Fault Terrace Research Articles

Tectono‐sedimentary evolution of high‐displacement crustal‐scale normal faults and basement highs on rifted margins: Klakk Fault Complex and Frøya High, Mid‐Norwegian Margin

AbstractCrustal‐scale high‐displacement (>10 km) normal faults are not captured in existing tectono‐sedimentary models of rift basins. We used 2D and 3D seismic reflection and well data to perform a structural and source‐to‐sink analysis of the southern part of the Klakk Fault Complex and the western part of the Vingleia Fault Complex, Mid‐Norwegian rifted margin. The north–south trending Klakk Fault Complex has a zig‐zag to sinuous plan‐view geometry, forming a series of structural recesses and salients along strike. In cross‐section, the fault complex has a listric to convex‐up or low‐angle planar geometry with displacements above 20 km. This fault complex exhumed basement highs, the Frøya High and Sklinna Ridge, in its footwall and created a series of supradetachment basins, for example, the Rås Basin, in its hanging wall. In contrast, the northeast‐southwest trending Vingleia Fault Complex has a zig‐zag geometry in plan view and planar to listric geometry in cross‐section and displacement of up to 5 km. This fault has the Frøya High in its footwall and the southern Halten Terrace in its hanging wall. Restoration of selected structural cross‐sections shows a prominent fault‐parallel ridge, up to 15 km east of the Klakk Fault Complex interpreted as a palaeodrainage divide. This divide separates steep drainages developed along the west‐dipping footwall scarp to the Klakk Fault Complex, from broader, gentler east‐dipping drainages up to ca. 10 km long developed on a back‐tilted dip slopes along the eastern side of the Frøya High and Sklinna Ridge. Progressive headward erosion of active flank catchments was enhanced around topographically elevated structural salients to the point of capturing previous dip‐slope‐directed drainages during the earliest Cretaceous. A network of submarine canyons develop down‐dip of the drainage catchments along the Klakk Fault Complex scarp, whose geometries and length are controlled by their location with respect to the structural salients or recesses, and the presence of fault terraces. The middle Jurassic‐earliest Cretaceous synrift deposits form two seismic sequences that are filled with five distinctive seismic facies that record the evolution from a linked normal fault during rift climax to a high‐displacement stage. During the high displacement stage, exhumed local continental core complexes formed structural salients, separated along strike by structural recesses at the heads of supradetachment basins. Key elements of the high‐displacement fault stage include (i) the development of structural salients at sites of rift climax displacement maxima, (ii) development of supradetachment basins in rift climax displacement minima and (iii) migration of major depocentres away from the centre of rift climax fault segments. We synthesise these observations into a generic tectono‐sedimentary model for high‐displacement faults.

Lacustrine shale lithofacies and depositional environment in the paleocene second member of the funing formation, subei basin, China: Insights into shale oil development prospects

Lacustrine fine-grained sedimentary rocks play a crucial role in the petroleum industry, particularly in the exploration of unconventional oil and gas resources. This study focuses on a petroliferous fine-grained succession deposited prior to the Paleocene–Eocene Thermal Maximum (PETM), specifically, the Paleocene second member of the Funing Formation. This formation represents the primary target for shale oil exploration in the Qintong Depression of the Subei Basin. Employing detailed sedimentological descriptions and utilizing multiple geochemical proxies, our investigation aimed to investigate lithofacies distribution and reconstruct the paleoenvironmental conditions prevalent in the Qintong Depression. Through the integration of lithological observation and well log data, the second member of the Funing Formation was categorized into three distinct system tracts: the early expanding system tract (EEST), the late expanding system tract (LEST), and the highstand system tract (HST). Within the established sequence stratigraphy framework, six lithofacies were identified, displaying comparable lateral and planar distributions. Results indicate that lithofacies exhibit greater thickness in the Deep Sag zone compared to the Slope and Fault Terrace. The EEST is characterized by laminated calcareous mudstone (LCM), laminated dolomitic mudstone (LDM), and massive argillaceous dolomite (MAD). Whereas the LEST predominantly comprises LCM and massive calcareous mudstone (MCM). These lithofacies were deposited under relatively arid climatic conditions, characterized by high salinity and strong terrigenous input, with a moderate abundance of organic matter consisting of type Ⅱ and type Ⅲ kerogen. In contrast, the HST is dominated by MCM with stable thickness, indicative of deposition under relatively humid climatic conditions with low salinity and terrigenous input. These conditions fostered an environment conducive to plankton and algae, resulting in a high abundance of organic matter composed of type Ⅰ kerogen. These new characterizations have significant implications for considering favorable lithofacies regarding the vertical orientation and target zones regarding the planarity of shale oil resources in the Qintong Depression of the Subei Basin.

