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382

6th International Conference & Exposition on Petroleum Geophysics “Kolkata 2006”
The Geochemical Composition of Oils and Sediments From
Ahmedabad-Mehsana Block of North Cambay Basin, India
P. Sivan*, K.P. Singh, M. Bhandari and R.R. Singh
KDM Institute of Petroleum Exploration, ONGC, Dehradun-248195, Uttaranchal, India Porphyrins and perylene biomarkers have attracted considerable attention as important constituents of source rocks and crude oils, in recent years. These biomarkers provide significant information on depositional environments, origin,and maturity of the oils in petroliferous basins. During present studies, both porphyrins and perylene pigments in oils fromdifferent structures and sediment samples belonging to Cambay Shale Formation from age Late -Early Eocene of NorthCambay basin were analysed and quantified. The results indicate that distribution and preservation of both petroporphyrinsand perylene pigments are mainly controlled by the redox conditions experienced by different sub units of the Cambay ShaleFormation. The anoxic events identified by the distribution of these pigments during Early to Late Eocene times can becorrelated with the sea level changes, which occurred during this period. The abundance of vanadyl porphyrins and perylenein the Younger Cambay Shale sequences suggests that high anoxic condition with low pH during deposition. On the contrarythe enrichment of nickel porphyrins in Older Cambay Shale sediments indicate suboxic conditions with high pH and Eh.
Furthermore, the enrichment of etioporphyrin (ETIO) series of nickel porphyrins with vanadyl porphyrins as well as lowconcentrations of vanadium in the oils of North Cambay basin suggests that these oils are mainly derived from the OlderCambay Shale (OCS) and Olpad Formation deposited under suboxic conditions.
Introduction
Geological settings
Accumulation and generation of hydrocarbons is a The Cambay basin, situated in the northwestern part complex process dependent on preservation of organic matter of Indian craton is a rift graben, which came into existence and its maturation in space and time. These processes involve in the earliest Paleocene. Basin is divided into five tectonic diagenesis and catagenesis of various organic facies, which Blocks, which resulted as a sequel to rifiting along the west lead to the formation of petroleum. The diagenetic products coast of India. These blocks are Narmada, Jambusar-Broach, of chlorophyll such as ‘Petroporphyrins’ found generally in Cambay-Tarapur, Ahmedabad-Mehsana and Sanchor-Patan the form of complexes of nickel and vanadium in sediments blocks (Pandey et al., 1993). In the Ahmedabad- Mehsana and oils are important biological markers to study and fine- block, two longitudinal axial trends parallel to the axis of tune the hydrocarbon occurrence models. Similarly, another the basin are consipcuous. These are Sobhasana- Kalol- biomarker ‘perylene’, a polycyclic aromatic hydrocarbon Ahmedabad trend in the east and Mehsana- Kadi- Viraj- preserved under identical depositional conditions as those Jhalora- Sanand- Dholka trend in the west. The longitudinal of porphyrins has also been used to decipher the mechanisms up-lifts and depressions are (i) Eastern margin depression involved in the genesis of petroleum (Aizenshtat, 1973). The (ii) The eastern anticline zone (Sobhasan in North and presence of both these pigments in a wide variety of Ahmedabad- Bakrol in south) (iii) The Central depression sediments, sedimentary products (petroleum) and terrestrially (iv) The Western anticline zones (v) The Western margin derived peats and coals is well documented in the literature depression. The lithostratigraphy of different blocks of the (Baker and Louda, 1986). Both porphyrin and perylenes have Cambay Basin has been described by various authors also been reported to be extremely sensitive indicators of (Chandra and Chaudhary, 1969, Bhandari and Chaudahary, time-temperature history of sedimentary organic matter 1975, Pandey et al., 1993). In Mehsana block the Deccan (Baker and Louda, 1986). Present work deals with the trap forms the technical basement, which is overlain isolation and quantification of both porphyrin and perylene unconformally by Olpad Formation. It is further subdivided pigments in oils from different structures and pay zones and in to two units designated as Older and Younger Cambay sedimentary source sequences of Ahmedabad- Mehsana Shale formations. The Younger Cambay Shale has block, North Cambay basin to evaluate the source organics, unconformable relationship with overlying Kalol Formation depositional environment and maturity.
and underlying Cambay Shale Formation. The Kadi Formation restricted to Ahmedabad-Mehsana block is thestratigraphic equivalent of the younger Cambay Shale. TheKalol Formation has conformable relationship with overlainTarapur Formation. The formation has been deposited inalternating regressive and transgressive marine influence ina deltaic depositional regime, its thickness is about 150-200min Mehsana block. The location map and generalizedstratigraphy of North Cambay basin is shown in Fig. 1 & 2.
