Author’s Copy

Lahiri, S.K., 2013. Near surface lateral inhomogeneities and oil

field locations – a strong connection observed in Namrup-

Sapekhati window, Assam. Insignia, V, 12-16

Near surface lateral inhomogeneities and oil field

locations – a strong connection observed in Namrup-

Sapekhati window, Assam

Siddhartha Kumar Lahiri

Department of Applied Geology

Dibrugarh University

Siddharthalahiri2@gmail.com

River valleys experiencing active tectonic controls bear typical geomorphic signatures. If

incidentally, the valley concerned is highly petroliferous and oil habitat is strongly governed

by tectonic forcings; expectations grow among the explorers from the recently developed

disciplines like remote sensing methods to play a meaningful complementary role. However,

when the rate of sediment influx is very high, a competitive damping effect sets in that

obliterates many important geomorphic signatures. Thus, in spite of highly potential remote

sensing techniques capable of offering different filtering options for suitable image

processing, the anticipated breakthroughs in ‘exploration leads’ might not be achieved. Under

such circumstances, it is observed that shallow subsurface data like uphole and shallow

seismic refraction, which provide a highly representative description of near surface lateral

inhomogeneities, are very useful. Work was done on high density shallow refraction data sets

in Namrup-Sapekhati window, situated in the south bank of the upper reach of the

Brahmaputra valley, Assam. Three parameters namely; lateral variations in the P-wave

velocities of the topmost low velocity layer (LVL) and the layer following immediately,

known commonly as sub-weathered zone and the thickness of the LVL were considered. A

new approach “Multiparametric microzonation” was applied to implement a zonation scheme

based on different combinations of the above said parameters in the GIS based environment

and oil field locations along with some of the geological elements like blind faults were

collated . It was observed that high velocity (both top and bottom layers) and thicker LVL

bearing zones are more closely associated with the oil fields.

Introduction

In the Brahmaputra valley of Assam, the future of oil exploration rests on how efficiently we

relate thrust belt tectonics with the possibilities of generation of petroleum systems in the

light of basin evolution in different stages. Thus, foredeep areas bordering the thrust belts are

definitely the main focus of present attention which will continue to be so in near future as

well. So far, none of the geophysical techniques can contest the outcome of the seismic

reflection methods. Resolution enhancing efforts by populating the energy source at greater

depths with high frequency contents meet tough challenges in practice. Multi component

seismic methods and increasingly wider range of variability in attribute analysis, thanks to the

fast growing computing ability, brings in the domain of subsurface modelling a kind of

‘magic realism’ like thing but increasing simultaneously the I/O (input versus output) cost

ratio. In essence, most of the high-tech current practices are engaged in approaching the

‘object at distance’ as closely as possible. However, if we assume that the ‘object’ due to its

interconnected neighbourhood is bound to cause certain perturbations at remote places as

well, say at the place where the observer stands, then we could have spent some of our

innovative energy to diagnose the disease by monitoring the skin, instead of depending

increasingly and almost exclusively on ‘test-packages’ which never claim of absolute

certainty yet inflict heavy toll on the pocket of the patient than perhaps the disease itself.

Here comes the role of the shallow subsurface geophysical data which might be used in

extending the scope of morphotectonics. The use of shallow refraction data is very old.

Pioneering papers by Barton (1929) and Heiland (1929) established literally the importance

of the refraction method. Muscat (1933) explained in detail the theory of refraction shooting.

Fan shooting technique that was used in the earlier days for shallow structural traps of oil was

mainly based on the refraction method. Later on the method was used for mapping subsurface

structures (Gardner, 1939), water prospecting and rock investigations (Hasserlström, 1969;

Bachrach and Nur, 1998; Grelle and Guadagno, 2009). Besides these, workers have used

this method to map ancient channels (Pakiser and Black, 1957) and in exploring Quaternary

deposits (Burke, 1973). In this paper, first, arguments are forwarded in favour of a

classification system which is based on the assumption that foredeep areas of the outcropping

thrust belt mountain ranges get highly affected due to the advancing leading edges of the

blind thrusts which influence considerably the sites of aggradation-degradation in the valley.

