Keywords: Shale, impedance, Clastic reservoirs ,Crossplots of the logs, Stratigraphic /Combination, Plays, Carbonates

IntroductionThe Mumbai offshore basin, a passive marginbasin on the continental shelf of western Indiacontinues into the on-land Cambay basintowards the northeast. On the north it is boundedby the Saurashtra Peninsula and on the east bythe Indian craton. Its southern limit is marked bythe east-west trending ridge south of Ratnagiri.

Hydrocarbon accumulations generally occur incarbonate reservoirs ranging in age from MiddleEocene to Middle Miocene which are structurallycontrolled. However stratigraphic / combinationplays in Paleocene - Lower Eocene andOligocene clastic reservoirs are also significant.The area under study covers part of south-eastern sub-basin of Saurashtra offshore towardssouth of Saurashtra craton (Fig: 1).

Present study is the result of interpretation of the3D seismic data and Well log caliberations andintegration of laboratory studies in theinterpretation.

Fig. 1: Location Map

The area under study lies to the east of Diu Archin Saurashtra offshore area (Fig: 2). An attempthas been made to analyze the availablegeoscientific data and bring out the responses ofvarious lithologies with its characteristic fluidwithin Daman formation in the area. The study

High Impedance shales of Daman formation towards east of Diu arch inSaurashtra offshore: their role in hydrocarbon exploration and exploitation.

M. Singh*, K.R.K. Singh , C. Mehta and S. Chandrasheker

Abstract: Amplitude anomalies have been the significant tool for exploration of hydrocarbon as bright spot/DHI for manyyears for the Geoscientists. This has proved to be important in some places and in some formations whereas it has failed atsome places. Since it is one of the quick look interpretation skills, it is still adopted by Geoscientists for obtainingconfidence. However validation of the anomalies is done through different petro physical analysis. Tapti Daman sector inthe Western offshore basin has been explored widely based on the amplitude anomaly analysis. Success and failures bothhave been observed from this sector of the basin. Late Oligocene Daman formation has been explored in this basin from2D seismic data to 3D seismic data. The amplitude anomalies which failed for the upper part of Daman formation hasproved successful for lower part. The Western part of Tapti Daman sector covering part of Diu Arch has been exploredwidely based on the amplitude driven exploration. High amplitude anomalies depicting presence of thick channel sandproved to be water bearing after drilling. The shales within the Daman formation are low impedance shales and waterbearing sands are high impedance sandstones. In this paper an attempt has been made to analyze the lithology and theirrespective impedance in the western part of Tapti daman sector on the eastern flank of the Diu arch. The lower part ofDaman formation having thick net gas bearing sand of 15-18m has been explored which has no impact on amplitudeanomaly whereas in upper part of Daman formation high amplitude anomaly corresponding to the channel sand hasbeen found water bearing. Cross plots of the logs have been attempted and the shales of the area are analyzed in finerdetail. It has been found that the shales are dominantly silty in nature, thus reducing fissility. Sandstones are also silty innature and found to be unconsolidated at some places, hence posing problem of sand incursion in well testing and wellcompletion. The analysis of the impedance of the lithology and pay sands from logs is then compared with seismic data.The analysis suggests that the shales in the lower part of Daman formation are predominantly high impedance shaleshaving impedance values close to that of Gas bearing sands. Thus it becomes difficult to differentiate between the Gasbearing sands and shales. The shales in the upper part of Daman formation are clean and devoid of silt, hence havinglower impedance values.

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will help to efficiently differentiate & identify thevaried lithology encountered along with its fluidcontent for better exploration of Damanformation in the area.

Fig. 2: Time relief map on top of Panna formationTectonic setting and StructuralframeworkBasement controlled NW-SE (Dharwarian trend)to N-S(Delhi trend) trending faults split the entireshelf area in longitudinal stripes. This has resultedin a horst-graben morphology which guidedsedimentation in the basin throughout theTertiary period up to Middle Miocene.

FFig. 3: Structural elements of Mumbai offshoreBasin

Five mega tectonic elements; viz. EasternHomocline, Graben system, Central RidgeSystem, Shelf Margin Depression and West MarginBasement Arch define the tectonic setup of theMumbai offshore basin. Each element isbounded by normal faults (Fig: 3).The 'highs' are dissected by NE - SW cross trends.The most prominent among basement highs overMumbai platform is the 'Mumbai High'.In the North and North East of Mumbai high Diuarch, Dahanu structure, Saurashtra low, Suratdepression, Daman low and Navasari lows havevarious Inversion structures formed due totranspressional and transtensional forces of strikeslip movements.

Stratigraphy and Depositional SettingMumbai Offshore basin is limited by theexposures of Deccan Trap in the east. A thinveneer of Neogene and Quaternary limestone,marl and clay form the outcrops along thecoastal belt of Saurashtra Peninsula in the north.The subsurface sedimentary section ranges inage from Paleocene to Holocene and overliesnon-conformably the Deccan Trap / Granitic /Metamorphic basement. Deccan Traprepresents the basin floor geology with a fewgranitic/metamorphic inliers. Seismic sectionsand Cretaceous exposures in Wadhawan andDhrangadhara areas of Saurashtra block revealthe presence of a sub Deccan Trap Mesozoicbasin. The lithostratigraphy of the basin is shownin Fig: 4.

