Relations between seismic signals and reservoir properties of deep gas reservoirs in Northwest-Germany [Elektronische Ressource] : Wustrow member, Rotliegend / von Peter Abram

Peter Abram

Informations

• Publié par: friedrich-schiller-universitat_jena
• Publié le: 01 janvier 2008
• Nombre de lectures: 16
• Langue: English
• Poids de l'ouvrage: 4 Mo

Extrait

Relations between seismic signals and reservoir properties
of deep gas reservoirs in Northwest-Germany
- Wustrow member, Rotliegend







Dissertation
Zur Erlangung des akademischen Grades doctor rerum naturalium
(Dr. rer. nat.)















Vorgelegt dem Rat der Chemisch-Geowissenschaftlichen Fakultät der
Friedrich-Schiller-Universität Jena

Von Dipl.-Ing. Peter Abram
Geboren am 28. Dezember 1974 in Bozen (Südtirol, Italien)




































Gutachter:

1. Prof. Dr. Reinhard Gaupp

2. Prof. Dr. Michael Weber

Tag der öffentlichen Verteidigung: 18. April 2007 Abstract

This study tries to find the petrophysical basis for correlations between reservoir properties
and seismic signals calibrated with geological information for Wustrow-sediments of the
Rotliegend in NW-Germany. A secondary objective consists in explaining the large scatter of
permeability for Wustrow reservoir rocks of comparable properties.
The correlations were ascertained in the DGMK-project 593-8, for instance to authigenic clay
mineral types and permeability.

The investigations demonstrate that the differences in seismic signals (amplitude, shape and
lateral coherency of seismic traces) are a product of distinct compressional field velocities
(from wireline logs) and attenuation processes as in-situ bulk densities are barely varying
within the investigated horizon.
The regions of different clay mineral authigenesis (illite and chlorite) diverge by their average
compressional in-situ speeds. The differences suffice to provoke relevant variations in
seismic signals. Total porosity appears not to affect the field wave characteristics distinctly as
high void volumes are encountered in the province of high compressional speed. The distinct
velocities for rocks of different authigenesis and the subsidiary effect of void volume are
ascertained also at plug dimensions. Other rock properties affecting wave characteristics
than porosity become thus relevant.
Plugs of comparable properties but largely varying compressional ultrasonic velocity are
analyzed for textural features which certainly or potentially influence wave characteristics.
These features represent, affect or indicate also reservoir quality. Consequently, the potential
is tested to extend the relevant influence of the identified properties from the plug to the field
scale.
clay minerals in positions important to the rigidity of the rock framework strongly reduce the
stiffness and hence the compressional velocity. The investigations show that sand/siltstones
of illite authigenesis demonstrate by far larger amounts of clay minerals between particle
contacts than reservoir rocks of chlorite authigenesis. Further, rocks with different amounts of
non-load bearing minerals but comparable porosity should differ by compressional velocity.
The illitized plugs feature larger amounts of solid components, particularly illite fibres (IM),
which do not contribute to rigidity. These textural properties appear to be laterally and
vertically consistent throughout the reservoir due to the distinct evolutionary conditions of
load bearing illite and chlorite minerals, their distinct relative proportions, the limited
occurrence of IM-fibres to illitized regions and the continuous core documentation of
corresponding clay mineral types. Hence, the characteristic textural distinctions could be co-
responsible for the dissimilar field velocities of the regions of illite and chlorite authigenesis.
The average transmissivity is severely reduced by the presence of illite fibres (IM). The
correlation to major permeability trends could thus be a product of the characteristic textural
properties in the illitized sandstones affecting velocity and the limited occurrence of the IM-
morphotype to illitized regions. Likewise, the predictability of seismic facies classes for
intense bituminisation could be based on the elastic features of illitized reservoir rocks as the
organic matter impregnation is widely restricted to the provinces of illite alteration.
The ascertained effects of particle contact conditions and pore geometrical attributes at the
plug scale could co-determine both velocities and transmissivities of the Wustrow rocks too.
Although in contrast for the other textural properties, evidences for such characteristic
distinctions at the field scale could not be identified, their influence, also if possibly not
prevailing, remains still plausible.
The reservoir rocks of the Wustrow horizon, whose cores were investigated for the concerning
textural features, affect volumetrically the seismic signals to the largest extent. Furthermore,
seismic signal modifications by other lithological units, internal reverberations, reflections on thin shaly or saline strata intercalating the reservoir and shale layers not resolved by GR-logs
are demonstrably not relevant.
Several features regarding geologic structure, gas contents in pores, seismic and logging
acquisition, etc. are discussed which could hamper the extension of petrophysical features
co-regulating wave characteristics at the plug scale to the field scale. Seismic energy loss
could be caused by eventual characteristic variations in reservoir temperature, particle
contact conditions and the relative motion of pore fluids and illite fibres (IM). The calculated
maximal modification of the seismic amplitude by potential distinct gas saturations appears to
be negligible. However, eventual characteristic fluid saturations could affect field velocities
significantly.
The pore geometrical studies and the modelling of permeability evidence that the strongly
diverging permeability for Wustrow-plugs of similar porosity and equal clay mineral type is
attributable to characteristic variations in pore throat size and distribution, degree of
interconnectivity, fractions of intragranular porosity and pore wall roughness. The void
geometrical differences are a result of dissimilar secondary dissolution and mechanical and
chemical consolidation, apparent at the micro scale.


