ABSTRACT The Monterey Formation of Central and Southern California has produced billions of barrels of oil since the early 1900s. The Monterey Formation in the Santa Maria Basin is a tectonically fractured reservoir, meaning that the fractures formed through natural geologic processes; they are not human-generated artifacts. Open natural fractures provide the effective porosity for oil storage and the permeability pathways through which oil flows from rocks to wells. Monterey strata are notable for a diverse range of lithologies characterized by contrasts in texture and composition. Not all Monterey rock types contain natural fractures. Structural geologists applied the concepts of mechanical stratigraphy to the Monterey Formation to explain fracture variability. Hard rocks, including chert, porcelanite, and dolostone, contain extensive open-fracture systems, while softer lithologies like siliceous mudstone and organic-rich mudstone have few or no open fractures. However, the words “hard and soft” or “strong and weak” are inexact and subject to interpretation. This report constrains these qualitative descriptions by using engineering-geology data to associate rock properties with quantitative measurements of rock mechanical strength.

ABSTRACT Rock properties play a critical role in dictating styles of deformation at all spatial scales, yet the effect of changes across and within diagenetic transition zones has been little studied, despite profound impact on resulting mechanical stratigraphy. Our analysis of the variation of fold strain at map scale and outcrop scale of the highly siliceous Monterey and Sisquoc Formations in the southern Santa Maria Basin, California, provides insight into the interplay among deformation, diagenesis, and rock composition. Diagenetic modification of these rocks has created intervals with high interstratal and interformational contrasts in competence. Map-scale analysis showed large variation in fold strain within the same area, with shortening values ranging from 5.5% to 21.1% between siliceous formations of different diagenetic grade and competence. Apparent shortening in the competent, diagenetically altered, thinly bedded Monterey Formation is twice as high as that in the overlying highly porous, diatomaceous, more massive Sisquoc Formation. The large difference in measurable apparent shortening suggests that the same amount of actual strain was chiefly accommodated by folding in the Monterey Formation versus horizontal compaction in the Sisquoc Formation, since there is no evidence of a detachment between the units. Strain analysis at outcrop scale provided insight into the ways in which both units express such different shortening ratios without having an unconformity or detachment fault between them.

Basins around the margin of the Puna Plateau in NW Argentina each record the onset over a few hundred thousand years of alluvial-fan facies sedimentation with the deposition of the Punaschotter conglomerates in the late Cenozoic, despite differing tectonic histories. Their striking similarity suggests that these conglomerates might have a common origin, such as the climatically driven coarsening and increase in sedimentation rate proposed to have occurred worldwide at 4–2 Ma. With this contribution, we present new U-Pb geochronology of intercalated ashes that bracket the onset of Punaschotter deposition along the margin of the Puna Plateau in the Fiambalá and El Cajon Basins. Combining this with existing data, we explore the relative roles of climate and tectonics in shaping sedimentation in this region. Our analysis reveals that Punaschotter deposition was diachronous and only occurred if both aridity and structural deformation were present. Development of orographic barriers and global climate change may have contributed to establishing regional aridity. In the eastern part of the study region (Santa Maria and Angastaco), onset of Punaschotter deposition tracks the establishment of aridity. In the already arid western basins (Fiambalá, Corral Quemado), a reactivation of structural deformation may have triggered the onset of Punaschotter deposition. Our study shows that initiation of coarse alluvial facies in this part of the Andes largely preceded the 4–2 Ma global climate change and emphasizes the coupled nature of local climate and tectonics in controlling sedimentation.

ABSTRACT Miocene Monterey Formation reservoirs of California contain unique reservoir rocks and additional complexity from fractures. The rocks contain a high proportion of biogenic silica derived from diatoms. Porcelanite, chert, and siliceous shale, along with dolomite, are the primary reservoir rocks of these fine-grained siliceous reservoirs. Strata are typically thinly-bedded and heterogeneous, and are difficult to adequately describe using standard reservoir characterization techniques. Our approach focuses on opal CT and quartz-phase rocks, and relies on an integration of tools to characterize both the matrix and the fractures. We attempt to quantify rock and reservoir properties, and examine the controls these factors exert on reservoir performance with field examples from Hondo (offshore), offshore Santa Maria, Elk Hills, and North Shafter. Matrix properties are affected by two primary factors, the ratio of silica to fine-grained detritus (mostly clay), and the silica phase of the rocks. The best matrix properties are found in quartz-phase porcelanite with low clay content. Rocks with higher clay volumes, or those with a high proportion of opal CT silica, have smaller pore throats, lower oil saturation, and lower permeability. In most cases, well-log techniques relying on porosity logs and spectral gamma ray logs successfully predict the clay volume and silica phase of rocks in the subsurface. Fractures are common in nearly all Monterey Formation reservoirs. Fracture distribution is controlled by mechanical stratigraphy and structural position. Cores and outcrops have traditionally supplied data and analogs for fracture characterization. This data set has been vastly expanded by the use of borehole image logs, particularly from horizontal wells. Organizing fracture data using a fracture network model enhances our predictive capability for fracture distribution and the ability to visualize likely flow paths in the reservoir. Fluid properties are also an important factor influencing Monterey reservoir behavior. Low-gravity, high-viscosity oils are generated by high-sulfur Monterey source facies, and are difficult to produce economically even from some reservoirs with excellent matrix and fracture properties. Knowledge of source facies distribution, burial history, and generation kinetics is needed to predict the hydrocarbon properties of Monterey reservoirs.