North slope transition zone of Songnan-Baodao sag in Qiongdongnan Basin and its control on medium and large gas fields, South China Sea

Based on analysis of newly collected 3D seismic and drilled well data, the geological structure and fault system of Baodao sag have been systematically examined to figure out characteristics of the transition fault terrace belt and its control on the formation of natural gas reservoirs. The research results show that the Baodao sag has the northern fault terrace belt, central depression belt and southern slope belt developed, among them, the northern fault terrace belt consists of multiple transition fault terrace belts such as Baodao B, A and C from west to east which control the source rocks, traps, reservoirs, oil and gas migration and hydrocarbon enrichment in the Baodao sag. The activity of the main fault of the transition belt in the sedimentary period of Yacheng Formation in the Early Oligocene controlled the hydrocarbon generation kitchen and hydrocarbon generation potential. From west to east, getting closer to the provenance, the transition belt increased in activity strength, thickness of source rock and scale of delta, and had multiple hydrocarbon generation depressions developed. The main fault had local compression under the background of tension and torsion, giving rise to composite traps under the background of large nose structure, and the Baodao A and Baodao C traps to the east are larger than Baodao B trap. Multiple fault terraces controlled the material source input from the uplift area to form large delta sand bodies, and the synthetic transition belt of the west and middle sections and the gentle slope of the east section of the F12 fault in the Baodao A transition belt controlled the input of two major material sources, giving rise to a number of delta lobes in the west and east branches. The large structural ridge formed under the control of the main fault close to the hydrocarbon generation center allows efficient migration and accumulation of oil and gas. The combination mode and active time of the main faults matched well with the natural gas charging period, resulting in the hydrocarbon gas enrichment. Baodao A transition belt is adjacent to Baodao 27, 25 and 21 lows, where large braided river delta deposits supplied by Shenhu uplift provenance develop, and it is characterized by large structural ridges allowing high efficient hydrocarbon accumulation, parallel combination of main faults and early cessation of faulting activity, so it is a favorable area for hydrocarbon gas accumulation. Thick high-quality gas reservoirs have been revealed through drilling, leading to the discovery of the first large-scale gas field in Baodo 21-1 of Baodao sag. This discovery also confirms that the north transition zone of Songnan-Baodao sag has good reservoir forming conditions, and the transition fault terrace belt has great exploration potential eastward.

New insights into the structure, geology and hydrocarbon prospectivity along the central-northern Corona Ridge, Faroe–Shetland Basin

The Faroe–Shetland Basin (FSB) is one of the only significant exploration frontiers remaining on the UK Continental Shelf. Over half of the basin's discovered reserves and resources lie along two intra-basinal highs, the Corona Ridge and the Rona Ridge. In contrast to the Rona Ridge, the central-northern Corona Ridge has received much less attention. To reveal new insights into the geology, structural configuration and hydrocarbon prospectivity of the central-northern Corona Ridge, we analyse 3D seismic data and data from the exploration wells 213/23-1 (Eriboll), 214/21a-2 (South Uist) and 213/25c-1V (North Uist). This study extends the Colsay T40–T45 sub- and intra-basaltic play concept from the Rosebank Field NE along the Corona Ridge, at least into well 213/23-1. Analysis also suggests that no Triassic strata are present within well 213/23-1, challenging the previous understanding of Triassic distribution within the central FSB. Our findings show that the central-northern Corona Ridge is structurally complex, comprising a series of discrete basement-bounding faults, downflank fault terraces and faults that are oblique to the dominant NE–SW-striking structural fabric of the FSB.

