ABSTRACT

In the Upper Permian to Lower Triassic Khuff Formation in the Arabian Gulf, a vast shallow-marine carbonate platform developed broad facies belts with little significant changes in the lithofacies. However, trace fossil assemblages and ichnofabrics, in combination with sedimentological observations, serve in subdividing this platform and in distinguishing sub-environments. From proximal to distal, these are sabkha and salina, tidal flat, restricted lagoon, open lagoon, platform margin, shoreface/inner ramp, slope/outer ramp and basin/deeper intra-shelf. In this way, changes in relative sea level can be better reconstructed and guide the sequence stratigraphic interpretation. Meter-scale shallowing-upward cycles dominate the succession and, in addition to conventional methods, bioturbation, trace fossil assemblages and tiering patterns aid in interpreting subtidal, lower and upper intertidal and supratidal portions of these peritidal cycles. Bioturbation (and cryptobioturbation) have an impact on the primary reservoir quality before diagenetic processes overprint the deposits. For instance, deposit-feeders (such as vermiform organisms) introduce a certain amount of mud and decrease porosity and permeability considerably, whereas others like the Zoophycos-producers fill their dwellings with ooid grains and turn a mudstone from a barrier to a flow unit. This novel study demonstrates the value of ichnological information in carbonate reservoir characterization and the significance of trace fossil analysis in facies interpretation, reservoir zonation and the impact of bioturbation on the reservoir quality.

INTRODUCTION

The study of trace fossils, ichnology, has been proven a valuable method in the reconstruction of paleoenvironments (e.g. Curran, 1985; Pemberton, 1992; Bromley, 1996; Pemberton et al., 2001; McIlroy, 2004; Miller, 2007; Bromley et al., 2007; MacEachern, 2007). The early concept of ichnofacies zonation has been continuously refined and the constituent ichnocoenoses were widely used in distinguishing sub-environments (McIlroy, 2008). Given the need in the oil and gas industry for improved reservoir characterization, the ichnofabric concept has been developed over the last 20 years in close association between academics and industry (Bromley and Ekdale, 1986; Bockelie, 1991). Ichnofabric analysis serves in solving problems when trace fossils and their tiering patterns are available in cored wells (Taylor and Goldring, 1993; Taylor et al., 2003; Goldring et al., 2005; Knaust, 2009b). Key stratigraphic surfaces become obvious when employing ichnology and can be used in sequence stratigraphic analysis (Taylor et al., 2003; MacEachern et al., 2007). Certain ichnotaxa are even useable for biostratigraphy, backed by the progress achieved in ichnotaxonomy (Seilacher, 2007). Recent advances in ichnology show the immense impact of infaunal organisms in generating biogenically enhanced porosity and permeability, with implications for hydrocarbon and aquifer reservoirs (Pemberton and Gingras, 2005; Gingras et al., 2007; Pemberton et al., 2008; Cunningham et al., 2008).

In the oil and gas industry, trace fossil analysis covers three key reservoir characterization aspects:

  • (1) Facies interpretation by utilizing the ecological information of trace fossils and ichnocoenoses;

  • (2) Reservoir zonation by identifying sequence boundaries and flooding surfaces for correlation;

  • (3) Reservoir quality and connectivity, which is directly effected by bioturbation.

Ichnological studies have typically played only a minor role in the sedimentological analysis of carbonate reservoirs (e.g. Goldring et al., 2005; Pemberton and Gingras, 2005), although the study of ichnofabrics has long been used in siliciclastic deposits (Taylor and Goldring, 1993; McIlroy, 2004), proving to be a valuable tool for the characterization and prediction of reservoir quality, the recognition of potential flow barriers and prediction of lateral depositional trends.

Using one of the most important reservoirs in the Middle East, the Middle Permian to Lower Triassic Khuff Formation, as a case study, this paper will demonstrate the feasibility and value in the application of ichnological concepts and methods in shallow-marine carbonate systems. This is the first comprehensive ichnological study carried out and published on the Khuff Formation. The results show that ichnology also has much to contribute in the understanding of carbonate reservoirs, e.g. in their zonation, heterogeneity evaluation and prediction of flow units.

STUDY AREA

The Middle Permian to Lower Triassic Khuff Formation is extensively distributed in outcrops and in the subsurface in the region around the Gulf and contains important gas reserves in the Middle East (e.g. the Ghawar field in Saudi Arabia, the North field in Qatar and the South Pars field in Iran). The carbonates of the Upper Khuff Formation in Abu Dhabi are separated by the Middle Anhydrite from the Middle Permian Lower Khuff Formation, and are equivalent to the K1 to K4 reservoir intervals in Qatar and Oman, the Khuff-A to Khuff-C units in Saudi Arabia, and the Kangan Formation and the Upper Dalan Member in Iran (Figure 1).

