Mapping of Fluvial Fairways in the Ten Boer Member, Southern Permian Basin
Published:January 01, 2011
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Marinus E. Donselaar, Irina Overeem, Johannes H. C. Reichwein, Clemens A. Visser, 2011. "Mapping of Fluvial Fairways in the Ten Boer Member, Southern Permian Basin", The Permian Rotliegend of the Netherlands, Jürgen Grötsch, Reinhard Gaupp
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The Ten Boer Member (ROCLT; Upper Rotliegend, Southern Permian Basin) is a claystone-dominated succession with thin sandstone beds, deposited on the margin of a large saline lake. The sandstone beds were deposited by fluvial channels and associated unconfined sheet floods in the distal part of a fluvial system. A recent re-perforation test in a depleted Rotliegend well successfully produced 30 Mm3 gas from a thin ROCLT sandstone bed at only 50K Euro re-perforation costs. This success triggered a detailed study to map the fluvial fairways and assess the lithofacies associations of the ROCLT.
Based on log and core analysis the ROCLT succession is subdivided into five cycles, each characterised by a high-to-low-to-high gamma-ray succession and an associated mudstone–sandstone–mudstone sediment sequence. The cyclic succession is interpreted as a wet–dry–wet climatic change. Log correlation panels and gamma-ray-log-derived net sand maps show that the sandstone deposits are concentrated in SSW–NNE-oriented belts 15–30 km wide. The belts are fairways for fluvial transport from the Variscan Mountain Range in the south to the basin centre in the north. The shape of the fluvial fairways changes from elongate belts to lobe shapes across a narrow east–west-oriented transition zone. Net-to-gross drops drastically north of the transition, and the sandy lithofacies changes accordingly from stacked sandstone in the elongate belts to thin sheet sandstone embedded in claystone in the lobe-shaped part. Over time the fluvial fairways show a lateral shift, and the entire fluvial system gradually progrades northward.
The net sand maps assist to further constrain the locus of isolated sheet-sandstone reservoir units, and thus aid in future reservoir architecture modeling.
The Southern Permian Basin (SPB) is a mature area for oil and gas exploration. Production in the Netherlands is declining in the smaller fields surrounding the giant Dutch Rotliegend Groningen Gas Field. To postpone the end of field life of these smaller fields, and to make optimal use of the existing infrastructure, the focus is on near-field exploration of small satellite reservoirs. Studies of the palaeogeographic setting and climatic cyclicity of the Upper Rotliegend (Lower Permian) have been published by, among others, Ziegler (1990), George and Berry (1994, 1999), Howell and Mountney (1997), and Sweet (1999). The Upper Rotliegend Ten Boer Member (ROCLT) is a claystone succession 40–110 m thick which overlies the Slochteren Sandstone gas reservoir of the Groningen Gas Field. Thin fluvial sandstone beds embedded in the claystone are considered a potential secondary reservoir. In the past the ROCLT was classified as a “waste zone” because of its low net-to-gross (N/G). However, a re-perforation test in a depleted Rotliegend well successfully produced 30 Mm3 gas from thin ROCLT sandstone at only 50K Euro re-perforation costs. The result showed that ROCLT sandstone may produce as isolated gas reservoirs, i.e., the sandstone is not connected to the main Rotliegend reservoir. The success boosted research in the depositional setting and spatial distribution of the sandstone facies in the ROCLT. The challenges were to map location of the fluvial fairways from widely spaced well data. The aims of this paper are to present: (1) a lithofacies interpretation of thin-bedded sandstone in the claystone-dominated ROCLT stratigraphic interval and (2) a comprehensive model for the areal distribution and development in space and time of the sand-prone intervals therein. The study is based on analysis and correlation of 150 well logs through the ROCLT, complemented with core analysis of well Grijpskerk-1A.
Data And Methods
The study area is located onshore and offshore the Netherlands and measures 300 km by 160 km (Fig. 1). The ROCLT is penetrated here by 150 wells, all of which were used for the present study. Analysis of gamma-ray logs allowed the subdivision of the ROCLT into five cycles, characterised by a high-to-low-to-high gamma-ray succession. The cyclicity is consistent in all wells and was used for the construction of correlation panels across the study area. The lithofacies characteristics of the ROCLT interval in a cored well, Grijpskerk-1A, was described in detail and the results used for log-to-core validation and for interpretation of the depositional environment and sequence strati-graphical succession. The well was selected for its overall claystone dominance and the occurrence of thin sandstone beds, comparable in thickness to the target of the successful re-perforation test. For each cycle the gamma-ray log correlations were used to create maps of the cycle thickness and net-to-gross (N/G) in the study area. Vshale values from normalised gamma-ray logs were used to establish N/G, whereby a cutoff of 0.5 was used to distinguish between potential reservoir (Vshale < 0.5) and nonreservoir (Vshale > 0.5). Density logs and cuttings descriptions were used to correct the N/G for the presence of salt. Next, net-sand maps were created by multiplying the thickness maps with the N/G property maps.
