Date of Graduation

12-2020

Document Type

Thesis

Degree Name

Master of Science in Geology (MS)

Degree Level

Graduate

Department

Geosciences

Advisor/Mentor

Glenn Sharman

Committee Member

T.A. McGilvery

Second Committee Member

Barry Shaulis

Keywords

Detrital Zircon, Heavy Minerals, Provenance, XRF

Abstract

The Stevens sandstone is a prolific hydrocarbon reservoir in the southern San Joaquin Basin that has produced greater than five billion barrels of oil equivalent and represents an important exploration target. The San Joaquin basin has a complex depositional and structural history related to the overprinting of a Mesozoic-Paleogene forearc basin by transtension and transpression along the San Andreas fault system that presently borders the southwestern side of the basin. The Stevens sandstone consists of quartzo-feldspathic, deep-water turbidite deposits and is primarily confined within the subsurface of the southern San Joaquin basin. Due to this mode of deposition and the changing paleogeography and paleobathymetry associated with active translation of the Salinian block along the San Andreas fault during deposition of the Stevens sandstone, the provenance could greatly vary across the basin which in turn can impact reservoir properties. Understanding the Stevens sandstone’s provenance is thus critical for predicting mineralogic composition and subsequent controls on reservoir characteristics which in turn could affect the future exploration and exploitation of the Stevens sandstone.

To address this, a multi-proxy provenance analysis of the Stevens sandstone was conducted to characterize sandstone mineralogy (thin section modal point-counts), geochemistry (handheld X-ray fluorescence), heavy mineral analysis, and detrital zircon U-Pb geochronology. In total, 36 original samples were analyzed that span both the basin margins (zones of sediment input and bypass) and basin center (zone of sediment mixing). The petrographic data revealed overall major framework mineralogy (42-54-4; Q-F-L) with slight variances across the basin, including quartz enriched sands on the northwestern side of the basin. The hXRF results reflect the major framework minerals and diagenetic factors such as cementation. hXRF data is thus influenced by both provenance and diagenetic factors, with carbonate cementation (calcite and/or ankerite) strongly influencing the abundance of Ca in four samples.

The heavy mineral assemblages provide a robust data set that differentiate different sediment sources around the San Joaquin basin. The eastern Stevens sandstone has elevated concentrations of titanite, hornblende, and epidote. The western Stevens sandstone contains the highest proportions of apatite, zircon, and rutile. The southern Stevens has the highest abundance of monazite. The central basin represents a mixture of heavy minerals derived from around the basin periphery, with greater representation of minerals derived from the southern and western margins. Detrital zircon U-Pb geochronology also differentiates the Stevens sandstone on the basis of varying abundances of late or mid-Cretaceous, Jurassic, and Eocene zircon grains. Jurassic zircon with elevated Th/U is present on the eastern and southern basin margins, whereas Th/U values are lower on the western basin margin.

The use of multiple provenance proxies suggests that Stevens sandstone provenance varied around the basin margin, with sediment sourcing from the Sierra Nevada highlands to the east, the ancestral Tehachapi and San Emigdio Mountains to the south and southwest respectively, the translocating Salinian Block to the west, and possibly the ancestral Temblor Range in the northwest. Sediment within the basin center displays a more consistent provenance signature than on the basin margins, suggesting progressive mixing and provenance homogenization from source-to-sink. Sediment sources from the south and possibly western margin of the basin appear to outweigh sediment contributions from the eastern side of the basin.

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