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The Mesnard Quartzite of Paleoproterozoic age is believed to represent among the first preserved cratonal continental rocks in the Lake Superior region, was deposited above Archean basement in the Marquette structural trough and is estimated to contain more than 99% sandstone (now quartzite) beds. The Mesnard Quartzite is interpreted to have been deposited largely by tidal currents. Evidence of tidal current deposition includes lack of wave-influenced structures such as hummocky cross-beds, and presence of strongly unidirectional paleocurrents from cross-beds, mud-drapes on cross-bed foresets, evidence of current reversal including herringbone cross-bedding and sigmoidal cross-bedding. Sandstone bedding types include homogeneous or structureless beds, parallel-laminated beds, cross-beds, and symmetric ripple marks. Unidirectional paleocurrents from cross-beds are largely subparallel to the axis of the Marquette structural trough, leading to the interpretation that the trough was subsiding during deposition. Cross-bed set thickness and symmetric ripple marks both support the interpretation of shallow water marine deposition, likely only a few meters. Although the Mesnard Quartzite is on the order of 110 m thick, there is no evidence of internal sequence boundaries or flooding surfaces, and low accommodation during deposition is inferred. The climate during deposition is interpreted to be humid, and tropical to subtropical based on the quartz arenite composition. Deposition likely occurred in a failed rift basin or in an intracratonic extensional basin.

Two end-member approaches to 3-D reservoir characterization modeling exist. The “Frankenstein” approach integrates hard and soft conditioning data while incorporating geologic concepts and interpretations, to construct a highly detailed deterministic model. The Frankenstein approach focuses on what is known about the reservoir, and is so named because construction of the model can be time-consuming, which can negatively impact project timelines, sometimes “killing” its creator. The “Gilligan” approach uses available data to build suites of models that are simple, and strives to address what is not known about the reservoir. Gilligan models are simplified, at least in the view of the Frankenstein modeler, and focus on quantitative characterizations of uncertainty. Reservoirs usually become more complex with new data from wells and production, a concept referred to here as “the Law of Increasing Reservoir Complexification”. The Law of Increasing Reservoir Complexification should influence earth modeling workflow choices. Currently, the Gilligan probabilistic modeling approach is more commonly employed in early phases of projects. The appearance of overly complex Frankenstein models when probabilistic models are needed remains an ongoing problem. Mature reservoirs typically possess uncertainties that could influence business decisions, and may warrant a Gilligan probabilistic approach as well. No single recipe exists for choosing the best 3-D modeling approach: a fit for purpose or Goldilocks modeling approach should depend on business needs and requirements. Recognition that uncertainty is a fundamental and prevalent driver of business value for all reservoirs in all phases of development demands widespread adoption of probabilistic or Gilligan approaches.

Unconventional opal-A and opal-CT diatomaceous reservoirs in the Monterey Formation of the San Joaquin Valley in California have produced for more than 30 yr. These opal-A reservoirs are characterized by high porosity (>50%) and low permeability ( 45% [Cymric and McKittrick fields]) or are very thick (>1000 ft thick [Lost Hills field]). These insights provide general guidance about characterization of unconventional diatomaceous reservoirs and the identification of factors that most dramatically influence performance.

The upper Miocene Kern River Formation at Kern River field is a thick succession of braided stream deposits. Because the oil field has been penetrated by more than 21,000 wells, with well spacing typically of 2.5 ac (104 m2), high-quality imaging of the reservoir can be accomplished using well logs. The field contains more than 650 observation wells recording temperatures and saturation, such that heating and drainage of the reservoir can be characterized over time. Oil and gas saturations in a 360-ac (1.5 × 106-m2) study area within the Kern River field were modeled in three dimensions over time, creating a four-dimensional (4-D) earth model. Static lithology and porosity properties were first created. Then, temperature, oil, and gas saturations were kriged using data from observation wells at different time increments. From these kriged models, difference models were created to show changes in oil and gas saturation with time. Changes in oil volumes in the 4-D model matched produced volumes within a 10% error. Next, categorical descriptions of net oil saturation were made (best oil, background oil, partially drained, and residual saturations), and temporal evolution of the volumes of the different categories was modeled. By characterizing the reservoir in detail using the well-log data to define a reservoir framework and the observation wells to characterize changes over time, significant challenges in steam-flood reservoir management can be addressed, including determining which parts of the reservoir are producing the most and recognizing parts of the reservoir that are not actively draining.

A continuous oil accumulation is one that is pervasive throughout a large area and is not affected by natural hydrodynamic influences. Three source rocks in the San Joaquin Valley of California are actively producing hydrocarbons and represent potential continuous oil accumulations: the Monterey, Kreyenhagen, and Moreno Formations. The Energy Information Administration announced in 2014 that there are potentially 15 billion bbl of recoverable oil in the Monterey Formation in California, spiking huge interest. Such a resource would make the Monterey by far the largest continuous oil accumulation in North America. This number has since been reduced dramatically to 600 million bbl of oil for the state and 21 million bbl of oil within the San Joaquin Valley. In this study, the subsurface character of the Bakken Formation of North Dakota is compared with the three source rocks in the San Joaquin Valley at oil window depths. To characterize these reservoirs, hundreds of well logs, core descriptions, and mud logs were studied. A technique to rank character of oil show data was developed in which interesting, possibly interesting, or not interesting wells were located on thermal maturity maps. Interesting wells have significant oil shows, whereas not interesting wells have minor or no shows. In the Bakken Formation, the character of the oil show correlates with well productivity. Applying this same classification to the San Joaquin Valley source rocks leads to a more disappointing conclusion. Although there are oil shows in the source rocks of the San Joaquin Valley at oil window depths suggesting the presence of a continuous oil accumulation, the distribution of shows is both laterally and vertically heterogeneous and not predictable. Moreover, recent attempts to produce from source rocks at these depths have not been economically successful. We conclude that the three source rocks in the San Joaquin Valley represent heterogeneous and discontinuous oil accumulations at oil window depths in the subsurface. Likely there are billions of barrels of oil in these discontinuous oil accumulations. Source rocks in the Bakken are rated as world class; source rocks in the San Joaquin Valley are good to excellent quality. However, the quality of the oil shows in the San Joaquin Valley appears poorer than the Bakken Formation. It is possible that effective drainage between the source rocks and the up-dip reservoirs has left large volumes of the source rocks at oil window depths with only residual oil saturation. Complex structural and stratigraphic architecture, heterogeneity, and continuity create issues of predictability for optimal areas to target. Rapid rates of subsidence over the past few million years and accompanying thrusting and folding resulted in a complex subsurface pressure regime. The lack of clear hydraulic fracture targets, analogous to the middle Bakken, further complicates drilling decisions and likely deliverability. In addition, the oil windows in the San Joaquin Valley are significantly deeper than the Bakken Formation, which would result in substantially higher well cost. Recent drilling results support this study and suggest that heterogeneous and discontinuous oil accumulations in the San Joaquin are unlikely to become economic without dramatic changes in technology.

