Facies Distribution And Paleo - Depositional Model Of Kalol Pay Sands In Nandej - Hirapur Area (Cambay Basin) 1C.P.Sharma*, S.P.Painuly*, S.K.Das*, N.K.Negi*, Ch.V.Satya Sai* ABSTRACT

Recently acquired high frequency seismic data coupled with geological and sedimentologi- cal information provided new insight into the depositional history of Kalol IX and X pay horizons in Nandej-Hirapur area. Structural style of the area is governed by the interplay of Dharwarian and Archean trends indicating an area of cross trend. The Kalol IX and X sands represent two parasequences which were deposited as tidal dominated sands in Nandej- Hirapur area. The sedi- ment input from two drainage systems from the north and north east was acted upon by tidal ener- gies from the south in the lower delta plain regime resulting into the deposition of good quality reservoir sands in the area. These reworked sands are inferred to be spread in about 15 sq.km. area to the west, south and east of Nandej and have reserve accretion to the tune of 10 MMt contained in them. This area needs exploratory thrust for quick reserve accretion.

INTRODUCTION: presents the results of integrated studies includ- The study area (Fig. 1) lies to the south ing the analysis of the pattern of facies distribu- and south east of Ahmedabad and is bounded by tion of pay horizons and the depositional model the Jetalpur depression in the west, ultimately leading to the identification of thrust Mehmedabad-Kheda low in the south and east- areas of exploration. ern margin of Cambay Basin in the east. Pro- ducing fields of Ahmedabad, Nandej, Wasna and The Ahmedabad-Nawagam area has been Nawagam are located in the vicinity. The paper the subject of an active exploration programme

Fig 1: INDEX MAP

1 GEOHORIZONS Vol . 1 No. 2 & 3 for the last many years, and a. number of earlier workers proposed various depositional models. The more prominent ones include; tidal chan- nels under high wave energy conditions (Ghosh et. Al 1984), destructive tide dominated deltaic environment (Sastry et. al. 1984), meandering channels (Kothe et.al. 1990), distributory chan- nel and bay fill deposits (Kundu et. al.1994). However, it was not until 1993 that high fre- quency seismic data was acquired in the area to facilitate detailed structural and stratigraphic in- terpretation of the area.

Structural Set up:

Structural style (Fig.2) of the area is gov- erned by the interplay of two prominent struc- tural trends i.e. Dharwarian and Archean. The Ahmedabad high and the Jetalpur depression are both aligned in north-northwest - south-south- east direction and so is the alignment of Mehmedabad low in the south. But the inter- vening area of Nandej-Hirapur exhibits shifts of corresponding to these zones(Fig. 3). Upper cycle the axes arid the zone of disconformity pointing is fairly continuous whereas the lower reflector towards the presence of a cross trend (east-north- shows interruptions at a number of places. east-west-south west). Some evidences sugges- tive of strike slip movement area also observed Parasequences I (corresponding to K- in the data. X):

The main producing sedimentary unit The K-X pay horizon is characterised by corresponding to the Kalol IX and X (Sertha argillaceous silty sandstone, interlayered carbon- Member) consists of two parasequences repre- aceous .shale and sideritic claystone with bur- senting two independent episodes that are sepa- rows, siltstone which exhibits pin-stripe, lami- rated from each other by a persistent transgres- nar and occasional lenticular bedding and coal. sive shale. Each parasequence has been analysed On logs (Fig. 4) the facies is characterized by: in detail in the light of all available data. Two strong reflectors are observed in the seismic data

