Early work indicates prospectivity in Bikaner-Nagaur basin, India

April 8, 2002
The first well drilled on an exploratory block in the Bikaner-Nagaur basin of northwestern India indicated the presence of hydrocarbons.

The first well drilled on an exploratory block in the Bikaner-Nagaur basin of northwestern India indicated the presence of hydrocarbons.

This event could open a new exploration frontier in India.

The results obtained by Essar Oil Ltd. (EOL) and Polish Oil & Gas Co. (POGC) in Rajasthan have come close to earmarking the Bikaner-Nagaur basin of Rajasthan as a potential generator of hydrocarbons. In this article we attempt to present the results and observations inferred from the geoscientific studies undertaken in this evaporite basin, the only one of its kind in India.

Introduction

EOL signed a production sharing contract with the government of India and Oil India Ltd. (OIL) as licensee for exploration blocks RJ-ON-90/4 and RJ-ON-90/5 (together measuring ~32,600 sq km) in 1996.

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The blocks are in the Paleozoic Bikaner-Nagaur basin (Fig. 1) in Rajasthan. Later, POGC was assigned 25% participating interest in these blocks.

The exploration activities EOL and POGC carried out in this part of the alluvium-sand covered Bikaner-Nagaur basin are first of their kind. The only data available before the start of the work area was a Bouguer gravity map, a couple of crooked seismic lines, and published literature. EOL-POGC performed seismic, gravity, and drilling.

The observations made herein are based on geoscientific data available from published literature, data provided by OIL, and the work carried out by EOL-POGC in 1996-2000.

Regional geology

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The Bikaner-Nagaur basin, part of the Indus basin, is limited by the Delhi-Sargodha subsurface ridge on the northeast and north, by the Aravallis on the east, and the Jodhpur-Pokhran-Chottan-Malani Ridge on the south. The Devikot-Nachna subsurface high separates it from the adjoining Jaisalmer basin on the southwest (Fig. 2).

The Bikaner-Nagaur basin is a shallow Paleozoic basin with a Precambrian basement comprised of the Malani Igneous Suite and Delhi metamorphites.

This basin is divided into two sub-basins, namely Jodhpur-Nagaur and Nagaur-Ganganagar (Fig. 1). Subdivision of the Bikaner-Nagaur basin was based on geophysical data and surface and subsurface geological data. The Jodhpur-Nagaur sub-basin in the south trends SW-NE to WSW-ENE, while the Nagaur-Ganganagar sub-basin in the north trends NNE-SSW, which is confirmed from the available seismic data (Fig. 3).

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The Jodhpur-Nagaur sub-basin indicated the presence of heavy oil whereas EOL-POGC's exploratory efforts in the Nagaur-Ganganagar sub-basin indicated the presence of lighter hydrocarbons.

The Bikaner-Nagaur basin has a stratigraphic succession of sediments of Cambrian, Permo-Triassic, Mesozoic, Tertiary, and Quaternary age. Lower Cambrian comprises sediments of the Marwar Supergroup. This supergroup is divided into the Jodhpur Group (comprised of predominantly sandstone interbedded with bands of shale and limestone), the Bilara-Hanseran Evaporite Group (comprised of limestone-dolomite-evaporites with thin bands of clay-shale), and the Nagaur Group (comprised of argillaceous and arenaceous facies).

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It is reported that the basin remained tectonically unstable during the deposition of the entire Marwar Supergroup. No remarkable change is apparent in the lithological features of sediments of the Jodhpur Group, Nagaur Group, Tertiary, and Quaternary that occur in the two sub-basins (Table 1).

The Jodhpur-Nagaur sub-basin, located in the southern part of Bikaner-Nagaur basin, has the Bilara Group (within Lower Cambrian age) overlying the Jodhpur Group and underlying the Nagaur Group. The Bilara Group is comprised of limestone-dolomite-stromatolitic dolomite lithology with some evaporites.

