A fundamental question in the correlation of 1-D sedimentologic data is whether to use a layer-cake or shingled correlation approach. The resulting reservoir geometry has important implications for the characterization of reservoir heterogeneities and fluid flow. On the Saiq Plateau in Oman, epeiric carbonate ramp deposits of the Triassic Sudair Formation are well exposed and can be investigated in detail over several kilometers. There, reservoir heterogeneities on different scales have been documented by creating various outcrop wall panels and 2-D correlations. Multi-level architectural elements with different depositional geometries were discovered, which were linked to a sequence-stratigraphic hierarchy consisting of three levels. Level 1: A “layer-cake”-type stratigraphic architecture with minor thickness variations over several kilometers becomes apparent when correlating fourth-order cycle set boundaries. Level 2: The correlation of fifth-order cycle boundaries reflects horizontally continuous geometries, within which, however, internal grainstone layers were discovered to be arranged in a shingled fashion. Muddy layers in between these shingles illustrate sixth-order mini-cycle boundaries. Level 3: Within sixth-order mini-cycles another scale of a shingle-like architecture can be observed. Amalgamated cm-thick grainstone units form thin wedges with subtle but clearly inclined dipping geometry.
Fourth-order cycle sets and fifth-order cycles can be traced over several kilometers, and therefore assumed to be related to allocyclic stratigraphic processes. The internal shingle geometries within fifth-order cycles are traceable over 100s of meters and presumably reflect an autocyclic lateral migration of a shoal complex. Cm-thick shingling grainstone wedges within sixth-order mini-cycles are interpreted as storm-related spill deposits. Their event-driven character is reflected by frequent amalgamation and reworking of the preceding deposits.
The results of this study of epeiric carbonate ramp deposits suggest that a “layer-cake” correlation approach is appropriate when correlating 10s of m-thick grainstone units over a distance of several kilometers. However in the documented example, these thick grainstone units consist internally of small-scale architectural elements, which show inclined geometries and require a shingled correlation approach. These small-scale heterogeneities within an overall “layer-cake” architecture might have an impact on fluid flow in similar subsurface reservoirs and should be taken into account for detailed reservoir correlations and static reservoir models.
Shoal grainstones form important carbonate reservoir facies in the Middle East such as in the Permian–Triassic Khuff Formation or the Jurassic Arab-D (e.g. Murris, 1980; Al-Jallal, 1991; Alsharhan, 1993; Bashari, 2005; Ehrenberg et al., 2007; Koehrer et al., 2011, 2012). Knowledge about the shape, geometry and architecture of shoal deposits is of great importance for the geological understanding of the flow units and to identifying potential seals or baffles in these reservoirs. Hence, various studies focus on modern carbonate systems aiming to quantify the geometry and architecture of shoals (e.g. Purser and Evans, 1973; Harris, 1977; Gischler and Lomando, 2005; Rankey et al., 2006; Reeder and Rankey, 2008; Harris et al., 2011). Compositional and textural variations are commonly investigated and linked to sedimentary processes delivering valuable information about the development of shoal complexes and explaining the lateral distribution and vertical thickness of modern carbonate sands (e.g. Lokier et al., 2009; Rankey and Reeder, 2011; Sparks and Rankey, 2013). However, epeiric carbonate ramps are absent in modern day environments making the investigation of reservoir geometries and dimensions in this particular geotectonic setting challenging. Furthermore, modern depositional systems can evolve over time into dynamically changing landscapes with laterally migrating depositional environments. The interaction of different sedimentary processes such as sedimentation and erosion can result in bed amalgamation and sediment reworking, which may lead to the preservation of sedimentary geobodies that differ in size and shape from those observed as geomorphic forms in modern analogs.
Outcrop analog studies permit the detailed analysis of geobodies preserved in the stratigraphic record and can help understanding the geometry and extent of reservoir units in the subsurface. Thus, outcrop investigations can contribute to subsurface hydrocarbon exploration and production. This study focuses on shoal and shoal-margin grainstone facies outcropping on the Saiq Plateau in the Oman Mountains (Al Jabal al-Akhdar) in north Oman (Figure 1). There, the Lower Triassic Middle Mahil Member, an outcrop equivalent of the subsurface Sudair Formation (Koehrer et al., 2010; Pöppelreiter et al., 2011), is well exposed and permits lateral tracing of individual grainstone units over a distance of several kilometers. Middle Mahil facies were deposited on an epeiric carbonate ramp and range predominantly between tidal flat and shoal facies, thus providing a depositional analog to Khuff reservoirs in the subsurface of Oman and in Qatar’s North Field.
