Subsurface Structural Data Research Articles

Identification of potential mineral exploration targets from the interpretation of aeromagnetic data covering the Sirwa region (Central Anti-Atlas, Morocco)

Situated at the junction between the Moroccan High Atlas and the Anti-Atlas, the Sirwa Massif has always been a perfect target for mining exploration for its unique geological characteristics that provide an economically valuable concentrations of metals. It corresponds to a crustal uplift at an elevation above 2000 m that were slightly folded and lifted during the Hercynian and Alpine orogeny. The structural settings of this area are very complicated due to the acute shortage of surface and subsurface structural data. The aeromagnetic data represents a powerful tool for the identification and the delineation of the poorly exposed faults, that could promote the development of new ore deposits.The processing of the residual magnetic map was carried out to locate anomalies, define the trends and boundaries of the buried structures and identify the magnetic contacts that might represent faults, fractures or lithologic boundaries. This is done by the reduction to the north pole (RTP) filter and several edge detectors processes (i.e., total horizontal derivative, the tilt angle derivative and the 3D-Euler deconvolution). The orientations of lineaments extracted from the total horizontal derivative and Tilt derivative maps predominantly suggest NE-SW to ENE-WSW trending structures, in agreement with the post-Pan-African rifting event. However, we have identified a few NW-SE and NNW-SSE trending lineaments probably related to the recent Variscan movements. The 3D Euler deconvolution (ED) suggests a depth solution that ranges from 600 m to 3400 m. The deepest lineaments have an ENE-WSW direction, and the shallowest with a depth of about 600 m, shows a NE-SW/NW-SE direction. These results reveal new tectonic features that can be useful for future mine exploration

An integrated approach using microtremor data for field development and reservoir monitoring: an example from Betung Field, Jambi

Reservoir monitoring is one of the most important aspects in oil and gas field management, because it provides industries with the required data in hydrocarbon optimization. This is often carried out in Indonesia by the TAC Pertamina-Pusako Betung Muaro Senami Jambi (PBMSJ), an organization responsible for the management and coordination of oil well activities. In 2012, the organization conducted passive seismic data acquisition and series of drilling campaigns, which successfully increased oil production in Betung Block from 43 to 652 bopd. This campaign was successfully carried out through one and two wells in 2013 and 2014, respectively. Furthermore, the pre-2017 interpretation describes the Betung Anticline as a relatively symmetrical and low angle N-S trending field, which defies the South Sumatra Basin Northwest-Southeast regional trend. This anticline, bound by two high angle normal faults, is unable to define the LKO/OWC position, leading to the consideration of a more integral interpretation approach by the PBMSJ team in early 2017. The procedure was conducted through the use of a 2d seismic line bounded with new wells and SRTM imagery, to combine subsurface structural data with the surface fundamental analogue. In 2017, a passive seismic study was repeatedly conducted in the Betung field, with only 30 measuring points. The urgency in identifying the reservoir is essential within the petroleum industry, in order to produce sufficient hydrocarbons. It is also essential in confirming the potential uses of microseismic data in storage decision-making, through a dataset acquired in Betung, based on reservoir management.

Wall-Rock Structural Controls on the Genesis of the Voisey's Bay Intrusion and its Ni-Cu-Co Magmatic Sulfide Mineralization (Labrador, Canada)

