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 Sequence stratigraphy has proven to be an invaluable tool for the analysis of coarse-clastic depositional systems and the integration of observations across scales from reflection seismic to scanning electron microscope. Applications to mudstone-dominated depositional sequences have been more limited, despite the fact that mudstones make up more than 60% of the global sedimentary volume and generally provide the most complete record of sedimentation in a basin. During the late 1970s and through the 1980s, Bob Garrison and his students at the University of California–Santa Cruz conducted numerous studies that revealed the basic sedimentary and stratigraphic framework of the Monterey Formation in California, advancing our understanding of the sedimentary processes at work in these deep-margin basins. We expanded on that framework using direct observations from outcrops and cores that have been integrated with other subsurface data, as well as a wide variety of information derived from paleontologic, chronostratigraphic, geochemical, and compositional analyses to illustrate a sequence-stratigraphic approach to interpreting fine-grained rocks and their associated depositional systems in these settings. These were some of the earliest investigations of mudstone sequence stratigraphy focused on slope and basinal environments. In this study, observations from outcrops in the Pismo Basin, California, provided the basis for developing a detailed sequence-stratigraphic framework for the Monterey Formation, expanding on the broad-scale characterization of Garrison and his colleagues. These outcrops represent deposition during different phases of basin evolution and in different borderland-type basin settings (slope and basin depocenters). Comparison of coeval strata from different depositional settings and locations documented variation at both the sequence and parasequence scale. Variation of parasequence character, in particular, provided a valuable tool for enhanced understanding of deposition and diagenesis in these margin basins. Extrapolation to the subsurface using gamma-ray logs greatly enhanced basinwide application compared to limited, partial-stratigraphic-section outcrops, and it facilitated the lateral characterization of mudstone depositional sequences. These elements served as the building blocks for improved models of deposition in margin-basin settings.