Fig. 2: Generalised stratigraphy
The mixture of nickel and vanadyl chelated MPP isolated from alumina column was concentrated and wereapplied to silica gel (100-200 mesh) column. Elution withhexane/toluene (1:1v/v) separated nickel complexes whereaselution with mixture of toluene/chloroform (1:1 v/v) isolatedvanadyl complexes. Perylenes were fractionated from theoils and bitumen on silica gel column by using toluene asmobile phase (Louda and Baker, 1984). HPLC (Waters 840HPLC) was used to isolate different structural isomers ofMPP. The solvent system acetonitrile + methanol (10:90v/v) was used on reverse phase C18 column (Waters 5ì, 150 Fig.1: Location map of Cambay basin
mm, Synergy RPC18 column) with a flow rate of 1.5ml/min. The detection of nickel porphyrin isomers was done at Methodology
a fixed wavelength detector set at 546mm. Structural isomers Crude oil samples (9-10 gms) were extracted with of nickel porphyrins were identified by using commercially 100 ml dimethyl formamide (DMF) on water bath. The DMF available standards. Thin layer chromatographic (TLC) extracts were reextracted with ether and was concentrated analyses of demetallated free base petroporphyrins were after the removal of DMF with distilled water. The extract carried out on Silica gel G plates using dichloromethane: of metalloporphyrins was purified by column chromatography using neutral alumina. The finely groundcutting/core samples (100-150 gms) were soxhlet extracted Results and discussion
with methanol + benzene (1:9 v/v) mixture. The extract wasconcentrated and metalloporphyrins (MPP) were purified The geochemical data and the distribution of by alumina column chromatography by using sequential perylene and porphyrins in oils and sediments are listed in elution with 100% toluene, a 20-80% gradient of toluene tables-1 and 2 and the distribution pattern of ETIO (C23- and chloroform and 100% chloroform. The fraction eluted C34) and DPEP series of porphyrins of oils and sediments with 100% chloroform contained MPP as ascertained by UV/ 6th International Conference & Exposition on Petroleum Geophysics “Kolkata 2006”
Table 1 : Geochemical data and concentration of metalloporphyrins in oils
CMBS-Cambay Shale; L. Kadi-Lower Kadi; U.Kadi-Upper Kadi; Nip-Nickel porphyrin Perylenes and porphyrins as indicators of
in lacustrine/brackish environment with high pH and Eh source organics
(suboxic) conditions. The oils accumulated in the marginareas at Sanand, Jhalora, Kadi, Balol, Lanwa, Santhal andIndrora and also part of Jotana and Nawagam fields Terrestrial source input is indicated by low originated from source rock equivalent of SR-7 in KL-B at concentration of petroporphyrins, generally less than 100 2670-2710m. The enrichment of Vop and absence of Nip in ppm (Lewan and Maynard, 1982). These are predominantly SR-8/9 (2845-2990m.) of KL-C indicates more anoxic nickel complexed and are associated with low sulphur influence, which is in contrast to the freshwater depositional content (HajiIbrahim et al. 1981). Presence of perylene and environment. The predominance of ETIO Nip in Cambay dominance of nickel porphyrin as compared to vanadyl Shale further confirms the diagenesis of source organics in porphyrin in both oils and sediments indicate the terrestrial a sub-oxic to oxic conditions (Barwise & Roberts 1984).
source input in the studied samples of Ahmedabad – Mehsanablock (Table-1 & 2). Yang & Cheng (1988) also reported Oil-to-oil correlation
presence of perylene in crude oils and sediments fromcontinental Salt Lake in China, derived from terrigenousorganisms.