The Lombardi reservoir at San Ardo oil field in California is extremely sand rich (>90%) and appears to be a relatively simple reservoir several hundred feet thick. The purpose of this study is to characterize stratigraphic complexities in the Lombardi reservoir that may affect later production and interpret how changes in stratigraphic architecture are tied to local syndepositional tectonism. Two parasequence sets are present in the Lombardi: the reservoir exhibits clinoform geometries along the western edge of the field and stacking patterns within the reservoir change across the field. Based on stacking pattern, core, and correlation studies, two possible sequence boundaries are recognized in the Lombardi sand. A lower sequence boundary is proposed approximately in the middle of the Lombardi sand, and an upper sequence boundary is proposed at the top of the Lombardi sand. In the southern part of the field, the Lombardi sand consists of two progradational parasequence sets, separated by the mid-Lombardi sequence boundary. The upper sequence boundary clearly truncates underlying folded Lombardi sand. Just a few miles (∼5 km) away, in the northwest part of the field, the Lombardi sand consists of a lower progradational parasequence set and an upper retrogradational parasequence set, separated by the mid-Lombardi sequence boundary. The upper sequence boundary at the top of the Lombardi sand shows little evidence of truncation of underlying units and is recognized by onlapping of younger strata. The difference in reservoir architecture between the south and northwest part of the field, a distance of only a few miles (∼5 km), is interpreted to be related to syndepositional uplift accompanying tectonism in the south. In core, the Lombardi sand consists mostly of homogeneous sand and bioturbated sand, arranged in upward-coarsening successions. The Lombardi sand is interpreted to be the product of deltaic deposition based on (1) the presence of homogeneous and laminated sandstones interpreted to represent mass flow deposits and (2) its restricted geographic distribution within the Salinas basin. No evidence of deposition in a wave-dominated or tidal-dominated shoreline is indicated. The interpretation presented here is that the sediments were deposited on the front of a delta emptying into a wave-protected estuary or bay, with minimal tidal effects. Complexities in stratigraphic architecture along the western edge of the field do not currently affect production but will likely become important during later stages of field development. That is, in the developed part of the field, reservoir continuity is not an issue, because of high sand content and sheet-like geometries. However, at the margins of the field, which will be produced in the future, clinoform geometries and sandstone tongues clearly affect reservoir continuity and will create vertical compartments during draining.

Creating earth models for deep-water appraisal and development studies is perhaps the most challenging task confronting the petroleum geologist today. Data are limited (few wells, limited core, untested seismic quality), time is limited and drilling, testing and facilities costs are very high. Uncertainty in geological characterization of the reservoir can have the greatest potential impact on project value. How can a thorough characterization of reservoir uncertainty be made based on limited data and in a timely fashion? A workflow for creation of suites of models for appraisal and development studies of deep-water reservoirs is described. The goal of the workflow is to rapidly construct suites of earth models based on limited data that capture the full range of uncertainty in reservoir characteristics and properties. After characterizing possible distributions for individual parameters, suites of earth models are built in a single step using an experimental design framework, aided by a powerful workflow manager which automates earth model construction. Earth models created using the experimental design framework are seamlessly linked to flow simulation software. Plackett–Burman, folded Plackett–Burman and Full Factorial experimental designs were used in different appraisal and development cases. Multiple experimental designs were produced by adding and modifying uncertainty parameters as additional data arrived, and ideas about the possible character of the reservoir evolved. In the appraisal case described here, 6 experimental designs were made, 388 earth models were created and studied, and 79 of those models were dynamically simulated. The process of quickly building and re-building suites of earth models using experimental designs to address changing perceptions and concerns about uncertainty in reservoir character is termed here ‘procycling’. Procycling is complementary to experimental design studies, in that multiple experimental designs are employed over time: procycling focuses on changes in predictions made by individual experimental design studies. The results of procycling are not necessarily to change the perception of uncertainty (for example the range of possible outcomes), but to anchor what the limits of uncertainty are and what the most important uncertainties are with the given data.

To address uncertainty, models created for appraisal and development studies try to capture the full range of possible rock and fluid properties, and reservoir and structural architecture. Using experimental designs, each parameter can be varied to indicate how uncertainty for the parameter affects the outcome (for example, recovery after 30 years). In numerous models in appraisal and development studies, stratigraphic or reservoir architecture is not recognized as a significant uncertainty. This is a concern for two reasons. First, reservoir architecture is known to be an important parameter influencing oil production. It is puzzling that experimental design studies indicate it is not a significant uncertainty. Second, building geologically realistic earth models can involve months of time and can impact timelines for development projects. If reservoir architecture is in fact a non-significant uncertainty, perhaps detailed modelling could be postponed. The current study addresses in what situations reservoir architecture is important in appraisal and development studies. Reservoir architecture characteristics considered are net to gross, channel orientation, channel stacking pattern, channel sinuosity, channel width/thickness, type of architectural element, influence of shale units within channel fills, influence of channel drapes at the base of channels, presence of other fades, influence of valley fills, and sheet geometries. Two field characterizations were studied using the same workflow, and results are compared with results of conceptual modelling studies. For studies of recovery uncertainty, it is shown here that models that look to be very different may have similar dynamic performances because of similar reservoir connectivities and path lengths. Moreover, it is shown that the elements of reservoir architecture investigated (channel width/thickness ratio, sinuosity, orientation, thickness, element type) do not control recovery efficiency to a large degree unless reservoir connectivity is impacted. Reservoir architecture may not be an important uncertainty in appraisal and development studies unless reservoir connectivity or tortuosity are affected. Although reservoir architecture may not represent a key uncertainty in appraisal and development studies, it may well be a significant uncertainty for well planning in mature fields.

Connectivity represents one of the fundamental properties of a reservoir that directly affects recovery. If a portion of the reservoir is not connected to a well, it cannot be drained. Geobody or sandbody connectivity is defined as the percentage of the reservoir that is connected, and reservoir connectivity is defined as the percentage of the reservoir that is connected to wells. Previous studies have mostly considered mathematical, physical and engineering aspects of connectivity. In the current study, the stratigraphy of connectivity is characterized using simple, 3D geostatistical models. Based on these modelling studies, stratigraphic connectivity is good, usually greater than 90%, if the net: gross ratio, or sand fraction, is greater than about 30%. At net: gross values less than 30%, there is a rapid diminishment of connectivity as a function of net: gross. This behaviour between net: gross and connectivity defines a characteristic ‘S-curve’, in which the connectivity is high for net: gross values above 30%, then diminishes rapidly and approaches 0. Well configuration factors that can influence reservoir connectivity are well density, well orientation (vertical or horizontal; horizontal parallel to channels or perpendicular) and length of completion zones. Reservoir connectivity as a function of net: gross can be improved by several factors: presence of overbank sandy facies, deposition of channels in a channel belt, deposition of channels with high width/thickness ratios, and deposition of channels during variable floodplain aggradation rates. Connectivity can be reduced substantially in two-dimensional reservoirs, in map view or in cross-section, by volume support effects and by stratigraphic heterogeneities. It is well known that in two dimensions, the cascade zone for the ‘S-curve’ of net: gross plotted against connectivity occurs at about 60% net: gross. Generalizing this knowledge, any time that a reservoir can be regarded as ‘two-dimensional’, connectivity should follow the 2D ‘S-curve’. For channelized reservoirs in map view, this occurs with straight, parallel channels. This 2D effect can also occur in layered reservoirs, where thin channelized sheets are separated vertically by sealing mudstone horizons. Evidence of transitional 2D to 3D behaviour is presented in this study. As the gross rock volume of a reservoir is reduced (for example, by fault compartmentalization) relative to the size of the depositional element (for example, the channel body), there are fewer potential connecting pathways. Lack of support volume creates additional uncertainty in connectivity and may substantially reduce connectivity. Connectivity can also be reduced by continuous mudstone drapes along the base of channel surfaces, by mudstone beds that are continuous within channel deposits, or muddy inclined heterolithic stratification. Finally, connectivity can be reduced by ‘compensational’ stacking of channel deposits, in which channels avoid amalgamating with other channel deposits. Other factors have been studied to address impact on connectivity, including modelling program type, presence of shale-filled channels and nested hierarchical modelling. Most of the stratigraphic factors that affect reservoir connectivity can be addressed by careful geological studies of available core, well log and seismic data. Remaining uncertainty can be addressed by constructing 3D geological models.

