Deccan Syneclise Research Articles

Basalt–sediment configuration in the Tapti rift of central Indian region as derived from magnetotelluric (MT) studies and hydrocarbon prospects

Geochemical studies in Deccan syneclise have identified anomalous concentration of light hydrocarbon gases in the surface soil from the Tapti rift region in Narmada–Son Lineament, India. We have reanalysed and modelled broadband MT data from this region to delineate basalts and underlying sediment configuration, which is favourable for hydrocarbon accumulation. We carried out 1D and 3D modelling of the MT data for this purpose. Modelling studies brought regions of thick sub-basaltic sediments towards NW and SW of Nepanagar. But very less anomalous light hydrocarbon gas concentration is observed along NW of Nepanagar. This may be due to the presence of thick basalt cover in that area, which prevents the leakage of gaseous hydrocarbons on to the surface soil. We observed a comparatively thin basalt layer and high sediment thickness with a high concentration of light hydrocarbon gases near to Burhanpur area, which strongly suggests better hydrocarbon prospects in the area. It is inferred that the relatively thin and fractured/vesicular nature of basalt in this region facilitates the upward migration of light hydrocarbon components to the surface soil.

3D upper crustal density structure of the Deccan Syneclise, Central India

ABSTRACTA constrained 3D density model of the upper crust along a part of the Deccan Syneclise is carried out based on the complete Bouguer anomaly data. Spectral analysis of the complete Bouguer gravity anomaly map of the study region suggests two major sources: short wavelength anomalies (<100 km) caused primarily due to the density inhomogeneities at shallow crustal level and long wavelength anomalies (>100 km) produced due to the sources deeper than the upper crust. A residual map of the short wavelength anomalies is prepared from the complete Bouguer anomaly using Butterworth high‐pass filter (100 km cut‐off wavelength). Utilizing the constraints from deep resistivity sounding, magnetotellurics and deep seismic sounding studies, 2.5D density models have been generated along 39 profiles of this region. The mismatch between the calculated response of the a priori 2.5D model with the residual (short wavelength) gravity anomalies is minimized by introducing high‐density intrusive bodies (≥2.81 g/cm3) in the basement. With these 2.5D density models, the initial geometry of our 3D density model, which includes alluvium, Deccan trap, Mesozoic sediment and high‐density intrusive bodies in the basement up to a depth of 7 km (upper crust), is generated. In the final 3D model, Deccan trap extends from 200 m to nearly 1700 m below the 90–150 m thick Quaternary sediment. Further down, the sub‐trappean Mesozoic sediment is present at a depth range of 600–3000 m followed by the basement. The derived 3D density model also indicates six intrusive bodies of density 2.83 g/cm3 in the basement at an average depth of about 4–7 km that best fits the residual gravity anomaly of the study area.

Sub-basalt Imaging of Hydrocarbon-Bearing Mesozoic Sediments Using Ray-Trace Inversion of First-Arrival Seismic Data and Elastic Finite-Difference Full-Wave Modeling Along Sinor\u2013Valod Profile of Deccan Syneclise, India

Imaging below the basalt for hydrocarbon exploration is a global problem because of poor penetration and significant loss of seismic energy due to scattering, attenuation, absorption and mode-conversion when the seismic waves encounter a highly heterogeneous and rugose basalt layer. The conventional (short offset) seismic data acquisition, processing and modeling techniques adopted by the oil industry generally fails to image hydrocarbon-bearing sub-trappean Mesozoic sediments hidden below the basalt and is considered as a serious problem for hydrocarbon exploration in the world. To overcome this difficulty of sub-basalt imaging, we have generated dense synthetic seismic data with the help of elastic finite-difference full-wave modeling using staggered-grid scheme for the model derived from ray-trace inversion using sparse wide-angle seismic data acquired along Sinor–Valod profile in the Deccan Volcanic Province of India. The full-wave synthetic seismic data generated have been processed and imaged using conventional seismic data processing technique with Kirchhoff pre-stack time and depth migrations. The seismic image obtained correlates with all the structural features of the model obtained through ray-trace inversion of wide-angle seismic data, validating the effectiveness of robust elastic finite-difference full-wave modeling approach for imaging below thick basalts. Using the full-wave modeling also allows us to decipher small-scale heterogeneities imposed in the model as a measure of the rugose basalt interfaces, which could not be dealt with ray-trace inversion. Furthermore, we were able to accurately image thin low-velocity hydrocarbon-bearing Mesozoic sediments sandwiched between and hidden below two thick sequences of high-velocity basalt layers lying above the basement.