This study aims to unravel the internal architecture of shoal deposits from an epeiric carbonate ramp addressing the fundamental question as to whether a layer-cake or shingled correlation should be used when correlating 1-D sedimentologic data in such a setting. Furthermore, this study documents lateral thickness and facies changes within shoal and shoal-margin grainstones and links the observed heterogeneities to sedimentary processes and sequence-stratigraphic interpretations.
The Lower Triassic Middle Mahil Member in outcrops of north Oman is part of a kilometers-thick carbonate succession, which developed on a flat epeiric carbonate ramp on the Arabian Plate bordering the western Neo-Tethys Ocean (e.g. Ziegler, 2001). The initial carbonate deposition in north Oman is reflected in outcrops by the Middle to Late Permian Saiq Formation (Glennie, 2005), which has been studied in detail with the results published by several authors (e.g. Koehrer et al., 2010, 2011, 2012; Zeller et al., 2011; Bendias et al., 2013; Walz et al., 2013; Haase and Aigner, 2013). The conformably overlying Mahil Formation was subdivided by Koehrer et al. (2010) into three members: (1) Lower Mahil Member, (2) Middle Mahil Member, (3) Upper Mahil Member.
The Lower Mahil Member and the underlying Saiq Formation are considered as time-equivalent to the subsurface Khuff Formation (Koehrer et al., 2010). According to Koehrer et al. (2010) and Pöppelreiter et al. (2011), the Lower Triassic Middle Mahil Member corresponds to the subsurface Sudair Formation. The Middle Mahil is unconformably overlain by the Upper Mahil Member, the uppermost part of the thick carbonate succession of the Permian–Triassic epeiric carbonate ramp system (Figure 2) and time-equivalent to the subsurface Jilh Formation (Koehrer et al., 2010; Obermaier et al., 2012). A distinctive regional erosional unconformity at the top Upper Mahil Member (Jilh) (Sharland et al., 2001; Forbes et al., 2010) indicates the end of the Permian–Triassic epeiric carbonate ramp system on the Arabian Peninsula.
The Middle Mahil Member, the focus of this study, was deposited on a uniformly subsiding passive continental margin during tectonic quiescence (Dercourt et al., 1993; Konert et al., 2001). Hence, vertical facies variations most likely are related to sea-level fluctuations of low amplitude and high frequency, typical for the prevailing transitional to greenhouse climatic conditions.
During the Early Triassic, the study area in the Oman Mountains was located in a seaward platform margin setting with increasing open-marine influence towards the north (Ziegler, 2001), leading to the deposition of a ca. 265 m-thick carbonate succession on the Saiq Plateau with m-thick shale beds in the basal 30 meters (Pöppelreiter et al., 2011).
The outcrop study on the Saiq Plateau (Al Jabal al-Akhdar) is based on sedimentologic descriptions and sequence-stratigraphic interpretations of the Middle Mahil Member. A 265 m-thick vertical outcrop succession, which covers the entire Middle Mahil, was described at a location 4 km away from outcrops investigated by Pöppelreiter et al. (2011), and correlated to them (Level 1). The succession consists of two overlapping sections. Bases and tops of the sections are given in Universal Transverse Mercator (UTM) grid data in Figures 3 and 4 (WGS 84, UTM Q40). Furthermore, 11 vertical sections from a ca. 5 m-thick distinctive grainstone unit in the lower part of the Middle Mahil were logged nearby on a cm-scale (Level 2 in Figure 1). On average the sections are 200 m apart and distributed over a total distance of approximately 2,500 m. Additionally, two wall panels of a m-thick grain-dominated interval were constructed (Level 3 in Figure 1). The more extensive wall panel was walked out over 240 m laterally with a detailed vertical sedimentologic description every 10 m. The smaller wall panel is one meter thick, extends over 13 m and involves a vertical sedimentologic log every meter.
In order to highlight the internal architecture of grain-dominated intervals, correlations and wall panels were flattened on a distinctive cycle boundary in the upper part of the section, which most likely was a roughly horizontal horizon at the time of deposition. The apparent dip angles of grainstone beds and cycle boundaries were derived from the geometric reconstructions. The bed inclination is considered as being a combination of the ancient sea-floor topography and differential sediment compaction. The dipping angles highlight the extremely flat and gentle depositional geometries of grainstone geobodies.