The ROCLT is part of the fill of the SPB. The basin formed as a result of thermal subsidence after the Saalian tectonic phase (∼ 270 Ma) and the start of a rifting period and associated upper-mantle basalt extrusion at the end of the Hercynian Orogeny (Gebhardt et al., 1991; Glennie, 1998). The basin was structurally bounded by the London– Brabant Massif and the Variscan Mountain Range in the south and the Mid North Sea High and the Ringkøbing–Fyn High in the north. East–west transtensional reactivation of NW–SE-oriented lines of weakness caused the subdivision into sub-basins, separated by subtle swells (Geluk, 2005). The lineaments are expressed by the Texel–Ijsselmeer and Zandvoort–Maasbommel–Krefeld Ridges in the Netherlands and by the Downsing Fault Zone in the British offshore (Fig. 2; George and Berry, 1994; Verdier, 1996; Geluk, 2005). The lineaments were inherited from the Variscan, Caledonian, and Cadomian orogenies (Verdier, 1996). The SPB extends from the east coast of the UK to Poland, with a length of more than 2000 km and a width of 300–600 km. The basin fill comprises some < 2500 m of Upper Rotliegend siliciclastic and evaporite sediments and < 2000 m of Zechstein siliciclastic, carbonate, and evaporite deposits (Ziegler, 1990). Stratigraphically, the ROCLT belongs to the Silverpit Formation (Upper Rotliegend Group) in the Central North Sea Graben (Fig. 3; see also Appendix A, Figs. A.3.I–A.3.II).
The Upper Rotliegend Group sediments were deposited during distinctive cycles. Large-scale tectonically driven cycles are interpreted as formations, whereas smaller-scale, climate-driven cycles make up the members (Gaupp et al., 2000; Roscher and Schneider, 2006). In the study area, the London–Brabant and Rhenish massifs were the main source of clastic sediment influx during Rotliegend times (Figs. 2, 4). Here, the SPB comprised distinct facies belts from the southern margin to the basin centre (Fig. 4; Kabel, 2002; Legler and Schneider, 2008). Coarse-grained alluvial-fan and fluvial deposits occurred along the margin of the London-Brabant and Rhenish Massifs. Towards the north extensive sandflats with aeolian dune deposits gave way to mudflats and perennial saline lake facies. South–north-elongated fluvial fairways interfinger with sandflat and mudflat facies (George and Berry, 1994; Kabel, 2002; Legler and Schneider, 2008). The relatively thin fluvial deposits onshore Germany may indicate recycling of fluvial sediments into aeolian deposits during dry phases of deposition. During the Late Permian the Zechstein transgression resulted in deposition of evaporite and carbonate rocks on top of the Upper Rotliegend sediments (Legler et al., 2005).
During the Early Permian the SPB was located at palaeolatitudes similar to those of the present-day North African and Arabian deserts (Glennie, 1998). The Base Permian Unconformity coincides with a change in climate from humid equatorial conditions into arid desert conditions. From the Late Carboniferous onwards the general aridization trend of the climate was interrupted by several wetter periods (Roscher and Schneider, 2006). The wet phases are interpreted as caused by waxing and waning phases of the Gondwana icecap (Roscher and Schneider, 2006). In the rock record of the Upper Rotliegend Group the wetter periods are characterised by the frequent occurrence of fluvial and lacustrine sediments and laterally extensive lacustrine and sabkha facies (Glennie, 1998). The SPB was located in the rain shadow of humid Tethyan trade winds, and the fluvial and lacustrine sediments suggest regular tropical rainfall in the mountains (Glennie, 1998), while to the north of the mountain range an arid desert climate prevailed (Sweet, 1999) with northeast winds parallel to the Variscan Mountain Range (Verdier, 1996). Inundation of the SPB in the Late Permian occurred as a result of a more efficient and greater connection to the north with the proto-Atlantic (Richter-Bernburg, 1955; Verdier, 1996, Legler et al., 2005).
The ROCLT and the Ameland Claystone Member (Fig. 3) consist dominantly of claystone formed in periods of lake-level highstand. Both members wedge out to the lake margin in the south of the basin. Drying-upward cycles in the ROCLT were defined from climatic fluctuations (George and Berry, 1994). These cycles were subdivided into a lower “wet” phase and an upper “dry” phase. In case of a gradual bounding surface between the individual drying-upward cycles, a transition phase is recognised (George and Berry, 1994). Sweet (1999), Bailey and Lloyd (2001), and Bourquin et al. (2009) recognised that the water table may be a major controlling factor on the occurrence of wet–dry cycles. George and Berry (1994) and Yang and Baumfalk (1994) attribute the alternation of wet and dry periods to Milanko-vitch cyclicity.
