ABSTRACT Mount Diablo Coalfield was the largest producer of coal in California from the 1860s to 1906. The now-depleted coalfield is located on the northeast limb of the Mount Diablo anticline. The mineable coal seams occur in the Middle Eocene Domengine Formation, which is predominantly composed of quartz-rich sandstone with several thin coal seams. As many as 26 mine operations were established to mine the coal, and it has been estimated that the total production exceeded 4 million tons. The coal fueled the industrial growth of the major cities of northern California. The mines closed at the turn of the nineteenth century as competition from better coals from Washington Territory and overseas entered the market. After coal mining was abandoned, sand operations were established in the early and mid-twentieth century to mine the silica-rich sandstone. The extraction methods used for sand were underground room-and-pillar mining and surface open-pit mining. The high-quality sand was used widely in the production of pottery and glass, and in foundries. Previous studies have interpreted the environment of deposition of these quartz-rich sandstone and coal deposits as barrier island with tidal channels or delta, tidal shelf, and marsh complexes along a north-south–trending shoreline. However, the excellent exposures in the sand mines display abundant evidence for their deposition in a fluvial/estuarine system. Their regional distribution indicates that they were deposited in a northeast-southwest–trending incised-valley system formed by fluvial incision during a lowstand. The incised valley was filled with fluvial and estuarine deposits made up of quartz-rich sand brought in by streams that flowed westward from the Sierra Nevada.

ABSTRACT The mid-Cenozoic succession in the northeast limb of the Mount Diablo anticline records the evolution of plate interactions at the leading edge of the North America plate. Subduction of the Kula plate and later Farallon plate beneath the North America plate created a marine forearc basin that existed from late Mesozoic to mid-Cenozoic times. In the early Cenozoic, extension on north-south faults formed a graben depocenter on the west side of the basin. Deposition of the Markley Formation of middle to late? Eocene age took place in the late stages of the marine forearc basin. In the Oligocene, the marine forearc basin changed to a primarily nonmarine basin, and the depocenter of the basin shifted eastward of the Midland fault to a south-central location for the remainder of the Cenozoic. The causes of these changes may have included slowing in the rate of subduction, resulting in slowing subsidence, and they might also have been related to the initiation of transform motion far to the south. Two unconformities in the mid-Cenozoic succession record the changing events on the plate boundary. The first hiatus is between the Markley Formation and the overlying Kirker Formation of Oligocene age. The succession above the unconformity records the widespread appearance of nonmarine rocks and the first abundant appearance of silicic volcanic detritus due to slab rollback, which reversed the northeastward migration of the volcanic arc to a more proximal location. A second regional unconformity separates the Kirker/Valley Springs formations from the overlying Cierbo/Mehrten formations of late Miocene age. This late Miocene unconformity may reflect readjustment of stresses in the North America plate that occurred when subduction was replaced by transform motion at the plate boundary. The Cierbo and Neroly formations above the unconformity contain abundant andesitic detritus due to proto-Cascade volcanism. In the late Cenozoic, the northward-migrating triple junction produced volcanic eruptive centers in the Coast Ranges. Tephra from these local sources produced time markers in the late Cenozoic succession.

Abstract Techniques for the study of outcrop geology have gone through a paradigm shift in the last decade. Traditional techniques such as analogue photography, logging and manual mapping using pens and pencils have been complemented and partly replaced by digital techniques. These digital techniques include LIDAR scanning, photorealistic mapping, remote sensing techniques and digital mapping tools. Recording methods have switched from paper to handheld computer tablets, GPS and digital data. The digital techniques allow for much higher precision and efficiency in acquisition of field geological data. The techniques complement but does not replace traditional scientific insight and empirical data when interpreting the outcrops. The combination of digital outcrop data with behind-the-outcrop borehole and geophysical data provides an unprecedented ability to move outcrop examples from analogues towards homologues. The ability to compare outcrop data, where much larger extents of geological systems can be viewed, with subsurface data where little primary data such as cores are available and geophysical data which may have unsatisfactory resolution, has revitalized outcrop geology in the last decade. Therefore, presently, and in the foreseeable future, outcrops will increase in their importance, both in terms of their independent value as the primary data source and training ground for geologists, but also for invaluable comparisons with subsurface data.

