Footwalls Of Normal Faults Research Articles

Characterising bed-parallel slip during gravity-driven deformation

Despite the recognition that bed-parallel slip (BPS) must operate during gravity-driven deformation of basinal sediments, there is a general paucity of detailed outcrop-based observations to characterise and detect such a process. We therefore present detailed timing relationships between BPS and steeper dip-slip faults that were both created during seismically-triggered downslope-directed movement of sediments. Using the late Pleistocene Lisan Formation that was deposited around the Dead Sea Basin as our case study, we show that ‘sub-seismic’ decametric scale BPS planes may pre-date, post-date, or operate coevally with steeper faults generated as sediments slip downslope towards the depocentre. Older BPS can be recognised by sediment injections and minor folds and fractures, whereas younger BPS displaces marker faults downslope towards the basin. BPS operating coevally with steeper faults results in complex overprinting and development of fault-bound lenses. BPS that forms along single surfaces in the footwall of normal faults becomes separated into two distinct planes in the downthrown hangingwall block, indicating broadly coeval development. Adjacent BPS planes that operate synchronously result in synthetic and antithetic faults that ‘hard-link’ and transfer displacement between BPS planes. Attenuated bedding between segments of BPS that overlap and terminate next to one another suggests that ‘soft-linkage’ also forms between coeval BPS planes. Displacement-length relationships of measured BPS planes plot in the same range as recorded for normal faults, although BPS with larger displacements have relatively ‘short’ lengths, suggesting that complete BPS planes are missing due to limitations of outcrop size. Although the lack of displaced bedding across BPS makes it largely invisible on seismic sections across large-scale gravity-driven systems, it does potentially contribute towards the apparent inbalance between net extension and contraction observed in many sections across mass transport deposits. In addition, the realisation that BPS interacts with dip-slip faults to create repeated and missing sections that are particularly focussed along earlier deformed horizons and turbidites, has implications for palaeoseismic studies that assume broadly continuous stratigraphy.

Late Jurassic Intracontinental Extension and Related Mineralisation in Southwestern Fujian Province of SE China: Insights from Deformation and Syn-Tectonic Granites

Late Mesozoic igneous intrusions and extensional structures in Carboniferous to Permian sequences in the SW Fujian region acted as important controls on the localisation of Fe-polymetallic deposits. Here we document the identification of extensional deformation at shallow crustal levels and syn-tectonic granites related to normal faults. Based on spatial distribution and structural features, the extensional deformation can be divided into cover-only and basement-intersecting styles. A series of syn-tectonic plutons were emplaced into the footwall of normal faults. Representative samples of the Tangquan Granite have high SiO2 (66.4 wt.%-73.9 wt.%) assays and Mg# values (37-59). The samples also have relatively homogenous initial 87Sr/86Sr (0.708 3-0.708 9) and eNd (-9.2--10.2) values. Geochemical and isotopic evidences indicate that the Tangquan granite originates from a hybrid source including lower crustal-derived felsic and lithospheric mantle-derived mafic magmas. Zircon U-Pb dating indicates that the granodiorite phase from the pluton crystallised at 161±4 Ma and the monzogranite phase crystallised at 159±1 Ma. Combined with the granitic rocks in a wider region of SE China, the widespread granitic magmatism and polymetallic mineralisation have been synchronous during the Late Mesozoic, probably resulting from extensional tectonics related to the lithospheric thinning.

