A reinterpretation of the only core (taken in 1988) through the reservoir sandstone in the Chestnut oilfield changed the interpretation of the reservoir from a base-of-slope turbidite system downdip from a known slope channel to a sand injectite reservoir. Using the base-of-slope turbidite model, four unsuccessful appraisal wells were drilled, and it was concluded that the reservoir was more complex than anticipated, or that the sedimentary model was incorrect. A new examination of the core identified numerous well-preserved characteristics associated with sand injectite reservoirs, including internal structures and hydrofractures. No firm evidence of depositional structures was found. The new interpretation led to changes in the field-wide reservoir model that transformed the Chestnut Field from an uneconomic discovery into a producing oilfield that exceeded expectations. Examples of the injectite facies, including evidence of erosion during sand injection, are provided, together with evidence of hydrofractures along lithological boundaries and in mudstone intervals that allow association between hydrofractures and sand injection.

Reinterpretation of the only core (taken in 1988) through the reservoir sandstone in the Chestnut oilfield changed interpretation of the reservoir from a base-of-slope turbidite system down-dip from a known slope-channel to a sand injectite reservoir. Using the base-of-slope turbidite model, four unsuccessful appraisal wells were drilled, and it was concluded that the reservoir was more complex than anticipated, or that the sedimentary model was incorrect. A new examination of the core identified numerous well-preserved characteristics associated with sand injectite reservoirs, including internal structures and hydrofractures. No firm evidence of depositional structures was found. The new interpretation led to changes in the field-wide reservoir model that transformed Chestnut from an uneconomic discovery into a producing oilfield that exceeded expectations. Examples of the injectite facies, including evidence of erosion during sand injection are provided together with evidence of hydrofractures along lithological boundaries and in mudstone intervals allowing association between hydrofractures and sand injection.

Natural surface gas seeps provide a significant input of greenhouse gas emissions into the Earth's atmosphere and hydrosphere. The gas flux is controlled by the properties of underlying fluid-escape conduits, which are present within sedimentary basins globally. These conduits permit pressure-driven fluid flow, hydraulically connecting deeper strata with the Earth's surface; however they can only be fully resolved at sub-seismic scale. Here, a novel 'minus cement and matrix permeability' method using three-dimensional X-ray micro-computed tomography imaging enables the improved petrophysical linkage of outcrop and sub-surface data. The methodology is applied to the largest known outcrop of an inactive fluid-escape system, the Panoche Giant Intrusion Complex in Central California, where samples were collected along transects of the 600 to 800 m stratigraphic depth range to constrain porosity and permeability spatial heterogeneity. The presence of silica cement and clay matrix within the intergranular pores of sand intrusions are the primary control of porosity (17 to 27%) and permeability (<= 1 to ca 500 mD) spatial heterogeneity within the outcrop analogue system. Following the digital removal of clay matrix and silica (opal-CT and quartz) cement derived from the mudstone host strata, the sand intrusions have porosity-permeability ranges of ca 30 to 40% and 10(3) to 10(4) mD. These calculations are closely comparable to active sub-surface systems in sedimentary basins. Field observations revealed that, at decreasing depth, the connected sand intrusion network reduces in thickness and becomes carbonate cemented, terminating at carbonate mounds formed from methane escape at the seafloor. A new conceptual model integrates the pore-scale calculations and field-scale observations to highlight the key processes that control sand intrusion permeability, spatially and temporally. The study demonstrates the control of matrix and cement addition on the physical properties of fluid-escape conduits, which has significance for hydrocarbon reservoir characterization and modelling, as well as subsurface CO2 and energy storage containment assessment.

