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Sand Distribution Pattern and Depositional Model of
Kopili Formation (Eocene) with Special Reference to Sequence
Stratigraphic
Framework from North Assam Shelf, Assam-Arakan Basin, India*
S.K. Roy Moulik1, H.J. Singh1, R.K. Singh Rawat1, Md. S.Akhtar1, S. Mayor1, and M. Asthana1
Search and Discovery Article 50196 (2009)
Posted August 11, 2009
*Adapted from manuscript prepared for oral presentation at AAPG Annual Convention, Denver, Colorado, June 7-10, 2009
1Oil and Natural Gas Corporation Limited, Dehradun,India ([email protected])
This study has incorporated enhanced quality 3D data
(PSTM/PSDM) along with high resolution biostratigraphic and available G&G
inputs to bring out the sand distribution pattern and building of a
depositional model within sequence
stratigraphic framework of the upper part of
middle Eocene to upper Eocene Kopili Formation for better understanding of the
reservoir distribution for the entire North Assam Shelf.
Sequence
stratigraphy has an important built-in
interpretation component, which addresses issues such as the reconstruction of
the allogenic controls at the time of sedimentation, and predictions of facies
architecture in yet unexplored areas. It is a process response model, thus
leading to the interpretation of paleoenvironments which are much more critical
for
sequence
stratigraphy than for lithostratigraphy. As a result, current
sequence
stratigraphic concept is being used globally in place of
lithostraigraphic concept, in particular, to bring out the facies distribution
pattern and depositional model in basinal scale
analysis
.
The North Assam Shelf is in the mature stage of exploration for relatively shallow horizons and has established an oil province from Neogene (Oligocene and Miocene) age. A good number of wells have been drilled through deeper sections(Eocene and Paleocene) and has established leads from strata of Paleogene age.
Kopili Formation (middle to late Eocene) of North Assam
Shelf could be further subdivided into three higher order sequences; viz.,
Sequence
I, II, and III, primarily based on logs and available biostratigraphic
and paleobathymetric data as the seismic imaging is found to be very poor
within the Kopili section.
Sequence
I is the lowermost
sequence
;
Sequence
III
is the uppermost
sequence
, whereas
Sequence
II is the intermediate
sequence
identified within Kopili Formation. Two types of system tracts (viz., TST
(Transgressive System Tract) and HST (High stand System Tract)) are found to be
present in each
sequence
. System tract
analysis
has brought out facies
distribution pattern and barrier bar- lagoon - tidal inlet - tidal bar - bay
head delta depositional model has been envisaged for Kopili Formation in the
entire North Assam Shelf. This model will certainly aid in proper planning for
exploration of Kopili Formation in North Assam Shelf.
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The study area lies in the North Assam Shelf of Assam and Assam-Arakan Basin, northeast India. (Figure 1). The North
Assam Shelf is in the mature stage of exploration for relatively shallow
horizons in Barail and Tipam groups and has established an oil province from
Neogene (Oligocene and Miocene) age. A good number of wells have been drilled
through the deeper section and has established leads from Paleogene age. In
Geleki area alone, a number of wells have been drilled to probe the HC
potential of Kopili Formation but except for #E, none has fully penetrated
the Kopili section, although Kopili sands in some of those drilled wells are
found to be HC-bearing. In adjoining Lakwa structure a total of 5 wells #S,
#U, #V, #W, and #Y have been drilled through the Kopili section, which show
encouraging electrolog characters, along with oil and gas shows during
drilling. Therefore, it was an urgent need to have a basinal scale study
applying
Assam Shelf, a part of Assam-Arakan Basin covering an area of about 40,000 square kilometers, has long been established as a hydrocarbon-producing basin (Figure 2). Most of the area is covered with alluvium except for a thin veneer of Cretaceous and Tertiary shelf sediments skirting Mikir Hills and Shillong Plateau. It extends westward beyond Bangladesh and West Bengal up to Orissa, including part of Burma in the east and south. The shelf consists of Shillong Plateau, Mikir Hills, the Garo, Khasi, Jaintia Hills and the Upper Assam Valley. Geologically, this is the northeastern prolongation of the Indian Peninsular Shield with Garo -Rajmahal traps in between (Murty, 1983). Metamorphics of Mishmi Hills of Arunachal Pradesh occur towards the northeast of the shelf. Hill ranges of Nagaland, Cachar, and Mizoram belonging to the mobile belt mark the southern and southeastern edge of the shelf. To the north, the Himalayan Foredeep, exposing Siwalik molasse, bound the shelf sediments. The Shillong Plateau and Mikir Hills, with relatively undisturbed cover of Cretaceous-Tertiary sediments, form the western and southwestern extension. The Assam Shelf has been further subdivided into two parts: North Assam/Upper Assam Shelf and South Assam Shelf by Jorhat fault. Present study area is the entire North Assam/Upper Assam Shelf (Figure 2).
