Tectonic control on sedimentation, erosion and redeposition of Upper Jurassic sandstones, Central Graben, North Sea - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 503

22/30c-13

Long
Te rm
Strat.
Cycle

Grain size

RHOB AT 60

Metres
MD

PHI-Spicule
(%)

1.95 2.95 0.2 2000

GR

NPHI AT90

0 100

0.45 -0.15

Heather Shale

6300 Franklin C Sands

FACIES CODE
Upper shoreface

Secondary
Porosity from
dissolved
Spicules

Middle shoreface
Lower shoreface
Tr ansition zone

6350

Franklin
Upper B Sands

6400

Franklin
Lower B Sands

6450

Franklin A Sands

Fig. 24. Secondary porosity development related to sponge spicule dissolution in the upper part of the Franklin B Sands.
GR: Gamma Ray Log; MD: Measured Depth (m); RHOB: Bulk Density; NPHI: Neutron Porosity; AT60: Medium Resistivity;
AT90: Deep Resistivity; PHI-Spicule: petrographically estimated core secondary porosity related to spicule dissolution.

Elgin Well 22/30c-13: Franklin B Sands Phi-K Cross-plot

35

30

“Upper B Sands”
Spicule porosity
(porosity increased
but not permeability)

25

20

Upper B Sands
Lower B Sands
Log. (Up B Sands)
Log. (Lw B Sands)

15

“Lower B Sands”
No spicule porosity

10

Fig. 25. Comparison of porosity-
permeability relationships for the
progradational Lower B Sands vs. the
retrogradational Upper B Sands (of
Fig. 17) where secondary porosity
related to sponge spicule dissolution
is developed.

5

0
0.001 0.01 0.1 1

Kh (mD)

10 100 1000 10000

Fulmar deposits display an increased marine
dinocyst component and relatively more abun-
dant saccate pollen. This could be attributed to
the more condensed character of the retrograda-
tional succession.

DISCUSSION

Key findings of this study challenge a number of
concepts concerning stratigraphy and sedimen-
tation in the Central Graben, most notably: