SIGNIFICANCE OF MICROBIAL ANOMALIES IN IDENTIFYING THEHYDROCARBON PROSPECTS IN PARTS OF CAMBAY BASIN, GUJARAT

M. A. Rasheed1*, P. L. Srinivasa Rao1, Annapurna Boruah1, Syed Zaheer Hasan1,

Bougi Mohamoud Mohamed Seddik2, and Disha Pandey3

1Gujarat Energy Research and Management Institute (GERMI), Gandhinagar, Gujarat, India.2Higher Institute of Biotechnology of Monastir, Tunisia

3Indian Institute of Technology (IIT), Roorkee

*Email: abdul@germi.res.in

Abstract

Microbial prospecting method is based on the detection of anomalous population of

hydrocarbon oxidizing bacteria in the surface and near surface soils that indicate the presence

of hydrocarbon accumulations. The technique is based on seepage of light hydrocarbon gases

such as C1 - C4 from the oil and gas pools to the shallow surface providing the suitable

conditions for the development of highly specialized bacterial populations. These bacteria

utilize hydrocarbon gases as the only food source and are found enriched in the near surface

soils above the hydrocarbon bearing structures. For the purpose a total of 90 soil samples

were collected and the propane oxidizing bacteria is used as an indicator in the present study.

The propane oxidizing bacterial (POB) indicators were isolated and enumerated using

enrichment culture technique wherein, the bacterial colonies were counted using the standard

plate count method. Microbial anomalies were also observed in the Northern and SW regions

of the study area. Incidentally, all the anomalous zones are near the major oil producing

fields. This demonstrates the usefulness of microbial prospecting techniques in hydrocarbon

exploration as a tool to locate the new prospects. The present research work employing

methodologies of high significance that bears potential to reduce risk in petroleum

exploration. This method could be well applied when integrated with other technology tools

or alone in frontier basins for the identification of hydrocarbon potential zones.

Key words: Hydrocarbon oxidizing bacteria, microbial prospecting, geochemical, exploration.

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Introduction

Surface prospecting for hydrocarbons comprises investigation of near-surface soils/sediments

for the occurrence of hydrocarbons that may indicate the location of subsurface reservoirs.

The basic assumption of all near-surface prospecting techniques is that hydrocarbons migrate

to the surface from the subsurface accumulations through faults/fractures or capillary rise

leave their signatures in surface or near-surface soils. Various hydrocarbon migration

mechanisms such as diffusion, effusion, advection with moving waters and permeation have

been identified (Price 1986; Tedesco 1995; Klusman and Saeed 1996). The detection of the

anomalous concentrations of the gases such as methane, ethane, propane and butane in the

surface soils serves as the direct evidence addressing the accumulation of petroleum system

in an area (Klusman, 1993; Tedesco, 1995). The anomalous concentration of C1-C4 in the

surface soils is used to assess the hydrocarbon generation potential. The adsorbed soil-gas

techniques are rapid and cost-effective for initial evaluation of a frontier basin for

hydrocarbon exploration. The activities of hydrocarbon-oxidizing bacteria cause the

development of near-surface oxidation-reduction zones altering the behaviour of soils and

sediments lying above the reservoirs. These specialized microorganisms largely depend on

light-hydrocarbon gases as the only energy source. Microbial prospecting method is based on

the detection of anomalous population of hydrocarbon oxidizing bacteria in the surface soils,

marking the presence of subsurface oil and gas accumulations. The technique is based on

seepage of light hydrocarbon gases such as C1 - C4 from the oil and gas pools to the shallow

surface providing the suitable conditions for the development of highly specialized bacterial

populations, hence the present study “topic”. The present when integrated with other

technology tools may evolve a holistic picture of reduced financial engagements towards the

exploration subsequent the development and management of hydrocarbon resource.

Geological Setting

The Cambay Basin while covering the Gujarat state lies predominantly onshore, with only the

south western corner offshore in the Gulf of Cambay. The basin is rich petroleum province

with active exploration history. The Cambay basin is a narrow elongated and intra-cratonic

rift basin of late Cretaceous age that contains different sub-basins with varying sediment fills.