The Voisey’s Bay intrusion of the Mesoproterozoic Nain Plutonic Suite hosts a world-class magmatic Ni-Cu-Co sulfide deposit. Here we document the brittle and ductile structure of gneisses surrounding the intrusion using field data, borehole intersections, and subsurface structural data obtained from acoustic and optical televiewer surveys. We elucidate the role that wall-rock structure played on controlling the geometry of the intrusion and the formation of structural traps that host mineralization. The Voisey’s Bay intrusion consists at first order of two km-scale magma chambers, interpreted to represent an upper and lower chamber of the intrusion, that are connected by a system of dikes. Three types of gneiss, associated with the Paleoproterozoic Torngat orogen, form the wall rocks of the intrusion: Archean orthogneiss, Paleoproterozoic Tasiuyak paragneiss, and Paleoproterozoic enderbitic orthogneiss. Three phases of ductile deformation (D1–D3), predating intrusion emplacement, are recorded in the wall-rock gneisses. D1 produced the dominant S1 fabric, subparallel to original bedding (S) in the Tasiuyak gneiss. D2 shortening caused shallowly plunging, upright to moderately inclined, F2 folds in the enderbitic gneiss, the prototype of which was likely emplaced toward the end of D1. D3 ductile deformation resulted in steeply dipping shear zones, and flat lying to shallowly dipping C3 shear planes. At least two later phases of brittle deformation (D4 and D5) are present. D4 is a preemplacement event characterized by fractures with orientations similar to D3 shear zones and shear planes. D5 faults and joints formed during syn- to postemplacement brittle deformation likely associated with E-W brittle sinistral transtension associated with the Mesoproterozoic Gardar-Voisey’s Bay fault zone. The geometry of the Voisey’s Bay intrusion and its mineralization was controlled by pre- to synemplacement brittle structures and preemplacement ductile structures, which acted as passive wall-rock anisotropies. Mineralization hosted within Tasiuyak gneiss is controlled by the intersection of a dike with S1/S, D3 shear zones and D4 fractures. Where the intrusion occurs within enderbitic gneiss, mineralization locally coincides with shallow fabrics associated with F2 fold closures. The margins of the Voisey’s Bay dikes are controlled by steeply and shallowly dipping D4 fractures. Changes in dike thickness, which control the locations of mineralization, can be explained by thermomechanical erosion of the walls of the dike, which occurs by thermal expansion and contraction of wall rock due to repeated magma pulses, and the mechanical spalling of this wall rock along anisotropies. The strong brittle structural control on emplacement, along with previous geothermobarometry, is consistent with a mid-crustal (9–11 km) depth of emplacement. Lithosphere-scale E-W–trending D5 faults may have acted as conduits for the crustal-scale transport of primitive Voisey’s Bay magmas from their source, rather than an older orogenic suture zone.

Three-armed rifts or masked radial pattern of eruptive fissures? The intriguing case of El Hierro volcano (Canary Islands)

Using new surface structural data as well as subsurface structural data obtained from seventeen water galleries, we provide a comprehensive model of the volcano-tectonic evolution of El Hierro (Canary Islands). We have identified, measured and analysed more than 1700 volcano-structural elements including vents, eruptive fissures, dykes and faults. The new data provide important information on the main structural patterns of the island and on its stress and strain fields, all of which are crucial for reliable hazard assessments. We conducted temporal and spatial analyses of the main structural elements, focusing on their relative age and association with the three main cycles in the construction of the island: the Tiñor Edifice, the El Golfo–Las Playas Edifice, and the Rift Volcanism. A radial strike distribution, which can be related to constructive episodes, is observed in the on-land structures. A similar strike distribution is seen in the submarine eruptive fissures, which are radial with respect to the centre of the island. However, the volcano-structural elements identified onshore and reflecting the entire volcano-tectonic evolution of the island also show a predominant NE–SW strike, which coincides with the main regional trend of the Canary archipelago as a whole. Two other dominant directions of structural elements, N–S and WNW–ESE, are evident from the establishment of the El Golfo–Las Playas edifice, during the second constructive cycle. We suggest that the radial-striking structures reflect comparatively uniform stress fields during the constructive episodes, mainly conditioned by the combination of overburden pressure, gravitational spreading, and magma-induced stresses in each of the volcanic edifices. By contrast, in the shallower parts of the edifice the NE–SW, N–S and WNW–ESE-striking structures reflect local stress fields related to the formation of mega-landslides and masking the general and regional radial patterns.

Tectonic controls on sedimentation in an intermontane hinterland basin adjacent to inversion structures: the Nuevo Mundo syncline, Middle Magdalena Valley, Colombia

Abstract A combination of new surface and subsurface structural data, new stratigraphic data on conventional provenance, facies and palaeocurrents, low-temperature thermochronology and detrital zircon U–Pb provenance data provides a comprehensive account of the timing of deformation in the intermountane Middle Magdalena basin of the Central Colombian Andes, and allows evaluation of the style of foreland basin deformation associated with tectonic inversion. This robust dataset enabled documentation of focused tectonic activity in two competing low-relief basement structures to the east and west of the present Middle Magdalena Valley during the Palaeogene, earlier than previously recognized. Cenozoic sediment accumulation of a sedimentary pile up to 7 km thick in the Middle Magdalena Basin created a large original taper angle in this part of the north Andes. At that time, when the detachment rocks were deeply buried, the original larger taper angle facilitated the forelandward advance of deformation instead of promoting its stagnation. Supplementary material: Raw data results from geochronometrial analyses are available at: http://www.geolsoc.org.uk/SUP18627

A structural study of western Anticosti Island, St. Lawrence platform, Quebec: a fracture analysis that integrates surface and subsurface structural data

