Sedimentary Architecture and Palaeogeography of Lower Slochteren Aeolian Cycles From The Rotliegend Desert-Lake Margin (Permian), The Markham Area, Southern North Sea
Published:January 01, 2011
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F.J.G. Van Den Belt, F.F.N. Van Hulten, 2011. "Sedimentary Architecture and Palaeogeography of Lower Slochteren Aeolian Cycles From The Rotliegend Desert-Lake Margin (Permian), The Markham Area, Southern North Sea", The Permian Rotliegend of the Netherlands, Jürgen Grötsch, Reinhard Gaupp
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The Rotliegend gas play in the Southern Permian Basin has yielded over 200 gas fields in the Netherlands; they are found in an E–W fairway along the southern flank of the basin. Sandstones generally pinch out basinward, but localized, isolated sands are present north of the main fairway. The Rotliegend of the Markham gas field and a number of smaller fields in its vicinity (“Markham area”) provides a good example of such an isolated sand occurrence, and it may serve as a model for exploration in the “feather edge” of the Rotliegend desert lake.
The reservoir interval (Lower Slochteren Member) is a diachronous sequence, 20–50 m thick, from aeolian-dune sandstones to desert-lake mudstones. Periodic fluctuations of lake level, probably controlled by short-period Milankovitch rhythms (precession or obliquity) resulted in the formation of desert-lake mudstone drapes that compartmentalize the reservoir over kilometers. The Lower Slochteren interval consists of four aeolian cycles, 5–15 m thick, which are retrogradational from sharp-based aeolian sandstone, via sandflat and mudflat deposits to desert-lake mudstone. Toward the south the clay-bearing facies pinch out and aeolian sandstones merge into a compound aeolian sandstone body 20 m thick. The aeolian cycles accumulated in an eastward-dipping, 10-km-wide palaeovalley in the Base Permian Unconformity. The cycles onlapped onto the valley margins until the entire valley was filled and a depositional plain came into place. The plain was flooded by the Rotliegend desert lake, followed by the formation of progradational cycles about 5 m thick, each consisting of a basal desert-lake mudstone grading upward into mudflat and sandflat deposits.
The change from retrogradational (fining-upward) cycles to progradational (coarsening-upward) cycles seems controlled by the rate of formation of accommodation space during lake-level rise. Initially palaeotopography restricted the creation of accommodation space, thus allowing sediment supply to keep up with rising lake level and forcing dune sands to stack up against rising palaeogeography, resulting in aggradational to retrogradational sequences. However, lake-level rise across the depositional plain caused regional flooding and rapid and far retreat of the lake-margin depositional system, causing accommodation space to be filled after the flooding and resulting in progradational sequences.
The Markham case shows that the presence of isolated Rotliegend sandstones is related to palaeotopography and that their internal architecture is controlled by periodic expansion and contraction of the desert lake. It emphasizes the importance of accurate seismic definition of the Base Permian Unconformity and detailed, sedimentology-driven correlation for future exploration at the fringes of the Rotliegend-play fairway.
The Markham gas field, discovered in 1984, is the main field in a cluster of Rotliegend (Late Permian) gas fields in what is here referred to as the “Markham area”. It is situated approximately 150 km northwest of the Dutch coast line in the Southern North Sea on the median line that divides the Dutch and United Kingdom (UK) continental shelf (Fig. 1). The gas-producing sandstones in the Markham area are from the (Lower) Slochteren Formation, which is equivalent to the Leman Sandstone Formation in the UK, and were deposited in aeolian-dune and sandflat environments (Myres et al., 1995). The sandstones alternate with and pinch out northward into sandy mudflat and clay-playa (desert lake) mud-stones. The abundance of mudstone interbeds in the Markham area is directly related to its relatively northern location, at the southern margin of the Rotliegend desert lake.
After its discovery, several other Rotliegend gas accumulations were found nearby, such as Windermere (Bailey and Clever, 2003) in the UK and J3-C and K1-A in the Netherlands (Fig. 2). There are numerous Rotliegend gas fields to the southwest in license blocks K5 and K6 that are characterised by a comparable stratigraphic and sedimen-tological setting (Ministry of Economic Affairs/TNO, 2010), but they lie in the main Rotliegend-play fairway (see Glennie, 1998). The Markham area is separated from the main fairway by an area where Rotliegend sandstones are absent and where gas is produced from Upper Carboniferous fluvial sandstone bodies (Fig. 2).
The Markham field is covered by offshore licenses J6 and J3b in the Netherlands and 49/10b and 49/5a in the UK. The field has been studied extensively by various partners participating in the Markham field development (e.g., Myres et al., 1995). Its location in two states and several offshore licenses required equity determinations and an international treaty (Sharples et al., 1994). With many wells and more than 500 m of cored section, the Markham case is a well-documented example of the Lower Slochteren deposi-tional system at the southern margin of the Rotliegend desert lake.
This paper deals with the sedimentary architecture of the marginal dune field in the Markham area and the effects and causes of repeated transgressions by the Rotliegend desert lake. In addition, the influence of palaeotopography on the distribution and stacking of aeolian sandstones is discussed. The Rotliegend play is highly mature, and most exploration wells are drilled in the vicinity of existing fields. At the same time, higher-risk exploration has shifted to the “feather edge” margin of the Rotliegend desert lake, and the data presented here may support such exploration efforts and provide quantitative input for reservoir-modeling studies.
