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A geometrical analysis is simply dividing a basin's sedimentary section into three-dimensional bodies of strata using regionally correlative surfaces or features as boundaries. The sequence stratigraphic approach uses unconformities or other genetically significant features to divide the section into depositional sequences, systems tracts, and/or parase-quences. Recognizing these correlation features is key to identifying depositional sequences properly.

'''Geometrical analysis''' is simply dividing a basin's sedimentary section into three-dimensional bodies of strata using regionally correlative surfaces or features as boundaries. The [[sequence stratigraphy|sequence stratigraphi]]c approach uses unconformities or other genetically significant features to divide the section into [[depositional sequence]]s, [[systems tract]]s, and/or [[parasequence]]s. Recognizing these correlation features is key to identifying depositional sequences properly.

==Procedure==

==Procedure==

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| Divide the sedimentary section into depositional sequences, systems tracts, and parasequences using the following:

| Divide the sedimentary section into depositional sequences, systems tracts, and parasequences using the following:

 

*  Seismic sequence analysis

*  Seismic sequence analysis *  Well data sequence analysis

*  Well data sequence analysis

 

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==Identifying unconformities==

==Identifying unconformities==

Unconformities are third-order sequence boundaries. They are generally regional onlap surfaces. In ''basinal settings'', they are characterized by onlap of allochthonous deposits (i.e., debris flows, slump deposits, turbidites), prograding deltas, carbonate platform deposits, or evaporites. In ''shallow-water or nonmarine settings'', they are characterized by onlap of strata deposited in fluvial, deltaic, or nearshore marine or peritidal environments.<ref name=ch21r48>Weber, L., J., Sarg, J., F., Wright, F., M., 1995, [[Sequence stratigraphy]] and reservoir delineation of the middle Pennsylvanian (Desmoinesian), Paradox basin and Aneth field, southwestern U., S., A., in Read, J., F., Weber, L., J., Sarg, J., F., Wright, F., M., eds., Milankovitch Sea-Level Changes, Cycles, and Reservoirs on Carbonate Platforms in Greenhouse and Ice-House Worlds: SEPM Short Course No. 35, 79 p.</ref> We can identify unconformities using stratigraphic evidence and individual well evidence.

Unconformities are [[third-order cycles|third-order]] sequence boundaries. They are generally regional onlap surfaces. In ''basinal settings'', they are characterized by onlap of allochthonous deposits (i.e., debris flows, slump deposits, turbidites), prograding deltas, carbonate platform deposits, or evaporites. In ''shallow-water or nonmarine settings'', they are characterized by onlap of strata deposited in fluvial, deltaic, or nearshore marine or peritidal environments.<ref name=ch21r48>Weber, L., J., Sarg, J., F., Wright, F., M., 1995, [[Sequence stratigraphy]] and reservoir delineation of the middle Pennsylvanian (Desmoinesian), Paradox basin and Aneth field, southwestern U., S., A., in Read, J., F., Weber, L., J., Sarg, J., F., Wright, F., M., eds., Milankovitch Sea-Level Changes, Cycles, and Reservoirs on Carbonate Platforms in Greenhouse and Ice-House Worlds: SEPM Short Course No. 35, 79 p.</ref> We can identify unconformities using stratigraphic evidence and individual well evidence.

 

* Reflection terminations in seismic sections (onlap, downlap, toplap, or truncation)

* Reflection terminations in seismic sections (onlap, downlap, toplap, or truncation)

* Bed truncation observed in detailed well log cross sections

* Bed truncation observed in detailed well log cross sections

* Abrupt vertical geochemical changes such as stable isotopes

* Abrupt vertical geochemical changes such as stable isotopes

'''Individual Well Evidence'''

===Individual well evidence===

 

* Dipmeter changes

* Dipmeter changes

* Gamma-ray log changes in response to increased uranium concentration at exposure surfaces

* Gamma-ray log changes in response to increased uranium concentration at exposure surfaces

* Vertical breaks in thermal maturity profiles (i.e., abrupt vertical change in vitrinite reflection values)

* Vertical breaks in thermal maturity profiles (i.e., abrupt vertical change in vitrinite reflection values)

* Changes in lithology as seen in cores that indicate subaerial exposure or nondeposition, as evidenced by the following:

* Changes in lithology as seen in cores that indicate subaerial exposure or nondeposition, as evidenced by the following:

 

** Paleosols and weathered horizons

<list-item><label>—</label>Paleosols and weathered horizons</list-item><list-item><label>—</label>Hematitic grain coatings or dissolution textures unrelated to burial diagenesis</list-item><list-item><label>—</label>Clam-bored hardgrounds such as Toredo borings</list-item><list-item><label>—</label>Thin lag deposits of bone, phosphate, or shell hash</list-item>

** Hematitic grain coatings or dissolution textures unrelated to burial diagenesis

** Clam-bored hardgrounds such as Toredo borings

** Thin lag deposits of bone, phosphate, or shell hash

* Fluid inclusion evidence for atmospheric gases (e.g., argon, helium)

* Fluid inclusion evidence for atmospheric gases (e.g., argon, helium)

==Example of unconformity analysis==

==Example of unconformity analysis==

Cores and samples should be examined for evidence of unconformities. These unconformity surfaces should then be calibrated to logs. Logs can then be used to correlate the surfaces to seismic and to other wells. The figure below (from <ref name=ch21r12>DolsonMuller, D., S., 1994, Stratigraphic evolution of the Lower Cretaceous Dakota Group, Western Interior, U., S., A., in Caputo, M., V., Peterson, J., A., Franczyk, K., J., eds., Mesozoic Systems of the Rocky Mountain Region, U., S., A.: SEPM Rocky Mountain Section, p. 441–456.</ref> shows an example of calibrating unconformity evidence from cores to logs. The Lower Cretaceous Cutbank Sandstone unconformably overlies the Jurassic Swift Formation. A major lowstand surface of erosion (LSE) is shown at [[depth::2957 ft]] (901 m) and was identified using the following criteria:

Cores and samples should be examined for evidence of unconformities. These unconformity surfaces should then be calibrated to logs. Logs can then be used to correlate the surfaces to seismic and to other wells. The figure<ref name=ch21r12>DolsonMuller, D., S., 1994, Stratigraphic evolution of the Lower Cretaceous Dakota Group, Western Interior, U., S., A., in Caputo, M., V., Peterson, J., A., Franczyk, K., J., eds., Mesozoic Systems of the Rocky Mountain Region, U., S., A.: SEPM Rocky Mountain Section, p. 441–456.</ref> shows an example of calibrating unconformity evidence from cores to logs. The Lower Cretaceous Cutbank Sandstone unconformably overlies the Jurassic Swift Formation. A major lowstand surface of erosion (LSE) is shown at [[depth::2957 ft]] (901 m) and was identified using the following criteria:

* Missing biostratigraphic horizons

* Missing [[biostratigraphy|biostratigraphic]] horizons

* Subaerial (weathered) zone beneath a channel

* Subaerial (weathered) zone beneath a channel

* Facies omission (abrupt change from marine shale to a fluvial sandstone)

* Facies omission (abrupt change from marine shale to a fluvial sandstone)

==See also==

==See also==