Stratigraphically, the entire Khuff Formation forms a second-order, transgressive-regressive sequence (Sharland et al., 2001; Strohmenger et al., 2002), built by third-order sequences, which are controlled by major sea-level changes (Sharland et al., 2001, 2004). In large parts of the Gulf, the KS4 through KS1 sequences are recognized and correspond to the K4 to K1 reservoir intervals (Insalaco et al., 2006). During the Late Permian and Early Triassic, Arabia was situated in a low latitude position in the southern hemisphere (Figure 2).

In the subsurface, the succession reaches a thickness of up to 970 m (Alsharhan, 2006) and mainly consists of limestones, dolomites and anhydrites. At a large scale, the depositional environment was a rimmed carbonate platform with an extensive inner platform in the south, which was separated from a more open-marine intra-shelf low in the north by a broad belt of grainy shoals (Al-Jallal, 1995; Dasgupta et al., 2002; Insalaco et al., 2006; Figure 2).

MATERIAL AND METHODS

Core material from a 445 m section through the Middle Permian to Lower Triassic Khuff Formation in three exploration wells was available for this study (well cores with a total length of 1,100 m), of which the cored part in one well is most complete and therefore is regarded as a key well. A comprehensive sedimentological, diagenetic and geochemical study (unpublished material; Svana et al., 2007; Garland et al., 2008, Eliassen et al., 2008; Ehrenberg et al., 2008) leads to a good understanding of the depositional environment and diagenetic history. Concurrent with this, an ichnological analysis was carried out on the medial cut of the core and by employing high-resolution core images from its counterpart. Numerous intervals are covered by thin sections, which partly reveal bioturbation patterns and discrete trace fossils.

Degree of Bioturbation

The degree of bioturbation is an important attribute in ichnological analysis. It indicates the amount of endobenthic colonization versus deposition rate, shows hiatuses in the sedimentation, and its dependence from available nutrients and oxygen. Animals (and plants) penetrate the substrate to successive depth because of size and/or their needs to utilize different feeding and respiratory strategies (Taylor et al., 2003). The resulting tiering pattern consists of the thoroughly bioturbated mixed layer at the top, the transition layer and the underlying historical layer without active bioturbation (Bromley, 1996). To achieve an overview of the degree of bioturbation and its change, the bioturbation index (BI) was logged in conjunction with the lithofacies. It represents the amount of reworking (sediment mixing by animals and plants) with respect to the original sedimentary fabric and ranges from zero (no bioturbation) to six (complete bioturbation; Reineck, 1963; Taylor and Goldring, 1993; Bromley, 1996; Taylor et al., 2003).

Logging of the BI is challenging due to a number of difficulties:

  • Different types of lithology reflect the BI in a different way as its development and visibility is dependent on grain sizes, colors, cementation, etc. (Figure 3). Therefore, a subjective factor (systematic error) in the process of estimating the BI cannot be excluded.

  • Identifying bioturbation trends and stacking patterns of ichnofabrics is scale-dependent. In accordance with the sedimentological log, logging of the BI was carried out in a scale of 1:50 with a lower limit of 5 centimeters for the smallest unit regarded.

  • The size of trace fossils is crucial for recognizing sediment mixing due to bioturbation. In addition to macrobenthos, meiobenthic animals (shortest body size less than 1 mm) are important bioturbators but their effect is limited by the grain size (Dashtgard et al., 2008). This, and diffuse bioturbation by macrobenthic animals are known as cryptobioturbation (Howard and Frey, 1975; Pemberton et al., 2001). Estimating the correct BI is depended on contrasts in grain size, colors and not least upon the imagination of the ichnologist.

  • The Permian – Triassic boundary is characterized by a major loss of organisms, which is reflected in a decrease of the degree of bioturbation. The content of the trace fossil assemblages and the size of the trace fossils changes dramatically across this boundary.

  • Sedimentary processes (e.g. dewatering) can produce structures (e.g. dewatering pipes) similar to simple trace fossils (e.g. Skolithos) and may lead to confusion. Moreover, some sediment mixing can result from physical re-sedimentation and dewatering as well as from biological bioturbation.

  • With continuous shallowing-upward trends, different carbonate lithofacies turn into caliche deposits, in which diagenetic features can overprint plant-sediment activities (e.g. rooted horizons).

Trace Fossil Content

Studying trace fossils in core is challenging since there are major differences compared to the observation of trace fossils in outcrop (Bromley, 1996; Chamberlain, 1978). Nevertheless, trace fossils were identified as accurately as possible for each lithological (sedimentary) unit.

