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3D Lithofacies Model Building of the Rotliegend Sediments of the NE German Basin*
By
Inga Moeck1, Heinz-Gerd Holl1, and Heinz Schandelmeier2
Search and Discovery Article #30039 (2006)
Posted February 1, 2002
*Adapted from an extended abstract prepared for AAPG International Conference, Paris, France, September 11-14, 2005
1GeoForschungs Zentrum Potsdam, Geothermics, Telegrafenberg, D-14473 Potsdam, Germany ([email protected])
2TU Berlin, Exploration Geology, Ernst-Reuter Platz 1, D-10587 Berlin, Germany
Abstract
Static 3D
geological models are essential to reservoir characterization and dynamic
models. We introduce an approach of combining pre-existing and newly generated
data to assess lithofacies distributions and 
sandstone
permeability of a clastic
reservoir within the Rotliegend II of the NE German basin. The target is at 4300
m depth and situated north of Berlin (Germany) in the vicinity of a former gas
exploration well, drilled in 1990 and currently acting as geothermal in-situ
laboratory.
This
second-hand well was re-opened in 2000 and deepened in two steps to 4309m TVD.
An extensive logging program was performed between various stimulations and
hydraulic tests. Porosity/permeability data are available from logging data as
well as from abundant porosity (290 samples) and permeability measurements (109)
on cores.
A basic 3D
structural geological model of an area of 120 km2
was
calculated using pre-existing well data and 2D seismic profiles. Detailed well
data provided information to develop a 3D lithofacies model, comprising five
lithotypes. The facies grids were calculated with a 3D minimum tension
technique. In this procedure each facies grid was normalized, calculated against
each other, and reconciled by creating a 0-isoenvelop, that clearly defines the
facies-type body. The isoshells were set in each fault block of the structural
model and processed to a comprehensive 3D structural lithofacies model. This
volumetric 3D model allows an assessment of both matrix-driven and
fracture-driven permeability. This approach can be applied to any region, where
detailed structural and sedimentological data are available.
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The
well Gross Schönebeck 3/90 (GrSk 3/90; NE of Berlin, Germany;
Figure 1) was drilled in 1990 assigned to
gas exploration in East Germany. The borehole was been closed due to
lack of gas. In 2000 the well was re-opened to use the borehole as
in-situ laboratory for geothermal low enthalphy reservoir studies. The
geothermal reservoir is situated in Rotliegend II strata in part of the
NE German Basin (NEGB). The NEGB is part of an extensive basin system
which extends from the North Sea towards Poland. It is bounded in the
north by the Baltic Shield and in the south by the Variscan fold belt.
Initial basin extension occurred between the latest Carboniferous and
the Early Permian and was accompanied by the deposition of volcanic
rocks which were subsequently covered by a siliciclastic sequence of
alluvial fans, ephemeral stream, playa deposits, and eolian sands (Rieke
et al., 2001). Thick cyclic evaporites and carbonates, deposited during
the Zechstein, are overlain by a thick section of Mesozoic and Cenozoic
sediments. The basic scientific function of the in-situ laboratory
consists in the development of hydraulic stimulation techniques in order
to enhance the
MethodsLogging ProgramThe
first geophysical logging program was carried out in 1991 to explore a
potential gas reservoir in the Rotliegend sediments. Erdöl Erdgas GmbH
(EEG) provided logging data in digital format for this project (caliper,
spectral gamma ray, resistivity, neutron, density, sonic, and dipmeter).
The GeoForschungsZentrum (GFZ) logging operations were performed by the
Operational Support Group (OSG) implementing caliper, electric, spectral
gamma ray, resistivity, and acoustic measurements. The last logging
campaign was executed by Schlumberger during the winter 2003. Two new
logging tools were used in addition to the GFZ logging program. The
latest
3D Model BuildingThe basic 3D structural model is calculated, based on data of 6 pre-existing, reprocessed 2D seismic sections and 15 wells (locations in Figure 1). This modeling procedure comprises the development of a 3D conceptual model interpreted from a time-thickness map of the Rotliegend II, and the calculation of a final 3D structural model of the reservoir integrating the seismic section and well data with the conceptual model (Figure 2; see also Figure 4, track lithology). The
calculation of grids has been made with an iterative minimum tension
algorithm, the fault model has been developed according specific fault
hierarchies and the horizon model was processed according geological
intersection rules. Lithofacies data are provided by the well data,
resulting in following 5 lithotypes: (1) mudstone, (2) siltstone, (3)
fine-grained