Giant sand injection complexes form, intricate, basin-scale fluid plumbing systems and document the rem obilisation and intrusion of several tens of cubic kilometres of sand within the shallow crust in stratigraphic units 100's metres thick. This is the first detailed and extensive account of the Panoche Giant Injection Complex (PGIG), aregionally significant outcrop (>300 km(2)) and part of a larger subsurface development (>4000 km(2)) identified in boreholes and on seismic reflection data. Magnificent exposure of the PGIC occurs along the north western margin of the San Joaquin Valley and presents the opportunity to examine the regional geological significance of a giant sand injection complex and its origin in the context of a late Cretaceous - early Paleocene forearc basin. Between 25 and 49 km(3) of sand were remobilised and injected, at least 0.35 km(3) of which extruded onto the paleo-seafloor. large sandstone intrusions often >10 m thick and laterally extensive on a kilometer scale formed saucer-shaped intrusions, wing-like intrusions and a variety of sill geometries along with volumetrically smaller randomly oriented dikes in a 200-300 m thick interval. Dikes prevail below and above this interval, some reaching the paleo seafloor and extruding sand. Networks of propagating hydrofractures form intensely brecciated host strata, some of which were intruded by sand. All intrusions formed in a single pulsed event in which the most intense hydrofracturing caused by supra-lithostatic fluid pressure occurred approximately 600 to 800 m below the paleo seafloor. A crudely orthogonal arrangement of dikes is preserved with most oriented normal, and less commonly oriented parallel to the oceanic trench associated with the late Mesozoic to early Tertiary North Pacific subduction. Dikes orthogonal to the trench opened against the minimum horizontal stress, which was parallel to the trench. Dikes parallel to the trench opened against the regional maximum horizontal stress along minor faults formed in extension caused by shallow crustal deformation. There is no evidence that compressional tectonics influenced the onset of elevated pore fluid pressure necessary to promote sand injection. However, tectonic compression was responsible for creating the basin physiography that locally increased subsidence and accelerated chemical diagenesis in the basin centre. PGIC outcrop, located along the basin margins, was unlikely to have experienced heating above 70 degrees C, equivalent about 2 km burial, so the effects of chemical diagenesis in the host strata of the injection complex had negligible potential to evolve significant pore water volume. In a deeper part of the basin approximately 150 km to the south, lateral equivalents of the host strata were subjected to heating >100 degrees C and would expel significant volumes of water displaced by quartz cementation and clay dehydration that caused lateral pressure transfer to the north and western margin of the basin where the PGIC formed. Estimates of the total volume of water expelled from the deep basin suggest that a fluid volume equivalent to a gross rock volume reduction <1% would have provided a fluid budget sufficient to fluidise and inject the sand that forms the PGIC. In terms of areal and vertical extent, volume and architecture the PGIC shares strong similarity with the regionally developed giant injectite systems of Tertiary age in the North Sea basin. In both cases regional sand injection is genetically linked to pressure transfer toward the basin margin from more rapidly subsiding basin centres. Aqueous fluid is derived from thermally driven chemical diagenesis of thick deep water clastic sandstone and smectitic mudstone or from deeper, stratigraphically older, aquifers.

Observation of basin-scale networks of sandstone intrusions are described from subsurface studies and outcrop locations. Regional scale studies are prevalent in the volume and two new regionally significant subsurface sand injection complexes are described. Higher resolution studies, both outcrop and subsurface, show the small-scale complexity but high level of connectedness of sandstone intrusions. Discordance with bedding at all scales is diagnostic of sandstone intrusions. The propensity of hydraulic fractures to develop and fill with fluidized sand in a broad range of host rocks is demonstrated by examples from metamorphic and magmatic basement, and lignite. Terminology used to describe sandstone intrusions and other elements of sand injection complexes is diverse.

The principle aim of this paper is to document well-preserved field examples of sandstone-filled faults in order to raise awareness of these poorly understood structures, and discuss their potential as fault seals within injection-prone, multilayered siliciclastic reservoirs. To achieve this goal, we have undertaken a detailed field survey in the Panoche and Tumey hills in Central California, which allowed us to recognize numerous faults filled with injected sand. In particular, sandstone-filled extensional, contractional and strikeslip faults are observed cutting the sandstone/mudstone successions. Sandstone-filled faults commonly display small offsets and apertures ranging from a few centimetres to some decimetres. Evidence of tectonic deformation is usually lacking, meaning that sand injection supported by overpressured fluids propped open the fault walls. In this paper we also describe the main mechanism leading to the emplacement of sand along a fault plane, and propose a predictive model of sandstone-filled fault distributions in different structural environments. Finally, we discuss the role of sandstone-filled faults, that although relatively small and not adding significant volume to the reservoirs, can markedly increase fluid transmissibility and thereby promote better reservoir connectivity.