Rotliegend deposition took place in the E–W-aligned Southern Permian Basin, an arid intracontinental depression between the Variscan thrust belt in the south and the Caledonian Highlands and Baltic Shield in the north (Fig. 3). It was located at a palaeolatitude of approximately 10° N with very low precipitation because the Variscan highlands created a rain shadow for humid Tethyan trade winds (Glennie, 1998). It was an elongate basin that by the end of the Permian was some 1500 km wide and extended from the UK to Poland (Ziegler, 1990; Verdier, 1996; Gast et al., 2010). The main depocenter was located in Germany, where a Rotliegend succession up to 2.5 km thick accumulated (Bachmann and Hoffmann, 1997; Gast et al., 2010).
After an initial phase of volcanic deposition centered in Germany (Lower Rotliegend) and long-lasting erosion with localised sedimentation (Upper Rotliegend 1), the basin was filled with desert-lake mudstones and evaporites in the center and with aeolian-dune and fluvial-fan sandstones along its margins (Glennie, 1998; Hedemann et al., 1984; George and Berry, 1993; Verdier, 1996) belonging to the Upper Rotliegend 2 (Glennie, 1997; Gast et al., 2010). The progressive widening of the basin resulted in an overall onlapping succession, with sandstone-dominated deposits at the base that are overlain by mudstone and evaporite-dominated deposits. The central-basin lake is thought to have been well below global sea level, maybe up to some 300 m (Glennie, 1997). The basal sands are diachronous and are particularly thick at the margins of the basin (Glennie, 1998; Bailey and Lloyd, 2001). Areas of aeolian and fluvial deposition were geographically separated, with large fan-shaped fluvial systems being linked to the tectonically controlled sediment-supply routes and aeolian-dune fields developing in areas downwind of these fan systems (Verdier, 1996; George and Berry; 1997; Mijnlieff and Geluk, this volume).
Rotliegend of the Netherlands
In the Netherlands the Upper Rotliegend Group is generally 200–400 m thick with more than 700 m of mudstone and evaporites near the basin axis in the northeast (Fig. 4; Van Adrichem Boogaert and Kouwe, 1997; Geluk, 2007). All sandstone facies of the Upper Rotliegend Group in the Netherlands are part of the Slochteren Formation, which is found on the southern flanks of the basin along an E–W trend that parallels the present-day barrier islands along the northern Dutch coastline (Fig. 4). The sandstones of the Slochteren Formation are overlain by mudstone and evaporites of the Silverpit Formation (Fig. 5). Markham is one of the more northerly-located Slochteren gas fields and is isolated from the main Rotliegend fairway by a narrow ridge; in the nearby license blocks E18 and F16 Lower Slochteren sandstones are located even more northerly (Figs. 4, 5).
Along the margins of the Rotliegend desert lake the Slochteren Formation consists of the Lower and Upper Slochteren members, which are separated by desert-lake mudstones and mudflat deposits of the Ameland Member (Fig. 5; Appendix A.3. The latter pinches out toward the south, where the Lower and Upper Slochteren members merge into a compound sandstone unit. At the northern location of the Markham area only sandstones of the Lower Slochteren Member are present (Fig. 4); Upper Slochteren sandstones pinch out into desert-lake mudstones a few kilometers south of Markham (Fig. 5).
Palaeogeographic maps prepared by George and Berry (1993, 1997) for the Southern North Sea show that a major fluvial supply system was located east of the Markham area, originating from the southeast (Fig. 4) and supplying sediment to the Rotliegend desert lake. This fan system may have been the source for the aeolian sands in the Markham area, being transported by easterly winds.
Palaeotopography of the Base Permian Unconformity
The culmination of the Variscan Orogeny during the Late Carboniferous resulted in uplift of the Variscan foreland, causing severe erosion of the Carboniferous sequence over a period of 10–20 My and resulting in an angular unconformity (Base Permian Unconformity; Gast et al., 2010) between Permian sediments and mildly folded and faulted Carboniferous strata (Ziegler, 1990; Glennie, 1998; Maynard and Gibson, 2001; Geluk, 2007). This resulted in an accentuated terrain at the level of the top Carboniferous, which influenced deposition during the Rotliegend. For instance, the total thickness of the Rotliegend was controlled by palaeotopography, with thicker sequences overlying palaeotopographic lows (e.g., Bailey and Lloyd, 2001; George and Berry, 1993). Crugnola et al. (1996) showed for the central Dutch offshore that palaeoscarps, with a 045°–225° strike and perpendicular to the main palaeowind direction (toward 280°–310°), influenced the distribution of aeolian sands, with aeolian-dune deposits being preserved at their lee sides. Also Maynard and Gibson (2001) concluded that aeolian sandstones are preferentially preserved in topographic depressions. For the Dutch offshore Geluk and Mijnlieff (2001) related palaeotopographic highs and lows to the lithology of the subcropping formations and their structural dip, with inclined sandstone formations forming “cuestas”.
For the Markham area the influence of palaeotopogra-phy is demonstrated in a NW–SE correlation panel (Fig. 5), which shows that Lower Slochteren sandstones are present in the lows to the north and south of a topographic ridge. Note also that the Upper Slochteren depositional system did not prograde beyond this ridge.
The Markham Field
The Markham field is an excellent example of an isolated Rotliegend sand accumulation. It is made up of a basal sandstone–mudstone alternation (Lower Slochteren Member), 20–50 m thick, at a depth of about 3500 m, overlain by some 200 m of shale and anhydrite of the Silverpit Formation (Fig. 5). The overburden comprises an Upper Permian– Lower Triassic sequence of carbonates, evaporites, and red beds and a Cretaceous–Tertiary sequence dominated by carbonates and marine siliciclastics; these sequences are separated by an unconformity caused by Jurassic rifting (Fig. 6).
