The Deccan Syneclise is considered to have significant hydrocarbon potential. However, significant hydrocarbon discoveries, particularly for Mesozoic sequences, have not been established through conventional explorati...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 pe- troleum micro-seepages. An analysis of 75 near-surface soil-gas samples was carried out for light hydrocarbons （C_1-C_4） 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 （∑C2＋ = 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 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.展开更多
The Kohima Syncline exposes the Eocene Upper Disang Formation in an extensive region that is composed of grey shales with subordinate sandstone and siltstone alterations.The shales contain low to fair amount of organi...The Kohima Syncline exposes the Eocene Upper Disang Formation in an extensive region that is composed of grey shales with subordinate sandstone and siltstone alterations.The shales contain low to fair amount of organic matters represented by Type IV/Type III terrestrial kerogens.The Biomarkers indicate presence of terrestrial angiosperms and microbially reworked organic matters with some marine phytoplankton.Rock-Eval maturity parameters indicate maturity within a wide range between immature to post mature wet gas window,while the biomarkers show maturity between immature to mid mature.The biomarkers indicate that the Disang shales deposited in an oxic deltaic-terrigenous environment.The trace elements characterize an oxic to suboxic warm and humid depositional environment.Few trace elements point to deposition in a low salinity continental rather than a marine environment.Thus,several biomarker and trace element parameters show that a continental milieu was already established during the deposition of the Upper Disang sediments.It indicates that either the Disang basin,that evolved as a marine basin,suffered rapid shallowing during the deposition of the Upper Disang sediments,or the Lower and Upper Disangs were coeval and Lower Disangs represented deep marine facies while the Upper Disangs represented peripheral shallow marine deltaic facies.Maturity parameters like Tmax and,sterane and hopane isomerization ratios indicate that the organic matters of the Disang Group were affected by sudden short duration thermal events followed by cooling.This is possible in an active tectonic region like the Kohima Syncline that bears many evidences of thrusting and folding.展开更多
文摘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 pe- troleum micro-seepages. An analysis of 75 near-surface soil-gas samples was carried out for light hydrocarbons （C_1-C_4） 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 （∑C2＋ = 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 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.
文摘The Kohima Syncline exposes the Eocene Upper Disang Formation in an extensive region that is composed of grey shales with subordinate sandstone and siltstone alterations.The shales contain low to fair amount of organic matters represented by Type IV/Type III terrestrial kerogens.The Biomarkers indicate presence of terrestrial angiosperms and microbially reworked organic matters with some marine phytoplankton.Rock-Eval maturity parameters indicate maturity within a wide range between immature to post mature wet gas window,while the biomarkers show maturity between immature to mid mature.The biomarkers indicate that the Disang shales deposited in an oxic deltaic-terrigenous environment.The trace elements characterize an oxic to suboxic warm and humid depositional environment.Few trace elements point to deposition in a low salinity continental rather than a marine environment.Thus,several biomarker and trace element parameters show that a continental milieu was already established during the deposition of the Upper Disang sediments.It indicates that either the Disang basin,that evolved as a marine basin,suffered rapid shallowing during the deposition of the Upper Disang sediments,or the Lower and Upper Disangs were coeval and Lower Disangs represented deep marine facies while the Upper Disangs represented peripheral shallow marine deltaic facies.Maturity parameters like Tmax and,sterane and hopane isomerization ratios indicate that the organic matters of the Disang Group were affected by sudden short duration thermal events followed by cooling.This is possible in an active tectonic region like the Kohima Syncline that bears many evidences of thrusting and folding.