Abstract The ultimate goal of collecting outcrop data is to be able to understand the underlying physical controls on the formation of the rock record, and these techniques provide an essential step in the right direction towards this goal. In particular, development of new techniques for acquiring data from outcrops has led to an ongoing evolution in how outcrop data can be analyzed and utilized in the petroleum industry. Understanding which technique to use depends strongly on the problem being addressed, as the choice of techniques provides different cost-benefit outcomes to different problems. The key to adopting these techniques successfully is to understand their limitations and where the successes have been in applying them. This brief review defines the problems being addressed in collecting outcrop data, examines the role of the “digital” geologist and the collection, application, and deployment of three-dimensional data, and the future of outcrop studies. Reference is made to case studies described in other contributions in this volume.

Abstract An outcrop’s geometry will always have an effect on how data from that outcrop are interpreted; therefore those data should be used only when the implications of that geometry have been taken fully into account. Problems inevitably arise whenever the geologist attempts to do this, the greatest of which stems from the uncertainty that necessarily exists in the position and orientation of the outcrop relative to the underlying rock body. This problem is best handled by means of a hypothesis-based approach for outcrop interpretation, for instance by using one of the three techniques outlined in this paper. The first of these techniques—the use of geometric probability arguments—is illustrated here in the context of recognizing clinoforms in outcrop. It is shown that clinoforms will rarely be seen as sigmoid-shaped curves on randomly positioned and randomly oriented outcrop faces; they are more likely to be seen as simple convex-up or concave-up curves. The second technique—the use of Monte Carlo methods—is illustrated in the context of interpreting dog-legged outcrops in flat-lying sedimentary successions. It is shown that the probability of failing to recognize simple features on the face of dog-legged outcrops can be high, and that this failure probability is highest for relatively long and sinuous outcrops with relatively many segments. This result conflicts totally with conventional geological wisdom. The third technique—the use of standard statistical tests—is illustrated by showing how isolated outcrops can be used to test correlation hypotheses in areas of broken exposure. The paper warns of the danger of conflating rock body and outcrop, then finally offers guidance on hypothesis selection. Key words: stratigraphy, correlation, model, hypothesis, Monte Carlo

Abstract Digital survey methods, including terrestrial laser scanning (LIDAR) and differential GPS, allow geological and topographic data from outcrops to be recorded very rapidly, in 3D, at detailed resolutions and with high spatial precision. Geological interpretations of outcrop datasets (e.g., fault or bedding traces) can be extended into the subsurface using geometric, probabilistic, or deterministic methods. Geometric methods based on interpolation and extrapolation of observed surfaces and surface traces are generally associated with high uncertainty. This can be reduced in areas of highly irregular topography. Another approach is to use geological heuristics to constrain the subsurface interpretation. This approach can help to limit the number of possible interpretations when creating multiple realizations. Deterministic methods encompass both invasive and non-invasive approaches. Invasive methods include mining and quarrying, as well as small-scale excavation of unconsolidated sediments. Behind-the-outcrop boreholes are only slightly invasive, and can provide very useful constraint of the subsurface. In contrast, geophysical methods such as near-surface seismics and ground penetrating radar (GPR) allow indirect imaging of the subsurface and are non-invasive. Excellent coastal exposures of Namurian turbidites near the Bridge of Ross in County Clare, western Ireland, provide a case study in which several different types of digital outcrop data are combined and co-visualized in a 3D model. In vertical sections on opposite sides of the outcrop a small-scale turbidite channel is marked by an erosional base and inclined interbedded sandstones and mud-clast conglomerates. The observed channel margins can be traced through the subsurface using 3D GPR.