Exhumation history of the La Caridad and Suaqui Verde porphyry copper deposits in the eastern Basin and Range province of Sonora: Insights from thermobarometry and apatite thermochronology

The Arizona-Sonora porphyry copper cluster is one of the most productive provinces of this type on Earth. There, upper-crustal deposits emplaced between Late Cretaceous and Early Eocene, subsequently exhumed and developed supergene enrichment profiles. Despite the Basin and Range regional extension, these shallow intrusions and associated mineralization were preserved in eastern Sonora in the exhumed footwalls of normal faults. In order to better constrain their exhumation history, we performed a thermobarometrical (Al-in-hornblende) and thermochronological (apatite U–Pb and fission-track) study on the La Caridad and Suaqui Verde porphyry copper deposits. On the one hand, thermobarometry allows us to determine that the Suaqui Verde porphyry copper emplaced at 4.8 ± 0.9 km-depth, which implies that Cu–Mo mineralization may occur deeper than the ~2–4 km depth classically proposed for porphyry copper. On the other hand, using thermobarometry and inverse modeling of thermochronologic data obtained for individual and drill core samples, we reconstruct the thermal history of these porphyry copper. After a very fast post-emplacement cooling, numerical models suggest that cooling of rocks was slow and mainly monotonous during exhumation. Indeed, our data do not record an acceleration of the exhumation in eastern Sonora during the Basin and Range regional extension (between ~25 and 12 Ma). However, inverse modeling of fission-track data obtained along a drill core in La Caridad allows us to reconstruct the evolution of the geothermal gradient through time, which is in agreement with the regional geodynamics. Overall, it cannot be excluded that Oligocene volcanic rocks protected the ore deposits from erosion, even if they remained relatively thin (<500 m-thick). We propose that eastern Sonora was characterized by Basin and Range extension distributed on numerous normal faults with small offsets, which allowed to preserve the upper crustal part of the Cretaceous-Paleogene magmatic arc and its numerous associated porphyry copper systems.

Structural variation within the Himalayan fold and thrust belt: A case study from the Kohat-Potwar Fold Thrust Belt of Pakistan

The Kohat and Potwar fold thrust belts (KP-FTB) in Pakistan exhibit structural variations over 250 km along strike within the Himalayan fold and thrust system. Our 3D deformation model shows that Kohat surface structures evolved above an active roof thrust in Eocene evaporites. The ramp-forming duplexes in the Kohat were stacked and passively transported toward the foreland above new ramps, resulting in up to 5 km of thickening between the two decollements. Ramps from the Kohat extend into the Potwar as thrust tips of fault propagation folds. The basement slope changes from flat (β < 1°) below the northern part to north-dipping (β > 1°) below the southern part, corresponding to the change in structural style and complexity of the KP-FTB. The Kalabagh Fault Zone, linking the two belts, is interpreted as a zone of complex dextral strike-slip rotational faulting. Salt expulsed from the hanging walls of normal faults and under synclines in the Kalabagh Fault Zone moved toward the footwall of normal faults, accumulated in the cores of anticlines, and formed lobe structures at the deformation front. The fundamental reasons for the variable structural styles are changes in decollement strength, basement slope, preexisting normal faulting, presence of a secondary decollement and spatially-variable salt mobility and accumulation.

Modelling of hydrothermal fluid flow and structural architecture in an extensional basin, Ngakuru Graben, Taupo Rift, New Zealand