The field is a structural trap with dip closure, sealed by mudstones of the Silverpit Formation and Zechstein (Permian) evaporites (Ministry of Economic Affairs/TNO, 2010). The gas was sourced from underlying Carboniferous coals and shales, although lateral migration played a role in the Markham area (Van Hulten, 2007).
Myres et al. (1995) described the sand-rich Lower Slochteren interval as the deposits of a sand sea (aeolian dune) that graded northward into sabkha (sandflat, mudflat) and playa (desert lake) environments. The Lower Slochteren thins to the south, which they attribute to onlap against an eastern extension of the Cleaver Bank–Inde High (or Inde pediment) located to the south(west) of the Markham field (George and Berry, 1993; Bailey and Lloyd, 2001; Maynard and Gibson, 2001).
The aeolian-dune sandstones from the Markham field are characterised by fairly good porosity (15–25%) and permeability (10–1000 mD; Myres et al., 1995). Sandflat to mudflat deposits in the northern part of the Markham Field have moderate to poor reservoir quality due to detrital clay and diagenetic overprint, as a result of proximity to the desert lake. With a Q50 (productivity under 50 bar drawdown) of over 1×106 m3/day in the southern part of the field (Myres et al., 1995) the productivity of the field is much higher than for comparable lake-margin sandstones from the Upper Slochteren Member of the Ameland gas field (0.03×106 m3/day); this is related to pervasive clay-mineral diagenesis (Crouch et al., 1996).
Sandstones in the Markham area are mostly mature arenites; the dominant grain size is fine to very fine. The petrological composition of Markham sandstones is typically 50% monocrystalline quartz and 10% polycrystalline quartz. The sandstones contain less feldspar than the 2.5% typical of Rotliegend sandstones (Almon, 1981), which is attributed to dissolution of feldspar grains (Myres et al., 1995). The abundance of lithic fragments (4%) is similar to the average value for Rotliegend sandstones in wells from quadrants K8 and K11. Sandflat sandstones and siltstones are characterised by substantially higher rock-fragment abundance and slightly higher feldspar abundance (2–3%). Authi-genic minerals in aeolian sandstones comprise quartz-overgrowth cement and kaolinite clays, which do not severely reduce primary porosity. Sandflat facies are characterised by ferroan dolomite and anhydrite cements due to proximity to basin-centre evaporites, leading to reduced reservoir quality with permeabilities of 0.1–10 mD (Myres et al., 1995).
Sedimentary facies from a wide range of depositional environments have been recorded in the Rotliegend of the Southern Permian Basin, including aeolian dune, fluvial fan, wadi, sandflat, and mudflat (sabkha) and desert-lake (playa) deposits (e.g., George and Berry, 1993, Glennie, 1998). The sedimentary evolution of the Lower Slochteren depositional system was studied based on the integration of core observations and wireline-log patterns (gamma-ray) from a total of 19 wells in the wider Markham area. The interpretations of sedimentary facies are based on core descriptions by Intergeos/TNO (1995) complemented by new core observations (Fig. 6; Appendix B3d. Subsequently sedimentary facies were assigned to the uncored intervals based on the typical gamma-ray patterns in the cored intervals.
Sedimentary Facies Observed in Core
Rotliegend core descriptions are presented for the wells J06-01, J06-02, J06-03, J06-A3, 49/5-2, 49/5-3, 49/5-5 and 49/5-5z (Fig. 8; Appendix B3d. The following main facies were observed: aeolian dune, aeolian sandsheet, sandflat, mudflat, and desert lake. The facies are described below; core photographs are shown in Figure 7 and in Appendix B3d.
This facies is observed in all cored sections but is most abundant in wells from the southern and central parts of the Markham field; it is dominated by cross-laminated sandstones (Fig. 7, photograph 1). The sand is moderately to well sorted and is mostly fine to medium grained, with localised, thin coarse-grained intervals. Beds of cross-laminated sandstone are stacked into compound units 5–20 m thick. Cross-sets are commonly 1–2 m thick, but are up to 5 m thick in places. Cross-sets display tangential cross-lamination with subhorizontal basal lamination and cross-lamina dips increasing upward to around 25°. Strings of pebbly sandstone in places characterise the base of cross-sets and are indicative of deflation by wind. The coarseness of the pebbles indicates fluvial activity, which is attributed to occasional (low-quantity) fluvial input from the exposed Carboniferous ridge just south of the Markham area (cf. Bailey and Lloyd, 2001). Locally, cross-sets alternate with thin intervals of horizontally laminated, wind-rippled deposits of interdune origin. Limited cross-set thickness and interdune facies point at stacking of small to moderate-size dunes. aeolian-dune intervals in places grade upward into aeolian-sandsheet deposits; the facies sharply overlies the Base Permian Unconformity and mudflat and sandflat deposits higher up in the sequence.
This facies comprises moderately to well-sorted sandstones of fine to medium grain size. It consists mainly of horizontally laminated to low-angle cross-laminated sandstones, with bimodal lamination in places (Fig. 7, photograph 2). Laminae occasionally contain thin wind-ripple trains. The deposits are present as beds or layers no more than a few meters thick and commonly overlie aeolian-dune facies, grading upward into sandflat deposits. The facies is interpreted as the deposits of sandsheets with poorly developed incipient dunes.