ResultsFigure 4 shows the lithology distribution and stratigraphic classification of the well Gross Schönebeck 3/90. The volcanic formation of the Lower Rotliegend consists of two different magmatic rock types. The upper series shows higher thorium contents than the lower series and is suggested to be more highly differentiated with a trachydacitic or trachyandesitic character. The lower series is characterized by geochemical properties of a more primitive source and is classified as basaltic andesite. Both of the volcanic rock suites are intersected by a crossbedded tuffaceous or tuffitic layer. The interbedded sediments consist of marls, marly limestones, and mudstones, subordinately interstratified by thin anhydritic evaporite layers. The sandstones and conglomerates of the Havel Subgroup show decreasing gamma ray values attributed to the depletion of mechanically unstable, chemically altered volcanic rock fragments within a fining-upward cycle of the siliciclastic sediments. The nearly clean sandstones of the Lower Dethlingen Formation (4130-4175m/Lower Elbe Subgroup) show the lowest gamma ray emissions of all measured clastic sediments, owing to their low clay content. The gamma ray values increase continuously upward within the interval of interbedded siltstones/sandstones and reach a maximum value in the mudstones of the Upper Elbe Subgroup. The
petrophysical properties of the target horizons are important for the
characterization of the geothermal reservoir. Figure 4 shows a composite
log of the sandstones of interest. We used the neutron The
comparison of logging data (Tphi RST) with measured core porosities as
well as a comparison of calculated
permeabilities with core permeabilities is shown in tracks 3 and 4 (core
The 3D lithofacies model reveals the spatial distribution of the five lithofacies types that are part of the Rotliegend II siliciclastics (Figure 5). The conglomerates and the medium-grained sand were only deposited south of a NW-SE-striking strike-slip fault and in NE-SW-trending graben structures. The thickness of these facies types increases toward the S to SW. The fine-grained sand type is distributed over the whole model area and shows increasing thickness toward the WSW. The facies type mudstone shows increasing thickness toward the NW and SSW. The
target horizon is in the fine- and medium-grained facies type at an
average 4150 m TVD due to its efficient
ConclusionsRecent investigations on the state of stress in the NEGB have revealed a constant ± N-S trend of the direction of the maximum horizontal stress SH in the area north of Berlin (Röckel and Lempp, 2003). The mean direction of SH was determined in the open-hole section of Gross Schönebeck from borehole breakouts and vertical hydraulically induced fractures to 18.5° ± 3.7° (Holl et al., 2004). Thus only a small acute angle exists between SH and the NNE-trending faults. Röckel and Lempp (2003) have shown that the current state of stress in the NGB is generally a normal-faulting state; however, transitions to strike-slip faulting-states may not be excluded. The recent stress regime in the Gross Schönebeck area is not fully known; however, in both possible cases (SH = s1 or SV = s 1) the potential kinematic behavior of the NNE-trending faults is transtensional, whereas the NW-trending faults may suffer frictional blockade. The N- to NNE-trending faults are considered to be hydraulically conductive due to its shear stresses and resulting high tendency of transtensional slip (see also Zoback and Healy, 1992). Based on this knowledge, a 3D hydrotectonic model is developed for the reservoir (Figure 6), indicating hydraulically conductive structures. This should be taken into account when new potential drilling sites are going to be localized. The combined use of pre-existing and newly generated data sets provides new understanding for the characteristics of the geothermal reservoir in the Lower Rotliegend formation of the North East German Basin. The multidisciplinary approach included implementation of existing well data, reprocessing of pre-existing 2D seismic sections, interpretation of various newly generated well log data, and reconciliation of all data to a coherent 3D geological model. The
petrophysical properties of the target, the depositional environment,
and the reservoir geometry are now well known. These data are crucial
for modeling the thermo-hydraulic conditions within the geothermal
reservoir during production. Future work will focus on the use of
geostatistical models, combining
ReferencesBaltrusch, S., and S. Klarner, 1993, Rotliegend-Gräben in NE-Brandenburg: Zeitschrift der deutschen geologischen Gesellschaft, v. 144, p. 173-186. Holl, H.-G., I. Moeck, and H. Schandelmeier, 2004, Geothermal well Großschönebeck 3/90: A low enthalpy reservoir (Rotliegend, NE Germany): Proceedings, 66th EAGE Conference, F032, Paris, France.
Pape, H., C. Clauser, and J. Iffland, 1999, Rieke, H., D. Kossow, T. McCann, and C. Krawczyk, 2001, Tectono-sedimentary evolution of the northernmost margin of the NE German Basin between uppermost Carboniferous and Late Permian (Rotliegend): Geological Journal, v. 36, p. 19-38. Röckel, T., and C. Lempp, 2003, Der Spannungszustand im Norddeutschen Becken: Erdöl Erdgas Kohle, v. 119, no. 2, p. 73-80. Ziegler, P.A., 1988, Evolution of the Arctic-North Atlantic and the Western Tethys: AAPG Memoir, v. 43, 198 p. Zoback, M.D., and J.H. Healy, 1992, In-situ measurements to 3.5 km depth in the Cajon Pass scientific research borehole: Journal of Geophysical Research, v. 97, p. 5039-5057.
AcknowledgmentsThis multidisciplinary project is a joint research project of GFZ Potsdam, BGR Hannover, GGA Hannover, TU Berlin, Ruhr University Bochum, University Stuttgart and industrial partners (GTN Neubrandenburg, BWG Neubrandenburg, MeSy Bochum). It is funded by BMWi, BMBF, BMU, MWI and MWFK. The 3D models are processed with EarthVision®, Dynamic Graphics Inc. (DGI). Our thanks are addressed to Erdöl Erdgas GmbH (EEG) for data release, and to DGI (namely P. Luxey) for support and advise . |
