KWAME NKRUMAH UNIVERSITY OF SCIENCE AND TECHNOLOGY.
COLLEGE OF ENGINEERING.
DEPARTMENT OF PETROLLEUM ENGINEERING.
NAME: ADELI JOEL
INDEX NUMBER: 8182919
REFERENCE NUMBER: 20689954
GEOLOGICAL FIELD TRIP REPORT
January 31, 2023.
Table of Contents
INTRODUCTION .....................................................................................................3
DISCUSSION ............................................................................................................3
RESERVOIR ROCKS ...........................................................................................3
Depositional Environment .....................................................................................4
SOURCE ROCK ....................................................................................................6
Depositional Environment .....................................................................................8
CONCLUSION ..........................................................................................................9
Figure 1; Elmina Sandstone. ......................................................................................4
Figure 2 Elmina Sandstone Outcrop ..........................................................................5
Figure 3; Takoradi Shale Basal Unit..........................................................................6
Figure 4; Takoradi Shale ............................................................................................7
INTRODUCTION
This report is a product of an account of a geological field trip at Takoradi visiting
places such as Elmina, Cape Coast, Takoradi and New Takoradi. The sole purpose
of the field trip is to understand the geological requirements for the formation of
petroleum. This process requires a source rock rich in organic matter with sufficient
oxygen, example: shale. A reservoir rock with suitable porosity; to enable fluid
storage, permeability; to allow for the transmission of fluids and saturation. A typical
required reservoir rock is sandstone. The presence of a cap rock or seal to trap fluids
and prevent upward buoyant fluid movement, which has to be ductile so it can break
while expanding. An example of a cap rock or seal is shale and salts. Upon coming
across an outcrop (a rock formation which is visible on the surface), the rock
formation is observed from a distance to ascertain the megascopic features of the
rock before moving closer to check the layering i.e. Thinly layered, medium or
massive layered. The colour of the formation is also noticed before a portion of the
rock is broken to see the inherent colour of the rock, this is because factors such as
environmental condition, human activities and that of other life forms such as fauna
and flora may affect the colour of the rock hence giving it a different coloration on
the surface. The places visited covers the Saltpond basin made up of a succession of
rocks. The structure of the basin is characterized by multiple faulting, which has
resulted in a complex set of horsts (a raised fault block bounded by normal faults),
and grabens (an elongated block of the earth’s crust lying between two faults and
displaced downwards relative to the blocks on either sides, as in a rift valley).
DISCUSSION
RESERVOIR ROCKS
Elmina Sandstone, which overlies the Ajua Shales, is one of the thickest formation
within the Sekondian, which were formed during the Ordovician to the Cretaceous
period. It is the dominant rock type from Takoradi to Elmina having more than 30%
of feldspar minerals present making it an arkosic sandstone It is uniform medium –
grained arkosic sandstone rich in mica and feldspar coupled with massive and poorly
sorted mineralogy. It is characterized by chocolate brown to purple colour. It is
coarser grained towards the base of the formation but thin – bedded and somewhat
shaly at the top. The homogeneity and preservation of unaltered feldspars the rock
suggest their derivation from an area devoid of vegetation and deposited over a wide
area.
Depositional Environment
The depositional environment of the Elmina Sandstone is made up of unaltered
feldspar is indicative that the sediments may have come from a topographic high
which suffered very little weathering in a probable arid condition. The somewhat
reflective nature of the sandstone imply the presence of mica with the feldspars
responsible for rock colouration. The Elmina Sandstone ( Late Ordovician – Early
Silurian) is followed by the Takoradi sandstones ( Devonian) , Takoradi Shales (
Middle Devonian – Early Carboniferous , Efia Nkwanta beds ( Late Carboniferous
– Permian ) , Sekondi sandstone ( Triassic – Early Jurassic ) and Essikado beds
(Lower Cretaceous sediments) from oldest to the youngest.
Figure 1; Elmina Sandstone.
Figure 2 Elmina Sandstone Outcrop
Takoradi Sandstone, which has the lower part sandy shale, shaly sandstone and thin
bedded finer grained micaceous sandstone. Above this is thick – bedded, medium
grained, well-sorted and highly friable sandstone. It displays planar to tubular cross
– bedding dipping to the southwest. It is predominantly quartz, which are angular in
shape. Accessory minerals include zircon, apatite, magnetite and tourmaline.
Trapping is both structural (fault bounded blocks) and stratigraphic (sandstones
interfingering into shales) with sealing provided by the Takoradi Shale formation. It
is the reservoir rock of the Saltpond field. (ocw.tudelf.nl).
Figure 3; Takoradi Shale Basal Unit.