Subsurface Data Analysis
Gamma-ray logs of the ROCLT show the cyclic succession of decreasing to increasing and again decreasing gamma radiation (Fig. 5). The cyclicity is present in all studied wells and comprises at least four complete cycles. The fifth cycle, in the uppermost part of the ROCLT, may be incomplete due to the erosional contact with the overlying Zechstein deposits. The cyclic succession is interpreted as the expression of a climate-driven expansion and contraction of the perennial saline lake (Sweet, 1999; Bailey and Lloyd, 2001; Bourquin et al., 2009). Lake regression occurred during base-level fall. Basinward shift of facies belts resulted in progradation of the fluvial system and deposition of coarser-grained fluvial sediment on top of mudflat claystone deposits. Subsequent base-level rise caused lake transgression, flooding of the coastal plain and landward retreat of the fluvial system. As a result finer material was deposited in this period (Vail et al., 1977; Shanley and McCabe, 1994). Therefore, the gamma-ray-log peaks correspond to periods with a relatively high lake level (Bailey and Lloyd, 2001), whereas the low gamma-ray readings correspond to periods with a relatively low lake level.
Core analysis of well Grijpskerk-1A shows that each gamma-ray cycle corresponds to a claystone-prone base and top and a middle part characterised by a heterolithic succession of thin siltstone and sandstone layers, 5–50 cm thick, embedded in claystone (Fig. 6). The sandstone layers are sharp based and consist dominantly of well-sorted very fine to medium sand. The tops of the sandstone layers show a gradual grain-size transition to overlying claystone. Cross-lamination and climbing-ripple lamination (Fig. 6A, B) are the most common sedimentary structures in the sandstone layers, and injected sandstone is a common feature (Fig. 6C). In the fluvial environment, deposition of low-angle climbing-ripple beds is a common feature of sediment-laden unconfined sheet flows over an inundated floodplain. The unconfined flows form sheet deposits that expand laterally (crevasse splays, sheetfloods) and dis-tally (terminal lobes) of fluvial channels. The flows rapidly lose momentum and dump their high-density suspension load and thereby form climbing-ripple lamination. Injection of sand into overlying clay is a process of expulsion of overpressured pore fluid during shallow burial by rapid sediment loading in a water-saturated environment. Sand injectites are commonly recorded in a deep-marine turbidite setting (Hurst and Cartwright, 2007). In a fluvial setting their occurrence is an indication of rapid sedimentation (crevasse splays or flash floods) onto a wet floodplain. Based on the lithofacies characteristics, and on the absence of thicker fluvial-channel sandstone bodies, the thin-bedded sandstone intervals in the mud-stone-dominated ROCLT are interpreted as ephemeral channel and terminal-lobe deposits at the termination of the fluvial system. The fluvial deposits interfinger with claystone, which was deposited in mudflats with a high groundwater table bordering the perennial saline lake in the centre of the SPB.
The cyclic gamma-ray pattern of the ROCLT was used to correlate 150 wells in the study area. The ROCLT thickness increases to the north, i.e., towards the subsiding centre of the SPB (Fig. 7; see also Appendix A, Fig. A.5.I). The gamma-ray-log correlation panels show the change from a sandstone-dominated ROCLT in the south (yellow colours) to a claystone-dominated succession in the north (blue and purple colours). The gamma-ray logs in the south are characterised by two-meter-thick low-gamma-ray intervals intercalated with thin peaks in the gamma-ray log responses. The low-gamma-ray intervals are interpreted as stacked fluvial channel sandstone. In most north–south correlation panels the change from proximal stacked fluvial-channel sandstone in the south to distal shale-prone sediments with thin sandstone beds in the north occurs within a narrow transition zone 10–20 km wide (box in Fig. 7). In east–west correlation panels the ROCLT thickness, and that of the individual cycles therein, is constant. From the Dutch–German border to the Dutch–UK border the number of intervals with low gamma-ray log readings increases. In the southwestern part, near the Dutch–UK border, the largest intervals with low gamma-ray log values are encountered, pointing to the thickest stack of fluvial sandstone. Parallel east–west panels show the decrease of sandiness towards the basin centre (Fig. 8). Sand-rich parts of the ROCLT are concentrated in narrow corridors surrounded by shale-prone successions (Fig. 8B).