Advantages of describing trace fossils in cores include:

  • Vertical continuity in long cores, overview of facies succession;

  • Limited weathering, core usually protected from excessive diagenesis by the passage through it of hydrocarbons;

  • Cores provide optimum subsurface information for correlation of facies;

  • Large amount of data (geological, petrophysical) generated from core studies are available to support the ichnological data;

  • Core data are often available from areas where outcrops are lacking.

Disadvantages include:

  • Narrowness of the sample;

  • Small likelihood that large trace fossils are represented in the sample;

  • 2-Dimensional expression on core slab;

  • Bedding-plane views are rarely available in core or of very limited areal extent.

For the reasons given above, trace fossil determination in this study is restricted to the ichnogenus level and thus differs from most ichnological studies with outcrop data. However, a few trace fossils are undetermined to identify in 2-D core sections. Moreover, different organisms can produce the same ichnotaxon (Bromley, 1996).

For example, sub-vertically oriented, passively filled burrows with a circular cross-section are commonly referred to as Skolithos. This simple trace fossil with only few features can be produced by a wide range of animals (Bromley, 1996) and plants (Gregory et al., 2006).

Long, vertically oriented spindle- to funnel-shaped burrows with a thick laminated mud wall around it resemble Rosselia, which is interpreted as a terebellid polychaete dwelling (Nara, 1995). In the studied cores, these structures are commonly present on micritic bedding planes (mudstone) but were more likely produced by plant activity in the form of root calcretes (see Alonso-Zarza and Jones, 2007). This interpretation is supported by the co-occurrence of more irregularly shaped structures and simple sand-filled root traces. They typically occur on top of shallowing-upward cycles in association with caliche deposits, dissolution features (e.g. breccia), dolomitization and cement-filled cavities (root cavities).

Chondrites consists of fine, downward-bifurcating branches and passive fill from the overlying sediment. It may also be produced by plant-rootlets, which were filled subsequently by sediment. Downward-reducing diameters of the branches would be the only different from animal-produced Chondrites. Small simple rootlets could, however, be easily confused with small fractures.

A major problem for the determination of ichnotaxa and the estimation of the BI is their common overprint by dissolution phenomena and subsequent differential cementation during various stages of diagenesis. This diagenetic overprint results in ichnoconcretions or patterned fabrics (Kirkham, 2004) and often obliterates the primary outline and structure of the affected trace fossil.

A total of 22 ichnotaxa was identified in addition to cryptobioturbation, mottling, small and large plant roots, equilibrium and escape traces (Figure 4, see enclosed poster). They build trace fossil assemblages and are plotted against the BI.

Trace Fossil Assemblages and Ichnofabrics

The identified trace fossils constitute trace fossil assemblages (TFA), of which ten were identified in the studied cores, each of which is characterized by its relationship to a specific substratum, degree of biotubation and ichnodiversity (Table 1, Figure 5). Trace fossil assemblages refer to all of the trace fossils in a bed that cannot be subdivided into component ichnocoenoses or suites (McIlroy, 2008) and are named according to the dominating ichnotaxon, which is typically accompanied by other trace fossils. They can constitute characteristic ichnofabrics, of which the most conspicuous were recorded by line drawings to highlight discrete trace fossils and their relation to each other.

Ichnodiversity

The ichnodiversity (ID) was logged by counting the number of recognized ichnogenera within each unit. Because ichnotaxa were artificially established following the International Rules of Zoological Nomenclature, but are not real species in the sense of biology, the ID must not be confused with biodiversity but serves as a biodiversity proxy. It provides a clue about the diversity of the benthic community within each sub-environment and thus enables to estimate influencing stress factors such as suspension, oxygen, energy, salinity, temperature etc. A further limiting factor is the overprinting of pre-existing trace fossils by repeated bioturbation. On the composite log, the ID equals the number of recognized ichnotaxa (maximum 8) for each horizon. This data is generalized into three grades of ID, e.g. 1 (low, 1–3 ichnotaxa), 2 (moderate, 4–6 ichnotaxa) and 3 (high, 7–8 ichnotaxa).

Ichnopermeability

Bioturbation directly impacts the petrophysical properties of sediment and two main processes can be distinguished, each of them either leading to enhanced or reduced porosity and permeability: (1) Bioturbation by macrobenthos, e.g. animals larger than 1 mm, which is relatively easy to recognize by its disturbance of existing sedimentary structures such as layering. (2) Cryptobioturbation mainly by meiobenthic organisms, such as many members of the phyla nematoda, platyhelmintha, sipuncula, annelida and arthropoda. This type of bioturbation is often related to the movement of organisms in interstices (pore space) without significant movement of grains and thus the sediment still retains the bedding or lamination. Therefore, cryptobioturbation is often hard to recognize due to the lack of reliable criteria and could be mistaken as post-depositional sedimentary structures such as dewatering features. In the presented study, only cryptobioturbation with unequivocal biological disturbance was mapped, though a combination with small-scale dewatering structures cannot be ruled out completely.

