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===Alluvial fan deposits===

===Alluvial fan deposits===

An ''alluvial fan'' is a wedge of clastic detritus that forms at the base of a mountain front as sediments eroding from the mountains are transported downslope by streams or debris flows and deposited at the base (Figure <xref ref-type="fig" rid="ClasticSystemsfig3">3e</xref>). The fan-shaped body is generally characterized by a gradation from coarser sediments at the apex to finer sediments at the toe. Alluvial fans are commonly divided into proximal, mid-fan, and distal fan subenvironments.

An ''alluvial fan'' is a wedge of clastic detritus that forms at the base of a mountain front as sediments eroding from the mountains are transported downslope by streams or debris flows and deposited at the base (Figure 3e). The fan-shaped body is generally characterized by a gradation from coarser sediments at the apex to finer sediments at the toe. Alluvial fans are commonly divided into proximal, mid-fan, and distal fan subenvironments.

Vertical sequences through the proximal fan are generally dominated by gravelly deposits with subordinate sandy deposits. Sequences through the mid- and distal fan are increasingly sand dominated. Gamma ray, SP, and resistivity log responses throughout a fan can generally be expected to be blocky to irregular, depending on the amount of clay.

Vertical sequences through the proximal fan are generally dominated by gravelly deposits with subordinate sandy deposits. Sequences through the mid- and distal fan are increasingly sand dominated. Gamma ray, SP, and resistivity log responses throughout a fan can generally be expected to be blocky to irregular, depending on the amount of clay.

Downdip from alluvial fans, rivers typically grade first into braided channels then, farther down the alluvial valley toward the coastal plain, into meandering channels. These different channel types can occur in the same river system and produce distinctly different kinds of sandstone bodies.

Downdip from alluvial fans, rivers typically grade first into braided channels then, farther down the alluvial valley toward the coastal plain, into meandering channels. These different channel types can occur in the same river system and produce distinctly different kinds of sandstone bodies.

''Braided rivers'' and ''braidplains'' form elongate, tabular, sandy and gravelly deposits composed of braided, sand-filled channels and sand and gravel bars (Figure <xref ref-type="fig" rid="ClasticSystemsfig3">3c</xref>). They typically consist of coarse sand and gravel with relatively minor amounts of clay. Vertical sequences are composed of stacked, upward-fining channel sands and sand and gravel bars. Lateral trends in these deposits are dominated by an overall tabular geometry bounded by floodplain muds with an internally complex geometry of cross-cutting sands and gravels with subordinate mud-rich beds of varying thickness and dimension. Bar and channel deposits are typically elongate in the paleocurrent direction.

''Braided rivers'' and ''braidplains'' form elongate, tabular, sandy and gravelly deposits composed of braided, sand-filled channels and sand and gravel bars (Figure 3c). They typically consist of coarse sand and gravel with relatively minor amounts of clay. Vertical sequences are composed of stacked, upward-fining channel sands and sand and gravel bars. Lateral trends in these deposits are dominated by an overall tabular geometry bounded by floodplain muds with an internally complex geometry of cross-cutting sands and gravels with subordinate mud-rich beds of varying thickness and dimension. Bar and channel deposits are typically elongate in the paleocurrent direction.

''Meandering rivers'' are different in that sand is restricted to a single channel and surrounded by fine-grained sediments (Figure <xref ref-type="fig" rid="ClasticSystemsfig3">3d</xref>). Sand is concentrated mainly in the channel bottoms and point bars. A vertical sequence through such a channel system frequently has an upward-fining character, starting from the channel lag at the bottom and grading upward into deposits of the adjacent levee and floodplain. Individual meander belts are built of cross-cutting and stacked individual upward-fining sequences often separated laterally by meander loop cutoffs and clay plugs. Multiple meander belts are built by abandonment of an entire river segment (''avulsion'') and by establishment of a new section in another position on the floodplain.