SOURCE ROCK
Takoradi Shale composed of hard, compact, black or dark gray fissile shale or sandy
shale, rich in carbonaceous matter. It is associated with Sulphur mineralization
occurring as pyrites in disseminated forms or as spherical nodules. The top of the
formation have large discoidal nodules of compact fine granular, gray siderite or
clay ironstones. It is the source rock of the saltpond field. Gas would be seen coming
out of these shales then hydrochloric acid was added to them. Fissile shales are
another name for these shales. Clay – sized particles were found in smaller samples
of the outcrop lower section. The particles were extremely minute with carbon as
part of the minerals.
The formation of the shales occurred during the Trans – tensional movement during
the separation of Africa and South America and opening of the Atlantic in the
Albian. Active rifting and subsidence during the period and other prevailing
conditions at the time was ideal was the deposition of of shales, thus thick organic
rich shale was deposited in the Cenomanian and Turonian. (epa.gov.gh )
The Takoradi Shales were found to be appropriate source rocks; however, this could
only be confirmed once the entire organic carbon content was established. To
confirm the nature of the source rock, the total organic content must be known. Poor
source rocks have total organic carbon (TOC) to be less than 0.5 percent. Source
rocks have a (TOC) of between 0.5 and 1 percent are regarded to be good. In
addition, those with a TOC of 2% to 4% are regarded as excellent source rocks.
Source rocks with a TOC of more than 4% are regarded are excellent. The Takoradi
shales characteristics. Many geological features are planes or lines, and their
characteristics is used to describe their orientation. These positions are defined from
two perspectives; strike and drip. When looking at the outcrop, it was clear that
tectonic forces had worked on it because it did not conform to the law of
horizontality. As a result, the rock’s strike and dip had to be attained. The direction
of the line created by the intersection of a fault, bed, or other planar feature and a
horizontal plane in geology is called strike. The position of linear structural elements
like faults, bed, joints and folds is indicated by the strike. A planar feature’s dip
(angle and direction) is the angle at which it is inclined to the horizontal plane,
measured in a vertical plan perpendicular to the feature strike.
The vertical angle formed between the horizontal plane and the axis or line of
maximal elongation of a feature is known as the plunge. The dip is measured along
the limbs, whereas the plunge is measured along the axis of a fold. Folds can be
classified using the plunge. The strike and dip can also be utilized to indicate the
direction of youngling.
Figure 4; Takoradi Shale
Figure 5
Depositional Environment
The depositional environment of the Takoradi Shale is highly fossiliferous and has
provided fossils of diverse groups. The Palynomorph (trilete spore and acritachs)
have a lot of implication for its past environment. The spores recovered are similar
to those that inhabit fresh water swamps in humid climates. The acritarch though
few as compared with the spores also confirms a near shore or brackish water
influence. The siderite is indicative of a reducing environment maintained by organic
matter, such as in deltaic swamps or mangrove with Carboniferous age assigned to
the sediments.
TRAPS
A trap is the definition for the configuration of reservoir seals or cap rock preventing
upward buoyant movement of hydrocarbons and halt their migration. Structural and
stratigraphic traps are the basic types of traps. When the reservoir rock and the
overlying seal are distorted by bending or faulting, structural traps form which
occurs over a long period of geologic time after sediment deposition. Trapping is
both structural ( fault bounded blocks) and stratigraphic ( sandstones interfingering
into shales) with sealing provided by The Takoradi Shale formation is a typical trap
as it provides sealing through both structural ( fault bounded rocks ) and stratigraphic
(sandstones interfingering into shales ) hence trapping the Takoradi Sandstone; a
typical reservoir .
CONCLUSION
The purpose of the field trip of the geological trip, which is to understand the
geological requirement for the formation of petroleum, i.e.; source rock with
sufficient organic matter buried at great depths, reservoir rock with good porosity,
permeability and saturation and cap rock / seals to prevent further migration of
hydrocarbons was met. The trip also helped broaden the scope and enabled students
relate courses taken such Basic Geology and Regional Geology as lecture images
were seen physically enabling students appreciate geology better.
The Western basin is divided into two section; a rift part with shallow marine to
continental deposits, and a thick upper cretaceous drift layer with basin floor fans,
stratigraphic traps and channel systems. The roundness of rocks is determined by
their provenance , journey travelled and manner of transit. Information on
depositional settings and even mineralization sites can be found in geological
structures.
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Walking Schemes, 3rd Edition Mountain Training
 https: // ocw.tudelft.nl
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Mapping in GIS. Handbook of Exploration and Environmental Geochemistry
11. Elsevier, 360 p.