaging of the Athel silicilyte may require 3-D pre-stack depth migration. The application of other geophysical techniques including gravity, aeromagnetics, seismic attribute analysis and borehole geophysics has also proven useful, particularly where conventional seismic data quality is poor.

Stratigraphic Interpretation of Magnetotelluric Data in Central Saudi Arabia

Abstract: The pre-Khuff interval of the Paleozoic sediments in central Arabia is poorly defined by conventional seismic techniques, although this layer has significant potential for hydrocarbon exploration. Model studies indicated that magnetotelluric methods could outline the regional changes expected in the pre-Khuff in the area, which are largely dependent on the topography of the Precambrian basement. The Hercynian orogenic event created an extensive block-faulted terrane of half-grabens and horsts. The Hercynian structural relief was infilled in the Permo-Carboniferous and faulting reactivated in Triassic and later time, but the relationship between pre-Khuff and post-Khuff structure was impossible to understand using seismic data alone. In this survey of almost 500 magnetotelluric (MT) stations, essential control on the shallower section was provided by seismic interpretations, in addition to well log data for depths and resistivities. The MT method was very successful in confirming the presence of significant pre-Khuff section over some basement structures, as well as defining areas where the section is thin or absent which may be suitable for further exploration for stratigraphic traps.

3-D Land Seismic Acquisition in Saudi Arabia

Abstract: Between 1991 and 1996, Saudi Aramco has acquired more than 8,500 square kilometers of 3-D seismic data in Saudi Arabia. During this time, a universal approach to 3-D acquisition has been developed. The resulting acquisition schemes use a dense source point grid with a low sweep effort per source point, and a high number of recorded channels distributed over a large surface aperture. This sampling strategy results in high fold data. Cost-effectiveness is achieved by ensuring that the source and receiver effort are balanced. Comparisons have shown that increases in surface aperture and fold, cross-line fold in particular, improve the data quality significantly at a marginal increase in cost. The cost per unit of data is made significantly lower even if the cost per unit of time may increase.

Targeting Infill Wells in the Densely Fractured Lekhwair Field, Oman

Abstract: The oil bearing reservoirs of the Lekhwair A-North field in Oman consist of layered low permeability chalky limestones of the Lower Shu’aiba and the Kharaib formations. These have been waterflooded since 1992 by a 200 well development project. However, the field is more faulted and fractured than was anticipated prior to full-scale development and prolific early water breakthrough occurred in many producers (~20%), due largely to direct alignment of injectors and producers along the principal orientation of faults and fractures in the field. As a consequence, the waterflood is currently being converted from a vertical inverted 9-spot well pattern to a line drive orientated parallel to the dominant orientation of faults and fractures in the field. This will result in significantly improved reservoir sweep and oil recovery. Although initially disappointing, production from the field is now well on the way to meeting original expectations. Several options were considered for implementation of the crestal pattern conversion. In heavily faulted and fractured areas, horizontal appraisal wells can aid the targeting of vertical infill wells by providing better definition of the local fracture network. Significant water-conductive features (faults) thus encountered can be targeted as natural injection planes by infill injectors, while infill producers can be better targeted with reduced risk of early water breakthrough. In relatively unfractured areas, the expense of such appraisal is not justified because the risk associated with infill drilling is small. Simple geometric pattern infill is adopted in these areas, with appropriate reference to reservoir simulation models. Significant scope for coiled tubing drilling exists for pattern infill activities, with potential cost savings. Identification of unswept reserves and improved mapping of the fault and fracture network has been achieved through extensive use of FMI/FMS logs in horizontal wells, correlation of production attributes, and production history matching using numerical simulation models.