Fracture Characteristics and its Role in Bedrock Reservoirs in the Kunbei Fault Terrace Belt of Qaidam Basin, China

Fracture characteristics at bedrock reservoirs in the Kunbei fault terrace belt in the southwest of Qaidam Basin, China, are investigated based on cores, thin sections, formation microscanner imaging, and production data. Results show that the weathered unit at the top of bedrocks is not an effective reservoir because of extremely low porosity. A semi-weathered unit is a potential reservoir, where formation water migrating along early effective fractures could form dissolution fractures and pores as primary storage space, and fractures can also act as seepage channels. Four fracture types can be identified from bedrock reservoirs, e.g., structural ones, diagenetic ones, weathering ones, and dissolution ones, while shear fractures related to faults are the most important ones. Fracture types and characteristics are different in granite and slate, e.g., high fracture density results in intensive dissolution in granite. Fracture density is closely related to tectonic stress. The bedrock reservoirs near fault zones and semi-weathered units are potential oil and gas exploration targets in the Kunbei fault terrace belt. Effective fractures govern dissolution behaviors as well as dominant seepage directions, which play a significant role in the development of bedrock reservoirs.

The hydrocarbon accumulation regularity and the model of hydrocarbon accumulation along the fault ridges in the slope zone of the continental fault basin

The slope zone of fault basin has a large area and high hydrocarbon potential. Its relatively highly scattered reservoirs and vastly different accumulation characteristics make it difficult to accurately determine the main controlling factors of hydrocarbon accumulation. To solve this problem, we adopt the 3rd member of the Funing Formation (Ef3) in the slope zone of the Qintong Sag as an example to identify the main controlling factors of hydrocarbon accumulation in the slope zone of the Paleogene continental fault basin. The results reveal that there are two critical intervals for Ef3 in the Qintong Sag slope zone, namely, the Late Paleogene to the Early Neogene (23.2–20 Ma) and the Late Neogene to the Early Quaternary (2 Ma until now), and the crude oil comes from the source rock of the second member of the Funing group related to the slope barrier and the inner slope zone. Instead of migrating directly into the traps in the slope zone, three main paths of hydrocarbon migration were developed: First from southwest to northeast and then from northeast to southwest; From northeast to southwest; From west to east directly. Oil and gas migrated along the sand body in the northeast and southwest directions and bypassed by the central antithetic fault terrace, thus forming the mode of accumulation along the fault edge.

Tectonic evolution of the central part of the East Anatolian Fault Zone, Eastern Turkey

The Eastern Anatolian Fault Zone (EAFZ), having a prominent place in the tectonic evolution of the Eastern Mediterranean, is a structural element of tectonic indentor due to the convergence between the African-Arabian plates and the Eurasian Plate. This study investigates the central part of EAFZ between Doğanyol (Malatya) and Çelikhan (Adıyaman). The geometry of the fault and the morphotectonic structures were determined by the field studies. Moreover, fault-slip data are measured according to the fault planes along the deformation zone for paleostress analysis. The paleostress analysis revealed three deformation phases that developed from the Late Eocene to the present due to the convergence between the Arabian Plate and the Anatolian Block. The first deformation phase is characterized by NW-SE compressional stress between Late Eocene and Late Oligocene periods. The second deformation phase is related to N-S compressional stress from the Middle Miocene to Pliocene. The most recent deformation phase shows the strike-slip faulting under the NNE-SSW compressional stress from the Late Pliocene to the present. The EAFZ developed during the last phase of these deformation stages. In addition, elongated ridges parallel to the fault, sinistral offsets of drainage networks, linear valleys, and fault terraces observed along the segment show that the study area exhibits active tectonic morphology of the EAFZ. The distribution of seismic activity that occurred during and after the recent mainshock (24 January 2020, Sivrice-Doğanyol earthquake) is compatible with the geometry of the segment and confirms strongly the active tectonics of the segment.

Controlling Factors Analyses of Sediment Dispersal From Source-to-Sink Systems of Lacustrine Rift Basin: A Case Study From Paleogene Shahejie-Dongying Formations of Nanpu Sag, Eastern China