There is a subtle controversy in the petroleum industry regarding the relationship between reservoir architecture and recovery efficiency from petroleum reservoirs. Stratigraphers tend to believe that facies architecture and geometric shapes strongly govern recovery, whereas engineers stress factors such as permeability heterogeneity and anisotropy. To understand the relationship between architecture and recovery better, a suite of conceptual models of different channelized clastic reservoir architectures was constructed and pore-volume replacement waterflood simulations were performed on each. Three 11-member suites of models were constructed at net: gross values of 35%, 60% and 85%. The reservoir architecture features that were varied were channel width to thickness, thickness, sinuosity, stacking patterns, orientation and reservoir element type (point bar vs. channel). The three sets of models appear visually very different from a reservoir architecture standpoint. Permeability and porosity were simulated using geostatistical techniques but share the same conditioning population. The same water saturation, well count (110 acre spacing), fluid properties and relative permeability were used in each waterflood simulation. In this situation, in which the only significant variable was reservoir architecture, only a few percent spread in recovery was noted for each net: gross suite. Variation in recovery efficiency can be shown to be associated with reservoir connectivity, which is generally high for all models but shows some variation. Additional studies were made to address the influence of well count, mobility ratio, permeability heterogeneity and geostatistical seed number on recovery efficiency. Two stratigraphic factors are shown to influence recovery efficiency: reservoir connectivity and permeability heterogeneity. Both factors influence the volumetric sweep efficiency of reservoirs. Models that appear to be visually different, but have similar connectivity, well count and permeability heterogeneity, have similar sweep efficiencies and, therefore, recovery efficiencies. The practical importance of this observation for development studies is that building and flow-simulating suites of models with different reservoir architectures may not help to characterize recovery uncertainty unless the models are constructed to emphasize differences in sweep efficiency.

Summary Fifty reservoir models were built by using seven different modeling methods and varying a combination of reservoir characterization parameters using experimental design. These 50 models were all subjected to unconstrained fluid-flow simulation with an active aquifer and limited peripheral waterflooding without history matching. The results show that oil recovery from existing wells at 95% water cut ranges from 29 to 34% in these 50 models despite the large apparent visual differences between them. However, there are large differences (some greater than 200%) in water-breakthrough time, water cut, and cumulative water production. The differences in oil recoveries and water cut among all models were found to be largely related to horizontal permeability heterogeneities perpendicular to the shoreline direction. Experimental design indicated that there were statistically significant differences in flow-simulation results associated with the definition of reservoir quality trends, 3D facies trends, and variogram range and direction. Do more complex, geologically realistic models provide the best frameworks for predicting flow performance in sandy shoreface reservoirs with good well control? Some of the simplest models we constructed provided flow-simulation results similar to those of the most complex and soft-data-conditioned models. What we do observe, however, is that the more soft conditioning data and the more complex geologic models tend to give a restricted range in full-field flow performance.

It is well established that uncertainty exists in simulated recovery forecasts due to the ambiguity in the measurement and representation of the reservoir and geologic parameters. This is especially true for immature projects, such as deep-water reservoirs, where the high cost of data limits the information that is available to build reservoir models. We present two strategies, based on Experimental Design, to quantitatively assess this uncertainty in recovery predictions for primary and waterflood processes. We apply the Experimental Design methodology to channelized sandstone systems because of their relevance to many deep-water projects. We choose to study synthetic geological analogs of channelized systems that are built from panoply of relevant parameters while taking into account the uncertainty that exists in the estimation of their ranges. We use the results of this study to generate type curves with neural networks. The trained neural networks can be used to rapidly predict reservoir performance where field data is very limited. We discuss applications of this methodology on field cases from western Africa.

The degree to which the depositional environment influences recovery from a reservoir is a topic of some importance, but one that has not been thoroughly addressed. One of the most widely known and cited examples relating stratigraphy to recovery efficiency is the study of Tyler and others (1984; Tyler, 1988, updated in 1991 by Tyler and Finley). In this study, 450 major Texas petroleum reservoirs were evaluated (Galloway et al., 1983). They showed that average recovery efficiency could be closely tied to depositional environment and drive mechanism (Figure 1). Reservoirs composed of fluvial rocks with water drive, or solution gas drive, or gas cap expansion may have recovery efficiencies of 35–50%, whereas barrier strand-plain reservoirs with gas-cap expansion may be characterized by recovery efficiencies of 50–70%. However, because they plot recovery efficiency against drive mechanism and depositional system, it is still unclear which of these factors has more important influence on recovery efficiency....

An irony of contemporary petroleum exploration and development is that, although technological advances have been significant, risk has not been reduced in all cases. In fact, risk may actually be greater in some environments—given costs associated with deepwater platform design and subsalt reservoirs. With risk remaining significant or perhaps even growing, the definition and understanding of uncertainty is a major challenge.

Three-dimensional geologic characterizations of reservoirs are used in waterflood simulation studies to predict oil and gas field recovery efficiencies, quantities of water, oil and gas production over time, and for well optimization and economic studies. Quantitative global characterizations of the static reservoir, for example, reservoir connectivity or permeability heterogeneity, can be used to help explain waterflood performance, but with a wide range in result quality. In the current study, aspects of dynamic simulation of waterfloods were replaced by static approximations using a quantitative localized approach, in which information about distances and properties within the reservoir is used to help predict waterflood performance. A “ladder” of waterflood flow simulators was considered, ranging from simulators using only static or rock property information, to simulators using static and dynamic information, to state-of-the-art dynamic simulators. Predictions based on each simulator type were compared with predictions using a state-of-the-art simulator. The simplest waterflood simulator uses a static model with only permeability and porosity information. These approximations to fluid flow can be improved by including dynamic considerations, such as an approximation of water saturation behind the waterfront (obtained by solving the Buckley-Leverett transport equation) or an approximation to the velocity of the advancing waterfront (obtained by solving the Darcy equation for a single pressure step). In the cases considered here, comparing swept volume or recovery factor using our simplest static flow simulators with results using a state-of-the-art simulator yielded R2 values in the range of 0.5 to 0.65. By better characterizing geometric aspects of the reservoir, R2 values of 0.65 to 0.75 were attained. By adding a single pressure step, R2 values of 0.8 to 0.85 were calculated. Ranking, or perhaps binning, can be achieved using simulations based on earth models with limited dynamic information. Ultralarge models containing a billion cells can be simulated with little difficulty using this approach.

