Unayzah Reservoir Research Articles

DEPOSITIONAL AND DIAGENETIC BARRIERS, BAFFLES AND CONDUITS: PERMIAN – CARBONIFEROUS UNAYZAH RESERVOIR, NUAYYIM FIELD, CENTRAL SAUDI ARABIA

The Unayzah Formation is one of the most important Palaeozoic reservoir systems in Saudi Arabia. In the Nuayyim field, Central Saudi Arabia, it produces light, sweet crude oil and consists of three main reservoir units, in descending stratigraphic order: Unayzah A, B and C. These reservoir units include a wide range of depositional facies deposited under a variety of climatic conditions, from high‐latitude glacio‐fluvial to more temperate playa/lacustrine, floodplain and braided‐fluvial to hot‐arid aeolian environments. Together with the diagenetic changes superimposed on the various depositional facies, this has produced complex reservoir heterogeneity.The effects of this diagenetic and sedimentologic complexity on reservoir quality and compartmentalization are the subject of this paper. Approximately 816 ft of core and 611 core plug samples were examined from three wells which penetrate, completely or in part, the Unayzah reservoir. We combine petrographic and scanning electron microscope analysis with porosity and permeability data and calculated pore throat dimensions to identify fluid conduits, barriers and baffles to fluid flow. A rock classification scheme is proposed which takes into account whether the pore‐level control on fluid flow is due to depositional or diagenetic processes and the composition of depositional or diagenetic phases within the pores. Distinguishing depositional versus diagenetic controls on fluid flow is important for predicting the three‐dimensional geometry of conduits, barriers and baffles, and a priori knowledge of potential reactions between injected fluids and reservoir rocks is important for designing enhanced oil recovery fluids.In the three wells studied, it appears that the Unayzah reservoir is compartmentalized vertically due to the occurrence of diagenetic and depositional barriers and baffles. There is insufficient data to assess the lateral or areal extent of the barriers, baffles and fluid conduits, but the understanding of pore‐level controls on reservoir quality and the rock classification schemes introduced here will provide a starting point for future studies. These studies should combine well logs, seismic and engineering data with data presented here to assist mapping conduits, barriers and baffles across the field. The proposed classification schemes may also prove to be useful for assessing reservoirs in other fields both within Saudi Arabia and elsewhere.

Illitization of Smectite in Sandstones: The Permian Unayzah Reservoir, Saudi Arabia

The transformation of smectite to illite through a mixed-layer illite/smectite is a major clay mineral reaction that occurs in a range of diagenetic environments and is commonly observed in siliciclastic sedimentary basins worldwide. This reaction is controlled by several chemical and physical factors. Two of the major controlling factors are temperature and potassium availability within the sediments. Illite coating in the Permian Unayzah reservoir has two main morphologies, platelet-like and fibrous illite. The former is interpreted as formed by transformation of infiltrated smectite during burial. The latter has likely been formed by precipitation from pore water.

Bimodal pore size behavior of the Shajara Formation Reservoirs of the Permo-Carboniferous Unayzah Group, Saudi Arabia

The sandstones of the Permo-Carboniferous Shajara Formation form the main part of the Unayzah Reservoir in the Greater Arabian Basin. It is divided into three reservoirs, namely from base to top Lower, Middle, and Upper Shajara reservoirs. Mercury intrusion technique was carried out on representative sandstone samples collected from the type section and the three reservoirs are generally characterized as heterogeneous megaporous reservoirs. The best reservoir quality is assigned to the lower sand unit of the Lower Shajara followed by the Middle Shajara Reservoir. One sample collected from the upper part of the Lower Shajara was described as low quality due to its fine grain characteristic and its proximity to the unconformity surface. Reservoir quality is controlled to a large extent by the depositional facies and specifically by rock texture illustrated by petrophysical description. The quality of the three reservoirs of the Shajara Formation, increases with the increase of grain size and grain sorting.

