Stylolites in Lower Cretaceous Carbonate Reservoirs, U.A.E.
Published:January 01, 2000
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Abdulrahman S. Alsharhan, James L. Sadd, 2000. "Stylolites in Lower Cretaceous Carbonate Reservoirs, U.A.E.", Middle East Models of Jurassic/Cretaceous Carbonate Systems, Abdulrahman S. Alsharhan, Robert W. Scott
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The major Lower Cretaceous reservoirs of the United Arab Emirates (U.A.E.) are characterized by stylolite-rich zones. Although their distribution and frequency are variable, stylolites tend to be most common and abundant toward the flanks of the fields. Three main types of stylolites are found in these reservoirs, each characterized by variations in amplitude, morphology, lateral continuity, and thickness of the accumulated insoluble residue. The recorded stylolites are classified as rectangular or high amplitude, solution seams or wave-like, and wispy seams or horsetail. The composition of the relatively insoluble seam material is variable, and depends mainly upon the composition of the nearby host rock. Most of the seams are composed of clay minerals, black bitumen, pyrite, and fine- to medium-grained calcite crystals or dolomite rhombs. Stylolites affect the petrophysical characteristics (porosity/permeability) and thickness reduction (compaction). The lower porosity and permeability values are found associated with the well-developed stylolites. Most stylolites observed in cores are parallel to subparallel to bedding (the horizontal type), indicating the predominance of vertical stress imposed by overburden pressure in stylolite formation. This also suggests a relative absence of tectonic or metamorphic activity in the area, which might produce inclined to vertical stylolites, although vertical to subvertical tension fractures are found associated with the well developed stylolites.
Stylolites are lithologic manifestations of pressure dissolution in carbonate reservoir rocks. Stylolitization usually results in reduction of unit thickness and also in porosity and permeability of the reservoir (Dunnington, 1967; Johnson and Budd, 1975; Koepnick, 1984; Alsharhan, 1985; Oswald et al., 1995). This paper focuses on stylolite-bearing zones in the Lower Cretaceous carbonate reservoirs in the U.A.E. The objectives are 1) to document the distribution and morphology of the stylolites, 2) to evaluate the importance of stylolites as barriers to fluid flow, 3) to evaluate the relationship between porosity, permeability, and stylolite distribution in the reservoir, and 4) to evaluate effects of stylolitization and fracturing on porosity and permeability of carbonate reservoirs in the Lower Cretaceous Thamama Group, one of the main producing carbonate formations of the U.A.E. Petroleum production is obtained from large anticlines in these limestone reservoirs (Fig. 1).
In the U.A.E., the Lower Cretaceous Thamama Group is characterized by interbedded porous and dense limestones (Fig. 2). Porous hydrocarbon-bearing limestones are divided into several zones. Each zone is subdivided into several reservoirs separated by dense impermeable to poorly permeable limestones. The latter act as barriers to vertical fluid migration and are of prime importance in oil production efficiency and reservoir development. The stylolites observed in the Thamama Group occur along lithologic transitions or partings between these two lithotypes.
The Thamama Group ranges in thickness from 800 to 1000 m, and includes beds of Berriasian to Aptian age, a time span of nearly 30 million years. It consists of four formations: the Habshan, Lekhwair, Kharaib, and Shuaiba formations, in ascending order (Fig. 2). This thick sequence mainly consists of shallow shelf carbonates, although some deep-water carbonates occur locally in association with an Aptian intrashelf basin (Alsharhan and Nairn, 1993).
Materials Studied and Methods Used
This study is based on Lower Cretaceous reservoir core samples from the U.A.E. fields shown in Figure 1. Data from these cores are presented to illustrate the field-wide distribution of stylolites in selected reservoirs. Also, an analysis of the wireline log response to stylolite-bearing intervals is presented to identify stylolite-bearing zones in uncored wells. Core porosity and permeability data from some wells are used to produce a composite log to show the relationship between the petrophysical characteristics (porosity and permeability) of the stylolites and log response. One hundred thin sections were studied using the polarizing characteristics of the stylolite zones in the studied cores.
The stylolite intensity (SI) approach of Johnson and Budd (1975) is also used to evaluate stylolite-bearing horizons. This relationship is expressed by the empirical formula where H = thickness of stylolite zone on logs (feet), and ϕ = porosity within stylolite interval (%). This assumes that the petrophysical character of the rocks is related to stylolite development, but in many cases, this is not the case.
A stylolite intensity value was obtained for each stylolite-bearing horizon in each well. These wells were related to adjacent ones, including those with measured vertical permeability, and a map was prepared for each major stylolite zone. This provides a means to analyze the effectiveness of the zone as a barrier or impediment to fluid movement (Fig. 3). These stylolite intensity maps also can aid the development of reservoir simulation and field development of plans, especially where secondary-recovery water or water-injection programs are envisioned.
The percentage of compaction due to stylolitization (which also gives the minimum thinning) of a stylolitized limestone unit is calculated by comparing the average porosity before and after stylolitization by use of the formula developed by Dunnington (1967): where ϕ is the present porosity outside the stylolite zone, is the present porosity within the stylolite zone, and T is the percentage thinning of the unit due to stylolitization.
The porosity outside the stylolite zone (ϕ) implies only short-distance transport of solute. In fact, stylolite-generated cementation may occur well away from an obviously cemented stylolite horizon. The formula implies that all porosity loss, relative to adjacent units, is stylolite related. In many cases, stylolites develop within or along beds that have been cemented at the sea floor. In this case, use of this formula requires caution.
The amplitude of stylolites was measured in the core samples, from the apex to the lowest point of the stylolites. It was found that the measured amplitude varied with the type of stylolite, e.g., the rectangular types had the largest amplitude and the wispy seams had the smallest.
In order to determine whether stylolite type is related to degree of pressure dissolution and loss of unit thickness, we examined many cores from the Shuaiba and Kharaib formations. Also, seven cores containing well-developed examples of the three stylolite types from the Bu Hasa, Bab, Asab, and Zakum fields (Fig. 1) were selected for analysis of mineralogy and trace-element geochemistry. Three cores from the Kharaib Formation in the Bab, Asab, and Zakum fields displayed rectangular stylolites and wave like stylolites, and one core from the Shuaiba Formation in the Bu Hasa Field displayed horsetail stylolites. Each core was first spot sampled using a 1 mm stainless steel drill to isolate portions of host micrite, sparry cement, and insoluble stylolitic residue within the core at various stratigraphic distances from a stylolite. Location of spot samples was aided using petrographic thin sections to locate relatively pure regions of each core. These spot samples were split analytically in preparation for mineralogie and trace-element analysis. One split was powdered by slow grinding in acetone using a diamond mortar, and analyzed on a General Electric XRD7 Xray diffractometer using copper Kα radiation and scanning over a 2θ range of 0 to 60 degrees. Refraction data were collected digitally and analyzed using dedicated software to correlate each peak with a specific mineral from the JCPDS files (American Society of Testing and Materials, 1967).