Integrated surface geochemical studies for hydrocarbon prospects in Deccan Syneclise, India

The presented study reports the geochemical investigation for hydrocarbon prospects in western and central regions of Deccan Syneclise to evaluate the near surface manifestations of hydrocarbons due to microseepage; identify the sources of light hydrocarbon gases and integrate the results with geophysical data to demarcate the prospective areas for hydrocarbons. The adsorbed soil gas analysis of soil samples from western and central regions show the presence of C1–C4 hydrocarbons with concentrations of C1 and C2+ varying from 2 to 626ppb, 0–261 ppb and 4–255ppb, 0–114ppb respectively. The propane oxidizing bacteria (POB) ranged between 1.0×102 and 2.65×105cfu/g of soil. The evaluation of compositional ratios of adsorbed soil gases indicates that the gas components are co-genetic and migrated from subsurface hydrocarbon accumulations/organic rich beds. The carbon isotopic compositions of adsorbed soil gas methane from western region of Deccan Syneclise range from −29.87 to −13.09‰ (VPDB) representing near surface oxidation of gases subsequent to migration and are derived from gas producing type III kerogen. The δ13C signatures of soil carbonates from central region of Deccan Syneclise falling in the range of −9.61 to −5.1‰ (VPDB) indicate diagenetic alteration of seeping hydrocarbon gases in the surface soils. The adsorbed light gaseous hydrocarbon, propane oxidizing bacteria and soil carbonate anomalies are representative of samples from Kopargaon, Shirdi, Rampur and Newasa in western region and Edlabad, Popetkheda and Khandwa in central regions of Deccan Syneclise. These anomalies are in good agreement with thick subtrappean Mesozoic sediments of up to 2km delineated by the magnetotelluric (MT) and gravity studies in Deccan Syneclise, which might have sourced the hydrocarbon gases. The anomalous areas are also associated with Barwani-Sukta, Tapti and Gaviligarh faults which might have provided migratory pathways for light gaseous hydrocarbons. The geochemical results of this study combined with the geophysical data show evidence of hydrocarbon potential of the study area.

Tomographic imaging of sub-basalt Mesozoic sediments and shallow basement geometry for hydrocarbon potential below the Deccan Volcanic Province (DVP) of India

Summary A shallow 2D tomographic P-wave velocity image is obtained in the Deccan Volcanic Province (DVP) of India using first-arrival seismic data along a 90 km long N-S trending profile in the Deccan Syneclise region. The tomographic image depicts smooth velocity variations of Quaternary (2.0-2.5 km/s) sediments, basalt (5.0-5.5 km/s) and sub-basalt Mesozoic sediments (4.0-4.5 km/s) as well as basement (5.8-6.0 km/s) configuration down to a maximum depth of 5.0 km. Due to late Cretaceous volcanism with outpouring of basaltic lava flows, this region is affected by numerous dyke intrusions and thick basaltic trap forming sills with the presence of horst and graben structures due to deep basinal faults. Although imaging below the basalt is a major challenge for the oil industry, with the help of tomographic imaging technique of long-offset seismic refraction data, we are able to image sporadic sub- and intra-trappean Mesozoic sediments (<0.75 km) deposited below thick columns (0-3 km) of basalt flows above the basement. The imaged Mesozoic sediments are expected to contain hydrocarbon due to their wide extension and suitable trapping mechanism of basaltic sills. The robustness of the velocity image is assessed through numerous tests like velocity perturbations, χ 2 estimates, RMS residuals of traveltime fit, resolution and uncertainty estimates through computation of ray-density or Hits.