Tectonic activity not only shapes the basic stratigraphic framework of rift basins, but also profoundly affects the sediment dispersal in rift basins. In this study, analyses of heavy mineral assemblages in different periods demonstrate that there are three obvious tectono-sedimentary evolutionary stages (Es3–Es2, Es1–Ed2, and Ed1, respectively) in the Paleogene provenance area of Nanpu Sag, and the volume of sand bodies increases from the bottom of the Paleogene Shahejie (Es) Formation to the top of the Dongying (Ed) Formation in Nanpu Sag. Besides, this study comprehensively utilize the analyses of seismic interpretation, palynology, heavy mineral, and borehole core samples to investigate the controlling factors of sediment dispersal in the rift basin. The assemblages of heavy minerals in different periods reflect the rock composition and catchment area of different provenance areas, and their vertical differences reflect the evolution process of the provenance area and reflect the uplift-denudation process of the provenance area. The results reveal that the synergy of the evolution of tectonic activity and the adjustment of topographical evolution are the main controlling factors of sediment dispersal in Nanpu Sag, while climate change is not the main controlling factor. We conclude that an increased sediment supply rate in the long term reflects the control of tectonic activity on basin topography, rather than climate fluctuations. The differences in morphological modification result in differential sediment dispersal, which is principally related to the differential extrusion of the fault system. The catchment area and provenance distance adjustment is evidenced by the vertical changes of heavy mineral characteristics of single-well and interaction and linkage of boundary faults, and the adjustment of topography evolution. A consideration is that the interaction and linkage of boundary faults and complex subsidence history are multi-directional, and differential evolution of provenance area is universal in lacustrine rift basins, all of this highlights the adjustment of sediment pathways generated by this characteristic of rift basins and emphasizes the importance of controlling factors analyses in understanding differential sediment dispersal that presents in the rift basins. Besides, four sets of sediment dispersal patterns were delineated based on different developmental regions in the rift basin, which are fault segmental point and multi-stage fault terrace, single-stage fault terrace and axial fault valley, axial fault terrace, and paleo-terrace and axial fault valley, respectively. This study has a certain guiding significance for the prediction of the spatial distribution of sand bodies in the rift basin and the exploration of potential oil and gas targets in the rift basin.

Classification of slope area types and hydrocarbon accumulation rules in Wuerxun-Beier Depression

After more than 50 years of exploration and development, Wuerxun-Beier depression has entered a high exploration stage. It is more and more difficult to find large-scale structural reservoirs. Therefore, structural-lithologic reservoirs in slope areas have gradually become the focus of exploration. Based on the overall study and dissection of the depression, the geological differentiation characteristics of the large-scale slope area are discussed. According to the occurrence of the slope, the intensity of tectonic activity, the intensity of source supply and the pattern of synsedimentary fault combination, the slope zone in the study area can be divided into six types, namely, the steep slope with strong source along the fault step type, the compound fault step type of medium slope with strong source, the reverse fault step type of the gentle slope with strong source, and the weak supply type. Different types of slope have different sand control mechanisms and reservoir forming characteristics, such as the source gentle slope forward fault terrace type, the weak source gentle slope reverse fault terrace type and the weak source middle slope forward fault terrace type. Source supply intensity and slope dip control the development type of sedimentary system, the combination of synsedimentary faults and valleys controls the distribution characteristics of sedimentary system. Through exploration practice, significant discoveries have been made on the southwestern, southeastern and northern slopes of Beixi, Wuerxun-Beier depression, which confirms that the slope area is the main breakthrough direction for oil and gas exploration and large-scale reservoir increase in continental fault-depression lake basins.

Influence Factors of Pressure Attenuation Gradient and its Reservoir Forming Control - A Case Study of Bonan Sag in Bohai Bay Basin

As the dynamic condition of oil and gas migration, the change rate of overpressure has a certain relationship with the mode of oil and gas migration. In order to quantitatively identify the mode of oil and gas migration in overpressure basins, the influence of different transmission modes on pressure attenuation gradient is simulated, and the application and discussion are carried out in Bonan sag of Bohai Bay Basin. The influence of different transmission modes on the pressure attenuation gradient is discussed by single factor simulation method. Taking Bonan sag as an example, the process of overpressure fluid migration and accumulation is discussed combined with the evolution process of pressure attenuation gradient. The results show that fault activity has the greatest influence on pressure attenuation gradient, followed by fault dip angle and fault distance. the influence degree of physical properties of skeleton sand body, sand body dip angle and sand body thickness on pressure attenuation gradient decreases in turn; For the fault-sand consistent transport, it mainly depends on the combined effect of the dip angle and resistance of the transport system. The critical value of the fault or sand body transport in Bonan sag is 58.8MPa/km. On this basis, combined with the evolution law of pressure attenuation gradient, it is considered that the overpressure fluid in the deep sag migrates along the oil source fault to the fault terrace zone and the overpressure center distribution of the oil and gas ring in the deep sag. The overpressure oil and gas mainly migrate laterally along the sand body, and the oil and gas are distributed contiguously. In gentle slope zone, oil and gas distribute sporadically at structural height.

