Field Development Geology: Low Permeability Reservoirs, Diagenesis, Hydrocarbon Traps

This article discusses low permeability reservoirs, diagenesis, and hydrocarbon traps in Field Development Geology. It explains the factors contributing to permeability and the mechanisms of hydrocarbon migration and accumulation. The article also covers the types of traps and their classifications, including structural and stratigraphic traps, and salt domes and diapirs.

Field development geology1
FIELD DEVELOPMENT GEOLOGY
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Field development geology 2
Question 1: low permeability reservoirs
Carbonate rocks produce about 40% of all gas and oil and comprise several of the
reservoirs in Western Canada and the huge reservoirs in the Middle East (Burnside and Naylor
2014, pp.2). Even though there are numerous types of carbonate reservoir, most fall with the
following groups; Grainstones with enhanced primary porosity, reefs, and carbonates slope
deposits, and chalks. The two principal properties needed from a rock to be viable reservoir are
permeability and porosity (Glover 2010, pp. 247). Permeability is the rock capability to transmit
a fluid and it depends importantly on the link between the pores. Darcy's law institutes the basic
connection between flow rate, pressure and permeability (Buiting and Clerke 2013, pp.267). One
of the important aspects of carbonate deposition is that of material being biogenic. Reservoirs
are deposited on shale’s, and quite often on waters that do not have high mud supplies
(Cuthbertson, Ibikunle, McCarter and Starrs 2016, pp. 868). As a consequence, carbonate
reservoirs have general low clay contents than sandstones.
Intuitively, it is apparent that permeability will rely on absorbency; greater the
permeability the higher the porosity (Ordonez-Miranda and Alvarado-Gil 2012, pp. 6736). But,
permeability also relies on the linkages of the pore spaces. The connectivity of the pores hinges
on several elements such as diagenesis, compaction, scope and nature of grains, and grain size
dispersal (sorting) (Glover and Walker 2009, pp. 18).

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Low permeable
carbonate reservoirs
Why they have low permeability Factors contributing their permeability
Chalk Fine grained nature of rock, less
rigid and undergo more compaction
Have sheet like deposit of great
lateral content
Although fine grained , they are
relatively stable during diagenesis
Less prone to early exposure to the
meteoric water during period of low
sea-level
Early oil migration, over pressure,
chalk lithofacies, burial depth, grain
size and mud content
Re-deposited chalk tend to form a
thicker masses
Diagenesis, compaction, scope and
nature of grains, and grain size
dispersal (sorting)
Grainstones with
enhanced primary
porosity
Composed of sand–size grains
The linkages of the pore spaces
Form pro-grading sheet or linear bars
Diagenesis, compaction, scope and
nature of grains, and grain size
dispersal (sorting)
Reefs They have vertical permeability
They can connect isolated porous
and permeable zones
Form a massive ribbon or sheets
Diagenesis, compaction, scope and
nature of grains, and grain size
dispersal (sorting)

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Question b
Diagenesis is the word utilised for all the modification that sediment experiences after
deposition and prior to the metamorphism change. The diverse procedures that come under the
word are physical, biological and chemical (Collin, Mancinelli, Chiocchini, Mroueh, Hamdam,
and Higazi 2010, pp. 228). They comprise cementation, deformation, compaction, dissolution,
replacement, bacterial action, hydration, authigenesis, and recrystallisation and concretion
growth (Moeck and Beardsmore 2014, pp. 245). The two significant diagenetic processes are
compaction and lithification. One of the key influences of the meteoric-water diagenesis is the
reordering of calcium carbonate by grains dissolution and the substitution of calcium carbonates
as cement in pore spaces. Carbonates which are not lengthily modified during the initial
diagenesis are specifically vulnerable to chemical procedure like grain-to-grain pressure solution
and stylolitization in the course of burial causes comparatively high calcium carbonate solubility.
Dissolution of metastable stages may frequently be choosy, and initially aragonite portions of
shells may be dissolved while magnesium calcites stay complete. Cementation by calcium
cement often happens in the deeper portions of the freshwater aquifers (Goater, Bijeljic and
Blunt 2013, pp. 319). Cementation by carbonate is not limited to the fresh-water phreatic zones.
It usually happens in pores spaces nearly after sedimentation of atoms. This style of cementation
is the route through which reefs are made firm as rocks. The procedure of replacement is termed
as neomorphism which comprises the diagenetic methods in which the mature minerals, whether
biogenic or abiotic in source, are used and in their place concurrently occupied by novel crystals
of the similar polymorph or mineral.
Question c

