Geoscience Frontiers 5 (2014) 419e428 Contents lists available at ScienceDirect China University of Geosciences (Beijing) Geoscience Frontiers journal homepage: www.elsevier.com/locate/gsf Research paper Surface geochemical data evaluation and integration with geophysical observations for hydrocarbon prospecting, Tapti graben, Deccan Syneclise, India T. Satish Kumar a,*, A.M. Dayal b, V. Sudarshan c a Centre of Excellence for Energy Studies, Oil India Ltd, Guwahati 781022, India b National Geophysical Research Institute (CSIR), Hyderabad 500006, India c Applied Geochemistry Department, Osmania University, Hyderabad 500006, India article info abstract Article history: The Deccan Syneclise is considered to have significant hydrocarbon potential. However, significant Received 28 December 2012 hydrocarbon discoveries, particularly for Mesozoic sequences, have not been established through Received in revised form conventional exploration due to the thick basalt cover over Mesozoic sedimentary rocks. In this study, 27 July 2013 near-surface geochemical data are used to understand the petroleum system and also investigate type Accepted 5 August 2013 of source for hydrocarbons generation of the study area. Soil samples were collected from favorable Available online 27 August 2013 areas identified by integrated geophysical studies. The compositional and isotopic signatures of adsorbed gaseous hydrocarbons (methane through butane) were used as surface indicators of pe- Keywords: Adsorbed gas troleum micro-seepages. An analysis of 75 near-surface soil-gas samples was carried out for light e Microseepage hydrocarbons (C1 C4) and their carbon isotopes from the western part of Tapti graben, Deccan Light hydrocarbon Syneclise, India. The geochemical results reveal sites or clusters of sites containing anomalously high Stable isotope concentrations of light hydrocarbon gases. High concentrations of adsorbed thermogenic methane Geophysical (C1 ¼ 518 ppb) and ethane plus higher hydrocarbons (SC2þ ¼ 977 ppb) were observed. Statistical Deccan Syneclise 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 À22.5& to À30.2& PDB, À18.0& to 27.1& PDB and 16.9&e32.1& 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 concentra- tions 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. Ó 2013, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved. 1. Introduction Visible seepage of oil or hydrocarbon gases in sedimentary ba- sins has played a major role in the discovery of many oil fields throughout the world (Link, 1952). In 1930s, the concept of a * Corresponding author. prominent macro seep environment in association with commer- E-mail address: [email protected] (T. Satish Kumar). cial subsurface hydrocarbons was extended to the micro seep level by measuring minute surface gas concentrations or secondary soil Peer-review under responsibility of China University of Geosciences (Beijing) alteration effects induced by gas seepage. Microseepage of hydro- carbon gases into near-surface environments is often interpreted as a direct indication of the presence of deeper hydrocarbons. Jones and Drozd (1983) and Severne et al. (1991) provided examples for Production and hosting by Elsevier such successful applications of soil gas surveys in exploration. The 1674-9871/$ e see front matter Ó 2013, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gsf.2013.08.003 420 T. Satish Kumar et al. / Geoscience Frontiers 5 (2014) 419 e 428 Figure 1. Adsorbed soil sample locations are plotted on geology of the study area, Tapti Graben, Deccan Syneclise (modified after Geological Survey of India, 2001). T. Satish Kumar et al. / Geoscience Frontiers 5 (2014) 419e428 421 Table 1 identification of seepage can add valuable information to the Generalized stratigraphy of the Deccan Syneclise Basin (After Deshpande, 1998). exploration process when the diagnosis of gas micro seep envi- Age Formation/ Anticipated ronments can be distinguished and separated from background range Group thickness (km) Lithology Geographic distribution data. Under certain conditions, active seepage may be recognized through measurement of concentration and composition of hy- Recent– Alluvium Nagpur, Bhandara , Pleistocene Laterite, sand Candrapur, Wardha drocarbon gases in soils (Jones and Drozd, 1983; Klusman, 1993). Jalgaon, This paper describes the application of surface geochemical Kolhapur