Syn‐rift sediment gravity flow deposition on a Late Jurassic fault‐terraced slope, northern North Sea

AbstractStructurally controlled bathymetry in rifts has a significant influence on sediment routing pathways and depositional architecture of sediment gravity flow deposits. In contrast to rift segments characterized by crustal‐scale half‐grabens, the tectono‐stratigraphic evolution of deep‐water rift domains characterised by distributed faulting on narrow fault terraces has received little attention. We use 3D broadband seismic data, calibrated by boreholes, from the Lomre and Uer terraces in the northern North Sea rift to investigate Late Jurassic syn‐rift sediment gravity flow systems on fault‐terraced slopes. The sediment gravity flow fairways were sourced from hinterland drainages via basin margin deltaic systems on the Horda Platform to the southeast. The deep‐water sedimentary systems evolve from initial, widespread submarine channelized lobe complexes, through submarine channels, to incised submarine canyons. This progressive confinement of the sediment gravity flow system was concomitant with progressive localization of strain onto the main terrace‐bounding faults. Although the normal fault network on the terraces has local impact on deep‐water sediment transport and the architecture of gravity flow deposits, it is the regional basin margin to rift axis gradient that dominantly controls deep‐water sediment routing. Furthermore, the gravity flow deposits on the Lomre and Uer terraces were predominantly sourced by rift margin deltaic systems, not from erosion of local uplifted footwall crests, emphasising the significance of hinterland catchments in the development of volumetrically significant deep‐water syn‐rift depositional systems.

Geochemical characteristics, origin, and mechanism of differential accumulation of natural gas in the carboniferous kelameili gas field in Junggar basin, China

Since the discovery of the Kelameili gas field, the exploration of volcanic gas reservoirs in the Junggar Basin has attracted considerable attention. However, owing to the limitations involved in studying the characteristics of the origin and distribution of the volcanic gas reservoirs in this area, the study of gas reservoirs in the South fault terrace area of the Kelameili gas field as a typical peripheral area is greatly restricted, and the exploration process is relatively slow. Therefore, it is necessary to analyse and compare the differences in the geochemical characteristics of the natural gas between the Kelameili gas field and the fault terrace area of the south belt by analysing the natural gas geochemistry, hydrocarbon generation evolution, and structural evolution. In addition, geochemical experiments involving the natural gas components, carbon isotopes, light hydrocarbons, hydrocarbon generation, and evolutionary history of the source rocks and the interpretation of seismic profiles reflecting the structural evolution, help clarifying the genetic type of the volcanic gas reservoirs and the source of the natural gas in this area. The results indicate the following. (1) The natural gas in the study area has undergone thermal genesis and is generated by kerogen cracking. It is a typical humic gas with high maturity. (2) The origin of the natural gas is the Carboniferous source rock in the depression, which is significantly different from the natural gas from the typical Permian and Jurassic. (3) The primary reason for the difference in natural gas accumulation is that the Carboniferous source rock entered the high-maturity stage in the Early Jurassic. The formation time of the trap is earlier than that of the high-maturity stage of the source rock evolution, which can capture multi-stage source rock products. (4) Based on the hydrocarbon source conditions that the hydrocarbon supply horizon of the Kelameili gas field has a high maturity of organic matter, and insufficient hydrocarbon supply, it is considered that traps capturing the multi-stage source rock products represent a larger scale of natural gas accumulation. The results of this study can deepen the understanding of volcanic gas reservoirs in the Junggar Basin and guidethe exploration and development of global volcanic gas reservoirs and the surrounding large-scale gas fields.

Control effects of the synsedimentary faults on the basin-marginal fans in the central part of the deep-water area of early Oligocene Qiongdongnan Basin, South China Sea

The synsedimentary faults and basin-marginal fans located in the central part of the deep-water area of the early Oligocene Qiongdongnan Basin have been investigated using seismic profiles, boreholes, and well-log data. Through the formations of the characterized paleogeomorphology, such as transverse anticlines, fault ditches, and step-fault belts, the synsedimentary faults are known to have controlled the development position, distribution direction, and extension scales of the basin-marginal fans. For example, at the pitching ends of two adjacent faults, transverse anticlines developed, which controlled the development positions and distributions of the fans. During the early Oligocene, the faults controlled the subsidence center, and fault ditches were formed at the roots of the faults. In the surrounding salient or low salient areas, which were exposed as provenance areas during early Oligocene, the fault ditches acted as the source channels and determined the flow paths of the clastics, where incised valley fills were obviously developed. The fault ditches which developed in the sedimentary basins were able to capture the drainage systems and influenced the distributions of the fans. The large boundary faults and the secondary faults generated two fault terraces and formed step-fault belts. The first fault terrace caused the clastics to be unloaded. As a result, fans were formed at the entrance to the basin. Then, the second fault terrace caused the fans to move forward, with the fans developing in a larger extension scale. The results obtained in this study will potentially be beneficial in the future prospecting activities for reservoirs and coal-measure source rocks in the basins located in the deep-water areas of the South China Sea.