Present-day geothermal activity on the margins of the Ngakuru graben and evidence of fossil hydrothermal activity in the central graben suggest that a graben-wide system of permeable intersecting faults acts as the principal conduit for fluid flow to the surface. We have developed numerical models of fluid and heat flow in a regional-scale 2-D cross-section of the Ngakuru Graben. The models incorporate simplified representations of two ‘end-member’ fault architectures (one symmetric at depth, the other highly asymmetric) which are consistent with the surface locations and dips of the Ngakuru graben faults. The models are used to explore controls on buoyancy-driven convective fluid flow which could explain the differences between the past and present hydrothermal systems associated with these faults.The models show that the surface flows from the faults are strongly controlled by the fault permeability, the fault system architecture and the location of the heat source with respect to the faults in the graben. In particular, fault intersections at depth allow exchange of fluid between faults, and the location of the heat source on the footwall of normal faults can facilitate upflow along those faults. These controls give rise to two distinct fluid flow regimes in the fault network. The first, a regular flow regime, is characterised by a nearly unchanging pattern of fluid flow vectors within the fault network as the fault permeability evolves. In the second, complex flow regime, the surface flows depend strongly on fault permeability, and can fluctuate in an erratic manner. The direction of flow within faults can reverse in both regimes as fault permeability changes.Both flow regimes provide insights into the differences between the present-day and fossil geothermal systems in the Ngakuru graben. Hydrothermal upflow along the Paeroa fault seems to have occurred, possibly continuously, for tens of thousands of years, while upflow in other faults in the graben has switched on and off during the same period. An asymmetric graben architecture with the Paeroa being the major boundary fault will facilitate the predominant upflow along this fault. Upflow on the axial faults is more difficult to explain with this modelling. It occurs most easily with an asymmetric graben architecture and heat sources close to the graben axis (which could be associated with remnant heat from recent eruptions from Okataina Volcanic Centre). Temporal changes in upflow can also be associated with acceleration and deceleration of fault activity if this is considered a proxy for fault permeability. Other explanations for temporal variations in hydrothermal activity not explored here are different permeability on different faults, and different permeability along fault strike.

Using the ASTER global DEM to derive empirical relationships among triangular facet slope, facet height and slip rates along active normal faults

The advent of global DEMs provided a uniform elevation dataset for studying geomorphic parameters in a variety of settings. In this paper ASTER DEM data (30-mpixel size) are used to derive empirical relationships between triangular facet attributes and slip rates along active normal faults. We sampled 232 triangular facets along 10 normal faults in Greece and Bulgaria that slip with rates from 0.1mm/yr up to 1.3mm/yr. The studied normal faults accumulate Quaternary tectonic strain in well-known extensional provinces, such as central Greece, Crete and SW Bulgaria. The normal fault footwalls analyzed herein have been developed under similar long-term climatic conditions. It is suggested that two key geometrical features of the youngest generation of triangular facets (slope angle and height) can provide useful metrics to assess rates of deformation when seismological and geodetic data are lacking or not found in sufficient quantity to make reasonable assessments. Our derived empirical relation between slip rate and facet slope angle is: Y=0.057×X−1 where Y is the fault slip rate (mm/yr) and X is the facet slope angle (degrees), with an R2=0.728. It is envisaged that our analysis may be helpful in assessing seismic hazard along normal faults with similar facet characteristics in other extensional settings.

Evolution of the NE Qinghai–Tibetan Plateau, constrained by the apatite fission track ages of the mountain ranges around the Xining Basin in NW China

The Laji Shan and Daban Shan which are located along the southern and northern margins of the Xining Basin in the northeastern Qinghai–Tibetan Plateau, experienced important multi-stage exhumations during the Cenozoic. The apatite fission track (AFT) dating of 19 samples along four north–south sections in the Laji Shan and Daban Shan showed that the AFT ages of the Laji Shan from 88Ma to 12Ma and the ages of the Daban Shan are between 60 and 32Ma. All these AFT ages are significantly younger than their host rock formation or sediment deposition ages. Thermal modeling of the AFT data indicated that the Laji Shan had undergone important exhumation events in the Late Cretaceous, ca. 50–30Ma, and 17–8Ma, whereas the only significant thermal event in the Daban Shan occurred at ca. 50–30Ma. The exhumation of the Laji Shan during the Late Cretaceous may result from the Lhasa–Qiangtang collision. During 50–30Ma, the Central Qilian Block rotated clockwise relative to the South Qilian Block, which led to the formation of the Xining Basin. The slow uplift of the Laji Shan and Daban Shan in this period was the isostatic adjustment of the footwalls of normal faults along the two margins of the Xining Basin. The exhumation of the Laji Shan during 17–8Ma resulted from compression due to the clockwise rotation of the South Qilian Block relative to the Central Qilian Block, causing the inversion of the Xining Basin, a marked change in the sedimentary environments and a rapid increase in sedimentation rates.