Abstract The Nukhul Formation (Suez rift) consists of fluvial and tidally influenced shallow marine strata that were deposited in fault-controlled seaways and tidal embayments during rift initiation. In this study, we create a half-graben-scale, high-resolution (typical grid cell dimensions 20 m x 20 m x <1 m), geocellular outcrop model of the Nukhul Formation. The evolution of the normal fault system in the study area is associated with the development of fault-parallel and fault-perpendicular folds. The changing nature of the structural template, and the resulting geomorphology, during deposition led to complex syn-rift stratigraphic architecture and facies distributions. We use a LIDAR-based digital outcrop approach to map this geological complexity to a high degree of accuracy, for export to reservoir modelling software. Software developed in-house was used to integrate field observations with the digital dataset, aid interpretation, and create realistic surface meshes from outcrop data. Facies modelling used a combination of sequential indicator simulation and object-based modelling approaches. Sedimentary logs were attached to the dataset and used as conditioning data. 2D probability maps, source points, and flow lines constrained the geocellular outcrop model to match the known geology. The approach leads to improvements in three areas: (i) geological knowledge of the study area, (ii) data portability, and (iii) geocellular outcrop modelling. Comparison between the final geocellular outcrop model, outcrop geology, and inferred palaeogeography shows that the geology of the Nukhul Formation is realistically modelled. The final reservoir model can be used as an analogue for similar geological settings. It can be applied to improve the prediction of subsurface geology in analogous reservoirs and to increase the accuracy of static connectivity and flow simulations. Ultimately this will improve knowledge of the impact of facies heterogeneities on reservoir performance and lead to increased efficiency of reservoir drainage.

Abstract Resolving the geological details hidden beneath the resolution of seismic reflection data has been a continuing challenge for decades. Forward seismic modeling of outcrop analogues provides an important scale link between the architectural geometries observed in outcrops and in seismic. Such models can potentially bridge the critical gap in resolution between datasets and provide new and improved insight to the interpretation of petroleum targets. The use of outcrop analogues can produce realistic models where petrophysical properties, lithology distribution, and reservoir architecture are all defined. The synthetic seismic model is compared with subsurface seismic data and adjusted to find the earth model that gives a seismic response that matches the real seismic amplitudes. A four-step workflow on how to build seismic models of outcrops is suggested and detailed in this paper. Information regarding how to construct high-resolution, complex, geologically realistic architectural elements such as mass-transport complexes is also described. Analysis of outcrop synthetic seismic has enabled us to study and understand the anticipated seismic character and imaging of depositional systems at different scales. Seismic models reveal the expected seismic architecture and character of similar geobodies and basin-fill sequences. They also increase the ability to predict hydrocarbon-bearing rocks in unexplored basins. Seismic interpretations of subsurface targets in both Angola and in the Gulf of Mexico have been quality controlled using this technique. This research contributes to better constraints on lithology predictions, fluid response, and geometry distribution in these areas.

Abstract Because of their fast acoustic velocity, their ability to create steep slopes, and their important postdepositional diagenetic modification, carbonate rocks are notoriously more difficult to image and interpret using seismic than siliciclastic rocks. This paper shows how building a 3-D synthetic seismogram based on well-constrained outcrop-based 3-D geocellular models can help in seismic interpretation and seismic-based reservoir characterization. Workflow to populate a 3-D geological model with velocity is presented that is based on building statistical distribution of velocity per facies or lithostratigraphic units or diagenetic features and extrapolating velocity through the model using stochastic Gaussian simulation. A 3-D model built from Lower Permian deep-water carbonate gravity flows is used to demonstrate the complexity of interpreting intricate 3-D geometries using 2-D planar seismic slices and to assess volumetric error associated with the intrinsic resolution loss of seismic. Modern karst morphology is used to assess the seismic response of caves, sinkholes, or karst topography in seismic. Finally, a Permian dolomitized ramp-crest grainstone complex is used to test the sensitivity of prestack techniques to pore-type changes in grainy carbonate rocks. These few examples illustrate the strength of building a well-calibrated 3-D synthetic seismogram based on a 3-D geocellular model so that some of the complexity of seismic response of carbonate rocks might be unraveled.

