Abstract:
The Permian Basin is a structurally complex sedimentary basin with an extensive history of tectonic deformation. As the basin evolved through time, sediments dispersed into the basin floor from surrounding carbonate platforms leading to various mass movements. One suchmass movement is observed on a 3D seismic survey in the Upper Leonard interval (Lower Permian) of the Midland Basin that is characteristic of a mass transport deposit (MTD). The 350 ft thick MTD mapped in the study area is 5 mi wide, extends up to 14 mi basinward, and covers only the translational and compressional regime of the mass movement. A unique sedimentary feature, unlike those observed previously, is mapped and interpreted as gravity spreading. MTDs have been extensively studied in the Delaware Basin of Permian-aged strata; however, only a few works have been published on the geomorphological expression of MTDs using seismic and seismic attributes to delineate the shape, size, and anatomy of this subsurface feature. The MTD in the study area exhibits an array of features including slide, slump, basal shear surface, and MTD grooves. In cross section, the MTD is characterized as chaotic with semitransparent reflectors terminating laterally against a coherent package of seismic facies, or the lateral wall. Sobel filter-based coherence, structural curvature, dip magnitude, and dip azimuth attributes are used to map thrust faults within the discontinuous MTD. Kinematic evidence provided by the Upper Spraberry isopach suggests that this MTD was sourced north of the Midland Basin and deposited on the basin floor fairway. Slope strata are interpreted from well-log analysis showing MTD as a mixture of carbonates and siliciclastics with a moderate to high resistivity response. Introduction Mass movements generate the most impressive deposits in terms of volume on the earth’s surface in subaqueous and subaerial settings. Nissen et al. (1999) are the first to document the various aspects of mass movements in seismic data using coherence attribute in the Nigerian continental slope including mass transport deposits (MTDs). Such mass movements are distinctive in deepwater depositional systems mostly due to their large size, geomorphology, and chaotic internal character (Shipp et al., 2011). Furthermore, MTDs have been known to play a significant role in petroleum exploration because they may be top and lateral seals or may have acted as paleobathymetric constraints on the deposition of overlying reservoir deposits (Amerman, 2009). Regardless of their architecture or their ability to hold hydrocarbons, MTDs are in essence earth’s modern and ancient deepwater stratigraphic record and are an important tool in our understanding of mass movements in slope settings around the world. At the time of MTD deposition, the Midland Basin was surrounded by carbonate platforms, which provided vast inputs of carbonates into the basin. The LeonardianSeries (Lower Permian) had shelf to open marine depositional environments in the Midland Basin, which included siliciclastic and carbonate rocks with detrital limestone restricted to slope and base-of-slope settings (Hamlin and Baumgardner, 2013). Handford (1981a) points out that the Leonardian-aged sediments (Spraberry Formation) were deposited as a large basin-floor submarine fan systemand are commonly interpreted as deposits of turbidity currents and debris flow. The present study observed a different spectrum of mass movement on 3D seismic in the Upper Leonard interval, which is representative of MTDs. In this study, anMTD is described as a gravity-flow deposit in which grains remain in contact with each other as opposed to turbidity deposits. The Permian Basin has been known to host vast amounts ofmassmovements, and studies have been conducted to understand the importance of the underlying paleobathymetry and its effect on sediment flow and reservoir facies distribution. Amerman (2009) investigates the structure and stratigraphy of deepwater MTDs in the Permian Cutoff Formation and overlying Brushy UT Permian Basin, Department of Geosciences, Odessa, Texas, USA. E-mail: paritoshb12@gmail.com; sumit.verma.geophysicist@gmail.com; verma_s@utpb.edu. Fasken Oil and Ranch Ltd, Midland, Texas, USA. E-mail: ronb@forl.com. Manuscript received by the Editor 12 March 2019; published ahead of production 30 July 2019. This paper appears in Interpretation, Vol. 7, No. 4 (November 2019); p. 1–14, 19 FIGS. http://dx.doi.org/10.1190/INT-2019-0036.1. © 2019 Society of Exploration Geophysicists and American Association of Petroleum Geologists. All rights reserved. t Special section: Permian Basin challenges and opportunities Interpretation / November 2019 1 Canyon Formation in the Delaware Basin to analyze the internal architecture and stratigraphic relationship of MTDs in succession. Allen et al. (2013) study MTD in the second Bone Spring Formation in the Delaware Basin of West Texas, USA, in which the authors use seismic and