Outer-rise normal fault development and influence on near-trench décollement propagation along the Japan Trench, off Tohoku

Multichannel seismic reflection lines image the subducting Pacific Plate to approximately 75 km seaward of the Japan Trench and document the incoming plate sediment, faults, and deformation front near the 2011 Tohoku earthquake epicenter. Sediment thickness of the incoming plate varies from <50 to >600 m with evidence of slumping near normal faults. We find recent sediment deposits in normal fault footwalls and topographic lows. We studied the development of two different classes of normal faults: faults that offset the igneous basement and faults restricted to the sediment section. Faults that cut the basement seaward of the Japan Trench also offset the seafloor and are therefore able to be well characterized from multiple bathymetric surveys. Images of 199 basement-cutting faults reveal an average throw of approximately 120 m and average fault spacing of approximately 2 km. Faults within the sediment column are poorly documented and exhibit offsets of approximately 20 m, with densely spaced populations near the trench axis. Regional seismic lines show lateral variations in location of the Japan Trench deformation front throughout the region, documenting the incoming plate’s influence on the deformation front’s location. Where horst blocks are carried into the trench, seaward propagation of the deformation front is diminished compared to areas where a graben has entered the trench. We propose that the décollement’s propagation into the trench graben may be influenced by local stress changes or displacements due to subduction of active normal faults. The location and geometry of the up-dip décollement at the Japan Trench is potentially controlled by the incoming outer-rise faults.

Contrasting strike-slip motions on thrust and normal faults: Implications for space-geodetic monitoring of surface deformation

Recent global positioning system (GPS) records of surface deformation caused by earthquakes on intracontinental dip-slip faults revealed in unprecedented detail a significant strike-slip component near the fault tips that is markedly different for thrust and normal faults. In the hanging wall of the thrust fault ruptured during the A.D. 2003 Chengkung (Taiwan) earthquake, a divergent displacement pattern was recorded. In contrast, a convergent slip pattern was observed in the hanging wall of the normal fault that produced the 2009 L’Aquila (Italy) earthquake. Such convergent slip patterns are also evident in field records of cumulative fault slip from central Italy, which underlines the coseismic origin of cumulative displacement patterns. Here we use three-dimensional numerical modeling to demonstrate that the observed fault-parallel motions are a characteristic feature of the coseismic slip pattern on normal and thrust faults. Modeled slip vectors converge toward the center of normal faults, whereas they diverge for thrust faults; this causes contrasting fault-parallel displacements at the model surface. Our model also predicts divergent movements in normal fault footwalls, which were recorded for the first time during the L’Aquila earthquake. During the postseismic phase, viscous flow in the lower crust induces fault-parallel surface displacements, which have the same direction as the coseismic displacements but are distributed over a larger area that extends far beyond the fault tips. Therefore, detecting this signal requires GPS stations in the prolongation of the fault strike. Postseismic velocities vary over several orders of magnitude depending on the lower crustal viscosity and may reach tens of millimeters per year for low viscosities. Our study establishes the link between coseismic and cumulative slip patterns on normal and thrust faults and emphasizes that understanding fault-parallel slip components and associated surface displacements is essential for inferring regional deformation patterns from space-geodetic and fault-slip data.

Fault-controlled alpine topography in Norway

Abstract: Alpine topography in Norway is largely fault-controlled. Linear and asymmetric ranges developed in the footwalls of normal faults that were reactivated after the main phase of Mesozoic rifting, but prior to the Late Cenozoic glaciations. Stark geomorphological contrasts developed across the faults, reflecting differential glacial exploitation of the pre-glacial drainage pattern. Alpine topography developed preferentially in the footwalls. Triangular facets mark the traces of the most recently active faults. At the base of deeply incised, alpine range-front escarpments, the best-exposed faults display metres-thick fault-rock successions and record multiple phases of fault movement. Juxtaposition of Precambrian and Caledonian basement rocks with Jurassic or Cretaceous sedimentary rocks provides evidence for fault activity in or after the Mesozoic for some of the faults. Late Cretaceous or younger reactivation is indicated by jumps in apatite fission-track apparent ages across the faults, and interferometric synthetic aperture radar and earthquake data attest to normal faulting at the present day. Two of the areas described host anomalous clusters of rockslides that may relate to tectonic activity. The most distinct landscape-forming faults in western Scandinavia were probably active in the Cenozoic, and imposed asymmetric landscape patterns from the scale of single mountain ranges to the whole of Scandinavia.