This facies consists of poorly to moderately sorted argillaceous and silty sandstones with a clay percentage up to 25%; the sediments are poorly to bimodally sorted. The deposits have a wavy-laminated to structureless or chaotic texture, resulting from adhesion rippling and disruptive processes such as salt precipitation–dissolution and sand injection. The facies is present as meter-scale units and alternates on a meter-scale with aeolian facies and mudflat facies (Appendix Fig. B.3.d. Thin sandflat intervals, typically no more than a few decimeters, may be present between successive aeolian-dune buildups. Sandflat deposits accumulated in the transitional area between dune fields and the desert lake where ground water reached the sediment surface, occasionally resulting in capturing of windblown clay.
The mudflat facies is the mud-rich equivalent of sandflat deposits and contains more than 25% clay. It is particularly common in cored sections from the north of the study area and from cores taken in the Silverpit Formation. The facies comprises red, muddy sandstones and sandy mudstones with a horizontally laminated to wavy laminated texture, reflecting alternating subaqueous and subaerial deposition. Mud cracks and adhesion ripples are common (Fig. 7, photographs 3 and 4). In places, bedding is disrupted due to sand injection and desiccation. The facies alternates on a meter scale with sandflat and desert-lake facies. It was deposited along the flanks of the desert lake, where the high water table resulted in accumulation of clays. Sand that was blown across the mudflat was preserved as adhesion ripples.
This facies is dominated by massive red mudstones, which alternate on a decimeter to meter scale with laminated silty mudstones and intervals of sandy mudstone with disrupted lamination (Fig. 7, photographs 3 and 4). Adhesion ripples and mud cracks are present in the sandy intervals. This facies alternates on a meter scale with mudflat facies. The desert-lake facies is abundant in the northernmost part of the Markham field, especially toward the top of the studied interval. The mudstones accumulated in shallow, marginal parts of the Rotliegend desert lake, under semipermanent high-water-table conditions. Mud cracks and adhesion ripples indicate that these shallow lacustrine areas dried out occasionally, pointing at contraction of the Rotliegend desert lake.
Sedimentary Facies in Uncored Intervals
Sedimentary facies were assigned to uncored intervals using wireline logs (Fig. 8). Bailey and Lloyd (2001) demonstrated for the Rotliegend in the Markham area that the gamma-ray (GR) log is a crude proxy for lithology and that it can be used for basic facies breakdown. Due to higher GR values for clay-rich facies there is a general trend toward higher GR values from aeolian sandstones, via sandflat and mudflat to desert-lake deposits. Because clay content is low in both aeolian-dune and aeolian-sandsheet facies, those facies cannot be distinguished based on GR values. They were grouped under the log-facies name “aeolian-dune”, although not necessarily cross-laminated. The facies was assigned to log intervals that are characterised by continuously low (minimal) GR values and a low-amplitude pattern. Typical GR values for pure sandstone vary by well: they are mostly 20–25 API, but may be as high as 40–50 API.
At the other end of the spectrum, intervals characterised by continuously high (maximum) GR values represent pure mudstones of desert-lake origin. Because these mud-stones are commonly thin they are characterised by distinct GR peaks, but where desert-lake mudstone intervals are thicker, GR values are relatively constant (in most wells between 125 and 150 API). Because of the limited vertical resolution of the GR tool, thin peaks are characterised by subdued values.
Sandflat and mudflat deposits, with their highly variable mud content, are characterised by intermediate values and by GR patterns that are more variable, displaying trends and higher-amplitude patterns. The cutoff value between sandflat and mudflat deposits was defined at 25% between the GR minimum and maximum, typically 50–60 API (reflecting a mudstone percentage of about 25%).
Wireline-log correlations of Rotliegend lake-margin deposits are often based on flooding surfaces (or “flood” surfaces, cf. Langford and Chan, 1988), i.e., mud-rich layers that are characterised by distinctive high-gamma-ray peaks (e.g., Martin and Evans, 1988; Crouch et al., 1996). Flooding surfaces in lake-margin areas break up aeolian- dune and sandflat-dominated sequences into meter-scale intervals (e.g., Crouch et al., 1996). Crugnola et al. (1996) used multiple flooding surfaces to correlate wells from the Rotliegend in the central Dutch offshore, which allowed them to demonstrate that Lower Slochteren sands infill a base-Permian palaeotopography.
Correlation and Datum Levels
The organic-rich Kupferschiefer (Copper Shale) at the base of the Zechstein (Upper Permian) evaporite sequence, represented in GR logs by a narrow peak (Fig. 5), defines a datum from which successive correlations in the Silverpit Formation can be hung in a top-down manner (Bailey and Lloyd, 2001); this allows the identification of datum levels within the diachronous Lower Slochteren Formation.
In the study area wireline-log correlations in and above the Lower Slochteren interval are based on two such datum levels. The upper datum is present in the lower part of the Silverpit Formation and constitutes a narrow, high-GR peak, which is sharply overlain by a 20-m-thick retro-gradational (fining-upward) sequence from moderate to high GR values (Fig. 8: Silverpit datum). The second datum is near the top of the Lower Slochteren Member, and it constitutes the sharp base of a meter-scale aeolian-dune unit that is sharply overlain by a progradational (coarsening-upward) mudflat sequence (Fig. 8: Lower Slochteren datum). This dune-mudflat sequence is a 10-m-thick unit that is distinctive throughout almost the entire study area; only in the southernmost, sandstone-dominated wells the unit consists of aeolian-dune sandstone entirely and loses its distinctive pattern.
