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The geological field visit was carried out along the inner Firth coastline investigating deposits of Hopeman and Burghead sandstone. The geological site visit to the inner Moray Firth region served to be resourceful in learning the various reservoir properties of sandstone formations investigated. This helped in understanding the possibilities of potential reservoirs offshore. The Hopeman sandstone formation which has an Aeolian desert depositional environment is found along the coast line. These formations can form poor to moderate reservoir characteristics. The Burghead sandstone formation investigated has a fluvial depositional environment and has good reservoir properties.
The Moray Firth is a triangular bay along the North-East coastline of Scotland between latitudes of 57' N and 58' 45' North and longitudinally 2' w and 4' w.
Firstly, it is to be understood that during the Permian Scotland was part of a large continent known as the Pangaea and the location of Scotland could have been along the equator where the current Sahara or Africa is located.
The Geological site visit included formation of Permian and Triassic age rocks. During the Permian and Triassic age a desert environment was present in Scotland and the Moray firth region was occupied by dune fields and wadi bound deposits of rivers. Well rounded sand grains and wind made pebbles also csn confirm desert environment on land. The moray firth basin was surrounded by desert uplands .During the late Permian these desert regions were flooded due to rise in sea level by melting of the polar ice caps.
During the early Triassic and late Permian, the basins were filled up with evaporated and detritus by erosion from highlands and continental condition were formed again. The tectonic activity re activated which led to deposition of sediments in the moray firth area. The North East trending faults east of the great glen fault which could have been active during the old red sand stone deposition, and later in the Permian were re-activated to form the Jurassic-Cretaceous Inner Moray Firth Basin.
The subsidence and faulting that occurred to formation of the Permian and Triassic resulted in large depositional basins which are present offshore. The subsidence of the basin is controlled by tectonics as well as by weight of the overlying sediment and water. The amount of sediment deposited depends on the erosion of rate on shore and amount of rainfall received.
The tectonic settings resulted in the formation of three main regional basin the Northern Permian basin, the southern Permian basin and the inner moray firth. The inner Moray Firth was stretched by tectonics and was sinking rapidly filling with desert sediments in the upper Permian. The later Jurassic extensional axis was to the east and west at the Atlantic opening. The axis of extension in Permian was east west and Jurassic was north south.
The upper Permian is known as Zechestein ,this is due to the fact that during the upper Permian the sea level had increased which resulted in an inflow of seawater..This increase in sea water levels was a result of melting of ice caps around the South Pole. An arid environment existed this caused evaporation and resulting deposition of Zechestein salts.
The source for this gas formation is coal deposits of the underlying carboniferous formation.The seal rock was formed by the Zechestein sequence .In the northern Permian basin roligened sandstones form oil bearing reservoirs with source rock being the Kimmeridge clay.
Objective of the field visit:-
The objective of the geologiacal field visit was to understand the environment of deposition and to correlate with formation offshore. This would help in identifying the possibile occurrence of a potential reservoir in the offshore subsurface and understand the depositional environment of these reservoir rocks ,its reservoir characteristics like porosity, permeability, mineral composition etc.The inderstanding of all these information could give a understanding about the potential of a reservoir buit up.
Geologiacal site 1(a) Hopeman sandstone formation
This outcrop is located along the silver sand beach coastline at Lossiemouth below the Covesea lighthouse and is part of the hopeman sandstone formation. The rock age is Permian around 255 Ma.It has a light brown, reddish orange colour,The reddish brown colour is due to the formation of iron oxide formed from mineral hematite during diagenesis,due to the breakdown of iron bearing detrital minerals thatgot oxidised toits ferric state. This kind of occurrence is common below the surface of desert environments.
Tectonic uplift and erosion are the two main factors that lead to the formation of this rock type at the present location. The uplift settings were mainly seen during the upper Jurassic and lower cretaceous .This could be due to the opening of the North Atlantic ocean plate.
Weathering was seen on the rock surface, honey comb type weathering due to presence of carbonates .This is known as carious weathering. The carbonates in the rock combines with rain water to form carbonic acid which would lead to such weathering. This was confirmed by doing an acid test using hydrochloric acid.
The sediments are primarily
Lower part of cliff contains lumps or pebbles which can be associated with barium sulphate cementing. The other cementing type is the carbonate cementing.
Various fractures are seen along parts of the rock. This was caused due to joint formation when deposited in the subsurface and as it uplifts to the surface the joints expands and opens up.
The Net to gross is very high no shale /mud were seen at this outcrop.