Abstract The Tumey Giant Injection Complex (TGIC) is a regionally developed sandstone intrusion complex emplaced into the deep-water Kreyenhagen Shale (Eocene) in the San Joaquin Basin, Central California. Detailed geological mapping, stratigraphic reconstruction and outcrop description, supported by structural analysis, allowed the architectural characterization of the TGIC. The complex is described as two main stratigraphically constrained intervals: (1) a lower interval (250 m thick) emplaced into clay-rich mudrock, consisting dominantly of sills with stepped and multilayered geometry; and (2) an upper interval (200 m thick) characterized by injection breccia and large wing-like intrusions (c. 600 m width × 100 m high) emplaced within predominantly biosiliceous mudrock strata. The intrusions in both intervals were derived from turbiditic channel fills intensely modified by sand fluidization. Sandstone intrusions and fractures affecting host strata are dominantly oriented sub-parallel to the basin axis striking between NW–SE and N–S, mainly dipping to NE and forming asymmetric saucer-shaped intrusions, suggesting structurally driven hydraulic fracturing and sand emplacement. The absence of a deep aquifer and potential sand sources underlying the complex suggests a lateral contribution of fluid flow. The TGIC occurs at a scale similar to injection complexes recognized in the subsurface and is a valuable reservoir analogue for hydrocarbon accumulations associated with sand injectites.

Mudstone samples from the Moreno (Upper Cretaceous-Paleocene) and Kreyenhagen (Eocene) formations are analysed using X-ray diffraction (XRD) and X-ray fluorescence (XRF) to determine their mineralogy. Smectite (Reichweite R0) is the predominant phyllosilicate present, 48% to 71.7% bulk rock mineralogy (excluding carbonate cemented and highly bio siliceous samples) and 70% to 98% of the <2 μm clay fraction. Opal CT and less so cristobalite concentrations cause the main deviations from smectite dominance. Opal A is common only in the Upper Kreyenhagen. In the <2 μm fraction, the Moreno Fm is significantly more smectite-rich than the Kreyenhagen Fm. Smectite in the Moreno Fm was derived from the alteration of volcaniclastic debris from contemporaneous rhyolitic-dacitic magmatic arc volcanism. No tuff is preserved. Smectite in the Kreyenhagen Fm was derived from intense sub-tropical weathering of granitoid-dioritic terrane during the hypothermal period in the early to mid-Eocene; the derivation from local volcanism is unlikely. All samples had chemical indices of alteration (CIA) indicative of intense weathering of source terrane. Ferriferous enrichment and the occurrence of locally common kaolinite are contributory evidence for the intensity of weathering. Low concentration (max. 7.5%) of clinoptilolite in the Lower Kreyenhagen is possibly indicative of more open marine conditions than in the Upper Kreyenhagen. There is no evidence of volumetrically significant silicate diagenesis. The main diagenetic mineralisation is restricted to low-temperature silica phase transitions.

The presence of sand injections has been shown to enhance the likelihood of hydrocarbon traps within siliciclastic successions. Through the development of large interconnected networks of sills and dykes, sand injection complexes provide a volume of porous and permeable rocks within the low permeability host units. Overall, the formation of sand injection complexes requires extensive fracturing and hydrofracturing, which can be particularly pronounced when sand injections are coupled with brittle tectonic deformation. In some circumstances, this process may threaten the integrity of the reservoir top seal thereby preventing further hydrocarbon accumulation. Studying exceptional exposures along the coastal area of Santa Cruz in California, we report evidence for top seal failure associated with injection episodes. Two distinct sand injection episodes are proposed. The first event, datable to the Late Miocene, resulted in large volumes of sand being emplaced within the top-seal units, and was followed by accumulation of hydrocarbons within the newly injected sandstones. Later, a series of brittle tectonic events, associated with the San Andreas/San Gregorio Fault System, caused remobilization and accumulation of sand along newly formed fault planes. Our case study documents this combination of pervasive brittle deformation and sandstone injection along fault structures, which can ultimately disrupt the integrity of a host unit leading top seal failure and leakage of hydrocarbons.