Within this basic correlation framework additional flooding surfaces were defined and correlated throughout the study area (Fig. 8). In the Lower Slochteren interval three major flooding surfaces were correlated (Slochteren flooding 1–3), of which the lower one is localised. In the Silverpit interval four major flooding surfaces were defined (Silverpit flooding 1–4). The lower three surfaces extend throughout the study area; the fourth, uppermost flooding surface is present in the northern wells only. The flooding surfaces could be traced from well to well throughout most of the study area, but flooding surfaces in the Lower Slochteren Formation lose their high-GR expression toward the aeolian-dune-dominated southern part of study area.
Thickness and Facies Trends
Lateral Thickness Variations
Thickness maps were drawn for (1) the interval between the base Lower Slochteren and the Lower Slochteren datum (Lower Slochteren interval) and for (2) the interval between the Lower Slochteren datum and the Silverpit datum (Silverpit interval); these maps are shown in Figure 9. The map for the Lower Slochteren interval displays strong lateral thickness variations. Assuming that the Lower Slochteren datum represents a flat palaeo-depositional surface (which is suggested by the constant thickness and the regionally consistent facies arrangement of the sedimentary cycle overlying this datum), the thickness pattern of the Lower Slochteren interval reflects the palaeotopography of the Base Permian Unconformity. This palaeotopography comprises a 10-km-wide, eastward- to northeastward-dipping valley that is bordered to the west, north, and south by gentle flanks. Along its axis this palaeovalley deepens eastward to a (post-compaction) depth of some 35 m.
The thickness pattern of the overlying Silverpit interval is different, displaying a gradual thickness increase from 18 m in the south to 24 m in the north, and then suddenly to 29 m. This map shows that the base-Permian palaeotopography had been leveled completely after deposition of the Lower Slochteren Member and that the depositional gradient had changed from eastward to northward. This northward gradient is in line with the overall E–W elongation of the basin and reflects increasing rates of subsidence toward the basin axis. That the Silverpit interval is about 5 m thicker in the northern part of the study area, with no indication of a gradual thickness increase, suggests that an E–W-aligned fault may have been active between the southern and northern areas, resulting in increased accommodation space in the north.
Vertical Facies Trends and Sedimentary Cycles
The studied interval is an overall retrogradational sequence from aeolian-dune and sandflat deposits to mudflat and desert-lake deposits. At its base aeolian-dune sandstones unconformably overlie the Base Permian Unconformity (Fig. 8).
The Lower Slochteren Member is dominated by aeolian-dune sandstones and sandflat deposits, and comprises four repetitive cycles 5–15 m thick. These cycles are retrogradational from aeolian-dune to mudflat and desert-lake facies (Slochteren cycles 1–4). The first cycle (Slochteren cycle 1) is localised, but Slochteren cycles 2, 3, and 4 are present throughout the study area. Because of the abundance of aeolian facies these cycles are referred to as “aeolian cycles”. The aeolian-dune sandstone bodies at the base of each cycle sharply truncate underlying lithologies and are approximately 5 m thick in most parts of the study area. Note that the basal part of Slochteren cycle 3 (in the northern part of the study area) is different from that in other aeolian cycles, with its gradational base from desert-lake mudstone to sandflat.
The Silverpit interval (above Silverpit flooding 1) consists of four to five cycles, about 5–6 m thick, which coarsen upward from desert-lake mudstone to mudflat or sandflat deposits. These cycles are referred to as “desert-lake cycles”. Toward the south some of these cycles show a transition toward sandier facies, such as sandflat and locally aeolian dune. Cycles 1–4 are present across the study area, the fifth cycle is present in the north only; this is the area where the Silverpit interval is some 5 m thicker, and it may be that movement along an E–W-aligned fault resulted in the formation of an additional cycle in the north.
In the south the upward transition from the sandstone-dominated Lower Slochteren Formation to the mudstone-dominated Silverpit Formation is sharp. There aeolian sandstones have stacked into a 20-m-thick compound sandstone unit, which is sharply overlain by a relatively thin interval of mudflat and desert-lake facies. This transition is more gradual in the north, where aeolian sandstones alternate with mudflat and desert-lake deposits.
Lateral Facies Trends in the Lower Slochteren Member
The study area extends approximately 8 km N–S and 10 km E–W. At this scale lateral facies transitions occur, especially in N–S transects, which are shown in the facies maps of Figure 10. For each of the Lower Slochteren cycles a map was drawn for (1) the dune phase (lower part of the cycle) and (2) the lake phase (upper part). In the following section the lateral and vertical facies distribution is described for each of the Lower Slochteren cycles.
Lower Slochteren Cycle 1.—
The first Lower Slochteren cycle is present only in the narrow E–W-aligned palaeovalley in the center of the study area (Fig. 10). During the dune phase it was characterised by aeolian-dune facies throughout with thicker accumulations in the deeper, eastern part. The map suggests that the northern flank of the valley was steeper than the southern flank. Note that the northern flank of the palaeovalley coincides with the approximate location of the (assumed) fault that is responsible for the sudden northward thickness increase of the Silverpit interval (see Fig. 9), which suggests that the palaeovalley may have been fault-bounded in the north. Subsequent transgression resulted in deposition of desert-lake mudstones on top of the aeolian facies, with thicker mudstone packages along the axis of the valley. Only on the southern flank a narrow strip of sandflat deposits accumulated, which indicates that the northern flank of the valley was cut off from the southeastern sediment source by the desert lake. Note that flooding of the area during the lake phase did not result in more widespread deposition than during the dune phase. This suggests that the basal, axial part of the palaeovalley may have been a prominent feature with relatively steep sides.
