Source rock

The sedimentary rocks, rich in organic matter, which are capable of generating hydrocarbons are known as source rocks. "Good" source rocks usually contain between 5% - 20% organic matter

Source rocks are deposited in three main settings:

Lakes - isolated basins with poor turnover of the liquid column, allow the accumulation of land-derived (gas prone) or algal-derived (oil prone) organic matter. The Eocene Green River Shale of the Western US and many of the SE Asian (particularly in China) source rocks were deposited in lakes (lacustrine).

Deltas - Deltas occur where rivers meet the sea (e.g., Nile, Mississippi). They are characterised by river channels with swamps and ponds (lagoons) in between. Organic matter can be derived from lagoonal algal concentrations or directly from plants growing on the delta plain. Coals in the Tertiary sequence of Indonesia, originally deposited in swamps, form important oil source rocks. The lagoonal shales in the Carboniferous (e.g. the Pumpherston Oil Shale outcropping at S. Queensferry, Lothian, Scotland, immediately underneath the Forth Rail Bridge) have been an oil source. More commonly, the coals in the Carboniferous are the source for gas, as occurs in the Southern North Sea.

Marine basins - Marine basins, especially those with restricted circulation, form ideal conditions for the accumulation of thick organic-rich source rocks. The Kimmeridge Clay (North Sea) is a good example of a rich source rock (e.g. high organic content) in a marine shale. The Posidonia Shale (Posidonia is a marine fossil), of Lower Jurassic age, is the oil source in the southern part of the Dutch Offshore and the Paris Basin and is a marine source rock. Zones of adundant upwelling in the oceans, where the fauna thrives because of the abundance of nutrients brought up as warm light water meets dense cold water, can also lead to the formation of organic rich beds.

Key SOURCE ROCK points:

• Source rocks for hydrocarbons are fine-grained organic-rich sediments (e.g., coals, shales).

• Source rocks may be quite different stratigraphic units, in a location far away, from the reservoir rocks.

• Shales tend to source oils and coals, gas.

Key MATURATION points:

• Pressure and temperature sustained for a period of time are needed to generate oil or gas from a source rock.

• Gas tends to be generated at higher pressures and temperatures.

• The region in which maturation takes place is called the kitchen.

Key MIGRATION points:

• Migration from the source rock into the carrier bed is called primary migration.

• Primary migration is driven by pressure build-up caused by hydrocarbon genera-tion.

• Migration from the source kitchen area to the reservoir trap is called secondary migration.

• Secondary migration is a gravity-driven processs controlled by pore-entry net-works.

Traps

Structural traps are those caused by tectonic, diapiric, gravitational and compactional processes. These form at some time after deposition of the reservoir as a result of Earth movements

Stratigraphic traps are those in which the geometry is inherited from the depositional morphology, subsequent diagenesis, facies changes (figure 21) or unconformities

Faults

A fault is a more-or-less planar surface or zone, across which the rocks on either side have been moved by shear displacement (i.e. displacement parallel to the fault surface). Faults can be sharp (infinitesimally-thick) planes, and they can also be wide zones consisting of an array of complex deformation features.

The types of faults are defined by the sense of movement along the fault plane. In dip-slip faults (the slip motion is parallel to the dip direction), if the hangingwall moves down (with respect to the footwall), this is called a normal fault. Normal faults are associated with extension (lateral increase in dimension). If the hangingwall rises over the footwall, this is called a reverse fault.

The uplifted blocks are called horsts, and the down-dropped blocks are called grabens.

Folds

A fold can be defined as the deflection of a marker surface (e.g. from a planar shape before folding to one that is non-planar after folding)

When a layer is flexed “up”, we call this structure an anticline; when the layer is flexed “down”, we call this structure a syncline

One fold classification scheme is based on variations or constancy in the thickness of the folded layers. Parallel folds have constant layer thicknesses (measured perpendicular to the layer boundaries). Concentric folds are special parallel folds that have more-or-less constant curvature centered on a point. Similar folds have a constant distance between layers (measured along a direction that is parallel with the axial surface).

Диагенез [Diagenesis] (с греч. диагенесис – перерождение) – это совокупность процессов преобразования рыхлых осадков в осадочные горные породы в верхней зоне земной коры. Выделяют ранний диагенез (физико-химическое уравновешивание состава осадка) и поздний диагенез (перераспределение вещества с образованием различных минеральных стяжений).

Diagenesis -Those processes affecting a sediment while it is at or near the Earth’s surface, i.e. at low temperature and pressure. Changes which take place as a result of Earth movements and increased pressure are metamorphic in character and are excluded from the definition; however it is clear that diagenetic changes grade into metamorphic ones. Sub-aerial weathering is also specifically excluded. Lithifaction is the term used to describe those changes which result in the formation of a solid rock from a loose sediment. Two important processes in diagenesis which appear to operate more or less continuously up to the completion of the lithifaction process are compaction (consolidation is approximately synonymous) and cementation.

Compaction involves the close-packing of the individual grains mainly by the elimination of pore-space and expulsion of entrapped water; this is normally brought about by the weight of the overlying sediments. Occasionally, compaction involves grain breakage. Cementation is the process by which the individual particles of sediment are held together by a secondarily developed material. This may either be a substance introduced by ground water percolating through the pores of the rock, or be derived from solution of part of the mineral matter of the rock followed by nearby redeposition.