Abstract During the last decade, the laser scanning of outcrop analogues for the construction of 3D geologic models has become commonplace. Many elements of both laser-scan data collection and photorealistic model creation have dedicated software and are semi-automated or fully automated. However, much of the subsequent geological interpretation process relies heavily on manual digitization. Consequently, only a small fraction of the inherent 3D and 2D information captured within photorealistic models is normally extracted and analyzed. This situation is perhaps most acute in the study of fractured reservoir analogues where hundreds of thousands or even millions of fractures can be captured in multi-kilometer-scale datasets, making full manual digitization of the dataset impractical. In order to maximize and standardize the extraction of quantitative data from digital outcrop models, we sought to develop new technologies specifically for the automated extraction of lithologic information, bedding, and fracture discontinuities from photorealistic data. This is achieved through the adaption of algorithms originally developed for automated fault interpretation from 3D seismic data. Projection of the 2D bedding and fracture discontinuities extracted from digital outcrop images onto 3D outcrop surface, using transform matrices calculated during the construction of a photorealistic model, permits calculation of the strike, dip, and a confidence value for every extracted feature. The high-resolution image-derived discontinuity data is then integrated with larger-scale 3D discontinuities extracted from topologic analysis of laser-scan-derived data. This combined approach maximizes the potential information from both (1) high-resolution 2D digital image data and (2) the 3D laser scan topologic data in order to extract and combine different scales of discontinuities into a single 3D dataset. Independent quality control of the automated bedding and fracture extraction methods is achieved through the third-party collection, analysis, and comparison of structural data collected in the test area. Comparison between automatically extracted and field-mapped and measured fractures demonstrates the accuracy of the methods developed and some limitations. The automated techniques developed are designed not to replace outcrop studies but to augment them. They limit the tedious and time-consuming manual interpretation tasks by providing unbiased, pre-generated geological primitives that can be edited and analyzed with a set of structural-analysis tools. These data and tools allow time to be better spent on the analysis of large numbers of quantitative data and their incorporation into 3D models. The main limitations on quality of the automatic mapping appear to be related to the coverage and quality of the original dataset.

Abstract Prediction of fractures in carbonate reservoirs represents a very significant challenge. We describe the use of a digital outcrop analogue from faulted and jointed Lower Jurassic rocks from Somerset, U.K., that provides exceptional exposure of fractured carbonates. The aims were to gather high-resolution and exact information about the fracture systems and to understand the mechanics of the fracture development. A 2.5 km section of coastline was digitally captured and built into a high-resolution photorealistic model. Faults were hand interpreted in an immersive virtual reality environment. A line sample of the faults in the photorealistic model compares well with a similar line sample taken in the field. The photorealistic data also include large bedding-plane exposures of joint systems. The joints were extracted semi-automatically using a combination of image curvature and ant tracking; ground-truthing of the resulting joint map confirms the validity of the interpretation. By using this semi-automatic technique it is possible to digitize far more joints than would be possible for a human interpreter. The detailed fracture data provide a rich source of data for modelling of fracture systems. However, in order to be predictive in the subsurface, it is not sufficient to have a purely statistical fracture description and so we turn to mechanical modelling. On the assumption that the joint system formed in the perturbed stress system around pre-existing faults, we performed boundary element modelling and were able to match to the joint system in the photorealistic model using an extensional stress regime and fluid-pressure perturbations along the fault plane.

Abstract Channel stacking pattern in deepwater reservoirs has a significant impact on reservoir producibility. These stacking patterns are the result of feedbacks between turbidity currents and associated supply variations and the evolution and aggradation of slope physiography. Turbidite flow events that leave significant physiographic relief due to underfilled within-channel deposition or local erosion tend to have greater influence on subsequent flow events, resulting in organized channel stacking patterns. Turbidite flow events that fill their channels do not confine subsequent flows and result in disorganized channel stacking patterns. High rates of system aggradation result in a greater degree of inter-element disconnection, low rates result in amalgamated elements. Event-based models are amenable to the integration of expert rules related to the influence of channel fill (fraction of active channel fill) and aggradation rate on channel stacking pattern. The event-based approach is part of a new subset of geostatistical modeling that focuses on greater integration of stratigraphic concepts and sedimentological process. In the event-based method stochastic models are constructed as a sequence of depositional events. The sedimentological process and allogenic forcing are approximated as a set of empirical and predictive rules. The resulting numerical laboratory efficiently constructs high-resolution reservoir models and allows calibration of model response to changes in input values. Event-based models are constructed based in part on outcrop examples with various channel stacking patterns. The resulting models are used to explore the relationship between stacking pattern, preservation potential of axis, off-axis and margin facies assemblages and reservoir producibility. In addition, these models can be used to illustrate hierarchical relationships, explore larger issues related to the value of architectural information and to aid in the discovery of new rules by induction combining outcrop observations and models results.

