The reduction of carbon emissions is a global challenge. Carbon capture and sequestration has been established as a viable solution to the problem of rising carbon emissions. Consequently, mature and depleted fields, some of which are friable reservoirs, are increasingly being used to store CO2. However, because it can be extremely challenging to obtain intact samples and prepare highly friable specimens for laboratory testing, very few studies have examined the permeability of highly friable rocks as compared to consolidated rocks. Therefore, there is a scarcity of data on the permeability of friable rocks. In this study, the permeability of the Erin Formation highly friable sandstone and thin-bed shale was measured under effective pressures, up to 130 MPa, in both perpendicular (kv) and parallel (kh) directions to bedding. The Erin Formation is a major reservoir from which oil has been produced in Southern Trinidad for the past century. The friable rocks exhibit low permeability, with ranges from 12.5 to 0.02 μD, and 0.2 μD to 2 nD for sandstone and thin-bed shale, respectively. The effective pressure has a substantial impact on permeability, causing the permeability to decrease as the effective pressure increases. The permeability is higher in the parallel bedding direction and the anisotropy (kh/kv) ranges from 17 to 421, which is greater in the sandstone. The results indicate that the flow of CO2 during injection will be predominantly lateral along the beds with the thin-bed shale acting as a reliable seal to flow perpendicular to bedding.

Compound clinoforms are well‐recognized in modern large muddy deltas and in some ancient deltas, but there is still a lack of understanding regarding their lithology variations and the process by which sand from the shoreline clinothem reaches the subaqueous clinothem foresets that are sometimes 100 km away. Net‐to‐gross, thickness and facies association evaluation show overall coarsening‐upward through a 191 m thick exposure of the late Pliocene Orinoco, Lower Morne L'Enfer Formation, with a distinct tripartite lithology distribution. The subaqueous clinothem records a lower, relatively muddy coarsening‐upward interval, 112 m thick, with net‐to‐gross increasing from zero to 60%. On the lower delta front, zero net sand units show graded beds of silt and mud with occasional spring–neap rhythmites, strongly suggesting gravity flows influenced by tidal currents. These foreset beds are overlain by structureless very fine sand, interbedded with deformed wavy to lenticular, grey fluid mud layers that rapidly accumulated near the subaqueous clinoform rollover point. The tidally dominated subaqueous platform (subaqueous delta topset), 1 to 4 m thick, shows zero net sand units with anomalously high mud content, >70%, due to the high near‐bed suspended sediment concentration of externally derived fluid mud that migrated littorally alongshore from the Amazon Delta. The interaction of freshwater river flood discharge with fluid‐mud banks gave rise to density stratification with fine sand hypopycnally dispersing as a turbulent layer above the denser fluid‐mud carpet. The shoreline clinothem (<8 m thick) has high net‐to‐gross, >85%, attributed to winnowing of sediment by waves and tides. Utilizing net‐to‐gross trends and facies changes provide useful criteria to identify compound clinoforms in the rock record. The Orinoco Delta deposits, however, are unusual, since fluid mud hinders sand deposition on the platform, allowing for easy identification of platform facies and a clear distinction between the subaqueous and shoreline clinothem in outcrop.

Durability, compactness, and natural strength of sandstones are pivotal properties for its subsequent application as a building material. If a sandstone is tight and contains petroleum, then hydraulic fracturing is necessary to achieve commercial production. The Casa Cruz Sandstone Member, southern Trinidad, is currently being explored as a potential petroleum target and building material. This study investigates the petrographic, petrophysical, and mechanical characteristics of the Casa Cruz sandstone to understand the factors that control the quality of the sandstone and to determine the sandstone’s suitability as a natural resource. A thin-section analysis was performed to investigate the mineralogy, texture, and diagenesis of the sandstone. Permeability, porosity, density, P- and S-wave velocities, unconfined compressive strength (UCS) and confined compressive strength were measured. The elastic properties (Young’s modulus, shear modulus, bulk modulus, and Poisson’s ratio) were estimated from inversion of the P- and S-wave velocities. The results show that the Casa Cruz sandstone is one of the tightest sandstone types reported in the literature with porosity of 0.7% and permeability of 4.7 x 10-22 m2 (0.47 nD). Authigenic cementation and mechanical compaction are the main diagenetic processes that jointly controlled the quality of the sandstone. The sandstone was classified as a very strong (UCS of 128 MPa) quartz-arenite and is barren of petroleum and bioclasts. Brittle failure behaviour was exhibited, accompanied by axial splitting fractures and shear fractures under unconfined and confined conditions, respectively. The Casa Cruz sandstone is suggested to be an excellent building material as its petrophysical and mechanical properties are comparable to those of commercial industry grade of Westerly granite and Naparima Hill argillite.

