The present research work presents a detailed analysis of electrofacies, cycles,
sequences, associations and limited core of the Neogene sedimentary sequence in the
well Shahbazpur-1 of the Shahbazpur structure, for reconstruction of
paleoenvironments of deposition. The Neogene sequence of the well in the Shahbazpur structure
is characterized by four sequences (Sequence 1-4) that consist of 15 para-sequence
sets and 27 para-sequences. The identified electrofacies in GR logs are bell, serrated
bell, funnel, serrated funnel, cylindrical, egg or bow and linear-shaped. The
Shahbazpur Sequence-4 (SBS4) consists of two para-sequence sets (one fining upward and
one coarsening upward) and 16 para-sequences (nine fining upward, six coarsening
upward, and one coarsening upward then fining upward). The lower part of the sequence
was deposited in marginal estuarine influence and the upper part in marshy
environments including shallow marine to transitional estuary tidal channel, mud flat, etc.
The Shahbazpur Sequence-3 (SBS3) consists of four para-sequence sets (one
coarsening upward then fining upward, one coarsening upward, one linear and one fining
upward) and five second-order (one coarsening upward and four fining upward). The
sequence (SBS3) might have been deposited under shallow marine (lower part) to deep
marine (upper part) environments. The Shahbazpur Sequence-2 (SBS2) consists of five
para-sequence sets (three coarsening upward and two fining upward) and two
para-sequences (coarsening upward). It might also have been deposited
under progradational delta or marine regression and marine transgressional
environments.
The Shahbazpur Sequence-1 (SBS1) consists of four para-sequence sets (two
coarsening upward and two fining upward), and four para-sequences (two coarsening upward
and two fining upward). Overall, the nature of the log suggests a phase of slow rate
of deltaic progradation and then marine transgressional during the deposition of
the sequence (SBS1). Although several cycles, marine transgressions and regressional
phase have been interpreted, yet the overall sequence
forms an upward coarsening progradational deltaic phase of deposition.Well logging is an important technique in studing subsurface lithofacies. This
technique has been widely used as a tool in the reconstruction of paleoenvironments of a
depositional basin. After a detailed analysis of digital seismic data, previous geological
and geophysical information, a well was drilled by Bangladesh Petroleum Exploration
and Production Company Ltd. (BAPEX) in 1995 to discover gas in the Shahbazpur structure.
A detailed sedimentological analysis of subsurface sequence from logs and cores,
for reconstruction of the paleogeographic setting of the Bengal basin, has been carried
out several times (IKML, 1991; Alam, 1993, 1995a and 1995b; Sultana and Alam, 2000;
Deeba et al., 2001; Alam et al., 2003; Devices et al., 2003; Gani and Alam, 2003; and Hossain et al., 2003). There is no detailed research on sedimentological and
electrofacies aspects based on wireline logs and cores in the Shahbazpur structure. A
sedimentary electrofacies always bears the evidence of depositional condition. Electrofacies analysis
is mainly done on the basis of different log signatures (gamma and Spontaneous
Potential (SP)).
The gamma log curve shows some distinctive shapes at different depths along
the well under study, by which it is possible to identify that the major (para-sequence set)
and the minor (para-sequence) cycles of sedimentation and distinctive shapes of the
Gamma Ray (GR) log curve for each cycle are indicative of different facies sequences. The
stratigraphic sequence, lithologic interpretation and analysis of the major and minor facies cycle
can provide information regarding environments of deposition of the sedimentary sequence
in the well. SP and GR logs are best suited for study of the log shape. The shape of GR
could be interpreted as grain size trend. Lower GR value indicates an increase in grain
size, whereas smaller grain size corresponds to higher GR value. The Shahbazpur structure
is situated in the middle of the Bhola island, in between the southern part of the
platform slope and the northern margin of Hatiya trough of the Bengal Foredeep of Bengal
basin (refer Figure 1). The main objectives of the present study are identification of
electrofacies, facies sequence, cycles and associations from the wireline logs and limited core samples
for reconstruction of paleoenvironments of deposition. |
