Geological periods indicated by geological time scale Essay
Geological periods indicated by geological time scale, 494 words essay example
Essay Topic: time
Geological time scale indicates 13 geological periods, which are in a 4000 Ma range. These periods are Quaternary, Neogene, Paleogene, Cretaceous, Jurassic, Triassic, Permian, Carboniferous, Devonian, Silurian, Ordovician, Cambrian, Proterozoic, Archean and Hadean. The Machar field shallow sections have Quaternary, Neogene, Paleogene and Cretaceous periods. In this study, especially, Neogene and Quaternary which are main interests may have potential shallow gas accumulations.
This period has Holocene and Pleistocene epochs. Quaternary and Neogene periods have a boundary which formed during a climatic deterioration. This is taken place in about 2.6 Ma ago. In between quaternary and Neogene, biostratigraphic life covered foraminifera.
Quaternary part of the North Sea Basin illustrates shallow marine, fluvial and deltaic facies with moderated glacial formations from Early Pleistocene to Late Pleistocene. Besides, continental shelf is surrounded by continentalscale glacial formations. Quaternary period have glacial and deltaic sections, which is understood due to seismic reflections. They show internal reflection patterns and noticeable geometries
This period is located under the Quaternary period and has two epochs Pliocene and Miocene. In this period erosions and uplifts took place and a great deal of sediment is deposited. These sediments are located on deltaic systems by building progradations (Evans et al., 2003).
Neogene period is lasted 20.4 Ma and its boundary at the bottom is Paleogene. In the North Sea, Zechstein Salt diapirs evolutions affected Neogene period by piercing. As a result of this, fluid flow started and shallow gas accumulations occurred in the North Sea. This is taken place in Miocene, Pliocene and Pleistocene sediments. Besides, related faults cause a number of leakage paths in some exploration fields with salt diapirs. Therefore, gas charged sediments and gas chimneys are abundant in the North Sea ( Evans et al., 2003).
Machar field has two reservoir rocks the first one is sand and the second and main one is chalk. The majority of the hydrocarbon was deposited in the fractured chalk of the latest Cretaceous Turonian to Earliest Paleocene Danian with little sandstones (C&C Reservoirs, 2009). The Machar oilfield lies on the upper Jurassic and Lower Cretaceous marine mudstones. In the Machar the chalk section is divided into 3 formations Tor, Hod and Ekofisk. Tor formation is the main one its thickness is 341 ft and has coccolith fragments with pelagic bioclastic shales (C&C Reservoirs, 2009).
The structure of the Machar oilfield was formed in the Cretaceous and Paleocene within salt diapir effects. The structure formed during the Triassic, when salt pillow, which is on the graben margin was stimulated by fault prevailed deposition. It kept growing in the Triassic and Jurassic. Then, sediments eroded over it. In the Paleocene and Cretaceous salt stopped to grow close to sea bed (Foster et al., 1993). During the Late Cretaceous chalk section was formed around the salt and its thickness is approximately 1000m. Probably, the salt diapir completed its form in the Paleocene. Besides, regional drawdown started during the late Miocene covered Machar and led high pressure with diapir caprocks diagenesis building the Celestite formation during the late PlioceneMiocene.