''Meandering rivers'' are different in that sand is restricted to a single channel and surrounded by fine-grained sediments (Figure 3d). Sand is concentrated mainly in the channel bottoms and point bars. A vertical sequence through such a channel system frequently has an upward-fining character, starting from the channel lag at the bottom and grading upward into deposits of the adjacent levee and floodplain. Individual meander belts are built of cross-cutting and stacked individual upward-fining sequences often separated laterally by meander loop cutoffs and clay plugs. Multiple meander belts are built by abandonment of an entire river segment (''avulsion'') and by establishment of a new section in another position on the floodplain.

Gamma ray, SP, and resistivity logs through braided channel complexes generally have a blocky character, whereas individual meandering channels have an upward-fining signature except where stacked and cross-cut, where they may exhibit more complex wireline log signatures.

Gamma ray, SP, and resistivity logs through braided channel complexes generally have a blocky character, whereas individual meandering channels have an upward-fining signature except where stacked and cross-cut, where they may exhibit more complex wireline log signatures.

===Eolian deposits===

===Eolian deposits===

Eolian sands develop in arid settings and commonly form extensive, blanket-like deposits (Figure <xref ref-type="fig" rid="ClasticSystemsfig3">3b</xref>). Wind transport removes fines and produces rounded and extremely well sorted grains often leading to favorable reservoir quality.

Eolian sands develop in arid settings and commonly form extensive, blanket-like deposits (Figure 3b). Wind transport removes fines and produces rounded and extremely well sorted grains often leading to favorable reservoir quality.

This combination of widespread occurrence and good reservoir properties makes eolian sandstones attractive exploration targets and many hydrocarbon accumulations have been discovered in such deposits (see <ref name=pt06r2>Ahlbrandt, T. S., Fryberger, S. G., 1982, Introduction to eolian deposits, in Scholle, P. A., Spearing, D. eds., Sandstone Depositional Environments: A APG Memoir 31, p. 11–47.</ref>.

This combination of widespread occurrence and good reservoir properties makes eolian sandstones attractive exploration targets and many hydrocarbon accumulations have been discovered in such deposits (see <ref name=pt06r2>Ahlbrandt, T. S., Fryberger, S. G., 1982, Introduction to eolian deposits, in Scholle, P. A., Spearing, D. eds., Sandstone Depositional Environments: A APG Memoir 31, p. 11–47.</ref>.

Rarely do deltas conform perfectly to these end-members. In general they are transitional, giving rise to complexity and variability in the geometry and reservoir heterogeneities of resulting sandstone bodies (e.g., <ref name=pt06r130>Sneider, R. M., Tinker, C. N., Meckel, L. D., 1978, Deltaic environment reservoir types and their characteristics: Journal of Petroleum Technology, Nov., p. 1538–1546.</ref>.

Rarely do deltas conform perfectly to these end-members. In general they are transitional, giving rise to complexity and variability in the geometry and reservoir heterogeneities of resulting sandstone bodies (e.g., <ref name=pt06r130>Sneider, R. M., Tinker, C. N., Meckel, L. D., 1978, Deltaic environment reservoir types and their characteristics: Journal of Petroleum Technology, Nov., p. 1538–1546.</ref>.

Distributary mouth bars and channel deposits (Figure <xref ref-type="fig" rid="ClasticSystemsfig3">3h</xref>) comprise the best reservoir quality bodies within a delta system. The general upward-coarsening character of distributary mouth bars tends to produce sandstone bodies that have their greatest permeability at the top. Conversely, distributary channel sandstone bodies are usually upward-fining and have their greatest permeability at the base<ref name=pt06r130 />). Preferred orientation of flow may be expected to follow paleochannel trends.

Distributary mouth bars and channel deposits (Figure 3h) comprise the best reservoir quality bodies within a delta system. The general upward-coarsening character of distributary mouth bars tends to produce sandstone bodies that have their greatest permeability at the top. Conversely, distributary channel sandstone bodies are usually upward-fining and have their greatest permeability at the base<ref name=pt06r130 />). Preferred orientation of flow may be expected to follow paleochannel trends.