The geomorphology and sedimentology of the Nariva River tidal-inlet complex, a microtidal fluvially influenced tidal-inlet complex, was analyzed. The complex comprises a recurved spit, an ebb-tidal channel, and an ebb-tidal delta. Morphological trends in the spatio-temporal evolution of the inlet complex were observed and recorded from Google Earth (TM) timelapse satellite images taken from 2003 to 2019. The two-dimensional internal architecture of the inlet complex and the sedimentary succession of the recurved spit, an ebb spit, the swash platform (of the ebb-tidal delta), a mouth bar (associated with the wet-season river-dominated inlet complex erosion), the ebb-tidal channel, and the adjacent foreshore were observed and documented from six shallow sedimentary cores.The Nariva River inlet width ranges from 17 to 40 m through its seasonal evolution, has a tidal prism of similar to 2.17 3 10(5) m(3), a cross-sectional area of 29.52 m(2), and a depth similar to 1.4 m (calculated at peak dry season near the inlet throat). The inlet complex undergoes an annual geomorphological evolution linked to the seasonally induced migration of the fluvial-to-marine transition zone (FMTZ). Increased fluvial discharge during the wet and hurricane seasons results in the basinward migration of the FMTZ rendering the inlet river dominated and resulting in the erosion of the inlet complex. During the dry season, low fluvial discharge, tidal dominance, and fair-weather conditions promote sedimentation in the inlet and the redevelopment of the inlet complex. The inlet has a complex (CX) internal architecture (fill pattern) defined by the laterally migrating recurved spit and ebb spit on their updrift margin, and conformable, mounded elements on their downdrift (e.g., mouth bar, swash platform, and foreshore). Two sedimentary successions were developed for ebb-tidal-delta deposits: off-axis of the ebb-tidal channel and on-axis. The off-axis succession is considerably similar to the adjacent foreshore-to-shoreface succession which can pose a challenge when attempting to identify these deposits in the rock record. The on-axis succession, however, despite thickness variability, showed a positive correlation to studied mesotidal tide-dominated inlet successions.

ABSTRACT Water as pore fluid is an important factor that influence rock deformation and failure behaviour. This paper presents an experimental study on the effects of water saturation on the failure behaviour of mudstones at high confining pressures. Triaxial compression experiments were performed at confining pressures up to 130 MPa, on dry and water-saturated mudstone samples. Results including stress-strain curves, failure strength, and failure modes, showed that the mudstones experienced brittle, brittle-ductile transition, and ductile failure behaviour as the confining pressure increases. The results also revealed that water significantly weakened the rock, thereby reducing the failure strength and causing the rock to become ductile at lower confining pressure compare to dry conditions. Furthermore, in water-saturated conditions, the brittle-ductile transition behaviour is accompanied by shear band fractures and ductile flow, as opposed to shear fractures to shear band fractures in dry conditions. The findings of this study could provide valuable information for optimizing hydraulic fracturing techniques and improving production efficiency in unconventional oil and gas reservoirs by identifying the conditions and depths of the transition from brittle to ductile failure behavior. INTRODUCTION Crustal rocks typically contain some form of pore fluid, and most of these fluids are water (Price, 1975). Understanding how pore fluids affect the mechanical properties of rocks is crucial in solving a range of problems in geotechnical and geological applications. For example, water's significant impact on the strength and deformability of rocks has been linked to many rock engineering hazards like landslides (Iverson, 2000). Water has been shown in numerous studies to weaken the mechanical strength of various types of rock, including mudstone, sandstone, granitic rocks, and quartzite. (Cai et al., 2019; Lu et al., 2017; Wasantha and Ranjith, 2014). When rocks are saturated with water, they become weaker and more prone to failure. This is because water can act as a lubricant between the individual particles, reducing the frictional forces that hold the particles together. As a result, the rock may experience reduced strength, stiffness, and cohesion, as well as an increased risk of creep, plastic deformation, and failure rates (Brantut et al., 2013; Chen et al., 2019; Wong et al., 2016).