well-log data to map the compressional features of the MTD along with the log responses to highlight the MTDs reservoir potential. Asmus and Grammer (2013) further investigate the architectural attributes of less than 3 ft thick turbidites andMTDs in the Delaware Basin of the Upper Bone Spring Formation. Based on their study on two cored wells, the authors conclude that more than 90% of MTDs (slumps and debris flows) observed in the cores are easily identified in image logs. Moreover, the MTDs were correlated to decreasing gamma-ray and increasing resistivity responses in conventional logs. In summary, several extensive studies conducted in the Delaware Basin highlight the importance of paleobathymetry and the subscale architecture and composition exhibited by MTDs. Even though mass movements have been well-documented in the lower Permian period, few works have been conducted to illustrate the geomorphology of these features using seismic expression. In this paper, we first review the geology of the Upper Leonard interval of the Midland Basin and understand the paleobathymetry of the Upper Spraberry Formation and its control on the morphology of the overlying MTD. Then, we move from available seismic data to detailed analysis of characterizing the MTD using seismic attributes, and we conclude with the overall interpretation of the shape, size, and anatomy of this subsurface feature. With integrated well control and 3D seismic data, new insights are put forward in our understanding of how paleobathymetry affect sediment flow and unfold the geologic evolution of the MTD mapped in the study area. Geology of the study area The extent of the Permian Basin spans an area of approximately 250 mi wide and 300 mi long in West Texas and Southeastern New Mexico of the United States. Before the Permian Basin completely formed, it was first described to be a shallow marine, slightly dipping basin referred to as the Tobosa Basin (Hoak et al., 1998). It was not until the upper Paleozoic time (Late Mississippian — Early Pennsylvanian) when the North American plate collided with the South American plate giving rise to the Marathon Ouachita Orogeny (Figure 1). This massive compressional event gave rise to the Central Basin Platform (a northwest-trending uplifted basement block) bounded by the Delaware Basin (to the west) and the Midland Basin (to the east; Kelly et al., 2012). The MTD mapped in this study using seismic was observed in the Upper Leonard interval of the Midland Basin, which lies in the Leonardian series of the Permian-aged strata (Figure 2). The Leonardian stratigraphy in the Midland Basin records deposition in an intracratonic deepwater basin, bounded by shallow-water carbonate platforms. Sea-level Figure 1. Paleogeography of the Permian Basin in early Permian time showing study area in the red box (modified from Ruppel et al., 2000). The blue polygon highlights the outline of the Central Basin Platform, and the dashed red/yellow line highlights the shelf edge. Figure 2. A simplified stratigraphic chart correlating the shelf to basin facies (modified from Handford, 1981a). The red box indicates the stratigraphic interval in which the MTD was deposited. 2 Interpretation / November 2019 fluctuations controlled sediment input into the basin by flooding or exposing the platform. Slope environments, which separate the basin floor from surrounding shallow-water platforms, are characterized by abrupt stratigraphic discontinuities, detrital carbonates, and clinoformal geometries (Hamlin and Baumgardner, 2013). This is evident in Figure 3, which shows an interpreted regional 2D line trending northwest–southeast from the Northern shelf into the Midland Basin illustrating the prograding carbonate platform (clinoformal geometries) basinward due to continuous sea-level fall. The Upper Leonardian interval in which the MTD deposited conforms on top of the Upper Spraberry Formation and is equivalent to the Glorietta Formation (Figure 2) on the platforms (Handford, 1981a, 1981b). Therefore, understanding the paleobathymetry of the underlying Spraberry Formation with the help of an isopach map can provide useful information on how the sediments were dispersed on the basin floor and how the underlying seabed exerted control on the morphology of the overlying MTD. Regional mapping of the 1000 ft thick Spraberry fan cone shows that the fan system was deposited in water depths of 600–1000 ft (Handford, 1981a, 1981b). Basinwide maps of sandstone distribution in the broadly defined lower and upper Spraberry clastic members (Handford, 1981a, 1981b) show that the principal sediment sources lay to the northwest, north, and northeast. This is evident in the Upper Spraberry isopach map, which shows depocenters around the Horseshoe Atoll in the north indicating probable entry points (Figure 4). The Horseshoe Atoll is an isolated carbonate platform in the northern Midland Basin that began in the Pennsylvanian as a broad carbonate buildup and was s
