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We have used three dimensional seismic, magnetic, and gravity data in an integrated approach to map the basement surface and the associated structural features in the Arkoma basin, Oklahoma and Arkansas. Analysis of the 3D seismic data including structural interpretation and seismic attributes has revealed a very intensive EW zone of deformation or crustal weakness influencing the northern part of the study area. This zone of weakness may represent a Late Paleozoic tectonic (structural) inversion of the normal faulting (block faulting) which developed during the Cambrian rifting. Seismic structural interpretation reveals also a compressive structural style of deformation related to Ouachita orogeny dominating the Late Paleozoic time. We have recognized a clear relationship between the Precambrian basement structures and the Paleozoic structural deformation and depositional history. Edge detector techniques of the magnetic data have delineated clear magnetic boundaries (faults or body edges) that extend in EW, NE-SW and NW-SE in the northern, southeastern, and western parts of the study area, respectively. The Euler magnetic depth estimation method has also delineated the previously mentioned faults by showing clustering of the solutions along these fault trends. The Euler's method shows a maximum depth value of about 3850 m to the faults that affect the basement and/or the intrabasement features with a preferable depth of approximately 2000 m. The trends of the faults obtained from seismic data interpretation show a very clear correlation with those determined by the Euler's method and the edge detector techniques.
The Precambrian basement structures in the continental United States have strongly influenced later Proterozoic and Phanerozoic tectonism within the continent. The aim of this study is to map the basement surface and the associated structural elements in the Arkoma basin area as well as to investigate the relationship between these structures and the Phanerozoic tectonic deformation in the area.
The basement surface is rarely well imaged because most seismic surveys are designed to image shallower targets for hydrocarbon exploration and there is a rarity of wells that penetrate deeply to the basement surface. Mapping the basement features is very important because Precambrian tectonics are responsible for the processes controlling the formation of natural fractures and faults in the overlying sedimentary cover in the basins and hence the distribution of petroleum resources. Precambrian basement structures in the continental United States strongly influenced later Proterozoic and Phanerozoic tectonics within the continent (Sims et al., 2005). The structures dominating the formation of the Ancestral Rocky Mountains, the Laramide and Sevier orogenies were strongly affected by intense deformation along zones of weakness in the Precambrian basement. During a new tectonism, the preexisting zones of weakness are preferentially reactivated at the expense of formation of new fractures (Holdsworth et al., 2001). The weakness zones of the basement contribute a prominent role in localizing magmatism and ore deposits (Sims et al., 2002). At a local basin-scale, the basement structures in the Fort Worth basin are responsible for the intrasedimentary features such as faults, fractures and collapse features in the Ellenberger Group and Viola limestone, (Sullivan et al., 2005; Baruch et al., 2009 and Elebiju et al., 2010). Moreover, hydrocarbon production from the fractured basement has attracted more attention to the need for better understanding of the basement. Many commercial hydrocarbon accumulations have occurred in reservoirs in fractured basement rocks in several countries (Gutmanis et al., 2010). New hydrocarbon plays are developing in the Arkoma foreland basin and within the Ouachita orogenic belt. Thus, there may be chances for petroleum accumulation in the Lower Paleozoic strata lying beneath the Ouachita thrust sheets (Keller, 2009). In this study, recently released 3D seismic data, gravity, and magnetic data have been integrated with geological information for mapping and illumination of the basement structures of western Arkoma basin, Oklahoma and Arkansas.
In southern Oklahoma, the basement rocks are represented by two different provinces; the Wichita and Arbuckle Mountains. The basement rocks have controlled the structural and stratigraphic evolution of the Paleozoic sediments in the two provinces in two different manners (Ham et al., 1965). Basement-involved structures are common in the foreland and subthrust platform of the Ouachita orogeny. The basement-fault-block developed before or during the Cambrian rifting. Many of these faults were reactivated by the Late Paleozoic tectonics and they cut upward through the sedimentary cover (Thompson et al., 1995).