The compressive and tensile strengths are essential parameters used for determining the safe mud weight window (SMWW) during borehole drilling, especially in friable rocks where the SMWW tends to be narrow; therefore, the margin for wellbore instability issues is high. However, these parameters are seldom reported for friable rocks because of the difficulties and challenges faced while recovering and preparing friable cores and outcrop samples for strength testing. This study evaluates the compressive and tensile strengths of friable rocks under dry and saturated conditions and investigates how strength anisotropy affects the SMWW. Unconfined compressive strength, confined compressive strength (up to 130 MPa effective pressure), and Brazilian (tensile strength) measurements were made perpendicular and parallel to the outcrop bedding of the friable sandstone and thin-bed shale lithofacies of the Erin Formation, Southern Trinidad. Although this formation is one of the main petroleum reservoirs, there are no published data on the rock strength. The results show that strength anisotropy exists and is larger under saturated conditions. Under confinement, the friable rocks accumulated large strain (an average of 20 σ_v, σ_h and σ_H ), of a normal faulting stress regime, at depths of 3400–6900 feet. The upper mud weight limit of the SMWW is not influenced by the tensile strength anisotropy. However, the lower mud weight of the SMWW was influenced by the compressive strength anisotropy, predominantly for σ_v and σ_h aligned wells that are penetrating through the saturated friable sandstone. Since the rocks are friable, drilling outside of the SMWW by 5–8

Cores samples are vital in the characterization of heterogeneous volcanic geothermal environments. Continuous cored intervals, representative of the subsurface formation, can help to validate geological and thermal classifications previously interpreted from drill cuttings. More importantly, core samples can help to improve our understanding of controls on fluid flow, as direct permeability measurements are attainable. In this study, we conduct petrological and petrophysical analyses on three cores (Core 1, Core 2 and Core 3) obtained from the well MON-3, drilled within the hydrothermal system located in Montserrat, Lesser Antilles arc, West Indies. We present a litho-stratigraphic model of the geothermal system. Contrasting lithologies (sandstone, limestone and interbedded silicified tuff) and mineralogies (calcite vs plagioclase) from Core 3 (2109 - 2116.5 mbsl) contributed to constraining a volcanic-sedimentary unit (-1900 - 2200 mbsl) while pumice textures of Core 1 (1478 - 1482.6 mbsl) and Core 2 (1700 - 1705.3 mbsl) helped to classify a subaerial volcanic unit (-1200 - 1900 mbsl). Integrating classified alteration assemblages in the model with downhole temperature logs, we propose that the thermal regime of the hydrothermal system was hotter in the past based on evidence of epidote and actinolite observed at depths less than 1200 mbsl. Lastly, we infer that the primary permeability of carbonate formations or secondary fracture permeability developed by a SW and WNW fault zone in a shallower volcanic sedimentary unit (-900 - 1200 mbsl) could be influences on potential feedzone locations in the wellbore. This could be supported by the relatively highest measured permeabilities (10(-18) - 10(-17) m(2)) in a carbonate sample of Core 3 and fractured samples of Core 2. Overall, the improved characterization of litho-units, alteration assemblages and permeable zones with possible influences on fluid flow, can be used to enhance previous numerical and geophysical models of Montserrat's geothermal system.

In the Holocene compound clinoforms are well recognized, but there are very few examples in the rock record. Utilizing 191 m of measured section of exposed highstand Orinoco delta deposits along the southwest coast of Trinidad, three main facies associations (FAs) were identified: FA1: up to 35 m of coarsening upward (CU) successions with a net to gross (N:G) < 60%; FA2: 2-4 m of a muddy subaqueous platform with a N:G < 10% and ; FA3: the shoreline clinothem consisting of sandy CU units up to 15 m with a high N:G, > 85%. This study illustrates how both grain size distribution and facies can be used to successfully distinguish compound clinoforms in the rock record.

