DYNAMIC PLANET WEEK 11 Flashcards
What is diagenesis?
Series of processes that modify sediment during its lithification (from greek “lithos”: rock) from loose sediment into rock.
Processes begin at surface T and P immediately following deposition of the sediment and continue during elevation of T and P during burial
Why is diagenesis important?
-Important processes in the destruction of primary porosity and generation of secondary porosity in reservoirs
- Critical to the maturation of organic material into oil and gas in source rocks.
What are the differences between diagenesis and metamorphism?
Metamorphism involves taking the rock up to a T and P in which individual minerals cannot exist at the same phase: it involves the solid state change in crystal structure of rocks
Begins at different T and P for different minerals
Diagenesis involves all changes to the sediment up to (but not including) the point at which T and P is too high for the mineral phases to exist.
What occurs during diagenesis?
Conversion of loose sediment grains into rock
-achieved by the growth of minerals in the pore spaces between the grains, which “cement” the grains together
pore space in sediments (and sedimentary rocks) are what make them (potential) reservoirs for aquafer systems, oil and gas, and CO2.
Diagenesis involves changes in the amount of pore space (e.g. the example above), so these processes are important for understanding porosity and permeability in fluid reservoir systems.
What is mechanical compaction? What occurs as a result of it?
Mechanical compaction by overburden causes grain reorganisation/rotation in order to reduce volume
rock volume is not reduced, but pore-space is reduced
grains are better “packed”
causes “dewatering” – expulsion of water that occupied the pore space. (sandstone dykes are evidence of this)
Grains typically reorganise such that the long-axis moves to become perpendicular to compaction
Compare sand to silts and clays composition? How does this influence compaction?
Sand is predominantly composed of quartz grains lacking habit (often rounded)
-compact into sandstones may lose 10% vertical thickness
-significant pore space maintained by the rigid quartz grains
Whereas silts and clays have larger quantities of clay minerals and micas which have a “platy” morphology.
-true clays may compact into mudstones, and lose up to 90% vertical thickness
-majority of original pore space can be lost
What is pressure solution?
another manner in which compaction by overburden can reduce rock thickness.
-material at grain contacts perpendicular to compaction / shortening dissolve
-material re-precipitates as cement at grain contacts parallel to compaction / shortening.
-net result is also porosity loss.
-common in carbonates
-requires greater depths of burial for clastic sedimentary rocks.
What is dissolution?
caused by changes in T and P, and or the composition of pore-water, which results in the mineral being more stable in solution
Dissolution without reprecipitation occurs in systems which are subjected to prolonged pumping of exotic pore-water, with an abundant fresh supply
-common in carbonates, less common in clastic sedimentary rocks
What is cementation? What are some common cements?
the process by which chemical precipitates (new crystals) form in the pores of a sediment or rock and bind the grains together.
Cementation reduces primary porosity by filling in the pore spaces between the grains.
Common= quartz, carbonates and clays
the sources of cement can be local or external (authigenetic)
What are concretions?
typically spherical masses of sediment more strongly cemented than the surrounding volume.
Typically spherical, because they grow from a point nucleus
Often associated with presence of organic material, which reacts (either during its lifetime, or during its decomposition) with the surrounding chemistry
What is recrystallisation? Why does it happen?
refers to the reorientation of the same crystal lattices in mineral grains
result of solution and reprecipitation of the same mineral phase already present.
does not usually involve net pore space change
When does mineral replacement?
occurs when a newly formed mineral replaces a preexisting one in situ
Changes in polymorph or ion exchange leading to the formation of new minerals, can lead to changes in volume, which can destroy or increase porosity
What is dolomitisation?
an example of mineral replacement
this diagenetic processes increases porosity (blue), which is good for hydrocarbon or water subsurface storage
The degree to these different diagenetic processes operate is a function of?
-original composition of the sediment
-pressure due to burial
-temperature due to burial
-composition and availability of pore fluids
-porosity and permeability
latter two determine the rate at which ion exchange can happen due to concentration gradients at grain boundaries
What are effects of diagenesis?
Burial diagenesis largely leads to destruction in porosity
Culminating in metamorphism: total porosity destruction
But is crucial to the maturation of oil and gas