The second split of each subsample was analyzed semiquantitatively for a variety of trace metals. Powdered samples were extracted for geochemical analysis using analytical microwave extraction in HN03 following the standard United States Environmental Protection Agency analytical test methods 6010/200.8, 7471, and 8080 (http://www.epa.gov/Standards.html) using a CEM MD2000 microwave digestion apparatus. Solutions were analyzed in vacuum using an ARL 2410+ ICP, and all results are reported in parts per million dry weight or as ratios normalized to calcium content. Twelve trace metals (Al, Be, Cr, Cu, Fe, Mg, Mn, Na, Ni, Pb, V, Zn) were found in analytically detectable concentrations (10x d.l.) in most spot samples, and were chosen for more detailed study of stratigraphic core subsamples. A 1-cm-wide longitudinal section was cut from each core oriented perpendicular to the stylolite, and this core section was then divided into 1 cm subsamples to allow analysis of portions of the core at various stratigraphic distances from the stylolite. These core subsamples were powdered and prepared for analysis of the concentration of the twelve selected trace metals. The mean concentration of each trace metal in the stylolitic zone was compared statistically to the mean concentration in the host rock above and below the stylolitic interval using the t-test (equal variance assumed). Results of these analyses, reported as ratios normalized to calcium content, are summarized in Table 1.
|Trace Element||Rectangular Stylolites||Wave-like Stylolites||Horsetail Stylolites|
|Be||very low||very low||very low|
|Na||very low||very low||very low|
|Mn||very low||1.6X||very low|
|Zn||1.5X||very low||very low|
|Trace Element||Rectangular Stylolites||Wave-like Stylolites||Horsetail Stylolites|
|Be||very low||very low||very low|
|Na||very low||very low||very low|
|Mn||very low||1.6X||very low|
|Zn||1.5X||very low||very low|
Origin and Classification of Stylolite Types
The timing of stylolite formation is still controversial. Several authors have accepted a pre-induration origin (Shaub, 1939; Prokopovich, 1952; Park and Schot, 1968a, 1968b), but others have postulated a post-induration origin (Stockdale, 1943; Dunnington, 1954; Brown, 1959; Manten, 1968). A third group has claimed that although some stylolites are of pre-induration origin, the well-developed, sutured, rectangular types are of post-induration origin (Wanless, 1979; Buxton and Sibley, 1981), whereas Shinn and Robin (1983) suggested that those in fine-grained muds form simultaneously with the onset of lithification.
An early idea on the formation of stylolites was provided by Marsh (1867), who refuted the “wood” theory of Eaton and the “salt” theory of Vanuxem, which had prevailed since the 1830s, and instead concluded that stylolites could be ascribed to differential vertical compression before lithification. The argument for pressure dissolution was analyzed by Stockdale (1922), who concluded that stylolites form in lithified rocks and that clay caps result as a solution residue. He calculated the magnitude of vertical shortening in rocks in Indiana (USA) by measuring the thickness of the clay cap and clay concentrations in host rocks. Shaub (1939) contradicted Stockdale’s (1922) solution theory and claimed that the clay cap is sedimentary and that stylolites form while sediments are still plastic and beginning to dewater. Prokopovich (1952) suggested that stylolites form in soft sediments on the sea bottom in local areas where pH is low. Deelman (1975) proposed that plastic deformation, due to crystal lattice dislocation rather than pressure dissolution, caused post-lithification stylolites. Garrison and Kennedy (1977) demonstrated that flaser structure in the Upper Cretaceous chalks of Southern England probably predated stylolite formation and resulted from interaction between compaction, dissolution, and partial lithification.
For each stylolite type, the mean Me/Ca ratio for each element in the stylolite zone was compared to the mean ratio in the host rock above and below the stylolite. If this difference was statistically significant (95% confidence level using t-test; equal variance assumed), the mean concentration factor from all cores analyzed is shown. Both rectangular and wave-like stylolites show significant concentration in the stylolite zone of a similar suite of trace elements, although the degree of concentration varies substantially for some elements (Fe, Ni, Al, Mg). Horsetail stylolites show no significant concentration values for all elements.
It would appear that stylolites commonly, but not always, form along lithologic transitions or partings as a result of stress due to either overburden pressure or tectonism. Stress causes an increase in pressure, and when a critical level is reached, rock components start to dissolve in order to relieve the stress. Although the irregular shape of stylolites is initiated by differential solubility of the rock components, they are not completely irregular, and some have a wave-like form or a periodicity in them.
The conditions that determine the initial generation of stylolites are not well understood. Experiments carried out by Rutter (1983) showed that it is extraordinarily difficult to reproduce natural microstructures, whereas Shinn and Robin (1983) were able to produce wispy stylolite-like seams of organic matter in a series of sediments by compaction processes. Buxton and Sibley (1981) showed that stylolites may also abut porous zones, implying low gradients and transport over relatively long distances away from the sites of dissolution. The type of stylolites observed by these authors reflect the degree of cementation at the time of dissolution.
In this study all stylolite-rich zones observed in the Thamama Group are parallel or subparallel to bedding (termed horizontal or stratiform). The presence of horizontal stylolites indicates the absence of tectonic activity. Tectonism would give rise to inclined and /or vertical stylolites, and this fabric is documented in many parts of the world by Dunnington (1954, 1967), Park and Schot (1968a, 1968b), Bathurst (1975), and Wanless (1979). Purser (1984) attributes the difference in intensity of stylolites between the flanks and crests of the Abu Dhabi oilfields to the distribution of early diagenetic processes. He suggested that the reservoirs situated on paleohighs were more susceptible to early cementation (or leaching) than those on the flanks of the structures, which remained submerged and received less preburial cement.
Characteristics of Stylolites and Their Types
Stylolites are undulose surfaces, often but not always highly irregular on a scale of centimeters. They occur at surfaces along which dissolution of calcium carbonate occurred as a direct result of stress exerted by the weight of overburden (Nelson, 1979; Burgess and Peter, 1985). Study of the development of stylolites in the Lower Cretaceous carbonate reservoirs in U.A.E. field areas is of importance to the understanding of the effective reservoir capacity. Clearly a better definition of the sedimentologic, petrographic, and diagenetic character of rocks would help in the evaluation of the total effective pore spaces in each reservoir so as to evaluate more accurately its hydrocarbon content.
Stylolite amplitude is generally considered to be a measure of the minimum thickness of the sedimentary section removed along a pressure-solution surface (Stockdale, 1922). As pressure solution proceeds, insoluble constituents gradually accumulate along the solution surface. As this residue increases, significant reduction in fluid cross-flow can occur. Measurement of stylolite amplitude generally provides a minimum estimate of section removal, i.e., an estimate of the minimum magnitude of section thinning caused by pressure solution. The amplitude of pressure-solution surfaces may not show a positive correlation with the degree of matrix cementation; high-amplitude stylolites may be weakly cemented, whereas low-amplitude stylolites may be strongly cemented (Koepnick, 1988). We agree with many workers such as Dunnington (1967), Koepnick (1984, 1987, 1988), Purser (1984), and Johnson and Budd (1975) that stylolite frequency and cumulative stylolite amplitude increase from the crest to the flanks of the anticlinal closure that forms the field. Koepnick (1988) concluded that the amplitudes of stylolites are controlled by the pattern of stress concentration developed during growth of the anticlinal structures, and by the inhibition of pressure solution by hydrocarbons along the anticlinal crest.
