GEOCHEMICAL STUDIES FOR PETROLEUM FROM MULTILAYERED RESERVOIRS OF NARIMANAM FIELD, CAUVERY BASIN, INDIA

Abstract

Cauvery Basin is a pericratonic basin which came into existence due to rifting in the Gondwana land during Late Jurassic-Early Cretaceous. The basin is characterised by NE-SW trending basement horsts and grabens. The Narimanam oil field lies on the southern side of the NESW trending Karaikal Horst which is flanked in the south by Nagapattinam depression and in the north by Tranquebar depression. Petroleum hydrocarbons are found in multilayered sandstone bodies of the Niravi Formation of Oligocene age and the Kamalapuram Formation of Eocene/Paleocene age. Although many geoscientific studies have been carried out in this area, only limited work has been done on the geochemical aspects related to the oils discovered in Narimanam field. The present work aims at carrying out detailed geochemical studies on source rock evaluation, generation and expulsion of oil, oil to oil and oil to source correlations and assessment of amount of hydrocarbons generated and possible locales for exploration. Tertiary and Recent sediments directly overlie the Archean basement (granite gneiss) in the Karaikal Horst area. However, the sediments deposited in the basement depressions around this horst range in age from Cretaceous to Recent. Petroleum hydrocarbons occur in the sandstone bodies of mainly deltaic environment on the margins of the shallow marine domains. The study is based on analyses of about 2300 samples collected from 23 wells in this area, covering Karaikal Horst, Nagapattinam depression (NGT DP), Central western sub-basin (CWSB), North western sub-basin (NWSB) and Tranquebar depression (TQB DP). The source rock studies were carried out with a view to determine the abundance of organic matter (total organic carbon percent, TOC), quality of organic matter (using maceral constituents, Rock Eval parameters and biomarker composition) and the thermal maturation of organic matter (using Tmax; thermal alteration index, TAI; mean random vitrinite reflectance, VRo% and biomarker epimerisation reactions). Source Rock Evaluation The argillaceous sediments from the Miocene, Oligocene, Eocene and Paleocene of Tertiary sequence have sufficient organic matter (TOC >0.5%) but the values of Tmax (< 430°C), TAI (< 2.5) and VRo (< 0.5%) indicate that the organic matter in these Tertiary sediments is immature and has not generated petroleum hydrocarbons commercially, though the Narimanam field, located on Karaikal Horst has oil bearing sandstone bodies belonging to Eocene and Oligocene. It is thus inferred that the oil was generated somewhere else, possibly by older Cretaceous sediments. The source rock analysis of the argillaceous sediments of the Upper Cretaceous belonging to Porto Novo Formation (Campanian to Maastrichtian), Nannilam Formation (Santonian to Lower Maastrichtian), Kudavasal Formation (Coniacian to Santonian) and the Bhuvanagiri Formation (Cenomanian to Turonian) indicate that they are immature (VRo< 0.5%, TAK 2.5 and Tmax<430° C) to generate petroleum hydrocarbons commercially. However, the shale units of Lower Cretaceous, Sattapadi Formation (Albian age) and Andimadam Formation (pre Albian to Early Albian), are found to have adequately matured organic matter (Type II and Type III) capable to generate petroleum hydrocarbons commercially. Sattapadi Formation (Albian age) The Sattapadi Formation, 80 to 100m thick, consists of mainly shale units with sandstone. The TOC content varies from 0.6 to 6.1%. The quality of organic matter is of mixed type. In the northern side of Nagapattinam depression and Central western sub-basin, TOC is higher and the organic matter is relatively richer in kerogen of type II. The terrestrial organic matter increases from Nagapattinam depression in the south to Tranquebar depression in the north through Central western sub-basin and North western sub-basin. The sediments of Sattapadi Formation have attained early to moderate maturity levels as indicated by various maturity parameters (TAI, 2.5 to 2.75; VRo, 0.55 to 0.75%). Andimadam Formation (pre Albian to Early Albian) The Andimadam Formation varies in thickness from 300 to 1500m. It consists of sandstone interbedded with shale units. TOC in the shale units of this formation ranges from 1.0 to 2.5%. The organic matter is of mixed type (Type I and Type III). In the sediments of Nagapattinam depression marine input is more in mixed organic matter. The terrestrial input increases from Nagapattinam depression (in the south) to Tranquebar depression (in the north). The shale units in the Andimadam Formation are moderately mature to mature (VRo 0.70 to 0.85%). In view of these findings, it is inferred that only Andimadam and Sattapadi formations of Lower Cretaceous in Nagapattinam depression, the Central