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PSThe Influence of Basement Structures from Devonian Black Shale Thicknesses in the Northern Appalachian Foreland Basin*

 

Gerald J. Smith1, Robert D. Jacobi1, Jodi L. Seever2, and Stu Loewenstein1

 

Search and Discovery Article #50203 (2009)

Posted Posted September 25, 2009

 

* Adapted from poster presentation at AAPG Annual Convention and Exhibition, Denver, Colorado, USA, June 7-10, 2009.

 

1Nornew, Inc., Amherst, NY. (stratigrapher@msn.com)

2Department of Geology, University at Buffalo, Buffalo, NY.

 

Abstract

 

Five thick black shales were deposited in western New York and northern Pennsylvania during the Middle and Late Devonian. Traditional models show the regional maximum black shale thickness successively steps farther west with the development of a gentle, structurally inactive clinoform. However, in the northern region of the Appalachian Foreland Basin, many of the areas of thickest black shale deposition coincide with areas of active faulting. From our outcrop studies in New York State and well-log analyses in New York and Pennsylvania we observed abrupt thickening of several of the black shales coincident with active faults that extend up from basement structures, primarily the Clarendon-Linden Fault System and Iapetan opening/Rome Trough structures. For example the regionally minor black shales the of Pipe Creek and the Hume formations are typically 1 meter or less thick and appear inconsequential as a reservoir/source rock. However, within the extent of the Clarendon-Linden Fault System, the Hume Formation averages 36m (120ft) thick, and the Pipe Creek Formation reaches 5.5m (18ft). More importantly for shale reservoirs, thick accumulations of the Geneseo (~45m/150ft) and Rhinestreet (91m/290 ft) formations coincide with basement structures of reactivated Clarendon-Linden, while greater thickness of the Marcellus (~56m/180ft) and Middlesex (~61m/200ft) correspond with the Iapetan-opening/Rome Trough structures.

 

We suggest that the combined stress of the Neo-Acadian collision and accompanying sediment loading reactivated the older basement structures, generating variable accommodation within the vicinity of the fault zones. In some cases, the thickening may result from thrusts that can be easily overlooked in the typical wireline logs if there are not distinctive marker units (as is typical in the black shales). However, such thrusts are recognizable in outcrop and FMI or similar logs. In addition to the increased localized accumulation of organic-rich shale, later fault reactivation would increase local fracturing, increasing the potential of these black shales as reservoirs and source rocks.

 

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fig01

Figure 1. Structural contour map of basement with major faults, modified from Schumaker, 1996

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Figure 2. Model of a structureless foreland basin. Thickness of the black shale is at its greatest at the deepest point. Subsequent basins shift farther away from the orogeny.

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Figure 3. Comparison between the structural complexities of New York State from almost no fault (Schumaker, 1996) to hundreds of structures (Jacobi, 2002).

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Figure 4. Devonian stratigraphic section for western New York State. Middle Devonian based upon Rickard, 1975.

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Figure 5. Simplified Devonian stratigraphy illustrating the relationship between the 5 major black shales.

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Figure 6. Foreland basins along the Appalachian-Ouachita orogenic front.

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Figure 7. Structural cross section A-A’.

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Figure 8. Structural cross section B-B’.

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Figure 9. Structural cross section C-C’.

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Figure 10. Stratigraphic cross section A-A’, flattened on the base of the Dunkirk Fm.

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Figure 11. Stratigraphic cross section B-B’, flattened on the top of the Geneseo Fm.

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Figure 12. Stratigraphic cross section C-C’, flattened on the top of the Tully Fm.

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Figure 13. Structural cross section D-D’.

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Figure 14. Structural cross section E-E’.

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Figure 15. Structural cross section F-F’.

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Figure 16. Structural cross section G-G’.

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Figure 17. Example of well log data from New York and Pennsylvania.

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Figure 18. Rhinestreet isopach map.

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Figure 19. Middlesex isopach map.

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Figure 20. Geneseo isopach map.

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Figure 21. Marcellus isopach map.

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Figure 22. All Devonian black shales isopach (Dunkirk, Pipe Creek, Rhinestreet, Middlesex, Geneseo and Marcellus).