Secondly, in one of the foremost oil provinces of the upper Assam valley, the Namrup-

Sapekhati window, the locations of some of the major producing oil fields are seen to follow

a distinct trend that is in conformity with the trend of the general regional strike direction.

Namrup-Sapekhati window

This is a NE-SW trending window with approximate area 880 km2 and located within the

four points the longitudes and latitudes of which are respectively 94.990/27.17

0;

95.340/27.36

0; 95.44

0/27.20

0; 95.10

0/27.00

0 (Fig.1). This is one of the richest oil-windows of

the upper reach of the Brahmaputra valley (Fig.2A). At least seven major oil fields namely,

Nahorkatiya, Jorajan, Tarajan, Sarojini, Rajgorh, Diroi and Dipling seem to lie one after

another in a belt and two more oil fields, Tinali in the western part of the belt and

Baruanagar is located in the eastern part of the belt. The alignment of the Naga Patkai Thrust

(NPT) belt, direction of flow of the Disang River, the direction of the regional blind faults

(B1-B1´ and B2-B2´) and above all the clusters of oil fields seem to be parts of an integrated

whole. Seismo-tectonic map covering the window and some of the adjacent areas show (Fig.

2B) there is heterogeneity in valley fill; sediment thickness (as inferred from the basement

depth variations) varies from 4.0 – 6.0+

km within the window. Thicker part is observed along

the foredeep adjacent to the accretionary complex and the Schuppen belt belonging together

to the NPT belt. Bouguer gravity anomaly, in conformity with the above situation varies from

-192 to -200 mgal. The window is very rich in palaeo channel marks and the presence of ox-

bow lakes (Fig. 2A). The amplitude and wavelength of the meandering palaeo-channels are

very much comparable to that with the present day channels and this gives a clear clue of

river migration along a specific direction.

Fig.1. Location map of the Namrup-Sapekhati window is shown on the IRS-P6-LISS-3

image in the south bank of the upper reach of the Brahmaputra valley, Assam.

Abbreviated geological elements are HFT-Himalayan Frontal Thrust, MBT-Main

Boundary Thrust, NPT-Naga Patkai Thrust.

Fig.2. (A) The NE-SW trending

Namrup-Sapekhati window has

plenty of palaeochannel marks.

Two major tributary rivers of

the south bank, Brahmaputra

valley, the Burhi Dihing and

the Disang pass through the

window. Two regional blind

faults and some of the major oil

fields of the area are shown.

Total number of oil fields is 9,

out of which seven are located

in a zone bounded by two blind

faults. (B) Seismotectonic map

covering Namrup-Sapekhati

window and the adjacent areas

show the thickness of alluvial

fill along the foredeep more

than 6km.

Data and Methodology

Shallow refraction data coverage for the Namrup-Sapekhati window is uniform and

extensive. In total 1228 points were covered (Fig. 3). For a general estimate, three (3) points

were investigated for every 2 km2 area. The data was collected systematically by Geofizyka

Toruń of Poland during January-February, 2008 field season for the OIL.

Velocity variation in the topmost LVL is 200 - 840 m/s. This shows a wide range of

variation. Velocity variation in the sub-weathered layer is 600 – 2100 m/s; suggesting thereby

a still wider range of lateral inhomogeneity. Thickness range of the topmost LVL is 1-14m. A

generalized approach of two component system was adopted with the assumption that the

channel-fill deposits and the older flood plain deposits have sharp velocity contrast.

Accordingly, after trial and error, ‘high’ and ‘low’ velocity zones are differentiated. For the

V1 values (that is, velocity of the LVL) the threshold was fixed as, V1 (low) = 200 – 383m/s

and V1 (high) = >383 – 840 m/s. For the V2 values (that is, velocity of the sub-weathered layer)

the threshold was fixed as V2 (low) = 600 – 1162m/s and V2 (high) = >1162 – 2100 m/s (Fig. 4).

Fig.3. Shallow refraction data

density in the Namrup

Sapekhati window having

1228 number of stations

covered.

Thickness range 1-5m for the LVL is taken as ‘thin’ and 5+-14m is treated as ‘thick’. Logic of

micro-zonation is developed based on the overlapping of high-low velocities of V1 and V2

and thick-thin characteristics of the LVL thickness. This results into eight type areas (Table

1). The entire exercise was done in the GIS environment that facilitates a comparative study

of the microzonation with the oil field locations as well as some of the well established

structural elements.