Organic material in specimens from surface and downhole sites and samples of oil and gas from Barbadian wells were studied for source potential and maturation history. The study involved: (1) visual examination of the kerogen; (2) measurement of total organic carbon content; (3) determination of vitrinite reflectance; (4) Rock-Eval pyrolysis assay; (5) geochemical measurement of pooled and extracted bitumens including carbon isotopic ratios (δ 13 C); and (6) gas chromatographic analysis of saturated gaseous hydrocarbons. Two types of kerogen occur in Barbados: terrestrial, presumably of South American origin, and marine planktonic. The two types are mixed in most samples. The terrestrial kerogens are dominant and occur in all wells in the basal complex and most of its outcrops. The planktonic organic matter occurs in exposed melange, hemipelagic rocks and rarely in terrigenous turbidites. The well specimens do not show significant maturation within the drilled depths, suggesting that pooled hydrocarbons originated from deeper levels of the prism and accumulated as a result of migration. Faults bounding the intraprism thrust sheets probably provided migratory paths for hydrocarbons and water, juxtaposed rocks of variable maturity, assisted in alteration and biodegradation, and led to the formation of structural traps. The composition of gases indicates that maturation conditions for gas generation may exist at great depths within the Barbados accretionary prism.

A popular misconception of island arcs is that they consist mostly of intrusive rocks belonging to the granite family, and volcanic rocks. Study of exhumed Caribbean island arcs indicates that they are composed mostly of sedimentary rocks such as epiclastic volcanic materials, and true plutonic and volcanic rocks are relatively minor. A simple model for the stratigraphic development of oceanic island arcs is proposed based on observations in the Caribbean region and intuitive reasoning. The lowermost part of the model arc is oceanic crust (oceanic arc depositional system 1, or OADS I). Built atop of the oceanic crust is the arc sequence, including volcaniclastic strata, lava flows with feeder dikes and plutons. Oceanic arc strata consist of a lower pyroclastic-absent volcanic unit (OADS II) formed by submarine volcanism, and an upper unit (OADS III) bearing volcanic, pyroclastic and epiclastic rocks (referred to here as the pyroclastic-present association or unit). Both OADS II and III may contain intercalated pelagic and/or biogenic sediments such as limestones and cherts. Submarine volcanic rocks include pillowed and sheet lava flows, lithic and hyaloclastic breccias and resedimented lithic and hyaloclastic debris. OADS III can be recognized by the presence of shallow-water fossils, stream- or beach-worn volcanic clasts and other epiclastic materials even in deep-water environments where such materials are resedimented. Shallow-water materials and epiclastic strata are never found in OADS II. Thickness of the units vary in accordance with initial water depth of oceanic crust prior to arc formation, amount of thermally induced uplift during early arc rifting, subsidence due to loading, and differential basinal subsidence or rates of volcanic aggradation. Examples of arc sequences from Aruba, Bonaire, Curacao, Grenada, Martinique, the Virgin Islands and Puerto Rico show significant variability. Aruba and Curacao show incomplete sequences of arc growth and may reflect arc development on thick oceanic crust (for example, a plateau). Submarine volcanic arc facies associations (OADS II) occur only in the northern Virgin Islands (Water Island Formation) and Bonaire (Washikemba Formation). Typically, great structural thicknesses (5–10 km) of arc-related strata are preserved, although condensed arc sequences built upon accreted terranes are present locally (southwest Puerto Rico, Grenada). On Grenada, the active magmatic platform is underlain by overthrust forearc(?) basin strata, indicating that magmatic platforms may grow by means other than volcanic aggradation.

Most accretionary complexes are submarine mountain belts consisting predominantly of sedimentary rock. Past studies of accretionary complexes have concluded that they have minimal petroleum potential, although they do contain potential source rocks. Owing to their great bulk of sedimentary rock, large amounts of petroleum must be generated in accretionary prisms, and these hydrocarbons must be trapped in the complexes, or vented into the ocean or on land. To understand the nature and abundance of organic material in accretionary complexes, organic matter in the shales, limestones and melange matrix of the Franciscan and Cedros subduction complexes was studied, mostly using Rock-Eval pyrolysis. The largest amounts of total organic carbon (TOC) were found in the Calera Limestone from the San Francisco area (1–6% TOC). Terrigenous sedimentary rocks of probable trench or trench-slope origin contain from

Sedimentary basins in the NE Caribbean Plate Boundary Zone are associated with the evolution of a Cretaceous to Early Tertiary island arc, and with carbonate banks which formed following cessation of volcanism in the Eocene to Oligocene. Neogene sedimentary basins are typically present where crustal blocks in the plate boundary zone have undergone divergence, with or without rotation. Cretaceous through Oligocene basins may also have formed during divergence between crustal blocks, although this is difficult to prove owing to limited exposure and subsequent tectonic deformation. Other basins are associated with localized overthrusting along the plate boundaries.

Heat Flow values, determined from temperature logs and estimates of thermal conductivity from geophysical logs range from 23 to 37 mW/m2 from 800 to 2500 m depth in the Toa Baja scientific drillhole on the north, central coast of Puerto Rico. Near the target seismic reflector at the base of the well, an active hydrothermal system was encountered in which heat flow of up to 90 mW/m2 was found in a mineralized zone beneath a volcanic sill or flow. The heat flow then dropped to 50 mW/m2 beneath this subhorizontal flow zone. The mining of heat from downdip is proposed to account for this thermal anomaly, as well as the scatter in the heat flow determined from the few other wells drilled into Puerto Rico. The time-temperature history of the well indicates that Eocene volcaniclastics of the lower 2 km were deposited into a geothermal gradient of 60°C/km north of an active arc (heat flow estimated to have been 120–180 mW/m2). Uplift, erosion and cooling occurred between 40 and 30 Ma. Reburial and deposition of Oligocene-Miocene Limestones produced the present-day geothermal gradient of 15 °C/km (heat flow of 30–50 mW/m2). Based upon comparisons with slab cooling models, the crustal thickness beneath Puerto Rico is estimated to be closer to continental than oceanic.

The authors propose that the Caribbean (Ca)-North American (NA) plate boundary zone (pbz) from the Puerto Rico Trench to the Venezuelan Basin from Mona Canyon east has been in left-transtension over the last 15-20 ma. A boundary-normal component of extension occurs throughout the pbz and is a principal cause of the Puerto Rico Trench. Such extension is due to WNW velocity of NA-Ca and the northward pullaway of NA from its S-dipping slab, which is below Puerto Rico. Strike slip motion may be taken up among terranes in the pbz by rigid CCW rotation and by oblique slip at their boundaries. Rotation of the largest terrane, Puerto Rico-Virgin Islands (PRVI), has caused such major structures as the Muertos thrust and Anegada Passage. The model implies NA-Ca velocity estimated from Cayman transforms is more accurate than that from slip vectors from seisms in the NA slab.

Paleogene deformation in south central Puerto Rico of Cretaceous-Eocene volcaniclastic sediments and volcanic rocks is characterized by west-northwest trending sinistral strike-slip faults, northeast directed thrusts, and fault-related mesoscopic folds. A west-northwest trending belt of predominantly Eocene age sediments is flanked by Upper Cretaceous rocks; a 8–15 km wide fault zone is spatially coincident with this lithologic belt. Structural data and deformational style indicate that the area is a sinistrally transpressive, slightly north vergent, asymmetric flower structure, of which the upper portion is presently exposed in the belt of predominantly Eocene sediments. Deformational characteristics, in combination with sedimentary and tectonic evidence, constrain evolutionary scenarios of the fault zone (specifically, age and amount of displacement) and the northeastern Caribbean, as discussed herein.

