The measured gravity field of the Earth contains two main components of short wavelength residual anomalies and long wavelength regional anomalies. In the petroleum industry, the most important component is the residual anomalies which correlate with the shallow density variations in sedimentary basins. The regional anomalies are caused by deeper density contrasts in the lithospheric mantle and the Asthenosphere. The current study is focused to determine residual and regional gravity anomalies for understanding basin architecture of the Cauvery Basin. In this study, the basement of the Cauvery Basin was determined using two-way-travel time seismic data and well log data obtained from eight exploration wells. Depth data from selected 2D sections were used as input data for a Mathematica® program which developed based on the iterative algorithm to calculate the residual gravity anomaly caused by a 2D polygonal body having a density contrast with the surrounding. The difference of the calculated and the observed gravity was then used to obtain the regional gravity along the arbitrary survey lines. Results show that the regional gravity ranged from about -50 mGal to 70 mGal with having a relatively high gravity anomaly in the central part of the Cauvery Basin. According to the regional tectonic settings, the high gravity anomaly can indicate a crustal thinning process that occurred during the rifting phase of the basin. The residual gravity anomaly of the Cauvery Basin ranged from about -60 mGal to 20 mGal and it follows the horst and graben structures. In summary, several undiscovered sub-basins were identified related to the rifting of (i) East-West Gondwana (ca. 167 Ma, NE-SW trending structures) and (ii) the East India-Antarctica (ca. 137 Ma, NW-SE trending). Therefore, this study can be productively used for mapping the structural elements and architecture in terms of considering the most favorable locations for future hydrocarbon exploration in the Cauvery Basin.

Oil and Gas production always ties with drawing out of naturally-occurring radionuclides deposited beneath the earth, which are referred to as “NORM”. Understanding the prevailing background levels of these elements in the sub-surface reservoir rock formations will be beneficial to all stakeholders, more importantly to regulatory authorities of the country. The drill cutting samples from 5 m sampling intervals of natural gas reservoir sand section in the depth range 3025m to 3095m from deep water exploratory well “CLPL-Dorado 91 H/1z” drilled in the Mannar basin offshore Sri Lanka were tested in the laboratory using high-resolution Gamma-ray detectors. Test results revealed that the activity concentration of 40K, 210Pb,226Ra and 232Th levels and the calculated outdoor annual effective dose rate varies between considerably lower range when compared with the global standard limits. NORM concentration ranges of the sedimentary rocks within the tested section were recorded on the lower side, when the test results compared with the International Atomic Energy Agency published data on NORM concentration ranges of the sedimentary rocks found elsewhere in the world. Study results proved that there is no harmful public exposure of NORM by disposing these drill cuttings to environment or storing at any site location as it is. Also, it can be predicted that there will be very low level of NORM contaminations occur, if Dorado reservoir taken in to the production stage and well operations conducted with proper solid control mechanisms in future.

Naturally-occurring radionuclides deposited beneath the earth, which are referred to as "NORM" and other toxicnon-radioactive elements transported to the earth surface with the oil and gas production. Hence, knowledge of the prevailing background levels of these elements in the subsurface reservoir formations is valuable to all stakeholders, most notably to regulatory authorities of the country. The drill cuttings obtained within depth range 3025m to 3095m of reservoir sand section in the deep water exploratory well (CLPL-Dorado 91 H/1z) drilled in the Mannar Basin offshore Sri Lanka were subjected to high-resolution Gamma-ray spectrometry and X-ray fluorescence (XRF) spectrometry.As test results revealed activity concentration of 40 K varies from 0.338 Bq/g to 0.514 Bq/g, 210 Pb from 0.007 Bq/g to 0.015 Bq/g, 226 Ra from 0.012 Bq/g to 0.0145 Bq/g while 232 Th levels are between 0.030 Bq/g to 0.040 Bq/g. According to the XRF testing levels of significantly hazardous non-radioactive elements are considerably lower, except for the level of the Barium. The Pb level varies between the 48 ppm to 22 ppm. The Thorium level varies between 9.6 ppm to 10.1 ppm. Manganese has a range of 5,173ppm to 653ppm.The barium levels are between 118,666 ppm to 24,400 ppm. NORM concentration of the tested section were on the lower side when results matched with the IAEA published data on NORM concentration in oil, gas and there byproducts and therefore there will be low level of NORM contaminations when the Dorado gas discovery proceeds to the production stage. Further there is no harmful public exposure from NORM by disposing these drill cuttings to environment or storing at any site location. But the disposal of the drilling mud and handling of the drilling mud should be conducted with cautious since extremely high Ba levels can potentially cause health problems.

The Mannar Basin extends over 45,000 km2 off the western coast of Sri Lanka. It has evolved due to the multiphase rifting between Indo-Lanka landmasses during the Barremian-Paleocene time. The sediment thickness of the basin ranges from about 4 to 10 km. The northern part of the basin is a targeted area for hydrocarbon exploration in Sri Lanka. Though two natural gas discoveries were made in 2011, the basin remains a frontier due to lack of well penetration and 3D seismic coverage. As a result, the depositional features of sediment in the basin are little known. This study focuses on identifying paleo depositional features in the Albian and Aptian strata using 650 km2 3D seismic data from the Mannar Basin. Root Mean Square (RMS) amplitude was used to characterize the depositional features in three-time windows on IHS Kingdom software (v.8.3). The results show the existence of a multi-level paleo submarine fan system in the Albian and Aptian strata. They are located relatively close to the western coastline compared to the fan system in the Eocene strata. The deposition of this paleo submarine fan system has taken place in a shelf marine environment and has been influenced by relatively high sea levels during the Albian and Aptian compared to the relatively low sea levels in the Paleocene and Eocene.

In exploration geophysics, seismic surveys and their interpretations provide the most reliable information of the subsurface structures. Extensive work on the determination of the subsurface structure has been carried out in both available satellite-gravity and magnetic data suggest that the Lanka Basin may have the necessary sediment volume for the occurrence of hydrocarbons. However, without seismic data, this suggestion cannot be tested further. As an alternative to acquiring additional seismic data in the region, this research focused on digitising 50,000 km of vintage seismic lines from the National Geophysical Data Centre (NGDC) data repository and converting them into Seg-Y format. Key seismic horizons on the digitised sections were interpreted to provide details of the seabed and acoustic basement in the time domain. Regional sedimentary thickness maps were compiled in the time domain interpolating the interpreted horizons using ' ' method. The velocity data acquired in the region were used to convert the maps from time to depth domain. The results indicate that average thickness of sediments in the region varies around 5000 m to 6000 m, where the maximum thickness is around 8000 m in the North Eastern part of the Lanka basin. The thickness maps can be used as a reference dataset to plan commercial seismic surveys in the future for hydrocarbon exploration in the Lanka Basin.