For each outcrop section the lithology, Dunham texture (M: mudstone; W: wackestone; P: packstone; G: grainstone; B: boundstone; F: floatstone; R: rudstone), sedimentary structures, components and rock color were recorded in order to determine lithofacies types and to assign the appropriate lithofacies association (LFA) (Table 1 and Figure 3). Rock descriptions were digitized using WellCAD 4.3. Facies descriptions and sequence-stratigraphic interpretations follow an earlier publication on the Middle Mahil (Pöppelreiter et al., 2011) and are briefly described below.
LITHOFACIES AND DEPOSITIONAL ENVIRONMENTS
The Middle Mahil Member on the Saiq Plateau has been studied in detail by Pöppelreiter et al. (2011), who identified 14 lithofacies types. Their sedimentologic and biostratigraphic analyses at the Saiq Plateau suggest that the Middle Mahil was deposited in a restricted inner-ramp setting lacking open-marine foreshoal and offshoal environments. Despite dolomitization, the primary depositional fabrics in the Middle Mahil are well preserved and permit detailed facies analysis. No locally produced fluid flow units or baffles due to dolomitization could be observed in the investigated stratigraphic interval.
Based on fauna content, carbonate texture, sedimentary structures, sorting and vertical facies stacking patterns, the lithofacies types of the entirely dolomitized succession were grouped into five lithofacies associations (LFA) across a carbonate ramp (Table 1, Figure 5). The terminology and numbering for the lithofacies associations follows the modified LFA scheme from Koehrer et al. (2010): (1) LFA 2: Coastal marsh; (2) LFA 3: Peritidal; (3) LFA 4A: Low-energy backshoal; (4) LFA 4B: Moderate-energy backshoal (leeward shoal margin); and (5) LFA 5: High-energy shoal environment.
Most proximal facies are typically of muddy texture and reddish color. They are marked by subaerial exposure features such as epikarsts and/or fine root structures and are interpreted as coastal marsh deposits. White-colored, microbial boundstones (e.g. Photo A, Figure 3), which are occasionally reworked and redeposited as intraclastic pack-and grainstones, were deposited in a peritidal environment close to the shoreface. Facies of the protected low-energy backshoal setting are of muddy texture and commonly bioturbated or thinly bedded with few gastropod and bivalve shells. Further seaward with increasing wave energy, facies become more grain-dominated. Graded packstones to wackestones with sharp erosive bases and ripple bedding as well as skeletal-and peloidal-rich packstones/grainstones (e.g. Photo C, Figure 3) are interpreted as moderate-energy backshoal environment. This depositional environment includes tempestites and spill-over deposits from an adjacent shoal complex, hence the term leeward shoal margin or shoal flank is also used in this context. Most distal lithofacies are light brown, m-thick cross-bedded grainstones (e.g. Photo D, Figure 3) and comprise well-sorted mm-size ooids and peloids and are assigned to high-energy shoal bars.
The cyclic alternation of muddy proximal facies and grainy distal facies is well reflcted in outcrop weathering profiles. Muddy intervals, which are more prone to disintegrating under weathering conditions, form recessive units in outcrop walls. In contrast, grainy intervals as more resistant units typically form prominent cliffs (Figure 3).
Following the systematic sequence-stratigraphic interpretations of Pöppelreiter et al. (2011) for the Middle Mahil Member, shoal-associated trough cross-bedded oolitic/peloidal grainstones were interpreted as zones of maximum floodings (mfz). Rooted mudstones and peritidal microbial boundstones, which commonly mark sea-level lowstands, are interpreted as cycle boundaries (Figure 6).
The complete succession of the Middle Mahil was sedimentologically described and interpreted in terms of sequence stratigraphy. The naming of sequences in this paper follows the nomenclature of Pöppelreiter et al. (2011). At the Saiq Plateau, the composite sequence of the entire Middle Mahil (MSS) is subdivided into three high-frequency sequences (HFS: MS 1, MS 2, MS 3) each of which comprises three fourth-order cycle sets (Figure 4). Decameter-thick cycle sets are named according to their high-frequency sequence, such as MCS 1.1, 1.2 and 1.3 for cycle sets, which occur within high-frequency sequence MS 1. Each fourth-order cycle set consists of a number of m-thick fifth-order cycles. The 46 cycles of the entire Middle Mahil Member can further be subdivided into several dm-thick sixth-order mini-cycles.