This volume concerns information learned by drilling the Toa Baja well on the north coast of Puerto Rico, and current studies of Puerto Rican geology and tectonics. The Toa Baja Drillsite is located in the North Coast basin of Puerto Rico about 10 km west of San Juan. The hole was spudded on August 23, 1989, and plugged and abandoned on November 7, 1989 at a total depth of 2,704m. Two lithologies were encountered during drilling: an upper series consisting of Oligocene-Miocene shallow-water limestone and sandstone facies, and a lower series consisting of Eocene deep-water volcaniclastic strata, including some lava flows or shallow intrusions, pelagic marls, and altered igneous rocks or coarse-grained sandstones. Principal findings made during drilling include: (1) the important unconformity separating the upper and lower series at about 579 m; (2) 8 faults defined clearly by dipmeter log; (3) changes in rock type probably associated with reflection events in seismic reflection profiles crossing the drillsite; (4) confirmation of overall low geothermal gradients and heat flow, but presence of a thermal anomaly near 2683 m; (5) documentation of high paleogeothermal gradients using petrographic, isotopic, X-Ray diffraction and electron microprobe studies; (6) presence of fractures indicating a current extensionalmore » tectonic setting. Current studies in the Puerto Rico region include: (1) paleomagnetic evidence for late Miocene counterclockwise rotation; (2) geochemical evolution of Cretaceous and Eocene igneous rocks; (3) evidence of transtension in the northeast Caribbean plate boundary zone; (4) results of studies of ancient fault zones on Puerto Rico; and (5) stratigraphic studies of the Tertiary of Puerto Rico.« less

The stratigraphy of the Eocene rocks of the Toa Baja drillhole is dominated by volcaniclastic sediments which are interbedded with marly pelagic limestones, especially in the lower part of the hole, and lava flows. Petrological, geochemical and paleontological evidence suggest that the site of deposition was a deep marine basin and the source of the volcanics were subaerial or shallow submarine island arc volcanoes.

Reflectors imaged in a multichannel seismic line at Toa Baja, Puerto Rico, were studied by comparing the positions of reflection events with changes in rock properties determined through analysis of logs and cuttings. Reflectors appear to be associated primarily with changes in rock properties across facies or formation boundaries, and more rarely unconformities and faults. Faults that seal versus faults that leak can be recognized on the basis of gas Chromatograph data. Several laterally discontinuous reflectors characterized by modest amplitude anomalies seem to correlate in depth with minor shows of methane gas, measured by onsite gas chromatography. Gas was not recognizeable using geophysical logs indicating small quantities. The gas is apparently associated with rock types and rock properties which affect reflector amplitude rather than directly enhancing reflector amplitude.

The Toa Baja Drilling Project was a broad, interdisciplinary experiment to document the in situ geology and geophysics of a non-volcanic island arc massif. This overview provides a brief summary of oil exploration on Puerto Rico that lead up to the present investigation, and summarizes some of the problems addressed by drilling.

A 55 km geologic cross section through the Toa Baja Drillsite, generated by integrating geologic mapping data from the foothills of the Central Mountains of Puerto Rico with onshore and offshore multichannel seismic reflection data, provides an opportunity to examine in profile from the arc interior northward to within 40 km of the current trench slope break. Three structural divisions are recognized. In the foothills of Puerto Rico, Cretaceous and Eocene rocks are separated by transpressional strike-slip faults. In the vicinity of the Toa Baja drillsite where both seismic reflection and borehole data are available, Eocene rocks, deformed by thrust faults, lie above a lower unit, interpreted to be of Cretaceous age. Offshore, north of the drilling site, seismic reflections suggest Eocene rocks onlap structural basement, thought to be Cretaceous rocks, and both units appear only slightly deformed. All Eocene and Eocene (?) rocks are overlain by little deformed Oligocene to Recent rocks. From south to north, or from the arc massif interior toward the present-day trench, there is an apparent decrease in amount of Late Eocene to Middle Oligocene strike-slip and shortening deformation. Deformation events occurred mostly in the arc-interior and were not directly associated with the plate boundary which was probably near the Puerto Rico Trench.

Left-lateral transcurrent (wrench) faulting through the Southern Puerto Rico Fault Zone (SPRFZ) in central southern Puerto Rico is indicated by fault orientations and motion direction indicators. The SPRFZ is a west-northwest trending, 3-8 km-wide, diffuse system of faults and folds that within the study area bounds Paleocene and Eocene volcaniclastic rocks (the Eocene Belt), and is flanked by Cretaceous volcanic and volcaniclastic rocks. Structural data show that much of central southern Puerto Rico is cut by strike- and oblique-slip faults, whereas locally in the southeastern part of the Eocene Belt, rocks are pervasively thrust northeastward over Cretaceous rocks of central Puerto Rico. Structural data and the distribution of fault types support two local evolutionary scenarios: (1) different structural horizons of the transpressional SPRFZ (i.e., upper, thrust dominated horizon and lower, strike-slip dominated horizon) were juxtaposed along post-transpressional normal faults that border the Ponce Basi...

These days "estimating uncertainty" is the mantra. As we do this, we ask ourselves which is better: an array of geologically simple rapidly history-matched models, or a single geologically comprehensive, carefully history-matched model. After all, uncertainty, which is normally characterized by a range of forecasts from techniques such as Experimental Design, is difficult to quantify using just one model however comprehensive it may be. Yet if forecasts are obtained from a series of simple models, how good are they? Choosing one over the other also has significant implications in the time required for modeling, and also reservoir management. Specific questions, that directly affect the cost of modeling, come to mind. These are: What is the optimal level of geological detail, especially if the uncertainty management plan includes history-matching and simulating a series of models? Can the oil trapped behind the flood front be estimated by a series of simple lateral (shales) barriers/baffles or do we always need an extensive sequence stratigraphy framework? A detailed model may not be as amenable to uncertainty estimation by the virtue of its size. Is field-scale history matching adequate or is well-by-well history matching a must? Perhaps the brute force approach of probabilistic forward modeling provides the panacea. After all the proof-of-the-pudding lies only in the model's ability to accurately predict field performance. Finally, we also ask - should the modeling strategy, i.e., comprehensive vs. simple, be dependent on the response variable of interest, e.g. ultimate recovery factor vs. infill drill locations. As such, if ultimate recovery is the objective, a simple model may suffice. To answer questions like these, we re-visit current reservoir modeling paradigms. As a datum, we use a comprehensively modeled waterflood from Western Africa. This reservoir was modeled using extensive sequence stratigraphic techniques. The model was scaled-up from about 14 million cells to about 280,000 cells using a flow-based scale-up algorithm, carefully preserving all the mappable mudstones above flooding surfaces. History-matching for a 30-year period was systematically conducted with a team of field engineers and simulation specialists. The whole process took about a year to complete. Against this datum, we compare a series of rapidly built geological models that still honor all the data and the overall depositional architecture, and yet are significantly different from the datum geological model by the virtue of the modeling strategies implemented. The different modeling strategies vary in complexity from changes in variogram lengths and direction to simple tank models with stochastic sandstones and mudstones conditioned by well data. Various geostatistical algorithms were also investigated for facies modeling and petrophysical properties population within facies. The new models were history-matched using the conventional manual method and two separate assisted-history matching methods that use sensitivity coefficients. The question being addressed was: does constraining the geological models to the same dynamic data always create an imprint over the underlying geological variation and result in similar predictions? Preliminary results indicate that the history-matching overprint tends to mask some of the dramatic geological variations. This can have significant ramifications in modeling strategies, especially when assessing uncertainty in presence of substantial history.