Dunnington (1967) recognized that at Bab Field stylolite frequency and stylolite-related reservoir thinning increase from the crest to the flanks of the structure. Stylolite distribution at this field is proposed to reflect a combination of stress concentration patterns developed during the growth of the reservoir structures, and of pressure-solution inhibition by hydrocarbon entrapment. The cumulative stylolite amplitude values cannot account for the total isopach variation, because the amplitude is only a measure of the minimum thickness of sedimentary section removed along the solution surface (Stockdale 1922, 1926) and because a certain thickness of the section was also removed during development of the wispy seams and solution seams.
In the Bab oilfield, high-amplitude stylolites of more than 35 cm amplitude can be observed in the most lithified facies, whereas the wispy stylolitic partings of less than 1 cm amplitude are present in the least lithified facies. The stylolite amplitude in this field is much greater in the water zone (up to 35 cm, with an average of 17.0 cm) than in the oil zones (maximum 5 cm). This probably reflects the inhibition of the wet chemical dissolution-reprecipitation reaction associated with stylolites developed during high hydrocarbon saturation.
The application of Johnson’s and Budd’s (1975) equation to the Bab Field revealed that stylolite intensity is inversely proportional to the porosity and permeability of the reported stylolites zone. Thus a high intensity of stylolites acts as a barrier to vertical fluid flow and reflects decreasing porosity and/or permeability, thus preventing the development of more continuous petroleum reservoir zones. In other petroleum fields, highly stylolitized carbonates can be excellent reservoirs.
The application of Dunnington’s (1967) formula for assessing compaction, which uses porosity outside and within the stylolite zone on the studied core samples from Bab Field (Fig. 4), revealed that the stylolite zone that lies between subzones BI and BII in the Kharaib Formation showed a reduction in thickness. Thinning of the subcropping section reached about 23% and was due to active stylolitization. The total loss of porosity within the zone with more intense stylolites was mainly a response to chemical compaction. A small part of the dissolved material was redistributed within this zone, but most of this material must have passed out of the system to be precipitated elsewhere. This thinning caused by stylolitization, when separated from sedimentary compaction by porosity occlusion, reveals that the stylolites are preexisting sedimentary structures.
The petrographic features attributable to the pressure-dissolution effect are very common in the studied core samples, and the low-amplitude and wispy solution seams occur both above and below the oil/water contacts in the Kharaib Formation in Bab and Asab fields. The seams reflect pressure dissolution prior to complete lithification of the carbonate sediments. The stylolites are thought to have formed as a result of pressure solution among carbonate rock masses. Assuming that stylolites developed from low-relief seams, the greater the amplitude of these stylolites the greater the volume of limestone dissolved.
Published articles on the classification of stylolites, in general, are numerous and confusing. For example, Park and Schot (1968a) classified the stylolites on the basis of the two-dimensional geometry expressed by the seam and its relation to sedimentary bedding. Wanless (1979) classified stylolites on the basis of the presence and degree of suturing of seam material. In contrast Trurnit (1968) based his classification on the relative solubility and size/shape considerations of the adjacent rock or grain unit. This paper uses a modified version of the classification of Koepnick (1984). It is based on amplitude and morphology of the pressure-dissolution surface within stylolites, its lateral continuity, and the thickness of the accumulated insoluble residue in the stylolites.
The detailed sedimentologic amplitude analyses and petrographic study made for this paper resulted in the recognition of three kinds of stylolites in the Lower Cretaceous (Thamama Group). These are: 1) rectangular or high-amplitude stylolites, (2) solution seams or wave-like stylolites, and (3) wispy seam or horsetail stylolites (Fig. 5).
Rectangular or High-Amplitude Stylolites.—
These stylolites are characterized by relatively high amplitudes (Figs. 5A, 6). They are associated with the latest stage of stylolitization and are associated with fractures that are unrelated to structural uplift. Dissolution of calcium carbonate associated with the high-amplitude stylolite causes weakening of the adjacent section, which often becomes a site of subvertical fractures. Insoluble residue appears to become concentrated at the stylolite peaks (crests), whereas typically less or no residue occurs along columns. This is because the insoluble residue is a measure of local dissolution, and this type of stylolite is considered to be a poor barrier to vertical fluid flow. In contrast, stylolites with thick, laterally continuous insoluble residues constitute more effective barriers to fluid cross-flow than stylolites with either thin, laterally continuous insoluble residue or with discontinuous residues as described by Koepnick (1988).
Solution Seams or Wave-Like Stylolites.—
This type of stylolite is characterized by a smooth or undulatory pattern with a continuous coating of seam material along its surface (Figs. 5B, 6). The amplitudes of these stylolites are less than 1 cm and the thicknesses of the insoluble residue accumulations can reach 1 cm. These seams are similar to the composite seams of Garrison and Kennedy (1977). These stylolites appear to form during the early stage of chemical compaction. They occur at the boundary between porous and less porous intervals of limestone, either singly or in groups. They are observed to subdivide and meet again, forming an anastomosing interconnecting network. Park and Schot (1968a, 1968b), reported that such an association indicates several periods of stylolitization. Solution seams are laterally continuous on the core scale and are often associated with networks of wispy seams and small stylolites, and are commonly associated with impermeable stratigraphic intervals (e.g., Koepnick, 1988). These seams are important barriers to fluid cross-flow because of their lateral continuity and the thick buildup of insoluble residue.
Wispy Seams or Horsetail Stylolites.—
This low-amplitude (< 1 mm) stylolite most commonly occurs in the very thinly laminated, argillaceous limestones (Figs. 5C, 6) and is often disconnected (e.g., Bab Member of the Shuaiba Formation). This type of stylolite is similar to the “horse-tail stylolite” of Mossop (1972) and the simple seams of Garrison and Kennedy (1977). Wispy seams may form significant barriers to fluid cross-flow and are commonly associated with impermeable stratigraphic intervals in the reservoir (Koepnick, 1988).
Factors That Control the Development of Stylolites
The variation in the style, intensity, and amplitude of stylolites in the Lower Cretaceous from one field to another is a response to many factors, the most common of which are pressure dissolution, fluid types, and lithology.
Pressure dissolution occurs in limestone under the stress of burial or tectonic movement. This begins with as little as 300 m of lithostatic overburden (Sellier, 1979). It is recognized that the overburden pressure is essential for the formation of stylolites (Dunnington, 1954, 1967), and thus the processes of stylolitization assumes greater importance in the moderate to deep burial setting (Choquette and James, 1987).
Bathurst (1980) suggested that pressure dissolution does not begin until a carbonate rock has undergone considerable burial. This idea is accepted (but not proven) by many workers. Factors such as water chemistry, presence of clay minerals, and presence of organic matter may also promote stylolitization regardless of burial depth. Dunnington (1967) placed the depth of pressure dissolution between 600 and 900 m for carbonate reservoirs in the Middle East. In contrast, Sellier (1979) suggested 300 m as the depth for the onset of pressure dissolution for the Aquitaine Basin of France. Schlanger (1964) presented evidence that in some cases burial need not to be greater than 90 m, particularly in limestones with a high clay content, i.e., primary lithology is as critical as depth of burial.
In south and southeast Abu Dhabi, where the top of the Lower Cretaceous is at shallow depths of burial (between 2000 and 3500 m), the stylolites are poorly developed or absent and have little influence on porosity or thickness reduction. They are best developed in fine-grained, porous and argillaceous limestones. Stylolite swarms are better developed in the central and western Abu Dhabi, with different stylolite intensities corresponding to different burial depths.