FACIES INTERPRETATION BY MEANS OF TRACE FOSSIL ASSEMBLAGES AND ICHNOFABRICS

In a larger regional scale, the depositional system is mainly understood as a rimmed carbonate platform for the Upper Permian part, which conforms to the Wilson model (Wilson, 1975). In Triassic time, the platform profile changed to a monoclinal ramp platform geometry (Insalaco et al., 2006). Considering the recognized sub-environments, which is based on the available core material as well as published data (Insalaco et al., 2006), the standard facies zones of the Wilson model could be modified accordingly (Figure 6, see enclosed poster). A generalized cross section from south (proximal, Saudi Arabia) to north (distal, Iran) simplifies the recognized facies zones together with the related trace fossil assemblages, bioturbation index and ichnodiversity. In the following, each facies zone (facies association) is briefly characterized with respect to trace fossil assemblages and ichnofabrics in order of their occurrence in a proximal to distal direction. Examples of representative trace fossil assemblages are illustrated in Figure 5.

Sabkha and Salina

This most proximal facies zone consists of supratidal mudflats that were affected by pedogenic processes and karstification. As a result, altered mudstone occurs in the form of breccia, laminated caliche crusts and nodules, and alternates with anhydrite and gypsum. The only traces of macroscopic organisms are those of plant roots in a varying density and depth penetration.

Two main types of plant roots can be distinguished: (1) Large, centimeter to decimeter-sized roots with a predominant vertical sediment penetration, minor branching and rootlets, carbonaceous material and common infill with anhydrite cement. (2) Small, millimeter- to centimeter-sized roots with irregular branching, preserved with carbonaceous material. The large root traces are abundant in the Permian interval, but the small root traces occur in association with the large ones or in isolation throughout the Permian – Triassic succession. The large root traces could be related to a group of plants that went extinct at the Permian – Triassic boundary, such as cordaitalean which thrives in a marine-influenced coastal habit (Falcon-Lang, 2005). The small root traces are partly related to the large ones or occupy the ecological niches of sea grass.

Root traces are often involved in building complex trace fossils, in which they occur in the latest colonization event and overprint more diverse trace fossil assemblages resulting from bioturbation in an intertidal environment (e.g. tidal flat, shallow-lagoon). In other cases, several meters of stacked successions of paleosols indicate repeated colonization surfaces in a relatively persistent supratidal environment.

Tidal Flat

Tidal flat deposits are quite extensive and variable in respect to the lithofacies. Dolomitic mudstones with a laminated and patterned fabric dominate together with unbioturbated biolaminites within the upper intertidal zone. The dolomitic mudstone contains root traces, which may be the nuclei for the genesis of patterned dolomite, a diagenetic feature common for intertidal to supratidal environments (see discussion in Kirkham, 2004).

In the lower intertidal zone, thin layers of rooted mudstone are intercalated with oolitic and intraclastic grainstone and packstone. The grainstone and packstone lithofacies results from a variety of intertidal to shallow subtidal sand deposition, such as beach sands, sand waves, tidal channels, tidal deltas and storm deposits. It is characterized by the Asterosoma TFA, in which thick mud-lined Asterosoma burrows occur in a highly bioturbated ichnofabric. Minor internal erosional surfaces indicate repeated reworking probably by tidal currents. Asterosoma (and the related trace fossil Rosselia) colonized a mid to deep tier and shifted upward in response to sedimentation. In modern nearshore environments, similar traces are produced by terebellid polychaetes (Nara, 1995). In the geological record, Asterosoma has been recognized as an opportunistic trace fossil in marginal marine facies, especially in tidal flats (Bromley and Uchman, 2003).

Shallow subtidal areas within the tidal flat, such as ponds and hypersaline lagoons, contain elements of the Taenidium TFA and Planolites TFA. They are characterized by low ichnodiversity and high abundance and commonly occur in the restricted lagoon.

Restricted Lagoon

This facies zone may be present more or less pronounced between the tidal flat and the open lagoon/platform margin and the tidal flat. Its lithofacies is composed of dolomitic mudstone with thin wackestone interlayers. Lamination and homogeneous bedding is common. Scattered pseudomorphs of evaporate minerals and small anhydrite nodules are characteristic of hypersaline and saline depositional and diagenetic conditions.

The Taenidium TFA is most prominent in dark grey to beige mudstone to wackestone and consists of a low to moderate diverse ichnocommunity. It may extent into the intertidal zone and is indicative of a low bottom oxygenation. The opportunistic colonization style affects the upper and mid-tier and seems to be related to the frequent influx of sand by currents or storms. Taenidium occurs in both, marine (e.g. Blissett and Pickerill, 2004; Savary et al., 2004) and continental deposits (e.g. Savrda et al., 2000; Minter et al., 2007), and is most common in fluvial floodplain mudstone and playa deposits, where it is probably produced by arthropods (including insects), polychaetes or oligochaetes (e.g. Savrda et al., 2000; Minter et al., 2007).