Location and geology
The Arkoma basin is an arcuate structural feature that extends from the Gulf Coastal Plain in central Arkansas westward 400 km to the Arbuckle Mountains in south-central Oklahoma (Fig. 1). The basin is bounded on the north and northwest by the Ozark uplift and the Northeast Oklahoma platform. The southern margin of the basin is marked in Oklahoma by the Choctaw fault and in Arkansas by Ross Creek fault, both of which define the cratonward margin of the Ouachita fold belt (Sutherland, 1988). The basin was once a part of a stable shelf along a passive continental margin and it developed as a foreland basin related to Ouachita orogeny. The basin is characterized by down-to-the-south normal faults that affect Lower Pennsylvanian and deeper rocks. Many folds and faults in the Arkoma basin were produced by horizontal compressive forces related to the Ouachita orogeny (Keller, 2009). The compressive forces were directed north and northwest and decreased in intensity away from the Ouachita Mountains region (Eissa and Castagna, 2003). The Ouachita rocks are divided to two major stratigraphic units known as Ouachita facies (Viele, 1989). The lower unit, known as preorogenic, was deposited in the Late Cambrian to Early Mississippian, and it includes two different facies (Fig. 2). The first (off-shelf facies) is composed of deep-water shale, sandstone and chert that was deposited on the continental slope and the second is composed of shallow-water carbonates that were deposited on the shelf. The upper, synorogenic unit is mainly composed of deep-water turbidites in the basin and deltaic sediments on the shelf (Viele, 1989). The Arkoma basin is considered mainly as an Atokan basin where its subsidence mostly took place during Atoka time. About 20,000 ft. of the Atokan sediments have been deposited in Arkoma basin. Generally, the thickness of the sedimentary section in Arkoma basin ranges from 3000 ft. on the shelf at the north to 30,000 ft. along the forefront of the Ouachita Mountains at the south (Branan, 1968).
Tectonic history of the Arkoma basin-Ouachita orogenic belt province
The collisions of the Archean continents formed the Canadian Shield (Whitmeyer and Karlstrom, 2007). The addition of volcanic arcs and the accretion of oceanic terranes in the Paleoproterozoic built the lithosphere of the area known today as southern North America. The continental growth of the southern North America ended with the Grenville orogeny (Mosher, 1998). A supercontinent known as Rodinia was the result of this continental growth. The geologic history important to this study began with the breakup of Rodinia around 600 Ma and the development of a rifted continental margin about 550 Ma (Keller et al., 1983; Kruger and Keller, 1986). The passive continental margin lasted until Middle Paleozoic (Fig. 3) (Houseknecht, 1987). The Laurentia has been rifted along trends which were followed later by the Appalacian-Ouachita orogenic belt and oceans opened along the newly formed continental margins (Viele and Thomas, 1989). During the period that extends from the Cambrian to the Lowermost Mississippian, two different sedimentary facies were deposited. The first facies is composed mainly of carbonate with minor shale and sandstone and is known as shelf facies, while the second facies, known as off-shelf or Ouachita facies is composed mainly of shale with minor limestone, sandstone and chert (Houseknecht, 1987).
The ocean basin began to close in the Devonian-Early Mississippian due southward subduction beneath an approaching continental landmass often referred to as Llanoria. A magmatic arc which can be recognized by the existence of subsurface volcanic rocks at the Sabine uplift developed along the northern margin of Llanoria (Houseknecht, 1987). Consequently, Ouachita orogenic belt started to form as an accretionary prism.
During the Mississipian and the Earlier Atoka, the shelf area kept a slow rate of sedimentation where carbonate, sandstone, and shale continue to be deposited in environments range from shallow to nonmarine. The deep Ouachita (trench) and the remnant ocean basin were characterized by rapid deposition of flysch sediments. The ocean basin was closed due to the subduction and the accretionary prism was obducted onto the rifted continental margin (Dickinson, 1974). The continental margin has experienced a flexural bending due to the attenuation of the continental crust and the load exerted by the obducted material. Consequently, many normal faults have been created in the foreland (Houseknecht, 1987). Atoka sediments which represent the transitional stage between passive margin and foreland basin sedimentation were deposited contemporaneous with the faults movement. Finally, close to the end of Atoka time, the continuous advancing of the subduction complex resulted in uplifting of the thrusted strata along the frontal thrust belt of the Ouachitas and the foreland basin has been completed (Gangopadyay and Heydari, 1995; Houseknecht, 1987).