Mehsana-Ahmedabad block are only enriched in nickel Depositional environment
chelated tetrapyrrole pigments. The visible spectral analysisof demetallated free base porphyrin of all the samples studiedindicate the presence of ETIO porphyrin as dominant The high concentration of nickel porphyrins in all structural type as reported earlier by Chandra et al. (1983).
the oils from different structures and pay zones in north These oils also show very poor V/Ni ratios. Distribution Cambay Basin suggests that suboxic to oxic conditions pattern of metal porphyrin as analysed by HPLC also show prevailed during diagenesis. The nickel porphyrin the dominance of C31 and C32 structural isomers of ETIO- concentration in oils increases from central part of the basin Nip with poor DPEP-Nip contents. Thin layer to western margins but decreases towards eastern margins, chromatography of demetallated porphyrins also gives which seems to be the combined effects of redox and thermal similar results. All these studies indicate that oils are derived conditions. It appears that these source rocks were deposited Table 2 : Geochemical data and distribution of metalloporphyrin and perylene pigments in sediments
NW-A OLPAD 1927-30 22.36 1.80 429 30.0 0 from similar source organics. Variation in porphyrin contents of Older Cambay Shale, specially sediments at the bottom in oils is mostly attributed to redox conditions during of ‘Sequence II’ is the main source rocks which might have filled the axial as well as the basinal reservoirs of NorthCambay Basin. The presence of Vop in SR-8/9 and its Oil to source correlation
complete absence in oils of North Cambay Basin furthernegates the contribution of hydrocarbons from this source Paleocene to Early Eocene are enriched mostly in nickelporphyrins in depressional wells UN-1 & SB-C of Mehsana Distribution of nickel porphyrins from both oils and block and in KL-C & SD-G of Ahmedabad block. Whereas sediments of Older and Younger Cambay Shale indicate that the Younger Cambay Shale sediments of Middle to Late C31&C32 alkylated ETIO porphyrins and other higher Eocene are mostly enriched in either perylene or vanadyl homologues are predominantly enriched in Older Cambay porphyrins. The low concentration of vanadyl porphyrins in Shale of SK-B and SR-7 of KL-C than in Younger Cambay Kalol, Kadi and Cambay Shale pay oils and the dominance Shale sediments of JT-C & SB-F (Table-3). Similarly, the of ETIO- Nip indicates that the source rocks at the bottom oils of BL-B, SN-B and NK-A are enriched mainly in C31 6th International Conference & Exposition on Petroleum Geophysics “Kolkata 2006”
Table 3 : Concentration of ETIO and DPEP isomers of porphyrins in oils and sediments Oils VJ-A 0.50 0.75 0.50 1.20 1.40 0.65 0 0.25 Older Cambay Shale Sediments UN-A 0.25 0.05 0.25 &C32 ETIO Nip. Further, the similarities between Nip from whereas Older Cambay Shale sediments are in the past peak oil of NK-A and source sequences SR-7 of KL-C and SK-B (2001-2009m) almost conclusively prove that the sedimentsbelonging to ‘Sequence II’ of Cambay Shale are the main Conclusions
prolific source rocks. However, contribution from YoungerCambay Shale source sequences wherever they have attained The distribution of petroporphyrins and perylene sufficient maturity in some oil pools of North Cambay basin biomarkers in the oils and sediments (Cambay Shale) of North Cambay basin is the function of redoxconditions. The dominance of ETIO series of nickel Maturity
porphyrins and absence of vanadyl porphyrins in theoils indicates that the source sequences of these oils On the basis of porphyrin maturity parameter, the are deposited in suboxic conditions.
ratio of DPEP/ETIO porphyrins the maturity of Kalol pay Dominance of higher carbon homologs of ETIO-Nip oils decreases in the order VJ-A> BE-A> KL-A> NK-A= in all the oils suggests it to be sourced from mature JH-A>BL-A= SN-B> LN-B>. SB-A oil is less mature than source. In Younger Cambay Shale, the presence of JT-A and even less than KL-A oil. In Older Cambay Shale vanadyl porphyrins and perylene shows their deposition sediments there is high concentration of higher carbon in anoxic conditions. The dominance of lower carbon isomers with small concentration of DPEP in all samples.