The Planolites TFA occurs in gray, homogeneous and laminated mudstone. It consists of a low ichnodiversity but the degree of bioturbation can be high. It results from a shallow to mid-tier deposit-feeding community (polychaetes and other worm-like organisms, Pemberton et al., 2001) in a stagnant (quiet) subtidal environment. The Planolites TFA interfingers with the Arenicolites TFA.

The Arenicolites TFA co-occurs with the Planolites TFA and is characterized by laminated, beige to brown dolomitic mudstone to wackestone. The mud-sand interlayering indicates a better water circulation than in the Planolites TFA, with frequent input of sand. The ichnofabrics are rather simple, especially in the Lower Triassic interval, and are dominated by small trace fossils with a low to moderate ichnodiversity and bioturbation. Suspension- and filter-feeding annelids or small crustaceans are known to produce similar traces in modern nearshore environments (Pemberton et al., 2001). High abundance and low ichnodiversity point to a short-term colonization of thin event beds.

Open Lagoon

An open lagoonal facies zone is locally developed between the platform margin and the tidal flat. In contrast to the restricted lagoon, normal marine conditions prevail with adequate circulation. The lithofacies is mud-dominated; however, wackestone, grainstone and packstone interlayers result from frequent washover and storm deposition.

Relatively quiet areas of the lagoon, perhaps situated in a more proximal position close to restricted lagoonal parts, are characterized by mudstone containing the Zoophycos TFA. The bioturbation degree as well as ichnodiversity are high and indicate a suitable environment for a wide endobenthic community with reduced sedimentation rates. The highly bioturbated sediment contains elements of a softground to firmground suite with slightly dysoxic bottom conditions (small Chondrites). It is overprinted by discrete trace fossils such as Zoophycos, Asterosoma or Siphonichnus.Knaust (2004, 2009a) describes similar Zoophycos form the Middle Triassic (Muschelkalk) of Germany and the lowermost Khuff Formation in the Huqf area of Oman, where it occurs in a marginal-marine environment in relation with the marine transgression. The supposed trace maker is interpreted to be a kind of polychaete.

In more protected, proximal lagoonal parts, the Zoophycos TFA intergrades with the Taenidium TFA, whereas in sandy areas behind the platform margin (more distal), transition to the Asterosoma TFA is common. In the latter case, sediment mixing within grainstone or packstone is common but often prohibits identification of certain ichnotaxa. The intense bioturbation is restricted to certain horizons with a generally lower degree of bioturbation than in the muddy part of the lagoon. The Asterosoma TFA is fully developed in sandy subtidal to intertidal facies belts related to sand waves and washover deposits just behind the platform rim.

Platform Margin

The interior platform is protected from the open sea currents and waves acting on the intra-shelf low by an extensive rim or platform margin, dominated by clean grainstone and packstone with local occurrence of thin mudstone interlayers.

The leeward side of the margin grades into the Asterosoma TFA of the open lagoon, of which sandy tidal flat deposits are subject to emersion during migration of the shoal in a landward direction. The resulting ichnofabric consists of bioturbated/mottled grainstone with mudstone interlayers and penetration of root traces. The incidence of root traces and associated caliche deposits increases toward the central part of the rim, which is built by elongated shoals and tidal bars with subsequent reworking by eolian processes.

In contrast to this relatively low-energy system, the seaward side of the margin is directly influenced by the action of waves and currents with high-energy deposition. Only few organisms are capable of coping with such harsh conditions in a foreshore environment, such as certain polychaetes and meiobenthic animals that are responsible for a high to intense bioturbation with a low ichnodiversity (Pemberton et al., 2001). The resulting Macaronichnus TFA is well constrained to such foreshore environments (Seike, 2007).

Storm-reworked sand shoals and sand waves in the foreshore/upper shoreface transition zone are dominated by packstone and grainstone with sporadic bioturbation and few discrete trace fossils. Thin mudstone layers are intercalated and represent breaks between storm events. They were subject to initial bioturbation (e.g. Arenicolites in an opportunistic colonization style) and firmground to hardground bioeroding (e.g. Balanoglossites, Gastrochaenolites, Palaeosabella), indicating a rapid consolidation of sediment within this Balanoglossites TFA (Knaust, 2007, 2008).

Shoreface/Inner Ramp

The packstone and grainstone facies of the seaward platform margin is connected to the shoreface or inner ramp deposits, which differ from the former by general higher mud content. The degree of bioturbation is commonly low, though some flooding events can be accompanied by intense mottling. The ichnodiversity is moderate. Various cross-bedding features dominate over homogeneous grainstone.