Sandstone intrusions form large bedding-discordant sandstones that intruded into finer grained, less permeable host strata. They form naturally sand-propped hydraulic fractures that constitute a connected network of permeable conduits through which fluids escape to the Earth's surface. Saucer-shaped sandstone intrusions are among the largest volume intrusions and are commonly resolved on seismic data. Outcrop analogues of seismically-resolved saucers-shaped intrusions reveal that many attendant intrusions, in particular dikes, are undetected in seismic data. Seismic forward modeling of a detailed outcrop description of a saucer-shaped intrusion demonstrates that intrusions steeper than 45° are undetected and that up to 40% of the entire volume of sandstone intrusions is not seismically imaged. Wedge geometry – associated with discordant contacts between different lithologies – causes constructive and destructive amplitude interference, creating imaging artefacts of sandstone thickness and geometry. Comparison of the outcrop seismic models with 3D seismic data from Volund oilfield demonstrate both the similarity of the saucer-shaped intrusions and the distribution and quantity of dikes that may be undetected (ca. 78%) using subsurface data. Lack of detection of dikes has direct implications on the valuation of upward migration of fluids and an overestimation of seal capacity. This therefore has major implications when using seismic data to evaluate waste sequestration or to execute hydrocarbon or groundwater exploration and production.

Despite the potential of sandstone-filled normal faults to significantly influence fluid transmissivity within reservoirs and the shallow crust, they have to date been largely overlooked. Fluidized sand, forcefully intruded along normal fault zones, markedly enhances the transmissivity of faults and, in general, the connectivity between otherwise unconnected reservoirs. Here, we provide a detailed outcrop description and interpretation of sandstone-filled normal faults from different stratigraphic units in central California. Such faults commonly show limited fault throw, cm to dm wide apertures, poorly-developed fault zones and full or partial sand infill. Based on these features and inferences regarding their origin, we propose a general classification that defines two main types of sandstone-filled normal faults. Type 1 form as a consequence of the hydraulic failure of the host strata above a poorly-consolidated sandstone following a significant, rapid increase of pore fluid over-pressure. Type 2 sandstone-filled normal faults form as a result of regional tectonic deformation. These structures may play a significant role in the connectivity of siliciclastic reservoirs, and may therefore be crucial not just for investigation of basin evolution but also in hydrocarbon exploration.

Petrographical microtextural analysis and conventional core-plug analysis of samples from a depositional sandstone and a sandstone sill are used to compare with similar data derived from micro-CT (MCT) analysis of the same samples. A remarkable richness of information derived from the MCT data identifies isotropic granular and pore fabrics at the micrometre-scale in the sandstone sill that contrast markedly with the laminated fabric in the depositional sandstone. In the sandstone sill, porosity and permeability are more homogenous than in the depositional sandstone, in which lamination creates approaching two-orders of magnitude difference in permeability and enhances horizontal permeability relative to vertical permeability. In the sill, lower pore- and throat-shape factors and larger pore coordinate numbers are present than in the depositional sandstone. Preservation of the isotropic pore and grain structure in the sandstone sill is indicative of significant fluidized flow normal to the fracture margin during emplacement.

Sandstone injections are created by the forceful emplacement of remobilized sand in response to increases in overpressure. However, the contribution provided by horizontal compressive stress to the build-up in overpressure, and the resulting emplacement of sand injection complexes, is still to be substantiated by robust field observations. An opportunity to address this issue occurs in Central California where a large volume of sandstone intrusions record regionally-persistent supra-lithostatic pore-pressure. Detailed fieldwork allows sandstone-filled thrusts to be recognized and, for the first time, permits us to demonstrate that some sandstone intrusions are linked to contractional deformation affecting the western border of the Great Valley Basin. Fluidized sand was extensively injected along thrust surfaces, and also fills local dilatant cavities linked to thrusting. The main aims of this paper are to provide detailed descriptions of the newly recognized syn-tectonic injections, and describe detailed cross-cutting relationships with earlier sandstone injection complexes in the study area. Finally, an evolutionary model consisting of three phases of sand injection is provided. In this model, sand injection is linked to contractional tectonic episodes affecting the western side of the Great Valley Basin during the Early-Middle Cenozoic. This study demonstrates that sand injections, driven by fluid overpressure, may inject along thrusts and folds and thereby overcome stresses associated with regional contractional deformation. It is shown that different generations of sand injection can develop in the same area under the control of different stress regimes, linked to the evolving mountain chain.