The Ouachita Mountains are divided into three regional provinces depending on the structural elements (Arbenz, 2008). Starting from the north to the south these structural provinces are arranged as follows: (1) southern Arkoma basin fold belt, (2) frontal Ouachita thrust and fold belt, and (3) the Ouachita allochthon (Fig. 1). Our study area lies at the transition zone between the Arkoma basin and the Ouachita orogenic belt. A brief description of these tectonic (structural) provinces is given in the next paragraphs.
The southern Arkoma basin fold belt represents the northernmost division of the Ouachita orogenic belt where it is located between the compressional structures at the far north and the traces of the Choctaw and Ross Creek thrust faults to the south. The province is characterized by two structural levels separated by detachment surface in the Atoka ductile shale (Arbenz, 2008). The first structural style is represented by block faulting due to tensional forces associated with the basin subsidence in the Early Pennsylvanian while the Ozark uplift remained as positive high feature to the north. As a result of this subsidence, the tensional forces have produced several normal faults trending EW (Branan, 1968). The faults were active during Lower to Middle Atoka resulting in a significant thickening of those strata especially over the downthrown blocks (Houseknecht, 1987). The faults are obviously exposed on the surface in the northern parts of the basin and they are obscured in the basin trough area. The majority of these faults are characterized by downthrows to the south (Branan, 1968). These faults strike parallel to the Ouachita orogenic belt and offset the basement and Lower Paleozoic sediments. Most of these faults broke previously undeformed continental crust, while few of them have been the result of the reactivation of the Early Paleozoic rifting related faults (Houseknecht, 1987). The second structural style is represented by compressional structures due to the Ouachita orogeny. The compressional structures include thrust faults and narrow, long anticlines and synclines parallel to the Ouachita orogenic belt strike (Branan, 1968). The folded section is underlain by thrust faults which ramp to the surface along many anticlinal crests (Houseknecht, 1987). In addition to the previous two structural styles, there are high angle thrust faults influencing the older and deeper rocks in the area of the basin trough. In contrast to the down-to the-basin faults, these faults are upthrown on the south and they are unrelated to them (Branan, 1968).
The frontal Ouachita thrust and fold belt's northern boundary is represented by both the Choctaw fault and the Ross Creek fault in Oklahoma and Arkansas, respectively (Arbenz, 2008). The Ti-Valley and the Y-City faults are defining the southern boundary of this belt. The structural deformation in this belt has been strongly controlled by the thickness of the Atoka sediments. The thick turbidite sandstone show broad synclines and massive thrust slabs while the thin sandstone sections show narrow faulted folds and laterally limited thrust slices. The frontal thrust and fold belt terminates near Atoka, Oklahoma where the Choctaw and the Ti-Valley faults merge together.
The Ouachita allochthonous province has structural styles which differ vertically in the stratigraphic column. The older section, known as preorogenic, is mainly composed of ductile shale of pre-Pennsylvanian time. The younger section, orogenic sequence, is represented by competent sandstone of Pennsylvanian age. The Ouachita allochthonous province is divided into four distinct sub-provinces (Arbenz, 2008).