About fifty reservoir characterizations were built using seven different modeling methods, and by varying a combination of reservoir characterization parameters using Experimental Design. The methods include: 1) simple sequential gaussian simulation (SGS); 2) SGS with stratification; 3) SGS with reservoir quality map trend and no stratification; 4) SGS with reservoir quality map trend and stratification; 5) sequential screening simulation with 3D trend; 6) truncated gaussian simulation with trend and 7) general marked point process. These fifty models were all subjected to fluid flow simulation with an active aquifer and limited peripheral waterflooding. The results show that oil recovery at 95% water cut ranges from 29% to 34% in these fifty models despite the large apparent visual differences between them. However, there are large differences, some greater than 200%, in water breakthrough time, water cut and cumulative water production. Based on fluid flow performance, these fifty models are classified into three groups. Group 1 includes models constructed using short variogram ranges in Methods 1 to 4, or medium variogram ranges without stratification in Methods 1 and 3. They behaved more homogeneously and produced flow characterized by later water breakthrough and lower watercut. Group 2 are those models constructed using long variogram ranges in Methods 1 to 4, or medium variogram range with stratification in Method 2 and 4. They behaved more heterogeneously and produced early water breakthrough and high overall watercuts. Even though the mean permeability was similar in the oil zone for the two groups of models, the effects of continuity imposed by the variogram and stratification controlled this disparate behavior. Models of Methods 5 to 7 comprise Group 3, which produced the earliest water breakthrough and the highest overall watercuts. We interpret the difference in water breakthrough time and watercut between Group 2 and 3 to be a result of the higher average permeability (by 100-200 mD) in the oil zone of Group 3 models. Do more complex, geologically realistic models provide the best frameworks for predicting flow performance in sandy shoreface reservoirs with good well control? Some of the simplest models we constructed provided similar flow simulation results as the most complex and soft-data conditioned models. What we do observe, however, is that the more soft conditioning data used and the more complex geologic models tend to give a restricted range in full field flow performance.

This paper was prepared for presentation at the 1999 SPE Reservoir Simulation Symposium held in Houston, Texas, 14-17 February 1999.

Amount and type of organic matter present in Cretaceous and lower Tertiary rocks deposited on Puerto Rico were determined by total organic carbon (TOC) analysis, visual kerogen identification and Rock-Eval pyrolysis. These tests show that the samples commonly contain greater than 1% TOC, and a mixed population of kerogen, with gas-prone kerogen predominating. Based on organic maturity data (vitrinite reflectance, thermal alteration index and Rock-Eval pyrolysis), the maturation history of Puerto Rico is consistent with a two stage thermal model. Before the Oligocene, the island was part of an active magmatic arc. High geothermal gradients and rapid burial left Cretaceous rocks overmature and mature with respect to generated liquid hydrocarbons. After the Eocene, volcanism ceased following collision with the Bahama platform, and lower geothermal gradients combined with slower burial resulted in little maturation.

The active portion of the Portuguese Bend landslide complex is approximately 3 km2 in area and 30-50 m thick. Measured displacement rates range from less than 1 to greater than 30 mm/day on different parts of the landslide, with total displacements over the last 30 yrs ranging from about 10 to greater than 150 m. Six types of breccia, each locally with a foliated matrix, were recognized in the active landslide complex and are absent outside the landslide complex. Slide-body breccias are of two types, the first formed by extensional fracturing during bulk "pure shear" at the top of the landslide (slide-top breccia) and the second by flow of tuffaceous shales and fracture of embedded siliceous shales during simple shear deep in the landslide to the basal decollement (slide-bottom breccias). Slide-margin breccias, also in simple shear, are produced on the lateral margins of individual slide blocks accompanying wrench-fault motion. Other breccias (fault-ramp breccias) are formed during motion over ramps. Coll...

Barbados provides the only subaerial exposures of the Barbados Ridge, an accretionary complex lying outboard of the Lesser Antilles volcanic arc. Within nine measured sections of quartzose turbidites, five major types of sequences are recognized: 1. (1) Progradational sequences (basin plain-outer fan) of uniformly fine-grained, flat-based, base-absent turbidites that thicken upward and gradually become coarser-grained and base-present at the top of the sequence. 2. (2) A single rejuvenation (rapid progradational) sequence was identified as an abrupt progradational sequence above abandoned-lobe deposits, and is not characterized by stacked, well-developed, upward-thickening successions as many progradational sequences are. 3. (3) Channelized successions of coarse-grained, base-present turbidites arranged in packets 5–20 or 50–140 m thick which are midfan and inner-fan deposits, respectively. The distinction between predominance of base-present and base-absent turbidites seems to correspond with the interpreted change from midfan channelized to outer-fan sheet flow deposition. 4. (4) One example of a major retrogradational sequence , of midfan deposits above inner-fan (?) sandstones, has been found on Barbados. Minor sequences of facies retrogradation are relatively common in the outer-fan environment where upward-thinning sandstone beds overlie upward-thickening units to define symmetric sequences. 5. (5) Abandonment successions are: major, as indicated by massive accumulations of mudstone; minor, as indicated by abrupt reversals of upward-thickening trends in outer-fan lobe deposits; and cryptic where indicated by thick (> 4 m) bioturbated zones of massive sandstone.

The provenance of Paleogene sandstones on the island of Barbados is relevant to various models of the tectonic evolution of the southern Caribbean area. Modal point counts of 26 sandstones from Barbados show that the constituent grains most likely were derived from a composite craton interior and orogenic margin. Abundant detrital polycrystalline quartz grains have affinities to a spectrum of low- to high-rank metamorphic/plutonic crystalline source rocks. Microprobe data suggest that feldspars were derived from metamorphic or plutonic source terrances. The complex heavy-mineral suite of the terrigenous sandstones, including glaucophane, chloritoid, titanite, zircon, and aluminosilicates, supports derivation from orogenic and cratonal terranes. Due to the complex history of the South American-Caribbean plate boundary and poor age constraints on timing of deposition, widely contrasting models have been proposed concerning the provenance of the sandstones. Convergence between the Aruba-Orchila arc and continental South America during the Paleogene resulted in the formation of an E-W trending foreland fold-thrust belt and foredeep. Dominant sediment dispersal was to the east–northeast along tectonic strike. The present study shows that the terrigenous sandstones on Barbados probably were deposited in deep-sea fans to the north of present-day Araya Peninsula during and shortly after the late middle Eocene. Probable source terranes include the Guayana Shield, the Caribbean Mountain System/interior foreland fold-thrust belt, and possibly (to a lesser extent) the Lesser Antilles magmatic arc or predecessor arc.

The Niagara fault is believed to represent the suture between a magmatic arc terrane and a passive margin assemblage which were tectonically juxtaposed about 1.85 Ga ago during the Penokean orogeny. Accretion-related deformation is concentrated within an ∼12 km wide belt that straddles the suture. A distinctive structural feature of this belt is the rotation of early fold axes toward parallelism with the direction of bulk extension, resulting in a great-circle stereonet distribution of fold axes within a vertical axial plane. Rocks of both arc and passive margin assemblages crop out in this more highly-strained belt, but there has been no material transfer between them across the fault. Structural style and metamorphic assemblages demonstrate deep-level (beneath the brittle—ductile transition) erosion of the terrane accretion boundary in the southern Lake Superior region. There is some evidence for pure shear during the accretion event, but we are unable to evaluate the amount or sense of simple shear.