homologs of ETIO series of nickel porphyrins suggests But in SD-G of SR-11/12 there is high concentration of DPEP. The presence of DPEP in all the samples show that The sediments of Older Cambay Shale are enriched in with increasing thermal stress DPEP/ETIO ratio decreases nickel porphyrins in Mehsana block, indicating the because of transformation of DPEP into ETIO. The prevalence of suboxic conditions during deposition.
alkylation index of petroporphyrins increases with increasing The predominance of higher homologs shows Older Tmax (Baker and Louda, 1986). The distribution of nickel Cambay Shale to be matured sediments.
porphyrins in Younger Cambay Shale and Older Cambay The presence of ETIO series of nickel porphyrins, both Shale sediments suggest that the degree of alkylation of in oils and source sequences at the base of Older porphyrin increases with increasing Tmax (425-442oC).
Cambay Shale and upper part of Olpad suggests these Whereas at Tmax value of 455oC (KL-C, SR-11/12) nickel are the most prolific source rocks, which might have porphyrins probably start dealkylating and therefore, contributed to most of the oil reservoirs of north showing the presence of only C32 and C33 ETIO isomers.
It can also be inferred from these observations that Younger The oil of VJ-A is the most mature, BL-A and SN-B Cambay Shale sediments are now in late diagenetic stage, oils are of similar maturity and NK-A and JH-A oils are of similar maturity and the LN-B is the least mature Bhandari, L.L. and Chaudhary, L.R., 1975 Stratigraphic analysis oil among the studied oils of Kalol pay from different of Kadi and Kalol formations, Cambay Basin, India; Chandra, P.K. and Chaudhary, L.R., 1969, Stratigraphy of Cambay Acknowledgements
Chandra, K. Kumar, P., Prakash,C., Mali, M.R. Dwivedi, P, Sharma, M.C., Datta, G.C. Rao, S. And Gupta, V., 1983 The authors thank the ONGC management for Geochemistry and genesis of Cambay Basin oils and granting permission to publish this paper. The views their geochemical interpretation; ONGC Report expressed in this paper are of the authors and not necessarily of the organization to which they belong. Authors express Hajibrahim, S.K., Quirke J.M.E. and Eglington, G., 1981, their gratitude to D. M. Kale, ED-Head, KDMIPE. Authors Petroporphyrins Structurally related porphyrin series are grateful to Dr. Anil Bhandari, G.M. (GRG) for valuable in bitumen, shales and petroleums- evidence from HPLC and mass spectrometry; Chemical Geology, 32,173-188.
Lewan, M.D. and Maynard, J.B., 1982, Factors controlling Views expressed in this paper are that of the enrichment of vanadium and nickel in the bitumen of author(s) only and may not necessarily be of ONGC. organic sedimentary rocks; Geochimica etCosmochimica Acta, 46, 2547-2560.
References
Louda, J.W. and Baker, E.W. , 1984, Perylene occurrence, alkylation and possible aources in deep ocean Aizenshtat, Z., 1973, Perylenes and its geochemical significance; sediments; Geochimica et Cosmochimica Acta, 48, Geochimica et Cosmochimica Acta. 37, 559-567.
Baker, E.W. and Louda, J.W., 1986, Porphyrins in the geological Pandey, J., Singh, N.P. Krishna,B.R. Sharma, D.D., Parikh, A.K.
record, In “Biological Markers in the Sedimentary and Nath, S.S., 1993, Lithostratigraphy of Indian Record” (ed. R.B. Johns) Methods in Geochemistry and petroliferous Basins, Vol I, KDMIPE, ONGC, Geophysics; 24, Elsevier Amsterdam 125-225.
Barwise, A.J.G. and Roberts, I., 1984, Diagenetic and catagenic Yang, Z and Cheng, Z., 1988, Isolation, Identification and pathways for porphyrins in sediments; Organic correlation of porphyrins from continental crude oils, source rocks, oil shales and coals of China, InGeochemical Biomarkers. Ed. T.F. Yen and J.M.
Moldowon. Harward, Academic Publications. 117-131.

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