Satara Thane, etc. Early Paleocene Deccan Traps 1–2 Basalts Most of the state from studies carried out in sub-trappean of Mesozoic age in Deccan Trap –Cretaceous west of Nagpur to cover, western part of Tapti graben for the prospecting of hydro- Arabian sea coast --------------------------------------------Unconformity---------------------------------------- carbons. The region has received wide attention for its hydrocarbon Late Lameta, bagh Arenaceous Nagpur, Candrapur prospects because of the thick GondwanaeMesozoic sedimenta- Cretacoous beds limestone, Dhule, Gadchiroli tion of 1e2.75 km buried under the lava flow of Deccan Traps (Kaila 2–3.5 Sandstone Yavatmal Middle Upper shale et al., 1989; DGH, 2006). Hence an attempt has been made to find Triassic Gondwana the presence of hydrocarbons in the reported Mesozoic sediment ------------------------------------------ Unconformity -------------------------------------- Triassic– Nagpur, Chandrapur areas. In the present work, the adsorbed soil gases and their carbon Carboniferous Lower Yavatmal isotope signatures have been analyzed to determine the concen- Gondwana tration of light hydrocarbons and to identify the seepage and source Proterozoic Penganga beds Gadchiroli, Ratnagiri Limetsones, shales of hydrocarbons. Archean Sausar, Sakoli, Nagpur, Bhandara Amagaon, Ratnagiri, Sindhudurg Unclassified gneisses 1.1. Geology of the study area The Narmada-Tapti rift system, which constitutes the western part of the NSL (Narmada-Son Lineament), is covered by a thick pile of Deccan lava flows and is characterized by several hidden tectonic Figure 2. (a) Trap thickness map in Deccan Syneclise with study area (modified after Harinarayana et al., 2007). (b) Map showing Mesozoic sediment thickness of the study area (after DGH, 2006). (c) The Narmada-Tapti seaway of Deccan volcanic province with study area (modified after Keller et al., 2009). 422 T. Satish Kumar et al. / Geoscience Frontiers 5 (2014) 419e428 structures, magmatic crustal accretion, sedimentary basins and Deep seismic studies (DSS) along the Thuadara-Sendhwa- complex geophysical signatures (Fig. 1). Major tectonic adjustments Sindad profile wherein the first arrival refraction data analyzed in various crustal blocks, in Narmada-Tapti graben occurred in based on 2D tracing technique revealed a graben between Narmada Precambrian/Gondwana times which must have been responsible and Tapti. This graben contains 1000e2800 m thick low velocity À for the formation of Vindhyan and Gondwana sedimentary basins. (3.2e3.6 km s 1) sediments under a thick cover of Deccan traps. Tapti Basin, an intracratonic half graben in western-central India And this study also revealed that the graben is bounded by two (Guha, 1995) is considered to be Mesozoic marginal marine basin faults, one south of Narmada i.e., Barwani-Sukta and the other (Biswas, 1987). The generalized stratigraphy of Deccan Traps is given north of Tapti river. The graben seems to be extending towards in Table 1. Tapti Basin forms a linear tract spread over a length of south-east. The low velocity Mesozoic sedimentary rocks with a P- À 350 km and an average width of 30 km covered by alluvium of Tertiary wave velocity of 3.5 km s 1 and thickness ranging from about 0.70 to recent with isolated inliers of the Deccan Traps. The alluvium to 1.6 km and 0.55e1.1 km lies along the east-west and north-south thickness from south to north, in general, extends to a depth of profiles, respectively (Murty et al., 2010). À w200 m to >400 m below mean sea level at places (Ravi Shanker, The first layer with a P-wave velocity of 5.15e5.25 km s 1 and 1987). The Deccan Trap thickness varies considerably from 100 m in thickness varying from 0.7 to 1.5 km represents the Deccan Trap the northeastern part to more than 1500 m towards the west coast of formation along the NarayanpureNandurbar profile. The Trap layer À India (Fig. 2a). In the Narmada-Tapti region, a hidden Mesozoic sedi- velocity ranges from 4.5 to 5.20 km s 1 and the thickness varies mentary basin underlying the Deccan Traps has been reported in the from 0.95 to 1.5 km along the KothareSakri profile. The second form of two grabens separated by a small horst of Satpura hills. In the layer represents the low velocity Mesozoic sediments with a P- À southern part a larger Tapti graben with sediment thickness of about wave velocity of 3.5 km s 1 and