Vertical facies variations within sixth-order mini-cycles can vary depending on the overall stratigraphic position. In the Middle Mahil Member, mini-cycles that occur around fourth-order maximum flooding zones are typically of larger thickness and more grain-dominated. If present, microbial boundstones are thinner and subaerial exposure features less distinct or completely absent. Identifying mini-cycle boundaries within these grain-rich intervals can be challenging. Typically, boundaries were chosen at the top of the most proximal lithofacies type in a vertical succession, which commonly forms a recessive unit in the outcrop wall due to the muddy nature. Towards a mini-cycle boundary, the bed thickness tends to decrease from dm-scale to cm-scale as a result of a limited accommodation space. Ooids and peloids become less abundant, while micrite content and microbial influence increases (Figure 6). This is normally accompanied by a color change from beige/brown to whitish. Common stratal surfaces which are interpreted as minicycle boundaries form prominent gaps/breaks in outcrop walls and can be followed over several hundreds of meters.
Similar mini-cycles have already been observed in Khuff-equivalent strata in the Oman Mountains (Haase and Aigner, 2013), which include dm-to m-thick vertical facies variations within grain-dominated intervals.
In this study, lateral heterogeneities are analyzed on three different sequence-stratigraphic levels. Fourth-order cycle sets are correlated to highlight facies and thickness variations in the Middle Mahil Member over several kilometers (Level 1). Fifth-order cycles are used to investigate the grainstone architecture and depositional geometries of shoals over a lateral distance of 100s of meters (Level 2 in Figure 4). For the analysis over a distance of 100s of meters the focus was put on a well-exposed fifth-order cycle, which lies in the lower part of the succession and is easily accessible over a large distance. Wall panels of a sixth-order mini-cycle from the same stratigraphic interval were constructed to illustrate small-scale geometries on a m-scale (Level 3 in Figure 4).
Analysis of Lateral Facies Changes
Level 1: Grainstone Architecture between Fourth-order Cycle-set Boundaries
A correlation of the 265 m-thick Middle Mahil log (section 1 in Figure 7) with an outcrop log from Pöppelreiter et al. (2011), located 4 km away, bears a striking resemblance in terms of vertical facies stacking patterns and number of fourth-order cycle sets. Cycle sets (fourth-order) seem correlatable over at least 4 km and individual grainstone units are persistent over the entire distance reflecting a “layer-cake”-type geometry. Outcrop section 2 (Pöppelreiter et al., 2011), which according to paleogeographic reconstructions is located in a slightly more seaward position (Ziegler, 2001), contains thicker grainstone packages with less mudstone interlayers. By vertically exaggerating the correlation panel to 1:150 and restoring the correlation by using the claystone beds at the base of the formation as flat datum, grainstones seem to slightly dip towards the east-northeast with a maximum apparent angle of 0.1° (Figure 7).
Level 2: Grainstone Architecture between Fifth-order Cycle Boundaries
Eleven vertical sections of a grain-dominated fifth-order cycle from the lower Middle Mahil (maximum flooding zone mfz MS-3) were logged in detail and correlated on a W-E transect across a distance of ca. 2.5 km. A distinctive whitish mudstone bed with exposure features at the top of the cycle was chosen as flat datum for the correlation. As this particular bed is present over the entire distance of 2.5 km, it is assumed that it has been most likely a roughly horizontal horizon at the time of deposition. The beds at the top and base of the cycle act as marker beds and help to locate the particular fifth-order cycle within the overall grain-dominated interval in the lower Middle Mahil. Since paleogeographic reconstructions suggest a general seaward trend towards the north (e.g. Ziegler, 2001), the W-E transect is approximately oriented in depositional strike direction.
The ca. 5 m-thick interval includes predominantly peloidal and oolitic grainstones. Thicknesses of grainstone beds vary from one section to another. Whereas sections in the east include up to 2 m-thick cross-bedded grainstone packages, the grainstone beds in the west are mostly of decimeter thickness and interbedded with bioturbated mud-wackestone layers (Figure 8). The m-thick grainstones are commonly well-sorted and show cross-bedding and erosive surfaces which indicate bed amalgamation. Thinner grainstone beds, which are interbedded with muddy layers, show sedimentary structures such as wave ripples, cm-thick fining-ups and sharp erosive bases. At the top of the cycle a thin white-colored mudstone/wackestone layer occurs, which occasionally exhibits exposure features such as fine rootlets, mudcracks or thin microbial laminae with tepee structures. Above, reworked sediments with cm-size muddy intraclasts are present.