Spatial variations in catchment-averaged denudation rates from normal fault footwalls

We test the spatial correspondence between rock uplift in active normal fault footwalls in the western USA, and catchment-averaged denudation rates from cosmogenic radionuclide (CRN) analysis. We find that denudation rates vary along strike, depending on the fault length and displacement. For the 18-km-long Sweetwater fault, foot-wall relief is largely inherited from the prefaulting topography, and denudation rates are only partly reflective of fault displacement. For the 130-km-long Wassuk fault, all inherited topography has been removed by erosion in response to fault displacement. Denudation rates, however, show little along-strike consistency and no correlation with catchment size or morphology. We argue that, in the early stages of fault growth, CRN-derived denudation rates reflect inherited prefaulting relief, with an overprint of fault-controlled erosion. In later stages of fault growth, rates are decoupled from fault displacement through the impact of stochastic landsliding events, and longer term measures of denudation (such as low-temperature thermochronometry) that integrate over multiple earthquakes may provide a better record of spatial variations in rock uplift.

Fault reactivation and active tectonics on the fore-arc side of the back-arc rift system, NE Japan

Positive basin inversion is documented using seismic reflection profiles and seismic data on the fore-arc side of the back-arc rift system of northern Honshu, Japan. This finding within a seismically active region is used to understand source fault geometry based on fault reactivation models. Seismic reflection profiles and Bouguer gravity anomalies along with available surface and subsurface evidence define a >300-km-long normal fault system, formed in Miocene time, that bounds the fore-arc side of the Miocene back-arc basin. Based on the geometry of the seismogenic source fault which was estimated from aftershock distribution and seismic reflection profiles, reverse fault reactivation of former Miocene normal faults occurred and is presently occurring across the entire seismogenic zone. This contractional event began in late Neogene time. The total amount of shortening as inferred from sequential cross section restoration is approx. 20% of the total amount of Miocene extension in the fore-arc side of the northern Honshu. Thus, the overall extensional architecture of the region inherited from Miocene rifting was only slightly modified by the superimposed contractional deformation. Reverse fault reactivation seems to occur only when the dip angle of precursor normal faults is 60° or less. The geometry and evolution of active thrust faults are strongly controlled by fault reactivation processes and are found in conjunction with the development of new thrust faults that propagate across the normal fault footwalls with shortcut trajectories.

03/02416 Investigations on optimizing large solar thermal systems: Krause, M. et al. Solar Energy, 2002, 73, (4), 217–225

The Laji Shan and Daban Shan which are located along the southern and northern margins of the Xining Basin in the northeastern Qinghai–Tibetan Plateau, experienced important multi-stage exhumations during the Cenozoic. The apatite fission track (AFT) dating of 19 samples along four north–south sections in the Laji Shan and Daban Shan showed that the AFT ages of the Laji Shan from 88 Ma to 12 Ma and the ages of the Daban Shan are between 60 and 32 Ma. All these AFT ages are significantly younger than their host rock formation or sediment deposition ages. Thermal modeling of the AFT data indicated that the Laji Shan had undergone important exhumation events in the Late Cretaceous, ca. 50–30 Ma, and 17–8 Ma, whereas the only significant thermal event in the Daban Shan occurred at ca. 50–30 Ma. The exhumation of the Laji Shan during the Late Cretaceous may result from the Lhasa–Qiangtang collision. During 50–30 Ma, the Central Qilian Block rotated clockwise relative to the South Qilian Block, which led to the formation of the Xining Basin. The slow uplift of the Laji Shan and Daban Shan in this period was the isostatic adjustment of the footwalls of normal faults along the two margins of the Xining Basin. The exhumation of the Laji Shan during 17–8 Ma resulted from compression due to the clockwise rotation of the South Qilian Block relative to the Central Qilian Block, causing the inversion of the Xining Basin, a marked change in the sedimentary environments and a rapid increase in sedimentation rates.