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Figure 23. Tully isopach map.

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Figure 24. (A) Faults are commonly interpreted as a single fault-scarp with maximum throw, however, (B) from field observations; we have repeatedly encountered faulting as a series of small step-faults, each with an offset of 2-15 cm. (C) Example of step-faults from Caneadea Gorge, Allegany County.

fig25

Figure 25. A simplified model of accommodation. 1) For any given area, the accommodation is the volume available for deposited sediment. 2 and 3) With constant sediment supply, deposition will fill in the available accommodation starting closest to the source and building outward. 4) With lower sea level, accommodation space decreases and the carrying capacity (typically) increases causing the erosion where the accommodation can not hold the surplus sediment. 5 and 6) In a faulted region, the accommodation is more variable and will reflect in a varied preservation potential.

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Figure 26. Hypothetical model of fault-block derived accommodation variability. A) In an area with pre-existing faulting, reactivation is likely during periods of high stress such as compression during an orogenic phase (orange arrows). B) To relieve the stress, the faults may move, but not necessarily uniformly, but at different rates creating highs and lows between faults blocks. Rotational motion will also create higher and lower regions. C) The overall effect on deposition will be to generate lows that accumulate sediment, and highs that will erode, deflect currents and create multiple trending sediment packets. If the fault activity occurs during deposition, then the changes may be observed within the sedimentary deposits. It is important to note that neither scale nor orientation is implied by this model, such this model could represent basins or outcrop jointing.

fig27

Figure 27. Contrasting the structureless foreland basin model of the first panel is a hypothetical foreland basin with periods of syndepositional structural activity. In such a basin, subsequent black shales may not trend parallel to the main basin axis and may not stack with a constant, basinward shift.

Introduction

 

Recent influx of interest in black shales suggests a closer examination of the structural setting in which many of the black shale plays occur. (Figure 1) A simple, structureless foreland basin will contain clinoform sediments grading basinward to thick accumulations of organic rich clays and silt-sized material (Figure 2). In such a basin, the thickest black shales would occur in the deepest regions. With continued sediment influx and sea level cyclicity, subsequent black shales would form further basinward but still parallel the first black shale. However, structureless foreland basins are unlikely, particularly in regions that have experienced several orogenic events. (Figure 3, Figure 4 and Figure 5) The pre-existing basement structure in a foreland basin will contribute an additional level of complexity to the basin topography.  The result is that in black shale, deposits may be structurally controlled within isolated areas, creating areas of enhanced preservation that follow structural trends oblique to the basin.

 

Many of the major black shale plays occur along the orogenic front. (Figure 6) If basement structures were reactivated in the Appalachian Basin, then it seems likely that other foreland basins will have also experienced structural reactivations of some level.

 

Cross Sections

 

Structural cross sections were created across southwestern New York State incorporating outcrop measured at the centimeter scale with gamma ray logs. Cross-sections A-A’ (Figure 7), B-B’ (Figure 8) and C-C’ (Figure 9) are a series of east-west cross sections highlighting the thickness variations in the black shales. Each east-west cross section is also shown flattened along a distinct stratigraphic horizon. (Figure 10, Figure 11 and Figure 12) Where a cross section line crosses a basement structure a red-dashed line is superimposed on the cross section. Cross section D-D’ is a north-south cross section that ties A-A’, B-B’ and C-C’. (Figure 13) To utilize as many well logs as possible only wells drilled into the Upper Devonian were included using the Rushford Formation as a key correlation unit distinctive in outcrop and well logs.

 

Deeper structures generally coincide with preservation of the Rushford Formation. The broad anticline in E-E’ occurs where a thicker, coarsening upward sequence was deposited. (Figure 14) In F-F’, the thrust fault in the Tully coincides with the disappearance of the Rushford Formation. (Figure 15)  G-G' is a north-south Upper Devonian cross-section showing units thickening towards the south. (Figure 16)

 

Black Shale Isopach Maps

 

Isopach maps were created from well log data in New York and Pennsylvania (Figure 17) for four of the major Devonian black shales (Figure 18, Figure 19, Figure 20, Figure 21and Figure 22). Basement structures from Jacobi (2002) are overlain to show the general coincidence between the thicker deposition and basement faulting. It is important to note that for different black shales, different trending structures appear to have more control. As an example, the Tully Formation (Figure 23) and overlying Geneseo Formation both have a broad, north-south trend following the north-south Clarenden-Linden Fault System (CLF). The stratigraphically higher Middlesex Formation however follows northeast trending structures.