The Skolithos TFA is common throughout but most conspicuous in the cross-bedded grainstone facies. The simple vertical shafts of Skolithos are associated with escape traces and indicate a relatively high-energy system due to frequent storm reworking. The Asterosoma TFA and the Cylindrichnus TFA often accompany the Skolithos TFA.

The Cylindrichnus TFA is more pronounced towards the seaward direction and occurs in association with thick sand packages probably representing the wave- and current-exposed margin of subtidal shoals. The thick mud-lined burrows often occur isolated as reworked clasts (ichnoclasts) within the cross-bedded grainstone facies and document a certain amount of sediment reworking by storms (see Goldring, 1996; Goldring et al., 2005).

The grainstone and packstone are occasionally interrupted by thin mudstone layers with a trace fossil association typical of firmgrounds and hardgrounds (Balanoglossites TFA).

Slope/Outer Ramp

The slope or outer ramp is characterized by lithofacies types with a reduced grain size, such as grainstone, wackestone and mudstone. The degree of bioturbation is relatively low and consists of a moderate ichnodiversity.

The Cylindrichnus TFA continuous from the shoreface/inner ramp but appears in a more distal expression with a higher mud content and wavy to flaser bedding instead of thick cross-bedded intervals.

The Skolithos TFA is also present, accompanied by Asterosoma in the proximal part (mid ramp).

Basin/Deeper Intra-shelf

Basinal or deeper intra-shelf areas are poorly represented in the studied cores. They consist of laminated mudstone/marlstone with intercalations of wackestone and grainstone, grading upward into grainstone and wackestone of an outer ramp origin. Overall, the deposits are weakly bioturbated with a moderate ichnodiversity.

PERITIDAL CARBONATE CYCLES (PARASEQUENCES) BASED ON BIOTURBATION, TRACE FOSSIL ASSEMBLAGES AND TIERING PATTERNS

The recognition of key stratigraphic surfaces for field-wide correlation, and the zonation of the reservoir as basis for geological modeling, is not straightforward in relatively uniform carbonate successions such as the Khuff Formation. Ichnological methods have the potential to contribute with the identification of stacked sequences build-up of typical facies units (with distinctive trace fossil content as described above) and their boundary surfaces (often characterized by intense and typical bioturbation patterns).

The complete cored section under consideration is organized into meter-scale shallowing-upward cycles (peritidal parasequences), defined by stacked sedimentary units (facies types) and bounded by flooding surfaces (transgression surfaces). The cycles are stacked on top of each other and may contain differing facies types and degree of completeness. In general, cycles are best developed within a long-term early TST of third-order sequence, but are more cryptic and complex during other periods (e.g. late TST, HST), where they might be overlooked (Spence and Tucker, 2007). The degree of bioturbation together with trace fossil assemblages and ichnofabrics can serve in recognizing peritidal cycles as exemplary shown in the Middle Triassic (Muschelkalk) carbonate ramp of Germany (Knaust, 1998).

For example, the top of a high frequency sequence in the key well (3,018–3,027 m) is composed of cycles with shallowing-upward trends from a subtidal to supratidal environment (Figure 7).

Subtidal

The subtidal portion of parasequences typically appears in one of two facies variations because of deposition in varying sub-environments by involving different processes.

  • (A) After a sharp, slightly erosive boundary, the cycles start with a thin sand layer (grainstone), commonly bearing a certain amount of mud and reworked mud clasts. The degree of bioturbation is high to intense (BI = 4–5) with a moderate to high ichnodiversity. Sediment mixing is common and results in a mottled grainstone fabric with intermingled mud. Occasionally, discrete trace fossils such as Arenicolites, Skolithos, Diplocraterion, Asterosoma, Rosselia, Planolites and equilibrium traces can be recognized. This clearly indicates a shallow subtidal environment with limited restriction of exchange with the open sea. The thin muddy sand layers partly represent lag deposits originated during transgression or result from moribund shoal deposits with original tidal mud drapes. Tiering by one or more ichnocoenoses is common. Sporadic penetration of root traces indicates a late colonization in a very shallow subtidal environment or during beginning of emersion.

  • (B) A different, rare variation of cycles starts with a mudstone to wackestone unit, consisting of internal erosion surfaces and graded sand to mud layers above. These surfaces act as colonization surfaces and yield an ichnocoenosis mainly consisting of Planolites and Palaeophycus, rarely also Skolithos. The overall bioturbation is high (BI = 4) but some surfaces are almost completely bioturbated with downward-decreasing intensity. The colonization surfaces represent omission surfaces and are related to the rapid drowning of the platform. Ichnodiversity is low and the ichnofabric documents a restricted subtidal environment within a lagoon.