The northern and central parts of the Arkoma basin lie in the continental foreland province which extends northward to include\cover the southern flank of Ozarak uplift and the southwest extension of the Northeast Oklahoma shelf. There are no compressional structures in this zone. The basement and the sedimentary rocks have been subjected to extensional faults due to the breakup and rifting of the southern Lurantia margin (Thomas, 1977). An active faulting phase influenced the platform rocks during the period from Early Atoka to the Desmoinesian synchronous (contemporary) with the down-bending of the basin flank. Faults striking NE-SW with downthrows to the south have been active during the deposition of the Atoka Formation. Consequently, the subsidence of the basin and the accumulation rates has been accelerated. Some of these faults can be recognized beneath the Ouachita thrust and fold belt strata (Arbenz, 2008). Figure (4) shows a structural cross section OK4 across the Ouachita orogenic belt-Arkoma basin province
The study area
The study area lies at the most western part of the Arkoma basin in southeastern Oklahoma where it is bounded by Arbuckle Uplifts to the west (Fig. 1). The area occupies part of the transition zone between the Ouachita frontal zone and Arkoma basin. Choctaw fault, the leading thrust of the Ouachita orogenic belt and the boundary between Arkoma basin and the Ouachitas, separates two different structural and stratigraphic styles. To the north of Choctaw fault, the Upper Atoka-Desmoinesian deltaic strata are mostly deformed by broad, open folds and few faults. To the south of the fault, shallow-water carbonate and deep-water turbiditie sediments of Morrowan-Lower Atoka are dominated by imbricate faults, isoclinal folds and overturned strata (Suneson, 1995). The blue rectangle (Fig. 1) shows the boundaries of the area which has been subjected to magnetic and gravity data analysis while the red polygon shows the boundaries of the 3D seismic survey (Atoka survey).
3D seismic data and seismic attributes
Our seismic data are represented by the three dimensional Arkoma basin seismic survey. The Arkoma basin survey lies at southeastern Oklahoma and it was acquired for imaging Woodford Shale, Hunton Limestone and other targets (Fig. 1). The 3D seismic volume has been subjected to structural interpretation of some major faults as well as picking the tops of the Woodford, Timbered Hill Group and the basement horizons. We generated some volumetric attributes including the dip magnitude, dip azimuth (combined dip-azimuth), coherence and most positive and most negative curvatures. We generated also the basement surface and we followed this by generating some seismic surface attributes along the basement surface as well as using horizon probes.
A panoramic 3D view shows the basement surface with intensive irregular topography due to long period of exposure and weathering (Fig. 5). The white arrow shows a clear unconformity between the basement rocks and the Paleozoic section. An area of severe deformation where some faults penetrate into and offset the basement is shown by the red ellipse. The seismic section AAÊ¹ shows a tectonic or structural reactivation of the basement normal faults (block faulting) as a compressional structure (Fig. 6). The high angle reverse fault (F3) seems to be a master or major rifting fault that was reactivated as reverse fault due to compressional tectonic event probably of Late Paleozoic time. At southern Oklahoma, the reactivation of the weakness zone of the Proterozoic age during the Cambrian rifting created normal faults which have been reactivated again as oblique strike-slip faults during the Pennsylvanian time (Thomas and Baars, 1995). High angle thrust faults which dominate the older and deeper strata exist near the Choctaw fault, the approximate forefront of the Ouachita Mountains, and they were created mostly due to Ouachita compressive forces (Branan, 1968). To the southwest of the study area, the south-dipping Washita Valley thrust fault exposed the sedimentary rocks and rhyolite of the Cambrian on the southern hanging wall and it exposes the Ordovician carbonates on the northern footwall (Puckett, 2011).
The seismic section AAÊ¹ shows also a relationship between the faults that dominate the basement and Lower Paleozoic rocks and those dominate the Upper Paleozoic rocks (Fig. 6). A thrust fault that influences the shallower Upper Paleozoic strata is aligned vertically above the major reactivated revers fault (F3) that dominate the deeper strata of the Lower Paleozoic and the basement. This interpretation can be supported by the fact which states that Choctaw and Ross Creek faults originated near basement faults (Arbenz, 2008). Moreover, there is a distinct thickening of the Atoka Formation and the Late Pennsylvanian strata over the downthrown blocks. Atoka Formation shows a considerable thickening on the downthrown side of growth faults developed during the subsidence of the Arkoma basin (Houseknecht, 1987; Sutherland, 1988). The seismic section BBÊ¹ (Fig. 7) confirms the geologic facts obtained from the previous figure. The section shows the same area of intensive deformation where the master rifting fault F3 was reactivated as a reverse fault while the fault F2 is normal in this seismic section. The section shows also thicker Late Paleozoic strata above the downthrown blocks than those in the section AAÊ¹. Moreover, the seismic section CCÊ¹ confirms the previously mentioned relationship between the shallower compressional structures and the deeper rifting (extension) faults as well as the relationship between the thickening of the Late Paleozoic strata and the downthrown blocks of the Early Paleozoic rocks (Fig. 8).