 

Conclusions

 

Syndepositional faulting or tectonic activity within foreland basins (Figure 24) has been shown to control/influence deposition and architecture in fluvial systems (Plint and Wadsworth, 2006), carbonate reefs (Dorobek, 1995), beaches (Hart and Plint, 1993; Smith and Jacobi, 2001) and offshore sand ridges (Nummedal and Riley, 1999). Just as relatively minor amounts of uplift (~1 m) may affect deposition and depositional pattern, so can minor amounts of subsidence (or relative subsidence) affect preservation through a localized increase in accommodation. (Martinsen, 2003) (Figure 25) Syndepositional faulting caused by the reactivation of basement structures would be expected within a foreland basin as forces from collisional tectonics, sediment load, development and migration of the peripheral bulge generate alternating periods of localized compression, extension and strike-slip stress. (Figure 26)

 

The effect of syndepositional faulting on black shale deposits is varied accumulation thicknesses following structural trends. In periods of tectonic quiescence, the black shale isopach will parallel the basin axis, but where cross faulting has occurred, the black shale isopach will trend obliquely to the basin axis. (Figure 27)

 

References

 

Dorobek, S.L., 1995, Synorogenic carbonate platforms and reefs in foreland basins: controls on stratigraphic evolution and platform/reef morphology; in Dorobeck, S.L. and Ross, G.M. eds. Stratigraphic Evolution of Foreland Basins SEPM Special Publication 52, SEPM (Society of for Sedimentary Geology) Tulsa, p. 127 – 147.

 

Ettensohn, F.R., 1985a, The Catskill Delta Complex and the Acadian Orogeny: in Woodrow, D.L., and Sevon, W.D., eds., The Catskill Delta:GSA Special Paper 201, p. 39-49.

 

Harper, J.A., 1989, Effects of recurrent tectonic patterns on the occurrence and development of oil and gas resources in western Pennsylvania ; Northeastern Geology, v. 11, p. 225-245.

 

Hart, B.S., and Plint, A. G., 1993, Tectonic influence on deposition and erosion in a ramp setting: Upper Cretaceous Cardium Formation, Alberta Foreland Basin: American Association of Petroleum Geologists Bulletin, v. 77, p. 2092-2107.

 

Jacobi, R.D., 2002, Basement faults and seismicity in the Appalachian Basin of New York State: Tectonophysics, v. 353, p. 75-113.

 

Martinsen, R.S., 2003, Depositional Remnants, Part 1: Common components of the stratigraphic record with important implications for hydrocarbon exploration and production; American Association of Petroleum Geologists Bulletin, v. 87, p. 1869-1882.

 

Nummedal, D., and Riley, G.W., 1999, The origin of the Tocito Sandstone and its sequence stratigraphic lessons; in Bergman, K.M. and Snedden, J.W., eds., Isolated Shallow Marine Sand Bodies: Sequence Stratigraphic Analysis and Sedimentologic Interpretation, SEPM Special Publication No. 64, p. 227-254.

 

Rickard, L.V., 1975, Correlation of the Devonian rocks in New York: New York Museum and Science Service Map and Chart Series 24.

 

Schumaker, R.C.,1996, Structural History of the Appalachian Basin; in: Roen, J.B., and Ealker, B.J., eds., The Atlas of Major Appalachian Gas Plays, West Virginia Geological and Economic Survey, Morgantown, p. 8-21.

 

Smith, G.J., and Jacobi, R.D., 2001, Tectonic and Eustatic Signals in the Sequence Stratigraphy of the Upper Devonian Canadaway Group, New York State; American Association of Petroleum Geologists Bulletin, v. 85, no. 2, p. 325-357.

 

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