Abstract Studies of turbidite channel complexes in outcrops, wells, and 3D seismic-reflection data suggest a general model of turbidite channel behavior related to three critical measures: (1) the thickness of channel elements; (2)the thickness of abandonment facies within each element; and (3) the thickness of overbank aggradation. These measures constrain channel stacking pattern and can be integrated into event-based geostatistical reservoir models that provide probabilistic predictions of net reservoir volume and element stacking pattern. Although channel and overbank thicknesses are measured routinely, this model provides a predictive framework that also emphasizes the importance of recognizing the presence and thickness of shale-rich abandonment facies at the top of sand-rich channel elements in outcrops. For a given flow composition, the deposits of thick channel elements (thickness of active fill plus abandonment facies) tend to have relatively low sand percentage, abundant bypass facies, and thick abandonment facies (underfilled channels). Underfilled channel elements with high topographic relief, from levee crest to channel thalweg, at the time of abandonment influence the location of subsequent elements, resulting in an organized channel stacking pattern. The deposits of relatively thin channel elements tend to have higher sand percentage, small volumes of bypass facies and thin/absent abandonment facies. Filled channel elements with low topographic relief at the time of abandonment had little influence on the location of subsequent elements, which resulted in a disorganized channel stacking pattern. Channel ‘‘relief ’’ corresponds to the depth of erosion plus the height of the levee crest above the initial sea floor. We observe that erosion relief can correlate strongly with downslope gradient. Flow composition also is critical because the rate of overbank aggradation is strongly influenced by mud volume. Muddy flows tend to produce thick overbank aggradation, high confinement, and under-filled channels with an organized stacking pattern. Sand-rich flows tend to produce relatively low overbank aggradation, low confinement (unless erosion relief is high), and filled channels with a disorganized stacking pattern.

Abstract Fault-related dolomite subsurface reservoirs are formed from fluid circulation that results in significant transformation of the reservoir properties. The geometry and internal organization of such dolomitic reservoirs remain difficult to image with seismics alone. A multi-scale approach is essential to understand and predict the diagenetic processes that control the exact 3D morphology of the dolomite with spatial precision and true dimensions, and consequently the reservoir properties. In this context, we propose an analytical workflow including field work, LIDAR scanning and numerical geology applied to dolomite outcrops in Mesozoic carbonates (SE France). The exposed dolomite-limestone contact exhibits sinuous, irregular and convolute shapes, which are either fault-parallel, bedding-parallel or chaotic. To characterize this complex distribution, we performed LIDAR scanning on 500 m x 150 m cliffs and road cuts with 4.5 cm to 1–1.5 cm average point spacing. The cloud is composed of 22 millions points comprising X, Y, Z, intensity, red, green, and blue attributes. Digitization of the limestone-dolomite boundary was performed in RiscanPro and GOCAD environments, for extracting the true 3D geometry of the dolomite body for further geostatistical and 3D facies modelling. This approach captures the large-scale geometry of the dolomite bodies. However, single RGB or intensity properties do not unequivocally reproduce small-scale (below ∼ 1 m) heterogeneities of the late diagenetic dolomite. Color changes induced by weathering or climatic conditions are of the same size range as the small-scale heterogeneities, thus they are not unique to allow automated tracking on the point set. As a result, the workflow remains time-consuming, and further work is needed to allow calibration of the LIDAR data points with mineralogy.

Abstract Carbonate platforms can have complex internal facies variations and stratal geometries expressed at length scales longer than all but the largest outcrops. The latter commonly form high and relatively inaccessible cliffs, and thus conventional field techniques (logging and photomontages) may not adequately capture the 3D geometry of surfaces and the details of the facies distribution. Because facies and stratal geometry control rock properties and connectivity in carbonate reservoirs, accurate outcrop data can be critical to reservoir and forward seismic modeling. The Gresse-en-Vercors cliffs (southeastern France) provide a seismic-scale slice though a Lower Cretaceous (Barremian) platform margin analogous to Lower Cretaceous reservoirs in the Middle East. The cliffs are 500 m high and extend for 25 km along depositional dip, straddling the transition from shallow-water platform to deeper basin. This paper describes the methodology developed to create a high-resolution stratigraphical digital outcrop model (DOM) integrating field measurements (logged sections, facies mapping) and high-resolution digital data (photomosaic and new LIDAR data acquired by a helicopter survey). Integration of the LIDAR and other point cloud data provide a high-resolution digital elevation model (DEM) on which georeferenced field observations were then posted. The “solid image” technique was used to extract precise x,y,z coordinates of stratigraphic surfaces from the DEM. The resulting numerical geological model allows a coherent restoration of the platform architecture, quantification of component surfaces (shape, angles, dimensions) and geobodies, and a better characterization of the relationship between facies and platform architecture