Abstract Field data from fracture sets, folds and faults affecting generally flat lying Lower Ordovician to Lower Silurian strata on western Anticosti Island, Quebec, are integrated with remote sensing and seismic reflection data. Spatial and temporal relationships among different structural features observed on the Island and in the Gulf of St. Lawrence are assessed with the aim of understanding the role of these structural features for petroleum migration through the Anticosti Platform. The tectonic impact of Appalachian orogenic events on Anticosti Platform is considered, together with Jurassic and Quaternary tectonics. Four fracture sets are identified in Lower Silurian rocks: 1) a NS-EW orthogonal fracture system (Sets I and II); 2) a N40–70 set (Set III); 3) and a N135–165 set (Set IV). Some irregular, randomly oriented fractures are also observed. The orthogonal system may be related to Early Acadian events, which may have induced extensional stresses within the Platform during Late Silurian-Early Devonian time. Normal faults observed in the Lower Silurian succession may be related to this extensional event. The N135–165 and N40–70 sets may have developed either during the Middle Devonian Acadian orogeny or later in the Jurassic. The irregular fractures could have formed during the final stages of fracture development in response to erosion of the Lower Silurian and younger succession, or in response to isostatic rebound following glaciation. Shuttle Radar Digital Elevation Model lineaments are consistent either with observed fracture set orientations, or subsurface Taconian fault orientations. We infer some Taconian faults and Silurian fracture zones were reactivated, probably during the opening of the Atlantic Ocean, as indicated by the intrusion of two Jurassic dikes. In the subsurface of the Island, major SW-dipping normal faults and extensional fractures affect the Lower to Upper Ordovician succession most. These probably formed in response to an extensional stress field in front of the Taconian deformation front during Late Ordovician time. WSW-trending typically open folds occur on southern Anticosti Island in Chicotte Formation and in the Gulf of St. Lawrence. They involve Lower Silurian to Devonian rocks, and may have formed in a compressive regional setting during the Middle Devonian Acadian orogeny. Thermal maturation and petroleum generation on Anticosti Platform are controlled mainly by the burial history. However, the structures described in this study, in particular Taconian and Early Acadian fractures and faults systems, may have played an important role in petroleum migration and in potential reservoir development.

Neotectonics of the Subandes-Brazilian craton boundary using geomorphological data: the Marañon and Beni basins

Active and abandoned fluvial traces are used together with neotectonic, seismotectonic and subsurface structural data to study the neotectonic evolution of the Peruvian and Bolivian foreland basins. The Marañón Basin to the north and the Beni Basin to the south are located near the ends of the Peru-Bolivia Andean segment. This segment lies above the flat-slab subduction of the Nazca plate beneath the Andes. The surface of these basins shows a complex network of present-day rivers and fossil river traces. A relative chronology of the river traces deduced from morphological criteria allows the identification of successive shifting of fluvial belts up to the present position of the main rivers. As it enters the Ucamara Depression, the Ucayali River is deflected to the northeast. Successive shifts of the deflection point are directed upstream along the upper reaches of the river along the foothills. Simultaneously the Marañón River is deflected to the north, lining up with the straight, NE-trending lower reaches of the Huallaga River. In the central part of the depression the rivers trend northeast-southwest, parallel to the main strike of the basement faults. This regional trend is also parallel to (1) the greatest principal stress observed both to the east and the west of the depression, and to (2) the neotectonic faults on the margin of the Brazilian craton. The Beni Basin is characterized by a counterclockwise shift of the Beni River. Successive shifts of the river involve the northward migration of the deflection point of the Beni River as it enters the basin. A fault connected to the foothill margin controls this downstream movement. Recent erosion of the flood plain surface to the east suggests a tendency of the craton margin to ascend. No coaxial stress resulting from either plate motion or from collapse of the Andean topography seems to control the directional shift of rivers in the basins.

A lotus 1-2-3 diamond drillhole structural manipulation spreadsheet: Drillcore structural data generation

Diamond drillcore contains useful structural relationships through large subsurface volumes of rock. Drillholes normally are oriented by a survey but the drillcore generally is unoriented around its core axis. In regions where one structural element can be assumed reasonably to be constant in orientation, drillcore can be reoriented uniquely permitting resolution of all its structural elements. The Diamond Drillhole Structural Manipulation Spreadsheet automates this procedure in the situation where one planar structural element has a constant strike. The spreadsheet can generate the orientations of up to two other planes (in addition to the constant strike plane), two lineations, the intersection lineation between two of the planes, and the apparent dips of the same two planes on a section of the user's selection. This allows rapid and relatively easy generation of large subsurface structural data sets that can provide insights that a 2-D surface structural analysis does not. The spreadsheet permits interactive, iterative modifications to the basic reorienting assumption in different portions of the drilled volume of rock (either blindly or as a result of other information) that can refine the accuracy of the structural output.