Fig. 4: Lithostratigraphy of Mumbai Offshore Basin

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During different stratigraphic units the geologicevents, sea level changes and geochronologyhas been depicted in (Fig: 5).

PALEOCENE

EOCENE

OLIGOCENE

MIOCENE

PLIOCENEPLEISTOCENE

PRE-CAMBRIANCRETACEOUS

UPPER

LOWER

UPPER

LOWER

UPPER

UPPER

MIDDLE

UPPER

LOWER

UPPER

LOWER

MIDDLE

LOWER

PALEOCENE

EOCENE

OLIGOCENE

MIOCENE

PLIOCENEPLEISTOCENE

PRE-CAMBRIANCRETACEOUS

PALEOCENE

EOCENE

OLIGOCENE

MIOCENE

PLIOCENEPLEISTOCENE

PRE-CAMBRIANCRETACEOUS

PALEOCENE

EOCENE

OLIGOCENE

MIOCENE

PLIOCENEPLEISTOCENE

PRE-CAMBRIANCRETACEOUS

UPPER

LOWER

UPPER

LOWER

UPPER

UPPER

MIDDLE

UPPER

LOWER

UPPER

LOWER

MIDDLE

LOWERUPPER

LOWER

UPPER

LOWER

UPPER

UPPER

MIDDLE

UPPER

LOWER

UPPER

LOWER

MIDDLE

LOWER

LOWER

UPPER

LOWER

UPPER

UPPER

MIDDLE

UPPER

LOWER

UPPER

LOWER

MIDDLE

LOWER

Eustatic SeaLevel Curve

transgression

regression

sequence boundary

Eustatic SeaLevel Curve

transgression

regression

sequence boundary

Eustatic SeaLevel Curve

transgression

regression

sequence boundary

AgeYears 106

3 Ma

11 Ma

16 Ma

29 Ma

39 Ma

49 Ma

54 Ma58 Ma

66 MaAge

Years 106

3 Ma

11 Ma

16 Ma

29 Ma

39 Ma

49 Ma

54 Ma58 Ma

66 Ma

Recent: Quaternary sea level fluctuationsCompression on lower slopeSiliclastics dominate shelf: up to 4,000 m thickcarbonate platform terminated11 Ma Mid-Miocene global sea level lowstand

Bombay High carbonate reservoirs depositedExtension & growth faulting on shelf margin

First episode of major gravity slides on slopeIndus canyon & fan initiated.First major Himalayan orogenic activity

Thermal cooling & subsidence of margin

Extensive carbonate platform establishedoffshore Mumbai region

First contact of India with Eurasia

Panna Formation: major source interval> 2,000 meters in Surat depression

66-65 Ma Deccan Trap Flood BasaltsSeychelles separates from IndiaLaxmi Ridge established as continental sliver

Geochronology Sea Level Curve Key Geologic Events

Fig. 5: Geo-chronological chart, Sea Level Curveand Key Geologic Events

G &G data observations and analysis

1: Seismic data showing the high amplitudeanomaly. The amplitude attribute indicatingpresence of channel sand in the upper part ofDaman formation were explored in some of thewells. These anomalies proved to be waterbearing thick channel sand in C-37 area(Fig: 6).

Fig. 6: Horizon slice of 3D seismic dataindicating High Amlitude anomaly as channelsand.

2: Seismic amplitude anomaly in upper damanformation of another seismic close to theabove 3D seismic has also indicated presenceof similar channel sands (Fig: 7) however thesechannel sands were validated by AVO analysis.In AVO analysis these amplitude anomaliesturned out to be Class-III anomaly suggestingabsence of hydrocarbon (AVO Analysis report).

Fig. 7: 3D Seismic line showing High AmplitudeAnomaly

3: Log correlation of 4 wells A, B, C and D wasattempted. Clean shale corresponding toMaximum flooding surface and flooding surfaceswere correlated in all the 4 wells. The sandstonesand limestones genetically related to each otherwere correlated with sequence stratigraphicapproach. Progradation of Sands from North Eastto Southwest direction has been clearly broughtout. Water bearing sand and gas bearing sandgenetically related to each other have beenmarked (Fig: 8). Between clean sandstones andclean shales there is abundant presence of shalysandstones, silty sandstones , silty shales andsiltstone. The differentiation of these lithologieshas been clearly demonstrated on the logs.However the impact of the lithological variationson seismic data has a great bearing onunderstanding their petrophysical relation in onedimension and lateral changes in twodimensions.

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Fig 8: Log correlation along dip direction withsequence stratigraphic approach

4: Crossplots of GR and Impedance againstclean shale for understanding the nature ofimpedance in clean shale encountered in thewells A, B, C and D has been attempted. Theclean shales are having low impedance & highGR values and are marked as Flooding surfaces.As depicted by cross plots the Clean Shalesbelonging to Upper Mahuva, Lower Daman andUpper Daman formation have similar impedancevalues (Fig: 9).