Seismic attributes such as coherence and curvature can be used as powerful tools for optimizing seismic interpretation with the ability of revealing structural deformation and depositional history (Chopra and Marfurt, 2007b). In contrast to the conventional amplitudes which have the ability to show the faults running perpendicular to the strike, the coherence volume can reveal faults running parallel and perpendicular to the strike. Low coherence lineaments along fault planes and deformed areas result from discontinuities of trace-to-trace coherence of seismic traces (Chopra, 2001b). A time slice through coherence volume can delineate lineaments of low coherence along faults, channels, salt edges, and unconformities planes (Chopra, 2001a). On the other hand, volumetric curvature attributes enable interpreters to delineate small flexures and folds (Chopra and Marfurt, 2007b). Curvature shows subtle flexures not seen by coherence because coherence is sensitive only to lateral discontinuities.
Time slices (1500 ms) through the coherence and variance volumes show the discontinuities (faults and deformed zones) as incoherent dark lineaments (Fig. 9). The fault planes striking EW, perpendicular to the page plane, show clear coincidence with the dark lineaments of the coherence and variance time slices. Similarly, time slices (1500 ms) through the most positive and most negative curvature volumes show the same fault trends striking EW (Fig. 10). More specifically, the most positive curvature shows maximum positive values in red over the upthrown blocks and the anticlinal features, while the most negative curvature shows maximum negative values in blue over the downthrown blocks and the synclinal features. Generally, both the volumetric coherence and curvature attributes have clearly contributed to enhance the occurrence of the EW striking faults in the northern part.
The three dimensional view through the combined dip magnitude-dip azimuth volume co-rendered with the coherence volume shows an EW trend of high dip angles shown in bright colors corresponding to the area of high deformation (see the ellipse) while the flatter areas are shown in pastel colors (Fig. 11). Similarly, the combined dip magnitude-dip azimuth co-rendered with the seismic volume as well as a horizon probe through the basement surface show the same previously mentioned EW trend of deformation (Fig. 12). The deformed rock blocks of the downthrown side shows a belt of bright orange to red which indicate dipping to the southeast (see the red arrow), while the upthrown blocks are shown in blue indicating dip to the north (see the blue arrow).
Gravity and magnetic data analysis
Gravity and magnetic data were (acquired) obtained from database maintained by the university of Texas at El Paso, Department of Geological Science through the website // research.utep.edu/paces. The two datasets were subjected to some reductions and corrections to obtain the final end product represented by Bouguer anomaly and the total magnetic intensity girds. The Bouguer gravity map has been constructed with grid size of 5000 m while the total magnetic intensity was gridded with 2000 m. Both of the two maps were subjected to some filtering and derivatives techniques to get more indicative maps for optimum interpretation. The Bouguer gravity grid has been subjected to regional-residual separation to get the residual map which has been used to build (construct) density models. The Bouguer gravity map shows very low gravity anomaly related to Arkoma basin (AB) and very high anomalies due to Wichita Uplifts (WU), Arbuckle Uplifts (AU) and Ouachita interior zone (OIZ) (Fig. 13). The total magnetic intensity map (Fig. 14) of the area shown by the black rectangle in (Figs. 1 and 13) has been subjected to edge detector techniques and Euler depth estimation method. The location of the 3D seismic survey is shown by the red polygon.
Edge detector techniques
The total horizontal derivative and the tilt derivative of the magnetic data are powerful tools for delineating the edges or boundaries of geologic bodies. The maps resulted from the application of these techniques show clear boundaries of geologic bodies or faulted blocks shown by the obvious contrast in the magnetic anomaly signatures on both sides of the boundaries.