The development of stylolites in the reservoirs under the same lithostatic pressure can vary in response to the water saturation. The presence of hydrocarbons prevents and localizes stylolite development. The stylolites are less well developed in gas-bearing zones of Bab and Asab fields than in oil-bearing zones. Probably differences in water saturation ratio and salinity are the controlling factors. Stylolites are better developed in the waterbearing formations at great depth, as in the Sahil oilfield.
When oil migrates into the oil-zone reservoir, it effectively limits or stops carbonate diagenesis, because cementation requires that a large number of pore volumes of water be moved through each pore to precipitate cement. In the water zone, in contrast, diagenesis and stylolite formation may continue freely, and it would be expected that carbonates below the oil/water contact are considerably reduced in porosity relative to those above (Burgess and Peter, 1985).
Stylolites are relatively rare in the coarse-grained facies of the studied fields areas. In compacted oolitic limestones pressure dissolution is commonly manifested as micro-stylolites at grain boundaries, though stylolites occur mainly in the fine-grained or argillaceous intervals. Most of the stylolites are found in matrix-supported textures (mudstone or wackestone), but a few were observed in cemented grain-supported sediments (packstone and grainstone). The wave-like and wispy stylolites are concentrated in the laminated facies rich in clay. Stylolites commonly occur at bed boundaries or where mechanical layering created local stress concentrations.
Effects of Stylolitization and Cementation on Porosity and Permeability
The pore pattern in the studied pay zones resulted from a complex interplay of factors. The pore geometry can be characterized on the basis of pore size, pore shape, nature of the connections between pores, character of the pore wall, and distribution and number of larger pores and their relationship to one another. The result of stylolitization is considerable loss of porosity, reduction of permeability, and overall lessening of bed thickness or rock volume. This relationship can diminish reservoir capacity (Kopenick, 1984; Oswald et al., 1995).
The importance of stylolites to reservoir geology is their association with a significant reduction in porosity and permeability and their effects on hydrocarbon fluid flow and migration. The limestone dissolved as the stylolite formed is reprecipitated as cement nearby, causing partial or total filling of the intergranular pore space. This phenomenon affects the reservoir capacity. Such reduction of pore space depends on the degree of stylolitization, as described by Dunnington (1967) and Bathurst (1975). Stylolites and the cements derived from dissolution have the potential to act as barriers to vertical fluid flow. Because the presence of barriers becomes important when a reservoir is exploited, it is necessary to consider whether the stylolites are well enough developed to constitute a barrier.
Numerous diagenetic phenomena affect the Lower Cretaceous reservoirs in U.A.E. (Fig. 7). In order to determine the relative importance of factors controlling porosity and permeability within the Thamama Group, all diagenetic phenomena, including stylolitization and fracturing, should be considered. In the Lower Cretaceous of the U.A.E. the initial void-filling cements were precipitated after mechanical compaction and grain breakage. These cements probably originated from solution transfer during development of wispy seams (the earliest pressure-dissolution features). Some cements are probably not related to pressure dissolution. It should be noted that petroleum residues coat the first-generation, post-compaction cements, suggesting that oil migration occurred after initiation of pressure dissolution. The formation of dense limestones probably led to progressive restriction of fluid movement away from the sites of stylolite generation and therefore to a decreased rate of cementation in surrounding lithologies. There was still a lot of fluid/cement movement in the vicinity of the stylolites even though it was decreased in surrounding lithologies. The dissolution rate along the stylolite increased with the amplitude, resulting in more calcium carbonate available to cement pores. The effect of dolomitization was limited because of the dispersed nature of dolomite rhombs in the studied intervals of some reservoirs, such as Zone II of the Kharaib Formation in Zakum Field. The effect of recrystallization and cementation can be observed only in mesopores and megapores, because of microscopic limitations.
Figure 8 documents the distribution of visually estimated thickness of cemented rock associated with the Dl and the D2 stylolite horizons in the Asab Zone B of the Kharaib Formation. Koepnick (1988) concluded that the Dl stylolite horizon constitutes a significant barrier to vertical fluid cross-flow by virtue of its lateral continuity and its associated zone of matrix cementation. The D2 and the D3 stylolite horizons appear to be less significant barriers to vertical fluid flow by virtue of their lateral discontinuity, weak cementation, and hydrocarbon saturation. Furthermore, Koepnick (1988) described the Dl stylolite horizon as largely devoid of hydrocarbon staining, which suggests that it underwent matrix cementation prior to substantial input of reservoir hydrocarbons. Without hydrocarbons in the pore system, abundant nucleation sites are available nearby for precipitation of solute derived from pressure-solution surfaces. In contrast, the D2 stylolite horizon is uncemented and is saturated with hydrocarbons. These characteristics suggest that growth of the D2 stylolite horizon in Asab began during or after hydrocarbon entrapment. Hydrocarbons probably inhibited precipitation of solute derived from pressure-solution surfaces by coating many nucleation sites within the reservoir.
Carozzi and Von Bergen (1987) showed from petrographic observations that stylolitic porosity is discontinuous and corresponds to dissolution of carbonate material between the stylolite walls, which contained the least amount of insoluble residue. The effect of circulation of dissolving fluids along sutured stylolites controls the generation of the other types of burial secondary porosity. The hydrocarbons obviously migrated after the development of stylolitic porosity within a completely cemented and stabilized carbonate rock. Stylolitization in all instances results in liberation of calcium carbonate by pressure solution, and in concentration of insoluble residue along the stylolites (Carozzi and Von Bergen, 1987). Some stylolites are found in zones of relatively low porosity in the core material. This association could be initiated either early in diagenesis and localize subsequent stylolite formation, or could be late in diagenesis and due to reprecipitation of stylolite related pressure-dissolved material (Nelson, 1981, 1983). Dolomite commonly occurs as a replacement halo around individual pressure-solution features. Dolomitization associated with pressure-solution features drastically reduces matrix permeability and often is of greater significance as permeability barrier than adjacent undolomitized stylolite seams.
In the Zakum Field Zone II of the Kharaib Formation some stylolite swarms show that the greatest reduction in porosity and permeability is usually confined to a horizon 0.8–1.7 m thick (Alsharhan, 1990). These stylolite-rich horizons are generally thought to be too thin to constitute effective barriers, but much reduction in porosity and permeability results from stylolitization. This is because in such beds fractures, however uncommon, are apparently sufficient to enable fluid flow across the stylolitized intervals.
Cementation that preceded stylolitization also affects matrix porosity. Therefore, not all the dense intervals (from the porosity log) are caused by pressure dissolution. Permeability reduction is due to development of networks of the insoluble residues and to cementation of adjacent pore space resulting from solution transfer (Koepnick, 1984). The processes giving rise to solution transfer are: (1) pressure solution, which causes the minerals to dissolve along grain contacts or along more extensive surfaces under the influence of pressure or stress, and (2) solution transfer, which results in matrix cementation adjacent to pressure-solution surfaces.
