Page 254 - The Geological Interpretation of Well Logs
P. 254
- THE GEOLOGICAL INTERPRETATION OF WELL LOGS -
CALI -= : 46 NPHI o
z
1.95
3] 92) =
2.7
,_RHOB
GR aoa| os | ILD I DT
10 l 139 1 19 1909) 40
; ( ¢ ‘-NPHI RHOB 7
CSAL GR i open ‘ILD
r of i marine $
‘ : C.U.
t organic succession 7 c.u a
L ? decreases Joi ; Us ?
: ' condensed / { TRANSGRESSIVE
SURFACE
Be
section
organi¢-rich
shale
} a {of erosion)
J.
' = a fe a = ss
‘ eroded bedform? Seid |. bloturbated C
; . anagieasies 5 =
, japos!
4 incised vallay fill q. o ° - ———*
:
L
'
.
Figure 15.7 Log example of a transgressive surface of erosion, The underlying sand sequence of probable estuarine deposits,
shows eroded bedforms at the top. The erosion surface is covered by thin, bioturbated shaly sands: transgressive deposits. These
are followed by wel] laminated, open marine, organic rich shales of a condensed section which form the base of a shallow marine,
coarsening-up, prograding sequence.
fill sediments and cut with a lag into the interfluves (ie. of the sediment deposited comes from a hemipelagic or
ravinement). The example (Figure 15.7) being a case of pelagic source and represents deeper water conditions.
erosion into valley fill. Such a sequence is typically rich in pelagic fauna and
With logs alone, the transgressive surface is difficult to microfauna, is finely laminated, has a low quartz content
identify as it shows considerable variety. Where coarse and is enriched in marine organic matter and exotic
valley fill is present it will be evident as the sands are elements such as sulphides, pelletised glauconite,
followed by shales (e.g. Figure 15.7). Where the trans- phosphates, and iridium. It may have a stable isotope
gressive surface and the subaerial erosion surface fingerprint (Loutit er ai, 1988) (Figure 15.8). However,
(sequence boundary) are coincident (Figure 5.1), the only considerable variation exists in the characteristics of con-
record of the wansgressive surface may be a coarse lag, densed sequences, strong bioturbation and mineralisation
a centimetre or two thick, beyond log recognition (except also occur so that they may be confused with subaerial
image logs). erosion surfaces. In sequence stratigraphic theory, the
maximum flooding surface (equally called the downlap
3) surfaces and intervals of slow deposition: surface when prograded over by highstand clinoforms)
condensed sequence, maximum flooding surface, falls within a condensed section. Condensation lasts
downlap surface, hardground longer in the offshore but at its inland limit, lasts for a
shorter time and therefore is the maximum flooding
A condensed sequence represents a long period of time
surface (Figure 15.1). The condensed section may in fact
during which land derived detrital input is small and most
ST. STEPHEN'S Planktonic Water Oxygen Substrate 50 (%,POB)
QUARRY forams % depth vegetation
20 40 © 300° 600° ft low ni CON Qe
MINT
SPRINGS i
RED
BLUFF *GR
; 86 57 5¢
30
BUMPNOSE
]
Red Blutt|
ae
BLUE CLAY W772 VIET WEED VZEZ. Zz ZZ Vili, 2 bao
SHUBUTA - — $
PACHUTA | “ sruues Lag
0
Pachuts
BLUE CLAY = CONDENSED SECTION
*GR = x1000 counts/30 sec.
Figure 15.8 Characteristics of a condensed section, the Blue Clay, showing faunal, chemical, water depth and gamma ray
signatures (re-drawn from Loutit e¢ al., 1988).
244