Heavy mineral stratigraphy of the Unayzah Formation and Basal Khuff Clastics (Carboniferous to Permian) of Central Saudi Arabia

ABSTRACT A study of heavy mineral assemblages in the Unayzah Reservoir sandstones of Central Saudi Arabia has identified successive changes in provenance signature. These define four heavy mineral units that are of regional extent and largely coincident with the four main depositional units defined by previous authors: Unayzah C, Unayzah B, Unayzah A and the Basal Khuff Clastics. Sandstone bodies with anomalous mineral signatures also occur, however, especially within the Unayzah B Member. These are attributed to local supply of sand from pre-Unayzah Paleozoic sandstones exposed on the Central Arabian Arch and on intra-basinal highs. The stratigraphic changes in mineralogy reflect successive developments in the geography and climate of the region and in the pattern of sand sourcing and transport. The Unayzah C sands and the majority of Unayzah B sands were derived from the south but whereas the southerly derived Unayzah C sands appear to have been derived from pre-existing mature sandstones, those of Unayzah B were sourced from a wider range of rock types including crystalline basement. This contrast is interpreted as indicating that a significant hiatus may separate the two units. The Unayzah B sands are also characterised by the common presence of apatite, indicating that the source rocks were relatively unweathered. This observation is compatible with the glacial origin attributed to many of the Unayzah B sediments. A further change in provenance signature takes place at the base of the newly recognised ‘un-named middle Unayzah member’, equivalent to the basal part of Unayzah A of previous authors. This is associated with the onset of red-bed sedimentation throughout the area. Unayzah A sedimentation was terminated by a fall in sea level that led to the formation of a widespread unconformity and to the development of deeply incised valleys along the western basin margin. In most of the study area this unconformity corresponds to the base of the Khuff Formation, but in the east of the area, where the succession is more complete, it is believed to occur within the Unayzah Formation, at a level equivalent to the base of the Upper Gharif Member of Oman. By identifying lateral and vertical changes in sand provenance, heavy mineral analysis provides an important additional tool in the stratigraphic analysis of the Permian sandstone succession of Saudi Arabia, both at the regional scale and at the scale of individual reservoir sandstone successions.

Petroleum inclusions atop Unayzah gas condensate reservoir: Signpost for an undocumented chapter of the Arabian Basin filling history?

The Carboniferous-Permian Unayzah gas condensate reservoir of the Ghazal Field in eastern Saudi Arabia contains abundant oil inclusions indicative of a palaeo-oil column. The molecular composition of a fluid inclusion rich zone at the top of the reservoir revealed a mid-mature biomarker distribution (0.80% vitrinite reflectance equivalent, VRE) that is considerably lower than that of the co-occurring reservoired fluid (1.41% VRE). Side chain cracking of the triaromatic steroids evident in the produced condensate is significantly less advanced in the inclusion oil, as is also reflected in their alkylnaphthalene distributions. The lower maturity signal in the inclusion oil is combined with calcareous related biomarkers that include relatively abundant 30-nor-17α-hopane, C 24 tetracyclic terpane and alkyldibenzothiophenes, in contrast to the reservoir hydrocarbon fluid derived from the Lower Silurian Qusaiba shale. Inclusion gas is enriched in the higher hydrocarbons, which might be attributed to a high proportion of oil (or wet gas) inclusions. Fluid inclusion ethane and propane δ 13C are very close to the mean of the produced gases, but carbon dioxide δ 13C is about 5‰, and methane δ 13C 2‰ less negative than the average of the produced Ghazal gases, which may signify more than one hydrocarbon generation/population within the fluid inclusions. An earlier contribution into the Unayzah reservoir from a non-Qusaiba source may therefore be inferred to have had preceded the volumetrically more significant and more mature Qusaiba charges. Further research is needed to elucidate the source of this oil charge and its extent, which could include deep (Cambrian or Neoproterozoic) sources that might be present in the region.

Residual oil as a tool in migration and filling history analysis of petroleum reservoirs, Ghazal Field, Saudi Arabia