Hydrocarbon occurrence characteristics and charging differences in Chengbei Fault Zone based on fluid inclusions

Hydrocarbon accumulates in multiple layers of the high, middle and low parts of Chengbei fault zone, in vicinity of Qikou Sag and Qinan Subsag, which represents a breakthrough for large-scale hydrocarbon exploration in recent years. Based on the size, abundance, and attitute of hydrocarbon inclusions, as well as their microscopic polarization and fluorescence characteristics, the hydrocarbon occurrence and the charging difference in the high, middle and low faulted terraces were analyzed systematically in this paper. There are two types of occurrence in the intergranular pores and fractures in the reservoirs, namely, the medium to light oil that shows green fluorescent light, and the solid bitumen charged with a certain amount of hydrocarbon that does not show any fluorescent light. This indicates that the zone has experienced at least two stages of hydrocarbon migration and accumulation, and that diversified forms of hydrocarbon exist in the reservoirs of the zone. The characteristics of hydrocarbon inclusions show that two stages of inclusions were found in the tectonic positions of different faulted terrace. The first-stage inclusions were developed in the early secondary enlargement of quartz grains, with a low degree of development abundance; while the second-stage inclusions were developed after the secondary enlargement of quartz grains, with a high degree of development abundance. The second-stage hydrocarbon inclusions of the reservoir samples obtained at tectonic positions of the high, middle and low fault terrace represent the main filling stage of the present-day reservoirs. Their fluorescence characteristics demonstrate the oil charging difference, which is the maturity of the hydrocarbon decreases as the terraces lift up along the slope. Identifying the hydrocarbon occurrence and charging differences from the perspective of inclusions is conducive to recognizing the regularity of hydrocarbon charging and migration and the distribution patterns in fault terrace zones, and lay the foundation for hydrocarbon exploration of slopes in the future.

Quantitative analysis of a footwall‐scarp degradation complex and syn‐rift stratigraphic architecture, Exmouth Plateau, NW Shelf, offshore Australia

AbstractInteractions between footwall‐, hangingwall‐ and axial‐derived depositional systems make syn‐rift stratigraphic architecture difficult to predict, and preservation of net‐erosional source landscapes is limited. Distinguishing between deposits derived from fault‐scarp degradation (consequent systems) and those derived from long‐lived catchments beyond the fault block crest (antecedent systems) is also challenging, but important for hydrocarbon reservoir prospecting. We undertake geometric and volumetric analysis of a fault‐scarp degradation complex and adjacent hangingwall‐fill associated with the Thebe‐2 fault block on the Exmouth Plateau, NW Shelf, offshore Australia, using high resolution 3D seismic data. Vertical and headward erosion of the complex and fault throw are measured. Seismic‐stratigraphic and seismic facies mapping allow us to constrain the spatial and architectural variability of depositional systems in the hangingwall. Footwall‐derived systems interacted with hangingwall‐ and axial‐derived systems, through diversion around topography, interfingering or successive onlap. We calculate the volume of footwall‐sourced hangingwall fans (VHW) for nine quadrants along the fault block, and compare this to the volume of material eroded from the immediately up‐dip fault‐scarp (VFW). This analysis highlights areas of sediment bypass (VFW > VHW) and areas fed by sediment sources beyond the degraded fault scarp (VHW > VFW). Exposure of the border fault footwall and adjacent fault terraces produced small catchments located beyond the fault block crest that fed the hangingwall basin. One source persisted throughout the main syn‐rift episode, and its location coincided with: (a) an intra‐basin topographic high; (b) a local fault throw minimum; (c) increased vertical and headward erosion within the fault‐scarp degradation complex; and (d) sustained clinoform development in the immediate hangingwall. Our novel quantitative volumetric approach to identify through‐going sediment input points could be applied to other rift basin‐fills. We highlight implications for hydrocarbon exploration and emphasize the need to incorporate interaction of multiple sediment sources and their resultant architecture in tectono‐stratigraphic models for rift basins.