Surface geochemical data evaluation and integration with geophysical observations for hydrocarbon prospecting, Tapti graben, Deccan Syneclise, India

The Deccan Syneclise is considered to have significant hydrocarbon potential. However, significant hydrocarbon discoveries, particularly for Mesozoic sequences, have not been established through conventional exploration due to the thick basalt cover over Mesozoic sedimentary rocks. In this study, near-surface geochemical data are used to understand the petroleum system and also investigate type of source for hydrocarbons generation of the study area. Soil samples were collected from favorable areas identified by integrated geophysical studies. The compositional and isotopic signatures of adsorbed gaseous hydrocarbons (methane through butane) were used as surface indicators of petroleum micro-seepages. An analysis of 75 near-surface soil-gas samples was carried out for light hydrocarbons (C1–C4) and their carbon isotopes from the western part of Tapti graben, Deccan Syneclise, India. The geochemical results reveal sites or clusters of sites containing anomalously high concentrations of light hydrocarbon gases. High concentrations of adsorbed thermogenic methane (C1 = 518 ppb) and ethane plus higher hydrocarbons (ΣC2+ = 977 ppb) were observed. Statistical analysis shows that samples from 13% of the samples contain anomalously high concentrations of light hydrocarbons in the soil-gas constituents. This seepage suggests largest magnitude of soil gas anomalies might be generated/source from Mesozoic sedimentary rocks, beneath Deccan Traps. The carbon isotopic composition of methane, ethane and propane ranges are from −22.5‰ to −30.2‰ PDB, −18.0‰ to 27.1‰ PDB and 16.9‰–32.1‰ PDB respectively, which are in thermogenic source. Surface soil sample represents the intersection of a migration conduit from the deep subsurface to the surface connected to sub-trappean Mesozoic sedimentary rocks. Prominent hydrocarbon concentrations were associated with dykes, lineaments and presented on thinner basaltic cover in the study area, which probably acts as channel for the micro-seepage of hydrocarbons.

Geo-microbial and light gaseous hydrocarbon anomalies in the near surface soils of Deccan Syneclise Basin, India: Implications to hydrocarbon resource potential

Deccan Syneclise Basin is considered geologically prospective for oil and gas reserves; however, a major part of the basin is covered by the Deccan Traps, hindering the exploration of Mesozoic hydrocarbon targets. Surface geochemical prospecting, based on micro-seepage of hydrocarbons from subsurface accumulations, could be advantageous in such areas. Surface geochemical prospecting method was applied in Deccan Syneclise for evaluation of hydrocarbon resource potential of the basin. The gas chromatographic analyses of adsorbed soil gasses showed the presence of C1 to C4 hydrocarbons. The concentrations of adsorbed soil gasses ranged for methane (C1)=1 to 483ppb, ethane (C2)=1 to 83ppb, propane (C3)=1 to 38ppb, butane (nC4)=1 to 60ppb and ΣC2+=1 to 129ppb, respectively. The scatter plots between C1 and C4 components depict linear trend indicating that all the gasses are derived from the same source.The carbon isotope composition of adsorbed methane was measured using Gas Chromatography–Combustion-Isotope Ratio Mass Spectrometry (GC–C-IRMS). The values for methane δ13C1 varied from −39.9 to −19.9‰ versus the Vienna PeeDee Belemnite (VPDB). The carbon isotope ratio distribution pattern represents isotopic characteristics pertaining to hydrocarbon gasses derived from thermogenic sources. The propane oxidizing bacteria (POB) ranged between 1.0×102 and 12.8×105cfu/g of soil. The adsorbed light gaseous hydrocarbons and propane oxidizing bacterial anomalies found in and around Aurangabad, Ahmednagar, Shirdi, Akola, Bhusawal and Indore areas are considered to be promising for future hydrocarbon research and exploration.

Extension of Godavari Gondwana sediments underneath Trap covered region of Satpura basin as evidenced from seismic studies in Deccan Syneclise, India

We present a shallow P-wave velocity structure of the eastern part of Deccan Syneclise, central India, from the refraction and wide-angle reflection data recorded along two ∼100 km long seismic refraction profiles in east–west (E–W) and north–south (N–S) directions using travel time inversion method. In addition to the P-wave velocity model, we have derived S-wave velocity model and estimated Vp/Vs and Poisson’s ratios along the east–west profile. Travel time analysis and inversion of the data revealed three layers above the basement. The first layer with a P-wave velocity of 1.4–1.5 km s −1 represents alluvium and the thickness of this layer is varying between 100–400 m in the E–W and 30–100 m in the N–S profile. The Deccan Trap thickness is observed to be varying between 850 and 1250 m constitutes the second layer. P-wave velocity of the Deccan Traps is 4.8–5.2 km s −1. Amplitude decay and travel time skips in the first arrival refraction data in some of the record sections indicate the presence of a velocity inversion i.e. low velocity zone (LVZ) beneath the Traps. Wide-angle reflection phases from top and bottom of the LVZ (top of basement) together with basement refraction phase are identified and modeled to derive LVZ thickness. The LVZ has been interpreted to be the Gondwana sediments sandwiched between Deccan Traps and basement. These sediments are about 200–450 m thick with a P-wave velocity of 3.6 km s −1. The basement lies at about 1.60 km depth near Khandala in the west and Rajola in the north shows an upward trend towards south–east. The P-wave velocity of the basement is about 6.0–6.2 km s −1. The inferred subtrappean sediments could be the possible extension of Gondwana sediments of the Godavari graben towards the Satpura basin.