A number of diagenetic properties were studied in sedimentary rocks exposed along a 25 km across-strike transect on Barbados, the crest of the Barbados accretionary complex. The island of Barbados consists of three structural levels: a lower (mostly Eocene or undated) basal complex composed of quartzose turbidites, hemipelagites and melange; an intermediate level (Eocene to Miocene) of nappes composed mostly of calcareous pelagite; and a Pleistocene coral cap. Diagenetic studies concentrated on the basal complex although preliminary data from the nappes are also presented. The following subjects were studied: (1) clay mineralogy (% illite interlayers in mixed-layer clays); (2) vitrinite reflectance; (3) organic type and maturation, by Rock-Eval pyrolysis; (4) thermal alteration index of spores and pollen; (5) dry bulk density; (6) silica mineralogy. Taken together, the data can be interpreted to indicate maximum temperatures attained during diagenesis of less than 80°C. Based on a uniform palaeogeothermal gradient of 15°C km −1 , and a temperature at the sediment/water interface of 10°C, maximum burial depths of basement complex rocks were about 2–5 km. Minimum possible burial depths are 1 %), and are immature. Organic matter was derived mostly from terrigenous or oxidized sources (type III kerogens), although some marine planktonic sources are indicated locally (type II kerogens). Organic matter is therefore primarily gas-prone.

Eocene siliceous rocks exposed on Barbados contain both opal-A (biogenic silcia) and opal-CT, and are interpreted as uplifted abyssal-plain or trench deposits, probably correlative to Horizon Ac beneath the Atlantic Ocean floor. Rocks of hemipelagic origin on Barbados are well layered to massive and consist of radiolarite layers (T layers), matrix-supported radiolarian mudstone (F layers), and layers with an intermediate abundance of radiolarian tests (N layers). Large d-spacings of opal-CT (4.08-4.11 A) and low height to width peak ratios indicate poor crystallinity. Opal-CT is most common in silica-rich T layers, and opal-A is best preserved in mud-rich F layers. D-spacing doesn't correlate with Al2O3 weight percent or layer type as defined by relati e abundance of radiolarians versus mud matrix. We infer that the harder, more indurated T layers had better initial permeability prior to induration and that diagenesis was probably influenced by both porosity and permeability differences and to a lesser extent by chemical and mineralogic differences between F and T layers. Comparable diagenetic states between Horizon Ac siliceous sediments below the Atlantic Ocean floor and accreted sediments on Barbados indicate that accretion of siliceous sediments was not accompanied by deep burial and/or elevated temperatures.

The early Proterozoic Menominee Group and equivalents of the southern Lake Superior region everywhere except in the Marquette area, Michigan, consist of quartzite gradationally overlain by iron-formation, with total thicknesses on the order of 0.5-1 km. In the Marquette area, the Siamo Slate of the Menominee Group is present conformably between the basal quartzite and overlying iron-formation, and the total stratigraphic thickness is on the order of 2 km. Based on variations in regional stratigraphic thicknesses and sedimentary facies analysis, it is suggested that the Menominee Group strata in the Marquette area were deposited in a dominantly deeper-water basinal environment, whereas elsewhere in the Lake Superior region sedimentation took place in platformal and broad, shallower-wat r basinal environments. The Marquette basin filled with material deposited by turbidity currents and pelitic sediments, whereas the platformal and broad-basinal strata accumulated in a wave-agitated offshore-shelf environment. Banded iron-formation occurs in both basinal and platformal environments, indicating that deposition of these chemical sediments was perhaps independent of local variations in sedimentary tectonic setting. The preferred regional tectonic setting evokes shelf and basin sedimentation on a rifted passive continental margin.

Quartzose sandstone and mudstone turbidites of Cenozoic and possibly older age constitute a large proportion of strata in the accretionary complex of the Lesser Antilles arc exposed at its structural high on Barbados. The Chalky Mount succession is the thickest (> 628 m) and best exposed of structurally continuous turbidite sequences on the island. Four classes of layer types are distinguished among Chalky Mount beds: single sandstones (between 0.2 and 12 m thick), amalgamated sandstones, multilayers of sandstone (< 20 cm thick) and mudstone, and thick mudstone units. Layer types are divided among 65 types based on Bouma sequences, grading within sandy divisions, and sand to mud gradation. Of these, less than a dozen types occur frequently. Although many layer types cannot be in erpreted uniquely as turbidites, the occurrence of classic turbidites throughout the section indicates the succession is of subsea sediment-gravity flow origin. The vertical succession of layer types can generally be interpreted as reflecting lateral facies migration of a radial fan, and includes from bottom to top: basin plain, distal outer fan, proximal outer fan, midfan, inner fan (?), midfan, abandoned fan, proximal outer fan. The sequence indicates initial progradation, followed by retrogradation and abandonment, then rejuvenation of the former fan. Paleocurrent directions at several horizons in the section indicate transport mainly between north and east relative to present coordinates. The applicability of the radial fan model to the Chalky Mount and other turbidite successions of Barbados suggests that a base-of-slope configuration of the depositional sites was likely.

The island of Barbados exposes the crest of the Barbados accretionary prism of the Lesser Antilles forearc. The sedimentology of Lower Eocene hemipelagite was studied in three fault-bounded packets near Bissex Hill. Three facies were identified based on coherent stratigraphic successions. Facies 1 hemipelagite strata are well layered to massive and consist of radiolarite layers (T layers), matrix-supported radiolarian mudstone (F layers), layers with an intermediate abundance of radiolarian tests (N layers), mud-rich (M) layers containing less than 20% radiolarian tests, rare quartzose sandstone intervals, mud-chip breccia beds, and wispy, laminated, bioturbated units. Many T and N layers have parallel laminations and sharp basal contacts. Few T and N layers have coarse-tail grading, cour structures, and floating mud clasts. Most T and N layers were deposited by turbidity or bottom-water currents. F layers probably represent waning-flow current or suspended-load deposits. Wispy, laminated units are most likely bioturbated T, N, and F layers. Facies 2 strata are wispy, laminated terrigenous mudstones greater than 10 cm thick. Thorough bioturbation indicates slower rates of deposition relative to facies 1 strata. Facies 3 is mudstone composed of sparse radiolarian tests floating in a terrigenous-mud matrix. Facies 3 deposits are interpreted as extensively bioturbated sediment which, prior to reworking, contained fewer intercalated T layers. Bulk-rock chemistry indicates that the hemipelagites were derived from terrigenous and biogenic sources. Weight-percent Al2 UB>O3 correlates with the percentages of clay-sized material in T, N, and F layers. Low total organic carbon (bitumen-free; 0.06-0.83%) and Type III kerogens indicate an oxic depositional environment. The hemipelagic rocks were deposited in abyssal-plain or outer-trench-slope environments.

