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The Role of Depositional Sequences in Creating and Controling Petroleum Systems – Basic Principles and Examples
Most of the essential elements and processes that create a petroleum system are controlled by the lithology and stratigraphy of the rock package involved. The vertical and lateral distribution of source, carrier/reservoir, and migrational-barrier/trap-seal rocks generally reflects an orderly pattern of lithologic and environmental facies that represent sequences of transgressive-regressive or deepening-shallowing water sedimentation. In cross section, these sequences are commonly represented by unconformity-bounded, wedge-shaped bodies of sediment that thin from depositional basin centers toward edges of non-deposition and erosion on bordering highlands.
An "ideal" depositional sequence contains an internal distribution of source rock, carrier/reservoir, and seal units. Overburden depths that cause source rock maturity may be produced by the thickness of overlying beds within the sequence itself or in an overlying sequence. Migration paths may be either upward and cross-stratal or lateral and updip within a given carrier/reservoir unit. Migration may continue until either a site of entrapment is reached within a reservoir indigenous to the sequence or a "leak" is encountered into an overlying sequence or to the Earth's surface. Stratigraphic and structural traps may be present within a sequence as a result of depositional complexity and syn-depositional deformation. Post-depositional structural trap configurations may also be superimposed on a sequence.
Not all sequences contain the stratigraphic elements that make petroleum systems, nor do they have the same lithologic geometries. Sequences may be barren of hydrocarbons due to source rock immaturity or leakage. Understanding how required elements of a petroleum system are represented within a depositional sequence should aid in explaining and predicting where oil and gas accumulations are found.
Examples of petroleum systems related to depositional sequences will be presented.
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