Soil Iodine Determination in Deccan Syneclise, India: Implications for Near Surface Geochemical Hydrocarbon Prospecting

The association of iodine with organic matter in sedimentary basins is well documented. High iodine concentration in soils overlying oil and gas fields and areas with hydrocarbon microseepage has been observed and used as a geochemical exploratory tool for hydrocarbons in a few studies. In this study, we measure iodine concentration in soil samples collected from parts of Deccan Syneclise in the west central India to investigate its potential application as a geochemical indicator for hydrocarbons. The Deccan Syneclise consists of rifted depositional sites with Gondwana–Mesozoic sediments up to 3.5 km concealed under the Deccan Traps and is considered prospective for hydrocarbons. The concentration of iodine in soil samples is determined using ICP-MS and the values range between 1.1 and 19.3 ppm. High iodine values are characteristic of the northern part of the sampled region. The total organic carbon (TOC) content of the soil samples range between 0.1 and 1.3%. The TOC correlates poorly with the soil iodine (r 2 < 1), indicating a lack of association of iodine with the surficial organic matter and the possibility of interaction between the seeping hydrocarbons and soil iodine. Further, the distribution pattern of iodine compares well with two surface geochemical indicators: the adsorbed light gaseous hydrocarbons (methane through butane) and the propane-oxidizing bacterial populations in the soil. The integration of geochemical observations show the occurrence of elevated values in the northern part of the study area, which is also coincident with the presence of exposed dyke swarms that probably serve as conduits for hydrocarbon microseepage. The corroboration of iodine with existing geological, geophysical, and geochemical data suggests its efficacy as one of the potential tool in surface geochemical exploration of hydrocarbons. Our study supports Deccan Syneclise to be promising in terms of its hydrocarbon prospects.

Basement configuration of the Jhagadia–Rajpipla profile in the western part of Deccan syneclise, India from travel-time inversion of seismic refraction and wide-angle reflection data

2-D velocity structure up to the basement is derived by travel-time inversion of the first arrival seismic refraction and wide-angle reflection data along the SW–NE trending Jhagadia–Rajpipla profile, located on the western part of Deccan syneclise in the Narmada–Tapti region. The study region is mostly covered by alluvium. Inversion of refraction and wide-angle reflection data reveals four layered velocity structure above the basement. The first two layers with P-wave velocities of 1.95–2.3 km s −1 and 2.7–3.05 km s −1 represent the Recent and Quaternary sediments respectively. The thickness of these sediments varies from 0.15 km to 3.4 km. The third layer with a P-wave velocity of 4.8–5.1 km s −1 corresponds to the Deccan volcanics, whose thickness varies from 0.5 km to 1.0 km. Presence of a low velocity zone (LVZ) below the high velocity volcanic rocks in the study area is inferred from the travel-time ‘skip’ and amplitude decay of the first arrival refraction data and the wide-angle reflection from top of the LVZ present immediately after the first arrival refraction from Deccan Trap layer. The thickness of the low velocity Mesozoic sediments varies from 0.3 km to 1.7 km. The basement with a P-wave velocity of 5.9–6.15 km s −1 lies at a depth of 4.9 km near Jhagadia and shallows to 1.2 km towards northeast near Rajpipla. The results indicate presence of low velocity Mesozoic sediments hidden below the Deccan Trap layer in the western part of the Deccan syneclise.