Distributary mouth bars typically contain a high percentage of interstratified clay that reduces vertical permeability. Hartman and Paynter<ref name=pt06r50>Hartman, J. A., Paynter, D. D., 1979, Drainage anomalies in Gulf Coast Tertiary sandstones: Journal of Petroleum Technology, Oct., p. 1313–1322.</ref> document an example of such behavior from a Gulf Coast deltaic reservoir undergoing natural waterdrive. After several years of production, the better quality channel sands watered out, whereas oil remained in the poorer quality delta fringe deposits. In this field, by-passed oil was accessed by recompleting wells only in the delta fringe interval.

Distributary mouth bars typically contain a high percentage of interstratified clay that reduces vertical permeability. Hartman and Paynter<ref name=pt06r50>Hartman, J. A., Paynter, D. D., 1979, Drainage anomalies in Gulf Coast Tertiary sandstones: Journal of Petroleum Technology, Oct., p. 1313–1322.</ref> document an example of such behavior from a Gulf Coast deltaic reservoir undergoing natural waterdrive. After several years of production, the better quality channel sands watered out, whereas oil remained in the poorer quality delta fringe deposits. In this field, by-passed oil was accessed by recompleting wells only in the delta fringe interval.

In shoreline systems adjacent to active deltas, the geometry and internal anatomy of sandstone bodies are controlled by an interplay of tidal and wave processes. Clastic, nondeltaic shorelines with a tidal range of 0–2 m (microtidal) tend to be wave-dominated. Resulting sand bodies are elongate barrier islands and strandplains. A tidal range of 2–4 m (mesotidal) tends to produce short (“drum stick”) barrier islands with extensive tidal flats and ebb tidal deltas. A tidal range of 4–6 m (macrotidal) tends to produce estuarine linear tidal sand ridges that are perpendicular to shoreline with associated extensive tidal flats.

In shoreline systems adjacent to active deltas, the geometry and internal anatomy of sandstone bodies are controlled by an interplay of tidal and wave processes. Clastic, nondeltaic shorelines with a tidal range of 0–2 m (microtidal) tend to be wave-dominated. Resulting sand bodies are elongate barrier islands and strandplains. A tidal range of 2–4 m (mesotidal) tends to produce short (“drum stick”) barrier islands with extensive tidal flats and ebb tidal deltas. A tidal range of 4–6 m (macrotidal) tends to produce estuarine linear tidal sand ridges that are perpendicular to shoreline with associated extensive tidal flats.

Barrier islands (Figure <xref ref-type="fig" rid="ClasticSystemsfig3">3f</xref>) illustrate the spatial variability in facies that affect reservoir properties. Sands in the beach or foreshore are very well sorted, lack interstratified clay, and exhibit excellent reservoir properties where not cemented. Tidal inlet and flood tidal delta deposits comprise another important grouping of reservoir quality rocks, particularly because they are most often preserved in the rock record.

Barrier islands (Figure 3f) illustrate the spatial variability in facies that affect reservoir properties. Sands in the beach or foreshore are very well sorted, lack interstratified clay, and exhibit excellent reservoir properties where not cemented. Tidal inlet and flood tidal delta deposits comprise another important grouping of reservoir quality rocks, particularly because they are most often preserved in the rock record.

Wireline log shapes through barrier island sequences vary depending on exactly where a well intersects the barrier island complex. Gamma ray, SP, and resistivity logs through the barrier core have an upward-coarsening motif (Figure <xref ref-type="fig" rid="ClasticSystemsfig3">3f</xref>). Logs through the back barrier and lower shoreface are typically highly serrate and often lack a well-defined upward-coarsening motif. Logs through the barrier inlet may exhibit upward fining.

Wireline log shapes through barrier island sequences vary depending on exactly where a well intersects the barrier island complex. Gamma ray, SP, and resistivity logs through the barrier core have an upward-coarsening motif (Figure 3f). Logs through the back barrier and lower shoreface are typically highly serrate and often lack a well-defined upward-coarsening motif. Logs through the barrier inlet may exhibit upward fining.