Fig.4. Velocity differentiation of layers into two-component systems

like high velocity (older flood plains, more consolidated) and low

velocity (incised valley deposits, newer flood plains, less

consolidated). (A) Topmost LVL, (B) second layer (sub-weathered).

Discussion

Lateral variability in the geophysical properties of the layers below the ground surface is

apparently contradictory to the Steno’s principles (1669) regarding original lateral continuity

and original horizontality and the superposition principle whereby older layers are

superposed by the younger layers. However, the fact of the matter is, in the ideal source-sink

relationship, say particularly in the deep marine environment (excluding of course the sea

floor spreading zones), these principles are very much valid and for falling sea level and

regressive conditions, when these marine depositional successions (without the intervention

of any structural modifiers) become part of the continents, the outcrop study is bound to

endorse Steno’s principles. Thus Steno’s observations are valid for most of the continental

stratigraphy which are genetically related directly to the marine depositional environments.

Table 1

A schematic description of tri-parametric micro zonation model

However, in the high elevation continental basins with huge mass transfer of sediments, the

landscape is mostly constituted of older floodplains incised by channels having different

stream power. Recently deposited sediments within the channels and floodplains are usually

poorly consolidated and are having lesser seismic velocities compared to the older flood

plains. But there are situations where tectonic controls are forcing the older rocks from deep

inside to rise up and subsequently become a member of the shallow subsurface family, say a

‘high’ having more compact sedimentary rock units and obviously this part of the shallow

subsurface will show much higher velocities and we will get a drastic lateral variation

change. Then, there can be regional ‘lows’ having much thicker low velocity depositions.

Thus, we are bound to get (i) different types of lateral discontinuity, (ii) lack of horizontality

of recently deposited strata and, (iii) a number of complex associations of older strata with

the younger ones. This apparent deviation from Steno’s principles has tremendous influence

on surface landform evolution for the present study window.

Fig.5. Tri-parametric micro-zonation based on the velocity discrimination of the top two layers and the

thickness variation of the top LVL. (A) V2-high, V1-high, H1-thick, (B) V2-high, V1-high, H1-thin,

(C) Both the situations described in (A)-(B) juxtaposed with the locations of the oil fields and the blind

faults. Most of the discovered oil fields seem to be situated on the V2-high, V1-high belt.

The microzonation of the study window shows very clearly a closer affinity of the Type-I and

Type-II zones with the oil fields of the area (Fig. 5). In contrast, Type-VII and Type-VIII

areas in general do not have that kind of affinity (Fig. 6). The observation suggests further

that in the Namrup-Sapekhati window, oil fields have strong structural control.

Moreover, the microzonation shows it clearly that in the foredeep areas of the frontal thrust

belts, thicker LVL does not indicate necessarily the sites of recent aggradation. Similarly, the

sites of thinner LVL do not stand for degradation. Sites of aggradation-degradation are to be

identified principally on the basis of the velocity combinations (as shown in Table 1).

Fig.6. Tri-parametric micro-zonation based on the velocity discrimination of the top two layers

and the thickness variation of the top LVL. (A) V2-low, V1-low, H1-thick, (B) V2-low, V1-

low, H1-thin, (C) Both the situations described in (A)-(B) juxtaposed with the locations of the

oil fields and the blind faults. The oil fields seem to avoid the low velocity zones.

Conclusion

Mapping lateral inhomogeneities of the LVL in the foredeep areas and ‘multiparametric

microzonation’ can provide useful information about late Quaternary sites of aggradation-

degradation and a means for comparing the same with the present day preferential sites.

Subsequently, the nature of changes can be modelled in terms of the structural controls

determining the probable sites of oil entrapment. The findings of known windows like

Namrup-Sapekhati can be applied to the new areas where the reliability of seismic data is

questionable.

Acknowledgement

The author is highly thankful to OIL, Duliajan for sharing the shallow subsurface geophysical

data for research purpose. The cooperation and encouragement extended by Dr. Rahul

Dasgupta, President SPG Duliajan Chapter, in promoting industry-academia relationship is

highly commendable.

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