Tectonics and Landscape of the Central African Plateau and their Implications for a Propagating Southwestern Rift in Africa

AbstractThe Central African Plateau (CAP) covers a million square kilometers of African lithosphere absent of recent volcanism and intense seismicity. Treating the CAP erosion surface as a reference frame for measuring continental deformation reveals an active landscape of normal fault systems and crustal flexures. Free‐air gravity anomalies over the CAP reveal both a short‐wavelength (100–200 km) flexural and a longer‐wavelength (>500 km) mantle convective signature. Apatite fission track thermochronometry records the onset of regional cooling of the erosion surface below 60 °C between 38 and 22 Ma. The erosion surface was formed by the Latest Miocene and elevated to its present altitude (1,200 ± 50 m) in the Latest Miocene/Pliocene. High‐resolution Shuttle Radar Topography Mission‐ and LIDAR‐based digital elevation models of the erosion surface show active fault terraces and alluvial fan deformation associated with pre‐existing rift border faults. Flexural modeling of the footwall uplift of the Luangwa Rift border fault yields an effective elastic thickness of the CAP lithosphere of ~35 km. The rifting initiated in the Pliocene with, or soon after, elevation of the CAP. Subsequent Plio‐Pleistocene deformation of the CAP surface controls the Congo and Zambezi drainage systems and wetland locations. The CAP rifts link southwestward through the Zambezi, Kafue and Muchili Rifts to the Pleistocene aged Okavango and Eiseb Rifts of Botswana and Namibia, defining a propagating Southwestern Rift cutting the Nubian Plate. This active rift system developed along relatively thin (~150 km) lithosphere between the Congo and Kalahari cratons within crust inherited from Neoproterozoic collisional tectonics.

The hydrocarbon accumulation potential of Upper Cretaceous to Paleogene in the northern Kaikang trough, Muglad Basin

This paper investigates the characteristics of sedimentary faces, source rocks, and hydrocarbon accumulation in detail within the Paleogene and Upper Cretaceous in the northern Kaikang trough, Muglad Basin. Analysis shows that the fluvial floodplain, delta, and shore-shallow lake facies are well developed in the main formations and several reservoir-cap assemblages are formed. Research on geochemical indicators, hydrocarbon generation, and expulsion potential of source rocks indicated that the thickness of good source rocks in Paleogene ranges from 50 to 200 m and TOC value can reach 0.5–1.3%, but it is immature and has no hydrocarbon generation potential. Nevertheless, the Upper Cretaceous source rocks are mostly matured, but its hydrocarbon generation is limited because of lower TOC (only 0.5–0.8%) and thinner thickness (only 10 m). The main effective source rock is the AG Group in the Lower Cretaceous, which is distributed throughout the area. The evolutionary history shows that most of structures in the central troughs lack hydrocarbon potential, because they were formed since the Paleogene, which are later than the main accumulation period of the AG source rock. The fault terrace zones on both sides of the Kaikang trough have obtained petroleum discovery, but the distribution of oil layers is much complicated. Hydrocarbon accumulation is controlled by formation dips, fault activity intensity, and fault lateral docking characteristics. The weaker active fault block in late period and more effective trap are the key factors to hydrocarbon enrichment at the fault terrace zones. There are two typical reservoir-forming modes; one is the small fault distance reservoir-forming mode of the lower primary reservoir, and the other is the secondary reservoir-forming mode of the upper and middle combination.

Axial and transverse deep‐water sediment supply to syn‐rift fault terraces: Insights from the West Xylokastro Fault Block, Gulf of Corinth, Greece