OIL had encountered a limestone and dolomite sequence overlying the Nagaur Group, designated them as Upper Carbonates, and assigned them to Upper Cambrian age. However, this sequence of Upper Carbonates is interpreted as a local phenomenon (Fig. 1) as the same sequence is reported as not encountered in the wells drilled by OIL in adjoining areas.

Those wells also encountered the Bap-Badhura boulder bed of Permo-Triassic age and argillaceous and arenaceous units of Jurassic-Cretaceous age within Mesozoic. OIL reported the presence of heavy oil within the Upper Carbonate, Bilara, and Jodhpur groups on the paleo Baghewala structure in the Jodhpur-Nagaur sub-basin.

It is reported that the heavy oil originated from sulfur-rich organic matter in low maturity marine carbonate source rock deposited under anoxic conditions with reworking of deposited algal organic matter.

The northern Nagaur-Ganganagar sub-basin was unexplored for hydrocarbons except for the Pugal-1 well drilled by Oil & Natural Gas Corp. in the 1960s based on gravity data. ONGC ceased drilling after encountering evaporites.

Some drilling for hydrocarbons occurred in the western part of this basin in Pakistan. Based on the available information, except for Karampur-1 that encountered heavy oil similar to that reported in the Baghewala area, the other wells found no substantial hydrocarbon indications.

The boreholes GSI drilled on the Indian side of this basin for potash exploration and oil wells in the Pakistan part of this basin encountered a thick sequence of evaporites. The sequence is comprised of halite, anhydrite, dolomite-limestone, and clay facies. GSI designated this evaporite sequence as the Hanseran Evaporite Group (HEG). The sequence is considered homotaxial with the Bilara Group in the south, and the rock units of HEG are thought to lie conformably over the Jodhpur Group.

GSI reported seven cycles of halite within HEG separated by intervening clay-shale, dolomite-stromatolitic, dolomite-limestone-anhydrite, and marl zones. The thickness of each cycle and units within it vary because of the shifting of depocenters within the basin due to tectonic instability.

The evaporites were deposited under shallow to moderately deep marine environment as indicated by the presence of algal stromatolites within the dolomites of HEG. The thickness of evaporites varies from 100-1,000 m. Thickness of stromatolitic dolomites varies from few meters to hundreds of meters.

The area explored by EOL-POGC is in the Nagaur-Ganganagar evaporitic sub-basin, which is the southeastern extension of the Salt Range in Pakistan.

Geophysical work

Taking into consideration available geological information and the Bouguer gravity map, it was planned to acquire seismic and gravity data towards the west of the blocks as the basin dips from east to west and the sediment cover was expected to be thicker.

Seismic survey

A total of 1,410 line km of 30-fold 2D seismic data was acquired employing the Vibroseis method and was spread over two campaigns.

Data were acquired in straight-line profiles using:

  • 4 vibrators in linear array as source.
  • I/O two digital recording equipment.
  • Remote signal conditioners to digitize every set of 6 groups in the active spread.
  • Symmetrical split spread.
  • Linear geophone array with group interval of 25 m.
  • DGPS for surveying-position fixing.

In-field processing of the acquired data was done which helped in fine-tuning the acquisition parameters with regard to geological objective.

In the first campaign (September 1997-April 1998) 1,062 line km of seismic and gravity data were acquired and processed along profiles in a regional grid of 20 by 12 sq km covering both blocks. Interpretation of the data led to identification of 11 leads of various sizes. One lead of small areal extent was on RJ-ON-90/4, and the other 10 leads were on RJ-ON-90/5.

During the second campaign (August-November 1998) data acquisition & processing of 348 line km was focused on RJ-ON-90/5 to detail the identified leads. Newly processed data when interpreted along with first campaign data led to identification of two prospects having fault-bounded, four-way anticlinal two-way travel time (TWTT) closures and areal extents of 12 and 20 sq km, respectively.

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We also re-processed 523 line km of seismic data acquired by CGG for OIL with a view of seismo-geological correlation of newly acquired data with the well information on Baghewala structure.