Stepwise extraction and molecular analysis of non-recoverable oil from intact reservoir core plugs is used to provide constraints on the accumulation history of petroleum in the Permo-Carboniferous Unayzah reservoirs at the Ghazal Field, Saudi Arabia. When integrated with existing tools such as oil geochemistry, gas geochemistry, stable carbon isotopes, fluid inclusions and thermal modeling, the sequential extraction approach helped decode a previously unrecognized component of the hydrocarbon accumulation history. In particular, it indicates that the region received early petroleum charges that were less mature than currently produced light oil and gas condensates, possibly from the prolific Early Silurian Qusaiba shale source rock and other sources. This has significant implications for prospecting for additional oil, particularly in the north/northwest of the Ghazal region where paleo-oil accumulations may have been displaced or spilled. The lower portion of the Unayzah reservoir (Unayzah-C) remained water-wet during early filling of the upper reservoir unit (Unayzah-A) and appears to have been filled only by a late oil charge. Anomalous molecular and carbon isotopic behavior of residual oil extracts from the uppermost segment of the Unayzah reservoir, where the free oil (first of the sequential extracts) is less mature (0.95% Rc) and isotopically lighter (δ 13C−31.15‰) than the adsorbed oil (last sequential extract, 1.05% Rc; δ 13C−29.65‰), suggests a late oil contribution or bitumen contamination from the overlying Khuff Formation.

Interpretation and mapping of a stratigraphic play using 3D seismic data and attributes in central Saudi Arabia

Interpretation of three-dimensional (3D) seismic data from the northern part of central Saudi Arabia allowed mapping of the Permo-Carboniferous Unayzah Formation's aeolian sand deposits and fluvial channels. These types of depositional environments have been associated frequently with better reservoir-quality sandstones within Saudi Arabia. The depositional setting of the Unayzah reservoirs can be highly variable and range from glacial, to lacustrine, to meandering fluvial, to aeolian, to braided stream. Seismic amplitude data were instrumental in mapping a truncation of the upper Unayzah reservoir against the overlying Pre-Khuff Unconformity. The use of seismic attributes, such as coherence and curvature, indicated the existence of faulting and a fluvial channel network at the top of the Unayzah Formation. The integration of well and seismic amplitude data, via acoustic impedance inversion, revealed the direction of potentially higher porosity fairways within the channel system. The structural and depositional architecture depicted in this study is being appraised through drilling of an exploration well.

A new technology for the characterization of microfractured reservoirs (test case: Unayzah reservoir, Wudayhi field, Saudi Arabia)

This article presents a test case of a new technology using artificially enhanced anisotropy of magnetic susceptibility (referred to here as EAMS) for the characterization of microfractured reservoirs. These are reservoirs in which microfractures are essential to porosity and/or permeability. A conventional geological characterization is costly, time consuming, and difficult to quantify in terms of assessing fracture impact on porosity and permeability. Therefore, an efficient and effective method is required to characterize these microfractures and to determine their contribution to porosity and permeability. The EAMS technology, which we developed and tested, allows rapid analysis that bridges reservoir geology and engineering. Using petrography, the margin of error to detect microfractures that impact porosity and/or permeability is 43%; however, it requires three times the sampling rate of the new EAMS technology. The lower part of the Unayzah reservoir (Unayzah-B/C) in the Wudayhi field, Saudi Arabia, where fractures were studied and microfractures are known to impact reservoir performance, is used to develop and verify the EAMS technology. The results show that EAMS-derived microfracture fabric strikes east-northeast–west-southwest, consistent with that obtained by geological means. The effective-porosity profile obtained from EAMS tests is similar to that of the conventionally acquired porosity. Open microfractures in tested samples increase mean values of reservoir effective porosity by 36–50% in Unayzah-B/C. The occurrence of connected microfractures is estimated to cause an increase in average permeability of 75% in Unayzah-B/C. This is in agreement with the fact that wells in microfractured Unayzah-B/C have 4.5–14 times the productivity of wells as nonfractured sections of this reservoir. A maximum permeability trend of northeast-southwest permeability anisotropy is detected. The implementation of the EAMS technology in other fields with microfractured reservoirs will directly impact operational and simulation effort.