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Initially, mature hydrocarbon has to migrate out of the source rock. At the time of burial,
the rock becomes compressed and its interstitial liquid becomes overpressured with respect to
nearby rocks that have an extreme permeability, and from which fluid can move with larger
comfort upwards. Hence, a fluid pressure gradient progress between the source and the
surrounding, more porous rocks cause the water, fluid and hydrocarbons to move along the
pressure gradient, normally upwards, although a down migration is probable (Dernaika, Kalam,
and Skjaeveland 2014, pp. 10). This process is what is termed as primary migration and it
generally takes place across the stratification.
Therefore, migration is the movement of gas and oil within the sub-surface. Primary
migration is the initial phase of the migration process which involves the hydrocarbon expulsion
from their low-permeable, fine-grained source rock into a carried bed having much greater
permeability. Secondary migration comprises movement of gas and oil within the carrier bed.
Currently, only three primary mechanisms are given a serious consideration by most geologists:
oil-phase expulsion, diffusion, and solution in gas (Peel 2014, pp. 225).
The importance of diffusion is probably limited to the thin source beds or edge of thick
units. Similarly, it is most effective in immature rocks, where preexisting light hydrocarbon
pinch-out of the rocks before onset of considerable generation and expulsion. The key problem
with the diffusion mechanism is a dispersive force, whereas the hydrocarbon accumulation needs
concentration. Therefore, diffusion mechanism has to be coupled with a powerful concentrating
force to yield accumulation of considerable size (Han, Lee, Lu and McPherson 2010, pp. 7).
Most popular mechanism today is the expulsion of hydrocarbon in a hydrophobic oily
phase. There are three ways in which oil-phase expulsion can happen. One is a result of micro-

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fracturing induced by over pressuring during hydrocarbon generation (Osborn, Vengosh, Warner
and Jackson 2011 pp. 8173). The second way is from very organic-rich rocks before the onset of
strong hydrocarbon generation (Eppelbaum 2017). Finally, oil-phase expulsion can take place
when bitumen forms a steady web that substitutes water as a wetting agent in the source rock.
The final primary mechanism is the migration by molecular solution in water. While the
aromatics are the most soluble in aqueous solutions, they are rare in oil accumulation, thus,
discrediting the importance of this mechanism though it may be locally crucial. It can be termed
that under compaction is important for primary migration. It will assist preserve the source rock
permeability to a greater extent than in equilibrium state, while reaching temperature appropriate
for considerable hydrocarbon generation.
The process where hydrocarbons move along permeable and porous layers to its final
accumulation is referred to as secondary migration. The mechanism is completely governed by
the buoyancy forces and much less controversial than primary migration. The buoyancy forces
are proportional to the differences in density between water and hydrocarbon. The key conduits
for secondary migration are permeable sandstones beds and unconformities (Velaj 2015, pp.
125).
Question 2
Hydrocarbon traps from where the permeable and porous reservoir rocks such as
sandstones and carbonates are surrounded by the rocks with little permeability that have ability
of averting the hydrocarbon from more upwards movement (Rashid, Glover, Lorinczi, Collier,
and Lawrence 2015, pp. 150). Thus, a trap has the function of allowing entry to hydrocarbon and
to obstruct their escape. Typically, low permeable rocks are compacted evaporates, shale's and