Pressure or stress on deeply buried sediments dissolves the mineral along grain contacts or along more extensive surfaces. This pressure may exceed the average lithostatic pressure. Adjacent free areas of grain boundaries are subjected only to the pressure of the pore fluid, which is generally less than lithostatic pressure (Robin, 1978). Because mineral solubility increases with pressure, a decreasing concentration gradient between high-pressure grain contact areas and adjacent low-pressure areas develops across folds. If the insoluble residue continues to develop, significant permeability reduction results. Matrix cementation adjacent to pressure-solution seams reduces permeability. Extensive permeability reduction may result when matrix cementation occurs immediately adjacent to the pressure-solution surface. But if dissolved materials are transported beyond the immediate vicinity of the pressure solution surface, then adjacent matrix permeability remains.
Seam Material and Diagenetic Phenomena
The composition of the relatively insoluble seam material varies as a function of the composition of the nearby host rock and diagenetic processes. Most of the seams are composed largely of black bituminous hydrocarbon residue, and also contain insoluble rock residue (clay minerals, quartz silt) and diagenetic minerals (dolomite, calcite, pyrite, or other metallic sulfides; Fig. 9A). In the example presented here, the concentration of iron sulfide residue is proportional to the size of the stylolite.
The clay residue is largely a recrystallized kaolinite with minor quantities of illite, chlorite, and montmorillonite. Clays and organic matter are often associated together, and their proportions appear to vary within Lower Cretaceous swarm stylolites. The relative proportions of quartz and pyrite also tend to increase with the shale content of the stratigraphic horizon. Dolomite is common along stylolites (Fig. 9A) in the studied cores. Wanless (1979, 1982) believed that similar dolomite is formed during stylolite formation when pressure dissolution releases magnesium ions, but Pratt (1982) concluded that the dolomite forms either before or after stylolitization, but not during pressure solution. Calcite is occasionally observed as a fracture filling toward the tops of stylolites and probably formed from the reprecipitation of the carbonate dissolved at pressure-dissolution contacts.
The thicker and more laterally continuous the insoluble residue portions of the stylolites, the more effective a barrier they are to vertical fluid flow. In some reservoirs, as in Zone B of Asab Field, Johnson and Budd (1975) and Koepnick (1984) believed that swarms of wispy seams may form significant barriers to fluid cross flow because of the lateral continuity and thick buildup of insoluble residues.
Because stratigraphic intervals with large-amplitude stylolites are not always effective barriers to fluid flow, the amplitude may not be a good guide to loss of matrix porosity. It is more likely that the dense bituminous residue, with its incorporated noncarbonate minerals, which forms on the contact surfaces of the stylolitic seam, is the principal determinant of the effectiveness to fluid flow.
Examination of slabbed core samples revealed several examples of laminated bedding and undulatory wavy/nodular bedding. Thin sections studied show that the interlaminae consist of an anastomosing shale or organic-rich partings in lime mud matrix (Fig. 9B). Some dissolution seams appear to coincide with shaly organic-rich partings, which suggests that these sedimentary laminae might represent the first step towards stylolitization. The presence of organic matter promotes stylolite development.
Several studies have discussed the difficulty of distinguishing between these partings and stylolites or laminae. These have been variously termed clay seams, wispy or wavy laminae, pseudostylolites, microstylolites, hummocky structures, shaly or residual clay partings, stylolitized joints, and nonsutured to differentiated planes (Dunnington, 1967; Park and Schot, 1968a, 1968b; Trurnit, 1968; Coocan, 1970; Lucia, 1972; Shinn et al., 1977; Wanless, 1979; Buxton and Sibley, 1981; Pratt, 1982).
Fractures Associated with Pressure Solution
Stylolite horizons commonly have a variety of associated natural and induced fractures, which originate from the same stress state that caused the stylolite. Nelson (1979, 1981) described two types of fractures associated with stylolites: tension gashes and unloading fractures (for more details see Nelson, 1981). The tension gashes are a set of parallel fractures associated with stylolites and also parallel to the maximum stress direction. These extension fractures are derived from the same overburden stress that controls stylolite development. This type of fracture is generally 5–10 cm long and parallel to columns of the stylolite. They are often wedge shaped and commonly filled with coarse, highly twinned calcite. Unloading fractures tend to be nearly parallel with the stylolite seam and are perpendicular to the maximum paleo-stress direction. They are either extension fractures or true tension fractures related to unloading or relaxation of the rock parallel with the maximum stress direction.
Numerous microfractures are commonly found associated with stylolites. Such an association might be important because of its effect on permeability. Uncemented fractures increase permeability but add relatively little to pore volume. Stylolite-related fractures are typically small (maximum length is around a few centimeters) and are parallel to stylolite peak or column orientation, i.e., perpendicular to bedding. The microfractures associated with stylolites may be open, partially cemented, or fully cemented (Fig. 9C). The cement is generally calcite, though in some intervals organic clay-rich material partially fills the fractures. Fractures are usually associated with the large well developed rectangular stylolites. Where a stylolite with a thick, continuous insoluble residue is crossed by fractures enlarged by dissolution (i.e., fractures are later than stylolite), the fracture counteracts the negative effect of the residue on vertical permeability. Opened fractures associated with stylolites therefore may increase permeability.
Cores from the Bab, Zakum, Asab, and Sahil fields in the Kharaib Formation show that the fractures are mainly subvertical, regularly spaced, and continuous, presumably because of tectonic extension. Another set of fractures, dipping about 60°, are more irregular but also more continuous, and usual extensions range from a few centimeters to a few decimeters. Fractures within oil zones are generally free of cement, except for a few isolated calcite crystals, and they are often enlarged by dissolution, thus improving reservoir quality.
Cores from the fields in the basinal Shuaiba Formation (Bab Member) show that most of the fractures are short, up to a maximum length of 15 cm, and have widths on the order of a few millimeters. They occur predominantly in the lime mudstone and wackestone intervals of the open and deep marine facies. These micro-fractures, which resemble calcite veins, are usually plugged, although some are open or only partially filled (Fig. 9C).
At Asab Field in Zone B of the Kharaib Formation, natural fractures are common, particularly in the muddy facies units, and in most cases closely associated with the D2 and D3 stylolite-rich horizons. The fractures are oriented at high angles to the bedding and are usually perpendicular to adjacent pressure-dissolution features. The orientation of the fractures, and their gradation into breccia horizons, suggest an extensional tectonic origin for these features. The extensional fractures and conjugate shear may develop within the same compressional stress regime, giving rise to pressure-solution features (Choukroune, 1969).
In some studied cores of the Kharaib Formation in the Zakum Field, the effect of fractures appears insignificant because no enhancement in vertical (compared to horizontal) permeability is observed (Alsharhan, 1990). This implies that fractures are important in other adjacent fields. The increase in vertical permeability in some wells is not due to fracturing because no major fractures were observed. Also, in Zakum Field, fractures are common throughout Zone V of the Habshan Formation. Horizontal, vertical, and inclined fractures were observed in cores. They vary from a few centimeters to a few decimeters. Microfractures were observed in thin sections. Some are filled with calcite or other material, but many often have good permeabilities.
The origin and timing of formation of the microfractures is controversial. Park and Schot (1968a) hypothesized a pre-lithification origin separate from that of stylolitization. Other studies have indicated that the microfractures occur simultaneously with stylolites and form after rock lithification (Groshanf, 1975; Nelson, 1981, 1983). These extensional microfractures are developed within the same pressure field as stylolites. The formation of the microfractures associated with stylolites is a mechanism for releasing stress caused by the overburden pressure, uneven chemical compaction, and uneven unbalanced stresses.