It is important to understand the mechanical properties (elastic properties and strength) of rocks as these properties are used to model the rock's deformation in civil and underground engineering projects. Determining these properties for friable rocks are very difficult and scarce due to the rock's poor cementation making it challenging to retrieve samples from outcrop and boreholes, and difficult to prepare specimens for laboratory testing. To improve on the lack of mechanical properties of friable rocks, this study aims to measure the mechanical properties and P- and S-wave velocities and determine if empirical relationships exist between them. Experiments were carried out at effective pressures up to 130 MPa under dry and saturated conditions on friable sandstone and thin-bed shale lithofacies from the Erin Formation. The static Young's modulus of the rocks is less than 8 MPa and the unconfined strength ranged from 1.0 to 2.2 MPa, which indicate that the rock is very weak. The confined compressive strength is relatively high due to large strain accumulation (average of 20% strain), which resulted in significant strain hardening. At low effective pressures, velocity measurements were highly attenuated, resulting in limited S-wave velocity measurements. Empirical relationships established between the mechanical properties and P-wave velocities were predominantly weak to strong. Moderate and strong linear relationships were established between the unconfined compressive strength and static elastic properties and between the tensile strength and static Poisson's ratio. The results of this study could be carefully applied to velocity data that are readily available from well logs to estimate the mechanical properties of friable rocks.

Argillite rocks are often anisotropic due to the presence of microfractures , clay minerals, laminations , and bedding planes . Understanding the anisotropic behaviour of organic-rich argillite rocks is critical for their successful exploitation as unconventional reservoirs. In this study, the anisotropic rock properties of the argillite in the Naparima Hill Formation, Trinidad, an unconventional reservoir, was studied in laboratory experiments. A series of laboratory measurements including permeability, P- and S-wave velocities, and compressive strength were conducted at effective pressures of up to 130 MPa. The rock samples tested were collected from the Naparima Hill outcrop described by three lithofacies : siliceous calcareous mudstones , calcareous mudstone, and siliceous mudstones. The anisotropy of the argillite was determined by the ratio of the rock property measured perpendicular to the outcrop bedding to the rock property measured parallel to the outcrop bedding. For all the studied argillites, the permeability anisotropy ratios is about 2, while the P- and S-wave velocities anisotropy ratios, and compressive strength anisotropy ratios are close to 1, indicating that the rocks are fairly anisotropic. With increasing effective pressure, the permeability, velocities, and strength anisotropy ratios all decreased marginally. A comparison analysis reveals that the anisotropies of permeability and strength, anisotropies of P-velocity and permeability and anisotropies of S-velocity and permeability are all unrelated. However, anisotropies of P-wave velocity and strength, anisotropies of S-wave velocity and strength, and anisotropies of P-wave velocity and S-wave velocity anisotropy all have a 1:1 relationship. Overall, the results of the study suggest that the Naparima Hill argillite is weak to fairly anisotropic, which can be attributed to the low clay content, poor laminations, lack of foliation and few microfractures in the rocks.

Mud volcanoes are often associated with violent eruptions which are hazardous and pose significant risks to lives and livelihoods. A pseudo-3D electrical resistivity tomography (ERT) method that is integrated with digital elevation model (DEM) is used to map and monitor the near-surface structures and morphology of a mud volcano in Piparo, Trinidad. The mudflow within the vent is characterized by very low resistivity of less than 2.5 Omega m. Most of the mudflow within the near-surface is located north of the main crater. A magnitude 6.9 earthquake, located near the northern coast of Venezuela, caused the vent to enlarge, the mud volcano to be displaced towards the north, the crater region to sink by 0.2 m, and the flanks to rise by 0.7 m. This study demonstrates that the integrated pseudo-3D ERT and DEM provide means to monitor the dynamic geological interplay between the surface and subsurface geometry and structures.