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tightly cemented carbonates and sandstone rocks (Han et al. 2018). If the upwards loss of
hydrocarbon is less than the provision of hydrocarbon from the cradle rocks to the reservoir, the
hydrocarbons may still accrue and create a trap (Roberts and Bally 2012, pp.43).
Traps are normally categorised according to the mechanism that processes the
hydrocarbon buildup. The two chief classes of traps are those that are created by (structural
traps) structural distortion of rocks, and those that are connected to diagenetic and depositional
sorts in the stratigraphic traps (sedimentary sequence). Several reap result from both combination
of above factors. For instance; stratigraphic pinch-out that is combined with tectonic tilting.
Other traps result primarily from fracturing to hydrodynamic processes (Al‐Qayim 2010, pp.
390).
Salt domes build when salt is less dense than the superimposing rock, and the salt travels
gradually upwards owing to its buoyancy. Thus, for the above to occur, there ought to be a
slightest burden and the salt deposit width must be more than or approximately 100m. The
upward drive of salt through the sedimentary strata and linked distortion is signified as salt
tectonics or halo kinetics (Archer, Alsop, Hartley, Grant and Hodgkinson 2012, pp. 2). It is
worth noting that the movement may proceed for many hundred million years.
Salt configurations or diapirs are type of geological distortion buildup from the travel of
mudstone, salt rock and other rock whose masses are lower than superimposing rock under the
regulation of buoyancy/ gravity and regional strain. Structural combination and growth fault
patterns generated from salt rocks and structures are important aspects in governing the type and
distribution of post-salt turbidities. Salt move led to various salt deformations which has an
influence on sedimentary sand dispersal which forms diverse structural and litholigic-

Field development geology 8
stratigraphic traps (Bowman 2011, pp. 97). It is worth noting that compressional and extension
ones are favourable for the hydrocarbon accumulation. As a result of salt piercement, covering
strata are likely to create vault traps. Thus, salt piercement assists oil and gas under the salt rock
to travel up to reservoir rocks along the fissures, ensuing hydrocarbon buildup in large measure.
Salt diapirs are simple to build unconformity assemblies which offer space for hydrocarbon
accumulation. When the diapir is further established, it will fall and change into a coarse flexible
deposit, which is valuable reservoir rock. In case there is a compact cap rock above, it would be
a perfect trap.
Salt edifices are favourable for the creation of sub-basin which are dispersed in
extensional and compressional parts, and are satisfactory for sandstone sedimentation
(Eppelbaum, Katz and Ben-Avraham 2012, pp. 5). Salt flow forms several arrangements,
lithological-stratigraphy traps and multifaceted traps and also gives numerous intricate fault
schemes which offer excellent ways for vertical short space hydrocarbon migration (Mousavi,
Prodanovic and Jacobi 2012, pp. 244). The salt structure is very solid and soluble in acid, high
viscoplasticity and perfect heat conductivity. Similarly, it enables the oil window to extend
down, so the basis rock is not low-mature or over matures. As a result of salt rock having heat
conductivity, the pre-salt strata temperature is lesser than that of strata rock with no salt rock.
Thus, the lower temperature prohibits the diagenesis of the pre-salt reservoir rock. An important
aspect of salt structure exploration is that the strata at depth over 6000m, they have favourable
permeability and porosity (Dernaika, Kalam and Skjaeveland 2014, pp. 9).
As earlier mentioned, the salt structure is unlikely to grow impulsively from a horizontal
sheet of evaporates due to buoyancy only (Dernaika 2015). Phases of salt diapirs are described as
active, passive and reactive. Reactive diapirs happen in reaction to brittle overburden extension.

Field development geology 9
The action can function irrespective of original overburden strength and thickness. The
extension forms space directly above the salt layer, which permits the salt to emplace into
superimposing regular fault grabens. The move from active happens when reactive diaper has
gained a suitable perpendicular scope and the overload has been weakened by extension Gu et al.
2017, pp. 100). The progression to the passive diapirs happens when the salt diaper has
encroached and strapped aside the burden to the stage of salt emergence. Salt edifices augmented
by shortening have distinctive features such as episodic progress, permitting dense arrays of
strata to be dropped above the diapirs in between the advancement interval (Tang et al. 2017, pp.
1437). Additionally, they are characteristic by the pinched off or narrow channels from the salt
layer and strata which are a little bit distorted, as the bend is taken up within the salt stratum.

Field development geology 10
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