Total horizontal derivative
The edges of magnetized or high-density bodies have been often delineated by the horizontal derivatives of the potential field data which show maxima above the body edges or over the vertical contacts. The total magnetic intensity should be corrected for the inclination of the Earth's magnetic field (Lahti and Karinen 2010). The total horizontal derivative peaks over the edges and is zero over the body (Miller and Singh, 1994). The method has been commonly used to locate the steep gradients associated with vertical boundaries, such as faults (Blakely and Simpson, 1986).
The total horizontal derivative (THD) magnitude of the magnetic data grid has been calculated using formula by (Verduzco et al., 2004):
where, T is the total magnetic intensity of the magnetic field data.
The total horizontal derivative map shows clear magnetic boundaries which may coincide with the EW trending Olney-Bromide fault, NE-SW Choctaw fault and NW-SE striking of unknown fault, see the white arrows (Fig. 15. a).
The tilt derivative can be defined as the ratio of the first vertical derivative to the total horizontal derivative of the potential field. The technique has the ability to delineate sources in areas of low gradients. In contrast to the total horizontal derivative, the tilt derivative has high positive value over the source body, close to zero value near the edge, and a negative value outside the source region (Miller and Singh, 1994). Salem et al., (2007) used the zero contours to locate the magnetic body edges and to calculate the depth to the magnetic body by measuring the distance between the zero and the +45Â° or -45Â° contour lines.
The tilt derivative (TD) technique has been also applied to the magnetic data using the formula of (Verduzco et al., 2004):
where, VD is the vertical derivative and THD is the total horizontal derivative of the magnetic data, respectively.
The tilt derivative map of the study area reflects the same boundaries shown by the THD map by showing a close to zero values at or near the edges of the magnetic source bodies or the fault planes (Fig. 15. b).
Euler depth estimation method
The Euler deconvolution method can be used to determine the location and the depth to the magnetic source (Thompson, 1982). The method has the ability to delineate the magnetic boundary or fault trends as well as the geometry of the magnetic source (Reid et al., 1990). Euler's homogeneity relation can be written as:
(x - x0)âˆ‚T/âˆ‚x + (y - y0)âˆ‚T/âˆ‚y+ (z - z0)âˆ‚T/âˆ‚z = N(B-T) â€¦â€¦â€¦â€¦â€¦â€¦â€¦â€¦â€¦â€¦â€¦â€¦â€¦â€¦â€¦...(3)
where (x0, y0, z0) is the position of a source, whose total field T is detected at (x, y, z). B is the regional field or background value and N is the degree of homogeneity, and it is also known as the structural index (Thompson, 1982).
Euler solution cluster plot using structural index (S.I = 0.0) for magnetic contact (faults) shows clustering of magnetic source solutions along an EW trend to the north which may coincide with the Olney-Bromide fault (Fig. 16). Another cluster striking NE-SW at the southeastern area may coincide with the Choctaw fault. There is also a NW-SE cluster to the west which may be related to the previously unrecognized fault. The depth to the basement has been calculated using the Euler depth estimation method. The depth shows a maximum value of about 3850 m that may reflect the depth to the faults penetrate deeply into the basement or may be related to intrabasement features. The more reasonable and accepted depth to the basement is about 2000 m and it is constrained by depth values obtained from some wells penetrate to the basement.
Euler depth estimation & edge detector techniques
We have superimposed the results obtained from the application of the Euler's method on both the total horizontal derivative and tilt derivative maps of the magnetic data. The resulting two figures show good correlation between the solution clusters of the Euler's method with the edges of the magnetic source bodies determined by the edge detector techniques (Fig. 17. a and b).
Integration of results from seismic and magnetic data
The results from Euler solution cluster plot and edge detector methods show a considerable correlation with those obtained from the interpretation of the 3D seismic data and seismic attributes. The EW striking Olney-Bromide and the NE-SW striking Choctaw faults shown by the time slice through the seismic volumes show a very good matching with those shown by the Euler solution cluster plot and the edges obtained by the edge detector methods (Fig. 18. a and b).
