Palynostratigraphy and Depositional Environment of Cambay and Olpad Formations in Nawagam - Asmali Area,

Cambay Basin, India*

Raju Grover1 and M. S. Murthy1

Search and Discovery Article #50403 (2011)

Posted April 4, 2011

*Adapted from extended abstract presented at GEO-India, Greater Noida, New Delhi, India, January 12-14, 2011

1
Regional Geoscience Laboratories, Oil and Natural Gas Corporation Ltd., Baroda, India. (rajushimona@yahoo.co.in)

Abstract

The area of present study covers Nawagam, Naika, Mahelaj, Dholka and Asmali Fields in the Ahmedabad Block of Cambay Basin.
A number of hydrocarbon discoveries in this block hold promise for exploration of pay sands of Olpad and Cambay Formations. The
wells have been drilled to explore the hydrocarbon prospects of Deccan Trap, Olpad, and Cambay Formations.

Palynological studies were carried out on the subsurface samples to bring out the palynostratigraphy and paleodepositional
environments. The detailed studies enabled mapping of the variations in palynofloral content and demarcating the age boundary
between Paleocene/Early Eocene. The palynofloral assemblage is dominated by angiospermous pollen, pteridophytic spores, and
marine phytoplankton. The palynofloral yield in general is moderate to rich. The stratigraphic ranges, appearance, and disappearance
levels of various marker taxa have been used for dating and correlation of the sediments in different wells.

Two palynozones have been recognized in all the studied wells. Palynozone-I corresponds to Paleocene age while Palynozone-II to
Early Eocene age. The palynofloral assemblages recorded belong to eight floral ecological complexes, namely, inland, freshwater,
fern, fungal, palm, low salinity water plant complex, mangrove, and marine phytoplankton. The subsurface sedimentary sequence has
been studied in detail and paleoenvironment at micro level has been deciphered.

Recognition of paleoenvironments in the subsurface sequence is a pre-requisite for understanding the depositional set up of any given
basin for hydrocarbon exploration. At the same time correlation of equivalent units with age relationship is also important in a
sequence stratigraphic framework using zonations based on microfossils. Palynological studies have been carried out on the subsurface
samples to bring out the variation in palynofloral content as well as precisely mark the age boundaries between Middle Eocene/Early
Eocene and Early Eocene/Paleocene. In order to achieve these objectives five wells viz., Nawagam#A, Naika#B, Dholka#C,

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Mahelaj#D, and Asmali#E (Figure 1) were taken up for study.

The material used for this study consists of 300 samples of cores and cuttings were collected and studied in detail. The samples
show various lithologies, such as claystone, siltstone, clay, shale, and sandstone. These samples were processed by using standard
processing techniques adopted by various laboratories in ONGC.

Palynostratigraphy

The palynofloral assemblage recorded from the subsurface samples of the wells under study consist of diverse spores-pollen and
dinoflagellate cysts. The assemblage is dominated by angiospermous pollen, pteridophytic spores, and marine phytoplankton. The
palynofloral yield is moderate. The stratigraphic ranges of various marker taxa have been used for dating and correlation of the
sediments in different wells (Figure 2). The top of Paleocene and Early Eocene has been marked on the basis of first down hole
appearance of the index marker taxa.

Palynozone - I:

Definition: This zone is characterized by the restricted occurrence of Rhombipollis geniculatus, Peninsulapollis gillii, and
Yeguapollis prolatus. This zone has been mainly recognized in all the studied wells.

Top: The upper limit of the this zone is marked by the first down hole appearance of Rhombipollis geniculatus, Peninsulapollis gillii,
Yeguapollis prolatus, Spinizonocolpites adamanteus, Milfordia homeopunctata, Polycolporopollenites calvus, Nuxpollenites sp.,
Mulleripollis bolpurensis, Echitriporites sp., Kapurdipollenites sp., Cricotriporites vimalii, Plicatopollis sp. Apectodinium
homomorphum, and A. parvum.

Bottom: The lower limit of this zone has been taken as the top of the Deccan Trap. Associated taxa: includes Palmaepollenites
kutchensis, P. nadhamunii, Florschuetzia sp., Zonocostites ramonae, Proxapertites operculatus, Proxapertites cursus,
Lycopodiumsporites sp., Lygodiumsporites sp., Cyathidites sp., Couperipollis rarispinosus, Longapertites vaneedenburgi,
Margocolporites tsukadai, Marginipollis concinnus, Adnatosphaeridium sp., Exochosphaeridium sp., and Apectodinium spp.

Reference section: Mahelaj#D: 2100m-3200m

Suggested Age: The palynofloral assemblage is suggestive of a Paleocene age.

Remarks: This zone corresponds to the Olpad Formation. Majority of the wells are terminated in this formation.

Palynozone - II:

Definition: This zone is characterized by restricted occurrence of Pellicieiroipollis langenheimii, Triangulorites bellus, and
Striacolporites striatus.

Top: The top of this zone is characterized by the first down hole occurrence of Pellicieiroipollis langenheimii, Triangulorites bellus,
and Striacolporites striatus.

Bottom: The bottom of this zone is characterized of the first down hole appearance of characteristic Paleocene marker palynotaxa,
namely Rhombipollis geniculatus, Peninsulapollis gillii, Yeguapollis prolatus, Spinizonocolpites adamanteus, Milfordia
homeopunctata, Polycolporopollenites calvus, and Nuxpollenites sp.,

Associated palynotaxa: include Polybrevicolporites cephalus, Ctenolophonidites sp., Meliapollis sp. Lakiapollis ovatus and Triorites
communis.

Reference section: Mahelaj#D: 1580m-2100m.

Suggested Age: The palynofloral assemblage is suggestive of Early Eocene age.