Some of the observations based on acquired and processed data are:

  1. The basin has NNE-SSW strike with general dip towards northwest.
  2. Overall data quality is good, but some seismic profiles in northern RJ-ON-90/5 and one profile on southernmost RJ-ON-90/4 pass over a geologically disturbed zone in the subsurface, as the zone is represented by poor continuity-character of reflectors along such seismic sections (Fig. 4).
  3. Seismo-geological correlation was attempted using reprocessed and newly acquired-processed data in conjunction with well data over the Baghewala structure, but it was not very helpful as the reflection character changes completely as one approaches the block area. Also the poor continuity of reflectors along some of the reprocessed sections over disturbed zones further hindered the correlation.
  4. The disturbed zones are clearly demarcated by seismic data as the faults seen on seismic sections separate good reflection zones from poor reflection zones (Fig. 4).
  5. The interval velocities along the sections clearly define the zone separating younger Tertiary sediments (<3,000 ms) from older Lower Cambrian sediments (>3,800 ms). The interval velocity studies also guided in mapping major lithologic boundaries.
  6. The pinching out of certain reflectors against TWTT lows and the time-to-depth conversion based on seismic velocities indicate a possible structural inversion on the first look and which requires further study.
  7. The various TWT highs and lows do not seem to be caused by paleo features but appear to be caused by the faulting in the recent past as the effect of these faults is seen up to the shallow seismic reflectors (Fig. 4).

Gravity survey

The gravity data were acquired along seismic profiles in the first campaign of data acquisition with a maximum station interval of 250 m to map the basement configuration.

Some of the observations based on Bouguer gravity data are:

  1. Unlike in the Jaisalmer basin towards southwest, which is deep and where the gravity highs and lows match actual basinal highs and lows, the gravity highs and lows here don't match with the highs and lows of this shallow basin.
  2. Gravity lows are observed towards eastern side of the basin towards which the basin shallows.
  3. It is very difficult to assign any dominant trend of the basin based on gravity data.
  4. The gravity data didn't corroborate seismic data with regard to basement configuration and hence were not employed during the second campaign.

Geological findings

The interpretation of seismic data along with GSI published literature facilitated in seismo-geological correlation and guided in ascertaining the top of the Hanseran Evaporite Group on the block.

Based on the available geological and geophysical data it is inferred that the Permo-Triassic Bap-Badhura Boulder bed and Mesozoic sediments would not be present within the block area and the Tertiary sediments directly overlie the Lower Cambrian sediments. These Permo-Triassic and Mesozoic sediments pinch out towards the western boundary of the block as inferred from the well data available from Pugal-1 and Pakistan wells. However, further study is required to define the pinchout boundary.

The stratigraphy (Table 1) as inferred is based on the analysis of data from the EOL-POGC N1 exploratory well and from the lithological description published in literature. Further study is required to infer correctly about formation ages and lithologic boundaries.

Some observations based on drilling data, seismic data, and gravity data are:

  • HEG thickens towards the west and could reach 1,000-1,100 m. The N1 exploratory well encountered the eight formations (having 7 halite cycles) as classified by GSI.
  • Based on seismic data it is seen that this part of the basin experienced severe wrench tectonics resulting in reverse faults. The tectonism was active until recent time as can be seen from the reverse faults affecting the shallowest reflector (Fig. 4).
  • Based on the log data it is observed that some of the zones show high uranium indications, implying the presence of fissures-fracture or unconformity. Heavy mud losses were observed during drilling through such zones within clastic sediments of Tertiary and sediments of HEG.
  • The poor reflection zones seen up to the shallowest reflectors along some of the seismic sections may be due to lower order movement of the evaporite-salt within HEG into the overlying sediments. The salt movement may have started during wrench tectonics(?).
  • The broad mismatch between the gravity data and seismic data with regard to basement configuration could be due to intra-basement features that are getting highlighted by gravity data.

Hydrocarbon prospectivity

Drilling of Well N1 to basement on RJ-ON-90/5 in the Nagaur-Ganganagar sub-basin was aimed at exploring for hydrocarbons in the Jodhpur Group and HEG dolomites, both of Lower Cambrian age, because the presence of heavy oil was reported in time-equivalent groups in the Baghewala area of the Bikaner-Nagaur basin.