Seismicity and geometry of a 110‐km‐long blind thrust fault 1. The 1985 Kettleman Hills, California, earthquake

The August 4, 1985, Kettleman Hills earthquake was the third in a sequence of moderate shocks to occur beneath the northern half of a 110‐km‐long fold chain bounding the eastern California Coast Ranges. The 1982 MW =5.4 New Idria, 1983 MW=6.5 Coalinga, and 1985 MW=6.1 Kettleman Hills events define a southward progression of seismic activity beneath the fold. We use teleseismic waveforms, geodetic modeling, hypocenters relocated in a three‐dimensional velocity model, and subsurface structural data to investigate the Kettleman Hills earthquake. The main shock results from motion on a shallowly dipping thrust fault buried at ∼10 km depth. Aftershocks and coseismic fault slip extend 20 km along the fold axis, nearly the full extent of the Kettleman Hills North Dome anticline. Aftershocks occur primarily several kilometers in front of the fault tip and in the core of the anticline. The main shock and several foreshocks occurred at a 2‐km right step in the Quaternary fold axis, which also corresponds to the southern end of the 1983 Coalinga and northern end of the 1985 Kettleman Hills aftershock zones. From this we infer that the step in the fold is caused by an offset or tear in the underlying fault. The scalar seismic moment is 1.6 × 1018 N m, consistent with the geodetic deformation, and the duration of rupture is 16 s, 3–4 times greater than for the average earthquake with this scalar moment. The slow rate of moment release provides an explanation for the low level of ground shaking and low local magnitude reported for the event. The peak of the geodetic uplift is located 5 km perpendicular to the Quaternary fold axis. We argue that the fault is propagating northeast into the undeformed San Joaquin Valley sediments and that the overlying fold is growing at about 0.5 mm/yr.

Identification of Wrench Faults Using Subsurface Structural Data: Criteria and Pitfalls: Reply (1)

Identification of Wrench Faults Using Subsurface Structural Data: Criteria and Pitfalls: Reply (1)

Identification of Wrench Faults Using Subsurface Structural Data: Criteria and Pitfalls: Reply (1)

Identification of Wrench Faults Using Subsurface Structural Data: Criteria and Pitfalls: Reply (1)

Identification of Wrench Faults Using Subsurface Structural Data: Criteria and Pitfalls (1)

Many structures imaged with reflection seismic can resemble a wrench fault system because of the great structural diversity of strike-slip faults and the wide range of contractional and extensional features associated with these faults. Because of these resemblances, numerous faults are misidentified as wrench faults by workers using subsurface structural data. Credible identification requires combinations of profile and map criteria: (1) narrow, long, straight, throughgoing, solitary master fault or zone of deformation, (2) steep to moderate dip of the master fault at depth, (3) offset of the top of basement, (4) changes in relative upthrown side, separation sense, and/or fault dip direction at depth or along the strike of the master fault, (5) narrow fault slices within the master fault zone that steepen and join at depth (negative and positive flower structures), (6) different separation sense and orientation of relative upthrown side at these fault slices, and (7) coeval, en echelon flanking structures. The identification should be confirmed with other criteria that refute alternative interpretations. The main pitfalls to a reliable identification are not considering alternative structural styles and the misinterpretation of features or patterns that superficially resemble wrench systems. The profile geometries of basement-involved reverse or normal faults, particularly those with structural inversion, can resemble the single strand of convergent or divergent wrench faults. Profile geometries of flower structures can be mimicked by narrow contractional horsts or narrow extensional grabens, complexly faulted anticlines or synclines, splay faults, fault zones intruded by salt or shale, and overprinted structures. Pitfalls in interpreting map relationships are possible when structural patterns combine several tectonic influences, are controlled by rock anisotropy, or include elements hat are not coeval, and if the patterns characteristic of other styles are misinterpreted. Many erroneous identifications of wrench faults can be eliminated by demonstrating that the investigated fault or feature lacks a straight trace, lateral length, or throughgoing continuity.

Relative effectiveness of geologists and computers in mapping potential hydrocarbon exploration targets

Contouring of data by men and machines produces a collection of maps which demonstrates differences between them; exploration geologists, computer-mapping-package-builders, and managers should be aware of these differences. Machines and geologists contouring subsurface structural data were compared in terms of the ultimate accuracy of their maps. The machine versions were consistently more similar to the carbonate reef surface used for the analysis than those produced by the geologists, given specific amounts of control. However, in situations where additional information was critical (paleoslope, channel vs bar sand, regional dip, etc.) the bias contributed by the geologist proved critical to the “correct” interpretation. Several illustrations in the form of actual exploration examples are included and discussed. The examples described and analyzed herein are presented to acquaint both the map-maker and map-user with each other's problems with the overall objective of increasing the explorational effectiveness of both.