The base of the fifth-order cycle was picked at a distinctively white-colored microbial boundstone, which shows karst-related brecciation in places (blue rudstones in Figure 8). These peritidal deposits turn vertically into an alternation of grainy and muddy dolomites of a backshoal to shoal setting indicating a landward migration of depositional environments. Maximum accommodation space is interpreted at the top of the thickest shoal-related grainstone package and marks the turnaround point from transgression to regression. Thinner grainstones and packstones above the interpreted zone of maximum flooding are assigned to the regressive part of the fifth-order cycle. A thin whitish mudstone/wackestone layer with exposure features (rootlets/mud cracks/microbial laminae) was selected as cycle top as it illustrates the most proximal facies in the 5 m-thick interval. The mudstone bed is interpreted as being deposited in a low-energy tidal flat (LFA 3) or coastal marsh setting (LFA 2) with intermittent subaerial exposure. However, due to the lack of fauna a detailed biofacies analysis could not be carried out for the marker bed.
Vertical facies variations within the cycle were used to further subdivide the interval into smaller scale cyclicity: sixth-order mini-cycles. The mini-cycles within the fifth-order cycle are interpreted based on vertical changes in depositional environment, commonly from low-energy to moderate-energy backshoal. Mini-cycle boundaries are picked at the base of erosive surfaces. The number of mini-cycles per section varies from two to three. Also, the stratigraphic position of mini-cycle boundaries differs from one section to another.
The cycle boundaries of the fifth-order cycle are correlatable over a distance of about 2,500 m, highlighting a layer-cake geometry with some minor thickness variation. However, within the fifth-order cycle the correlation of the individual grainstone layers is challenging, especially between sections that are hundreds of meters apart. Both, layer-cake and shingled correlation scenarios are equally feasible when outcrop conditions do not permit following individual beds laterally over the entire distance between two sections (Figure 8a, b).
The correlation of mini-cycle boundaries within the fifth-order cycle seems to be key to constrain the different correlation scenarios and results in inclined grainstone geometries (Figure 8c). Grainstones within mini-cycles dip in a westward direction reflecting a shingled architecture with a maximum dipping angle of 0.14°. Downlap occurs onto the fifth-order transgression surface and toplap along the upper fifth-order cycle boundary. The subtle inclination of the grainstone layers most likely indicates the ancient shoal topography when neglecting differential sediment compaction.
Due to the shallow dipping angle, the shingled pattern is only revealed in plots using high exaggeration. In outcrops, the shallow shingled geometries cannot be detected visually (Figure 9).
Highlighting the different depositional environments in a W-E profile (Figure 8d) suggests the presence of two coeval shoal bodies, which migrated in a westward direction (Figure 10). The conceptual 3-D facies model depicts an internally complex shoal body with intershoal areas of moderate-and low-energy environments. A lateral migration of two grain-dominated high-energy shoal bars over mud-dominated intershoal areas could result in patterns akin to those observed in the outcrop. Whether the lateral migration is related to transgressive-regressive processes or to sediment distribution due to storms, or a combination of both cannot be determined. The roughly strike oriented correlation panel makes the determination of a landward or seaward movement of facies belts somewhat ambiguous.
Level 3: Grainstone Architecture between Sixth-order Mini-cycle Boundaries
(a) 240 m Wall Panel
Outcrop conditions permitted a lateral tracing of a m-thick sixth-order mini-cycle over 240 m in a SE direction (Figure 11). The interval lies stratigraphically below the fifth-order cycle, which was described on Level 2 (Figure 8).
The 1 m-thick interval comprises an alternation of cm-thick bioturbated wackestone and packstone beds in the lower part. These turn vertically into dm-thick grainstone packages with cm-thick muddy interlayers. Grainstones comprise mm-size peloids, coated grains and occasionally well rounded mud intraclasts. Ripple bedding is common in grainstones. Erosive surfaces within grainstone layers indicate bed amalgamation. Upsection, a prominent whitish, laminated microbial boundstone occurs which is karstified at the top and appears brecciated in outcrops. This bed was chosen as flat datum in order to highlight depositional geometries in the sixth-order mini-cycle (Figure 11).