The microfractures observed in the Lower Cretaceous carbonates appear to be of post-lithification origin contemporaneous with stylolites, because all the fractures observed terminate at the stylolitic surface and no fractures were observed to cut across the stylolites. Also, the presence of organic, clay-rich material and few quartz grains in the fracture indicate the simultaneous formation of stylolites and fractures. These are not formed by pressure solution but by tension and were therefore filled with stylolite residues.
Stylolite Occurrences in the Study Area
Stylolites in the Lower Cretaceous of the U.A.E. reservoirs are of post-lithification origin; these stylolites cut or penetrate micrite, fossils, and cemented pores indiscriminately. Some stylolites that pass through micrite and metasparite patches are of post-lithification origin.
The axial trends of increasing stylolite frequency and increasing cumulative stylolite amplitude from crest to the flank of Asab Field may partly explain by differential inhibition of pressure solution by reservoir hydrocarbon and emplacement. Reservoir hydrocarbons are an important component of stylolite insoluble residues, indicating post-oil migration growth of stylolites. Post-oil migration stylolite development very likely has been affected by increasing hydrocarbon saturation toward the crest of the structure (Koepnick, 1984). However, hydrocarbon inhibition of pressure solution does not explain the cross-axial trend, the preferential stylolite development on the southern structural nose, or the vertical sequencing of contour patterns. The strati-graphic alternation of stylolite contour patterns probably reflects differential response of stress of successive reservoir units developed during the structural growth of Asab Field.
In the oilfield the studied stylolite distribution is controlled by paleo-stress fields existing during development of the anticlinal closure, and by differential inhibition of stylolite growth by hydrocarbon saturation.
The hydrocarbons that were entrapped before stylolite formation prevented or at least localized the stylolite development, as is observed in “Zone B” of the Kharaib Formation of Asab and Sahil fields. This observation can be related to the counter-action of the reservoir pressure created by the accumulation of hydrocarbons. If the stylolite formation preceded hydrocarbon entrapment, however, it might have influenced the migration and accumulation of petroleum, as in the case of “Zone C” of the Kharaib Formation in Asab Field.
In the IA and II reservoirs zones (lower Shuaiba and upper Kharaib formations) of Zakum Field, all the stylolite-rich intervals exhibit stylolites that are oriented parallel to the bedding and their peaks are orthogonal to the seams. Crosscutting stylolites are also observed. Solution seams commonly converge into residual clay grooves that are slightly stylolitized. These are congruent to the bedding and commonly occur with horsetail structures.
Habshan Formation (Berriasian-Valanginian)
The deposition of this formation started with a transgressive cycle, in a restricted or semi-restricted shallow-marine setting. The lower part is composed of argillaceous, dense lime mudstones and wackestones, which grade upward to dolomitic peloidal packstones and dolomitic peloidal grainstones. This unit is water-bearing, and stylolites are absent. The absence of stylolites in southeast Abu Dhabi fields (e.g., Mender) is due to insufficient burial. Here, the upper part of the Habshan Formation is composed of lagoonal to intertidal lime mudstones and wackestone, with subordinate dolomite. Habshan cores from Asab Field show a very poor stylolite development, but the stylolites are well developed in the Habshan of the Bab and Sahil fields.
To the north in offshore Abu Dhabi, stylolites are, in contrast, developed throughout the upper part of the Habshan reservoir (Zone V) of Zakum Field. Stylolites tend to be more common in the mud-supported limestones rather than in the grainstones and packstones. More than 80% of the stylolites in this reservoir are in mud-supported limestones, and their amplitudes are generally in the range of 1.5 cm. The majority of the stylolite in this reservoir are of small magnitude and do not constitute barriers to the vertical permeability.
Lekhwair Formation (Hauterivian-Lower Barremian)
The Lekhwair Formation was deposited in an unrestricted marine shelf environment. It is composed of a number of cyclic sequences of alternating nonporous and porous limestones. In the onshore of Abu Dhabi, this formation splits into five reservoir zones (D-H), but in the Abu Dhabi offshore it is divided into two reservoir zones (III, IV). The onshore reservoir Zone H contains stylolites of wispy-seam type, which are associated with microfractures. Zone G has some stylolite development but less than the underlying Zone H reservoir in most fields. Zone F contains the most porous and permeable horizon, and the stylolites are more poorly developed than in Zones G and H.
Stylolite swarms occur in the lower part of the Lekhwair Formation in offshore (Zone IV or Zakum Member) of the Zakum Field (Fig. 10). Four stylolite rich horizons (IVA-S, IVB 1-S, IVB 2-S, and IVC-S) are described by Hassan and Wada (1979). Although stylolites are present elsewhere in the reservoir of Zone IV, they are not laterally continuous and do not have a significant effect on porosity and permeability.
Within reservoir zone IVA-S, a stylolite-bearing interval some 1.0–2.5 m from the base of subzone IVA effectively subdivides this reservoir into two subzones of unequal thickness (IVA-1 and IV-2) Subzone IVA-S is well defined stratigraphically and is correlative throughout the Zakum Field, although its thickness and porosity are variable. The stylolite-rich horizon IVB 1-S is thin but can be correlated using porosity logs from a number of wells. This stylolite horizon causes only a small reduction in porosity and permeability and is characteristically present in mud-supported limestone. The stylolite-rich horizon IV B2-S has a constant thickness and can be correlated throughout the field. It occurs some 1.5–3 m above the base of subzone IVB and is too thin to form a permeability barrier within the reservoir. Although the stylolitic-rich horizon IVC-S occurs some 4.5 m below the top of subzone IVC, it is well developed along the flanks of the structure. Where it is well developed there is a significant reduction in porosity and permeability as a result of stylolitization across this interval. However, it is not sufficiently well developed that IVC-S constitutes a barrier to fluid communication between the upper and lower parts of subzone IV C.
Kharaib Formation (Barremkn-Lower Aptian)
This formation consists of a series of upward-shallowing cyclic units. The basal transgressive parts of the cycles are characterized by argillaceous lime mudstones/ wackestones with subordinate peloidal packstones, which represent normal open marine shelf conditions. The succeeding regressive parts of the cycles begin with a mud-supported limestone deposited as inner-shelf or deep-lagoon facies, which shallows upward into grain-supported limestones representing higher-energy conditions. Reservoir Zones B and C are the predominant hydrocarbon-bearing zones in the Bab and the Asab fields. These zones are divided into subzones by well-developed stylolitic intervals.
The Zone B reservoir is oil-bearing in most fields (e.g., Zakum, Bab, Asab, and Sahil). It is considered to be the principal reservoir in the onshore Abu Dhabi. This zone is characterized by a porous inner-shelf limestone capped by a regressive shoaling sequence of packstone and peloidal grainstone. It is divided into subzones by well-developed stylolitic intervals along muddy horizons. In the southeast area at the Mender, Qusahwira, and Shah fields, the stylolites in Zone B are poorly developed or absent, perhaps because of the shallow burial depths.
Stylolite development increases with depth of burial in the Bab and Asab fields, and the D1 stylolite swarms form a barrier to vertical fluid flow in most of the onshore producing fields. In general, compaction, cementation, and stylolitization are less in the Asab Field than in the Sahil and Bab fields because of the shallower depth of burial and finer grain size of the Asab reservoir sediments.
