Geology and Hydrocarbon Potential of Bombay Offshore Basin

Abstract

ABSTRACT DESCRIPTION : The Western Continental Margin is a passive margin having six sedimentary basins namely Kutch, Saurashtra, Bombay Offshore, Konkan, Lakshadweep and Kerala. Out of the six basins Bombay Offshore is the biggest basin having an aerial extent of 1,20,000 Sq. Km. Bombay Offshore basin which is an intracratonic basin, is the southern extension of Cambay basin and encompasses the Gulf of Cambay and adjacent shelf part of the Arabian sea. It came into existence during Upper Cretaceous time as a result of faulting in the Deccan t['ap basement. This basin is divided into several tectonic units such as Cambay Gulf Shelf to the north followed to the south by Dahanu depression, Bombay Ratnagiri shelf occuyping central and southern part of the basin and Western clastic basin fringing Bombay Ratnagiri shelf. RESULTS: As a result of seismic surveys carried out five different reflectors have been met. They are H-1, H2, H-3, H-4 & H-5. Of these reflectors horizon-5 is confined only to the north and north Western region and probably marks the top of the basement. Geophysical surveys both reconnaissance, semi-detail or detailed carried out have helped in the delineation of big and small structures in the Bombay Offshore basin. The important structures delineated are Bombay High, Panna, Bassein, Dahanu, South Tapti, Heera, Ratna and other small structures indeed, area. A full sedimentary section from Palaeocene to Recent of over 4 Km. resting on trappean basement has been met. In few places Pre-Cambrian basement instead of trappean basement also occurs. The oil and gas bearing horizons of Bombay Offshore basin, range in age from Middle Miocene to Early Eocene and are mostly wackstone/ mudstones. and chalky limestones. Evaluation by drilling of important structures in Bombay Offshore Basin like Bombay High, Panna, Bassein, Heera, Ratna, South Tapti, etc., have resulted in the discovery of big and small oil and gas fields. Besides this oil and gas indications have also been met in several other structures, too. The Bombay Offshore Basin has only Tertiary sediments with Eocene being absent in Bombay High. However, west of Bombay High the Palaeogene and Miocene are represented by a thick sequence of limestones and thin shales having poor source rock characteristics. In the south east, along the Panna-Bassein structural trend, Palaeogene consists of limestone and shale. In the north east, Palaeogene is represented by thick shales, which are considered to be source rock for Bombay High (Miocene oil accumulation), Panna, Heera and Ratna (Palaeogene oil accumulation). SIGNIFICANCE OF SUBJECT MATTER : This paper briefly gives the review of exploration activity in the Bombay Offshore basin, which has been a prolific hydrocarbon bearing province in the Offshore area of India and where big oil and gas fields have been already found.