Lower Slochteren Cycle 2.—
During the dune phase of cycle 2 aeolian-dune facies accumulated across the entire study area, except in the immediate vicinity of well 49/9b-2, which suggest that regional subsidence had brought the entire area near or below base level. The thickest aeolian-dune accumulations again are found within the area of the palaeovalley, but the map for the lake phase indicates that the palaeovalley had become filled prior to flooding by the desert lake. The flooding resulted in deposition of desert-lake mudstones and mudflat deposits in the north of the area. Toward the south the desert-lake area was bordered by a sandflat and a narrow strip of aeolian dunes.
Lower Slochteren Cycle 3.—
The map for the dune phase of cycle 3 shows a 5-km-wide, E–W-aligned belt of aeolian-dune deposits in the south, which grades northward into sandflat deposits. In the east the sandflat grades into a mudflat, which may indicate the presence of the desert lake more to the east. Subsequent lake-level rise resulted in transgression of the area by the desert lake some 6 km inland. This pushed the aeolian depositional system back to the south and resulted in a change from aeolian-dune to sandflat deposition at the southern edge of the study area. This cycle represents the moment of maximum transgression in the lower part of the studied interval.
Lower Slochteren Cycle 4.—
The dune phase of the final Lower Slochteren cycle is dominated by aeolian-dune sandstones, which are thickest in the south. Northward and upward through the cycle aeolian-dune deposits grade into sandflat deposits (see Fig. 8). Note that for this cycle the facies distribution was mapped for the mudflat phase, which represents the minor flooding halfway up the cycle and not for the major flooding surface at the top (Fig. 8). In the north a 5-km-wide belt of mudflat deposits is present, which grades southward into sandflat and then into aeolian-dune deposits. The facies belts for this cycle are completely E–W aligned and show fewer irregularities than those for the previous three cycles. This suggests that the palaeovalley in the Base Permian Unconformity, which initially influenced facies patterns, had been filled completely by the time that the Lower Slochteren interval was flooded.
Depositional Model and Discussion
The observed sedimentary facies represent the range from an aeolian-dune environment to a desert-lake environment (Fig. 11A) (cf. Martin and Evans, 1988; Myres et al., 1995). The mudflat and sandflat environments are transitional between areas of low groundwater and areas of high groundwater or slight inundation; the clay content is associated with the capacity of a wet sediment surface to capture wind-blown clay and silt. The preservation of aeolian-dune sands in the northern and relatively remote Markham area was controlled by the presence of an eastward-dipping palaeovalley in the Base Permian Unconformity, which served as a trap for sand that blew into the area from an eastern source.
Its shape suggests that the palaeovalley may have originated as an erosional feature along the axis of an E–W-aligned fold. The palaeovalley may have been a tributary to a large-scale valley system that drained the Base Permian Unconformity, possibly toward the basin axis in the north.
The overall retrogradational pattern of the Lower Slochteren is in agreement with regional observations and reflects onlap of the sedimentary system in response to the progressive expansion of the Southern Permian Basin and the associated, overall rise of lake level. The progressive change from sandstone domination to mudstone domination reflects marginward retreat of the sediment-supply system, but may be related to progressive drowning of the hinterland source areas as well (cf. Bailey and Lloyd, 2001).
Sediment Bypassing and Deposition
The correlatable, meter-scale alternation of aeolian sands and desert-lake mudstones points at repetitive, small-scale expansion and contraction of the desert lake (Fig. 11B). That the aeolian cycles, which make up the Lower Slochteren Formation, are mostly sharp-based retrogradational cycles from aeolian sandstones to mudflat and desert-lake deposits (Fig. 8) is evidence of nondeposition and sediment bypassing during retreat of the desert lake. It seems likely that desert-lake lowering resulted in lowering of the ground-water level in lake-margin areas, thus preventing the preservation of mudflat or sandflat deposits that require a wet sediment surface. At the same time wind-blown sand seems to have been bypassed in a basinward direction, where it was incorporated in progradational, lake-fill sequences at the margins of the shrinking desert lake. That this may have been the case is suggested by the coarsening-upward trend at the base of Slochteren cycle 3 in the north of the area (Fig. 8: e.g., well 49/5-5(z), 49/5-B3); it suggests that bypass surfaces in proximal, aeolian sequences correlate with coarsening-upward sequences in the desert-lake area. During subsequent lake-level rise, accommodation space was created in lake-margin areas, allowing the accumulation of aeolian-dune deposits, in particular against rising palaeo-topography. Ongoing base-level rise resulted in progressive retreat of the sedimentary system, causing the formation of a retrogradational sequence from aeolian-dune sandstones to sandflat and mudflat deposits. Comparable retro-gradational aeolian cycles, with basal “sand-drift” surfaces and flooding surfaces at their top, have recently been described by Rodríguez-López et al. (2011) for the mid-Cretaceous of the Iberian Basin.