Seismic modelling is necessary to understand elasticwave propagation in the subsurface. Modelling is costeffective and insightful, as long as adequate methods are used. An ideal seismic-modelling strategy is to generate complete synthetic seismograms for realistic earth models, then process them as performed with real seismic data. These complete seismograms are best obtained by full-wavefield (FW) approaches. FW methods are therefore used in extensive benchmarking studies, which may require the joint effort of several institutions owing to high resource costs. Though computer power continues to increase, we are still a long way from applying ideal modelling to all cases where synthetic data is necessary to constrain results. This is especially true in interpretation and sensitivity analysis, where the influence of multiple parameters must be assessed. In many situations, ray-based (RB) methods are suitable alternatives, especially when rays, traveltimes, etc., are useful information for the problem at hand (Gjøystdal et al., 2007). However, standard RB methods do not allow modelling of detailed target structures owing to smoothness requirements. Geoscientists needing seismic modelling of such targets will then often resort to 1D convolution (Lecomte et al., 2015). 1D convolution has been successfully used for decades, and is still the method behind most standard well calibration, seismic inversion, etc. However, its conceptual validity is in reality very limited.

The Metaponto Coastal Plain (MCP), in southern Italy, stretches 60 km-long and 5 km-wide along the Gulf of Taranto in the Ionian Sea, and is presently subject to strong anthropogenic pressure. A multidisciplinary study reviewed the geomorphology, lithostratigraphy and sedimentology of the MCP and its subsurface. Incorporating both borehole and radiocarbon-dating information in the review, this paper focuses on comparisons and differences between present-day and buried Late Pleistocene landscapes (LGM and MIS 3).The modern coastal plain is the top of a late Holocene coastal wedge prograding on a very narrow-shelf, that is connected to a deep basin (the Ionian Sea) by a steep slope. This scenery likely resembles those produced during earlier late Quaternary relative highstands and is in marked contrast with that produced during the last sea-level fall and lowstand, and buried in the MCP subsurface. The last scenery corresponds to the LGM landscape, where river-valleys deeply dissected a previous highstand coastal wedge (MIS 3) whose remnants represented interfluve areas. Thanks to resonance properties of the subsurface, this buried landscape was obtained in a 3D visualization, highlighting location and shape of incised valleys and interfluve areas during the LGM. (C) 2013 Elsevier B.V. All rights reserved.

A non-invasive and low-cost geophysical method for the H/V spectral ratio (HVSR) of microtremors was employed for the first time in the Metaponto coastal plain (Basilicata region, southern Italy) in order to draw 3D unconformities within the subsurface. Through the stratigraphical analysis of several boreholes, the occurrence of two irregular erosional surfaces, bounding three main sedimentary units, was inferred. The upper unit fills and covers some paleovalleys that were incised during the Last Glacial Maximum (LGM). Filling was induced by a sea level rise and a high stand that followed the LGM. According to the stratigraphy of some boreholes, a 4-layer model of the Metaponto coastal plain subsurface was used in the geophysical investigation. The inversion of the HVSR data has been performed using the velocities of the shear waves calculated by some down-hole tests, and the main geophysical unconformity was recorded below the uppermost unit, corresponding to the topmost two layers of the 4-layer model. A 3D view of this main geophysical unconformity shows a surface with the occurrence of some deeper, narrow, and sinuous zones running roughly perpendicular to the present-day coastline and at depths of up to 90 m below the present-day sea level. These narrows likely correspond to the paleovalleys that developed in the region during the LGM and are buried below the Metaponto coastal plain. The satisfactory fit obtained by the comparison of geophysical sections with geological ones highlights the reliability of the HVSR method for reconstructing the geometry of buried paleomorphologies characterized by an appreciable contrast of seismic impedance between "bedrock" and "cover".