Fig. 9: Impedance against clean shales

5: Crossplots of GR and Impedance againstwater bearing sands for understandingimpedance of Water bearing sand in the wellsA, B, C and D has been attempted. This hasbeen done in two parts. The cross plots forupper and lower Daman sandstones has beendone separately. The upper daman waterbearing sand have high impedance valuesand range from 7000 to 8000 (Fig. 10). Asdepicted by cross plots the Water bearing sandbelonging to Upper Daman sectioncorresponds to Tidal channel and Bar complexhaving higher impedance.

Fig. 10: Impedance against Water Bearing &Gas Bearing Sands in Upper Daman Section

The higher impedance of these water bearingsands against the low impedance Floodingsurface clean shales generates a highamplitude anomaly in seismic data. LowerDaman water bearing sands also have highimpedance but they are encased in silty shalesand siltstones spatially and temporaly. Thisassociation of lithology does not have animpact on seismic amplitude changes thusthese donot generate amplitude anomalies(Fig: 11).

Fig. 11: Impedance against Water bearingsands, Silty Shales & Shaly Sands of LowerDaman Section

6: Crossplots of GR and Impedance against siltyshale and shaly sand are having highimpedance value (Fig:11). The relation of thesesilty shales to clean sands and clean shalesgenerate different amplitudes depending onthe relative impedances. The impedance ofthese silty shales and shaly sands are high thus itmakes difficult to differentiate between waterbearing sand and silty shales.

7: Crossplots of GR and Impedance against siltyGas bearing sand suggest lowering ofimpedance compared to water bearing sands.The impedance of these gas bearing sands arehowever not as low which can differentiate thewater bearing and Gas bearing sands (Fig:12).Under these circumstances it becomes difficultto delineate these sands spatially andtemporaly.

AB C D

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Fig. 12: Impedance against Silty Gas BearingSand, Water bearing sand & Clean shales

8: Crossplots of GR and Impedance against twodistinct gas producing sands of well A shows aslight difference in their impedance values. Thedeeper sand-1 has a pore pressure ofHydrostatic + 10% causing more compaction ofthe sand resulting in higher impedance values(7000-8000) as compared to shallower sand-2which is less compacted due to nearhydrostatic pore pressure having lowerimpedance values (5900-6900) (Fig: 13). Theinteresting fact is that the flow rates and FTHP ofdeeper sand-1 are higher than that of shallowersand-2.

Fig. 13: Comparison of impedance betweentwo different gas bearing sands

9: Side wall core and conventional cores cut intheses sandstones were investigated in detail inRegional Geological Laboratory Panvel. Thesandstones are milky white, transparent totransluscent, fine to medium grained,occassinaly coarse grained, moderately to wellsorted, with polycrystalline quartz grains. Thequartz arenite comprising of fine to mediumgrained lithclasts, Occassionally coarsegrained, subangular to sub rounded,polycrystalline quartz have poor sorting. Thematrix is argillaceous.

Conclusions:

1.The Western part of Tapti Daman sectorcovering part of Diu Arch has been exploredwidely based on the amplitude drivenexploration. High Amplitude anomaliesdepicting presence of thick channel sandproved to be water bearing after drilling. Theunderstanding of these amplitude anomalieshas been judgemental and biased.

2.The shales within the Daman formation aretaken as low impedance shales and waterbearing sands as high impedance sandstones.An attempt has been made to analysis thelithology and their impedance in the westernpart of Tapti Daman sector on the easternflank of the Diu arch.

3. In the western part of Tapti Daman sector andin the eastern flank of Diu Arch, the lower partof Daman formation having thick net gasbearing sand of 15-18m has been exploredwhich has no impact on amplitude anomalywhereas in upper part of Daman formation,high amplitude anomaly corresponding to thechannel sand has been found water bearing.Cross plot of the logs has been attempted andthe shales of the area are analysed in finerdetail. It has been found that the shales aredominantly silty in nature thus resulting in loss offissility.

4.Sandstones are also silty in nature and at someplaces are found to be unconsolidated,causing sand incursion problem during welltesting and well completion. The analysis of theimpedance response of the lithology from logis then compared with seismic data. Theanalysis suggests that the shales in the lowerpart of Daman formation are predominantlyhigh impedance shales which are close to SiltyGas bearing sands. Thus it becomes difficult todelineate & differentiate the Gas bearingsands and shales. The shales in the upper partof Daman formation are clean and devoid ofsilty matter thus lowering the impedance ofthe shale. Validation of windowed attributesby AVO analysis and other advanced seismicmethods are done.

Acknowledgement:The authors are thankful to Sri G.C. Katiyar (ED-Basin Manager, WOB, Mumbai) for his keeninterest and support. The authors sincerely thankto the colleagues of WOB, Mumbai for theirfruitful discussions in the process of interpretationprojects in the area.

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