Rock Cycle

An illustration of some of the more common methods of rock formation and alteration.

Igneous rocks are formed from the cooling and crystallization of molten rock (or magma)below the crust. When magma cools slowly within the crust, it forms coarsely crystalline rocks, such as granite and gabbro. When magma erupts at the surface of the earth, it forms glassy and microcrystalline volcanic lavas, such as basalt and rhyolite.

Metamorphic rocks are formed by the modification of pre-existing rocks by high temperature and/or high pressure. Regional metamorphism occurs at great depths in the crust. Thermal or contact metamorphism occurs where magma intrudes shallow rocks, and it is characterized by high temperature but low pressure.

Igneous and metamorphic rocks both have crystalline fabrics and lack porosity. They form petroleum reservoirs only in rare instances and are commonly referred to as ``basement.''

Sedimentary rocks are formed from the breakdown of pre-existing rocks: igneous, metamorphic, and sedimentary. They are formed both by the direct deposition of fragments of pre-existing rock (detrital) as well as from the precipitation of their solutes (chemical sediments, such as evaporites).

Sedimentary rocks, by their very nature, tend to be deposited with voids between the sediment particles. Thus most petroleum reservoirs are found in the coarser sediments. Fine-grained sediments serve as impermeable seals to petroleum migration and also as petroleum source beds. Sediments are classified by both grain size (clay, claystone; silt, siltstone; sand, sandstone; gravel, conglomerate) and chemistry (shale is composed principally of clay minerals; sandstone is composed principally of quartz; limestone is composed principally of calcium carbonate; dolomite is composed of the mineral dolomite; evaporites are salts formed from the evaporation of sea water: gypsum, anhydrite, halite, etc.).

Sedimentary rocks are thus often the result of a long history involving weathering, erosion, transportation, deposition, and subsequent physical and chemical changes during burial (called diagenesis). These various processes are defined below.

Other Images in Appendix A

An example of the sailing path of a marine vessel in a towed-streamer survey

Images - Chapter 8 An example of the sailing path of a marine vessel in a towed-streamer survey. Note that the time for turning from one sailing line to another is about nine hours for vessels carrying streamers that are 10 km long. The dotted line indicates the turning legs of the sailing path.

This figure illustrates a typical sailing path of a 3D survey; the vessel travels back and forth, shooting and collecting data along many parallel lines, resulting in seismic data generated along lines 25 to 50 meters apart. Note that it takes about nine hours to turn from one sailing line to another for a vessel carrying 10-km-long streamers. Today, data are recorded even when the vessel is turning.

A shot diagram

Images - Chapter 7 A display of source and receiver distribution of a 2D seismic line in the so-called shot diagram. The rows corresponding to common-shot gathers and columns to common-receiver gathers. The diagonal is the zero-offset section, and all the other lines parallel to the diagonal are common-offset gathers (also known as common-offset sections). The lines perpendicular to the diagonal are the CMP gathers.

Another illustration of towed streamer acquisition

Images - Chapter 7 Another illustration of towed streamer acquisition

Interference noise

Images - Chapter 7 An illustration of interference noise in seismic data before and after. This figure shows the stack of seismic interference noise contaminated shots from another line in the Gulf of Mexico. Interference noise is clearly visible. Attenuation of seismic interference noise can be achieved by the use f-x prediction filters. Courtesy of Western Geco.

Other examples of structural traps

Images - Chapter 1 Structural traps. (A) Tilted fault blocks in an extensional regime. The seals are overlying mudstones and cross-fault juxtaposition against mudstones. (B) Rollover anticline on thrust. Petroleum accumulations may occur on both the hanging wall and the footwall. The hanging wall accumulation is dependent on a subthrust fault seal, whereas at least part of the hanging wall trap is likely to be a simple, four-way, dip-closed structure. (C) Lateral seal of a trap against a salt diapir and compactional drape trap over the diapir crest. (D) Diapiric mudstone associated trap with lateral seal against mud wall. Diapiric mud associated traps share many common features with that of salt. In this diagram, the diapiric mud wall developed at the core of a compressional fold. (E) Compactional drape over a basement block commonly creates enormous low-relief traps. (F) Gravity-generated trapping commonly occurs in deltaic sequences. Sediment loading causes gravity-driven failure and produces convex-down (listric) faults. The hanging wall of the fault rotates, creating space for sediment accumulation adjacent to the fault planes. The marker beds (grey) illustrate the form of the structure that has many favourable sites for petroleum accumulation. Adapted from Gluyas JG and Swarbrick RE (2003) Petroleum Geoscience. Oxford: Blackwell Science.