Wind direction was from was from North East
The rock characteristics investigated include:-
a) Rock age:- Permian age rock formation.
b) Depositional Environment:- Aeolian Desert environment.
c) Rock type: - Sedimentary rock relatively friable and soft.
d) Structural type:-Cross bedding with bed thickness of 2-3 m. very low angle of dip 10-15Ëš towards North (Dune bedded sandstone)
c) Grain size: - Fine grained.
d) Grain shape: - Rounded.
f) Permeability: - Good permeability.
g) Porosity: - Moderate to good porosity.
i) Sorting: - Well sorted.
j) Mineral Composition: - Quartz, Barite, Iron, Calcium carbonate
Geological site 1b):- Hopeman sand stone formation located at Hopeman East beach(Daisy rock)
The rock has erosion patterns caused by flow of water through the joints. Syn- sedimentary deformation are seen in this rock caused an ' s' shaped pattern on the rock surface. These dunes fields were flooded by flash floods and caused the dunes to collapse and the dunes slumped .A occurrence of this pattern is possible only in the Zechestein due to rise in sea level. And similar disturbed level is seen in the north sea and can be used to correlating wells.
Rock type :- Sedimentary rock ,not friable ,hard rock type
Rock age:- Permian age
Depositional Environment:-Aeolian Sandstone possibly an arid desert environment.
Grain size-medium grade sand
Structural type or Facies:-Cross bedding
Sedimentary Structure:- cross bedding with an angle of dip towards north-west inclination
Sorting- well sorted
Porosity :- Poor
Colour:- Light brown
Net to gross:- High
This is a poor reservoir rock.
Geological Site :-1c) Hopeman Sand stone formation located at Hopeman East beach(50m East of daisy rock)
The directional setting of this formation shows the sand dune to be the bottom of a slip face of the sandune formation .The slip face is angled downward towards the sea which can be inferred that the possible wind direction was towards North.This can help us co relate that this kind of formation could be possible offshore as well with the dune that formed during the Permian. This is a poor reservoir rock.
The aeolian sand dunes were flooded with water during the Zechestein .This resulted in silica cementing .The tectonic settings resulted in uplifting ,fractiure formation and resulted in the formation of step like structures seen in the present time.
The dune shows two types of cross bedding due star dune formation This is a upper Permian age rock.Its a sedimentary sandstone formation
Rock type:-Sedimentary Sandstone with presence of quartz
Grain size:-Medium grade sand
Cementing:-Quartz cementing ,very hard to undergo weathering
Depositional Environment:-Aeolian Sand dune formation
Colour:-Reddish orange due to the presence of iron oxide
Net to Gross:-low
Geological Site 1d)Hopeman sandstone formation at cove bay
A high angle fault is visible with an orientation of fault in East West directon. The initial extension caused occurrence of the fault with the hanging wall moving down. The tectonic settings later re- activated with compression to the existing fault which caused the hanging wall to move up the this high angle re-activated fault. The hanging wall has an angle of dip towards the south.
The cementing shows variable cementing that is two sections one on top which shows presence of carbonate cementing and below very porous with no cementing features. The fault was hard, very low fluid movement caused this variable cementing. Near the fault plane the formation is hard. This is due to Silica and Iron oxide cementing
In the cave the fault plane has a broken up black material, which is natural glass formed by frictional kinetic energy due to the movement of the planes. Scrap marks caused due to movement of plane during fault formation can be seen near the cave. This is known as slicken sides.
Lines or fractures which are cemented and shows fluid leaking can be seen near th close proximity of the fault and decreases ahead of the fault.
This is an impermeable fault and has blocked the Hopeman Sandstone formation which explains the less cementing. This caused the rock to be less friable, highly porous and permeable.
On either sides of the fault plane different rock hardness is present with one on the left is harder and the other is relatively soft. The hardness is due to the cementing characteristics with iron oxide cement at surface and silica cementing.
The disadvantage of this rock type as a reservoir rock could be that compaction of the reservoir would occur due to unconsolidated sand. This hindrance can be avoided by using a sand screen to prevent the accumulation of sand in the annulus while drilling and use of gas injection system to prevent the collapse of the reservoir. Very good reservoir rocks.
Rock type:-Sedimentary friable rocks surface on the right side of the fault plane with a harder surface on the left
Rock Age:-Upper Permian
Depositional Environment:-Aeolian Sand dune formation
Structural type:-Low angle cross bedding
Grain Size:-Fine to medium
Net to gross:-Very high
Geological Site: - 2)Burghead sandstone formation
The burghead formation is located located 7-8 km west of hopeman town at the Burghead harbour..This formation is Triassic in age, The layering of different type of deposits is seen with the presence of pebbles or conglomerates, shale which is olive green in colour with presence of caly or silt,fluvial cross bedding at the very bottom of this formation,and an erosional channel at the top was visible which conforms the fluvial environment.
The deposition of mud could be due to the drying up of a river/ puddle causing this depositional region to dry up later when the water level is low. Later when the river level rises the dryed up mud gets carried away by the river. This is know as rip up clasts.
This type of depositional layering can be caused by the physical force of the river channel to cause such a depositional environment.
Also less tectonic activity and less fractures are senn int his formation.
Rock type:-Sedimentary sandstone friable in nature
Sorting :-Medium sorting/Poor sorting
Grain size:-Coarser to larger grain pebbles
Grain shape:- Rounded in smaller sized pebbles to angular in larger size pebbles
Depositional environment: - Fluvial environment
Structural type:- Cross bedding
Cementing:-Silicate Very less cementing
Net to gross is high
Porosity: - Good
Permeability: - High