Precarious Rock Evidence for Low Ground Shaking on the Footwall of Major Normal Faults

Preliminary interpretations of precarious rock observations suggest relatively low near-fault (<3 km) footwall ground motions for normal faults relative to strike-slip faults, as reported for a dynamic foam rubber normal fault model by Brune and Anooshehpoor (1999). Since for large normal fault earthquakes there are no instrumental data at such close distances from the fault trace, precarious rocks may provide important data for estimating the ground motion for these earthquakes. Use of empirical curves in the literature, based mainly on strike-slip data, may lead to overestimates of seismic hazard for the near-fault footwall of normal faults. These preliminary results may warrant more comprehensive and more quantitative followup studies.

Methodology for using precarious rocks in Nevada to test seismic hazard models

AbstractFields of precariously balanced rocks indicate that strong earthquake motions have not occurred at that site since the precarious rocks developed. These fields can be characterized with an estimate of the peak acceleration that would be sufficient to topple the rocks and an estimate of how long the rocks have been precarious. This article uses this information to test the input to probabilistic seismic hazard analysis. The fundamental assumption is that the probability of exceeding a ground motion capable of toppling a precarious rock during a time period equal to the age of the rock is equal to the confidence level at which the inputs to the probabilistic seismic hazard analysis can be rejected.We performed a probabilistic seismic hazard analysis for 26 sites of observed precarious rocks in Nevada, using preliminary estimates of the toppling acceleration and the age of the features. Two standard models are rejected with over 95% confidence by most of the precarious rock observations. We consider several possible explanations for the inconsistency. A probabilistic seismic hazard analysis that eliminates the area sources and only includes faults is consistent with the precarious rock observations at most of the sites. Spatial clustering of small-magnitude events could maintain this effect without contradicting the precarious rock observations. However, it may not be necessary to completely reject the area sources from our probabilistic seismic hazard analysis. The physics of rock stability may allow increasing the minimum magnitude to 6.0 in the area sources, because the short duration of high-frequency accelerations in smaller events may not topple all precarious rocks. The precarious rocks could generally have small site effects. Attenuation models may overestimate ground motions on the foot wall of normal faults, and a different attenuation model in general might decrease the ground-motion predictions. The ergodic assumption that is made in estimating the uncertainty in attenuation models may be contributing to overestimate the hazard. Individually, each of these effects might allow more of the precarious rock sites to be consistent with the area source zones.

Petroleum Field Evidence for Fluid Migration across Faults in Abnormally Pressured Siliciclastic Systems: ABSTRACT

