Editing Petroleum geology (section)

==Sedimentary basin analysis==
Petroleum geology is principally concerned with the evaluation of seven key elements in [[sedimentary basin]]s:

[[Image:OilReservoir.png|thumb|400px|A [[structural trap]], where a fault has juxtaposed a porous and permeable reservoir against an impermeable seal. [[Oil]] (shown in red) accumulates against the seal, to the depth of the base of the seal. Any further oil migrating in from the source will escape to the surface and seep.]]
* [[Source rock|Source]]
* [[petroleum reservoir|Reservoir]]
* [[caprock|Seal]]
* [[Petroleum trap|Trap]]
* Timing
* [[Maturity (geology)|Maturation]]
* [[Catagenesis (geology)|Migration]]

In general, all these elements must be assessed via a limited 'window' into the subsurface world, provided by one (or possibly more) exploration [[oil well|wells]].  These wells present only a one-dimensional segment through the Earth, and the skill of inferring three-dimensional characteristics from them is one of the most fundamental in petroleum geology.  Recently, the availability of inexpensive, high-quality 3D seismic data (from [[reflection seismology]]) and data from various electromagnetic geophysical techniques (such as [[magnetotellurics]]) has greatly aided the accuracy of such interpretation.  The following section discusses these elements in brief.  For a more in-depth treatise, see the second half of this article below.

Evaluation of the ''source'' uses the methods of [[geochemistry]] to quantify the nature of organic-rich rocks<ref name=":0">{{Cite journal |last1=Ferriday |first1=Tim |last2=Montenari |first2=Michael |date=2016 |title=Chemostratigraphy and Chemofacies of Source Rock Analogues: A High-Resolution Analysis of Black Shale Successions from the Lower Silurian Formigoso Formation (Cantabrian Mountains, NW Spain) |url=https://www.sciencedirect.com/science/article/abs/pii/S2468517816300053 |journal=Stratigraphy & Timescales |volume=1 |pages=123–255 |doi=10.1016/bs.sats.2016.10.004 |via=Elsevier [[Science Direct]]}}</ref> which contain the precursors to hydrocarbons, such that the type and quality of expelled hydrocarbon can be assessed.

The ''reservoir'' is a porous and permeable [[lithological]] unit or set of units that holds the hydrocarbon reserves.  Analysis of reservoirs at the simplest level requires an assessment of their [[porosity]] (to calculate the volume of ''[[in situ]]'' hydrocarbons) and their [[Permeability (fluid)|permeability]] (to calculate how easily hydrocarbons will flow out of them).<ref name=":1">{{Cite journal |last1=Richardson |first1=Ethan J. |last2=Montenari |first2=Michael |date=2020 |title=Assessing shale gas reservoir potential using multi-scaled SEM pore network characterizations and quantifications: The Ciñera-Matallana pull-apart basin, NW Spain |url=https://www.sciencedirect.com/science/article/abs/pii/S2468517820300010 |journal=Stratigraphy & Timescales |volume=5 |pages=677–755 |doi=10.1016/bs.sats.2020.07.001 |isbn=9780128209912 |s2cid=229217907 |via=Elsevier Science Direct}}</ref>  Some of the key disciplines used in reservoir analysis are the fields of [[#References|structural analysis]], [[stratigraphy]], [[sedimentology]], and [[reservoir engineering]].

The ''seal'', or ''cap'' rock, is a unit with low permeability that impedes the escape of hydrocarbons from the [[reservoir rock]].  Common seals include [[evaporite]]s, [[chalk]]s and [[shale]]s.  Analysis of seals involves assessment of their thickness and extent, such that their effectiveness can be quantified.

The ''[[Petroleum trap|geological trap]]'' is the [[Stratigraphy|stratigraphic]] or structural feature that ensures the juxtaposition of reservoir and seal such that hydrocarbons remain trapped in the subsurface, rather than escaping (due to their natural [[buoyancy]]) and being lost.

Analysis of ''maturation'' involves assessing the thermal history of the [[source rock]] in order to make predictions of the amount and timing of hydrocarbon generation and expulsion.

Finally, careful studies of ''migration'' reveal information on how hydrocarbons move from source to reservoir and help quantify the source (or ''kitchen'') of hydrocarbons in a particular area.

[[Image:Mudlogging.JPG|thumb|[[Mud log]] in process, a common way to study the [[lithology]] when drilling [[oil well]]s.]]