Robert C. Shumaker

Paleozoic Structure of the Central Basin Uplift and Adjacent Delaware Basin, West Texas (1)

Structural cross sections and a regional tectonic map show that the Central Basin uplift consists of upthrust blocks of alternating vergence. The uplift is separated from the adjacent Delaware basin by a major fault that changes throw and structural style along strike, accompanying changes in block vergence found along the uplift. The west-facing basement blocks were thrust westward over the Delaware basin, whereas the adjacent blocks face eastward. The distribution and style of structures suggest that small amounts of left-lateral movement occurred along west-trending and northwest-trending faults that separated rising blocks of the uplift. Transfer of lateral movement from vertical, west-trending faults on the Central Basin uplift and the Ozona arch to the west-trending Mid-basin fault in the Delaware basin rotated blocks of the uplift in a clockwise sense. Faults and folds within the study area have trends and slip directions that are generally similar to those within the Reagan and Washita Valley fault zones in the Arbuckle uplift adjacent to the Anadarko basin. The difference in the style of deformation between the two uplifts relates to varied angular relationships between preexisting structure and the direction of late Paleozoic stress. The identity of structural trends within these two uplifts suggests that other late Paleozoic basins and uplifts of the Ancestral Rocky Mountains may have formed in a similar stress field and that crustal rotation was pervasive along the southwestern margin of the continent during the late Paleozoic Ouachita-Maratho deformation.

Along-axis segmentation and growth history of the Rome trough in the Central Appalachian Basin

The Rome trough, a northeast-trending graben, is that part of the Cambrian interior rift system that extends into the central Appalachian foreland basin in eastern North America. On the basis of changes in graben polarity and rock thickness shown from exploration and production wells, seismic lines, and gravity and magnetic intensity maps, we divide the trough into the eastern Kentucky, southern West Virginia, and northern West Virginia segments. In eastern Kentucky, the master synthetic fault zone consists of several major faults on the northwestern side of the trough where the most significant thickness and facies changes occur. In southern West Virginia, however, a single master synthetic fault, called the East-Margin fault, is located on the southeastern side of the trough. Syndepositional motion along that fault controlled the concentrated deposition of both the rift and postrift sequences. The East-Margin fault continues northward into the northern West Virginia segment, apparently with less stratigraphic effect on postrift sequences, and a second major normal fault, the Interior fault, developed in the northern West Virginia segment. These three rift segments are separated by two basement structures interpreted as two accommodation zones extending approximately along the 38th parallel and Burning-Mann lineaments. Computer-aided interpretation of seismic data and subsurface geologic mapping indicate that the Rome trough experienced several major phases of deformation throughout the Paleozoic. From the Early(?)-Middle Cambrian (pre-Copper Ridge deposition), rapid extension and rifting occurred in association with the opening of the Iapetus-Theic Ocean at the continental margin. The Late Cambrian-Middle Ordovician phase (Copper Ridge to Black River deposition) was dominated by slow differential subsidence, forming a successor sag basin that may have been caused by postrift thermal contraction on the passive continental margin. Faults of the Rome trough were less active from the Late Ordovician-Pennsylvanian (post-Trenton deposition), but low-relief inversion structures began to form as the Appalachian foreland started to develop. These three major phases of deformation are speculated to be responsible for the vertical stacking of different structural styles and depositional sequences that may have affected potential reservoir facies, trapping geometry, and hydrocarbon accumulation.

Three-Dimensional Structural Interrelationships Within Cambrian-Ordovician Lithotectonic Unit of Central Appalachians

A block diagram of the Cambrian-Ordovician lithotectonic unit illustrates three-dimensional structural relationships within that sequence along the length of the central Appalachian Valley and Ridge and High Plateau provinces. The diagram shows that the Valley and Ridge province is divisible into areas within which shortening is relatively constant in the Cambrian-Ordovician lithotectonic unit. These areas are bounded by zones across which significant differences in shortening occur. These transition zones contain major cross-strike structural discontinuities in surface structure; in some instances, these discontinuities extend across the Valley and Ridge province and into the High Plateau province. Increases in fold amplitude and number occur in the cover of the Plateau, across strike from the more intensely deformed areas of the Valley and Ridge, where shortening within the Cambrian-Ordovician unit is significantly greater than elsewhere within that province. Structurally controlled gas accumulations are more prevalent in these areas of the Plateau.