In general, barrier islands have the best reservoir quality rocks at the top of the sequence. [[Reservoir quality]] drops off as one moves either seaward down the foreshore and shoreface into muds of the marine shelf or landward into the lagoon. High reservoir quality is also developed within the tidal inlet sandstones. Two major trends in directional permeability are suggested by (1) the shore-parallel nature of foreshore and shoreface sandstones and (2) shore-perpendicular tidal inlet and delta sandstones. In coastlines dominated by tidal processes, extensive interbedded mud and sand “flats” occur in the intertidal area of the coast and sand bars in estuarine channels in the subtidal area. The reservoir quality of tidal flat environments varies as a function of sand to mud ratio of the deposits. Reservoir quality of estuarine channel deposits also varies as a function of sand to mud ratio and degree of bioturbation.

In general, barrier islands have the best reservoir quality rocks at the top of the sequence. [[Reservoir quality]] drops off as one moves either seaward down the foreshore and shoreface into muds of the marine shelf or landward into the lagoon. High reservoir quality is also developed within the tidal inlet sandstones. Two major trends in directional permeability are suggested by (1) the shore-parallel nature of foreshore and shoreface sandstones and (2) shore-perpendicular tidal inlet and delta sandstones. In coastlines dominated by tidal processes, extensive interbedded mud and sand “flats” occur in the intertidal area of the coast and sand bars in estuarine channels in the subtidal area. The reservoir quality of tidal flat environments varies as a function of sand to mud ratio of the deposits. Reservoir quality of estuarine channel deposits also varies as a function of sand to mud ratio and degree of bioturbation.

===Shallow marine clastic deposits===

===Shallow marine clastic deposits===

The marine shelf is an environment affected by storm- and tidal-driven waves and currents and sometimes by oceanic currents. Although shelf sand ridges of either storm or tidal origin formed during transgression are the best known examples (Figure <xref ref-type="fig" rid="ClasticSystemsfig3">3g</xref>), sand bodies associated with the marine shelf also include reworked delta front and barrier sands, amalgamated storm sheets, and oceanic current deposits<ref name=pt06r10>Barwis, J. H., 1989, The explorationist and shelf sand models—where do we go from here?: 7th Annual Research Conference Proceedings, Gulf Coast SEPM, p. 1–14.</ref>.

The marine shelf is an environment affected by storm- and tidal-driven waves and currents and sometimes by oceanic currents. Although shelf sand ridges of either storm or tidal origin formed during transgression are the best known examples (Figure 3g), sand bodies associated with the marine shelf also include reworked delta front and barrier sands, amalgamated storm sheets, and oceanic current deposits<ref name=pt06r10>Barwis, J. H., 1989, The explorationist and shelf sand models—where do we go from here?: 7th Annual Research Conference Proceedings, Gulf Coast SEPM, p. 1–14.</ref>.

Most marine sand bodies are upward coarsening with the best reservoir quality rocks at the top of the body. Gamma ray, SP, and resistivity logs have a corresponding upward-coarsening character. In the case of storm-deposited sheet sands either attached or detached from the shoreface, amalgamation of individual storm deposits at the top of the bodies produces the greatest permeability and porosity and the most laterally continuous units<ref name=pt06r8>Atkinson, C. D., Goesten, B. G., Speksnijder, A., vander Vlugt, W., 1986, Storm-generated sandstone in the Miocene Miri Formation, Seria Field, Brunei (N., W. Borneo), in Knight, R. J., McLean, J. R., eds., Shelf Sands and Sandstones: Canadian Society of Petroleum Geologists Memoir 11, p. 213–240.</ref><ref name=pt06r38>Gaynor, G. C., Scheihing, M. H., 1988, Shelf depositional environments and reservoir characteristics of the Kuparuk River Formation (Lower Cretaceous), Kuparuk field, North Slope, Alaska, in Lomando, A. J., Harris, P. M., eds., Giant oil and gas fields—A core workshop: Society of Economic Paleontologists and Mineralogists Core Workshop 12, p. 333–389.</ref>. In the case of tidal- and storm-generated shelf sand ridges, best reservoir quality is also at the top in the form of several different types of large scale cross bedding.