2 GEOHORIZONS Vol . 1 No. 2 & 3 Isopach map (Fig.5) of the parasequence shows marginal thickness variations indicating low relief at the time of deposition and negli- gible subsidence. Sand isolith map (Fig. 6) indi- cates sand maxima/concentration lying in the southern part of Ahmedabad and in Hirapur area. Incidently this corresponds to the plunge of high axis (nosing part). The sand geometry indicates thickness variation from 2 m to 6 m and sand input from the north and northeast. Shallow ma- rine conditions had prevailed during the initial phase resulting into the deposition of marine - Large separation between neutron-density log. shale and subsequent retreat of sea accompanied - Resistivity in the range of 4-5 ohm. m. by tidal action might have generated high-energy - Sonic travel time in the range of 120-125 u conditions. This is evidenced by the seismic sec./ft. events as well (Fig. 3). Lithological association and sedimentary structures suggest the deposi- The porosity in this zone varies from 20- tion in tidal flat regime. The silts might have been 30% having V clay in the range of 30-40%, the deposited by tidal action. The coal beds were permeability is low. deposited in the swampy/marshy environment.

3 GEOHORIZONS Vol . 1 No. 2 & 3 form of pin stripe laminar and lenticular bed- Parasequence II (corresponding to K- ding, wavy lamination (Fig. 10). It is character- IX): ized by good porosity (20-30%), relatively higher volume clay and moderate water saturation. Deposition of this parasequence started with marine transgressive shale which persists This facies is developed throughout and appears throughout the area (Fig. 4). Isopach map of this to have been deposited in tidal regime. unit shows relatively larger range of thickness variation indicating rapid subsidence (Fig. 7). CONCLUSIONS: Sand geometry of this unit is also suggestive of two input directions i.e. north and north-east 1.Kalol IX and X pay zones of Kalol Formation (Fig.8). The unit is divisible into two facies ‘A’ represent two parasquences separated by a and ‘B’. The ‘A’ facies is lithologically transgressive shale. characterised by fine quartzwacke with sideritic clay (kaolinite) occupying the intergranular pore 2.Deposition of the parasequences in Nandej- space. The sedimentary structures include “wavy Hirapur area is a result of two drainage sys- and flasser bedding, interlayer carbonaceous tems operating from the north & north-east lamination of clays (Fig. 9 & 10). On logs, the acted upon by the tidal energies from the south facies is characterized by : in the lower delta plain regime.

- Neutron density curves show small separa- 3.K-IX reservoir facies has widespread devel- tion. opment and has better petrophysical charac- - Good resistivity of 10-40 ohm. m teristics than K-X. - Negative SP development. - Sonic travel time around 108-110 u.sec./ ft.

This facies is also characterised by low volume clay content, good porosity, permeabil- ity and low water saturation. The facies is well developed in the central part of Nandej and south of Ahmedabad area and might have been depositd as tidal bars & channels.

The ‘B’ facies is characterised by interlayered carbonaceous shale, sideritic claystone and siltstone. The later occurs in the 4 GEOHORIZONS Vol . 1 No. 2 & 3 4.The area lying in the west, east and south west Rehmani et. al. 1988 Estuarine tidal channel and is promising for hydrocarbon exploration in near shore sedimentation of a late cretaceous view of the development of reservoir facies epicontinental sea, Alberta, Canada (Tide influ- as a result of reworking processes. enced sedimentary environments and facies).

5.The prospective area may be about 15 sq.km. Sharma C.P., Painuly S.P., Das S.K., 1993 Inte- with likely reserve accretion of 10 MMt. grated basin analysis for delineation of subtle

6.Further south and southeast, reservoir quality traps in the area between Sanand-Motera & is likely to deteriorate in view of prodeltaic Dholka Meh-medabad. Unpublished ONGC re- conditions. port.

ACKNOWLEDGEMENTS: Upadhyay H., Anand S.R., Minz P.K 1994: The authors are grateful to the authorities of EBG, Lithofacies, depositional environment and res- ONGC for providing the opportunity to present ervoir characteristics of Kalol Formation in the and publish this paper. area aro- und Jetalpur depression. Unpublished ONGC report. REFERENCES Alien & Allien 1991 Principles of Basin analy- Well Log Interpretation 1994 Integrated field sis - Blackwell scientific publication. study of Nandej Section, Baroda. field. Unpub- lished ONGC report. BanlingT.P & PreyaJ.A. 1989 Exploring for subtle traps with high resolution paleogeographic maps, Ro Klow-l, AAPG.

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