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General Log Response of Different

Formations of Upper Assam

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DEPTH (M) FORMATION LITHOLOGY

0 - 1700 ALLUVIUMUNCONSOLIDATED

SAND/CLAY

17002100 GIRUJANSSOFT MOTTLED CLAY

WITH THIN SANDSTONE

BANDS

21002600TIPAMS &

SURMA

MEDIUM TO FINE

GRAINED SANDSTONE

26003000 BARAILS

MUDSTONE, COAL AND

FINE GRAINED

SANDSTONE.

OIL BEARING

3000 - 3400 KOPILISSPLINTERY SHALE WITH

VERY FINE GRAINED

SANDSTONE

34003470 PRANGS LIMESTONE

34703530 NARPUHSSPLINTERY SHALE AND

SILTSTONE

3530 - 3640 LAKADONGLIMESTONE,SANDSTONE, HARD

SHALE AND

CARBONACEOUS SHALE

3640 - 3700 LANGPAR

COARSE GRAINED

SANDSTONE WITH

SHALE

OIL BEARING

3700 - BASEMENTGRANITE BASEMENT

ROCK

Sub Surface Geology

Upper Assam Basin

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Girujan

Tipams

B

arails

Kopilis

Prang

Narpuh

Lakadon

g

Langpar

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Girujan Formation

Lithology is mainly Clay with thin sand bands

GR serrated but helps to identify lithology

Mixed type clay, high in montmorillonite

Thickness varies, increases in SE direction

Low formation water salinity (200-600 ppm)

Low Density (2.2 gm/cc)

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Girujan Log Response

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Tipam Formation

Lithology is mainly thick sand (>100m) with intervening

shale, sands are silty

Abundance of radioactive material

Difficult to differentiate lithology from GR log

Shales are made up of mainly montmorrilonite, kaolinite clay

Illite present at deeper zone

Formation water salinity increases downwards (1000 to 2000

ppm)

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Tipam Log response

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Barail Formation

Barail is divided into two

Upper Argillaceous unit

Mainly Shale facies

High density calcareous bands

Kaolinitic /Illite type of clay

Coal bands

Thin channel sands

Lower Arenaceous unit

Thick sand with fining up sequence

Kaolinite dominant clay

Formation water salinity- 2500-3500ppm

A

rgillaceousUnit

Arenaceous

Unit

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Barail Log response (Argillaceous)

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Barail Log response (Arenaceous)

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Kopili Formation

Monotonous shale, splintary in nature, deposited in shallowmarine condition

Thin silty sand present

Regionally extensive

Characterized by high GR, Mixed type of clay

Highly enlarged borehole due to unstable nature of theformation

Formation water salinity 3600-4000 ppm

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Kopili Log response

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Prang Formation

Limestone bands with splintary shale and siltstone.

Low GR, no SP deflection, high resistivity

Low neutron porosity, high density (2.71 gm/cc)

Laterally continuous, good marker bed

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Prang Log Response

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Narpuh Formation

Lithology similar to Kopili formation

Splintary shale and siltstone (sand facies in type area)

More sandy towards NE part of the basin (Baghjan

Mechaki)

Kaolonite Illite dominant clay.

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Narpuh Log Response

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Lakadong Member

Highly variable lithology

Broadly subdivided to three distinct units:

Upper calcareous zone

Middle sandy zone

Bottom carbonaceous zone

Clay type is mainly kaolinite

Sands are clean low GR

Shales areHotat bottom zone! GR upto 200 API

Formation water salinity 3500-4000 ppm

Thickness varies from 120-160m

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Lakadong Log Response

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Lakadong

Top calcareous zone

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Lakadong

Middle Sand zone

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Lakadong

Bottom Carbonaceous zone

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Langpar Formation

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Geological Time Scale

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