The brittleness of unconventional source rock reservoirs is an essential parameter that controls the effectiveness of hydraulic fracturing operation. Brittleness can vary with effective pressure (equivalent to burial depth) and can be influenced by fluid saturation. Therefore, a detailed evaluation of rock brittleness is necessary prior to field applications. The late Cretaceous Naparima Hill Formation is the primary source for most of the 4 billion barrels of oil and 10 trillion cubic feet of gas produced in the last hundred years from conventional reservoirs in Trinidad, West Indies. This Formation is now being considered an unconventional reservoir in order to prolong and increase current productions. However, despite being a prolific Formation, there are no available geomechanical data, including brittleness. Brittleness index (BI) is often used to measure rock brittleness. In this study, we determined elastic-based BI from P- and S-wave velocity measurements, at effective pressures up to 130 MPa, for dry and water saturated outcrop mudstones from four lithofacies within the Naparima Hill Formation. The experimental results show that the BI for all four lithofacies, is independent of effective pressures in both dry and water saturated conditions. All four lithofacies can be considered brittle under dry conditions. However, when saturated, the BI reduces, which proves significant for softer lithofacies (siliceous calcareous mudstones and siliceous mudstones). In this saturated state, the harder lithofacies (calcareous mudstone interbedded with black chert and carbonate rich mudstone with nodular chert) are considered marginally brittle and the softer ones, are considered ductile. The results highlight that porosity is the main factor that controls the magnitude of reduction in the BI when the mudstones are fully saturated.

The upper Cretaceous Naparima Hill Formation, a primary source rock for conventional oil and gas reservoirs in Southern and Columbus basins, Trinidad is now considered an unconventional reservoir. Effective fracability evaluation is critical to the entire process of unconventional reservoir exploitation. At present, there exists no information on the fracability of the Naparima Hill Formation. A recent study have shown that the Formation consist of four lithofacies (siliceous-calcareous mudstone, calcareous mudstones, carbonate-rich mudstone and siliceous mudstone) that are highly brittle, implying that they are easily fractured. This is debatable because brittle rocks can have a higher fracture toughness, making them more difficult to fracture. In this study, an existing fracability evaluation model that incorporates brittleness, fracture toughness, and minimum horizontal insitu stress was used to evaluate the fracability of the four lithofacies within the Naparima Hill Formation. Through a series of laboratory testing (measurements of P- and S-wave velocities, uniaxial compression tests and Brazilian tests) of the dry outcrop samples, elastic properties (Young's modulus and Poisson's ratio), uniaxial compressive strength and tensile strength were determined. X-ray diffraction analysis (XRD) was also performed on the outcrop samples to determine their mineral compositions. Brittleness indices based on the rock elastic properties, rock strength and mineral compositions were used to evaluate the rock brittleness, and the fracture toughness was estimated from the tensile strength. The study results indicate that all lithofacies are highly brittle, which is consistent with the previous study. The fracability evaluation results showed that the siliceous calcareous mudstones and siliceous mudstones are more fracable than the calcareous and carbonate-rich mudstones. The key factors that control the fracability of the Formation were found to be the amount of quartz, and the rock strength that is influenced by calcite cementation.

Bulk modulus is one of the mechanical properties used in any rock engineering related project. This property can be measured statically, where it is derived from stress–strain data, and dynamically, where it is derived from P- and S-wave velocities. The static bulk modulus is generally different from the corresponding dynamic bulk modulus. As the static bulk modulus is required in computation or modelling of the deformation of the rock when it is stressed hydrostatically, empirical relationships between the static and dynamic bulk moduli are needed to convert the dynamic bulk modulus to static values. In this study, the static and dynamic bulk moduli were measured simultaneously, at effective pressures up to 130 MPa, for dry and fluid-saturated argillites. The samples were collected from four different lithofacies within the upper Naparima Hill Formation, Trinidad. The results show that the dynamic bulk modulus is greater than the static bulk modulus, except for hard lithofacies under dry conditions where the static and dynamic moduli are approximately equal. Under saturated conditions, the porosity plays a key role in increasing the difference between the static and dynamic bulk moduli. A linear relationship with high correlation (R 2 greater than 0.85) was established between the static and dynamic bulk moduli, which is dependent on the effective pressure and saturation state.

