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Characterization and Correlation of the Kreyenhagen Formation in the Northern San Joaquin Basin, California: A Chemostratigraphic Perspective

Abstract

Acquisition of bulk inorganic elemental data, and chemostratigraphic characterization and correlation have become routine analyses in resource plays over the last 10 years. This analysis allows detailed information about depositional facies, mineralogy, redox conditions and sediment provenance to be gathered and placed in stratigraphic context. In this study, the same approach is used to help define, characterize, and correlate the Kreyenhagen Formation. The Kreyenhagen Fm is Middle-Late Eocene in age,-and comprises siliceous shales and siltstones with occasional carbonates. Over the last five years, the Kreyenhagen has become a notable target within the San Joaquin Basin, for both conventional and unconventional exploration and development. However, data relating to the Kreyenhagen is limited. Unlike many other onshore US shale plays, the Kreyenhagen is not overburden to any geographically extensive and prolific conventional reservoirs and has never been extensively drilled. As such, the interval is less studied and has less data and samples available for analysis than other, more “famous” US shales. In this paper we aim to answer three primary questions; A) Can the Kreyenhagen be characterized using elemental data? B) What does the data tell us about the depositional environments? C) How laterally homogenous is the Kreyenhagen? Data gathered from cutting samples in 7 wells within the northern San Joaquin Basin show that the Kreyenhagen can be readily differentiated from over and underlying formations using a chemostratigraphic approach. Additionally, it is possible to subdivide the Kreyenhagen into chemostratigraphic units that reflect differing redox conditions, facies, biogenic silica contents and abundances of detrital minerals, suggesting that this is a relatively heterogeneous and dynamic depositional system, in a stratigraphic sense. Detailed core analyses, integrating elemental and magnetic susceptibility confirm that not only do these sequences show heterogeneity down to a 6-12 inch scale, but that they also show a certain degree of cyclicity at this scale. Furthermore, the geochemical units defined within the Kreyenhagen, appear to be laterally continuous, allowing the chemostratigraphic correlation to be applied to wells analyzed across the northern San Joaquin Basin. Importantly, this work also provides an understanding of spatial and stratigraphic heterogeneity within the Kreyenhagen Fm and allows for comparison to other US shale plays.