Offshore Abu Dhabi (in the central part of Zakum Field) stylolites in reservoir Zone II are poorly developed. The degree of stylolitization decreases from the flank toward the top of the structure, and stylolites are thought to be responsible for the overall reduction in thickness on the east and west flanks. Stylolite development in the northeast part of this field is diminished, because of either an anomaly in the oil-water contact, high hydrocarbon saturation, or probably post-oil-emplacement differential structural tilting. The highest-intensity and highest-amplitude stylolites occur in the mud-supported sediment intervals. The intensity and thickness of these stylolitic intervals are not uniform throughout the field. The five stylolite-rich horizons, S1-S5, separating the different reservoirs in Zone II of Zakum Field (Fig. 11) were defined by Alsharhan (1990) using both log data and core observations. The majority of stylolites in this zone exhibit low amplitude, and the few intervals that are stylolite-rich are not recognized on either electrical or porosity-permeability logs.
The Zone C reservoir is oil-bearing in the Asab and Sahil fields, gas-bearing in Bab Field, and water-bearing in the Bu Hasa and southeast Abu Dhabi fields. The zone is subdivided into two sub-reservoirs (CI and CII/CIII) by a well developed stylolite swarm (S1) that occurs between CI and CII (Fig. 12). In the fields where the zone is water saturated and burial depth is shallow, stylolites are not well developed. In Asab and Sahil fields and in the fields located in southeastern area, the lower reservoir (CII/CIII) is further subdivided into subzones CII and CIII by the stylolite swarm CS2. At the Bab Field, the CS2 stylolite swarm is present in the northeastern part of the field and delineates subzone CII from CIII. At the crest and in the southern part of the structure, however, the CS2 stylolite swarm is poorly developed or absent and subzones CII/CIII are undivided. In Bu Hasa Field, the CS1 stylolite swarm is well developed, whereas the CS2 swarm is poorly developed or absent. This zone is water-bearing and deeply buried.
Shuaiba Formation (Early to Mid Aptian)
The Shuaiba is the youngest formation of the Lower Cretaceous of the U.A.E. It is characterized by rudistid and algal carbonate platform sediments that have very good reservoir characteristics (e.g., Bu Hasa Field). Rudist/algal buildups are associated with the inner rims of the Abu Dhabi intrashelf basin, which has its center in the central part of Abu Dhabi and extends into western Oman and includes most of the offshore areas in Qatar. Sedimentary rocks in the intrashelf basin are known as the Bab Member and consist mainly of microporous lime mudstones and wackestones. Basinward and upward this Bab Member progressively increases in density and argillaceous content and exhibits calcareous shale intervals with deep, open marine fauna (Alsharhan, 1987).
The basal Shuaiba Formation is called the Zone A reservoir. It is composed of algal packstones and boundstones and is overlain by microporous lime mudstones and wackestones. The zone persists over most of the U.A.E. region. In the onshore area of Abu Dhabi, the Zone A reservoir is subdivided into three main subzones (AI-AIII) by two well-developed and regionally correlative stylolite swarms, ASI and AS2 (Fig. 13).
In the Zone IA reservoir of Zakum Field, all the stylolites observed are parallel to the bedding, with peaks orthogonal to the seams (stratiform stylolites). Wave-like or undulatory, sutured and /or rectangular types are recognized but vary in intensity and amplitude. The wave-like forms are abundant and occur at pressure-dissolution contacts between grains. This type of stylolite commonly has converging solution seams, which themselves are slightly stylolitized. These are usually congruent to the bedding and occur with “horsetail” structures. It may be difficult, in some cases, to recognize whether or not certain clay grooves associated with bedding planes are actually dissolution residues or discrete depositonal shale-marl partings.
In the Bab Member, stylolites occur mostly in the lime mudstone and wackestone intervals of the deep and open marine facies. Wave-like or sutured stylolite types, oriented parallel to bedding (stratiform stylolites), are the most common. The maximum amplitude observed in Zakum field in this member is about 17 cm (Fig. 14). The residue in the stylolite seam consists of organic matter, clay, pyrite, and quartz. Dolomite and microfractures associated with stylolitization are rare. The Bab Member is characterized by predominance of linear or undulatory laminae consisting mainly of organic-rich material.
The Lower Cretaceous Thamama Group is divided into several reservoir zones (Fig. 2). The Zone A in Asab, Sahil, and Bab fields is divided into three subzones (Al, A2, and A3) by three intervening horizons of stylolites “swarms” (SO, SI, and S2). A poorly developed stylolite swarm (ASO) divides the zone to two subzones, AIU and AIL, except in SE Abu Dhabi at the Mender Field, where it is hard to separate the reservoir into these subzones (Fig. 13) because of the observed stylolite-related marker horizons. The ASI swarm lies between subzones AI and All, and the AS2 swarm lies between subzones All and AIII.
The Zone B reservoir of the Asab and Mender fields has three important stylolite swarms (D1-D3), which subdivide the Zone B reservoir into four subzones (BI-BIV) (Fig. 15). At the Bab, Sahil, Bu Hasa, and Rumaitha fields the Zone B reservoir contains six main stylolite swarms (D1-D5); they subdivide the reservoir into seven porous subzones, (these are subzones BI to BVI) (Fig. 15). The Dl horizon is a stratigraphic interval containing stylolites. The typical low porosity is partly a product of sedimentology and partly pre-stylolite diagenesis.
Zone B reservoir of the Bab Field is cited as an example of how the stylolite intensity (SI) is distributed. Johnson and Budd’s (1975) empirical formula is used. The D1 seam containsthe most intense stylolite swarm of all major stylolites in Zone B (Fig. 16), and is present over the entire field. The Dl horizonis, in fact, a stratigraphic interval containing stylolites. The typical low porosity is the result partly of sedimentological effects and partly of pre-stylolite diagenesis. Stylolite intensity values vary from moderate to very strongly developed and increase generally from southwest to northeast. The highest intensity values, 220, occur on the northern flank; the lowest value, 4, was calculated for the southern flank. The amplitude of Dl increases toward all the flanks, except to the north and northeast. The highest amplitude, about 20 cm, occurs on the western flank, and shrinks to about 2 cm at the northeast. The amplitude of the Dl stylolite seam is low, whereas the stylolite intensity is high.
The D2 stylolite seam is present only locally and is poorly developed over most of the reservoir (Fig. 16). Stylolite development improves towards the northeastern and western flanks; the highest intensity, about 6, was calculated on the northeastern flank, and the lowest value is less than 1 on the crest. The stylolite amplitude of the stylolite seam D2 is much greater than of the Dl stylolite seam. Stylolite amplitude, in general, increases down flank, except on the northeastern flank, where the lower value is 1 cm, and the highest amplitude of 35 cm is encountered on the southern flank.
The stylolite intensity value of the D2A stylolite swarm increases generally down flank except along the southern edge and part of the western flank (Fig. 16). However, the highest intensity calculated is about 15 for the northeastern flank and the lowest value is less than 1, and is near to the crest. The amplitude of the D2A stylolite swarm shows high amplitudes on the southern and northern flanks, whereas on the southwestern and northwestern flanks low-amplitudes have been observed and the amplitude varies from less than 1 cm to more than 30 cm.