Subsurface Geology of the Warfield Structures in Southwestern West Virginia: Implications for Tectonic Deformation and Hydrocarbon Exploration in the Central Appalachian Basin

Data from over 6000 wells and five multichannel reflection seismic lines were used to constrain the subsurface geometry of the Warfield structures in southwestern West Virginia within the central Appalachian basin. Based on their vertical differences in geometry and structural styles, we divided the Warfield structures into shallow (above the Devonian Onondaga Limestone), intermediate (between the Devonian Onondaga Limestone and the Silurian Tuscarora Sandstone), and deep (below the Ordovician Trenton horizon) structural levels. Shallow structures are related to the Alleghanian deformation above the major detachment horizon of the Devonian shales and consist of the Warfield anticline with a 91.5-m closure and southeast-dipping monoclines, which aided the northwest migrati n and entrapment of oil and gas. At the intermediate level, the closure of the Warfield anticline is lost because the Alleghanian deformation is obscured below the major detachment of the Devonian shales, which explains the reduced production from the Devonian and Silurian reservoirs. Deep structures are characterized by an asymmetric half graben within a continental rift system known as the Rome trough, in which a thick sequence of sedimentary rocks exists to provide sources for overlying reservoirs. Although stratigraphic traps may be associated with thickness and facies changes, the deep level is structurally unfavorable for trapping hydrocarbons. Based on changes we found in map trend, we divided the Warfield structures into a middle segment and southern and northern bends. The middl segment is parallel to the New York-Alabama lineament (a northeast-trending magnetic gradient); the southern and the northern bends are linked to the 38th parallel lineament (a west-trending fault system) and the Burning Springs-Mann Mountain lineament (a north-trending magnetic gradient), respectively. We propose a wedge tectonic model to explain (1) northern and southern bends of the Warfield structures and hydrocarbon distribution in the subsurface of southwestern West Virginia; (2) vertical changes in geometry and structural style from the deeply buried half graben to the overlying asymmetric anticline; and (3) geometric and kinematic relationship among the Warfield structures, the 38th parallel, and the Burning Springs-Mann Mountain lineaments.

Reinterpreted Oriskany structure at the North Summit field, Chestnut Ridge anticline, Pennsylvania

A widely accepted structural model for folds in the outer central Appalachian foreland is partially based on the geologic structure of the North Summit field. The model includes a simple surface anti cline that is detached in Silurian Salina Group salts and cored by imbricated Devonian Tully-Helderberg rocks thrust inward toward a depressed axial low. New data from wells at North Summit show that the core of the Chestnut Ridge anticline is not filled with imbricated reservoir rocks but that the reservoir is deformed into a series of faulted folds. Gas was trapped by a combination of closure and sealing faults. Space problems within collapsing synclines above competent reservoir rocks, the Huntersville-Helderberg lithostructural unit, are resolved by distortion and evacuation of overlying, incompetent Hamilton rocks. Huntersville-Helderberg rocks deformed into a variety of structural shapes, not solely the imbricated model that traditionally has been applied to Plateau folds of the central Appalachian foreland. Disparity in relief between pre- and post-Salina rocks indicates that primary detachment occurs in evaporite beds of the Salina Group. Erratic dips in presalt units and unresolved differences in structural relief between synclines flanking the Chestnut Ridge anticline, however, suggest that the basal detachment lies within the Martinsburg-Reedsville shales under the Allegheny Mountains. Regional decollement in Martinsburg shales ends in a triangle zone at the Intra-Plateau Front, at the Chestnut Ridge anticline, whereas the Salina decollement continues westward under the Pittsburgh Plateau.

