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Role of Time and Temperature in Petroleum Generation and Oil Properties of Mehsana Block, Cambay Basin, India

Soumitri S. Dash and Apoorva Sahoo
Sub Surface Team, ONGC, Mehsana, India

More than 26 small to medium size oil and gas fields have been established around Mehsana area in the Ahmedabad-Mehsana tectonic block of the Cambay basin.The oil properties in the Mehsana block widely vary from light oil in the fields of Linch, Nandasan,Langhnaj and Sobhasan to very heavy oil in the fields of Santhal,Balol & Lanwa.It is observed that properties like oil API, GOR consistently decrease from Sobhasan,North Kadi field in the south through Santhal, Balol to Lanwa field in the north within a distance of about 40 kms.Viscosity of oil gradually increases from 10cp in North Kadi through 450 cp in Santhal,500cp in Balol to 1250cp in Lanwa.GOR gradually decreases from 30 in the south to 6 in the north.

It has been well established that both time and temperature play a crucial role in the generation and the properties of hydrocarbons.In this study,the systematic variation in oil properties in the Mehsana block was analyzed using Time-Temperature Index(TTI) by considering a number of deep wells from south to north.TTI values of each were calculated using N.V.Lopartin’s method.TTI values vary systematically from 1064 in the south to 16 in the north thus explaining the occurrence of lighter hydrocarbons in the south and heavier in the north.Systematic integration of TTI values along with other geochemical parameters like Vitrinite reflectance leads to the conclusion that the onset of generation of hydrocarbons probably initiated within the Olpad and Younger Cambay Shale at a temperature range of 100-110 deg C.The TTI values also explain the absence or marginal occurrence of hydrocarbons in some of the wells considered in the study.The present study will help in understanding the hydrocarbon occurrences/accumulations and add value to the exploratory efforts being undertaken in this Basin.

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