The D3 stylolite swarm is better developed than the D2, D2A, and D4 swarms and is locally absent on the crest. The stylolite intensity value ranges from moderate on the flanks to absent on the crest (Fig. 16). The highest stylolite intensity of about 8 is on the west flank, and the lowest is less than one at the crest. The stylolite intensity trend decreases toward the northeastern and southern flanks compared to the western and eastern flanks. The maximum amplitude of 35 cm is found on the eastern flank, and the smallest (2 cm) on the anticlinal crest and northeastern flank.
The D4 stylolite seam is somewhat better developed than the D2 and D2A swarms but not as well developed as the D3 swarms. It is not present over the whole field area. The highest intensity values, 13, were encountered on the northern flank, and the lowest (less than one) are at the crest (Fig. 16). The amplitude of this stylolite shows a trend similar to the D3 swarm, with an increase in amplitude towards the flank except on the northeastern and northwestern flanks. Again, the maximum amplitude (> 30 cm) is present on the eastern flank and the lowest (< 2 cm) is at the crest.
The D5 stylolite swarm is the second best developed stylolite in Zone B (after Dl swarm). The stylolite intensity values vary from weak to strong and generally increases from southwest to northeast (Fig. 16). The highest calculated stylolite intensity is 36 at the northern flank, and the lowest is 1 on the southern flank. The greatest amplitude measured is about 23 cm at the eastern flank, and the smallest is about 5 cm at the crest.
Johnson and Budd (1975) and Koepnick (1984) concluded that the Dl stylolite horizon in Asab Field constitutes a significant barrier to vertical fluid migration because of its excellent lateral continuity and associated matrix cementation. The D2 and D3 stylolite swarms constitute less significant flow barriers because of their lateral discontinuity, cementation, and hydrocarbon saturation. The Dl stylolite swarm was developed largely before hydrocarbon entrapment, whereas the D2 and D3 stylolites grew mostly during and after hydrocarbon entrapment (Koepnick, 1984). Johnson and Budd (1975) reported that the Dl stylolite swarm in Zone B, over much of the Asab Field and particularly down flank, is a complete barrier to the movement of fluids but less effective toward the crest. The D2 and D3 stylolite swarms represent no real barrier over much of the field, even at their greatest intensity, where they probably impede vertical fluid movement.
Zone C in Asab, Sahil, and Mender fields is subdivided into three subzones (CI through CIII) by two stylolite swarms (Fig. 12). These are the SC1 swarm at the base of subzone CI and the SC2 swarm at the base of subzone CII. Stylolite swarm SCI is well developed over all the field areas and is considered to have a significant impact on inter-reservoir communication between subzones CI and CII. In Bab Field, Zone C is divided into two subzones, CI and CII/CIII (Fig. 12), by a stylolite swarm SC1 at the base of subzone CI. The stylolite swarm CS2 is also locally developed and consistently separates subzone CII from subzone CIII.
Mineralogy and Trace-Element Geochemistry
Not all of the measured trace metals were found in stylolitic horizons in the studied wells at statistically significant concentrations when compared to the host rock, but where significant differences occur there is a relative concentration within the stylolitic interval (Fig. 17). In general, elements displaying the greatest degree of concentration include Al, Cr, Cu, Fe, Mg, and Ni. Less, but significant, relative concentration was detected in Mn, Pb, V, and Zn ratios; there is no significant concentration of Be and Na in any core. Trace-element concentrations associated with both rectangular and wave-like stylolites are common and roughly equal in magnitude. Horsetail stylolites do not show significant concentration for any element. This suggests that, considering a constant stylolite amplitude, the degree of dissolution and intraformational loss of carbonate minerals is much greater for rectangular and wave-like stylolites than for horsetail stylolites, as evidenced by the elevated ratios for trace elements (Mn, Fe, Zn, Pb) that tend to substitute for Ca in the calcite crystal lattice (Reeder, 1983). It suggests that the wave-like and rectangular stylolites may have formed under equilibrium conditions of diagenetic alteration different than conditions in which the horsetail stylolites formed. It is also possible, however, that diagenetic alteration took place in contact with a different subsurface water.
There also tend to be slightly greater mean concentrations in samples with wave-like stylolites than in those with rectangular stylolites (Table 1). This suggests that processes that produced wave-like forms in these fields have been somewhat more chemically aggressive than those that produced the rectangular stylolites. Further study of the trace-element chemistry at the crystal level using electron microprobe would be helpful in determining the degree to which this apparent difference is related in general to the processes that result in formation of these two different types of stylolites, because trace-element ratios of bulk subsamples could be strongly affected by variations in mineralogy and mineral composition.
The mineralogy of the subsamples provides some clues to the differences in some trace-element ratios, and the chemical extraction technique used in this study may have partially dissolved some noncarbonate minerals. For example, rectangular stylolite seams appear to be richer in metallic sulfides than seams in cores with wave-like stylolites, and this is consistent with the relatively higher Fe/Ca and Ni/Ca ratios in samples with rectangular stylolites (Table 1). Similarly, wave-like stylolitic intervals appear to be richer in clay minerals and euhedral dolomite, which is consistent with the somewhat higher Al/Ca and Mg/Ca values in these samples. No minerals were detected using either XRD or petrography which contain significant copper or chromium, so there is no clear explanation for the elevated Cu/Ca and Cr/Ca ratios detected in these cores.
Study of stylolitization in the Lower Cretaceous carbonate reservoirs in the U.A.E. oil field areas is of importance in the evaluation of the capacities of the reservoir characteristics for petroleum.
The presence of hydrocarbons prevents and localizes the stylolite development. The stylolite-rich horizons in the studied field areas constitute the effective barriers for petroleum migration and accumulation, because of the pronounced reduction in permeability resulting from stylolite formation and development within the fields and/or the geologic formation stratigraphic levels.
Matrix cementation and continuity of insoluble residue seams in the vicinity of stylolites is significant. It is true that some stylolite seams have not contributed to localized matrix cementation, but they are not the ones of concern. It is the cemented ones that could effect the reservoir.
Mineralogic and trace-element geochemical analysis of the three stylolite types from the fields studied suggests that reduction in unit thickness from diagenetic dissolution is significantly greater for wave-like and rectangular stylolites, with little detectable loss associated with horsetail stylolites.
The authors would like to thank Dr. C.G.St.C. Kendall for reading the paper and also to Drs. R. Koepnick and R. W. Scott for their detailed review and valuable comments which enhanced this manuscript.
Figures & Tables
Middle East Models of Jurassic/Cretaceous Carbonate Systems
This volume will interest tectonic modelers, stratigraphers, sedimentologists, and explorationists. It is the product of the international conference of “Jurassic/Cretaceous Carbonate Platform-Basin Systems, Middle East Models” that was convened in December 1997 jointly by SEPM (Society for Sedimentary Geology) and the United Arab Emirates University in Al Ain, United Arab Emirates. The twenty-three papers present new data and interpretations arranged in three sections: 1) sequence stratigraphy, cyclostratigraphy, chronostratigraphy, and tectonic influences, 2) depositional and diagenetic models of carbonate platforms, and 3) hydrocarbon habitat and exploration/development case studies. New tectonic models of the Arabian Basin, new stratigraphic and sequence stratigraphic reference sections, new geochemical and source rock data, and new reservoir data are presented. New geologic models make this set of papers relevant to geoscientists working outside of Arabia also.