The basin is a north-south trending graben with an average width of 50 km and 450 km long

with a maximum depth of about 7 km. The maximum depth of the basin may exceed 11 km if

it includes the Deccan Trap lava flows. The basin bears more than 40 years of active

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hydrocarbon exploration history, covering an area of 53,500 sq. km including 6,880 sq. km in

the shallow waters of Gulf of Cambay, the basin extends between the latitudes 21° 00’ and

25° 00’ N and longitudes 71° 30’ and 73° 30’ E (Fig.1). The origin of the Cambay and other

basins on the western margin of India are related to the breakup of the Gondwana super-

continent in the Late-Triassic to Early-Jurassic (215 m.y.a.). As India drifted away from

Africa and Madagascar the rift grabens began to form on the western margin, the boundary

faults and grabens were initiated through reactivation of Pre-Cambrian faulting. On the north-

western margin of the Indian shield, three craton-margin-embayed basins came into

existence; Kutch, Cambay and Narmada aligned with major Pre-Cambrian tectonic trends

Aravalli, Dharwar and Satpura.

Fig 1: Location of Cambay Basin. Source: DGH, India

Study area:

The study area form an integral part of the Cambay Basin (Fig.1). Entire Cambay Basin

experienced the different stages of extensional tectonic episodes, essentially confined to

Cenozoic era, and related stratigraphic evolution. Hydrocarbon exploration in the basin dates

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back to 1958 and giving rise to the persistent geoscientific search leading to the discovery of

existing 79 oil and gas fields. This makes the study area highly vulnerable for hydrocarbon

prospects, the Kadi in the north and Nawagam in the south, and is traversed by the Sabarmati

River. The sedimentary sequence starts with the trap conglomerates succeeded by claystone,

trap-wacke, sideritic marls constituting the Lower Cambay shale. The sequence is sub-areally

exposed from time to time creating localised development of reservoir facies with

hydrocarbon accumulations in Nawagam (Kundu and Wani, 1992).

Materials and Methods

Sampling Techniques

The grid 2 x 2 km pattern was adapted to collect the soil samples to optimize the number of

sampling points spread over the study area in Cambay Basin. A total of 90 samples were

collected for microbial analysis.

Isolation of Hydrocarbon Oxidizing Bacteria

Isolation and enumeration of propane oxidizing bacteria for each sample was carried out

enrichment culture method followed by serial dilution and Standard Plate Count (SPC)

method (Wittenbury et al., 1976; Rasheed et al., 2008).

Results and Discussion

The population of Propane Oxidizing Bacteria (POB) has been counted using colony counter

and the reported in colony forming units per gram (cfu/g) of sample and the POB population

is found ranging between 1.0 x 103 and 1.59 x 106 cfu/g. The anomaly distribution map of

POB population shows distinct microbial anomaly in the North Eastern and South Western

parts of Study area.

The possibility of locating hydrocarbon reservoirs through microbiological method has been

emphasized by the fact that the population of hydrocarbon-oxidizing bacteria where ranges

between 103 and 106 cfu/g in soil/sediment confirms the hydrocarbon microseepages

depending on the ecological conditions (Rasheed et al. 2008; Wagner et al. 2002). In the

study area the population of hydrocarbon-oxidizing bacteria ranges between 1 x 103 and 1.5

x 106 cfu/g of soil which of significantly substantiate the seepage of lighter hydrocarbon

from the reservoirs (Rasheed et al. 2008; Wagner et al. 2002; Nimmi et al. 2003). Figure-2

exhibit the existence of oil fields in the area confirms the presence of hydrocarbon in the SW

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part of the region. The anomalies observed in the study coincide with the location of the oil

fields highlights the indicators used in the study are working as reliable indirect methods for

detecting hydrocarbon seepage in an area.

Fig 2: Microbial anomaly map of the study area in Cambay Basin.

Conclusions

Microbial anomalies are observed in the NE and SW regions of the study area. These

anomalies are found close to the producing oil fields. The hydrocarbon oxidizing bacterial

population used as proxy indicators have shown good understandable correlation with the

existing wells. The study authenticates the methodology adapted can work as an efficient and

cost effective indicators to identify the hydrocarbon seepage. This will be great significance

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when applied in conjugation with other geoscientific techniques to confirm the zones of

hydrocarbon accumulation through seepage.

Acknowledgements

The authors are thankful to Mr. Samir Biswal, Director Exploration, GSPC for his support to

carry out the field work and publication of the research output. Thanks are also due to Dr.

T.Harinarayana, Director, GERMI and other colleagues for their continuous support and

encouragement throughout this work.

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