It is now recognised that deltaic and associated sands, together with porous marine limestones, form the vast majority of the reservoirs in the major accumulations of hydrocarbons throughout the world.
The source of the hydrocarbons is now thought to be kerogen which is generated from the organic content of principally marine shales which are formed in or near the continental shelves.
The Trinidad area contains several sedimentary sub-basins, most of which consist in large of deltaic and associated sediments. These sediments, like most of the ancient deltas of the world, contain major reserves of oil and gas. Other less important reserves should occur in sporadic (time-wise) porous limestones.
The total probable reserves of the Trinidad areas exceed 7 billion barrels of oil and 35 trillion cubic feet of gas.
Current thinking assumes that in large part petroleum is generated in marine shales which are relatively rich in organic content Briefly, the organic content is changed under reducing conditions to kerogen or a kerogen-like substance which is the presursor of petroleum and petroleum like products. The kerogen petroleum and petroleum like products. The kerogen under the correct conditions of pressure and temperature, is then converted to hydrocarbons. These conditions are generally met if the shales are buried to several thousand feet in an area of normal temperature gradients. The hydrocarbons are generally thought to migrate soon After generation into whatever reservoir rocks lie nearby.
In broad terms two types of reservoir rocks trap over 95% of the known hydrocarbon accumulations. These are sandstones and limestones.
The majority of these accumulations are found to be those associated with deltas and porous limestones. The reasons for this are:
They generally interfinger with organic marine shales which act as source rocks and
They are broadly either lenticular in nature or affected by growth faulting and/or folding caused by plastic flow of underlying shales, i.e. shale diapirism plastic flow of underlying shales, i.e. shale diapirism (in the case of deltaic sands) so that traps exist even before hydrocarbon generation and migration begin.
In other words, all the pre-requisites for hydrocarbon generation and trapping are generally present in areas of deltaic deposition or where limestone deposits interfinger with highly organic marine shales.
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