Stratigraphic succession of Assam Shelf has been widely discussed by Evans (1932), Mathur and Evans (1964), Dasgupta (1977), and Deshpande et al. (1993). Regionally, Assam Shelf exhibits a “Weimerian layer cake” (Weimer et al., 1965) stratigraphy typical of epicontinental basins overlying the Precambrian granite basement. Up to this time, more than 1200 wells have been drilled on Upper Assam Shelf, and subsurface sedimentary records have been confirmed from Paleocene to Recent in age with a major break in sedimentation during Oligocene, late Miocene, and Pliocene times. The standardized lithostratigraphic succession (Deshpande et. al., 1993) for Assam Shelf, shown in Figure 3, depicts the generalized stratigraphy of Upper Assam.
In Assam Shelf area, the Paleogene sediments are mostly
deposited in shallow marine to transitional environments. Thickness of these
sediments increases from northwest to southeast, suggesting the basinal
system towards the southeast (Bhandari et al., 1973; Murty, 1983). The end of
Paleogene is marked by the widespread
Tertiary rocks rest directly over the granite gneiss and are divisible into two supergroups (Bhandari et al., 1973). The older Naga Supergroup of Paleogene age is further divided into Jaintia and Barail groups.
Jaintia Group (Paleocene- Eocene) is composed of:
The primary objectives of this study were:
In the present study concerted efforts have been made,
based on
Forty-five deep wells in the area have been studied, and 6 regional electrolog profiles (Figure 4; showing 3 dip and 3 strike profiles) were prepared for 22 wells; in most of them the entire Kopili sections are represented.
Thickness Distribution Trends
Overall, Kopili isochronopach map suggests four thickness maxima (Figure 5a and b). Three thickness maxima are Amguri, Charli-Rudrasagar, and Geleki-Nazira low areas. Another maximum is indicated towards the northwest and northeast of Lakwa area. The isochronopach map shows gradual thinning in the updip direction and thickening in downdip direction. The thickness of Kopili Formation indicates a gentle slope towards northeast, whereas towards south and southeast directions the dip is quite steep at places, as shown in Figure 5. The areas shown as red has thinner Kopili sediments indicative of paleohighs. Areas shown as yellow received more sediments and green shows maximum Kopili thickness, suggesting paleoslope in southeast and northeast directions (Figure 5b).
Reflection configuration of Kopili Formation
The deposits of sand/shale alternations that broadly
constitute a major part of the Kopili Formation overlie the Sylhet Formation
. Seismically the Kopili
Application of
The lithostratigraphic
Figure 6
shows the
inherent difference between lithostratigraphy and
Van Wagoner (1990) performed
Palynological marine index (PMI) has been calculated for a
few wells. PMI helps in the recognition of
Biostratigraphically, Kopili is characterized by the presence of calcareous foraminifera. The overall faunal frequency and the size of the forams decrease towards the upper part. A few arenaceous forams, like Trochammina and Haplophragmoides (normal growth), are also seen. The diminutive calcareous forms and the presence of the above arenaceous forams suggest the possible deposition in estuarine conditions, with salinity ranging from 100-20,000 ppm. (Mohan and Pandey, 1973).
Three higher order sequences have been identified within
Kopili Formation.
It is the lowermost
Seismic Response Close to
As stated earlier,
Sedimentary thickness of this, the middle,
Sand thickness ranges from 10m in #L to 33m in #A. In
Amguri area thickness varies from 22-33m and from 14-27m in
Charali-Demulgaon-Kuargaon area. In Lakwa-Moran-Sonari area thickness of sand
ranges between 14-20m, whereas in Rudrasagar-North Rudrasagar-Rajmai area
sand thickness is between 10-18m. In Panidihing, sand thickness is in the
range of 10-26m. Again in Dikhowmukh-Disangmukh area comparatively greater
thickness of sand (22-29m) is observed. Sands are found to be distributed as
elongate, discrete bodies particularly in Amguri-Geleki area,
Charali-Demulgaon-Kuargaon area, and Lakwa-Moran-Sonari area along a linear
trend (Figure 14).