Influence of Palaeotopography on Cycle Trends
As already noted in previous paragraphs, there is a marked difference in cycle character between the aeolian cycles from the Lower Slochteren interval and the desert-lake cycles of the Silverpit interval: the aeolian cycles are aggradational to retrogradational (fining upward), whereas the desert-lake cycle are progradational (coarsening upward, see Fig. 8). It has been shown that this change occurred at the stratigraphic level at which the base-Permian palaeotopographic lows had been completely sediment filled, and it seems likely therefore that cycle trends were controlled by palaeotopography. It is thought that under the condition of existing palaeotopography a rise of lake level caused only limited retreat of the lake-margin sedimentary system, thus allowing sediment supply to keep up with lake-level rise and quickly fill newly available accommodation space, until the system was finally flooded. Thus, rising lake level forced sediments to stack up against palaeotopographic highs, in this case the flanks of the basePermian palaeovalley, resulting in aggradational (“blocky”) to fining-upward sequences. However, onwards from the moment that any existing palaeotopography became leveled and a flat sediment surface came into place, even a small rise of lake level would result in far marginward retreat of the lake-margin system and in immediate and laterally extensive flooding of the area. The resulting accommodation space would then be filled after the flooding, resulting in progradational, coarsening-upward sequences.
Controls on Sedimentary Cyclicity
Lake-level fluctuations are commonly attributed to climate-controlled variations in aridity caused by shifting of climate belts (e.g., George and Berry, 1993; Yang and Kouwe, 1995; Howell and Mountney, 1997), with periods of low precipitation and high evaporation resulting in contraction of the desert lake. That mechanism could well explain the sedimentary sequences observed in the Markham area. In any case, the presence of sediment bypass surfaces between desert-lake or mudflat deposits and overlying sharp-based aeolian-dune sandstones indicates that desert-lake expansion and contraction was not controlled by climate-driven variations of sediment supply, with increased outbuilding causing (apparent) lake retreat. Under such conditions progradational sequences from desert-lake, to mudflat, to sandflat, to aeolian dune would have been preserved, and that is not observed here.
The sedimentary cyclicity in Rotliegend sequences is related to astronomical forcing by some (e.g., Yang and Kouwe, 1995; Gast et al., 2010), but reliable determination of cycle periods is problematic due to absence of biomarkers, and time is therefore poorly constrained (Glennie, 1997; Gast et al., 2010). For the Dutch offshore Yang and Kouwe (1995) proposed that the entire Rotliegend sequence comprises twelve “third-order” cycles that are internally composed of higher-order cycles with periodicities within the Milankovitch band. Also Howell and Mountney (1997) and Maynard and Gibson (2001) recognised 12 major cycles, but since thickness patterns for each of the 12-fold sequences are markedly different, the reliability of these subdivisions and their relevance in terms of time therefore seems low. Among others, Glennie and Provan (1990) and George and Berry (1993, 1997) subdivided the Upper Rotliegend in the UK and Dutch offshore into five large-scale units.
Large-scale subdivisions, other than the lithostrati-graphic subdivision into the Lower Slochteren Member and the Silverpit Formation, are not particularly obvious in the Markham area. Large-scale coarsening-upward and fining-upward patterns can be observed, but patterns are not always consistent between wells (Fig. 5). On the other hand, small-scale cycles are quite prominent, as discussed above. Based on maximum flooding surfaces, sedimentary cycles are either 5–7 m thick or approximately twice that thickness (10–14 m). Note, however, that some of the thicker cycles are clearly composed of two subcycles. For instance, Lower Slochteren cycle 4 is characterised by a minor flooding surface halfway up the cycle. And particularly in the northern wells, where the influence of the desert lake was strongest, a stacking of well-defined cycles 4-6 m thick is evident (Fig. 8, panel 1). Based on the above it seems that the studied sequence is composed of stacked cycles, each approximately 5.5 m thick, which were occasionally merged together when lake-level rise did not extend far enough southward to deposit desert-lake muds entirely across the marginal dune field. Note that the aeolian cycles are slightly thicker than desert-lake cycles (Fig. 8), which is most likely related to differential compaction of sand and mud. The 5.5 m cycle well matches results of spectral analysis by Maynard and Gibson (2001), who detected a 16–20 ft (5–6 m) cyclicity, as well as a 51–60 ft (15–18 m) cyclicity, in wells from the UK desert-lake margin just west of Markham.
If the 5.5-m-thick cycle indeed reflects a Milankovitch rhythm, then the 265-m-thick Rotliegend sequence in the study area (maximum thickness in well J06-03, Fig. 5) represents the duration of about 48 such cycles. Cycle periodicity then depends on the duration of the Upper Rotliegend 2 interval (UR2), which however is poorly constrained (e.g., Glennie, 1997; Gast et al., 2010). Recent work has suggested that the duration of UR2, originally estimated at some 10 My, may represent only 4–5 My in Germany, where the interval is most complete (Gebhardt et al., 1991; Glennie, 1997). This means that the Rotliegend interval in the UK and Dutch offshore, where approximately a third of the UR2 interval is present (Glennie, 1997) possibly represents no more than 1.3–2.7 My (George and Berry, 1997).
If the 5.5 m cycles are obliquity controlled (35–44 ky; Berger and Loutre, 1994) the Rotliegend sequence in the Markham area would be about 1.9 My long, which well matches the estimated duration of UR2. This gives a (post-compaction) subsidence rate of approximately 14 cm/ky, which is in agreement with expected rates of thermal subsidence for epicontinental basins (Gast et al., 2010). If controlled by precession (17–21 ky), however, the sequence would be about 0.9 My long, which is much shorter than the estimate of 1.3–2.7 My.