Lower Intertidal

The following unit commonly consists of grainstone/mudstone alternations, in which thin mudstone layers are intercalated between grainstone beds with varying thickness. The grainstone is homogeneous, rarely cross-bedded and may contain reworked mud clasts. The degree of bioturbation is sparse to low (BI = 1–2), consisting of mottled intervals, Palaeophycus, Skolithos, Diplocraterion and escape traces, overprinted by root traces. In thicker grainstone beds, cryptobioturbation can dominate. The mudstone interlayers are homogeneous and only contain root traces. In some intervals, it becomes apparent that root penetration took place through the sand and terminated within the mudstone, where it rested for stabilization reasons. The grainstone/mudstone alternations are typical for a lower intertidal setting, where sand bodies (e.g. small beach sand ridges) are continuously in motion and migrate over muddy depressions (troughs) between them. The latter are subject to early compaction and transformation into firmground. Cross-bedded grainstone facies with a basal lag deposit (intraclasts) originated by gently migrating tidal channels.

Upper Intertidal

The upper intertidal portion of parasequences typically is expressed by one of the following two facies variations.

  • (A) Dolomite with microbial laminates (microbialite) occasionally occurs but is not bioturbated. Laminated to patterned dolomitic mudstones to wackestones are mottled by root traces.

  • (B) Patterned dolomite is a characteristic lithofacies type and consists of a mottled appearance due to concentration of microcrystalline iron sulphide (Kirkham, 2004). The pattern often follows internal irregularities in form of grain size differences, intra-formational contortion and root penetration. The latter seems to be an important agent for the origin of patterned dolomite in the Khuff Formation and usually the outline of the roots are still preserved. Root traces are the only trace fossils within this lithofacies and the degree of bioturbation is often moderate to intense (BI = 3–5). The sulphide concentration in the patterned dolomite is a result of sulphate-reduction by bacteria and typically occurs under hypersaline conditions. Subtidal through supratidal environments are favored as loci of origin of patterned dolomite by various workers (see Kirkham, 2004), whereas its position in the Khuff cycles is between lower intertidal and supratidal deposits and clearly indicates an upper intertidal environment (following Walther’s law).

Supratidal

The top of many cycles is capped by dolomudstone with typical caliche features. The crusts are crudely laminated, often distorted, cracked and in-situ brecciated. Root structures (rhizogenic calcretes, alveolar structures) are common and lead to a moderate to intense degree of bioturbation (BI = 3–5). The caliche unit either develops gradually from the underlying intertidal facies by continuous exposure (shallowing), overprinting the laminated or patterned dolomites, or it forms as a distinct unit of supratidal deposits.

THE IMPACT OF BIOTURBATION ON RESERVOIR QUALITY

The reservoir quality of the studied cores is partly controlled by bioturbation, which modifies the primary sedimentary fabric, although a subsequent diagenetic alteration (e.g. recrystallization, dolomitization) can overprint this biogenic modification in places (Insalaco et al., 2006; Ehrenberg, 2006; Ehrenberg et al., 2007; Garland et al., 2008; Eliassen et al., 2008). The process of biogenically enhanced permeability was reviewed by Pemberton and Gingras (2005), and burrow-related porosity in carbonates was recently discussed by Cunningham et al. (2009).

In the Khuff material studied herein, four aspects related to bioturbation are important as an impact on the primary reservoir quality before alteration by diagenesis.

Porosity/Permeability Reduction by Bioturbation

Deposit-feeding animals such as the producers of Cylindrichnus, Asterosoma and Phycosiphon introduce a certain amount of mud when colonizing the top surface of sandy waves or shoals. Dwelling burrows like Palaeophycus, Rosselia and Siphonichnus commonly contain a certain amount of mud and reduce the sand/mud ratio within the affected interval. As a result, horizons with a considerable decrease in porosity and permeability can occur within highly permeable units and act as relatively thin baffle or barrier (Figure 8). Those horizons with reduced properties can either occur locally or follow flooding surfaces at the base of shallowing-upward cycles and then are widespread.

Porosity/Permeability Reduction by Cryptobioturbation

Cryptobioturbation, mainly related to the sediment modifying activity of meiobenthic animals, may also result in a considerable reduction of the reservoir quality (Figure 9), although the opposite is commonly the case. However, the sediment stirring activity of meiobenthos is very complex and organisms of completely different trophic groups can be involved. Similar to the porosity/permeability reduction by bioturbation, the process of cryptobioturbation can incorporate a substantial amount of mud, for instance by defecation (e.g. fecal pellets).