Over 1,000 muddy beds were analyzed to determine variations in sequences of sedimentary structures between various fan facies associations in the Paleogene terrigenous turbidites exposed on Barbados. Terrigenous muds of the study area exhibit textural and compositional similarities to fine-grained turbidites accumulating in the deep sea today. A coding scheme was developed in the field in order to tabulate common and recurring sedimentary subunits of Bouma Tcde intervals. Markov chain analyses were performed on 100 vertically successive beds per facies association to determine the significance of various transitions of sedimentary subunits in the Tcde intervals. Although variation in sequences of sedimentary structures within associations is great, discriminating differences are obser ed. Key variations are 1) presence of starved ripples in mud-rich sections of the middle fan and rarity elsewhere, and 2) variation in thickness of green hemipelagic muds, whose thickness is greatest in the inner-fan and basin-plain environments and thinnest in the middle fan. Enigmatic thinning-upward microcycles of four or more muddy turbidite beds occur commonly in outer-fan and basin-plain associations on Barbados.

The Golconda allochthon in Nevada consists of structurally complex mid to late Paleozoic basinal sedimentary and volcanic rocks deformed under low temperature and pressure conditions and emplaced onto the North American shelf in the Permo-Triassic. Above the basal thrust in the Antler Peak area, the allochthon consists (in ascending structural order) of late Early to early Late Permian thick bedded chert (‘Pumpernickel subunit 1 ’); Pennsylvanian and Early Permian thin bedded chert, argillite and siltstone (‘Pumpernickel subunit 2’); Mississippian siliciclastic turbidites (‘Jory unit’) and undated chert and argillite of the ‘Trenton unit.’ Open to tight folds in subunit 1 trend north-south and are east-vergent. Folds in thinner bedded subunit 2 vary considerably in style. Fold axes in this unit are distributed within the mean axial plane (oriented N-S, 60°W) but exhibit a west-northwest trending data gap. Boudin axes measured in zones of highly disrupted layering (interpreted as zones of movement or shear) exhibit similar patterns without data gaps. These girdled distributions in part reflect rotation of these elements upon greater strain towards the east-west trending direction of tectonic transport or towards parallelism with the X axis of the strain ellipsoid. Asymmetry of folds indicate movement was west to east. Locally developed late stage folds in subunit 2 are subhorizontal, trend north-south, and are compatible with fold-fault solutions that indicate east-directed thrusting. Striations measured within fault zones yield bidirectional east-west slip lines. Our fold and fault data indicate that east-west shortening of these basinal sediments was accomplished by folding and east-directed thrust faulting during a single progressive deformational event. This conclusion is supported by the presence of a major west-dipping thrust fault in the map area that places Mississippian over Permian rocks. The allochthon formed as an east-vergent submarine fold and thrust belt that was subsequently thrust on the continental shelf. Autochthonous rocks beneath the Golconda thrust are virtually undeformed, suggesting that most of the structure in the allochthon formed prior to its ultimate emplacement.

The Florence-Niagara terrane is an elongate fault-bounded block ∼ 10 km wide and 80 km long, composed of miogeoclinal assemblage lithologies, bounded on the north by a generally coherent miogeocline extending to the Superior craton, and on the south by a probably exotic, magmatic arc terrane. The Florence-Niagara terrane is composed of a collage of eight fault-bounded packets. All fault packets are characterized by faults that trend west to west-northwest, with a dominant west-northwest-striking, deeply dipping foliation. Elongation lineations plunge down the dip of foliations in all cases except in packet 1, where bulk extension locally parallels shallowly plunging fold axes, a probable product of superimposed strains. Fold axes are subhorizontal, or girdled in the mean foliation plane, or plunge steeply. Packets containing girdled fold axes probably were subjected to greater strain than were packets with subhorizontal fold axes. Steeply plunging fold axes are not easily explained, but they were, perhaps, formed by noncoaxial deformation. One of the packets contains a younger generation of folds formed during east-west shortening. Another packet contains folds associated with later strike-slip faulting. Convergence between the magmatic terrane and the miogeocline probably resulted in accretion of miogeoclinal sediments in the forearc area of the magmatic terrane. The Florence-Niagara terrane may represent such a zone of forearc accretion formed prior to, and during, collision, and thus it may be referred to as an accretionary complex.

The island of Barbados exposes the crestal zone of the remarkably broad accretionary prism of the Lesser Antilles foreacrc. The architecture of Barbados is three-tiered: an upper arched cap of Pleistocene reefs that record rapid and differential uplift of the island, an intermediate zone of nappes of mainly abyssal or deep bathyal pelagic rocks, and a basal complex whose lithotypes extend to substantial depth and may be representative of the bulk of the western or inner accretionary prism. The exposed basal complex consists of generally steeply dipping ENE to NE-striking fault-bounded packets which contain rocks of one of three lithic suites: terrigenous (quartzose turbidite and mudstone), debris flow, and hemipelagic (chiefly radiolarite). Present but imcomplete rock dating indicates that the terrigenous and hemipelagic suites and the pelagic rocks of the intermediate zone are age overlapping in Early and Middle Eocene time. Deformation within packets of the basal complex is systematic, pre- or synfault, and indicative of shortening that is generally normal to packet boundaries. A unit of terrigenous materials that probably underwent local resedimentation in the Miocene is recognized in wells, but its relationship to exposed rocks is uncertain. The packet-bounding faults of the basal complex are interpreted to have been primary accretionary surfaces which may have been reactivated by later intraprism movements. Exposed sedimentary rocks of Barbados can be successfully assigned to contemporaneous depositional sites associated with an accretionary prism: terrigenous beds to a trench wedge that was connected to South American sediment sources, debris flow to trench floor or slope basin accumulations of material derived from the lower slope, hemipelagic to Atlantic plain strata, and pelagic rocks of the intermediate zone to deep outer forearc basin sites. The decollement at the base of the intermediate zone is probably due to uplift and arcward motion of the crestal zone of the accretionary prism with respect to the forearc basin during progressive prism growth. Principal uplift of the prism seems to have started, apparently abruptly, in the Miocene. Quaternary uplift of Barbados may be due partly to local diapirism. Paleogene subduction that created the arcward region of the prism probably occurred in a differently configured zone from the present one.

The early Proterozoic Chocolay Group in the Lake Superior region is a cratonal sequence consisting of a basal unit overlain successively by quartzite, dolomite, and slate formations. Two regional depositional environments are described: (1) present-day structural troughs (Marquette, Menominee, and Felch, Michigan) show evidence of having sedimentary-basin precursors; and (2) other areas (Gogebic Range, Michigan) show evidence of sedimentation in platform settings. The basal unit in the Marquette and Menominee troughs is as much as 160 m thick and was probably deposited by alluvial processes. In the Gogebic Range, the basal unit is less than 10 m thick and resembles lag deposits at the base of transgressive sequences. The overlying quartzite contains cross-beds that show two regional sedimentary-vector (paleocurrent) distributions: (1) unimodal low-variance distributions with sedimentary vectors oriented subparallel to local structural trough axes (Marquette, Menominee, and Felch troughs); and (2) polymodal distributions with sedimentary vectors showing greater dispersion about the mean (Gogebic Range). As indicated by many varieties of sedimentary structures (silicified evaporites, cryptalgal structures, intraclast breccias, and so on), the carbonates and interstratified siliciclastics of the Chocolay Group were deposited in subtidal to supratidal conditions. Taylor noted, in 1972, that certain members of the Chocolay Group carbonates thicken toward the center of the Marquette trough, with concomitant facies changes. The uppermost pelite unit of the Chocolay Group was probably deposited below normal wave base. It is concluded that the basins and platforms were formed on a craton undergoing rift-type tectonism.