western sub-basin, North western subbasin and the Tranquebar depression have generated petroleum hydrocarbons in this part of the basin. Time of Generation and Expulsion of Petroleum The time of onset of oil generation in various parts of the study area was assessed using Waples's TTI and Sweeney and Burnham's Easy%Ro models. The study indicates that: (i) Nagapattinam depression was the first to start generation of hydrocarbons from Andimadam source rocks about 25 Ma. Sattapadi Formation started generating petroleum in this subbasin at around 10 Ma. (ii) In the Central western sub-basin the Andimadam source rocks started generating hydrocarbons around 10-15 Ma and Sattapadi Formation around 5Ma. (iii) In Tranquebar depression, generation of petroleum hydrocarbons from Andimadam Formation started at about 20 Ma and Sattapadi Formation started generation of hydrocarbons 5 Ma. (iv) In the North western sub-basin the Andimadam Formation started generation of hydrocarbons around 13Ma. However, the Sattapadi Formation in North western sub-basin has just entered in generation stage. Considering a minimum vitrinite reflectivity 0.70% VRo (TTI = 20) as the lowest limit for expulsion of hydrocarbons (based on hydrous pyrolysis study), the time of hydrocarbon expulsion from this area was assessed using burial history curves for different areas. The expulsion of hydrocarbons from the Andimadam Formation started about 15-20 Ma and Sattapadi Formation 5 IV Ma in Nagapattinam depression and in the Central western sub-basin, the expulsion started from Andimadam Formation around 10 Ma and from Sattapadi Formation around 3 Ma. In the North western sub-basin the Andimadam Formation started petroleum expulsion around 5-7 Ma and the Sattapadi Formation has just entered the expulsion stage. In Tranquebar depression the Andimadam Formation started expulsion around 15 Ma and Sattapadi Formation around 3 Ma. Oil to Oil and Oil to Source Rock Correlation The studies of °API, group composition, isotopic composition of saturates, Pr/nC17 ratio and GC profiles of oils from Tertiary sequence of the Narimanam field indicate that there are two types of oils in the Oligocene reservoir. One of these two types appears to be similar to one found in Eocene reservoir. The source specific parameters indicate that all oils are genetically correlatable. However, some oils of Oligocene reservoir are biodegraded. The biomarker based parameters namely hopane and sterane distributions also indicate that the oils of Oligocene and Eocene reservoirs are similar and have been generated by the source rocks deposited in bay/estuarine environment with substantial marine organic matter. The investigation of oil to source rock correlation reveals that Narimanam field oils were generated by Lower Cretaceous (Andimadam and Sattapadi formations) sequence in Nagapattinam depression. The organic matter in Central western sub-basin and North western sub-basin is relatively more terrestrial and does not match with these oils . The oils discovered in the satellite fields in the Central western sub-basin show gradual variation in the type of organic matter- more marine in eastern part to more terrestrial in the western part of the sub-basin. These oilsowe their generation to the shale units in Sattapadi and Andimadam formations (Lower Cretaceous) of Central western sub-basin. Thus, the source rock kitchen in the Nagapattinam depression has mainly generated the oils discovered in Narimanam field, located in the Karaikal Horst in the north. However, the oils found in Central western sub-basin have been generated by the Lower Cretaceous sediments in Central western sub-basin itself. The basement faults served as conduits for migration of hydrocarbons in V this rift basin. Hydrocarbons Generated and Expelled The assessed amount of generated hydrocarbons is of the order of 848MMT of which the Andimadam Formation has contributed 692MMT and Sattapadi Formation 156MMT. The expelled hydrocarbons are assessed to be of the order of 587MMT. The estimated inplace oil and oil equivalent gas reserves are of the order of 35MMT. The balance amount of oil in this part of the basin needs to be accounted for by making exploratory efforts in the possible traps along the inferred migration path ways. The prospective areas for future exploration are the updip slopes of Nagapattinam depression, Central western sub-basin, North western sub-basin and Tranquebar depression close to these basement faults which have cut through the petroleum kitchens. Besides these areas, the fractured basement of the kitchens are also important locales of petroleum accumulation. The lensoidal sandstone bodies of prograding deltaic environment as stratigraphic traps and the rollover anticlines adjoining fault zones as structural traps, need to be explored for petroleum hydrocarbons.