Argillites are indurated mudstones and have relatively high strength and low permeability. These rocks are: often used for aggregates in the construction industry, used as armour rock for sea walls, good sites for the waste repository (e.g. Cuisinier et al. 2009; Jia et al. 2009; Zhang et al. 2014), and unconventional petroleum reservoirs provided that the organic content is high (e.g. Rodnikova et al. 1968). In the mining of most rocks including argillites, several engineering issues are encountered, such as rock fracturing around mining pits and the support and control of the fractured rock masses, during drilling, blasting, and tunnelling (e.g. Aladejare 2020). The Uniaxial compressive strength (UCS) and tensile strength are important prerequisite parameters in the engineering design to minimize these issues (e.g. Ng et al. 2015). The lack of these data will result in poor engineering design that can lead to the collapse of the mining and tunnelling sites. Therefore them is imperative to quantify the UCS and tensile strength by direct measurement or estimating it from other available data such as velocity, and elastic properties. The UCS measurement can be time-consuming, expensive and impossible, especially for rocks that has well developed foliation and those that are highly fractured. The challenging aspect of determining the UCS is in the sample preparation, as the measurement requires sample with a length-to-diameter ratio (L/D) of 2.5–3 and their ends has to be parallel within ± 0.02 mm in accordance to ISRM (1983) standards. The Brazilian test, which is the most common indirect method used to measure the tensile strength, requires a circular disk with a thickness-to-diameter ratio (t/D) between 0.2 and 0.75 (ASTM D3967 2008). The preparation of circular disks is much easier to achieve compared to the sample dimensions required for UCS. In view of the fact that highly fractured rocks are unsuitable and high-quality rock core sample required for the UCS test are unavailable (Karaman et al. 2015; Ribeiro et al. 2016), empirical equations with a strong correlation will be more practical and efficient to estimate the UCS (Nazir et al. 2013). There are various studies in the literature proposing relationships between UCS and tensile strength, UCS and velocities, and UCS and elastic properties on different rock types but no such relationships exist for argillites to the authors’ knowledge (e.g. Arslan et al. 2008; Çobanoğlu and Çelik * O. O. Blake oshaine.blake@sta.uwi.edu

Understanding the failure behaviour of rocks is very important for many engineering applications. While conceptual models to predict failure behaviour of rocks have been proposed, very few models exist to quantitatively predict the brittle–ductile characteristics of mudstones at high confining pressures. In this study, we performed triaxial compression tests at confining pressures up to 130 MPa on dry indurated mudstones from the Naparima Hill Formation, Trinidad. Experimental results including stress–strain curves, failure modes, and strength parameters of the mudstones were obtained. Analysing the stress–strain curves, shows that the mudstones experienced brittle, brittle–ductile transition, and ductile failure behaviours as the confining pressure increases. The failure behaviour at confining pressures less than 50 MPa is accompanied by axial splitting fractures and shear fractures. At confining pressures greater than 50 MPa, shear band and ductile flow fractures are observed in the mudstones. A failure behaviour model was developed using the failure behaviour and failure strength of the mudstones. This model predicts the brittle, brittle–transition and ductile zones for mudstones at high confining pressures. The brittle to ductile transition is not sudden, but a gradual process that is controlled by the confining pressure and failure strength. The model indicates a narrow brittle-ductile transition zone for weak mudstones as compared to strong mudstones.

The late Cretaceous Naparima Hill Formation in the South Trinidad Basin (southeast Caribbean) is a prolific source rock with a potential for unconventional hydrocarbon exploration. It is exposed across the Central Range and extends to the city of San Fernando. This study characterized the Naparima Hill source rock using petrographic analysis techniques (X-ray diffraction and thin section analysis) and petrophysical techniques. These techniques were applied to outcrop samples that were collected from 11 locations. Firstly, petrographic analysis was used to determine the mineral composition and lithofacies. Secondly, plugs were taken perpendicular to bedding and the bulk density was determined in dry conditions. Thirdly, porosity and grain density were determined using a porosimeter at ambient conditions. Finally, permeability measurements were obtained at 10 MPa to 130 MPa effective pressures using the transient pulse decay technique. The results show that the source rock is dominated by silica with carbonate as secondary minerals. Four lithofacies were identified: (a) siliceous-calcareous mudstone (b) calcareous mudstones interbedded with black chert, (c) carbonate-rich mudstone with nodular chert and (d) siliceous mudstone. Ranges of 5.9 to 30.8%, 2362 to 2668 kg/m(3) and 1804 to 2350 kg/m(3) were determined for porosity, grain density and bulk density, respectively. The permeability ranges from 3.96 x 10(-20) to 5.94 x 10(-18) m(2) and shows very little dependence on the effective pressure. Permeability and porosity are moderately correlated (R-2 = 0.6). There is no relationship between permeability and mineralogical composition. The observed microfractures, diagenetic processes and grain sorting, are factors that have affected the permeability-porosity relationship to varying degrees. However, none of these factors can be considered as the main intrinsic factor controlling the permeability of the samples. Overall, it is difficult to predict the permeability of the Naparima Hill Formation with any of the proxies (density, mineralogy and porosity) determined in this study.