Porosity/Permeability Enhancement by Bioturbation

The studied material provides clear evidence of biogenetically enhanced properties. Firstly, Zoophycos is able to increase porosity and permeability in a c. 30 m thick interval of mudstone in lagoonal to tidal flat facies (Figure 10). The mudstone is interrupted by thin grainstone layers and it contacts the underlying grainstone unit, which is characterized by excellent reservoir quality. Extensive burrowing by Zoophycos trace markers destroys the homogeneous mudstone and the fill of their dwellings consists of ooid grains that contrast to the dense mudstone fabric. From the available behavior models of the Zoophycos producing animals, the refuse dump model (Bromley, 1991) may apply to Zoophycos from the Khuff Formation. It implies deposit feeding deep in the sediment and excreting the feces on the sediment surface, followed by introducing surface material (ooids) into the burrow to compensate the space created during the feeding process. By this process, permeability is created and enables fluids to flow from the underlying grainstone through the mudstone baffle. Moreover, mixing zone dolomitization (Svana et al., 2007) preferably takes place in the mudstone unit by following the burrow paths.

Secondly, Macaronichnus is the product of deposit-feeding polychaetes and improves the reservoir quality extensively if bioturbation is high (Figure 11). Macaronichnus commonly occurs in foreshore to shoreface deposits (e.g. outer platform margin) and seems to be restricted to this narrow facies belt (Seike, 2007; Bromley et al., 2009). The worms process the sand intensely, mainly in horizontal but also in oblique directions and thus destroy the sedimentary features such as bedding and lamination almost completely. The result is homogenized sediment with an isotropic behavior. Furthermore, the fluid flow is enhanced by separation of clean sand grains in the burrow cores and fine-grained heavy minerals and clay material within the surrounding mantle.

Porosity/Permeability Enhancement by Cryptobioturbation

Cryptobioturbation (see above) often increases permeability considerably in many parts of the succession due to the preferential colonization of instable sand shoals in high-energy settings.

CONCLUSIONS

The studied upper part of the Khuff Formation (Upper Permian to Lower Triassic) comprises relatively monotonous lithologies including limestones, dolomites and anhydrites that were deposited as broad facies belts. A differentiated carbonate ramp with changing sub-environments was responsible for centimeter- to meter-scaled variations in lithology, which impact reservoir quality and flow behavior. Beside well-recognized diagenetic processes, which overprint the complete succession, sedimentological features are also responsible for these small-scale heterogeneities. For the first time it can be shown, how the analysis of trace fossils and bioturbation can improve our understanding of the reservoir in the Khuff Formation.

Ichnological methods are demonstrated to be valid in three major aspects of reservoir characterization: (1) Facies interpretation and reconstruction of depositional environments; (2) sequence stratigraphy and reservoir zonation; and (3) the impact of bioturbation on reservoir quality and flow behavior.

A total of 22 trace fossils were identified in the cored sections and plotted against the bioturbation index, ichnodiversity and lithological/petrophysical data. They comprise ten trace fossil assemblages, which aid in the identification of the following sub-environments: Sabkha and salina, tidal flat, restricted lagoon, open lagoon, shoal, platform margin, shoreface/inner ramp, slope/outer ramp and basin/deeper intra-shelf. Based on this facies distinction, trace fossil assemblages and tiering patterns are important elements to recognize meter-scale shallowing-upward cycles (peritidal parasequences) as a basis for correlation and reservoir zonation a helpful procedure considered to be complementary with conventional methods. Finally, bioturbation is able either to reduce or to enhance porosity and permeability of the reservoir by means of different and complex organism-sediment interactions.

ACKNOWLEDGEMENTS

I would like to thank Stephen N. Ehrenberg (PanTerra) and Joanna Garland (Cambridge Carbonates Ltd) for providing information and for stimulating discussions. Stephen N. Ehrenberg and Tore A. Svånå (StatoilHydro) kindly provided me with their sedimentary core logs and interpretations. I thank Geraint W. Hughes (Saudi Aramco) and an anonymous reviewer for their constructive comments on the manuscript. The final design of the paper by Arnold Egdane is appreciated.

ABOUT THE AUTHOR

Dirk Knaust is presently a Principal Geologist with StatoilHydro ASA in Stavanger, Norway, where he works in the Geological Reservoir Characterization group. He received a PhD from the University of Greifswald, Germany, in 1998. His geological experience, prior to joining Statoil in 2006, includes 8 years technical services in Norway as a consultant for different companies operating fields in offshore Norway and internationally (Shell, Statoil, Hydro, BP, Amoco). Dirk’s professional activities include the application of sedimentological, stratigraphical and paleontological methods in the characterization of siliciclastic and carbonate reservoirs. His special focus is on ichnology and its implication on facies distribution, reservoir zonation and impact on reservoir quality. Dirk is a member of SEPM and the Palaeontological Association.

dkna@statoilhydro.com