In the deep portions of sedimentary basins, faults commonly form portions of boundaries between domains of contrasting datum fluid pressure. Fluid escape from high-pressure domains may be achieved by either hydraulic fracturing of an essentially impermeable seal (fault or top) or fluid-flow along or across a semi-permeable fault zone. In this study, hydrocarbon distribution patterns in petroleum fields within Tertiary siliciclastic strata of the offshore Gulf of Mexico and Trinidad areas are used to distinguish between these two possibilities. Hydrocarbon columns in Vermilion 14 field (Louisiana) and Flambouyant and Poui fields (Trinidad) are trapped in partial sand-against-sand juxtaposition in the footwall of normal faults. Fluid pressure differences of 700-5000 psi are observed across some portions of these faults. Column heights are several times longer in reservoirs that are underpressured relative to juxtaposed strata than along fault segments that juxtapose reservoirs with equivalent datum fluid pressures (either normal or abnormal). Sandstones that are juxtaposed against lower pressure strata contain anomalously short oil or gas columns. In these cases, buoyancy pressures generated by the hydrocarbon columns are insufficient to elevate the reservoir pressures to those necessary for hydraulic fracturing of the seal. The hydrocarbon distribution in these fields is most consistent withmore » a hydrodynamic model in which fluid flow occurs across semi-permeable fault zones from high to low datum fluid-pressure domains. Superposition of hydrodynamic effects on the capillary properties of the fault zones creates the observed enhancement or degradation of seal capacity (inferred from column heights) relative to the hydrostatic case. This model provides a simple mechanism for fluid migration out of the abnormally pressured regime, without invoking hydraulic fracturing of the seal.« less

Physical modelling of fold and fracture geometries associated with salt diapirism

Abstract The generation of salt diapirs is associated with a protracted deformation history in the surrounding overburden, which may be difficult to image seismically due to the highly inclined and fractured nature of sediments adjacent to the salt. To further the understanding of overburden deformation related to diapir development, a series of physical models has been produced incorporating a viscous SGM polymer source layer which is overlain by a granular, glass bead overburden. Model diapirs are initially triggered (to the pillow stage) via a basal indentor, but subsequently develop by a passive downbuilding process. The maximum widths of withdrawal basins and diapir-related upturn are approximately equivalent to one another. Experiments in which the salt analogue is allowed to extend laterally result in listric normal faulting of the overburden, to define graben structures centred on the diapir. Injection of the source layer into the footwall of normal faults results in reactive dispirism on the flanks of the major structure, whilst the adjacent central sections illustrate geometries which could be attributed to both active and passive diapirism. Multiple generations of normal faults produce graben with an inward-stepping sequence of fault propagation towards the central block. Complete evacuation of the source material at the keel of the normal fault simulates localized salt welds and results in extensional displacement being transferred into the hangingwall block. Sand-clay mix layers act as relatively competent horizons and display radial fracture patterns which may also be associated with a central ring fault and circular collapse graben above the diapir. Normal faults defining graben structures converge on the radial fracture pattern overlying the diapir. Circular thrust faults surrounding the diapir are developed when more competent horizons temporarily arrest vertical growth resulting in diapiric ‘ballooning’.

Fault-Zone Seals in Offshore Trinidad Oil Fields

Hydrocarbon columns in Pliocene sands of offshore SE Trinidad occur in 3-way closure, primarily in the footwall of normal faults. Multiple reservoir sands and numerous fault blocks result in a large number of individual hydrocarbon accumulations. Fault-plane sections demonstrate that fault sealing is unrelated to juxtaposition of intercalated shales. These fault-zone capillary seals were studied by (1) inferring their fluid-flow properties from the pattern of trapped hydrocarbons and (2) direct examination and measurement of cored faults. Buoyancy pressures for hydrocarbon columns were calculated from fluid property data for each reservoir and fault block. Buoyancy pressures range widely, increasing nonlinearly with fault displacement and percent shale in the faulted section, but do not vary systematically with stratigraphic position or depth. Small-displacement faults observed in core are narrow zones of cataclasis within porous sandstone. Mercury injection tests indicate fault-zone displacement pressures that coincide with buoyancy pressures calculated for hydrocarbon columns sealed by large-displacement faults. The agreement between measured displacement pressures and calculated buoyancy pressures indicates that (1) the reservoirs are filled to their capacity, dictated by the displacement pressure of the fault zones, and (2) the fault-zone seals are primarily the product of deformation of the sands, with some enhancement by incorporation more » of argillaceous material into the fault zones. The observed relationship between fault displacement and calculated buoyancy pressure of the hydrocarbon columns implies that fault-zone continuity is a factor that needs to be assessed in fault-zone seal analysis. « less