Assalam o alaikum,
Sir , I have a confusion regarding offshore reservoirs. I want to know how the offshore drilling is feasible as we learned that the thickness of sediments in the oceanic floor is v.less and also the sediments are unconsolidated. And the sediments have less time elapsed for the organic matter to get matured.
You are right that thickness of the sediments below sea floor is very limited in majority of the oceanic areas specially in the middle of the oceans. However, petroleum exploration is usually confined to the shallow to relatively deeper continental shelves straddling along the continents where sediments thickness has been quite adequate to continue petroleum exploration.
There are very large continental areas, specially in the passive margins of the continents such as continental shelves of eastern coast of United States and Canada, western and eastern Africa (specially west and east Africa)eastern South America and north western and eastern Australia where exploration is actively taking place since many decades and thousands of wells have drilled so far with several oil and gas discoveries.
The sedimentary package on the continental shelves are varying in thickness from few kilometers to tens of kilometers and this thickness is quite normal for a source rock reaching to the thermal maturation to spill hydrocarbon. Thermal maturation depends on the present day basin geothermal gradient and the past temperature history which can be determined with the basin modelling technique.
Every offshore area has different thermal and tectonic history which controls the petroleum habitat. The offshore areas where large river prograd their delta system on the shelf are usually found very prolific for oil and gas such as the deltas of the river Mississippi,(USA in Gulf of Mexico) Nile,(Egypt in Mediterranean Sea) Mahakam (Indonesia in Strait of Makassar), Niger (Nigeria in Gulf of Guinea)etc. Delta brings along with it lots of source material that forms source rock and also carries a lot of sand which forms the reservoirs.
As far as the sediments compaction is concerned the shallower and recent sediments are usually unconsolidated due to the less overburden and less time for compaction and for that reason drilling in the shallower section of the offshore wells is not always problem free. However, due to extensive drilling activity throughout the world people now know very well about the foreseeable drilling problems in every area of the world so they can plan well in advance accordingly to meet all challenges of the complicated geology and drilling.
These days, due to advanced drilling technology drilling time has been reduced considerably and whole operation stays safe accident free but still serious accidents happen with time to time in various part of the world but safety record of offshore drilling has been generally outstanding and every company invest massive funds for safety and to stop accidents as slight negligence may cost them almost everything what they got.
Not all offshore sediments are unconsolidated. Rocks strata after certain depth are usually consolidated their hardness and density is comparable as any hard and dense rock on land. For example, limestone are generally firm and consolidated but keep in mind that rock age is very important factor controlling the compaction as the younger rocks (Tertiary)are less compact than ancient rocks.(Mesozoic and Paleozoic).
Hope, this answer helps you to get away your confusion.