Remarks: This zone corresponds to the Cambay Formation. This zone has been demarcated in all the studied wells.

Depositional Environment

The palynofloral assemblage recorded from different wells, has been grouped into eight floral ecological complexes. These
complexes are inland, freshwater, fern, fungal, palm, low salinity water plant complex, mangrove, and marine phytoplankton.

The palynofloral assemblage recorded from the lower part of Olpad Formation, in the well Nawagam#A, is dominated by inland
complex, fresh water complex, fern complex, low salinity water complex, marine phytoplankton along with very low frequency of
mangrove complex. The fair occurrence of marine phytoplankton along with very low frequency of mangrove pollen in the
palynofloral assemblage suggests that the sediments were deposited under subtidal environment. In the middle part of Olpad
Formation similar complexes continue to occur excepting marine phytoplankton, which is attributable to intertidal conditions,
during Middle to Late Paleocene period. As the deposition continued, subtidal conditions again prevailed in the upper part of Olpad
Formation towards the end of Paleocene age. Subtidal followed by intertidal conditions continued during the deposition of lower
part of Cambay Formation in Early Eocene times. The sediments of the upper part of Cambay Formation show dominance of
marine phytoplankton, suggesting, inner shelf environment towards the end of Early Eocene period.

The well Naika#B was terminated in the upper part of Olpad Formation. The subsurface section has yielded dominantly inland
complex, fern complex, palm complex, low salinity water complex, along with moderate frequency of marine phytoplankton and
sporadic occurrence of mangrove pollen. The palynofloral association is suggestive of an intertidal environment. Similar conditions
prevailed almost during the deposition of Cambay Formation. The sediments of the upper part of Cambay Formation show
dominance of marine phytoplankton, suggesting an inner shelf environment towards the end of Early Eocene period.

The well Dholka#C was also terminated on reaching the Olpad Formation. The Olpad sediments have yielded dominantly inland
complex, fern complex, palm complex, low salinity water complex, along with moderate frequency of marine phytoplankton and rare
occurrence of mangrove pollen. The palynofloral assemblage is suggestive of an intertidal environment. Increase in
phytoplankton percentage indicates rise in the relative sea level during the deposition of lower part of Cambay Formation. The
associated palynofloral assemblage include, mangrove complex, fern complex, palm complex, low salinity water complex. The low
frequency of mangrove palynotaxa and dominance of marine phytoplankton is suggestive of inner shelf environment. During the
deposition of middle part of Cambay Formation due to the relative fall in the sea level, low frequency of marine phytoplankton are
recorded, besides, very low frequency of mangrove pollen in the palynofloral assemblage, suggesting that the sediments were
deposited under subtidal environment. Towards the end of Early Eocene, the upper part of Cambay Formation was deposited
under inner shelf conditions.

The well Mahelaj#D (Figure 3) was drilled still deeper into the Olpad Formation. The palynofloral assemblage include inland
complex, fern complex, palm complex, and 10% low salinity water complex, mangrove complex and marine phytoplankton. The
sediments of the Olpad formation were deposited under intertidal conditions. Towards the end of Early Eocene times, the
palynofloral assemblage is dominated by marine phytoplankton complex along with fair occurrence of mangrove complex in the
upper part of Cambay Formation. The low frequency of mangrove palynotaxa and dominance of marine phytoplankton is
suggestive of inner shelf environment.

In the well Asmali#E the depositional environment of Olpad Formation oscillates between intertidal to supratidal. During the
deposition of Cambay Formation subtidal to inner shelf conditions prevailed.

Schematic representation of paleoenvironment in Nawagam – Asmali area shows various depositional environments existed during
Paleocene and Eocene when the Olpad and Cambay Formations were deposited (Figure 4).

Olpad Formation :

Mainly subtidal environment prevailed during the deposition of Olpad Formation. Asmali area experienced supratidal
environments whereas in Mahelaj and Nawagam areas intertidal environment prevailed during early part of Paleocene.

Cambay Formation :

During Early Eocene times the areas such as Asmali, Mahelaj, Naika, Nawagam mainly experienced subtidal environment whereas
Dholka area was under inner shelf conditions. At the end of Eocene, supratidal environment was followed by inner shelf
environment in Mahelaj area, while Nawagam and Dholka areas experienced inner shelf environment during this time. This
condition was followed by subtidal environment in Asmali area.

Conclusions

Two correlatable palynozones have been recognized in the studied wells, viz., Nawagam#A, Naika#B, Dholka#C, Mahelaj#D,
and Asmali#E. Palynozone-I has been dated as Paleocene while Palynozone-II as Early Eocene in age. Palynozone-I corresponds
to the Olpad Formation, while Palynozone-II corresponds to Cambay Formation.

Paleocene/Early Eocene and Early Eocene/Middle Eocene boundaries have been precisely marked in these studied wells. The
palynofloral association suggests that the Olpad Formation was deposited mainly under intertidal environment, while Cambay
Formation was deposited under subtidal to inner shelf conditions.

The studies have not only brought out age and correlatable stratigraphic units but also helped to conceptualize the model of
paleoenvironments in space and time for future exploration leads. The wells have been drilled to explore the hydrocarbon prospects
of Deccan Trap, Olpad and Eocene pays of Cambay Formation. The longitudinal and transverse fault blocks at these levels and the
pinch out termination of Eocene sands are the probable locales of hydrocarbon entrapment where the depositional environments
in the present study are interpreted as subtidal to inner shelf.
Figure 1. Location Map.
Figure 2. Stratigraphic ranges of age marker taxa in Cambay Basin.
Figure 3. Paleoenvironment of Mahelaj#D.
Figure 4. Schematic Representation of Paleoenvironment in Nawagam-Asmali area.
Table I. Age Boundaries of the wells in Nawagam – Asmali areas.