Paleozoic petroleum systems of Saudi Arabia: a basin modeling approach

ABSTRACT The major Paleozoic petroleum system of Saudi Arabia is qualitatively characterized by a proven Silurian (Qusaiba Member, Qalibah Formation) source rock, Devonian (Jauf Formation), Permian and Carboniferous (Khuff and Unayzah formations) reservoirs, a laterally extensive, regional Permian seal (basal Khuff clastics and Khuff evaporites), and four-way closed Hercynian structures. Hydrocarbons found in these systems include non-associated gas in Eastern Arabia and extra light oil in Central Arabia. A basin modeling approach was used to quantify important aspects of the petroleum system. Firstly, seventeen regional wells were selected to establish a reference tool for the three-dimensional (3-D) basin model using multiple one-dimensional (1-D) models. This was accomplished by studying core material from source rocks and other lithologies for thermal maturity and kerogen quality. The major emphasis was on the Silurian section, other Paleozoic intervals and to a lesser extent on the Mesozoic cover from which only few samples were studied. Although vitrinite macerals, solid bitumen, and other vitrinite-like particles were not abundant in most of the investigated samples, enough measured data established valid maturity-depth trends allowing for calibrated models of temperature history. Sensitivity analyses for maturity support the view that thermal boundary conditions and Hercynian uplift and erosion did not greatly influence the Paleozoic petroleum systems. Secondly, a 3-D basin model was constructed using major geologic horizon maps spanning the whole stratigraphic column. This model was used to gain insight into the general maturity distribution, acquire a better control of the model boundary conditions and investigate charge, drainage, migration and filling history of the main Paleozoic reservoirs. The 3-D hydrocarbon migration simulation results qualitatively account for the present gas accumulations in the Permian-Early Triassic Khuff and Carboniferous-Permian Unayzah reservoirs in the Ghawar area. This kind of study illustrates the importance of basin modeling when used with other geologic data to describe petroleum systems. It provides a predictive exploratory tool for efficiently modeling hydrocarbon distribution from known fields. Real earth models can only be described in 3-D as pressure variations and fluid movements in the subsurface are impossible to address in 1-D and 2-D domains.

ABSTRACT: Data Integration for Unayzah Reservoir Characterization: Saudi Arabia

ABSTRACT: Data Integration for Unayzah Reservoir Characterization: Saudi Arabia

Geochemical variations among eastern Saudi Arabian paleozoic condensates related to different source kitchen areas

Large reserves of gas and condensate are found in Paleozoic reservoirs (Khuff, Unayzah and Jauf) in the Ghawar area of eastern Saudi Arabia. The source rock for these hydrocarbons is the basal organic-rich “hot shale” of the Qusaiba member of the Silurian Qalibah Formation. This is the same prolific source rock that generated the sweet light crude oil and condensate found in the Unayzah reservoirs in central Saudi Arabia. Thermal maturity modeling of the source rock identifies two kitchen areas: to the east and west of the Ghawar structure. The west kitchen was well into the oil window by 210 Ma, while the east kitchen was just entering the oil window. Large areas of both kitchens were in the wet gas window by 125 Ma and by 90 Ma most of the area north of Ghawar was generating dry gas. In eastern Saudi Arabia, Paleozoic oil and condensate are found in early-formed structures (Carboniferous and early Triassic growth), whereas late Cretaceous and Tertiary structures contain only dry gas. The geochemistry of the oils and condensates indicates two groups, each generated from a different hydrocarbon kitchen, suggesting that the two kitchens differ slightly in organic facies. Within each group, variations among hydrocarbons are related to the thermal maturity of the source rock during generation. Within the Khuff reservoir, gas souring is caused by thermochemical sulfate reduction.

Degradation of Reservoir Quality by Clay Content, Unayzah Formation, Central Saudi Arabia

ABSTRACT Reservoirs in Central Saudi Arabia are important new sources of light sulfur-free crude oil. Development work on these wells has shown the adverse effect of authigenic clay minerals on their productivity or injectivity. The clay minerals may be affected during drilling of the well, during completion, during production, and/or injection. Kaolinite, chlorite, illite, and ordered and random mixed layer illite/smectite are commonly occurring authigenic clay minerals within the Permian Unayzah reservoir rock. As little as 2 weight percent clay causes permeability to be drastically reduced. Formation damage due to clay minerals results from: (1) swelling of smectite caused by injection water or drilling fluids of low salinity; (2) clay mineral migration and subsequent pore blockage; (3) precipitation of gelatinous pore-blocking iron hydroxides caused by the dissolution of chlorite by acid; (4) high water saturations; and (5) disaggregation of poorly consolidated parts of the reservoir into its component sand grains if the weak clay cements are disturbed. The implementation of clay control measures has resulted in increased production while minimizing formation damage.

Stratigraphy and Sedimentation of the Unayzah Reservoir, Central Saudi Arabia: ABSTRACT

Stratigraphy and Sedimentation of the Unayzah Reservoir, Central Saudi Arabia: ABSTRACT