Most marine sand bodies are upward coarsening with the best reservoir quality rocks at the top of the body. Gamma ray, SP, and resistivity logs have a corresponding upward-coarsening character. In the case of storm-deposited sheet sands either attached or detached from the shoreface, amalgamation of individual storm deposits at the top of the bodies produces the greatest permeability and porosity and the most laterally continuous units<ref name=pt06r8>Atkinson, C. D., Goesten, B. G., Speksnijder, A., vander Vlugt, W., 1986, Storm-generated sandstone in the Miocene Miri Formation, Seria Field, Brunei (N., W. Borneo), in Knight, R. J., McLean, J. R., eds., Shelf Sands and Sandstones: Canadian Society of Petroleum Geologists Memoir 11, p. 213–240.</ref><ref name=pt06r38>Gaynor, G. C., Scheihing, M. H., 1988, Shelf depositional environments and reservoir characteristics of the Kuparuk River Formation (Lower Cretaceous), Kuparuk field, North Slope, Alaska, in Lomando, A. J., Harris, P. M., eds., Giant oil and gas fields—A core workshop: Society of Economic Paleontologists and Mineralogists Core Workshop 12, p. 333–389.</ref>. In the case of tidal- and storm-generated shelf sand ridges, best reservoir quality is also at the top in the form of several different types of large scale cross bedding.

Reservoir quality sand bodies form on both the continental slope and at the base of the slope. Slope environments include sand bodies formed within submarine canyons and gullies cut into the slope and as spillover sheets<ref name=pt06r120>Slatt, R. M., 1986, Exploration models for submarine slope sandstones: Transactions of the 36th Annual Meeting of the Gulf Coast Association of Geological Societies, Continental Slope—Frontier of the 80's, p. 295–304.</ref>. Sands can also accumulate on tectonically formed small basins within the slope itself.

Reservoir quality sand bodies form on both the continental slope and at the base of the slope. Slope environments include sand bodies formed within submarine canyons and gullies cut into the slope and as spillover sheets<ref name=pt06r120>Slatt, R. M., 1986, Exploration models for submarine slope sandstones: Transactions of the 36th Annual Meeting of the Gulf Coast Association of Geological Societies, Continental Slope—Frontier of the 80's, p. 295–304.</ref>. Sands can also accumulate on tectonically formed small basins within the slope itself.

Submarine fans may form at the base of slopes that have a delta-like appearance in plan view (Figure <xref ref-type="fig" rid="ClasticSystemsfig3">3i</xref>). Internal facies vary from channelized sand and gravel bodies to sheet-like, thin, graded beds deposited by turbidity flows in distal parts of the fan. Vertical sequences through channelized portions of the fan typically show an upward-fining character accompanied by an upward-fining wireline log motif. Vertical sequences through more distal parts of the fan show an alternation between sandstone and mudstone beds, so that wireline logs are typically interdigitate and irregular. Reservoir quality varys accordingly. Many variations of morphologies and internal facies configurations occur in submarine fans as a function of sediment supply, sea level, type of continental margin, and local tectonic features.

Submarine fans may form at the base of slopes that have a delta-like appearance in plan view (Figure 3i). Internal facies vary from channelized sand and gravel bodies to sheet-like, thin, graded beds deposited by turbidity flows in distal parts of the fan. Vertical sequences through channelized portions of the fan typically show an upward-fining character accompanied by an upward-fining wireline log motif. Vertical sequences through more distal parts of the fan show an alternation between sandstone and mudstone beds, so that wireline logs are typically interdigitate and irregular. Reservoir quality varys accordingly. Many variations of morphologies and internal facies configurations occur in submarine fans as a function of sediment supply, sea level, type of continental margin, and local tectonic features.

==See also==

==See also==