Laterally the sixth-order mini-cycle boundaries are correlatable over at least a distance of 240 m. Individual cm-thick layers are NW-dipping with an apparent angle of 1.7°. Grainstone wedges turn laterally into packstones and downlap onto the lower mini-cycle boundary. The wedge-like grainstone geometries are draped by a microbial laminite which forms a horizontal layer on top.
(b) 13 m Wall Panel
To investigate the grainstone geometries on small-scale in more detail, a m-scale 2-D wall panel of the same sixth-order mini-cycle was created over a distance of 13 m with detailed vertical sections every meter. The W-E oriented wall panel is located at the eastern end of the aforementioned NW-SE 240 m wall panel. A recess in the outcrop wall reveals the 3-D geometry of the grainstone unit.
By flattening the m-scale wall panel on the upper mini-cycle boundary and by using a high vertical exaggeration, a lobe-shaped architecture of individual grainstone packages is revealed. Cm-thick mudstone-wackestone beds separate individual grainstone packages and show apparent dipping geometries towards the E (< 7°) and towards the W (< 3°) when flattened on the top of the microbial boundstone marker bed (Figure 12). The bioturbated mud-dominated layers can be reworked and appear in some grainstone deposits only as mud intraclasts (e.g. section 9).
The grain-rich beds with sharp erosive bases, fining-ups and wave ripples most likely represent storm-induced lobate tempestites, which fill accommodation space in an intershoal area (Figure 13). In this context muddy interbeds represent post-event sedimentation during low-energy conditions and drape the sea-floor topography at the time of deposition. When the accommodation space was completely filled with sediment, microbes started to colonize the area. A subsequent relative sea-level fall presumably caused subaerial exposure and resulted in the karstification of the microbial boundstone.
Thus, on the scale of a sixth-order mini-cycle, dipping grainstone geometries are interpreted as a result of event-related sediment distribution as opposed to the lateral migration of entire facies belts. Hence, the dip direction of the grainstone wedges within a mini-cycle does not necessarily indicate a progradational or retrogradational pattern but rather the direction of sediment distribution during storm events. Currents may have caused additionally randomization of sediments.
DISCUSSION AND CONCLUSIONS
Epeiric carbonate ramps are absent from modern day environments. Hence, reservoir architecture and dimensions observed in modern analogs can be used only in a limited way for well correlations and static reservoir models from epeiric carbonate ramps (e.g. Khuff Formation). Reservoir geometries investigated in outcrops from a similar geotectonic setting may serve as more suitable analogs for subsurface correlations or modeling approaches. This study documented complex, but hierarchically organized heterogeneities in grainstone bodies from outcrops of a Triassic epeiric carbonate ramp. The determination of the sequence-stratigraphic level seems key to identify the predominating architectural geometry of grainstone deposits on this epeiric carbonate ramp.
Grainstones, which seemed on fourth-order cycle set scale laterally homogeneous and continuous, turned out to be internally composed of different architectural elements on different hierarchical levels (Figure 14).
Level 1: Fourth-order Cycle Sets
In this study, grainstones within a fourth-order cycle set appear at first sight as continuous “layer-cake” bodies over a distance of several kilometers. These up to 20 m-thick units show thickness variations of up to 30% over a distance of 4 km. Similar observations were made by Koehrer et al. (2011, 2012) on grainstone units from outcrops of the Oman Mountains, which were deposited on a Permian–Triassic epeiric carbonate ramp. These grainstones also form a layer-cake architecture with pinching and swelling geometries between fourth-order cycle boundaries over an area of 8 x 8 km.
Level 2: Fifth-order Cycles
The layer-cake framework of fourth-order cycle sets could be subdivided into several 4–5 m-thick fifth-order cycles. Between fifth-order cycle boundaries shingled grainstone geometries are present with an apparent dip angle of about 0.14°. In order to identify their architectural geometry, the interpretation and correlation of m-scale sixth-order mini-cycles was crucial. A systematic approach of identifying the most proximal facies by means of textural and compositional analyses, in combination with vertical changes in bed thickness allowed to determine mini-cycle boundaries with good confidence in grain-dominated intervals.
Without the framework of sixth-order mini-cycles, grainstone beds could have been correlated in alternative ways and result, for instance, in conventional pinching and swelling layer-cake geometries. The shingled architecture is interpreted to be associated to the lateral migration of facies belts which was either caused by transgressive/regressive processes and/or storm-related sediment distribution.