These elongate bodies strike NE-SW, the direction which is also parallel to
the strike of the basin. Based on
Seismic Response Close to
As the seismic reflection event corresponding to the top
of
This is the uppermost
Seismic Response Close to
The reflector ranges from 2600 msec in Disangmukh area to 5100 msec in Sonari–Nazira low area. It is discontinuous and patchy in the whole area. TWT relief (structure) map with 3-D view has been prepared, after correlation of this event (Figure 17a and b).
Kopili Formation is primarily an argillaceous section with some isolated, scattered, discrete sands distributed on the entire North Assam Shelf. Sand thickness is found to be on the order of 8-30m. The depth of occurrence makes it difficult to resolve its heterogeneity and spatial geometry with confidence. Delineation of these individual sands within the formation is difficult due to the following reasons:
To identify the prospective areas, locales of sand maxima
coupled with structural advantageous positions have been our main interest.
To achieve this objective, the sand thickness trends were calculated from
logs and a reasonable horizon window was calculated to accommodate most of
the sands from the Kopili package to avoid tuning constraints. Accordingly, a
window of 100 msec above
Instantaneous frequency and amplitude attributes were extracted, and spectral decomposition technique was also attempted. Several discrete sand geometries are observed in the area. Kopili sands could be recognized by the moderate to high amplitude and low to moderate frequency anomalies in the above-mentioned attributes. Low to moderate frequencies have been identified to be the anomalous pattern corresponding to the sands within thick shales of Kopili Formation (Figure 18).
As there is hardly any core data available for the Kopili
section for the entire North Assam Shelf, depositional environment has been
estimated on the basis of
It is basically a shoreline strike system, sediments transported by longshore currents. In Upper Assam Shelf a linear NE-SW trend of Amguri-Geleki-Charali-Lakwa–Moran consists of the elongate, discrete offshore bars. Relative lowering of sea level or increase in rate of sediment supply causes depositional regression (Curray, 1964) or normal regression (Posamentier, 1992) .Net effect was the seaward accretion of littoral sediments which were subsequently redistributed by longshore currents and deposited as offshore bar sands. These sand bodies grade landward into lagoonal muds and basinward into shelf muds and act as a barrier between estuary and the open sea. Towards the west of the mapped area (Figures 12, 14 , and 16) lay a lagoonal environment.
Tidal Inlet-Tidal Bar System
Intermittently, along its length the barrier bars were cut by tidal channels, or tidal inlets. Two such tidal inlets are observed between Charali and Lakwa bars and Lakwa and Moran bars. Area around wells R-134, R-156 and D-31was influenced by tidal bar deposits. Landward, towards the west, lagoonal conditions prevailed.
Bayhead Delta
Area farther west; i.e., the whole stretch from Dikhowmukh in the southwest to Panidihing in northwest, was under the influence of bayhead delta environment. A shore-parallel sand geometry marks the imprint of bayhead delta deposits.
Overall Pattern
Based on the above
Hydrocarbon
Prospectivity
Geleki field is the only established commercial
hydrocarbon producer from the sands of Kopili Formation in the entire Upper
Assam Shelf. A very few wells have produced significant amounts of liquid
hydrocarbon from Koplil sands. In Lakwa, a few deep wells have reported
hydrocarbon shows during drilling, and Kopili sands are also interpreted as hydrocarbon-bearing
in logs of a few such wells. Apart from these two areas Kopili sands did not
show encouraging results. West of Geleki-Lakwa trend, sands are found to be
water-bearing. It appears that structures closest to the kitchen (farther
southeast of Geleki-Lakwa trend) were charged and the structures farthest
from the kitchen were not charged. System tract
With the established leads from the Kopili play in Geleki
and Lakwa areas this study has incorporated enhanced quality 3D data
(PSTM/PSDM) inputs along with the G&G data within
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The authors are highly indebted to ONGC Limited for giving an opportunity to work on this topic Authors express their deep gratitude to D. K. Pande, Director (Exploration) for permission to publish this article. This work could not have been successfully completed without the valuable support and guidance provided by P.K.Bhowmick, ED–Head KDMIPE. The authors also highly appreciate the constructive suggestions of Shri. Jokhan Ram, the then - Executive Director and Head KDMIPE towards the progress of the project. Continuous inspiration and motivation from Dr. Manoj Asthana,GGM&Head BRG and Shri Sanjive Mayor,GM(Geology) are highly acknowledged. Authors also acknowledge the encouragement given by peers and colleagues. System group KDMIPE is thankfully acknowledged for their cooperation in carrying out the present work on IIWS. Support/inferences taken from the reports of various authors is also gratefully acknowledged. Views expressed in this paper are those of the authors only and may not necessarily be those of ONGC.
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