AbstractDeep‐water syn‐rift systems develop in partially‐ or transiently‐linked depocentres to form complicated depositional architectures, which are characterised by short transport distances, coarse grain sizes and a wide range of sedimentary processes. Exhumed systems that can help to constrain the tectono‐stratigraphic evolution of such systems are rare or complicated by inversion tectonics. Here, we document a mid‐Pleistocene deep‐water syn‐rift system fed by Gilbert‐type fan deltas in the hangingwall of a rift margin fault bounding the West Xylokastro Horst block, on the southern margin of the Gulf of Corinth, Greece. Structural and stratigraphic mapping combined with digital outcrop models permit observations along this syn‐rift depositional system from hinterland source to deep‐water sink. The West Xylokastro Fault hangingwall is filled by two distinct sediment systems; an axial system fed by coarse‐grained sediment gravity flows derived from fault‐tip Gilbert‐type fan deltas and a lateral system dominated by mass transport deposits fed from an evolving fault‐scarp apron. Abrupt changes in stratigraphic architecture across the axial system are interpreted to record changes in relative base level, sediment supply and tectonics. Locally, depositional topography and intra‐basinal structures controlled sediment dispersal patterns, from bed‐scale infilling of local rugose topography above mass transport complexes, to basin‐scale confinement from the fault scarp apron. These acted to generate a temporally and spatially variable, heterogeneous stratigraphic architecture throughout the basin‐fill. The transition of the locus of sedimentation from a rift margin to a fault terrace through the syn‐sedimentary growth of a basinward fault produced regressive surfaces updip, which manifest themselves as channels in the deep‐water realm and acted to prograde the system. We present a new conceptual model that recognises coeval axial and transverse systems based on the stratigraphic architecture around the West Xylokastro fault block that emphasizes the lateral and vertical heterogeneity of rift basin‐fills with multiple entry points.

Selection of Suitable Dam Axis Location Considering Permeability and Grout Curtain Optimization

ABSTRACT This study investigated the effects of active tectonism on the construction of hydraulic structures. Apart from the effect of dynamic loads, active tectonism can cause permeability problems. The focus of this study was on overcoming the permeability problems considering the economic yield in terms of grout curtain design and optimum dam axis location. The ground permeability of dam locations is one of the decisive factors affecting the feasibility and economic yield of dam construction. Techniques (such as grouting) for ensuring ground permeability reduction are generally expensive and time-consuming; the experience at the first location of the Turgutlu Dam is a case in point. Construction of the first location of Turgutlu Dam was planned on a fault terrace that is part of the North Anatolian Fault Zone, which is the most important active tectonic feature in Turkey. A grout curtain was planned to provide permeability reduction and ground treatment for the terrace material. However, the base elevation of the adjacent valley is deeper, and thus potential leakage to this valley was expected. In spite of its big reservoir area, the Turgutlu Dam Project became infeasible in terms of economic yield due to the ground remediation (construction of grout curtain) costs for this location. To solve this problem, the dam was relocated downstream to avoid the fault terrace, choosing an optimum grout curtain to minimize cost and improve water retention of the reservoir, and increase economic yield of the dam.

Architecture of tectonic sequences, depositional systems, and tectonic controls of the sedimentary fills of the rift‐related Wenchang Formation in the Lufeng Depression, Pearl River Mouth Basin, China

Different sequence types were formed in various stages associated with tectonic variations and changes of the sediment supply in the rift‐related Wenchang Formation. Based on cores, wire logging, and seismic data, the sequence stratigraphy, depositional systems, and controls on the depositional systems of the Wenchang Formation in the Lufeng Depression were studied. The Wenchang Formation developed braided‐delta, lacustrine, fan‐delta, and nearshore subaqueous fan deposits and represents one first‐order sequence and three second‐order sequences, indicating three stages of tectonostratigraphic evolution. (a) The early syn‐rift sequence (SSQ1) developed fan‐delta and shallow lacustrine deposits. (b) The rift climax sequence (SSQ2) contains the early rift climax systems tract and the middle‐late rift climax systems tract. The early rift climax systems tract formed primarily braided‐delta and shallow lacustrine deposits, whereas the middle‐late rift climax systems tract developed mostly semi‐deep to deep lacustrine deposits. (c) The late syn‐rift sequence (SSQ3) developed braided‐delta and shallow lacustrine deposits. In the SSQ3 transgressive systems tract, retrogradational parasequence sets developed along the southern dip slope because of the weak sediment supply from the Huilu low uplift, whereas westward aggradational–progradational parasequence sets developed along the long axis because of the stronger sediment supply from the Lufeng eastern low uplift. Tectonic activity primarily controlled the early syn‐rift and rift climax deposition; syn‐depositional faults formed the transfer zones and slope break zones. Three types of transfer zones for sedimentary transport into the basin were distinguished, including synthetic approaching transfer zones, strike ramps, and collateral transfer zones. Three types of slope break zones were also recognised, including fault scarp zones controlling fan deltas and nearshore subaqueous fans, fault terrace zones controlling low‐stand fan deltas, and antithetic fault terrace dip‐slope zones controlling braided deltas.