Precession is the more likely candidate to explain climate-driven variations of runoff and evaporation, and subsequent periodic contraction and expansion of the desert lake, because it primarily influences climate at low latitudes (de Boer and Smith, 1994). This explains why precession is widely recognised in aeolian environments (Clemmenson et al., 1994), unlike obliquity, which dominates at high latitudes (de Boer and Smith, 1994). On the other hand, lake-level variations in the deep continental depression of the Southern Permian Basin may have been indirectly driven by (obliquity-controlled) glacio-eustasy (e.g., Glennie, 1997), also considering that the Permian was characterised by south-pole glaciation (e.g., Crowley, 1994). For instance at successive eustatic highstands sea water may have flowed into the basin (one-way), which would better explain the presence of marine foraminifer tests (Spirillina sp.) and glauconitic grains in thin sections from the Rotliegend of the UK West Sole gas field (Butler, 1975; Gast et al., 2010), as well as the huge volumes of salt in the basin center, although their low bromide content could indicate a nonma-rine source (Holster, 1979).
Note that if the 5.5 m cycle indeed represents obliquity (35–44 ky), the 15–18 m cycle of Maynard and Gibson (2001) would match eccentricity (95–123 ky). Although beyond the scope of this study it is finally noted that the pronounced high-frequency cyclicity in the GR pattern of the Silverpit Formation (Fig. 5, see well J06-01: 0–200 m), reflecting a meter-scale alternation of anhydrite (low-value GR spikes) and mudstone beds (high-value GR spikes), could be precession controlled. A quick count shows that 200 m of Silverpit mudstones contain 80–90 anhydrite spikes; hence Silverpit anhydrite–mudstone cycles are on average 2.4 m thick, and based on the above time estimates represent 12-24 ky each.
The Lower Slochteren sandstones in the Markham area were deposited in a narrow dune field at the southern fringe of the Rotliegend desert lake. The presence of aeolian sand in this northern area is exceptional in the Dutch offshore and seems related to localised sand accumulation in a small palaeovalley incised within the Base Permian Unconformity. The studied interval is a retrogradational sequence from aeolian-dune sandstones at the base to mudflat and desert-lake deposits at the top.
Internally the sequence comprises stacked cycles that are between 5 and 15 m thick. The Lower Slochteren interval consists of four “aeolian cycles”, which are retrogradational from sharp-based, cross-laminated aeolian sandstones, via sandflat, to mudflat or desert-lake mud-stones. The first two cycles fill an eastward-dipping palaeo-valley within the base-Permian inconformity; during deposition of the next two cycles the base-Permian palaeotopog-raphy influenced deposition only slightly and was ultimately leveled by sediment. The drapes of mudflat or desert-lake facies at the top of each cycle (flooding surfaces) are traceable over distances of about 6 km and are evidence of periodic desert-lake expansion. The Silverpit interval consists of four to five approximately 5-m-thick prograda-tional cycles that coarsen upward from desert-lake mud-stone to sandflat deposits. The aeolian and desert–lake cycles are in the range of short-period Milankovitch rhythms (precession or obliquity).
Dune sands have stacked into a 20-m-thick, compound sandstone body in a narrow strip near at the southern edge of the Markham area, constituting good-quality, homogeneous sandstone reservoir. The area where dune sands are stacked is characterised by rapid southward thinning, which is the result of onlap onto the southern flank of the base-Permian palaeovalley. The reservoir quality decreases northward as aeolian-dune sandstones grade progressively into sandflat and mudflat deposits.
The observation that the sedimentary cycles are asymmetric, and are either retrogradational (aeolian cycles) or progradational (desert-lake cycles), indicates that periods of deposition and nondeposition alternated, but oppositely in dune and lake areas. This seems to be linked to progressive leveling of the base-Permian palaeotopography, with fining-upward cycles forming near palaeotopographic highs and coarsening-upward cycles forming on depositional plains.
It is clear from the Markham case that palaeotopographic lows in the Base Permian Unconformity at the fringe of the Rotliegend desert lake may be attractive sites for exploration, but this study also shows that good-quality aeolian sands may easily be compartmentalised vertically due to laterally extensive deposition of desert-lake mudstones, and that homogeneous, stacked sandstones may be laterally restricted in a narrow strip along the desert-lake margin. On the other hand, the good correlatability of the aeolian-sandstone units and intervening desert-lake mud-stones shows that the reservoir architecture in similar settings may be relatively predictable.
Exploration for Markham-type aeolian reservoirs obviously requires a good understanding of palaeotopographic trends in the subcrop. Accurate seismic definition of the Base Permian Unconformity is therefore prerequisite, with state-of-the-art, long-streamer-surveying technology opening new possibilities. Palaeotopographic lows may enhance the preservation of aeolian sand, but their orientation, dip, and opening with respect to (north)easterly palaeowinds may be crucial for sand to have been preserved, which stresses the importance of regional sedimen-tological analysis.
Energie Beheer Nederland (EBN) and TNO-Geological Survey of the Netherlands are acknowledged for permission to publish this paper. Reviewer Jan De Jager, editor Reinhard Gaupp, and an anonymous reviewer are thanked for useful suggestions and constructive comments on earlier versions of this paper, as well as Rob De Wilde for facilitating our core-viewing visits to the TNO-Geological Survey core-storage facility and Harmen Mijnlieff for coordinating core photography of well J06-A3 for the appendix of this volume.
Figures & Tables
The Permian Rotliegend of the Netherlands
More than 50 years ago, the discovery of the giant Groningen Gas Field in the subsurface of the Netherlands by NAM B.V. marked a turning point inthe Dutch and European energy market initiating the replacement of coal by gas. Despite the fact that the Rotliegend dryland deposits in the Southern Permian Basin are one of Europe's most important georesources, no sedimentological overview is available to date for the subsurface of the Netherlands. This SEPM Special Publication presents for the first time such a summary of the present-day knowledge, including a comprehensive core atlas from on- and offshore wells.