Static elastic properties, derived from stress–strain data, and dynamic elastic properties, derived from P- and S-wave velocities, are significantly different for rocks. Most rocks are deformed nearly statically (due to tectonic forces and reservoir compaction during production of the reservoir) but static measurements are not as readily available as dynamic measurements. Hence, empirical relationships between the static and dynamic elastic properties are needed to convert the dynamic elastic properties to static values. In this study, the static and dynamic Young’s moduli and Poisson’s ratio were measured simultaneously for dry and fluid-saturated mudstone samples. The samples were axially loaded only within the elastic region to determine the static elasticity. The samples were from four different lithofacies within the Naparima Hill Formation, Trinidad, West Indies. Experiments were carried out at effective pressures up to 130 MPa to determine if the relationship, if any, is influenced by effective pressure. The results show that the dynamic Young’s modulus is greater than the static Young’s moduli. Saturation of the samples causes a decrease in the Young’s modulus and an increase in Poisson’s ratio. Saturation also increases the difference between the static and dynamic Young’s moduli and Poisson’s ratio. A linear relationship with high correlation (R2 greater than 0.9) was established between the static and dynamic Young’s moduli. The gradient of the linear relationship increases, while the intercept decreases, with increasing effective pressure and axial loading. No clear trend was observed between the static and dynamic Poisson’s ratio.

ABSTRACT Icehouse continental-shelf-margin accretion is typically driven by high-sediment-supply deltas and repeated glacio-eustatic, climate-driven sea-level changes on a ca. 100 ky time scale. The paleo–Orinoco margin is no exception to this, as the paleo–Orinoco River Delta with its high sediment load prograded across Venezuela, then into the Southern and Columbus basins of Trinidad since the late Miocene, depositing a continental-margin sedimentary prism that is > 12 km thick, 200 km wide, and 500 km along dip. The Cruse Formation (> 800 m thick; 3 My duration) records the first arrival of the paleo–Orinoco Delta into the Trinidad area. It then accreted eastwards, outwards onto the Atlantic margin, by shallow to deepwater clinoform increments since the late Miocene and is capped by a major, thick flooding interval (the Lower Forest Clay). Previous research has provided an understanding of the paleo–Orinoco Delta depositional system at seismic and outcrop scales, but a clinoform framework detailing proximal to distal reaches through the main fairway of the Southern Basin has never been built. We integrate data from 58 wells and outcrop observations to present a 3-D illustration of 15 mapped Cruse clinoforms, in order to understand the changing character of the first Orinoco clastic wedge on Trinidad. The clinoforms have an undecompacted average height of 550 m, estimated continental slope of 2.5° tapering to 1°, and a distance from shelf edge to near-base of slope of > 10 km. The clinoform framework shows trajectory changes from strong shelf-margin progradation (C10–C13) to aggradation (C14–C20) and to renewed progradation (C21–24). Cruse margin progradational phases illustrate oblique clinothem geometries that lack well-developed topsets but contain up to 70 m (200 ft) thick, deepwater slope channels. This suggests a high supply of sediment during periods of repeated icehouse rise and fall of eustatic sea level, with fall outpacing subsidence rates at times, and delivery of sand to the deepwater region of the embryonic Columbus channel region. Also, evidence of wholesale shelf-edge collapse and canyon features seen in outcrop strongly suggest that deepwater conduits for sediment dispersal and bypass surfaces for Cruse basin-floor fans do exist. The change to a topset aggradational pattern with a rising shelf trajectory may be linked to increased subsidence associated with eastward migration of the Caribbean plate. The Cruse-margin topsets were dominated by mixed fluvial–wave delta lobes that were effective in delivery of sands to the basin floor. The preservation of a fluvial regime of the delta may have been impacted by basin geometry which partly sheltered the area from the open Atlantic wave energy at the shelf edge. Ultimately, understanding shelf-edge migration style as well as process-regime changes during cross-shelf transits of the delta will help to predict the location of bypassed sands and their delivery to deepwater areas.