Fractured Devonian Shale Reservoir, Appalachian Basin: ABSTRACT

Detailed structural analysis along the west flank of the Appalachian basin in Kentucky and West Virginia demonstrates the importance of detached and basement deformation in developing fracture permeability within Devonian shales. A porous fracture facies of regional extent within the organic-rich lower Huron Member of the Devonian shale partially relates to unique physical properties of the organic sediments, but an important factor for widespread gas production is fractures caused by differential shortening of sediments above a detachment surface in the lower Huron Member. Mineralized, uniquely oriented, and slickensided fractures, and increased fracture intensity within the organic lower Huron shales perpendicular to Alleghanian stress support this interpretation. The p rous fracture facies is most permeable (commercial) beyond the region of major tectonic transport where permeability is only local in extent. Linear trends of abnormally high final open flows in the producing area relate to trends of intensely fractured organic shale. These fracture zones seemingly reflect unique, complex, and perhaps more intense shear stress within organic shale found in flexures above basement faults. Gas migrated updip along open fractures placing the best wells slightly updip along the fracture trend or on the flank of adjacent low-relief flexures. This unique reservoir forms its own source and seal, and the lithologically restricted fracture facies imparts the permeability. Tailoring completion techniques which limit the vertical extent of induced fractures and whi h enhance recovery in the more common orthogonally fractured shale of the mid-continent region will be important for future development of this huge resource. End_of_Article - Last_Page 630------------

Importance of Regional and Local Structure to Devonian Shale Gas Production from Appalachian Basin: ABSTRACT

Over 1 Tcf of high-quality natural gas has been produced from Devonian shales during the past 100 years. The shale is its own source, seal, and reservoir; natural features within the shale form the reservoir. Organic shale is the prime requisite for production, but productivity relates to the presence of open fractures so that one can presume that abnormally productive trends correspond to open fracture zones. This presumption is supported by the direct association of linear zones of high production with linear basement structure detailed in three fields studied. The blanketlike nature of the lower production ubiquitous in southern West Virginia and eastern Kentucky probably relates to a fracture facies developed within the shale and controlled by shale lithology and Appa achian tectonics. End_of_Article - Last_Page 1671------------

Paleozoic Disruptive Deformation in North American Continent and Its Relation to Formation and Development of Interior Basins and Deformation Within Orogenic Core: ABSTRACT

The two major disruptive tectonic events during the Paleozoic which affected the North American craton seem to be associated with Appalachian-Ouachita orogenic events. The first Paleozoic cratonic disruption was tensional rifting, which occurred during the Avalonian intrusive-metamorphic event (± 560 m.y.). Evidence continues to mount that these Cambrian rifts of the Appalachian-Ouachita foreland, such as the Rome trough of Kentucky and West Virginia, are not Cambrian aulacogens. By time and position, the rifts seem to be incipient basins along a developing back-arc trough. However, this disruptive deformation was not restricted to the developing arc trough, but extended far into the craton where it commonly involved reactivation of older rift zones. These zones form d the axial portion of the subsequently developed Paleozoic basins. The Paleozoic basins developed by epeirogenic movement after a period of relative quiescence during Late Cambrian through Early Ordovician (pre-Taconic) time. The second Paleozoic continental disruption created large upthrust blocks in the craton during the Pennsylvanian and early Permian, probably by compressional deformation. This event ties, both by time and position, to deformation within the Ouachita part of the orogenic core. Upthrust crustal blocks in the craton may be bounded by reactivated faults of precursor rifts. When they formed, the upthrusts often developed near the axial part on the middle Paleozoic basins to form the late Paleozoic yoked basins. The occurrence of axial rifts within interior and foreland basins, and of axial upthrusts in the craton-margin basins, suggests an interrelation among rifts, basin formation, and the late-forming yoked basins. The developing foreland trough (the Appalachian-Ouachita geosyncline) has a tectonic history similar to that of the cratonic basins but, along its trend, tensional bending of the basement predominated. End_of_Article - Last_Page 1588------------

Disharmonic Folding in Iran: ABSTRACT

Exploration for petroleum in the Iranian Zagros folded belt has revealed spectacular disharmony between surficial folds in the terrigenous clastic Fars-Bakhtiari sediments and deeper folds in and below the Asmari Limestone, the major producing formation. Anhydritic marl, and locally thick salt of the Lower Fars stage I mobile unit, separate the two disharmonic fold sets. Some geologists have interpreted the disharmonic folding to have developed essentially in place without significant differential movement between the two fold sets. An alternate interpretation more compatible with the structural details proposes differential movement of two uniquely folded litho-structural sequences. A time-lapse movie of a dynamic model illustrates how such disharmonic folds may develop. /P> End_of_Article - Last_Page 209------------