Some of the interesting results & discussions are mentioned below:

  • Five interesting zones in HEG and the Jodhpur Group were identified based on mud logging and wireline logging data, which could be potential hydrocarbon bearing zones. Four are in HEG, and one is in the Jodhpur Group.
  • Before spud we expected to encounter oil similar to the 17.6° gravity oil of Baghewala; however, the N1 crude is light, which suggests that the hydrocarbon source is different in the two sub-basins of the Bikaner-Nagaur basin. This observation along with the tectonic trend from seismic data and GSI literature guided in the subdivision of the Bikaner-Nagaur basin.
  • Since thick sections of stromatolitic dolomites are reported by GSI and encountered in N1, the extent and nature of these within this sub-basin needs to be identified as these stromatolitic dolomites (which are hydrocarbon bearing in N1) would be the source as well as reservoir rocks.
  • There was a big hiatus before the Tertiary sediments got deposited above Lower Cambrian sediments, and published literature indicates that this part of the area was a high during this time. Present-day sedimentary cover in this sub-basin is not expected to exceed 2,200 m, and the sedimentary cover above zones with hydrocarbon presence is expected to be 1,000 m. Though the geothermal gradient in this area is more than normal, this along with overburden thickness and pressure may not have been enough to cook source sediments. Thus, the oil generation may have been during wrench tectonics (?) in the recent past (Eocene-Oligocene boundary?). Further studies of burial history of sediments would give a clear picture about time of generation.
  • Jodhpur sandstone despite having good reservoir characteristics was not promising. However, elsewhere in the sub-basin it may be prospective.
  • The evaporites underlying and overlying dolomites provide good capping.

The N1 well results have established that source rock, reservoir rock, seal, and cap exist in the explored area to facilitate generation and accumulation of hydrocarbons.

The future

The presence of hydrocarbons indicated by the N1 well necessitates a re-look at the hydrocarbon plays in the Paleozoic Bikaner-Nagaur basin.

Bikaner-Nagaur was designated a Category II basin after detection of heavy oil in the Jodhpur-Nagaur sub-basin. However, after the detection of light crude in the evaporitic Nagaur-Ganganagar sub-basin, Bikaner-Nagaur could be upgraded with further exploration.

More detailed seismic surveying might bring out new interesting features. Extensive wrench tectonics could have caused poor seismic results and require better velocity models.

Acknowledgments

The authors thank S.R. Agrawal, executive director, and Dr. Vijay K. Dharia, chief executive officer, and personnel of Essar Oil Ltd.'s E&P Division; K.N. Bhave, consulting geophysicist; OIL; the Directorate General for Hydrocarbons; and POGC.

Bibliography

Contact S. Rajagopalan for a complete listing.

The authors

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S. Rajagopalan (s_rajagopalan [email protected]) is joint general manager-geoscience with Essar Oil Ltd.-E&P Division. He is a former chief geologist with Oil & Natural Gas Corp. and previously worked with Larsen & Toubro Ltd. and Tullow Oil PLC. He has an MS degree in geology from the University of Pune.

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T.V. Santaram is deputy manager-geophysics with Essar E&P, where he has been employed more than 5 years. He interprets seismic, gravity, and wireline logging data and oversees their integration with geology. He has a master of technology degree in petroleum exploration from the Indian School of Mines.

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Jaydip Guha is deputy manager-geology with Essar E&P. He is responsible for interpretation and reservoir characterization based on geological, seismic, and wireline logging data for identification and analysis of hydrocarbon plays. With the company more than 5 years, he has a master of technology degree in applied geology from Indian Institute of Technology.

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D. Pavithran is a junior geologist with Essar E&P, where he has been employed 2 years. He is involved in interpretation and analysis of geoscientific data. He has also worked as a wellsite geologist. He holds an MS degree in geology from the University of Madras.