Processing of the gamma-ray logs yielded net-sand maps of each cycle (Fig. 9), from which the main fluvial fairways could be interpreted. All maps show well-defined SSW–NNE-oriented, sand-rich fairways,10–30 km wide, which spread out to irregular lobe shapes across the east– west-oriented transition zone in which a rapid change in N/G occurs. This transition zone coincides with the boxed area in Fig. 7. South of the transition zone the fairways consist of stacked fluvial-channel sandstone. The lobe-shaped sandstone accumulations north of the transition consist of thin sheet sandstone formed by unconfined flow at the termination of the fluvial system (Fig. 6). From the lithofacies interpretation, the overall low N/G, and similarity with the setting of the re-perforation test it is concluded that these thin sandstone sheets north of the N/G transition zone comprise isolated potential reservoir-sandstone units embedded in floodplain fines. The Texel–IJsselmeer High (TIJH) on the maps is a structural high that formed an obstacle for the fluvial fairways for part of the time (notably during Cycle 3; Fig. 9C) and served as a sand source in other periods (Figs. 9B, D, E). The ROCLT development in space and time is characterised by the lateral shift of preferential fluvial pathways, and by the gradual northeastward pro-gradation of the fluvial environment.
The locus of unconfined-sheet-flow sandstone is represented by lobe shapes in the net-sand maps (Fig. 9). It should be noted that the lobe shapes cover areas of several hundreds to thousands of square kilometers, whereas the sizes of individual crevasse-splay and terminal sheet sandstone in outcrops of unconfined flow deposits at the termination of fluvial systems are much smaller, and are measured in hundreds of meters width in the case of amalgamated sheet deposits, with thicknesses generally less than 1 m (Kelly and Olsen, 1993; Fisher et al., 2007; Hampton and Horton, 2007; Cain and Mountney, 2009). We interpret the lobe shapes as formed by the lateral amalgamation of individual crevasse-splay and terminal sheet-sandstone bodies in a very low-gradient environment, where multiple nodal avulsions created a network of small fluvial channels and associated sheets (see North and Warwick, 2007, for a discussion on the origin of these networks). To develop a realistic reservoir-architecture model for the fluvial sheet sandstone in the ROCLT, the size and shape data of individual sandstone sheets should be taken into consideration. Donselaar et al. (2009) proposed to establish a quantitative data base of size, shape, spatial distribution, and grain-size distribution of deposits in an analogue setting of the endorheic Miocene Ebro Basin with the aim to carry out process-driven forward modeling experiments. The experimental results provided insight into sheet-sandstone depositional thickness, area, volumes, and distribution. These hard data can serve as input for reservoir architecture modeling studies.
Subsurface data analysis shows that the Ten Boer Member (ROCLT) can be subdivided into five cycles on the basis of gamma-ray log response and core analysis. Each cycle consists of a claystone-prone lower part, a sandstone- to siltstone-prone middle part, and a claystone-prone upper part. The cyclic succession is interpreted as the result of wet–dry–wet climate cyclicity and associated expansion and contraction of the saline lake in the centre of the Southern Permian Basin (SPB). From core analysis the sand-rich middle part of each cycle is interpreted to comprise sheet sandstone bodies 5–50 cm thick formed by uncon-fined flow at the termination of the fluvial system. Log correlation panels demonstrate that net sandstone thickness in all cycles rapidly changes from high-N/G stacked fluvial-channel sandstone to low-N/G unconfined, thin-bedded sheet sandstone across a narrow, east–west-oriented transition zone. For each depositional cycle, net-sand maps are constructed from the gamma-ray logs. The net-sand maps show the development in space and time of the fluvial deposits, which formed well-constrained, SSW– NNE-oriented fairways ending in an irregular lobe shape. Combined with core and log analysis it is interpreted that the fairways consist of stacked fluvial-channel sandstone in their proximal, upstream part, and that the fluvial system ends in lobe-shaped sandstone accumulations which are the locus of thin sheet sandstone bodies embedded in mudflat claystone. The combined lithofacies interpretation, overall low N/G, and similarity with the setting of the re-perforation test leads to the conclusion that the thin sandstone sheets north of the N/G transition zone comprise isolated potential reservoir sandstone units embedded in mudflat claystone. Over time the fairways shifted laterally and the entire fluvial system extended progressively northward towards the basin centre. The deposi-tional model will assist in more accurately targeting ROCLT thin sheet sandstone as secondary reservoir.
The Nederlandse Aardolie Maatschappji (NAM) is kindly acknowledged for the permission to use the data and for the financial support to carry out this study. We especially thank Jan Penninga and Daan den Hartog Jager (both at NAM) for their assistance with the core analysis. Two anonymous reviewers are thanked for their constructive comments.
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.