Oil Gas Deposits in the Deltaic Sequence of Sediments in Cambay Basin and Upper Assam Valley in India

Two important hydrocarbon bearing basins of India, namely (i) Cambay Basin, and (ii) Upper Assam Valley, are producing from typically deltaic sequences.

In the Cambay Basin the hydrocarbon pools are mainly associated with the Ankleswar and the Mehsana-Ahmedabad-Nawagam upthrown blocks and are located where reservoir rocks are developed over synsedimentary structures. These synsedimentary structures are related to faulting movements and growth of incipient diapiric structures.

The source rocks are the Hazira Shales, and the characteristics of the oil and sedimentation waters indicate a more marine influence than in the northern delta. The oils are lighter, less waxy and the associated waters are calcium-chloride type, whereas the oils of the northern delta are waxy and are having high specific gravities, associated with sodium bicarbonate waters as characteristic of oil generated in a fresh water environment. The most important character of the hydrocarbon accumulations in the proto-Narmada delta, however, is that a clear structural control exists, with the structures running east-west, i.e., parallel to the Narmada fault and other synsedimentary faults in this area.

In the Upper Assam Valley, a regional slope towards southeast prevalent during Paleocene and Oligocene was largely responsible for the coming into existence of a drainage pattern with streams flowing SE. The Brahmaputra arch itself took shape later as a result of a strong opposing tilt towards northwest during the Upper Miocene. A nut-cracker type of movement which resulted due to forces from NW and southeast acting simultaneously as a result of Himalayan orogeny resulted in discrete piano-key type of block movements, in the intervening basement block with folding in the overlying sequence of sediments. These structures have largely controlled the distribution of hydrocarbon pools. Simultaneously with the structures, the stratigraphy of the sequence has played an important part in the distribution of hydrocarbons.

The structures are largely of idiomorphic type. The accumulations both in the Barail and in the Tipam sequence are controlled by the presence of (1) thick reservoir rocks and (2) reliable cap rocks. The reservoir rocks both in the Barail and Tipam sequence which are developed on the southeastern flank of the Brahmaputra arch show a thinning down dip, while the cap rocks in the same section show a thinning updip. The development of the coal-shale sequence in the case of Barail and the development of Girujan clay in the case of the Tipam respectively, on the southeastern flank of the Brahmaputra arch, thinning updip in both the cases has controlled the northwestern limit of the accumulations.

The time of the formation of the structure, i.e., immediately after deposition of Barail and Tipam sequences during the Miocene and the post-Miocene respectively, was quite early and either largely preceded or was simultaneous with the time of the migration of the hydrocarbons. This is considered to be a very important factor which favoured the accumulations.