Kurzfassung

Die vorliegende Studie versucht die petrophysikalische Basis für Beziehungen zwischen
geologisch-kalibrierten seismischen Signalen und Reservoireigenschaften zu finden für
Wustrow-Sedimente des Rotliegenden in NW-Deutschland. Ein Ziel sekundärer Bedeutung
besteht darin, die starke Schwankung von Permeabilitäten bei den Wustrow-
Lagerstättengesteinen mit vergleichbaren lithologischen Eigenschaften zu erklären.
Die Korrelationen wurden festgestellt im DGMK-Forschungsprojekt (593-8) und zwar zu
wichtigen Charakteristika wie u.a. durchschnittliche Permeabilität und Vorkommen von
authigenen Tonmineraltypen.

Die Unterschiede in den analysierten seismischen Signalen (Amplitude, Form und laterale
Verfolgbarkeit von seismischen Spuren) des Wustrow-Horizontes sind ein Produkt von
bedeutenden Divergenzen in der longitudinalen in-situ Geschwindigkeit (aus Bohrlochlogs)
und in Dämpfungsprozessen, da die in-situ Rohdichte im untersuchten Horizont kaum
schwankt.
Die Provinzen der Illit- und Chlorit-Authigenesen divergieren in deren durchschnittlichen
longitudinalen in-situ Geschwindigkeiten. Die Unterschiede reichen aus um die seismischen
Signale signifikant zu verändern.
Die durchschnittliche Gesamtporosität kann die Kontraste in den Laufzeiten nicht bedingen,
da die höheren Geschwindigkeiten in den Provinzen höherer Porosität gemessen wurden.
Die klaren Unterschiede in der Geschwindigkeit und die fehlende Dominanz der Porosität
sind auch im Proben-Maßstab offensichtlich. Diese Tatsachen heben die Relevanz anderer
textureller Gesteinseigenschaften für die Ursache der Korrelationen hervor.
Ausgewählte Kernproben vergleichbarer Eigenschaften aber von stark abweichender
Kompressionsgeschwindigkeit und Permeabilität werden eingehendst analysiert in Bezug auf
Gefügemerkmale, welche die Wellencharakteristika sicher oder potentiell mitbestimmen. Diese
Gesteinsmerkmale sind, beeinflussen oder indizieren auch Lagerstätteneigenschaften.
Anschließend wird untersucht, ob ein maßgeblicher Einfluß der identifizierten Eigenschaften
auch im Feld-Maßstab besteht.
Tonminerale welche sich in einer texturellen Position befinden, in denen sie die Festigkeit
des Gefüges beeinflussen, verringern die Kompressionsgeschwindigkeiten drastisch. Die
Untersuchungen zeigen, dass Sand- und Siltsteine von Illit-Authigenese bedeutend mehr solcher kornstützenden Tonminerale aufweisen als chloritisierte Gesteine. Weiters
unterscheiden sich Gesteine mit unterschiedlicher Menge an nicht kornstützenden Mineralen
aber gleicher Porosität in den Geschwindigkeiten. Die illitisierten Proben besitzen größere
Mengen dieser Komponenten, wozu vor a