Level 3: Sixth-order Cycles
Within a m-thick sixth-order mini-cycle, dm-thick grainstones build inclined wedge-like deposits. The low-angle wedges are interpreted to be the result of storm events distributing grain-rich sediments from an adjacent shoal complex into low-energy backshoal areas. Their event-driven character is reflected by frequent amalgamation and reworking of the preceding deposits. Cm-thin mud beds commonly pinch out after a few 10s of meters and are occasionally preserved as mud clasts only. On this level of detail, the amalgamation of individual event beds would result most likely in fluid flow communication between grainstone beds in similar subsurface configurations.
The observed grainstone geometries in this study vary in complexity and dimension depending on the scale of observation. The smallest scale of observation (Level 3) has shown that, by walking out individual cm-dm-scale layers over a distance of 240 m, grainstone beds are arranged in an inclined fashion. Due to this observation it must be assumed that a grainstone detected in two separate vertical sections with a lateral spacing of several hundreds of meters might not be simply uniform but could reflect similar internal dipping geometries. As a consequence, a simple layer-cake correlation of grainstones over hundreds of meters (Level 2) is questionable. In this study, the determination of mini-cycles provided the framework to identify the orientation of the inclined shingle-like grainstone geometries in meter-thick intervals (Level 2). On a larger scale (Level 1), these inclined geometries might not be noticed in a seemingly homogeneous layer-cake pattern of 10s of meter thick grain-rich units which appear to be continuous over distances of several kilometers (Figure 14).
While the large-scale geometries (i.e. fourth-order layer-cake grainstones) may also be detectable in the subsurface via the integration of common subsurface investigation tools (seismic, well logs, cores, ditch cuttings), the small-scale heterogeneities may remain undetected. Yet, the sub-seismic grainstone shingles and wedges can have a crucial impact on fluid flow. Despite the challenge to detect these geometries in the subsurface, similar reservoir configurations should be considered as correlation scenario for epeiric carbonate ramp settings in the subsurface. These subtle shingle geometries could be tested in dynamic flow simulations against conventional layer-cake scenarios to evaluate their impact on fluid flow.
The authors gratefully thank Petroleum Development Oman (PDO) for sponsoring the project and together with the Ministry of Oil and Gas of the Sultanate of Oman for permission to publish this paper. We thankfully acknowledge Joachim Amthor, Jean-Michel Dawans, Aly Brandenburg, and Gordon Forbes for technical discussions during field visits. Daniel Bendias and Lisa Walz are especially thanked for their assistance in the field. We are grateful to Shuram Oil and Gas (Muscat) for fieldwork logistics support. The Sedimentary Geology Research Group of the University of Tübingen is thanked for feedback and ideas to this paper. Access to WellCAD 4.3 software was kindly provided by Advanced Logic Technologies (ALT). Anonymous reviewers are thanked for their helpful comments. GeoArabia’s Production Co-manager, Arnold Egdane, is thanked for designing the paper for press.
ABOUT THE AUTHORS
Michael Obermaier studied Geosciences at the Universities of Tuebingen (Germany), and Miami (Florida, USA). His PhD thesis (2013), a research cooperation between the University of Tuebingen and Petroleum Development Oman was on Triassic reservoir and seal characterization in outcrops and subsurface of Oman. Since 2013 Michael has been working as a Carbonate Geologist for Shell Global Solutions in Rijswijk, The Netherlands.
Nicklas Ritzmann studied Geoscience with emphasis on Carbonate Sedimentology at the University of Tuebingen, Germany where he was awarded his MSc diploma in June 2012. He is currently a Geoscientist in the research department of Baker Hughes in Celle, Germany. His main research focus is borehole image processing & interpretation and surface logging applications (advanced analysis of cuttings, mud gas, etc.).
Thomas Aigner studied Geology and Paleontology at the Universities of Stuttgart, Tuebingen/Germany and Reading/UK. For his PhD dissertation on storm depositional systems (1985) he worked at the Senckenberg-Institute of Marine Geology in Wilhelmshaven (Germany) and spent one year at the University of Miami in Florida (USA). He then became an Exploration Geologist at Shell Research in Rijswijk/Holland and Houston/Texas focussing on basin analysis and modelling (1985–1990). Since 1991 Tom has been a Professor and Head of the Sedimentary Geology Group at the University of Tuebingen. In 1996 he was a “European Distinguished Lecturer” for the AAPG. His current projects focus is on sequence stratigraphy and reservoir characterisation/modelling in outcrop and subsurface.