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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.

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.

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.

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.

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.