Abstract Terrestrial LIDAR data were acquired at four sites in the Acacus Formation in southwestern Libya in order to better understand the sedimentary architecture and to create outcrop models to aid ongoing subsurface exploration. The outcrop models comprise up to 8 individual scans merged to produce panels up to 1.2 km wide and c. 200 m high. The panels highlight the gross stratigraphic architecture including shallowing- and coarsening-upwards cycles, major channel bodies, and subtle prograding units. The scan resolution was chosen to resolve the bed-scale stacking that typifies hydrocarbon reservoirs in this formation. Surfaces and facies observed in the outcrops and constrained by the LIDAR data were then used to construct 2-D synthetic seismic panels of the outcrop. These models were parameterized with densities and velocities from subsurface examples and used to gain insight into the seismic resolution of depositional architecture.

Abstract Cross-well or borehole ground-penetrating radar (GPR) tomography is commonly used to map subsurface aquifer heterogeneity; however, the influence of sedimentary architecture and anisotropy on GPR signal transmission is largely unknown. To address uncertainty in GPR tomography interpretation, we developed a method to combine GPR and LIDAR surveying to characterize lateral heterogeneity behind an outcrop of braided-stream deposits. Methods included using black paint to mark a regular grid of GPR shot points on opposing outcrop faces for the transmitter and receiver intervals. A 3D model of the outcrop was then constructed using LIDAR scans to determine distances between shot points. GPR tomography was conducted through the outcrop, and velocity calculations from these data were used for tomographic inversions to determine heterogeneities within the outcrop. We were able to successfully combine GPR shot-points with LIDAR data to calculate distances between transmitter and receiver points, and we were able to see first arrivals through the outcrop. Complications, however, were evident at both short offsets (∼ 3–5 m), when the air wave overprinted the first arrival, and longer offsets (> 8 m) when the signal dissipated before reaching the receiver. Air-time arrivals that were observed compared favorably to model predictions based on LIDAR-derived outcrop geometry, supporting the utility of LIDAR for constructing a digital, 3D outcrop. Furthermore, estimated values of radar velocity for this outcrop of 0.09 to 0.12 m/ns reasonably match known velocities for similar sediment. Had the GPR data acquisition been more successful at this outcrop site, we could have used detailed 3D facies architecture derived from the LIDAR data to assess the influence of sediment heterogeneity and anisotropy on GPR tomographic signals.

Abstract Accurate representation of heterogeneity at varying scales is vital for modeling solute dispersion in groundwater aquifers and petroleum reservoirs. Dispersion, the result of varying velocities in a flow field, is, in part, due to material heterogeneity. In order to represent the influence of heterogeneity at the outcrop scale, a series of terrestrial LIDAR scans at millimeter-scale point spacing were recorded in sediments located in braided stream exposures west of Albuquerque, New Mexico, and outside the Hanford Site in Washington. Scans are projected onto a vertical plane and converted to a high-resolution TIFF image. Using the mean and standard deviation of the ‘‘stacked’’ images, the data are processed through a series of filters to enhance textural information and distinguish between lithologies. The product is converted to a grid with numerical color values for each lithology (e.g., sandstone, gravel). Each lithologic class is assigned reasonable values of hydraulic conductivity. Groundwater flow and transport time are simulated using MODFLOW and MODPATH, respectively. Simulations show that flow and solute transport are focused in the coarser-grained laminae of cross-bedded units. Flow may be focused into some areas in finer-grained beds as well, if the adjacent gravel bed has been cut. Thus, most of the flow may be focused into a smaller volume of the material making up the aquifer. This result shows that terrestrial LIDAR can be successfully applied to produce synthetic stratigraphy for use in fluid flow models.