The interpretation of the gravity and magnetic data should include the modeling process as a final step. The interpretation process should include all relevant and reliable data that may help constrain the model (Williams and Finn, 1985). Modeling aims to use all the obtained results from the different techniques for designing and describing one or more possible geological property such as density or magnetic susceptibility distributions across a specific region in the study area and the cause of such distribution. Any potential field model provides a non-unique solution, that is, the observed data can be fit in many different ways (Jones-Cecil, 1995).
Two local density models along two selected profiles across the residual gravity map after upward continuation to 40 km have been constructed (Fig. 19). The models have been constrained by geological and geophysical information as well as the depth obtained from well data to get the optimum results. Two well datasets penetrate deeply to the tops of the Arbuckle Group and the basemen provided by Oklahoma Geological Survey have been used. The approximate thicknesses of the different stratigraphic units and their densities have been obtained from literatures and regional gravity models. The two models were chosen along profiles AB and CD in the NE-SW and NW-SE, respectively. The models reflect the subsurface distribution of the different blocks with their densities as well as the structural styles influencing the area. The first model along the profile AB shows that the basement depth increases from SW to NE (Fig. 20). The basement is exposed at the surface in the Arbuckle Mountains area to the far southwest and it becomes deeper to the northeast where its depth reaches a value of about 4000 m. The high gravity anomaly at the far southwest may be the result of the deeply buried mafic igneous rocks of the Cambrian rifting beneath the Arbuckle Mountains. On the other hand, the low gravity anomaly corresponding to the area located between the two wells named Arb-7 and Arb-16 (wells penetrate to top of the Arbuckle), may be related to the Lehigh syncline filled with thick cover of the fewer/less dense clastic sediments of the Pennsylvanian (Arbenz, 2008). The second model shows a distinct positive gravity anomaly coincides with or near the Coalgate anticline at the northern part (Fig. 21). The positive anomaly may be the result of thinning of the fewer/less dens Middle-Late Pennsylvanian clastic rocks and due to shallow Precambrian rocks. A negative anomaly exists directly to the south of the positive one and it occupies the central part of the area. This anomaly may be the result of the thickening of the less dens Middle and Late Pennsylvanian Atoka and Desmoinesian clastic sediments due to the thrusting of the deep-water frontal Ouachita sediments. To the far south, there is a positive anomaly which may be related to the existence of mafic rocks in the core of the Arbuckle Mountains or due to igneous and metamorphic rocks of the Ouachita interior zone. Another possible reason for this positive anomaly may be due to the nearness to the Cambrian rifting or the transition zone between the continental and oceanic crust.
The integrated approach adopted in this study has led to optimization of the interpretation process and the results. The basement rocks were subjected to severe and intense deformation as well as deep erosion. Seismic structural interpretation reflects different styles of structural deformation influencing the study area. Normal faults (extensional block faulting) dominate the basement and the Lower Paleozoic rocks while compressional structures dominate the Upper Paleozoic rocks. An EW zone of crustal weakness or intense deformation has been recognized in the northern part of the study area. The weakness zone may represent a tectonic (structural) reactivation of the Cambrian rifting faults as compressional thrust faults of the Late Paleozoic Ouachita and/or Arbuckle orogeny. The basement structures clearly influenced the Paleozoic structures and the depositional history of the Arkoma basin. Seismic attributes as well as Euler solution cluster plot and edge detector techniques show and enhance the occurrence of the EW zone of the intensive deformation and the other fault trends. Fault trends determined by the Euler depth estimation method show very good correlation to those obtained from seismic data. The depth to the basement increases generally from the southwest to the northeast with more reliable depth of about 2000 m. The maximum depth value of about 3850 m may due to faults that penetrate deeply into the basement or due to intrabasement features or structures.
The authors would like to thank Cgg Veritas and PABLO ENERGY II, LLC for providing the 3D seismic data and giving the permission to publish this work. The authors would like also to thank the Oklahoma Geological Survey staff especially Mr. Richard D. Andrews for providing the well data and Dr. Neil H. Suneson for kind discussion and sharing his knowledge. Special thanks to James Anderson for preparing the index map of the study area. Seismic interpretation was done by Petrel software provided by Schlumberger for use in research and education.