Delineation of Subtrappean Mesozoic Sediments in Deccan Syneclise, India, Using Travel-Time Inversion of Seismic Refraction and Wide-Angle Reflection Data

2-D shallow velocity structure is derived by travel-time inversion of the first arrival seismic refraction and wide-angle reflection data along the E–W trending Narayanpur–Nandurbar and N–S Kothar–Sakri profiles, located in the Narmada–Tapti region of the Deccan syneclise. Deccan volcanic (Trap) rocks are exposed along the two profiles. Inversion of seismic data reveals two layered velocity structures above the basement along the two profiles. The first layer with a P-wave velocity of 5.15–5.25 km s−1 and thickness varying from 0.7–1.5 km represents the Deccan Trap formation along the Narayanpur–Nandurbar profile. The Trap layer velocity ranges from 4.5 to 5.20 km s−1 and the thickness varies from 0.95 to 1.5 km along the Kothar–Sakri profile. The second layer represents the low velocity Mesozoic sediments with a P-wave velocity of 3.5 km s−1 and thickness ranging from about 0.70 to 1.6 km and 0.55 to 1.1 km along the E–W and N–S profiles, respectively. Presence of a low-velocity zone (LVZ) below the volcanic rocks in the study area is inferred from the travel-time ‘skip’ and amplitude decay of the first arrival refraction data together with the prominent wide-angle reflection phase immediately after the first arrivals from the Deccan Trap formation. The basement with a P-wave velocity of 5.8–6.05 km s−1 lies at a depth ranging from 1.5 to 2.45 km along the profiles. The velocity models of the profiles are similar to each other at the intersection point. The results indicate the existence of a Mesozoic basin in the Narmada–Tapti region of the Deccan syneclise.

Sub-basalt imaging from gravity studies over Deccan volcanic province of Central India

Large part of the northwest and central India is covered with Deccan volcanic of Cretaceous origin. Presence of thick sequence of Mesozoic sediments underneath the volcanic cover has opened a new frontier for hydrocarbon exploration in India. Recently, detailed gravity measurements were carried out over the Deccan Syneclise of central India as a part of integrated geophysical studies for the delineation of subtrappen Mesozoic sediments. Bouguer anomaly map of the region depicts number of significant short wavelength anomalies due to shallow sources superposed on long wavelength regional anomalies due to deep-seated sources. Important among them are (i) an E-W trending broad relative gravity high associated with the Satpura mountains belt indicating presence of high density magmatic material at the deeper level (ii) a large wavelength regional gravity low in the southern part bears an inverse correlation with topography of Ajanta hills indicating mass deficiency beneath the excess topography load due to isostatic compensation (iii) short wavelength nearly E-W trending gravity low suggest the presence sediments below the volcanic cover. The residual gravity field obtained using frequency domain filtering reveals number of linear positive anomalies due to basement upwarp apart from short wavelength negative anomalies aligned in E-W direction probably due to sediments. Quantitative modeling of residual gravity field constrained by other geophysical information has brought out prominent subtrappean Mesozoic sub-basins along the Tapti rift. It is observed that thickness of sediments increases towards the east reaching to a maximum of about 2.0 Km. It is therefore inferred that the Deccan Syneclise region of central India has large potential for hydrocarbon exploration.

An integrated geophysical approach for imaging subbasalt sedimentary basins: Case study of Jam River Basin, India

An integrated geophysical strategy comprising deep electrical resistivity and gravity data was devised to image subbasalt sedimentary basins. A 3D gravity inversion was used to determine the basement structure of the Permian sediments underlying the Cretaceous formation of the Jam River Basin in India. The thickness of the Cretaceous formation above the Permian sediments estimated from modeling 60 deep-electric-sounding data points agrees well with drilling information. The gravity effect of mass deficit between the Cretaceous and Permian formations was found using 3D forward modeling and subsequently removed from the Bouguer gravity anomaly along with the regional gravity field. The modified residual gravity field was then subjected to3D inversion to map the variations in depth of the basement beneath the Permian sediments. Inversion of gravity data resulted in two basement ridges, running almost east to west, dividing the basin into three independent depressions. It was found that the Katol and Kondhali faults were active even during post-Cretaceous time and were responsible for the development of the subsurface basement ridges in the basin. The inferred 3D basement configuration of the basin clearly brought out the listric nature of these two faults. Further, the extension of the Godavari Basin into the Deccan syneclise and the fact that the source-rock studies show the presence of hydrocarbons in the Sironcha block in the northern part of the Godavari Basin also shed some light on the hydrocarbon potential of the Jam River Basin.